Amorphous electron-accepting materials for organic optoelectronics
Ganesan, P. - \ 2007
Wageningen University. Promotor(en): Ernst Sudhölter; Han Zuilhof. - [S.l.] : S.n. - ISBN 9789085046875 - 124
elektronenoverdracht - elektronica - optica - macromoleculaire stoffen - synthetische materialen - electron transfer - electronics - optics - macromolecular materials - synthetic materials
The importance of organic materials for use in electronic devices such as OLEDs, OFETs and photovoltaic cells has increased significantly over the past decade. Organic materials have been attractive candidates for such electronic devices because of their compatibility with high-throughput, low-cost processing techniques and their capability to be precisely functionalized to afford desired performance attributes. This has already lead to commercial applications of OLED in car-audio, cell phones and digital cameras. To further improve the performance of these materials, many research groups are focusing on controlling the morphology of the organic films by carefully fine tuning the degree of crystallinity. Synthetic organic chemistry plays a pivotal role in this, as the toolbox of organic chemistry allows the formation of tailor-made materials that form a uniform film in their solid state.
Organic photovoltaics is a rapidly growing field since the exponential growth of energy needs and the rapid depletion of the fossil fuels have led to a compelling demand for alternative sources of energy. The traditional inorganic solar cells are based on silicon. Although energy efficiencies around 25 % have been reached in silicon-based solar cells, for many climatological conditions they are not cost effective since the production of such photovoltaic device requires demanding conditions like high processing temperature, clean room facilities, etc., which prevents the commercially attractive bulk production. Therefore, a significant research efforts are focused on easily processable organic materials for use in photovoltaic devices. Usually the organic photovoltaic devices consist of an electron donating polymer (p-type material) and a fullerene-based electron-accepting material (n-type material). For such devices energy efficiencies up to 5.2 % have been reported. A significant issue in these devices is the crystallization of fullerenes, which easily leads to excessive phase separation p and n-type materials.
Amorphous molecular materials may exhibit isotropic properties due to the absence of grain boundaries. Naphthalene diimides (NDI) are known to have a high conductivity and electron-accepting capability from a variety of electron donors. This thesis aims at making use of the properties of novel amorphous materials with NDIs to obtain uniform films without phase separation and crystallization for use in organic solar cells.
Chapter 1 gives an overview of the organic (opto-)electronic materials and of the working principles of several devices that are based on such materials. Solar cells, in particular organic heterojunction cells, are described in detail. The importance of the nanoscale morphology in such heterojunction devices is discussed, together with expected advantages of amorphous materials to obtain films with the desired morphology. Finally the outline of the thesis is given.
A new approach towards the design and synthesis of amorphous n-type materials with NDIs is presented in Chapter 2. The tetrahedral shape of the molecule yields the amorphous material properties, which are decoupled from its optoelectronic properties. In the first tetrahedral molecule the non-directionality available via tetrahedral cores, as present in tetra(phenyl) methane, is used. This tetrahedral material with 4 NDIs has been characterized for its steady-state and transient optical behavior and for its ground-state electrochemical properties. It has been shown to display a conductivity of 0.03 cm2 V^s"1 in neat film, and exhibited a near-complete quenching of the p-type. polymer fluorescence. The blended films of this tetrahedral molecule with polymeric p-type materials have a very uniform morphology and demonstrated high transient charge carrier mobility.
The photophysical properties of the tetrahedral molecule with naphthalene diimide (NDI) moieties and of two model compounds are described in Chapter 3. One of the model compound is a symmetrically dialkyl substituted NDI and the other model compound is an NDI with an alkyl chain and a phenyl ring substitution. The steady-state absorption and fluorescence spectra of dialkyl-substituted NDI are in agreement with literature. While the absorption spectra of the phenyl-substituted molecules are similar to all other NDIs, their fluorescence showed a broad band between 500-650 nm. This band is sensitive to the polarity of the solvent, and is attributed to a charge-transfer (CT) state. The absorption spectra and lifetime (10 ± 1 ps) of the electronically excited singlet state of a dialkyl-substituted NDI was determined by femtosecond transient absorption spectroscopy, and the latter was confirmed by picosecond fluorescence spectroscopy. Nanosecond flash photolysis showed the subsequent formation of the triplet state. The presence of a phenyl substituent on the imide nitrogen of NDI resulted in faster deactivation of the singlet state (lifetime 0.5- 1 ps). This is attributed to the formation of a short-lived CT state, which decays to the local triplet state. The faster deactivation was confirmed by fluorescence-lifetime measurements in solution and in a low-temperature methyl-tetrahydrofuran (MTHF) glass.
Another new class of amorphous materials with NDIs is described in Chapter 4. Cyclic siloxanes are known to exhibit a size-dependent structure with amorphous properties. Novel cyclic siloxanes with pendent naphthalene diimides were synthesized via a hydrosilylation reaction, to form amorphous electron-accepting materials. These materials were studied for their basic photophysical properties using steady state and time-resolved techniques. The fluorescence spectra revealed the formation of excimers, which was shown to be solvent dependent. Fluorescence quenching studies of blends of these siloxanes with p-type polymers (P3HT, MDMO-PPV) showed>99.9 % fluorescence quenching of the latter polymers. Mixtures of these siloxanes and p-type polymers gave homogeneous amorphous films from chloroform solution, and films with micro-crystallinity were obtained from o-dichlorobenzene solutions. The time-resolved microwave conductance in films formed from o-dichlorobenzene was higher than in films formed from chloroform, which is attributed to nanoscopic phase separation that enhances the interfacial charge separation. Due to this reason, they also showed a better conductivity than the tetrahedral molecule.
For a good charge transport in the active organic heterojunction films, it is necessary to have a bicontinuous film with nanoscale phase separation. For this reason it is essential that the NDIs are interacting with each other. In order to achieve this, four novel naphthalenediimide (NDI) side-chain polymers were synthesized by grafting NDI onto poly(R-alt-maleic anhydride) backbone polymers with various R groups and molecular weights [R= styrene, 1-octene and 1-octadecene]. The synthesis and other characterizations of these materials are described in Chapter 5. These polymers were obtained with a degree of substitution up to 60 %, and showed a high solubility in solvents like chloroform. Their absorption and fluorescence spectra were studied both in solution and in thin films, with specific attention to the fluorescence quenching of P3HT in thin films. The results show that in all four polymers the NDI chromophores form n-stacked dimers in solution exhibiting excimer fluorescence. The morphology of the blends of the grafted polymers with P3HT was studied at various weight ratios, and revealed phase separation into domains of um dimensions. These blends were also studied using time-resolved microwave-conductivity for their photo-induced charge carrier generation efficiency, which showed appreciable generation of charge carriers, although significantly lower than observed in blends of P3HT with PCBM or oligomeric n-type siloxanes described in the previous chapter.
Overall it could be summari2ed that the formation of amorphous films with structural elements based on a tetrahedral organization or flexible siloxane rings provide a novel way to construct materials that can be used as p-type or n-type materials in optoelectronic devices. Use of these elements with appropriate aromatic systems containing more extended xc-systems seems a viable route to further improve the potential of organic optoelectronic materials.
|Kunstgras even effectief als strooiselbak
Rooijen, J. van - \ 1998
De Pluimveehouderij 28 (1998)8. - ISSN 0166-8250 - p. 24 - 25.
dierenwelzijn - huisvesting, dieren - hennen - stallen - roostervloeren - vloeren - ligstro - producten - latex - rubber - afdeklagen - macromoleculaire stoffen - animal welfare - animal housing - hens - stalls - grid floors - floors - litter - products - latex - rubber - coatings - macromolecular materials
Om te kijken wat beters is voor het welzijn heeft het Praktijkonderzoek Pluimveehouderij onderzoekgevens van de strooiselbak en kunstgrasmat vergeleken
Kunststof roosters: er zijn verschillen!
Hoofs, A. - \ 1995
Praktijkonderzoek varkenshouderij 9 (1995)1. - ISSN 1382-0346 - p. 12 - 14.
huisvesting, dieren - dierhouderij - dierlijke meststoffen - dierenwelzijn - afdeklagen - kraamstallen - vloeren - roostervloeren - latex - biggen - varkens - productiviteit - producten - rentabiliteit - rubber - drijfmest - stallen - macromoleculaire stoffen - animal housing - animal husbandry - animal manures - animal welfare - coatings - farrowing houses - floors - grid floors - latex - piglets - pigs - productivity - products - profitability - rubber - slurries - stalls - macromolecular materials
In kraamhokken met volledig roostervloer voldoet het Tendernova-rooster beter aan de eisen dan de Tenderfoot, de MIK-Soft en de rond geplastificeerde metalen driekantroosters
Adsorption of charged diblock copolymers : effect on colloidal stability
Israels, R. - \ 1994
Agricultural University. Promotor(en): G.J. Fleer, co-promotor(en): F.A.M. Leermakers. - S.l. : Israels - ISBN 9789054852315 - 100
adsorptie - sorptie - kunststoffen - industrie - colloïden - dispersie - macromoleculaire stoffen - adsorption - sorption - plastics - industry - colloids - dispersion - macromolecular materials
In this thesis we present Scheutjens-Fleer (SF) calculations on the adsorption of diblock copolymers. More specifically, we restrict ourselves to adsorption at uncharged surfaces, while the specific type of block copolymers we consider have one uncharged adsorbing "anchor" block and one non-adsorbing charged "buoy" block. We compare these systems with a more simple one, that of the charged brushes. A polymer brush is the structure that is formed when polymer molecules are attached by one end to a surface, with a density high enough so that the chains are obliged to stretch away from the interface. Complementary to the numerical computations, the scaling behaviour of these systems is discussed. We study the structure of the adsorbed layer, and try to answer ultimately the question what the effect of the adsorption is on colloidal stability.
In the introductory Chapter 1 we explain the most important terms and discuss the relevance of this study. Furthermore, we introduce the SF model and compare it to two other approaches: Monte Carlo and Scaling. Finally, we briefly present the available information on the two systems under consideration, and compare them to a number of related systems.
The body of this work is divided in two parts. In Chapters 2 and 3 we discuss charged brushes, systems that are simpler than diblock copolymer adsorption, but still exhibit similar characteristics. In the subsequent two chapters we then proceed to the adsorption of diblock copolymers (Chapter 4) and its effect on colloidal stability (Chapter 5).
In Chapter 2 we present numerical results from the SF model for the structure and sealing behaviour of charged brushes and compare these with predictions of an analytical model on the same system. The relevant parameters are the chain length N , the average anchoring density σ, the average segmental charge αon the chains, and the salt concentration φ S .
