Theory of structural relaxation and vitrification in permanent and associating polymer networks

The microscopic, force-based Elastically Collective Nonlinear Langevin Equation theory of activated segmental relaxation in polymer melts has been generalized to treat the effects of chemical and physical crosslinks in the rubbery and supercooled regimes down to the glass transition temperature Tg. Permanent crosslinks are modeled as locally pinned sites along a semiflexible polymer chain which intensify dynamical constraints on mobile segments. Key results include: (i) crosslinking strongly increases the relaxation time and glass transition temperature Tg in distinctive manners, (ii) Angell plots of the alpha relaxation time nearly collapse onto a master curve for all crosslink densities and diverse vitrification timescale criteria, (iii) crosslinking enhances the coupled local cage and long range collective elastic barriers in different manners. The same theory also addresses associating polymer melts with strong and long lived physical bonding. A generic increase of Tg and a supra-exponential growth of the alpha time with crosslinking is predicted for telechelic polymer melts, but with little dynamic fragility change. The different behavior of associating polymers with weak sticker bonding is analyzed by introducing local bond fluctuations that dynamically weaken their effect on mobile segments. Quantitative comparisons to experiments on PDMS, PPG, PnBA and other networks show good agreement with theory. This work was performed in collaboration with Baicheng Mei, Ashesh Ghosh, Chris Evans, Charles Sing and Alexei Sokolov.