|Title||Adsorption of water-soluble polymers onto barium titanate and its effects on colloidal stability|
|Author(s)||Laat, A.W.M. de|
|Source||Agricultural University. Promotor(en): G.J. Fleer. - S.l. : De Laat - ISBN 9789054854265 - 150|
|Department(s)||Physical Chemistry and Colloid Science|
|Publication type||Dissertation, externally prepared|
|Keyword(s)||colloïden - dispersie - polymeren - colloids - dispersion - polymers|
|Abstract||Ceramic products are usually made from powders which are processed into a green body, with a shape dictated by the final product. Organic binders are used to give the green product sufficient mechanical strength. A sintering process at high temperature converts the green body into the final ceramic product. In electronic ceramics, a high density and a homogeneous microstructure are required to obtain high quality products. For that purpose solid state sintering, in which no liquid phase is present, is applied. The result of the sintering process is highly dependent on the structure of the green body. Small pores will disappear during sintering but large ones will remain, resulting in a lower quality.
The ideal ceramic powder has small (submicron) particles with a narrow size distribution and no hard agglomerates. Unfortunately, hard agglomerates often occur in ceramic powders. Milling of the ceramic powder in a liquid is often used to break down hard agglomerates. Voids or holes in the ceramic product can be due to these hard agglomerates or due to inadequate processing of the powder, which leads to a green body with many large pores.
The study of the properties of powders dispersed in liquids is a branch of colloid chemistry. By using colloid chemical methods, control over the particle interactions can be achieved, which then allows the production of dense and homogeneous green bodies.
Van der Waals attraction between particles of the same composition may cause flocculation. A dispersion with such a flocculated material will give a highly porous, low density green body. Roughly speaking, two methods are available to make the particles repulsive, allowing the production of highdensity homogeneous green bodies. The first is the adsorption of ionic species onto the surface of the particles. The particles will then repel each other electrostatically. The second method is to adsorb polymers onto the surface of the particles. If two particles with adsorbed polymer layers approach each other, the polymer concentration in the contact region between the particles is increased. The higher osmotic pressure in that region leads to repulsion. This so-called steric stabilisation can be quite effective if the adsorbed polymer layer has a sufficient thickness. The repulsion should start at a particle separation where the Van der Waals attraction still is weak.
Adsorbed layers with sufficient thickness can be made by adsorption of homopolymers, provided the molecular weight and the coverage are high enough. Long tails then develop due to crowding on the surface. However, block copolymers with only one adsorbing block can be much more effective. Very thick adsorbed layers occur if the non-adsorbing block is highly stretched, a situation that occurs at a suitable ratio in length between the adsorbing and non-adsorbing blocks.
Clearly, the adsorption of polymers and their conformation in the adsorbed layer are crucial in steric stabilisation. In this thesis the adsorption of polyvinyl alcohol (PVA) and polyacrylic acids salt (PAAS) onto BaTiO 3 are studied, with the objective to make sterically stabilised. dispersions in an aqueous environment. BaTiO 3 is an important material for electronic ceramics; it is used in the manufacture of capacitors and in resistors with a positive temperature coefficient.
All adsorption experiments are analysed by Size Exclusion Chromatography (SEC). Using SEC, not only can the adsorbed amount of polymer be determined but also the fractionation in chain length upon adsorption. Even the separate adsorbed amounts from mixtures of polymers can be analysed. The principles of SEC, its possibilities, and the methods and equipment used, are presented in Chapter 2.
In Chapter 3, the adsorption of PVA and PAAS on BaTi03 is studied. Adsorption of PVA occurs over the whole molecular weight range including short chains. The rounded shape of the adsorption isotherm indicates competition between chains of different length, pointing to preferential adsorption of the longer chains. This contradiction is probably due to the nature of PVA, which can be considered as a copolymer with short polyvinyl acetate blocks and branches distributed over the chain. Moreover, the fraction of acetate groups depends on the chain length. Accordingly, PVA must be considered as a mixture of several polymers with slightly different chemical compositions. Pure homopolymer adsorption behaviour cannot be expected with such a polymer. The adsorption of PAAS results in a rather peculiar fractionation behaviour: an intermediate molecular weight fraction adsorbs preferentially. This phenomenon is analysed in more detail in Chapters 4 and 5. In the adsorption onto BaTiO 3 from mixtures of PAAS and PVA, no adsorption of PVA occurs if enough PAAS is present to cover the surface of the particles completely. On the other hand, pre-adsorbed PVA cannot be displaced from the surface by addition of PAAS afterwards. Mixed adsorbed layers are found in this situation.