At high anchoring densities, three regimes of brush behaviour may be distinguished. In the salt-free case, the behaviour of the brush is dominated by electrostatic interactions if the charges are high (the so-called Osmotic Brush) or by non-electrostatic excluded volume interactions if the charges are low (the quasi-Neutral Brush regime). Upon adding salt a third regime can be found: the Salted Brush. The behaviour in this regime, although resulting from electrostatic interactions, is very similar to that in a neutral brush and can effectively be described using an electrostatic excluded volume parameter vel ≈ φ S-1α2. We find excellent agreement regarding structure as well as scaling relations between the two theories in these three (high anchoring density ) regimes. At extremely low anchoring densities, the agreement with the analytical theory is less good. This is due to the breakdown at low densities of the mean-field approximationpresently used in the numerical model.
In between, at intermediate anchoring densities, the analytical theory predicts a very peculiar regime, where the thickness H scales as H ≈N3σ-1α2. This so-called " Pincus Brush ", named after the author who originally described it, is not recovered with the numerical theory. For the wide range of parameters used, we find the Pincus regime is too small to be detected. This is probably true for any reasonable set of parameters.
In Chapter 3 we consider the acid-base equilibrium of the charged brush segments, so that grafted weak polyacids may be studied. For these systems the charge of a brush segment depends on its local environment and on the pH in the solution. The scaling dependence of the thickness H on the salt concentration φ S for such a brush is very different from that for a conventional charged brush with constant charge density.
In Chapter 4 we proceed to the adsorption of ionic diblock copolymers. One block, the "anchor", consists of N A uncharged adsorbing A segments, whereas the "buoy" block has N B segments which carry a fixed charge and are non-adsorbing. Upon adsorption these sorbed amount and layer thickness as a function of the block lengths N A and N B , the charge αe on the B segments, and the salt concentration φ S in each of the four regimes. The scaling relations axe checked using SF calculations.
The existence of two regimes for uncharged diblock copolymer adsorption has been reported previously. We argue that those HU and LU regimes are closely related to the two regimes HC and LC we find for charged molecules. Scaling relations can be translated from the uncharged to the corresponding charged regimes by replacing the excluded volume parameter v of the buoy segments by an effective electrostatic excluded volume parameter ve = α 2/φ S .
In the LC regime the chain density σscales as σ α( N A /N B ) 3/2ve-1and the layer thickness H as H α ( N A /N B ) 1/2. The latter scaling is independent of ve . Using the SF model, these relations axe found to be valid for an adsorbed amount of A segments below 10% of monolayer coverage.
In the HC regime the adsorption is dominated by the anchoring block and the scaling relation σ α1/ N A for the chain density is identical to that for uncharged molecules. The SF calculations show that this regime will not be reached in practical situations.
Finally, we address in Chapter 5 the effect of the adsorption of charged diblock copolymers on colloidal stability. Using again a scaling as well as the SF approach, we focus on the LC regime and find that the adsorbed layer may cause a significant repulsive interaction between two surfaces, despite the very low adsorbed amounts. The magnitude of this repulsion is well within the range that could be mea, sured using a surface force apparatus. Moreover, we estimate that the repulsive interaction may be strong enough to induce kinetic stability, provided the particle radius is large enough. Upon lowering the salt concentration, however, a critical concentration φ S * is reached eventually, below which the repulsion is no longer strong enough to effect colloidal stability. The scaling analysis predicts that this critical concentration scales as:
where R is the radius of the particles and the other parameters have been defined above. Thus the repulsive interaction decreases when the relative importance of charge effects increases, i.e., with decreasing salt concentration, and increasing buoy block length or buoy block charge. This counterintuitive behaviour can be explained from the effect that electrostatic interactions have on the adsorbed amount: stronger interactions lead to a lower adsorbed amount, which, in turn, leads to a weaker repulsion. The SF calculations confirm these scaling predictions.
Copolymers at the solid - liquid interface
Wijmans, C.M. - \ 1994
Agricultural University. Promotor(en): G.J. Fleer; F.A.M. Leermakers. - S.l. : Wijmans - ISBN 9789054852100 - 184
adsorptie - sorptie - oppervlakten - grensvlak - vloeistoffen (liquids) - kunststoffen - industrie - vaste stoffen - oppervlakteverschijnselen - tweefasesystemen - macromoleculaire stoffen - adsorption - sorption - surfaces - interface - liquids - plastics - industry - solids - surface phenomena - two-phase systems - macromolecular materials
Copolymers consisting of both adsorbing and nonadsorbing segments can show an adsorption behaviour which is very different from that of homopolymers. We have mainly investigated the adsorption of AB diblock copolymers, which have one adsorbing block (anchor) and one nonadsorbing block (buoy). The anchors adsorb from solution onto a surface and the buoys protrude into the solution, Thus, a polymer brush is formed. This name is derived from the resemblance between the protruding chains of B segments and the bristles of a brush. The presence of the adsorbing segments can be neglected when studying the characteristics of such a polymer brush, which is then modelled as (B-) homopolymer molecules which are terminally attached to the surface of a solid interface.
In chapter 1 two self-consistent field (SCF) theories are introduced which give a description of such a polymer brush. The first of these theories is a lattice model. It takes into account all possible conformations that can be generated on a lattice; the molecules are treated as freely jointed chains. The overall volume fraction profile (that is, the polymer volume fraction φas a function of the distance z to the surface) is then found by weighting each conformation with an appropriate Boltzmann factor. This theory can both be applied for systems with end-attached polymer molecules and for systems with freely adsorbing chains. The volume fraction profiles for any given system must be found using a complicated numerical procedure.
The second theory explicitly assumes that the polymer molecules are strongly stretched. Under this assumption only a fraction of all possible molecular conformations need be taken into account to find the volume fraction profile. Although this approach is less exact than the lattice model, it has as a major advantage that an analytical expression can be derived for the shape of the volume fraction profile. A simple algebraic expressions is also available for the brush height, if only the second and third order terms of a virial expansion of the free energy of mixing polymer and solvent are taken into account. If this free energy is accounted for in a more exact manner, one must (numerically) calculate the brush height from a (simple) integral equation.
In the first chapter we make a detailed comparison of the predictions of both theories for a polymer brush at a flat surface in a low molecular weight solvent. In general an excellent agreement is found between the results of both theories. Significant deviations only occur very close to the surface and at the periphery of the grafted layer. In the lattice model there is a small depletion zone near the grafting surface, which is caused by the entropical restrictions imposed upon many polymer conformations by this impenetrable surface. The lattice calculations further show a "foot" of the volume fraction profile, which extends further away than the brush height as calculated from the strong-stretching approximation. The relative importance of these deviations increases with decreasing chain length, decreasing grafting density, and decreasing solvent quality. In order to find good quantitative agreement between the lattice calculations and the strong-stretching theory, one must incorporate the full Flory- Huggins expression for the mixing free energy of polymer and solvent into the latter theory. The derivation of elegant, analytical expressions for the layer structure by expanding this free energy in a virial series is only valid for low grafting densities.
In all chapters except the second, the polymer chains are treated as freely jointed chains in a potential gradient. In chapter 2 more elaborate models are introduced for the polymer chains. Chain stiffness is incorporated by reducing the flexibility of the segment bonds. Stiffer chains give larger brush heights. Over a large range of chain stiffnesses the volume fraction profiles agree well with analytical expressions based on the incorporation of chain stiffness into the Gaussian approximation for the local stretching of a polymer chain. A further modification is a first order correction to the excluded volume interactions in the generation of the chain conformations. This correction slightly reduces the brush height. The opposing effects of this correction on the one hand, and chain stiffness on the other, suggest that the freely jointed chain is a good model for "real" polymers.
Chapter 3 considers polymer brushes on cylindrical and spherical surfaces with a radius of curvature R. On such surfaces the dependence of the brush height H on the chain length N differs from that of a flat brush. SCF lattice calculations are presented to investigate this dependency as a function of R. For large values of R the scaling law H - N is recovered for both spherical and cylindrical surfaces. For R = 1 good agreement is found with the scaling laws H - N 0.6(spherical surface) and H - N 0.75(cylindrical surface). Polymer brushes on spherical surfaces can be seen as a model for AB diblock copolymers adsorbed onto small colloidal particles. For R = 1 a star-branched polymer molecule in solution is modelled.
The volume fraction profile of the brush is also studied as a function of R. For this purpose we focus our attention on spherical brushes immersed in athermal solvents. For large radii of curvature we make the assumption that the potential energy profile of the segments can be approximated by a parabolic function, as for flat surfaces. Applying this approximation, we derived an analytical expression for the volume fraction profile which agrees reasonably well with the lattice calculations. For very small radii of curvature the lattice calculations predict volume fraction profiles which follow the scaling prediction (φ- z -4/3for spherical brushes in athermal solvents). For intermediate curvatures we propose an analytical expression for the volume fraction profile which is a combination of the parabolic potential near the surface, and the scaling form farther away from the surface. Thus, over the whole range of radii of curvature, analytical expressions for the volume fraction profiles are available which give reasonably good correspondence with the lattice calculations.
We also studied the "dead zone" from which the free ends are excluded near the grafting surface. The lattice calculations show such a dead zone under all solvency conditions, both for spherical and cylindrical surfaces. The extension of this zone is a non-monotonic function of the surface curvature. The relative size of this zone (with respect to the brush height) is a decreasing function of R. No easy analytical expression is available for the size of the dead zone.
In chapter 4 the adsorption equilibrium of AB diblock copolymers is considered for adsorption from solution onto small spherical particles. For adsorption onto flat surfaces it is known that the adsorbed amount shows a maximum as a function of the size of the adsorbing block, if the total chain length is kept constant. The thickness of the adsorbed layer shows a similar behaviour. Assuming that the adsorption energy is independent of surface curvature, we showed that the maximum in the adsorbed amount increases when the surface curvature increases. The hydrodynamic layer thickness of the adsorbed layer decreases strongly with increasing surface curvature. This increase occurs for all ratios of anchor to buoy sizes. On the other hand, the root- mean-square layer thickness changes much less as a function of the surface curvature. Depending on the anchor to buoy size ratio, it may either increase or decrease when the surface becomes more strongly curved.
Chapter 5 treats the interaction between two polymer brushes, both in the presence and absence of free polymer in the solution. In this chapter we first study the effect of free polymer chains in solution on the height and volume fraction profile of an isolated polymer brush. Using self-consistent field and scaling arguments, diagrams of state are constructed, which indicate different regimes with different scaling laws for the brush height and for the interpenetration of free and grafted polymer chains, as a function of grafting density, free and grafted chain length, and bulk volume fraction of the free polymer. These scaling laws are again corroborated by SCF lattice calculations. Predictions are also given for the volume fraction profiles of free and grafted chains based on the strong-stretching approximation. In the derivation of these expressions it is explicitly assumed that the free chain length is far smaller than the brush height. When this condition is satisfied, the volume fraction profiles from the lattice calculations agree excellently with those predicted by the strong-stretching theory. When this condition is not satisfied, both approaches still predict the same height, but the strong-stretching theory gives a far too sharp interface between the grafted layer and the free polymer.
