Phase transition in polymer gels induced by visible light

Abstract

PHASE transitions and critical phenomena in polymer gels have attracted much attention because of their scientific interest and technological significance^1–3. Phase transitions accompanied by a reversible, discontinuous volume change as large as several hundred times, in response to infinitesimal changes in environmental conditions, have been observed universally in gels made of synthetic and natural polymers^1–9. Phase transitions have been induced in gels by varying temperature, solvent composition, pH, ionic composition and a small electric field¹⁰. Recently, gels sensitive to ultraviolet light were also reported¹¹. The ultraviolet light initiates an ionization reaction in the gel, creating internal osmotic pressure which induces swelling. In the absence of this light, the equilibrium tends towards the neutral polymer system and the gel collapses. This transition process is slow, as it depends on the photochemical ionization and subsequent recombination of ions, and it is technologically desirable that the transition be induced by faster mechanisms. Also, visible light is less harmful and more abundant than ultraviolet in sunlight. Hence we now report the phase transition of gels induced by visible light, where the transition mechanism is due only to the direct heating of the network polymers by light, which is an extremely fast process. Such systems might be used as photoresponsive artificial muscles, switches and memory devices.