科研之友
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摘要
We formulate a theoretical mechanism for the physical origin of the massive dynamic fragility range observed in long chain glass-forming polymer melts within the context of the force-level elastically collective nonlinear Langevin equation theory of coupled local-nonlocal activated segmental relaxation. The hypothesis involves how the cage scale barrier hopping process on the three Kuhn segment length scale is quantitatively coupled to the longer range collective elastic distortion required to sterically allow large-amplitude events to occur. The key nonuniversal aspect is proposed to be an effective microscopic jump distance, a dynamical quantity associated with the activation barrier, which is influenced by nanometer-scale conformational transition physics and monomer chemistry. By introducing a single numerical factor that breaks the universality of the jump distance in our mapping of polymers to liquids of disconnected Kuhn-sized hard spheres, one can account rather well, and simultaneously, for the-vitrification temperatures and dynamic fragilities of 17 polymer liquids of diverse chemistry. The very low fragilities of polyisobutylene (Pm) and polyethylene are suggested to be a consequence of suppression of the collective elastic distortion effect. The large fragility variations displayed by polymeric materials appears special to long chain melts, consistent with their absence in molecular and oligomer liquids. Connections between cooperativity and fragility are identified. The theory very accurately captures segmental relaxation time experimental data for PIB, polypropylene, and polycaibonate melts over 11-13 decades. The present work sets the stage for attempting to understand the failure of time-temperature superposition in the deeply supercooled regime.
- 出版日期2016-12-27