The repulsive interaction between two compressed brushes starts at slightly larger separations according to the lattice calculations than one would expect from the strong- stretching approximation. This is caused by the "foot" of the volume fraction profile. This phenomenon occurs both in the absence and in the presence of free polymer in the solution. When free polymer is present the free energy of interaction can have an attractive part, caused by the depletion of the free chains.
Chapter 6 deals with the interaction between two surfaces bearing adsorbed multiblock copolymer layers. We first study ABA triblock copolymers. Grafted layers of B chains with an end A block ("brushes with stickers") are used to model an adsorbed layer of such polymers. When the A adsorption energy of such a grafted layer is small, the free energy of interaction between two surfaces is purely repulsive. When this adsorption energy increases, a minimum appears, which reaches a limiting value at a certain adsorption energy. The minimum adsorption energy needed to find an attraction increases with increasing grafting density σ, and chain length N. The absolute value of this minimum also depends on N and σ. It scales as or σ 1/3N -1. The minimum always occurs at a separation d that is larger than the separation 2h at which the brushes are just in contact if the "feet" in the profiles are neglected. The difference d-2h scales as Nσ 1/3. The attraction has an entropic origin. When the surfaces are far apart, the grafted chains form loops, with the A blocks adsorbed to the grafting surface. When the surfaces are brought together, the A block of a grafted chain can either adsorb onto the surface to which this chain is grafted, or it can adsorb onto the other surface. This freedom to choose between two surfaces leads to an entropically driven attraction.
The interaction between adsorbed layers of ABA triblock copolymers (where the adsorbed amount is determined by the equilibrium between free and adsorbed chains) has an attractive part if the copolymer chains are symmetric. The interaction curve is the same as that of a grafted layer ("brush with stickers") with a grafting density corresponding to the adsorbed amount of the triblock copolymers. If one of the adsorbing blocks is larger than the other block, the attraction decreases. For a relatively low asymmetry (one block roughly 20% larger than the other) the attraction disappears completely.
Multiblock copolymers consisting of more than three blocks can form bridges between two surfaces comprising several blocks. We studied the interaction between two surfaces bearing adsorbed multiblock copolymer layers. The overall composition of the polymer chains was kept constant, but the chains were divided into different numbers of A and B blocks (so that the blocks become shorter when there are more blocks per chain). Chains with smaller blocks give smaller adsorbed layer thicknesses, so that the interaction starts at smaller separations. In all cases an attractive part is found in the interaction curve. Copolymer chains consisting of alternating small blocks of A and B segments very much resemble homopolymers (with properties that are some average of the A and B segments). These copolymers show a strong attraction at small separations (<10 layers), and repulsion at very small surface separations (around 2 layers).
So far, we have only considered situations were the solvent is a good solvent for both blocks. The A blocks adsorb preferentially with respect to the B blocks, because the former have a stronger intrinsic affinity for the surface. We also consider the adsorption of an ABA triblock copolymer were both blocks have the same intrinsic affinity for the surface, but where the solvent is poorer for the A block. Now the A blocks adsorb preferentially, because of the selectivity of the solvent. We also pay attention to the interaction between two surfaces bearing adsorbed layers of such copolymers. When the interactions between the A and B segments and the solvent differ only slightly, the interaction curve resembles that of an adsorbing homopolymer, with an attraction at small separations. When these interactions differ a great deal, the interaction resembles that of a "conventional" triblock copolymer, with an attractive part at a large separation and repulsion at smaller surface separations. In the intermediate situation a more complicated interaction curve is found.
The subject of chapter 7 is the interaction between two small particles bearing adsorbed polymer layers. An extended version of the lattice SCF theory was introduced, which takes account of gradients in two directions. In this version a cylindrical coordinate system is used, so that the volume fractions can vary both parallel to the axis connecting the centres of both particles, and in planes perpendicular to this axis. Results are presented for terminally attached polymer layers. It is first shown that this cylindrical model gives an isotropic profile around one isolated particle. This profile agrees well with the profile calculated from the "conventional" SCF lattice model, where a concentration gradient can exist in one direction only. Various free energy of interaction curves are presented for two spherical particles with terminally attached chains.
If two spherically curved surfaces bearing adsorbed polymer layers interact, then the Derjaguin approximation relates this interaction to that between two similar flat surfaces, as long as the radius of curvature is far larger than the adsorbed layer. In chapter 7 we deal with systems where this condition does not hold. That is why we find interactions that are far less repulsive than the interaction according to Derjaguin's approximation. For increasing radii of curvature R, the interaction does move in the direction of the interaction predicted for very large R by the Derjaguin approximation. On a molecular level the decreased repulsion can be explained by the freedom of the grafted chains to mover laterally out of the gap between the two particles. Whether or not the grafting segments themselves can also move over the surface plays only a minor role.
Kinetics of polymer adsorption, desorption and exchange
Dijt, J.C. - \ 1993
Agricultural University. Promotor(en): G.J. Fleer. - S.l. : Dijt - ISBN 9789054851356 - 168
kunststoffen - industrie - adsorptie - sorptie - macromoleculaire stoffen - plastics - industry - adsorption - sorption - macromolecular materials
The aim of the study in this thesis was to gain more insight in the kinetics of polymer adsorption. To this end some well-characterised polymers have been systematically investigated.
In the process of polymer adsorption one may distinguish three kinetic contributions: transport to the surface, attachment, and reconformation of the adsorbing and adsorbed chains. In order to assess the role of each of the three contributions it is necessary to measuri the adsorption kinetics under well-defined hydrodynamic conditions. For such measurements the transport (convection and diffusion) can be calculated and therefore it becomes possible to study unambiguously the interfacial processes, i.e., attachment and reconformation.
For this study two experimental techniques were used that both fulfil the requirement that the adsorption occurs under well-defined hydrodynamic conditions: reflectometry in a stagnation point flow (chapters 2,3 and 5-7) and a streaming potential method (chapter 4). With both techniques it is possible to follow directly and continuously the buildup of an adsorbed layer. Reflectometry is a relatively new and simple optical technique for the measurement of adsorption on (optically flat) solid surfaces. In a reflectometer a linearly polarised light beam is reflected from the (adsorbing) surface, and the reflected beam is split into its parallel and perpendicular components. The intensity ratio between the two components is continuously measured. This ratio changes upon adsorption, and after calibration the adsorbed amount (mass/area) is obtained. For reflectometry there are only few restrictions on the choice of adsorbate, adsorbent and solvent.
The applicability in this study of the streaming potential method is limited to adsorption of uncharged polymers from aqueous solution. For that case, the streaming potential can be related to the hydrodynamic layer thickness of the adsorbed polymer layer. This thickness is mainly determined by loose ends of adsorbed chains, and it is sensitive to very small changes in the adsorbed amount of long chains near saturation. Such small changes occur for desorption of long chains into solvent, so that the streaming potential method is especially suitable for the measurement of the desorption kinetics.
In chapter I the aim and scope of this study of this study are explained, and a general introduction to adsorption of polymers is given.
In chapter 3 reflectometry is used to investigate the kinetics of adsorption of poly(ethylene oxide) (PEO) from water onto oxidised silicon. For the stagnation point flow the maximum rate of mass transfer of polymer to the surface is calculated. This rate is compared with the observed adsorption rate, and it is concluded that mass transfer is ratelimiting up to or nearly up to saturation, depending on the chain length. Only for long chains ( M >100 kg/mole) near saturation the adsorption rate is lowered by surface processes.
In chapter 4 a model is discussed for the desorption rate of polymers into a flow of pure solvent. This model is based on the assumption that near the surface there is a rapid equilibration between free and adsorbed polymer, and that transport of free polymer away from the surface is ratelimiting for the desorption. Due to the shape of the (high affinity) isotherm, the equilibrium concentration of free chains even after a minute desorption is extremely low, so that the transport -and thus the desorptionproceeds slowly. Thus, in spite of the rapid local equilibration, the desorption is slow because of the slow mass transfer. For a logarithmic adsorption isotherm of the polymer (for which the adsorbed amount Γincreases linearly with the log of the concentration c in solution) an explicit expression for the adsorbed amount as a function of time is derived: the desorbed amount increases proportionally with log t. The model predicts that the absolute value of the slope of the (kinetic) desorption curve Γ(log t ) and the (static) adsorption isotherm Γ(log c ) are the same.
Using the streaming potential method it is shown in chapter 4 that the above model gives an adequate description of the desorption kinetics in aqueous solutions of PEO on glass, even for high molar mass polymer (M = 847 kg/mole). Again, this shows that the equilibration of adsorbed layers of PEO is rapid as compared to the rate of mass transfer through solution.
Chapter 5 describes the adsorption kinetics of polystyrene (PS) from decalin on oxidised silicon. On a bare surface the adsorption rate of PS is limited by mass transfer from solution, like for PEO. For PS, the adsorption rate decreases gradually with increasing coverage. This is due to a decreasing probability of attachment during a collision of a free chain with the (covered) surface. From experiments in which the chain length, the solvent quality and the adsorption energy were varied, the picture arises that the adsorption probability during a collision is the result of a balance between a gain in adsorption energy on the one hand, and repulsive interaction with the adsorbed layer on the other.
Exchange between polymers that differ in chain length only is the subject of chapter 6. Displacement of adsorbed short chains of PEO by longer ones in solution is limited only by transport of long chains to the surface. The adsorbed layer is continuously in equilibrium with the solution near the surface. The same conclusion was drawn from the desorption kinetics of this polymer in a flow of pure solvent (chapter 4). For PS also surface processes play a role. During exchange of short by long chains of PS there is a temporary overshoot of short chains in the adsorbed layer. This overshoot may desorb either during adsorption of long chains, or by relaxation of the adsorbed layer. By interrupting the transport of long chains to the surface, this relaxation could also be directly observed. The higher chain stiffness of PS as compared to PEO possibly explains the slower equilibration of adsorbed PS.
Finally, we present in chapter 7 some results on the exchange kinetics between three chemically different polymers: polystyrene (PS), poly(butyl methacrylate) (PBMA) and polytetrahydrofuran (PTHF). Displacement of adsorbed layers of the rather stiff polymers PS and PBMA by the very flexible PTHF is limited only by transport of the displacing polymer from the bulk solution. For mutual exchange between the two stiff polymers, surface processes play an important role: the displacer PBMA adsorbs quickly, whereas PS desorbs slowly. Possibly, the slow exchange kinetics is caused by the low mobility of the adsorbed polymers. The displacement rate of PS by PBMA increases considerably after addition of a displacer of low molar mass. The faster exchange kinetics is probably due to the lower binding strength and, consequently higher mobility of the adsorbed polymers.
|Verandering van de smaak van mineraalwater als gevolg van wisselwerking met de verpakking.
