How can photoisomerization of azobenzene induce a large cloud point temperature shift of PNIPAM?

Abstract

We present a comprehensive study of the photo-induced phase transition of azobenzene-containing poly(N-isopropylacrylamide) (PNIPAM) in block copolymers (BCPs) upon the isomerization of azobenzene in the mixed solvent of water and dioxane. The BCP is composed of a poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) block and a PNIPAM block decorated with ∼2 mol% of azobenzene groups (P(NIPAM-co-MAzo)). In the mixed solvent of water and dioxane, the cloud point temperature (CPT) for the phase transition of the PNIPAM block displays a large shift >10 °C induced by the photoisomerization of azobenzene under UV or visible light. The CPT shift of PNIPAM is highly dependent on the solvent composition and the large shift only occurs at high dioxane concentration (>30%). The mechanism of the photo-induced large CPT shift by the isomerization of azobenzene was investigated by means of ¹H NMR spin relaxation studies. With the addition of dioxane, the exchange of hydrogen bonding from PNIPAM–water to PNIPAM–dioxane was detected in the neighborhood of the CPT, thereby demonstrating the better solvation of the BCP chains and the azobenzene moieties by dioxane. We further demonstrate that the spin relaxation of azobenzene in the trans–cis isomerization only displayed a notable change at higher dioxane concentrations where the large shift of the CPT was observed. An improved understanding of the mechanism of photoinduced phase transition offers new insights into the rational design, synthesis and application of photoresponsive polymer materials.