Microenvironments at Specific Chain Sites in Atactic Polystyrene Gels Studied by Photoisomerization

Microenvironments in atactic polystyrene solutions of a wide range of viscosities in either toluene or tetrohydrofuran (THF) are investigated as a function of temperature from -60 to 25 °C by the studies of the photoisomerization of azobenzene, attached to specific sites on a polystyrene (PS) chain. The sites investigated are the chain center (CPS), the chain end (EPS), and the side chain (SPS). A free probe, 4-(phenylazo)aniline, is studied for comparison. The photoisomerization behavior is explained by a biphasic process, involving fast and slow processes. The fraction of the fast process is used as a useful indicator of the microenvironments. Also, the fast fraction is correlated with the macroscopic viscosity of the solution to incorporate the temperature and concentration effects. The site dependencies both in dilute solution and near the gel temperature were observed and interpreted to shed light on the gelation mechanism of atactic polystyrene.