Degradation study on structural and optical properties of annealed Rhodamine BÂ doped poly(vinyl) alcohol films

We report the observations on optical and structural properties of thermally treated undoped and doped poly(vinyl) alcohol (PVA) films as a function of doping and annealing temperature. The crystalline peaks of both PVA and Rhodamine B are seen in the X-ray diffraction (XRD) of the as prepared samples where the crystallinity of Rhodamine B increases with doping. At 2 wt% doping, Rhodamine B crystals segregate and are seen at the surface. However, after annealing only PVA peaks remain and Rhodamine B becomes amorphous as seen from XRD and micro-Raman measurements. It is shown how the thermal energy imparted to host PVA and dopant Rhodamine B molecules results in the movement of molecules, breaking the bonds inside the material. The intensity and frequency of Raman active modes is observed to be modified with increasing temperature due to structural changes in polymer bonds. It is also observed that thermal annealing leads to a shift in the optical absorption edge towards lower energies. This is attributed to the modified electronic states. The overall results are interpreted in terms of modifications in structural and optical properties.