Linssen, J.P.H. ; Janssens, J.L.G.M. ; Roozen, J.P. ; Posthumus, M.A. - \ 1992
Voedingsmiddelentechnologie 25 (1992). - ISSN 0042-7934 - p. 24 - 26.
dranken - ingeblikte producten - inblikken - afdeklagen - uitrusting - geur en smaak - voedselbewaring - voedingsmiddelen - latex - mineraalwaters - verpakking - verpakkingsmaterialen - verpakken - producten - rubber - sensorische evaluatie - smaak - water - macromoleculaire stoffen - machines - beverages - canned products - canning - coatings - equipment - flavour - food preservation - foods - latex - mineral waters - packaging - packaging materials - packing - products - rubber - sensory evaluation - taste - water - macromolecular materials - machines
Mineraalwater verpakt in polyethyleen gecoat aluminium blikjes en de wisselwerking met de verpakking
Displacement of adsorbed polymers : a systematic study of segment - surface interactions
Beek, G. van der - \ 1991
Agricultural University. Promotor(en): G.J. Fleer; M.A. Cohen Stuart. - S.l. : Van der Beek - 174
kunststoffen - industrie - adsorptie - sorptie - anorganische verbindingen - macromoleculaire stoffen - anorganische scheikunde - plastics - industry - adsorption - sorption - inorganic compounds - macromolecular materials - inorganic chemistry
This thesis describes a method to determine the thermodynamic (reversible) adhesion strength of polymers on inorganic solids.
The key idea of the method is that the thermodynamic adhesion strength can be obtained from polymer displacement experiments. Polymers adsorbed from solution on an inorganic adsorbent can be desorbed by adding a more strongly adsorbing solvent component (a so-called displacer ). At a certain critical displacer concentration (the critical point) the polymer is entirely desorbed by the displacer. Cohen Stuart et al. showed that this critical solvent composition can be related to the effective adsorption energy per segment. Using an apolar solvent, the thermodynamic work of adhesion of a polymer can be estimated from this effective adsorption energy, the segmental cross- section, the solvent surface tension, and the dispersion contribution of the surface free energy of the substrate.
In this study, critical points have been measured for various systems by 5 different methods. Some methods do not only give the critical point, but also specific information about the conformational state of adsorbed polymers. The following procedures were used:
By means of the methods described above, we determined adhesion strengths for 5 different polymers on silica and alumina. In order to study the effects of the functional group and the chain structure of the polymer on the adhesion strength we choose the following polymers: polystyrene, poly(butyl methacrylate), poly(tetrahydrofuran), poly(methyl methacrylate), and poly(ethylene oxide). The given order of these polymers corresponds to an increasing segmental adsorption energy on both substrates. We find that polymers with more methylene groups per segment in the main chain or with larger alkyl side groups have a smaller adsorption strength. The adsorption energy for each individual polymer is higher on silica than on alumina.
The displacer concept may also be used to determine the adsorption energy of solvents but this time polymers are used as a standard. Doing so, we obtained the same trends for the adsorption energy of solvents as for polymers with respect to the functional group and the size of the monomer unit.
In chapter 6 we estimate the reversible work of adhesion for polymers in vacuum from the segmental adsorption energy. For pure polymers on silica, this work is of the order of 100-200 mJ/m2. It turns out that the contribution of specific interactions to the total work of adhesion ranges from about 10% for polystyrene to about 50% for poly(ethylene oxide). The largest contribution to the work of adhesion on silica with respect to vacuum is thus due to nonspecific, dispersive interactions.
Statistical thermodynamics of block copolymer adsorption
Evers, O.A. - \ 1990
Agricultural University. Promotor(en): G.J. Fleer; J.M.H.M. Scheutjens. - S.l. : Evers - 140
kunststoffen - industrie - chemie - colloïden - adsorptie - oppervlakten - macromoleculaire stoffen - oppervlaktechemie - plastics - industry - chemistry - colloids - adsorption - surfaces - macromolecular materials - surface chemistry
The aim of this study was to develop a statistical thermodynamic theory for the adsorption of linear flexible block copolymers from a multicomponent solution. This has been accomplished by a more general derivation of the self-consistent field theory of Scheutjens and Fleer for adsorption of homopolymer from a binary mixture, introducing local segment potentials for any type of segment.
In chapter 1 the statistical thermodynamic analysis for a multicomponent mixture (including block copolymers) near a surface is given in detail. Near the surface, a density gradient for every type of molecule is found due to spatial restrictions and mutual interactions between segments and between segments and the surface. Every individual segment is subjected to a local (segment) potential, which depends on the distance from the surface and on its chemical nature. We use a lattice model to evaluate the contact energies and the conformation count. The segment potential is derived from the maximum term in the canonical partition function. Like in the original derivation of Scheutjens and Fleer we maximize the canonical partition function with respect to the number of molecules in each particular conformation. However, to perform the necessary partial differentiations under the appropriate boundary conditions we apply the method of Lagrange multipliers. From the segment potentials we can calculate for every particular conformation its statistical weight as a multiple product of Boltzmann factors (one for each segment) and its contribution to the overall segment density profile. In Appendix III of chapter 1 a set of equations is formulated from which the segment potentials can be found in a self-consistent manner by standard numerical techniques.
A number of results on the segment distribution of di- and triblock copolymers is given. Diblock coplymers are found to adsorb with the adsorbing block rather flat on the surface and the less or non-adsorbing block in one dangling tail protruding far into the solution. A comparison with terminally anchored chains shows overall agreement but also typical differences.
In chapter 2 the physical background of the theory is briefly reviewed. Results on the adsorbed amount and the hydrodynamic layer thickness of adsorbed di- and triblock copolymers are given. We find a strong dependence of these parameters on the chain composition. When the total length and bulk solution volume fraction of a diblock copolymer are kept constant, a maximum is found in the adsorbed amount as a function of the fraction of adsorbing segments. The fraction of adsorbing segments corresponding to this maximum could be named "the optimal fraction"; it is found to decrease with increasing chain length, increasing bulk solution volume fraction, increasing surface affinity of the more strongly adsorbing block, and decreasing surface affinity of the weakly adsorbing block. From these results we have been able to relate in a simple way the adsorbed amount of an AB-diblock copolymer (where A adsorbs more strongly than B) to the adsorbed amount of an A-homopolymer of equal length. A linear relation is obtained between the adsorbed amount of AB-diblock copolymer (as compared with an A-homopolymer) and the block length ratio r B /r A . where r A and r B are the lengths of the A-block and the B-block, respectively. Usually, diblock copolymers form thick adsorbed layers, with a hydrodynamic layer thickness that depends strongly on the adsorbed amount. This thickness is of the order of 10 to 30 % of the length of the B-block. For BAB-triblock copolymers with adsorbing A-segments and non- adsorbing B-segments we find lower adsorbed amounts as compared to an AB-block copolymer with the same total number of A- and B-segments
The interaction between adsorbed layers of block copolymers is examined in chapter 3. The calculation of the free energy of interaction is straightforward. We elaborate the concept of full equilibrium and that of restricted equilibrium for a multicomponent mixture. Full equilibrium refers to the case that all molecules in the mixture are free to diffuse out or into the gap between the surfaces. Hence, in full equilibrium all molecules have a constant chemical potential when the surfaces are brought closer. If one or more of the components are unable to leave the gap when the surfaces come closer we have a restricted equilibrium and the chemical potentials of those molecules will not be constant. Usually, the interaction between adsorbed layer of adsorbed diblock copolymers at full equilibrium is found to be repulsive. in contrast to the case of homopolymers where the interaction is always attractive. At full equilibrium, when the surfaces are brought closer, homopolymers desorb and form bridges resulting in attraction between the surfaces. Since diblock copolymers hardly form any bridges when the surface affinities of both blocks differ enough, no attraction is found at full equilibrium. For the same reason we find always repulsion in a good solvent when the amount of diblock copolymer is kept constant (restricted equilibrium). The onset of the repulsion increases with increasing adsorbed amount and with increasing length of the non-adsorbing block. The interaction curves at various lengths of the adsorbing- and non-adsorbing block could be scaled onto approximately one master curve. When the solvent quality for the non-adsorbing block becomes poor (χ>0.5). there is an attraction at large separation as a result of osmotic forces (phase separation), even at restricted equilibrium. In fact, adsorbed diblock copolymers behave like infinitely long homopolymer chains in solution, which phase separate when χis above 0.5. For ABA-triblock copolymers with adsorbing A-segments and non-adsorbing B-segments, we find attraction at not too small separations in a good solvent for the B-blocks, because now bridging is again possible: adsorbing segments are found at both extremities of the chains.
This model has provided a detailed insight in the properties of adsorbed block copolymer layers and should be a useful tool for the development and optimization of experiments and products in which copolymer adsorption plays a role.
The adsorption of weak polyelectrolytes and polyampholytes : an experimental study
Blaakmeer, J. - \ 1990
Agricultural University. Promotor(en): G.J. Fleer; M.A. Cohen Stuart. - S.l. : Blaakmeer - 115
adsorptie - sorptie - kunststoffen - industrie - macromoleculaire stoffen - adsorption - sorption - plastics - industry - macromolecular materials
The objective of this study was to collect systematic data on the adsorption behaviour of weak polyelectrolytes and polyampholytes. The measurements were performed on well-defined systems in order to be able to compare the results with the recently developed theories of Evers et al. and Böhmer et al. for the adsorption of weak polyelectrolytes. The adsorption of polyampholytes was studied in order to help bridge the gap between the theoretically well understood behaviour of polyelectrolytes at interfaces, and the adsorption characteristics of proteins, which are, so far. mainly experimentally documented.
In order to compare experiment with theory it is essential to be able to vary the surface charge and the polymer charge independently. In Chapter 1 we describe the synthesis of a monodisperse, positively charged polystyrene latex with fixed surface charge. The positive charge results from quarternary ammonium groups and proved it to be independent of the pH in the range 4-10, as shown by streaming potential measurements on plugs.
In Chapter 2 the adsorption data of poly(acrylic acid) onto this positively charged latex are reported and compared with theory. The agreement between theory and experiment is remarkably good. The theoretically predicted maximum in the adsorbed amount as a function of pH is fully confirmed experimentally. The maximum occurs because of two opposing trends. With increasing pH, the increased negative charge on the polyacid leads to a stronger attraction between surface and polyelectrolyte groups and, thus, to a higher adsorption. On the other hand, the higher repulsion between the segments opposes accumulation of polymer near the interface. At relatively low pH the latter effect is small and the electrostatic contribution to the adsorption energy gives high adsorbed amounts. At higher pH, the intersegmental repulsion leads to a decreased adsorption.
