Effect of polymer-substrate interactions on the glass transition of polymer thin films

Abstract

It has been suggested that when the polymer-substrate interaction, ys, in a polymer film supported by substrate is strongly favorable, the Tg of the polymer film may increase with decreasing film thickness; but the opposite prevails if the interaction is less than weakly favorable. We present a quantitative study of the glass transition temperature, Tg, in thin films of polystyrene (PS) as a function of ys by measuring the change in the thermal expansion using x-ray reflectivity. Using random copolymer of styrene and methylmethacryalte anchored to the substrate, ys could be varied by varying the styrene content,/With a fixed PS film thickness of 33 nm, the Tg was depressed by -20 C as/was decreased from 1 to 0.7. An analysis analogous to the Gibbs-Thompson model indicated that the surface energy was not a suitable parameter to use to describe the effect of interfacial interactions on the Tg of polymer thin films. Instead, an associated local fractional change in the polymer mass density at the substrate interface was introduced to describe the observed change in Tg with different ys.