Postexposure evolution of a photoinduced grating in a polymer material with phenanthrenequinone

The specific features of the postexposure evolution of photoinduced (holographic) gratings in a model medium that consists of poly(methyl methacrylate) with distributed phenanthrenequinone and is characterized by a diffusion enhancement are investigated using the holographic relaxation technique. It is established that the evolution process occurs in four stages controlled by different mechanisms: (i) initial nonmonotonic changes governed by the diffusion of an intermediate radical photoreaction product and its transformation into a stable product due to attachment to the macromolecules, (ii) hologram enhancement through the diffusion of phenanthrenequinone molecules, (iii) rapid partial degradation associated with the spatially confined motion of polymer chain segments, and (iv) slow degradation as a result of macromolecular diffusion. In the course of polymer relaxation after a temperature jump, the processes associated with the mobility of molecules are retarded and the depth of rapid degradation decreases. The effective diffusion coefficient characterizing the destruction of reflection gratings is smaller than that for transmission gratings. This can be explained by the spatial inhomogeneity of the polymer.