Optical switching in polymer gels

Soft materials have long been sought after for use in devices such as actuators, artificial muscles, separators, switches, sensors, memories, and so forth. We developed a soft, optically transparent material using polymer gels that can not only be activated by visible light (switched on) but also deactivated (switched off) by altering the local environment using three different means: pH, temperature, and light. This copolymer gel is a covalently cross-linked network of N-isopropylacrylamide, sodium acrylate, and a chromophore, which is found to undergo phase transitions exhibiting large hysteresis in the degree of swelling in response to pH, temperature, and light. In each system, between the transitions for swelling and shrinking, the gel can show either a swollen or a collapsed state, which can be selected according to the history of the variables. It has been established that a thermoresponsive gel with chromophore exhibits a local volume phase transition upon illumination with visible light. By making use of this phenomenon, we have successfully controlled the phase in which a gel exists with visible light: Without light illumination the gel stays in the swollen state. Upon illumination beyond a threshold intensity, however, a volume transition is locally induced, thereby forming a material in which both phases coexist stably for at least several hours after the light source has been removed. The phenomenological stability of the material in the coexistence state is discussed on the basis of the Landau theory. (C) 1996 American Institute of Physics.