This research is aimed at a fundamental understanding of dynamic processes - diffusion, viscosity, viscoelasticity - in any polymer material containing more than one chemical repeat unit. Such multicomponent polymers . a class that includes miscible and immiscible blends, random and block copolymers, and their mixtures . are ubiquitous in modern macromolecular science and technology. Description of the dynamics of single component polymers has advanced considerably, primarily via reptation-based models, but the extension to multicomponent systems lags behind. There are two main obstacles: (i) limited understanding of the composition and temperature dependence of the local segmental dynamics that underly all mechanisms of chain relaxation, and (ii) incomplete understanding of the effects of mixing on the mechanisms of chain dynamics. The proposed research will address both of these issues through strategic measurements on model systems, using a unique combination of three powerful techniques: rheology, oscillatory flow birefringence, and diffusion. Intellectual Merit Recent research has revealed a variety of fascinating phenomena in the dynamics of binary polymer mixtures, attributable to the fact that the temperature dependences of the segmental mobilities of the two components are distinct. This "thermorheological complexity" arises both from intrinsic differences in flexibility, an intramolecular contribution, and from differences between the bulk, average composition, and that of the local environment surrounding a segment. A partial explanation for these observations based on the idea of self-concentration, the local enrichment of the environment in like monomers due simply to chain connectivity, has proven capable of describing much of the phenomenology. Furthermore, it provides an organizing principle by which the behavior of various systems can be classified, and it highlights those future experiments that are likely to be most revealing. The three experimental techniques in combination will allow precise extraction of the dynamics of each component, in carefully selected model blends, over broad ranges of composition and temperature. These data should enable full characterization of the local dynamics, which in turn will direct the development of an improved, predictive model. Then, with an understanding of local dynamics in hand, it will be possible to address several longstanding issues concerning chain dynamics, via the same experimental protocol. In particular, measurements of the composition and temperature dependent viscosity on the same model blends will allow detailed testing of proposed models, such as "double reptation", for the effects of polydispersity on chain dynamics. Furthermore, the extraction of the full frequency dependent response and diffusivity of a dilute component in a mixture will lead to an experimental resolution of the longstanding "constraint release versus contour length fluctuations" controversy in linear viscoelasticity. Broader Impact The vast majority of commercial polymer materials are processed in the liquid state, where the viscoelastic response (both linear and non-linear) ultimately dictates what can and can't be done. As the fraction of commercial materials that may be considered "multicomponent" increases steadily, the importance and utility of both "correlative" (i.e., take what is observed and relate it to molecular variables) and "predictive" (i.e., predict properties based on molecular variables) schemes grows as well. The research described herein shows promise of providing the first reliable predictive scheme for the viscosity of a miscible polymer blend. Furthermore, the underlying concepts should contribute to predictive schemes for the viscoelasticity of other multicomponent systems, and in the longer run for the non-linear flow properties as well. This program integrates teaching and research over the full range of polymer science . synthesis, characterization, morphology, rheology, dynamics, and theory . for students in chemistry, chemical engineering, and materials science programs. This breadth of training has made group alumni attractive in both academic and industrial settings, and to companies extending well beyond the traditional polymer industry. Regular opportunities for mentoring undergraduates, and for presenting research results at national scientific meetings and to audiences of industrial scientists, constitute an essential component of graduate student education in this program.
