Confinement of Surface Patterning in Azo-Polymer Thin Films
Yager, K.G.; Barrett, C.J. "Confinement of Surface Patterning in Azo-Polymer Thin Films" Journal of Chemical Physics 2007
This paper analyzes the azo-polymer surface patterning effects in extremely thin films. The spatial confinement of the thin film dramatically changes surface patterning efficiency and dynamics. The lengthscale of the effect allows us to establish that azo motion is a cooperative effect.
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 photo-motions, 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 us to probe the length-scale of mass transport, which is found to be ~150 nm. Furthermore, surface mass transport is completely arrested in thin films less than 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.