INFRARED STUDY ON MOLECULAR-ORIENTATION AND PHASE-TRANSITION IN LANGMUIR-BLODGETT-FILMS OF AN AMPHIPHILIC MICROGEL COPOLYMER WITH THE BRANCHING OF OCTADECYL GROUPS

Molecular orientation, structure, and phase transition in Langmuir-Blo dgett (LB) films of an amphiphilic polymer consisting of a flexible hy drophilic epichlorohydrin-ethylenediamine slightly cross-linking micro gel and a number of hydrophobic stearic chains (ES-3) have been studie d by infrared (IR) transmission and reflection-absorption (RA) spectro scopy. The infrared study indicates that the LB films of ES-3 possess crystalline, tightly packed methylene chains and extended interchain h ydrogen bonds between the amide groups, which connect the alkyl chains with the microgel. A comparison of the band intensities between the t ransmission and RA spectra suggests that the hydrocarbon chains are hi ghly oriented with a small tilt angle and that N-H and C=O bonds of th e 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 infr ared spectra of LB films of ES-3 prepared under different surface pres sures that the alkyl chains are highly ordered even in the film deposi ted under nearly zero surface pressure, while the microgel core shows some mobility. The phase transitions have been observed in narrow temp erature ranges near 65, 105, and 140 degrees C 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-assembl ing (SA) films previously reported. This may be due to structural diff erences in the alkyl chains between the two kinds of films; the SA fil ms have partial interdigitation of the alkyl tails, while the LB films do not have it. In contrast to LB films of low molecular weight organ ic dyes, even a one-monolayer LB film of ES-3 shows clear phase transi tions 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.