Thermal stability of nano-structure in polymeric Langmuir-Blodgett films studied by the energy transfer method

Abstract

The thermal stability of Langmuir-Blodgett (LB) films of poly(vinyl octanal acetal) (PVO), poly(diisopropyl fumarate) (PDiPF), poly(dicyclohexyl fumarate) (PDcHF) and poly(isobutyl methacrylate) (PiBMA) has been investigated. The energy transfer method was employed to study the structural relaxation of these films by heating, in which a pair of energy-donating and -accepting PVO layers were incorporated as the probes. In the temperature range between 30 and 120 °C, all these polymer LB films showed structural relaxation. A close relationship between the structural relaxation of polymer LB films and the glass transition temperature, T g , of the corresponding polymer bulk was observed. Although the fumarate polymers did not show clear T g in the temperature range observed, the structural relaxation of LB films was also observed and this was attributed to the local motions of the polymer chain; the transition was also detected as a small signal of DSC. After the thermal treatment, the miscible LB films showed an increase of energy-transfer efficiency expected for a random distribution of the chromophores, whereas immiscible polymer LB films induced a further increase in energy-transfer efficiency due to the phase separation of the probing layers from the examined layers after the thermal treatment. In the case of PiBMA-LB film, the rapid progress of phase separation was observed at temperatures higher than T g . In PDiPF-LB and PDcHF-LB films, the phase separation proceeded gradually because the segment motions of the polymer were not so vigorous in this temperature range. The effects of molecular weight and the initial spatial arrangement of LB layers indicated that the relaxation process is governed by the rate of diffusion of each polymer.