Professor M. P. Fontana (University of Parma, Italy) and his colleagues have recently reported X-ray photon correlation spectroscopy (XPCS) studies on poly[[4-pentiloxy-3'-methyl-4'-(6-acryloxyexyloxy)]azobenzene], which is a kind of photosensitive azo-polymer and is softened by photoisomerization. XPCS uses coherent X-rays to measure small angle scattering, called a speckle pattern, which is caused by some inhomogeneities. It gives information on the slow dynamics of various equilibrium and non-equilibrium processes in condensed matter systems. The main advantage of using X-rays instead of other direct methods such as scanning probe microscopy is that it provides statistical information averaged over the whole sample as a function of the momentum transfer. This is essential for the analysis of dynamical heterogeneity and of nonequilibrium and aging effects in the observed dynamics. The research group measured the time correlation functions at different temperatures and momentum transfers (q) and under different illumination conditions (dark, UV or blue light). It was found that the correlation functions are well described by the so-called stretched exponential function with relaxation times that are proportional to the inverse of q. They were able to determine the scaling laws for equilibrium and nonequilibrium fluctuations on local space scales. For more information, see the paper, "Slow dynamics in an azopolymer molecular layer studied by x-ray photon correlation spectroscopy", D. Orsi et al., Phys. Rev. E82, 031804 (2010).
