Polymer thin film properties as a function of temperature and pressure

We report on evanescent wave optical measurements of the glass transition t emperature, T-g, of spin-cast PMMA films as a function of film thickness an d molecular weight. It was found that for films of high molecular weight PM MA (M-n > 100,000 g mol(-1)) a strong T-g depression occurs for films that are thinner than 100 run in case they are deposited on hydrophobic substrat es. This strong T-g depression of up to 25 degrees 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 m aterials 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 con siderations. 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 measureme nts at different pressures up to ca. 100 MPa to further vary the free volum e of these polymer films in constrained geometry. Some first results obtain ed with this setup are also described.