Molecular Orientation and Phase-Transition Behavior of Langmuir−Blodgett and Casting Films of Reversed Duckweed Polymer ES-1 Studied by Infrared Spectroscopy

Abstract

A kind of amphiphilic polymer, ES-1, is transferred onto solid substrates as so-called “reversed duckweed” polymeric Langmuir−Blodgett (LB) films. Their orientations, structures, and phase-transition behaviors were studied in detail by infrared (IR) transmission and reflection−absorption (RA) spectroscopy. The IR studies have revealed that the alkyl chains are nearly perpendicular to the film surface and assume highly ordered conformation. The first monolayer has a different orientation and arrangement from other layers in a multilayer ES-1 LB film deposited on a Au-evaporated glass slide because of the interaction between the gold surface and hydrophilic headgroup of ES-1. IR−RA spectra of an ES-1 LB film on a Au-evaporated glass slide at evaporated temperatures have shown that the order−disorder phase transition of the LB film takes place at about 60 °C after the clear pretransitional alterations. The packing density of alkyl chains decreases gradually with temperature after the order−disorder transitions in the LB films. The alkyl chains in ES-1 casting films on a CaF2 plate and a Au-evaporated glass slide are tilted considerably. The studies on the phase-transition behavior have revealed that the two casting films give rise to the order−disorder transition at about 75 °C after undergoing the pretransitional changes. For the casting film of ES-1 on the Au-evaporated glass slide, there is a phenomenon of reorientation that the alkyl chains change from more tilted orientation at room temperature to more perpendicular orientation at higher temperature before the order−disorder transition takes place and turn tilted again above the transition temperature.