Relaxations in Complex Polymer Systems

Abstract

Many applications that employ polymeric materials rely on mixtures (polymer/polymer, polymer/nanoparticle, polymer/filler). A key challenge of using these materials is understanding interrelations between the physical properties and the local and macroscale morphologies. The most common systems are mixtures of two homopolymers, A and B, which can exhibit properties that are more desirable than those of the pure components. Unlike miscible small molecule systems, miscible A/B polymer/polymer blends, while macroscopically homogeneous, can be spatially compositionally heterogeneous at the molecular level, which can cause deviations in physical properties. Applications in the areas of organic electronics, membranes, and nanoscale coatings can also require these materials to function under various conditions of geometric confinement, such as thin films. This introduces an additional complication because the proximity to an external interface (free surface or substrate) influences the local composition and morphology, leading to film thickness-dependent behavior. To this end, this dissertation explores three problems involving the role of morphology on dynamic processes in polymeric systems of different local intermolecular environments.

First, we investigate the role of local spatial compositional heterogeneity on the dynamics of the A component in bulk, miscible A/B polymer/polymer blends. The dynamics of the faster, lower glass transition temperature component A, at temperatures sufficiently high above the blend glass transition, manifest the behavior of chains relaxing in a compositionally homogeneous environment. For temperatures lower than the blend glass transition, the A component chains relax in two distinctly different local compositional environments, manifesting the influence of spatial compositional heterogeneity.

Having investigated the role of spatial compositional fluctuations on the relaxations of the A component in A/B polymer/polymer blends, the additional effect of geometric thickness confinement at the nanoscale – confining the A/B mixture between two substrates – was studied. In thin film blends, the concentration of the A component may differ from the macroscopic average composition at different depths into the film, largely due to its preferential interactions with the confining substrates. In this case, the compositional changes driven by the interfacial interactions become dominant when the films are sufficiently thin. A key finding is that, whereas thickness confinement effects modify the dynamics of pure homopolymer chains for thicknesses up to a few nanometers, the effects on these A/B blends extend over hundreds of nanometers.

The third problem is based on the recognition that in most applications, polymer thin films can be required to contact other polymers or different “hard” materials. The vast majority of studies that investigate physical properties examine either free or supported films. Here we investigate the dynamics of a homopolymer A confined between a hard substrate C and a soft, immiscible polymer film B. A surprising finding is that the presence of the soft polymer B has the effect of increasing the relaxation rates of polymer A significantly, and over unusually large length-scales, not observed in polymers confined between two hard substrates C. These findings implicate the sensitivity of polymer dynamics to the modulus of the confining environment.

The works described in this dissertation provide a comprehensive view of how physical properties of polymers can change significantly in different environments – compositional changes, changes in mechanical moduli of the surrounding environment, and geometric constraints. Insights gained from these studies can be used to understand and control the physical properties of polymer-based materials for future applications.