Anisotropic behaviour of the glass transition temperature in thin films of Langmuir–Blodgett deposited side chain polymers

Langmuir–Blodgett (LB) films of side chain polyamic acids were formed, deposited onto optical waveguides and imidized via heating to the corresponding polyimides on the optical waveguides. The LB film formation of the side chain polyamic acids, the imidization reaction and the glass transition behaviour of the polyimide thin films were investigated. Polarisation dependent waveguide mode spectroscopy was applied therefore. It was found that the glass transition temperature (Tg) of bulk and LB thin film samples of these polymers are identical and only depend on the spacer length between the side chain and backbone, not on film thickness. Our investigation shows that in the case of the ultrathin film geometry, the glass transition temperature exhibits an anisotropic behaviour. We find the classical kink in the temperature scan (a change in slope of the free volume vs. temperature plot) due to the thermal expansion change at Tg, in p-polarisation only, which is perpendicular to the main chain orientation. In the s-polarisation measurements (parallel to the main chains) no kink was found, only an offset in the vicinity of Tg. We attribute the anisotropic thermal behaviour to a transition, the freezing of segmental motion, in the side chains only.