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

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.