Infrared Study on Molecular Orientation and Phase Transition in Langmuir-Blodgett Films of an Amphiphilic Microgel Copolymer with the Branching of Octadecyl Groups

摘要

Molecular orientation, structure, and phase transition in Langmuir-Blodgett (LB) films of an amphiphilic polymer consisting of a flexible hydrophilic epichlorohydrin-ethylenediamine slightly cross-linking microgel and a number of hydrophobic stearic chains (ES-3) have been studied by infrared (IR) transmission and reflection-absorption (RA) spectroscopy. The infrared study indicates that the LB films of ES-3 possess crystalline, tightly packed methylene chains and extended interchain hydrogen bonds between the amide groups, which connect the alkyl chains with the microgel. A comparison of the band intensities between the transmission and RA spectra suggests that the hydrocarbon chains are highly oriented with a small tilt angle and that N-H and C=O bonds of the amide groups and the interchain hydrogen bonds between the N-H and C=O bonds are nearly parallel to the surface. It is indicated from infrared spectra of LB films of ES-3 prepared under different surface pressures that the alkyl chains are highly ordered even in the film deposited under nearly zero surface pressure, while the microgel core shows some mobility. The phase transitions have been observed in narrow temperature ranges near 65, 105, and 140℃ for multilayer LB films of ES-3. The three temperatures are very close to those for the phase transitions of bulk materials of ES-3, but the transitions occur more sharply for the LB films. Of note is that the transition temperatures of the LB films are markedly different from those of the self-assembling (SA) films previously reported. This may be due to structural differences in the alkyl chains between the two kinds of films; the SA films have partial interdigitation of the alkyl tails, while the LB films do not have it. In contrast to LB films of low molecular weight organic dyes, even a one-monolayer LB film of ES-3 shows clear phase transitions at slightly higher temperatures. Probably, the microgels prevent the direct interaction between the substrate and the alkyl chains in the first layer, providing the one-monolayer LB film with clear phase transitions.