Is the Molecular Weight Dependence of the Glass Transition Temperature Driven by a Chain End Effect?

WF Drayer and DS Simmons, MACROMOLECULES, 57, 5589-5597 (2024).

DOI: 10.1021/acs.macromol.4c00419

The immense dependence of the glass transition temperature T-g on molecular weight M is one of the most fundamentally and practically important features of polymer glass formation. Here, we report on molecular dynamics simulations of three model linear polymers of substantially different complexity demonstrating that the 70-year-old canonical explanation of this dependence (a simple chain end dilution effect) is likely incorrect at leading order. Our data show that end effects are present only in relatively stiff polymers and, furthermore, that the magnitude of these end effects diminish on cooling. We find that T-g(M) trends are instead dominated by shifts in T-g throughout the entire polymer chain rather than through a chain end effect. We show that these data can be rationalized via a generic two-barrier model of T-g and its M-dependence, motivated by the Elastically Collective Nonlinear Langevin Equation theory. More broadly, this work indicates need to reopen the question of the origin of the T-g(M) dependence in linear polymers (and macromolecules at large), as well as an opportunity to reveal new glass formation physics with renewed study of M effects on T-g.

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