STRUCTURAL RELAXATION OF ULTRATHIN POLYMER-FILMS PREPARED BY THE LANGMUIR-BLODGETT TECHNIQUE - CHARACTERISTICS OF THE 2-DIMENSIONAL SYSTEM

Thermal relaxation of ultrathin polymer films prepared by the Langmuir -Blodgett (LB) technique was studied by a comparison with a three-dime nsional bulk system. Poly(vinyl alkanal acetal)s (PVAA) with different side chain lengths were synthesized and the thermal stability of thei r LB films was investigated by the energy-transfer phenomenon between fluorescence probes labeled to the polymer chains. The multilayer stru cture was irreversibly disordered and mixed by the thermal treatment. A close relationship was found between the glass-transition temperatur e (T-g) of the polymer bulk and the temperature at which the layered s tructure of LB films started to disorder. Theoretical calculations bas ed on Forster kinetics were applied to measure the diffusivity of poly mer segments, which was successfully evaluated by assuming time-depend ent Gaussian distributions of chromophores in the direction normal to the layer plane. The apparent activation energy Delta E-a of the diffu sion constant for the LB films, however, was much smaller than that fo r polymer bulk obtained by the viscoelastic measurement. It was conclu ded that the relaxation process of the ultrathin polymer films occurs by conformational changes from the two-dimensional nonequilibrium stat e given by the LB deposition. The confined form could be released by t he local motion of the polymer segments under the heating to temperatu res higher than T-g.