Orientation-independent spectra for biaxial systems in polarized Raman microspectrometry.

From an extension of the scattering intensity expressions in polarized Raman macrospectrometry to the case of micro-Raman backscattering experiments, we have established theoretical expressions to calculate orientation-independent intensity sums. This approach makes use of the K2 Raman invariant and of correction coefficients due to integration of the Raman scattering over the cone of collection of the objective lens, and it may be applied to uniaxial as well as to biaxial symmetry systems. The intensity sums thus obtained are expected to be orientation insensitive and allow one to compare conformational changes in various biaxially oriented polymer samples, either amorphous and/or semicrystalline thin films. As application examples we have compared the polarized Raman results obtained for various biaxially oriented amorphous and semicrystalline polystyrene (PS), poly(ethylene terenaphthalate) (PEN), and poly(ethylene terephthalate) (PET) thin film samples with their respective "isotropic spectrum" and have considered the different main conformational changes in related polymer systems. The method is thus tested on these macromolecular systems and appears quite successful in providing evidence for the molecular conformational changes without interferences from spectral contributions due to orientation.