Photochemical-induced phase transitions in photoactive semicrystalline polymers

TL;DR

This paper models the phase transitions in photoactive semicrystalline polymers induced by light, revealing temperature-dependent reaction kinetics, phase behavior, and strain responses, aiding future material development.

Contribution

A novel phase field model capturing the complex photo-induced phase transitions and kinetics in semicrystalline azobenzene polymers.

Findings

Photoreaction rate depends on temperature and light intensity.

Reentrant crystallization observed below melting temperature.

Phase diagram illustrates temperature and light effects on actuation strain.

Abstract

The emergent photoactive materials through photochemistry make it possible to directly convert photon energy to mechanical work. There is much recent work in developing appropriate materials and a promising new system is semi-crystalline polymers of the photoactive molecule azobenzene. We develop a phase field model with two order parameters for the crystal-melt transition and the trans-cis photo-isomerization to understand such materials, and the model describes the rich phenomenology. We find that the photo-reaction rate depends sensitively on temperature: at temperatures below the crystal-melt transition temperature, photoreaction is collective, requires a critical light intensity and shows an abrupt first order phase transition manifesting nucleation and growth; at temperatures above the transition temperature, photoreaction is independent and follows first order kinetics. Further, the phase transition depends significantly on the exact forms of spontaneous strain during the crystal-melt and trans-cis transitions. A non-monotonic change of photo-persistent cis ratio with increasing temperature is observed accompanied by a reentrant crystallization of trans below the melting temperature. A pseudo phase diagram is subsequently presented with varying temperature and light intensity along with the resulting actuation strain. These insights can assist the further development of these materials.