Confinement of surface patterning in azo-polymer thin films.
Azobenzene polymer thin films are known to spontaneously generate surface patterns in response to incident light gradients. This peculiar process is investigated in terms of the dynamics of the various azobenzene photomotions, which occur on different length scales. In particular, the formation and thermal erasure of surface relief gratings are measured as a function of film thickness and by using combinatorial samples with thickness gradients. The thermal erasure of gratings in this system provides a direct measure of the glass-transition temperature, which is found to deviate substantially from the bulk value. Thin azo films exhibit a glass transition up to 50 K higher than the bulk. These dynamical measurements allow the authors to probe the length scale of mass transport, which is found to be approximately 150 nm. Furthermore, surface mass transport is completely arrested in thin films<40 nm. According to these results, mass transport involves the coordinated motion of many polymer chains in the depth of the sample, rather than surface diffusion of individual chains.
Yager
Kevin G
KG
Department of Chemistry, McGill University, Montreal, Quebec H3A 2K6, Canada.
Barrett
Christopher J
CJ