Polymer thin film properties as a function of temperature and pressure

Abstract

We report on evanescent wave optical measurements of the glass transition temperature, T g , of spin-cast PMMA films as a function of film thickness and molecular weight. It was found that for films of high molecular weight PMMA (M n > 100,000 g mol -1 ) a strong T g depression occurs for films that are thinner than 100 nm in case they are deposited on hydrophobic substrates. This strong T g depression of up to 25°C decreases if similarly thick films of PMMA of low molecular weights are investigated and vanishes completely for PMMA with M n < 12,000 g mol -1 . For films made of these materials T g is found to be identical to that of the bulk even for films as thin as 5 nm. The results might be interpreted in terms of free volume considerations. To check this assumption we also designed and built a pressure cell that can be used together with the evanscent wave optical techniques for similar measurement, but with the additional option to do the measurements at different pressures up to ca. 100 MPa to further vary the free volume of these polymer films in constrained geometry. Some first results obtained with this setup are also described.