Structural relaxation of ultrathin polymer films prepared by the Langmuir-Blodgett technique: Characteristics of the two-dimensional system

Thermal relaxation of ultrathin polymer films prepared by the Langmuir-Blodgett (LB) technique was studied by a comparison with a three-dimensional bulk system. Poly(vinyl alkanal acetal)s (PVAA) with different side chain lengths were synthesized and the thermal stability of their LB films was investigated by the energy-transfer phenomenon between fluorescence probes labeled to the polymer chains. The multilayer structure was irreversibly disordered and mixed by the thermal treatment. A close relationship was found between the glass-transition temperature (T-g) of the polymer bulk and the temperature at which the layered structure of LB films started to disorder. Theoretical calculations based on Forster kinetics were applied to measure the diffusivity of polymer segments, which was successfully evaluated by assuming time-dependent Gaussian distributions of chromophores in the direction normal to the layer plane. The apparent activation energy Delta E-a of the diffusion constant for the LB films, however, was much smaller than that for polymer bulk obtained by the viscoelastic measurement. It was concluded that the relaxation process of the ultrathin polymer films occurs by conformational changes from the two-dimensional nonequilibrium state given by the LB deposition. The confined form could be released by the local motion of the polymer segments under the heating to temperatures higher than T-g.