Mechanistic Origins of Glass Transition Dependence on Molecular Weight in Linear Homopolymers

The dependence of the glass transition temperature (Tg) on molecular weight in linear homopolymers is canonically understood by the Flory-Fox equation.

From this perspective, chain ends that exhibit enhanced mobility become diluted in the infinite molecular weight limit.

Here we present results from simulations of molecular dynamics for several polymer models, ranging from the simple bead-spring model to atomistic polystyrene,

that indicate that the Tg molecular weight dependence is not driven by chain end effects.

Instead, chain end mobilities deviate weakly relative to the rest of the chain,

and Tg reduction from the infinite molecular weight limit manifests nearly uniformly along the backbone in all models.

We also find that molecular weight effects on Tg are encoded at high temperature even before the onset of glassy dynamics,

suggesting that quantities such as the size of the dynamical unit and chain stiffness effects may instead be leading order.

Our results suggest a need to reconsider textbook explanations for the molecular weight dependence on Tg.