Biaxial Orientation Induced in a Photoaddressable Azopolymer Thin Film As Evidenced by Polarized UV−Visible, Infrared, and Raman Spectra

The main goal of this spectroscopic study was to obtain relevant information about the symmetry properties for a thin film of a new class of photoaddressable azopolymers, which are known to display a huge birefringence (Δn exceeding −0.45 at 632.8 nm) and a high stability and are attractive materials for rewritable recording media and optical memories. First, various real-time birefringence experiments are performed using a linearly polarized irradiation at 532.0 nm and probing the thin film sample at 675.0 nm to confirm the largest Δn values ever reported for such amorphous polymers. Then, polarized transmission measurements in the UV−visible and mid-infrared spectral ranges are compared for an isotropic and anisotropic sample. They provide strong evidences for cooperative orientational effects and for the establishment of a biaxial symmetry; the two average orientation factors 〈F 2,0〉 and 〈F 2,2〉 are thus extracted from infrared data for several vibrational modes. The remaining and related fourth rank factors, namely 〈F 4,0〉, 〈F 4,2〉 and 〈F 4,4〉, are then determined from the confocal micro-Raman spectra recorded under various polarization configurations, more precisely from the two Raman intensity ratios, R 1 = I XY /I XX and R 2 = I YX /I YY . Reasonable values of the different orientation order parameters are thus obtained allowing to determine the overall orientation distribution function of the chromophores, in particular for the ω(CC) ring (8a) vibrational mode at 1599 cm-1. Definitive conclusions are thus drawn about the effective biaxial symmetry in this copolymer sample.