Computational Insights into the Molecular Origins of the Chain Length Dependence of Polymers' Glass Transition*

The question of the precise molecular origin of the profound molecular weight dependence of the glass transition temperature (Tg) remains inconclusive despite the central importance of this phenomenon to polymer science.

Namely, whether this trend emerges from chain end mobility or free volume as proposed by the Flory-Fox model, or whether it emerges from other mechanisms such as the chain-length dependence of chain stiffness as reflected by the characteristic ratio (Cn) and encoded in theories such as the Elastically Collective Nonlinear Langevin Equation theory of glass formation, or a combination of the two, is unknown.

Here we report on results of bead-spring and all-atom simulations of polymer chains over a wide range of molecular weight, examining the relationship between chain length, local dynamics, and global dynamics during polymer glass formation.

We specifically report on results regarding the extent to which Tg trends with chain length can be attributed to averaging over locally accelerated dynamics near chain ends versus the impact of chain stiffness from the perspective of the entire chain.