It was found experimentally that the adsorption is almost independent of the salt concentration. This is also predicted by theory. The computations showed that the segments in contact with the surface are dissociated to such a degree that the surface charge is just balanced. The segments in loops and tails are dissociated to a much lower degree. The effective charge of the colloidal particle plus the adsorbed polymer layer is therefore small, so that salt will hardly affect the adsorbed amount. The adsorbed amount increases slightly with increasing molecular weight of the polyelectrolyte and with increasing surface charge. These trends also correspond to theoretical predictions; the agreement is again semi- quantitative.
A next step was the adsorption of simple polyampholytes. Because well-defined homodisperse polyampholytes are not commercially available. we decided to synthesize such a macromolecule ourselves. The synthesis of a model polyampholyte is described in Chapter 3. The starting point for the polymerization was the tripeptide L-lysyl-L-glutamyl-glycine, with fully protected aminoand carboxylic functions. Coupling of two tripeptide molecules to a hexapeptide, two hexapeptide molecules to a dodecapeptide, etc. had to be attained by activation of the caboxylic terminal amino acid (c- terminus) with dicyclohexyl carbodiimide. It is known that this activator causes racemisation of the c-terminus. Therefore glycine was chosen as the c-terminus. The controlled coupling had to be given up at the dodecapeptide level due to the extremely low solubility of the fully protected peptide derivative.
In Chapter 4 adsorption studies are described with this dodecapeptide (L-lysyl-L-glutamyl-glycine) 4 and with a commercially available random copolymer of lysine and glutamic acid, both on positively and on negatively charged polystyrene latices. It was found that the dependence of the adsorption on pH was the same for adsorption on the positively and on the negatively charged latex: high adsorbed amounts at low pH and virtually no adsorption at high pH. This behaviour is easily understood for the negatively charged latex: at low pH the positively charged macromolecule adsorbs easily on the negative surface, whereas at high pH the molecule and surface repel each other. However, for the positively charged latex one would expect just the reverse so that the observation is difficult to understand. A possible explanation is that at low pH the "chemical" affinity of the adsorbate for the adsorbent is larger than the electrostatic repulsion. while at high pH the very good solubility of the polyampholyte counteracts the electrostatic attraction with the surface to such an extent that the adsorption is only small.
In conclusion, the adsorption behaviour of weak polyelectrolytes with only one type of charged group is now understood reasonably well, and the agreement between theory and experiment is excellent. For polyampholytes with anionic and cationic groups in the molecule there Is no symmetry in the behaviour with respect to positive and negative surfaces. Since, for purely non-specific electrostatic Interactions. such symmetry must exist. other factors such as solubility or formation of internal ionic bonds within the polyampholyte must be held responsible for the observations. The latter conclusion may be important for the understanding of proteins at interfaces.
Macromolecules at interfaces : a flexible theory for hard system
Scheutjens, J.M.H.M. - \ 1985
Landbouwhogeschool Wageningen. Promotor(en): G.J. Fleer, co-promotor(en): J. Lyklema. - Wageningen : Scheutjens - 168
industrie - grensvlak - kunststoffen - oppervlakten - macromoleculaire stoffen - oppervlakteverschijnselen - industry - interface - plastics - surfaces - macromolecular materials - surface phenomena - cum laude
A statistical theory for flexible macromolecules at interfaces has been developed. The theory is based on a lattice model in which the equilibrium set of molecular conformations in a concentration profile is evaluated, using a selfconsistent procedure. In this way, the Flory-Huggins theory for polymer solutions is extended to inhomogeneous solutions of macromolecules without any additional assumption. Apart from the Flory-Huggins polymer-solvent interaction parameter χ, a similar parameter χ s is used to describe the interaction of polymer segments with a solid interface. The average number of molecules in each particular conformation can be computed, so that a very detailed picture of the interfacial structure is obtained. Thus also the train, loop, and tail size distributions of adsorbed polymer can be calculated. In principle, there are no adjustable parameters in the theory. Moreover, there are no restrictions on the system parameters such as polymer concentration, chain length, number of species in a mixture or solvent quality, although in some cases numerical problems may occur. Results are given for adsorption of homopolymers, polydisperse polymer, polyelectrolytes, and star-branched polymer, for the structure of lipid bilayers and of the amorphous phase of semicrystalline polymer, and for the interaction between surfaces due to the presence of adsorbing or nonadsorbing polymer. Available experimental data on adsorption isotherms, bound fraction, layer thickness, surface fractionation, steric stabilization, and polymer bridging agree very well with the theoretical predictions.
An experimental and theoretical study of oligo- and polyelectrolyte adsorption
Schee, H.A. van der - \ 1984
Landbouwhogeschool Wageningen. Promotor(en): J. Lyklema. - Wageningen : Van der Schee - 189
adsorptie - sorptie - kunststoffen - industrie - oppervlakten - grensvlak - elektrochemie - macromoleculaire stoffen - oppervlakteverschijnselen - adsorption - sorption - plastics - industry - surfaces - interface - electrochemistry - macromolecular materials - surface phenomena
The purpose of this study is the description of the behaviour of a model polyelectrolyte near a model interface.Chapter 1 gives a general background and the outline of the investigations.The chapters 2 and 3 comprise the theoretical parts. Chapter 2 considers the applicability of equilibrium thermodynamics to polymer adsorption, an important prerequisite. We focus our attention on the applicability of Gibbs' law to polymer adsorption. In many cases results from surface tension measurements do not obey the classically formulated Gibbs' law, which is based on equilibrium thermodynamics. Thus, the use of equilibrium thermodynamics was questioned. We show that it is important to take the heterodispersity of the polymers used in adsorption studies into account when Gibbs' law is applied. Although not all problems could be solved completely, this result encourages the use of equilibrium thermodynamics in the polyelectrolyte adsorption theory.Chapter 3 describes the polyelectrolyte adsorption theory. Expressions are derived for the potential distribution in and the free energy of a double layer containing polyelectrolyte charge. In the case of the Debye-Hückel approximation of the Poisson-Boltzmann equation analytical expressions are obtained. When the full Poisson-Boltzmann equation is used, we have to rely on numerical procedures. The electrostatic interactions are incorporated in the phase equilibria theory of Flory and the Roe theory and the Scheutjens-Fleer theory of polymer adsorption. Electrostatic interactions strongly influence the conformation of the adsorbed polyelectrolyte. Because of the mutual repulsion of the charged segments the formation of loops and tails is strongly suppressed. Thus very flat adsorbed layers are predicted. For uncharged polymers the increase of the adsorption with chain length and polymer concentration mainly takes place in the loops and tails. As for polyelectrolytes, these are all but absent, their adsorption is nearly chain length-independent and the plateau value of the adsorption isotherms is much more constant than with uncharged polymers. Polyelectrolyte solutions are resilient against phase separation in much poorer solvents than uncharged polymers, depending on chain charge density and indifferent electrolyte concentration.The chapters 4 and 5 make up the experimental part. As the polyelectrolyte adsorption theory does not only apply to long polymer chains, but also to the adsorption of oligomers, it is important to test the theory for short chains, too.Chapter 4 describes the synthesis of these oligomers. Repeated coupling of chains of equal length yields a series of two, four, eight, sixteen and thirty-two segments long. For this kind of coupling reactions peptides are suitable compounds, since standard methods of synthesis have been developed for the benefit of protein research. Using only one type of amino acid, L-lysine, we obtained poly-L-lysine type oligomers.Chapter 5 presents characteristics for the adsorption of the model-polyelectrolyte poly-L-lysine on the model-colloid silver iodide. Several colloid chemical techniques can be applied to in vestigate this system both in the absence and in the presence of poly-L-lysine. We investigated adsorption isotherms, charge potential curves, coagulation kinetics and electrophoretic mobil ities. From the results the following picture of polyelectrolyte adsorption emerges:
(i) At low ionic strength only a thin layer of adsorbed poly-electrolyte is formed.
(ii) Under these conditions the adsorption is chain length in dependent, provided the chains are not too short.
(iii) Because of the shielding of indifferent electrolyte some loops and tails are formed at high salt concentrations.
(iv) Oligomer adsorption increases with chain length.This picture is in agreement with the theoretical predictions of chapter 3.Chapter 6 gives a quantitative comparison of the experimental and theoretical data. The quantitative agreement between theory and experiment is also satisfactory. This chapter also gives some, suggestions for further improvement of the theory and for additional experimental tests.
Adsorption of polylysines at solid-liquid interfaces
Bonekamp, B.C. - \ 1984
Landbouwhogeschool Wageningen. Promotor(en): J. Lyklema. - Wageningen : Bonekamp - 208
adsorptie - sorptie - kunststoffen - industrie - aminozuren - chemie - macromoleculaire stoffen - adsorption - sorption - plastics - industry - amino acids - chemistry - macromolecular materials
Adsorption properties of the polyelectrolytes poly-L-lysine (PL-L) and poly-DL-lysine (PL-DL) on hydrophobic (polystyrene latex, silver iodide) and hydrophilic (silica) negatively charged solid particles were studied.
Adsorbed amounts as a function of concentration, ionic strength, surface charge, PI, chain length and chain charge density were determined. The adsorption of PI, on negatively charged polystyrene latex and silica was also monitored conductometrically and potentiometrically. Further flocculation and coagulation measurements were performed. Information about the secondary structure of adsorbed PL-L was obtained from proton titrations of PL-L and PL-DL adsorbed at polystyrene particles.
The stereoregularity and secondary chain structure of PI, does not influence the adsorbed amount. At low constant adsorbed amount, there is no coil to helix transition in adsorbed PL-L, because of the charge contrast and hydrophobic interactions between PL and the surface. Only when the adsorbed amount is high at each pH, a transition takes place and adsorbed PL-L can be partly helical.
At low pH and low ionic strength the adsorbed polyelectrolytes show a rather flat conformation and there is no pronounced effect of the hydrophobicity of the adsorbent. All negative surface groups form ion pairs with an -NH 3+ group of PL, but the reverse is not the case. The adsorption of the basic polyaminoacids increases (i.e. loops and tails start to develop) if the pH is increased or if the ionic strength is raised. With the hydrophobic adsorbents the electrolyte effect persists up to very high concentrations. In the case of the hydrophilic silica no increase above 0.01 M salt was observed, because here hydrophobic interactions are absent. The ionic strength and pH dependence of the adsorption on hydrophobic substrates are in satisfactorily agreement with theoretical predictions.
