In situ Study of Photoinduced Orientation in Azopolymers by Time-Dependent Polarization Modulation Infrared Spectroscopy

Abstract

Infrared spectroscopy has been coupled with the polarization modulation technique in order to characterize the molecular orientation in films of azopolymers irradiated in situ with polarized visible light. The results obtained on disperse red 1-containing amorphous azopolymer (pDR1A) demonstrate the high efficiency of polarization modulation infrared linear dichroism to determine quantitatively the time dependence of the orientation function of several chemical groups during the orientation (laser on) and relaxation (laser off) processes. The difference dichroic spectra show that polarized visible light induces a preferential orientation of the azobenzene groups perpendicular to the direction of the polarization of the writing laser. This orientation of the side chains also results in a slight cooperative orientation of the C=O ester groups of the main chain of the polymer. The time dependence of the orientation function shows that the orientation and relaxation processes could be described by a biexponential function involving “fast” and “slow” response modes. The rate constants associated with these modes are the same for the main chain and the side chains of the polymer. Finally, we have observed that the level of orientation and the rate constants of the orientation and relaxation processes depend strongly on the film thickness.