Surface mobility of a glass-forming polymer in an ionic liquid

TL;DR

This study investigates how ionic liquids affect the surface mobility of glassy polymers, revealing that they slow surface flow but do not eliminate the inherent dynamical heterogeneity of the polymer surface.

Contribution

It provides experimental and numerical evidence that ionic liquids slow but do not suppress the surface mobility of glassy polymers, highlighting the persistence of dynamical heterogeneity.

Findings

Ionic liquids significantly slow surface leveling of polystyrene films.

Surface mobility in ionic liquids can match that in vacuum under certain conditions.

The liquid-like surface layer below Tg persists even in ionic liquids.

Abstract

The free surface of glassy polymers exhibits enhanced segmental dynamics compared to the bulk, forming a liquid-like layer that lowers the glass transition temperature (Tg) in nanometersized polymer samples. Recent studies have shown that immersing polymers in ionic liquids can suppress this enhanced surface dynamics. To investigate how ionic liquids influence polymer dynamics near the ionic-liquid-polymer interface, we measure the surface leveling of nanometersized stepped polystyrene films immersed in ionic liquids, and compared the results to the case of films in vacuum. Our results reveal that ionic liquids significantly slow the leveling process both above and below Tg. However, our results indicate that the liquid-like surface layer below Tg does exist in ionic liquids. Numerical solutions of the thin-film equation, incorporating appropriate boundary conditions, show that the surface mobility of PS films in ionic liquids can match that of PS films in vacuum. Thus, while ionic liquids alter the polymer flow process, they do not eliminate the dynamical heterogeneity inherent to glassy polymers.