Diffusion of dye molecules in the dye/polymer systems by the holographic-grating-relaxation technique

Abstract

Laser induced holographic grating relaxation studies of thymorquinone (TQ) dissolved in amorphous poly(-methyl styrene) (PaS) and comphorquinme (CK) in poly(methyl methacrylate) (PMMA) have been carried out. Different types of lineshape associated with the relaxation of the diffraction intensity are observed and are interpreted in terms of various diffusion processes. Diffusion coefficients are obtained as a function of TO concentration from 0.3 to 20 weight percent in PcS and from 0.5 to 15 weight percent for CQ is PMMA. Above 0.5% the diffusion coefficient increases rapidly with increasing dye concentration. The temperature dependence of the self-diffusion coefficient follows the Williams-Landel-Ferry equation, whereas the mutual diffusion coefficient at higher dye concentration is Arrhenius. In addition, the self-diffusion coefficient of the dye molecule in polymer decreases very rapidly as the sample temperature is traversed across Tg. However, there is no detectable effect due to the onset of the glass transition on the mutual diffusion coefficient at finite dye concentration. The difference is interpreted as due to the plasticizing effect of the dye molecules. Free volume theory is used to interpret the concentration dependence of the diffusion coefficient.