Glass transition in Ultrathin Polymer Films : A Thermal Expansion Study

TL;DR

This study investigates how ultrathin polymer films exhibit a broadened and detectable glass transition temperature using thermal expansion measurements, revealing effects not seen in thicker films.

Contribution

It demonstrates the use of thermal expansion and x-ray reflectivity to observe glass transition in films as thin as 4 nm, challenging previous detection limits.

Findings

Systematic broadening of Tg with decreasing film thickness.

Detection of glass transition in 4 nm films, previously thought undetectable.

Reversible thermal expansion behavior around Tg.

Abstract

Glass transition process gets affected in ultrathin films having thickness comparable to the size of the molecules. We observe systematic broadening of glass transition temperature (Tg) as the thickness of the polymer film reduces below the radius of gyration but the change in the average Tg was found to be very small. Existence of reversible negative and positive thermal expansion below and above Tg increased the sensitivity of our thickness measurements performed using energy dispersive x-ray reflectivity. A simple model of Tg variation as a function of depth expected from sliding motion could explain the results. We observe clear glass transition even for 4 nm polystyrene film that was predicted to be absent from ellipsometry measurements of thicker films.