Local free volume and structural relaxation studied with photoisomerization of azobenzene and persistent spectral hole burning in poly(alkyl methacrylate)s at low temperatures

Abstract

The final extent of trans-to-cis photoisomerization of an azobenzene probe in various amorphous polymers has been used in previous studies for estimating local free volume and its fluctuation in polymer solids. However, there have been few studies on what kinds of molecular motion cause the fluctuation of local free volume at low temperatures. The onset of local structural relaxation (molecular motion) can be observed with temperature cycling experiments in persistent spectral hole burning (PSHB). Thus, in the present article, the fluctuation of local free volume observed in trans-to-cis photoisomerization of azobenzene is related to the local structural relaxation observed in PSHB for poly(alkyl methacrylate)s with various ester groups, i.e., methyl (PMMA), ethyl (PEMA), n-propyl (PnPMA), isopropyl (PiPMA), and isobutyl (PiBMA) groups. In the final cis fraction, rapid decrease, from 20 to 4 K in PEMA, PnPMA, and PiPMA, and from 86 to 20 K in PiBMA, is observed. These temperature regions of the rapid decrease in final cis fraction in these polymers agree well with those where the hole width in PSHB temperature cycling experiments begins to increase for the same polymers. For example, PEMA begins its ester ethyl group rotation at 17 K, which was primarily observed with PSHB, causing the drastic decrease in final cis fraction of azobenzene from 20 to 4 K. The final cis fractions at 4 K for these poly(alkyl methacrylate)s reflect the intrinsic sizes of the local free volume, except in the case of PMMA, and are compared with the reported results of positron annihilation lifetime measurements. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3098–3105, 2000