Study of cooperative side group motions in amorphous polymers by time dependent infrared spectroscopy

Amorphous copolymers of 4'-[(2-(methacryloyloxy)ethyl)ethylamino]-4-nitroazobenzene (DR1M) and 4-nitrophenyl 4-[2-(methacryloyloxy)ethyl] benzoate (BEM) have been studied by time dependent polarization modulation infrared spectroscopy. Bands due to several chemical groups of the copolymers have been analyzed in order to follow simultaneously the time dependence of the photoactive (DR1M) and inert (BEM) groups during the orientation (linearly polarized laser on) and relaxation (laser off) periods. The results show clearly the cooperative orientation of the BEM groups with the azobenzene side chains. The level of orientation and the rate constants of the two types of structural units have been determined for different copolymer compositions by fitting the orientation and relaxation curves with biexponential functions. The fast ''writing'' rate constant of DR1M increases Linearly with the DR1M content while the slow ''writing'' rate constant is almost constant. The orientation kinetic of BEM groups is rather described by st monoexponential function and is mainly governed by that-of photoactive side chains at high DR1M content. During the relaxation process, the fast and slow rate constants of DR1M groups are fairly constant over the whole range of copolymer composition while the degree of orientation of BEM groups is almost completely maintained.