Investigation of Azobenzene Photoisomerization Effect on Morphologies and Properties of Nanostructured Thermosets Involving Epoxy and a Diblock Copolymer.

Abstract

This work highlights the effect of azobenzene photoisomerization on the morphologies and properties of the nanostructured thermosets involving epoxy and a diblock copolymer. First, a diblock copolymer composed of poly(ethylene oxide) (PEO) and poly(6-(4-(4-cyanophenylazo)phenoxy)hexyl methacrylate) (PCPHM) was synthesized, and this diblock copolymer was composed of an epoxy-philic block (i.e., PEO) and an azobenzene moiety-beating block (viz., PCPHM). This diblock copolymer was introduced into epoxy to obtain the nanostructured thermosets via reaction-induced microphase separation approach. To control the configuration of azobenzene moieties of the PCPHM block, the curing reactions were performed in the absence and/or presence of ultraviolet (UV) irradiation, respectively. It was found that, without UV irradiation, the PCPHM microdomains were generated with the trans isomers of azobenzene. Under UV irradiation, however, the PCPHM microdomains were formed with the cis configuration of azobenzene moieties. The ultraviolet-visible light (UV-vis) spectroscopy showed that the trans and cis configurations of azobenzene moieties were significantly fixed with the occurrence of curing reactions. The photoluminescent measurements showed that the nanostructured thermosets with trans-azobenzene moieties can emit fluorescence, which was in sharp contrast to those with cis-azobenzene moieties. The results of small-angle X-ray and atomic force microscopy showed that the nanostructured thermosets with trans and cis isomers of azobenzene moieties had quite different morphologies. It was found that the sizes of the PCPHM microdomains with cis configuration of azobenzene moieties were significantly larger than those with trans configuration. The difference in configuration of azobenzene moieties also resulted in the difference in glass-transition temperatures and dielectric properties of the materials. The results suggest a new approach to modulate the morphologies and physical properties of the nanostructured thermosets by means of photoisomerization of azobenzene moieties.