Nanoindentation studies to separate thermal and optical effects in photo-softening of azo polymers

Abstract

Mechanical characterization of an azobenzene dye-containing polymer (p4VP(DY7)_0.50) by nanoindentation shows a significant photo-softening effect under visible irradiation at 532 nm. Both strong rate-dependent plastic softening, as well as a rate-independent elastic modulus decrease are observed. Indentation at elevated, sub-glass transition temperatures results in only a rate-independent decrease in hardness however. These findings indicate, from a mechanics standpoint, for the first time a distinct mechanism for the photomechanical softening in azo-materials that is different from a simple thermal effect. The main hallmark of the photosoftening effect was significant viscoplastic flow with strong rate dependence. The presence of the viscoplastic softening in azo-polymers was accounted for in analysis of nanoindentation data to obtain an accurate elastic modulus unaffected by the presence of creep in the unloading curves. These results provide considerable insight into the long-observed and long-debated mechanism of all-optical surface patterning below T_g.