IN-SITU STUDY OF PHOTOINDUCED ORIENTATION IN AZOPOLYMERS BY TIME-DEPENDENT POLARIZATION MODULATION INFRARED-SPECTROSCOPY

Infrared spectroscopy has been coupled with the polarization modulatio n technique in order to characterize the molecular orientation in film s of azopolymers irradiated in situ with polarized visible light. The results obtained on disperse Fed I-containing amorphous azopolymer (pD R1A) demonstrate the high efficiency of polarization modulation infrar ed linear dichroism to determine quantitatively the time dependence of the orientation function of several chemical groups during the orient ation (laser on) and relaxation (laser off) processes. The difference dichroic spectra show that polarized visible light induces a preferent ial orientation of the azobenzene groups perpendicular to the directio n of the polarization of the writing laser. This orientation of the si de 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 pro cesses could be described by a biexponential function involving ''fast '' and ''slow'' response modes, The rate constants associated with the se modes are the same for the main chain and the side chains of the po lymer. Finally, we have observed that the level of orientation and the rate constants of the orientation and relaxation processes depend str ongly on the film thickness.