A SPECTROSCOPIC STUDY OF POLYMERS CONTAINING LONG FLEXIBLE SIDE-CHAINS AT AN AIR-LIQUID INTERFACE

The structures, in particular the amount of conformational order in te rms of the trans/gauche ratio of the methylene units, associated with monolayer films at the air-water interface, of a phospholipid, 1,2-dip almitoyl-sn-glycero-3-phosphochol a vinyl comblike polymer, poly( octa decyl acrylate) and a hairy-rod co-polyglutamate with long alkyl side chains, all have been studied by external reflectance Fourier transfor m infrared spectroscopy. The structures of these monolayers were monit ored as a function of molecular area/surface pressure and subphase tem perature using a microcomputer-controlled Langmuir trough interfaced t o an infrared spectrometer. The unique features of this combination of techniques permit the direct evaluation of the molecular dynamics ass ociated with the formation and collapse of spread monolayer films at t he air-liquid interface, as well as the description of the adsorption kinetics of amphiphile molecules at an air-solution interface. The con formation sensitive methylene stretching vibrations of the hydrocarbon side chains proved to be extremely sensitive indicators of conformati onal change during slow dynamic compression of the Langmuir films chan ging from expanded to more condensed monolayer states up to their coll apse region. The three systems exhibit significantly different spectro scopic features that can be directly associated with differences in th e intermolecular packing. The effect of temperature (from + 8 to + 27 degrees C) on the molecular order for compressed monolayers of poly(oc tadecyl acrylate) was also studied. The frequencies of the methylene s tretching bands for the liquid crystalline co-polyglutamate showed tha t at high compression, the long flexible hydrocarbon side chains were mostly in an all-trans configuration. At low compression, the side cha ins became disordered and contained a large fraction of gauche configu rations. (C) 1997 John Wiley & Sons, Inc.