A Semicrystalline Poly(azobenzene) Exhibiting Room Temperature Light-Induced Melting, Crystallization, and Alignment

Photomechanical materials powered by light-induced changes in crystalline lattices offer promise for improved performance due to the high degree of coordination between the shape changes of individual molecules. While photoswitchable semicrystalline polymers present an attractive combination of molecular ordering and material processability, systems developed to date typically show high glass transition (Tg) and melting (Tm) temperatures, limiting their ability to undergo rapid and complete photoswitching under ambient conditions. Here, we prepare a semicrystalline poly(azobenzene) containing an ethylene glycol chain extender, denoted as P(EG-azo). Because of its backbone flexibility, P(EG-azo) shows values of Tg (-20 degrees C) and Tm (74 degrees C) that are substantially lower than the previously reported analogous polymer prepared with an alkyl chain extender. This decrease in Tg and Tm translates to rapid and thorough photomelting and photocrystallization at room temperature with high reversibility. Reversible photoactuation of P(EG-azo) fibers is demonstrated, with bending deformations corresponding to an estimated specific work of 0.6 kJ m-3, 30 times larger than for previous semicrystalline poly(azobenzene) photoactuators at room temperature. In addition, photoalignment of P(EG-azo) through selective melting and templated crystallization provides a convenient and energy efficient route to rewritable orientation with an order parameter of up to S = 0.35.