Syneresis of curd
Dijk, H.J.M. van - \ 1982
Landbouwhogeschool Wageningen. Promotor(en): P. Walstra, co-promotor(en): J. Schenk. - Wageningen : Van Dijk - 87
melkeiwitten - colloïden - coagulatie - uitvlokking - kunststoffen - industrie - wrongel - macromoleculaire stoffen - milk proteins - colloids - coagulation - flocculation - plastics - industry - curd - macromolecular materials
This study deals with the syneresis of curd. Rennet gels are primarily considered; some comparisons with acid milk gels are given.After curdling the milk, the curd tends to shrink; in other words, the network of aggregated paracasein micelles (PCM) will be under stress. If the curd is cut or - as was the case in our expirements - a curd surface is wetted, syneresis starts. The rate at which the whey is expelled depends on the pressure gradient in the whey and on the permeability of the network.In Chapter 2 the materials and methods generally used are described. Unless mentioned otherwise, standard conditions were used in the experiments. By standard conditions is meant: reconstituted skim milk with the saw dry matter content as the original milk, to which 500 ppm rennet was added; the temperature during the whole experiment was kept at 30 °C; no CaCl 2 was added.The endogenous syneresis pressure ( Ps) appeared to be very low, about 1 Pa. In Chapter 3 two methods are described which give an order of magnitude of the stresses involved. Moreover, the weight of the network can cause an additional pressure. The maximum pressure caused by the weight ( Pg) at a level hc below the interface is (ρ curd - ρ curd ) ghc ≈75hc Pa (hc in m).The permeability measurements are described in Chapter 4. Two methods were used; in both, the flow of whey through a vertical column of curd was measured as a function of head pressure. A problem is that the curd is deformed during the experiment. In the "tube" method, deformation is a function of the pressure gradient (d Pt /dx), the diameter of the tube holding the curd (d t ), and the rigidity of the gel. In the second method the "torsionflux" method, the deformation was adjustable. 'The tube method led to the following results.
- The permeability is of the order of 10 -13m 2.
- Permeability increases with time, which is ascribed to "microsyneresis", i.e. syneresis at local sites in the gel. The rate of increase is approximately constant.
- The increase in permeability (d B /d t ) is higher for a higher pressure gradient or a wider tube; both lead to larger deformation of the curd.
- The change of the permeability with time in the absence of deformation (d Be /d t ) was obtained by applying the head pressure at different times after addition of rennet. Shortly after clotting permeability increases fastest. Between 1 and 24 h
d Be /d t was constant.
- The permeability of curd made from ultrafiltered skim milk ( B ( i )) and its change with time (d B ( i )/d t ) were determined. This
yielded the permeability as a function of concentration and time ( B ( i,t )).
- The permeability also depends on temperature, CaCl 2 concentration, acidity, fat content and type of skim milk.
- In acid milk geld permeability was of the same order of magnitude, but it hardly changed with time.The rheological behaviour of curd is discussed in Chapter 5. The dynamic measurments with the "Den Otter" rheometer show that the moduli G ' and G " kept increasing for a long time (~3 h) after rennet addition. From the dependence of G' and G" on the angular frequency it was deduced that G" is due to the relaxation of bonds and that the relaxation time is a few times 10 s.The instantaneous shear modulus ( G0 ) was determined as a function of protein concentration. The obtained relation can be explained in term of an only partly effective contribution of the casein to the network; this contribution being relatively smaller at lower concentrations. Also from the creep measurements it was concluded that the endogenous syneresis pressure was less than 10 Pa.If both permeability and pressure are known for all values of concentration (or relative remaining volume ( i )) and time ( t ), the syneresis can in principle be calculated. This is in the model described in Chapter 6, in which the equation of Darcy is combined with the equation of continuity. A numerical procedure is developed, for a one dimensional case; the syneresis of a thin slab.The pressure in the whey is the sum of the endogenous syneresis pressure ( Ps) and the pressure caused by the weight of the network ( Pg). For Ps( i ) and Pg( i ) some trial functions were considered.In Chapter 7 the syneresis of slabs is studied. The results of the experiments show that initially Γ= dlogΔH/dlog t is about 0.5. For t >0.5 h Γincreases to ~0.78. Γis independent of the original thickness of the slab ( H0 ) during a certain period (penetration period). The length of this period depends on H0.After one day H did not change any more and H∞ / H0 was about one third. The best fit between model calculations and experimental results was obtained if it was assumed that:
- the permeability increases with time ( t ) and decreases with i , as was found in the experiments,
- endogenous syneresis pressure (Ps) decreases only with shrinkage, - maximum gravitational pressure ( Pbg) is constant,
- P0s= Pbg= 1 Pa ( H0 = 10 mm).P0swas found to be a function of time after renneting, at first increasing, then (after 1 - 2 h) decreasing. However, the introduction of such a relation in the model did not improve the fit to the experimental results. After all, the pressure cannot relax twice, both by shrinkage and by "ageing".The effects of several parameters (pH, temperature, Ca concentration, etc.) on milk clotting, gel permeability, syneresis and curd rigidity are interrelated. A survey is given in Table 7.2 and a tentative explanation is summarized in Table 7.3.In Chapter 8 it is shown that external pressure has a dramatic effect m the syneresis rate. Extrapolation to zero external pressure yields, again, an endogenous syneresis pressure of about 1 Pa.
Flexible polymers at a solid-liquid interface : the adsorption of polyvinyl pyrrolidone onto silica
Cohen Stuart, M.A. - \ 1980
Landbouwhogeschool Wageningen. Promotor(en): B.H. Bijsterbosch. - Wageningen : Cohen Stuart - 151
industrie - grensvlak - kunststoffen - oppervlakten - macromoleculaire stoffen - oppervlakteverschijnselen - industry - interface - plastics - surfaces - macromolecular materials - surface phenomena
We undertook the present study in order to evaluate techniques which are devised to assess the comformations of adsorbed macromolecules. Since recent theories deal with these conformations, we also wanted to investigate to what extent these theories are supported by experimental data.
In chapter 1 we outline the scope of this study and we give reasons for using polyvinyl pyrrolidone / silica as the model system.
Chapter 2 deals with the general aspects of polymer adsorption. Some trends found from experimental work are mentioned, and the four main theories which have treated the problem of interacting chain molecules at a plane interface are briefly discussed. Arguments are presented which lead to the conclusion that, from a theoretical point of view, the best theory available at present is the one recently given by Scheutjens and Fleer (1979). However, it is shown that even this theory fails to take into account both the volume ratio between polymer and solvent and the flexibility of the polymer in a consistent way. We propose a simple way to modify the theory such that these properties are both incorporated; this requires the introduction of a new parameter into the theory: the segment-to-solvent volume ratio or bulkiness parameter b. A few calculations show how the results depend upon b. At the end of chapter 2 we also consider briefly the reversibility of polymer adsorption in connection with polydispersity effects.
Chapter 3 presents a selection of relevant properties of the polymer, the solvents and the adsorbent.
Adsorption experiments are discussed in chapter 4. Adsorbed amounts for two solvents (water and dioxane) are given as a function of chain length, properties of the adsorbent surface and polymer heterodispersity. It is found that PVP adsorbs strongly from both solvents, but in amounts that are substantially higher from dioxane than from water. Chapter 4 also deals with the desorption which occurs if suitable low molecular weight displacers are added to the solution. By means of a simple expression, the amount of displacer needed to just cause complete desorption is related to the strength with which the polymer is bound to the surface. With 5 different displacers an effective adsorption energy is estimated and its meaning is discussed. This type of measurements is entirely new in polymer adsorption studies. Although a detailed interpretation of the results is not straightforward, we obtain reasonable values for the adsorption energy parameter.
Measurements of conformation parameters for adsorbed PVP are described in chapter 5. Bound fractions are determined spectroscopically (infrared, nuclear magnetic resonance) and completed with data from the literature (electron spin resonance). Calorimetry is used as a means to study the surface occupancy. The discrepancies between results from the various methods are considerable, and it is concluded that the techniques do not measure the same property. Nevertheless, the results are useful in gaining a further understanding of the adsorption phenomenon.
In chapter 6 we compare in some detail experimental and theoretical results. The theory used is the aforementioned modification of the Scheutjens-Fleer theory and the experimental data comprise adsorbed amounts, bound fractions and interaction parameters. The adsorbed amounts, which can be fairly accurately measured, agree within 10 tot 20 % with the theoretically calculated values, over the whole molecular weight range. To our knowledge, this is the first time that quantitative agreement is obtained to such an extent. Bound fractions determined with ESR also agree fairly well with the theoretical values, and NMR gives a result in the right range. Infrared data lead to a serious underestimation of the bound fraction, which is easily explained on the basis of the chain structure. To a lesser extent, microcalorimetric results also tend to underestimate the bound fraction, although the results agree surprisingly well with enthalpies of wetting. The dependency on the solvent type is also correctly predicted.
The conclusion is drawn that the introduction of the bulkiness parameter is an essential step towards a complete theory and that the modified Scheutjens-Fleer theory is strongly supported by experiment. Finally, we picture in some detail the conformation of adsorbed PVP, as it emerges from this study.
|Advies over de verpakking van trosanjers in kunststoffolie
Molenaar, W.H. - \ 1978
Wageningen : Sprenger Instituut (Rapport / Sprenger instituut no. 2009) - 4
caryophyllaceae - cellulose - uitrusting - sierplanten - verpakking - verpakkingsmaterialen - verpakken - kunststoffen - eiwitten - macromoleculaire stoffen - synthetische materialen - machines - caryophyllaceae - cellulose - equipment - ornamental plants - packaging - packaging materials - packing - plastics - proteins - macromolecular materials - synthetic materials - machines
Inference of polymer adsorption from electrical double layer measurements : the silver iodide-polyvinyl alcohol system
Koopal, L.K. - \ 1978
Landbouwhogeschool Wageningen. Promotor(en): J. Lyklema. - Wageningen : Veenman - 141
adsorptie - sorptie - zilver - jodide - polymeren - zuurstof - kunststoffen - industrie - chemie - colloïden - oppervlakten - macromoleculaire stoffen - oppervlaktechemie - adsorption - sorption - silver - iodide - polymers - oxygen - plastics - industry - chemistry - colloids - surfaces - macromolecular materials - surface chemistry
The purpose of this study was to investigate how the double layer properties of charged particles are modified by the presence of adsorbed polymer molecules and to obtain information on the conformation of the polymer layer from the observed alterations in the double layer properties.
In chapter 1. the use of double layer investigations to obtain insight in the adsorbed layer conformation is briefly outlined. Some theoretical and experimental aspects of the studies of polymer adsorption are shortly reviewed.
For the experiments the AgI-PVA system is chosen. Double layer charge and potential of AgI dispersed in aqueous electrolytes can be determined and controlled. Much is known about the surface area determination and the stability of Agl. PVA is a water soluble, flexible and uncharged polymer of which the concentration in solution can be determined readily. The combined AgI-PVA system is well suited for the purpose of this study. For the interpretation of the results recourse can be made to similar information previously obtained with low molecular weight alcohols.
