Understanding chain motion at glass-forming polymer interfaces: dynamical gradients yield rubbery surfaces*

Over 30 years of research has probed alterations in the dynamical response of polymers confined to nanoscale domains. These alterations include profound changes in rheological response that are central to these materials’ performance and poorly understood at a fundamental level. A central question is how alterations in Tg and segmental dynamics in these settings drive changes in polymer chain motion and rheology. Because the Rouse theory that underpins polymer dynamics assumes homogenous segmental dynamics, addressing this question demands development of a new fundamental understanding of chain motion in massively heterogeneous environments. Beyond its fundamental implications, this problem has received broad interest because of its implications for the rheological response of nanostructured and nano dimensioned polymers ranging from thin films to polymeric nanocomposites.

Here we report on simulations and theory, supported by experiment, providing the first predictive understanding of polymer chain relaxation in a Tg gradient at the surface of a film. Our simulation results indicate that the gradient in segmental relaxation timescales near a polymer interface drives emergence of rubbery-like surface behavior, even in unentangled polymers, with a lifetime that grows upon cooling. As a consequence, time temperature superposition (TTS) breaks down at surface of polymer films for both entangled and unentangled polymers. We establish a new theory of Rouse motion near polymer interfaces involving large Tg gradients, with this theory predicting that the Tg gradient produces a diffuse transient tethering effect that in turn leads to the emergence of surface rubbery-like behavior on cooling. These results provide a new basis for understanding and analyzing rheological response data in polymer films in the glass-formation range and likely has major implications for the rheological response of nanocomposites as well.