Dynamics of Mass Transfer Caused by the Photoinduced Spatially Inhomogeneous Modulation of Mobility in a Multicomponent Medium

The reasons behind the three-dimensional mass transfer of the inert components of a mixture in which photoinduced spatially inhomogeneous polymerization takes place are discussed. Such a mass transfer is responsible for the laser-induced formation of controllable diffraction gratings in polymer composites containing liquid crystals [R. L. Sutherland, V. P. Tondiglia, L. V. Natarajan, and T. J. Bunning, Chem. Mater. 5, 1533 (1993); R. L. Sutherland. V. P. Tondiglia, and L. V. Natarajan, Appl. Phys. Lett. 64, 1074 (1994); R. L. Sutherland, V. P. Tondiglia, L. V. Natarajan, et al., J. Nonlinear Opt. Phys. Mater. 5, 89 (1996); R. Caputo, A. V. Sukhov, and C. Umeton, Mol. Mater. 12, 192 (1999)]. The semiphenomenological model proposed in this work is based on the assumption that mass transfer results from conventional Fickian diffusion which, however, does not occur over the entire volume of the sample, but only in regions free from clusters of long polymer chains. It is shown that such a "restriction of the active volume" is responsible for the conventional diffusion of inert components of liquid crystals as well as reagents and low-molecular products (short chains), in spite of the initial spatial homogencity of concentrations in the sample. The qualitative predictions of the model coincide with the experimental results