The general procedures and the preparation of the Agl precipitates and sols are given in chapter 2.
Chapter 3. deals with the characterization of the specific surface area of the AgI samples. Several independent methods are used: capacitance measurements, N2- adsorption, adsorption from solution and electron microscopy. Accepting an uncertainty margin of 10 to 20% the areas obtained by the last three methods compared well mutually. However, the capacitance areas were always 3 to 4 times greater. This disparity was observed before by VAN DEN HUL and LYKLEMA. Surface areas of AgI sols strongly reduce upon coagulation or precipitation. A subsequent heat treatment enhances this effect.
We used the capacitance area in electrochemical studies, whereas for the adsorption of organic molecules the methylene blue adsorption area is chosen, otherwise unrealistically low adsorption values were found.
In chapter 4. the properties and solution characteristics of PVA are described. The samples used, differing in molecular weight and acetate content, are characterized by IR and UV spectroscopy. It could be concluded that our samples are atactic, contain no or very little 1,2-glycol units and one or two conjugated carbonyl groups per molecule. The acetate groups in PVA 88 (12 mole % acetate groups) are predominantly distributed in blocks along the polymer chain, whereas in PVA 98 the distribution of these groups is probably random. Molecular weights and the molecular weight distributions were determined by viscometry and gel permeation chromatography.
The solution properties of the polymers in water have also been studied by viscometry. Unperturbed dimensions, linear expansion factors and polymersolvent interaction parameters are calculated, taking heterodispersity into account. The relative magnitude of the steric hindrance or the characteristic ratio, combined with the fact that water is a poor solvent for PVA suggest that intramolecular interactions occur in the polymer chain.
Chapter 5. covers the measurement of the mass of PVA adsorbed per m 2 AgI, Γ p , as a function of the PVA concentration. Special emphasis is. given to the influence of molecular weight, acetate content and of the surface charge and state of dispersion of the Agl. The adsorption isotherms show a high-affinity character, leading to a maximum amount adsorbed of 1.5 to 2.6 mg m -2 . The saturation adsorption increases with increasing molecular weight and acetate content. The surface charge of the Agl and its state of dispersion have no measurable influence on the adsorption. The reduction in adsorption by the addition of KNO 3 UP to 10 -1 M is due to a decrease in available surface area.
No desorption of the polymer could be detected upon dilution with solvent, but the increase in adsorption with time shows that the adsorbed segments are reversibly bound.
In chapter 6. the principles of the double layer investigations are explained, whereafter a description is given of the potentiometric titrations and electrophoresis studies. The titrations reveal three important features upon adsorption of PVA:
- the double layer capacitance decreases,
- the p.z.c. moves to more positive values,
- the curves pass through a common intersection point, characteristic for the type of PVA.
These features reflect changes in the Stern-layer, caused by adsorbing polymer trains. Adsorbed polymer trains and low molecular weight adsorbates having a composition comparable with that of the polymer segments behave very similarly in the Stern-layer. From this resemblance it could be concluded that in PVA 88 segments with an acetate group adsorbed preferentially in the first layer. This is promoted by the blocky distribution of these groups in PVA 88. The similarity has further been used to develop a theorem to obtain the degree of occupancy of train segments in the Stern-layer, θ. The obtained result is confirmed by electrophoresis studies. A measure of the effective layer thickness can be found from the slope of the electrophoretic mobility against pAg curve around the isoelectric point. It was shown that this procedure is superiour to the classical one, in which the effective layer thickness is deduced from the reduction of the mobility in the plateau region.The main conclusion is that the fraction of polymer adsorbed in trains and the effective layer thickness are a function of the adsorbed amount only. Molecular weight and acetate content affect the adsorbed amount and thus indirectly influence the occupancy in the first layer and the effective thickness. Except for PVA 3-98, where probably preferential adsorption of higher molecular weight species occurs, the layer thickness is proportional to the square root of the degree of polymerization, as expected theoretically. The dependence of the train segment adsorption on the surface charge, though in principle present, is
too small to be practically important. The layer thickness is also independent of the surface charge.
In chapter 7. some further parameters of the adsorbed layer are calculated, such as the fraction of segments adsorbed in trains, p , and the amount adsorbed in loops and tails. It can be concluded that molecules adsorbed in the isolated state (Γ p ->0) do not completely unfold, probably due to the presence of intra molecular interactions. The average segment density in the loop region is relatively high, indicating that the average loop length is much shorter than the total chain length.
The obtained adsorption parameters are used for a quantitative check of the Hoeve model for polymer adsorption. The adsorbed amount at large polymer concentration is reasonably well predicted, including the molecular weight dependence. Also the trends in p (Γ p ) and θ(Γ p ) were confirmed. However, the adsorbed layer thickness is very poorly predicted and the physical significance of the model parpameters is obscure. Our general conclusion therefore is that the Hoeve model cannot fully describe the adsorption of AgI on PVA.In conclusion, this study shows that double layer investigations combined with polymer adsorption measurements provide a valuable tool to investigate the conformation of adsorbed polymers. The degree of occupancy of the first layer, the effective thickness of the adsorbed layer and the kind of segments directly adsorbed onto the surface could be determined over a wide adsorption range.
Copolymers at a liquid-liquid interface and their retarding effect on mass transfer between both phases
Scholtens, B.J.R. - \ 1977
Landbouwhogeschool Wageningen. Promotor(en): B.H. Bijsterbosch, co-promotor(en): S. Bruin. - Wageningen : Veenman - 125
industrie - grensvlak - vloeistoffen (liquids) - vermenging - kunststoffen - oppervlakten - macromoleculaire stoffen - oppervlakteverschijnselen - industry - interface - liquids - mixing - plastics - surfaces - macromolecular materials - surface phenomena
In this study we have investigated the interfacial properties of several polyvinyl alcohol-acetate (PVA-Ac) copolymers, which only differ in the content and the intra-molecular distribution of their vinyl acetate monomers (VAc).
In chapter I the relevant theoretical and experimental aspects of studies on polymer adsorption are reviewed. Because of experimental difficulties, studies on polymer adsorption at liquid-liquid interfaces have, up to this moment, only resulted in qualitative information on the adsorption mechanism and the structure of polymer adsorption layers. One of the objectives of the present study was to further elaborate a recently proposed method to determine the degree of coverage of a liquid-liquid interface from the retarding effect of polymers adsorbed at that interface, on mass transfer through it. To that end, KCl transfer measurements between water saturated with 1-butanol (wabu) and 1-butanol (buOH) saturated with water (buwa) have been performed, as described in the chapters 4 and 6.
With the exception of the PVA(-Ac) (co)polymers, all materials and their relevant physical properties are described in chapter 2.
In chapter 3, a short review is given on the properties of PVA-Ac, with special emphasis on the influence of the VAc monomers on the (dis)solution and interfacial properties of the copolymers. Since it was supposed that it is the intra- molecular VAc distribution that determines the interfacial properties of PVA-Ac to a large extent, this aspect has been investigated systematically. To that end, five blocky (B1-B5) and two random (R1 and R2) PVA-Ac copolymers have been prepared, the intra-molecular VAc distributions of which have been analysed in several ways (with complexometry, IR spectroscopy and thermal analysis).
The solution properties of these copolymers in water have been studied by viscosimetry. Particular attention has been paid to the methods of processing the experimental data. For the random copolymers, the linear expansion factor in water steadily decreases with increasing VAc content, whereas it passes through a maximum for the blocky copolymers. These differences can not be explained by the often assumed inhibition of inter- and intra-molecular H-bonding between VA segments due to the size of the acetate groups. It, is suggested that the incompatibility of VA and VAc sequences causes the expansion of the blocky copolymers. The decrease in expansion with (a further) increase in VAc content is related to the more hydrophobic character of the VAc monomers.
BuOH has a stabilizing influence on PVA in aqueous solution, probably due to preferential adsorption of buOH molecules with their hydrophobic part onto the C-C backbone of the polymers. This influence decreases with increasing VAc content of the copolymers, which is ascribed to the prevention of this adsorption on those parts of the chain to which the acetate groups are attached. An attempt is made to estimate the peripheral solvent quality for the copolymers from their Huggins coefficients. Probably the mean peripheral VAc content is less than the mean overall content of the copolymers.
The interfacial activities of the copolymers have been studied by measuring the interfacial tension between a copolymer solution of wabu and buwa, both with the static drop-shape and the dynamic drop-volume method. It is shown experimentally that the more accurate and much simpler drop-volume method is also applicable to these solutions, provided the copolymer concentration is not too low. In this system, the interfacial activity of PVA-Ac increases with the VAc content and, in particular, with the average VAc sequence length of the copolymers. It is concluded that these VAc sequences are the anchors that adsorb at the wabu-buwa interface. The observed time effects are ascribed to the unfolding of the adsorbed copolymers, so that the longer VAc sequences of the inner part of the coil can adsorb.
In chapter 4, the KCl transport from wabu to buwa is investigated. With a semi- empirical method it is attempted to separate the hydrodynamic from the physico-chemical contributions to the overall mass transfer coefficient, K b . Although no unambiguous quantitative results are obtained, it can yet be concluded that the partial buwa mass transfer coefficient, k b , is the rate determining step in the transfer process. This implies that the explanation of the retarding effect of PVA-Ac can not be found in a simple reduction of the interfacial area available for transfer. The effect is assumed to be mainly of a hydrodynamic nature. The experimental set-up used does not enable more quantitative results to be obtained, but the hypothesis that adsorption is the cause of the retardation is confirmed.
Since the effect of PVA-Ac: can only be properly explained when the interphase mass transfer process itself is understood in all details, the fundamentals of those transport phenomena that are relevant to this process are treated in chapter 5. In addition, attention is paid to the experimental results found in the literature.
In chapter 6, a transport vessel is described that has more systematic and better defined flow patterns at both sides of the interface. By assuming two simple but realistic models for the laminar boundary layer flow, k b is estimated theoretically for both a 'clean' (mobile) and a completely covered (stationary) interface. This makes it possible to draw more definite conclusions on the presence of any interfacial resistance.
The agreement between the theoretical and experimental mass transfer coefficients for a 'clean' interface allows the conclusion to be made that the interfacial resistance is negligible for the system studied. This must mean that kb is changed drastically by the adsorption of PVA-Ac, which is confirmed by the agreement between the theoretical and experimental mass transfer coefficients for a completely covered interface. However, this method can not provide any direct information on the degree of coverage of a liquid-liquid interface.