The typical molecular weight fractionation with PAAS is studied in more detail in Chapter 4. Several PAAS samples with different molecular weights are used. It is shown that the molecular weight distribution (MWD) of the polymer has a significant influence on the result. The electrostatic repulsion between adsorbed chains on the surface and chains in solution prevents polymer chains above a certain length from reaching the surface. If the whole MWD is below this critical value, the longest chains adsorb preferentially, which is the expected behaviour for homopolymer adsorption. If the critical value lies within the MWD of the PAAS, the longer chains cannot participate in the exchange process, leaving an intermediate fraction adsorbed on the surface. PAAS samples with a MWD above the critical value and with only a few short chains show adsorption over a wide molecular weight range. Increasing the salt concentration of the solution decreases the electrostatic barrier, allowing longer chains to reach the surface. The preference in adsorption is then shifted to higher chain lengths.
In Chapter 5 we report on the kinetics of the adsorption of two PAAS samples. The adsorbed amount and molecular weight fractionation change relatively quickly over the first two days and more gradually over a longer period. At low salt concentrations changes occur for up to 24 days. Theoretical calculations predict a depletion layer with a minimum in polymer
In Chapter 6 several PVA-based copolymers are tested for their ability to sterically stabilise BATiO 3 . Regular PVA is shown to be ineffective. The results obtained in Chapter 3 make it possible to define potentially suitable steric stabilisers for BaTiO 3 based on PAAS and PVA. The first choice is a block copolymer with PAAS as the anchor block and a PVA block as the stabilising moiety. Three types, with different ratios in block length, were evaluated. Each of these proved to be suitable for stabilisation. Random copolymers of PVA containing a small fraction of carboxylic acid are a possible alternative. Seven of these types were tested; only two were successful. An evaluation of the stabilising mechanism showed pure steric stabilisation with a block copolymer and combined steric and electrostatic stabilisation with one of the random copolymers. Green bodies with an improved homogeneity could be made with both the block and random copolymers, provided steric or electrosteric stabilisation is realised.
Finally, in Chapter 7, two of the ineffective random copolymers are compared with one of the successful types. In addition, one of the block copolymers was included for comparison. The chemical compositions were studied with IR spectroscopy, and SEC was used for comparing the MWDs and for studying the adsorption behaviour. The IR analysis showed deviating chemical compositions for the ineffective random copolymers. One of these has lactone groups in the chain while the other one has a very high acetate content. SEC analysis showed a significantly lower chain length for both ineffective PVA random copolymers in comparison with the one suitable for steric stabilisation. This is the most probable reason for the ineffectiveness of these random copolymers. With short chains the steric layer thickness is too low for steric stabilisation. The block copolymer has an average molecular weight comparable to the stabilising random copolymer. Moreover, the longest chains of the block copolymer adsorb preferentially, probably resulting in a thick adsorbed layer. In the fractionation upon adsorption of the block copolymer there is no further increase in the relative amount of adsorbed long chains above a certain surface coverage. Further increase in adsorption above this coverage is possible if an increased stretching of the adsorbed chains occurs at equal relative amounts of the various chain lengths.
In this thesis it is shown that the design of suitable block copolymers for steric stabilisation of particles can be based on adsorption experiments with the separate homopolymers. The chemical composition of stabilising random copolymers can be derived from the same experimental results. Moreover, it is shown that the adsorption kinetics of polyelectrolytes is highly influenced by the electrostatic barrier between the chains in solution and the surface of the particles.