Microspectrometric study of azobenzene chromophore orientations in a holographic diffraction grating inscribed on a p(HEMA-co-MMA) functionalized copolymer film

An amorphous copolymer of methyl methacrylate (MMA, 60%) and hydroxyethyl methacrylate (HEMA, 40%), i.e. p(HEMA-co-MMA), was prepared by atom transfer radical polymerization (ATRP) and functionalized by complete esterification with the 2-[4-(4-cyanophenylazo)phenoxy]acetic acid. An isotropic thin film (0.5 mum) of this copolymer with a 40% (molar fraction) azobenzene content was irradiated by interfering two orthogonal (+/- 45 degrees) linearly polarized laser beams (lambda = 488 nm; irradiance = 1 W cm(-2)) and a permanent holographic diffraction grating was thus inscribed. AFM measurements revealed a nearly sinusoidal relief profile with a 2.5 tm period spacing and an amplitude of about 150 nm in the surface relief modulation (SRM). Using a confocal microspectrometric instrument we recorded various pre-resonance enhanced polarized Raman spectra from a large grating area and obtained spatially resolved images of the Raman intensity variations, in order to gain new insight into the photoinduced orientation effects and the grating formation mechanisms. From analyses of the experimental data we extracted values of the second <P-2> and fourth <P-4> coefficients in the chromophore orientation functions at several positions of the SRM, in particular at the top and bottom regions: in general, we found that the weakly absorbing chromophores do not undergo strong orientation effects and, as expected, they keep an isotropic distribution at half-slopes where the incident electric field was circularly polarized. Then, the corresponding normalized distribution functions were calculated using the information entropy theory. With respect to the sinusoidal relief surface, complex and broader distributions are found in the bottom and in the top regions, respectively, so that it appears that the mass transport phenomena have perturbed to some extent the primary photoinduced orientations. These results are compared with those we recently obtained using other highly birefringent copolymer samples. Copyright (C) 2001 John Wiley & Sons, Ltd.