Dynamics of associative polymer networks

Abstract

Associative polymer networks are a versatile class of materials as demonstrated by their use in a wide variety of applications including biomaterials, viscosity modifiers and underwater adhesives. The viscoelastic and transport properties displayed by these associative networks can be tuned by careful design of the polymers that make up the network, as well as the transient interactions between them. Thus, elucidating the relationship between the molecular level details and the observed macroscopic properties is of high importance to further advance our understanding of associative networks. However, the complex dynamics displayed by these materials over a wide range of length and time scales present a significant challenge in studying this relation. This thesis aims to provide insights into the molecular origin of the dynamics of associative polymer networks.

The first part of this thesis investigates the molecular origin of shear thinning in associative networks through the design of a model associative polymer and a custom-built set-up referred to as "rheo-fluorescence" to quantify force-induced bond dissociation under shear flow. Comparison to existing models in transient network theory then demonstrate that retraction of dangling chains alone is insufficient to account for shear thinning in the model associative polymer network. Additional modes are likely contributing to the observed shear thinning behavior. The second part of this thesis focuses on the effect of sticker density, sticker clustering and entanglements on the dynamics of the associative networks through combined studies of self-diffusion performed using forced Rayleigh scattering (FRS) and rheology.

All three studies were performed using well-defined polymers with the same chemical composition such that the observed effects are solely due to the changes in sticker density, clustering and entanglements introduced during synthesis. The combined FRS and rheology studies on the effect of sticker density using a set of random copolymers revealed apparent superdiffusive scaling for chains with up to 15 stickers. This finding demonstrates that molecular hopping and thus, deviation from predictions of mean-field models persists to a higher limit than expected. To study the effect of sticker clustering, a polymer with clustered stickers was synthesized such that the molecular weight and sticker density were comparable to the random copolymers. It is demonstrated that the network topology is significantly altered by sticker clustering as evidenced by the opposite trends observed in the FRS and rheology measurements.

Finally, the onset of entanglements was examined by performing FRS and rheology measurements on a high molecular polymer, prepared at concentrations that span the unentangled to the weakly entangled regime. A clear transition seen in the self-diffusion measurements demonstrates the advantage of FRS to study the transition regimes where other techniques like rheology only show subtle changes.