Multiple relaxation modes in associative polymer networks with varying connectivity
Bohdan, M. ; Sprakel, J. ; Gucht, J. van der - \ 2016
Physical Review. E, Statistical nonlinear, and soft matter physics 94 (2016)3. - ISSN 2470-0045
The dynamics and mechanics of networks depend sensitively on their spatial connectivity. To explore the effect of connectivity on local network dynamics, we prepare transient polymer networks in which we systematically cut connecting bonds. We do this by creating networks formed from hydrophobically modified difunctionalized polyethylene glycol chains. These form physical gels, consisting of flowerlike micelles that are transiently cross-linked by connecting bridges. By introducing monofunctionalized chains, we can systematically reduce the number of bonds between micelles and thus lower the network connectivity, which strongly reduces the network elasticity and relaxation time. Dynamic light scattering reveals a complex relaxation dynamics that are not apparent in bulk rheology. We observe three distinct relaxation modes. First we find a fast diffusive mode that does not depend on the number of bridges and is attributed to the diffusion of micelles within a cage formed by neighboring micelles. A second, intermediate mode depends strongly on network connectivity but surprisingly is independent of the scattering vector q. We attribute this viscoelastic mode to fluctuations in local connectivity of the network. The third, slowest mode is also diffusive and is attributed to the diffusion of micelle clusters through the viscoelastic matrix. These results shed light on the microscopic dynamics in weakly interconnected transient networks.
Supramolecular networks of telechelic polymers
Bohdan, M.A. - \ 2016
Wageningen University. Promotor(en): Jasper van der Gucht, co-promotor(en): Joris Sprakel. - Wageningen : Wageningen University - ISBN 9789462578678 - 117 p.
supramolecular chemistry - networks - polymerization - gels - mechanical properties - separation technology - rheology - supramoleculaire chemie - netwerken - polymerisatie - mechanische eigenschappen - scheidingstechnologie - reologie
This thesis focuses on the fundamental understanding of phenomena associated with the gelation of end-functionalized polymers and the dynamic processes occurring inside of the gel network. To address particular questions we use two types of telechelic polymers, in which the assembly occurs due to the solvophobic interactions and due to the metal-ligand coordination, respectively. In this research we employ a number of methods, mostly rheology and light scattering.
In Chapter 2 we revealed new insights into the complex microscopic dynamics of transient networks, assembled by hydrophobic forces. Using light scattering experiments we show how these materials exhibit complex multimodal relaxation spectra. To shed light on the nature of such relaxation processes we systematically changed the network architecture by gradually reducing the network connectivity while keeping the polymer concentration constant. This strategy allows us to disentangle the roles of concentration and connectivity on the dynamic modes of these systems.
In Chapters 3 and Chapter 4 we experimentally explored the pathways of network formation from telechelic polymers association by means of metal-ligand complexation. Interestingly, while some networks exhibit near-ideal Maxwellian behavior, as expected for transient networks, we find certain cases where we observe scale-free critical mechanics. To date this latter behavior was only identified close to a covalent percolation transition. The critical behavior observed for these end-functional self-assembled polymer networks, however, is robust to changes in concentration, temperature and crosslinking degree. Our studies show that such a self-organized and robust critical state is the results of arrested phase separation that kinetically traps the network-forming system at its percolation point. The system thus remains trapped in a critical state resulting in robust power-law scaling of shear and relaxation moduli. We also show how this state depends sensitively on the relaxation kinetics of the nodes by demonstrating an intermediate case where initial critical behavior slowly relaxes over the course of several days to the ideal linear Maxwell case. With our research we highlight the complex pathway where self-assembling systems reach their equilibrium ground state, involving persistent and long-lived kinetically arrested states which give rise to unusual mechanics and highly heterogeneous spinodal structures.
Chapter 5 brought us towards more applicable materials where we develop a highly tunable composite network based on orthogonal supramolecular interactions. For such a design we generate multivalent nanoparticle tectons, which are subsequently linked together into network structures, using metal-coordination interactions. Materials built this way are highly tunable with moduli and viscosities spanning many orders of magnitude.
In the remainder of this chapter, we focus on some unresolved and outstanding questions regarding the physical chemistry and properties of supramolecular networks and we will discuss some preliminary data obtained in our efforts to resolve them.
Supramolecular assembly of self-healing nanocomposite hydrogels
Gerth, M. ; Bohdan, M.A. ; Fokkink, R.G. ; Voets, I.K. ; Gucht, J. van der; Sprakel, J.H.B. - \ 2014
Macromolecular Rapid Communications 35 (2014)24. - ISSN 1022-1336 - p. 2065 - 2070.
poly(ethylene glycol) - aqueous-solution - gel transition - terpyridine - coordination - chemistry - gelation - driven - light
Hierarchical self-assembly of transient composite hydrogels is demonstrated through a two-step, orthogonal strategy using nanoparticle tectons interconnected through metal–ligand coordination complexes. The resulting materials are highly tunable with moduli and viscosities spanning many orders of magnitude, and show promising self-healing properties, while maintaining complete optical transparency.