Yet it appears a sensitive method to distinguish between the interfacial activities of the blocky and random copolymers: the effects are diverse, depending on the total amount of PVA-Ac spread at the interface, on the VAc content and in particular on the average VAc sequence length. These differences are interpreted qualitatively in terms of the irreversibility of the desorption processes and the compressibility of the adsorption layers. Finally, a method is suggested to investigate more quantitatively the behaviour of adsorption layers in a variable, stationary shear-stress field.
Interactions between adsorbed macromolecules : measurements on emulsions and liquid films
Vliet, T. van - \ 1977
Landbouwhogeschool Wageningen. Promotor(en): J. Lyklema. - Wageningen : Veenman - 131
kunststoffen - industrie - chemie - colloïden - adsorptie - oppervlakten - suspensies - emulsies - macromoleculaire stoffen - oppervlaktechemie - moleculen - intermoleculaire krachten - plastics - industry - chemistry - colloids - adsorption - surfaces - suspensions - emulsions - macromolecular materials - surface chemistry - molecules - intermolecular forces
The aim of this study was to gain more insight into the factors, determining the inter- and intramolecular interactions between adsorbed macromolecules. To that end several experimental and theoretical approaches were followed, using well-defined systems. It was shown that these interactions could conveniently be studied by measurements on emulsions and thin free liquid films. Two different macromolecules were used: a nonionic one: polyvinyl alcohol (PVA) and an ionic one: a copolymer of methacrylic acid and the methyl ester of methacrylic acid (PMA-pe) in the molar ratio 2:1.
The characterization of the used materials has been described in chapter 2. The conformational transition, occurring in dissolved polymethacrylates was briefly discussed. At low pH, the molecules occur in a compact form, the hypercoiled form, or a-conformation. At pH above ~ 6 the molecules occur in the common. more extended b-conformation. From viscometry on PVA solutions conformational parameters, such as the root mean square end-to-end distance, the length of a statistical chain element and the linear expansion factor were determined. These conformational parameters were determined in a I M aqueous glycerol solution because in the film experiments 1 M glycerol was present in the PVA solutions in order to lower the water vapour pressure.
In chapter 3 the experimental methods have been described. In the first part attention was paid to the preparation of the emulsions and to the determination of basic properties, such as specific area and adsorbed amount. A variety of rheological measurements were described in the second part. A more detailed description was given of the apparatus for the dynamic measurements (the rheometer) and of that for creep measurements.
The end of chapter 3 deals with the thickness measurements of polymerstabilized free liquid films. First, a description of the apparatus and the experimental procedure was given. Subsequently, a discussion followed of the calculation of thicknesses from the intensity of the reflected light. It was shown that, for the calculation of the correction to be applied to the equivalent aqueous solution thickness, the smeared out adsorbed polymer segment layers may be formally replaced by a block distribution.
The inter- and intramolecular interactions between the PMA-pe segments and the effect of these interactions on the conformation of the polyelectrolyte molecule and on the rheological properties of emulsions stabilized by this polyelectrolyte, have been discussed in chapter 4. As possible attractive forces responsible for the compact conformation at low pH, VAN DER WAALS attraction and hydrophobic bonding between the methyl groups in the main chain were considered. In addition, the strength of the COULOMBIC interaction between the carboxyl groups also plays a role in the conformational transition.
The conformational transition from the a- to the b-conformation in free and adsorbed PMA-pe, was studied by potentiometric titration. Data for adsorbed PMA-pe were obtained by titrating polyelectrolyte-covered emulsion droplets. It was found that the conformational transition also occurs in adsorbed PMA-pe. This conformational transition is reflected in the rheological properties of paraffin in water emulsions, stabilized by PMA-pe. It could be concluded both from viscosity and dynamic data, that strong attraction between the emulsion droplets occurs only at a low degree of neutralization α, that is, if a substantial part of the adsorbed PMA-pe is in the a-conformation. Then both the dynamic moduli and the viscosities are very high. On the contrary at high a the emulsions were very fluid with little or no indication of attraction between the adsorbed polyelectrolyte sheets.
The main conclusions from the potentiometric titration data and the rheological measurements are:
a. the attraction between the polyelectrolyte segments, observed at low αin solution occurs also between loops and/or tails, adsorbed on one emulsion droplet;
b. the high values of the dynamic moduli and of the viscosities at low αare due to attraction between extending loops and/or tails, adsorbed on different droplets;
c. the two types of interaction are very similar.
This conclusion was confirmed by the influence of methanol on Na-PMA-pe stabilized emulsions and the effect of temperature. Moreover, from these experiments it could also be concluded, that probably the hyper-coiled conformation at low α, is to a large extent due to hydrophobic bonding.
The influence of Ca ++ ions on the properties of the polyelectrolyte was also investigated. Potentiometric titration showed that, in the presence of Ca ++ ions, the conformational transition is moved to higher a. Again the transition is reflected in the rheological properties of emulsions, stabilized by Ca-PMA-pe. The balance between the inter- and intramolecular interaction forces and the interactions themselves are more complicated than in the case of Na-PMA-pe. This complex character is reflected in the more complex rheological functionalities (η(α), G' (α) curves) of emulsions stabilized by Ca-PMA-pe.
The interactions between adsorbed macromolecules were further investigated by studying the properties of polymer stabilized thin free liquid films. Measurements on films, stabilized by PVA or PMA-pe, were reported in chapter 5.
The interaction forces which must be taken into account in a PVA film are VAN DER WAALS attraction, hydrostatic pressure and steric interaction. The VAN DER WAALS attraction over a film can be calculated. The equilibrium film thicknesses of the films were determined at varying hydrostatic pressure. Then the steric repulsion force Fs between the two adsorbed PVA layers was obtained by equalizing - Fs with the hydrostatic pressure and the VAN DER WAALS attraction. So the steric repulsion force could be calculated for different equilibrium thicknesses. Next the free energy of steric interaction was found by graphic integration of the force-distance curve. These values can be compared with theoretical predictions.
In order to calculate the free energy of steric interaction theoretically, a model of the segment density distribution had to be developed. The proposed semiquantitative model was based on the consideration that the molecular weight distribution of the used PVA samples is wide and the presumption that a large fraction of the segments is adsorbed as tails. Indications for this presumption were found by comparing the extrapolated ( Fs - 0) film thickness with the ellipsometric thickness of an adsorbed layer. This model leads to the conclusion, that the properties of the outer part of the adsorbed layers are dominated by a few extending tails. The free energy of steric repulsion, thus calculated with the HESSELINK et al. (1971b) theory of steric repulsion, between two adsorbed PVA ( M v = 27,000 or 86,000) layers, agrees well with the experimentally determined values for reasonable lengths of the tails.
In chapter 5 also the drainage behaviour and the equilibrium thicknesses of PMA-pe films, made at different values of the degree of neutralization a, were discussed. The measured equilibrium thicknesses correlated well with ellipsometric measurements of an adsorbed layer. The drainage pattern changes if or is varied. At low αthe films are rigid, whereas at high αthey are mobile. Also the dilatational modulus decreased from α= 0.1 to α= 1.0. Probably the interaction forces between the polyelectrolyte segments which are responsible for these phenomena, are the same as those which induce the conformational transition in the molecule or which are responsible for the drastic changes in the rheological properties of emulsions stabilized by PMA-pe if αis varied.
A more elaborate discussion of the rheological properties of PMA-pe stabilized emulsions is given in chapter 6. Both dynamic and creep measurements were reported.
In the dynamic experiments the storage modulus G' and the loss modulus G'' were measured as a function of the frequency ω. The degree of neutralization, the polyelectrolyte supply and the salt concentration were variables. By comparing the gel point concentration of free PMA-pe with the polyelectrolyte concentration in the layer between two emulsion droplets, it was concluded that there also a gel could be formed if attractive forces between the polyelectrolyte segments dominate. This conclusion is supported by analyses of the G' (ω) and G'' (ω) curves. In cases where such a kind of gel is formed, it is possible to relate G' to the number of polyelectrolyte cross-links between two droplets. Equations were given for the case of an ideal network model and for an aggregate model of the emulsion structure. For both models equations were also derived relating G' to the VAN DER WAALS attraction between the droplets. It was found that the ideal network model was good enough to interpret semiquantitatively the results obtained for the viscoelastic emulsions. The VAN DER WAALS attraction between the emulsion droplets proved to be much less important than the interactions between the polyelectrolyte sheets. It was calculated that at αPMA-pe = 0.1, about 400-1000 polyelectrolyte bonds were formed between two emulsion droplets at interparticle distances of 30 to 50 nm. It implies that about 10-20% of the polyelectrolyte molecules, present in the contact region between two emulsion droplets, are directly involved in the formation of these bonds.
A short discussion was given of the unusually high values of the loss factor tg δ. The suggestion was put forward that these high values follow from the fact that liquid must move in and out of the micro gels between adjacent emulsion droplets or from the relaxation of the polyelectrolyte cross-links, during a deformation cycle.
The creep curves were analysed by assuming the existence of both strong and weak bonds between the emulsion droplets. If measured under the proper conditions the weaker (secondary) bonds are broken, but the stronger (primary) bonds are not. Then it is possible to calculate from a non-linearity in the deformation as a function of the shear stress, if any, the contribution of the secondary bonds to the shear stress. The secondary bonds were identified as VAN DER WAALS attraction between the emulsion droplets and the primary bonds as interactions between polyelectrolyte molecules adsorbed on different droplets. Again it was found that the VAN DER WAALS attraction is relatively unimportant. From the found contribution to the shear modulus of an emulsion due to VAN DER WAALS attraction and steric repulsion between the droplets the interparticle distance was calculated to be 25-30 nm. This value was of the same order of magnitude as the results of the film thickness measurements. A semiquantitative assessment of the activation energy necessary to break a polyelectrolyte-polyelectrolyte bond showed that the interactions between the methyl groups must have a cooperative character.
It was concluded that the results of creep and dynamic measurements support each other.In conclusion, this study shows that both rheological measurements of sterically stabilized dispersions and the investigation of polymer stabilized thin liquid films are excellent tools for investigating the interactions between adsorbed macromolecules. Intramolecular interactions and interactions between macromolecules adsorbed on different interfaces are very similar. The latter interactions are dominated by the outer part of the adsorbed macromolecule layers.
|Schade aan kunststoffen door knaagdieren en bestrijdingsmiddelen hiertegen
Anonymous, - \ 1976
Wageningen : [s.n.] (Literatuurlijst / Centrum voor landbouwpublikaties en landbouwdocumentatie no. 3886)
bibliografieën - biodegradatie - cellulose - woningen - insectenbestrijding - lagomorpha - materialen - kunststoffen - eiwitten - knaagdieren - ongediertebestrijding - macromoleculaire stoffen - synthetische materialen - bibliographies - biodegradation - cellulose - dwellings - insect control - lagomorpha - materials - plastics - proteins - rodents - vermin control - macromolecular materials - synthetic materials