Fast Photochromic Dye Response in Rigid Block Polymer Thermosets

The behavior of photochromic spiropyran (SP) dyes is greatly affected by the local environment in which they reside. Fluid microenvironments such as liquidlike polymers grant favorably fast dye decoloration whereas rigid microenvironments such as glassy polymers inhibit fast dye response. We study the use of disorganized but microphase-separated block polymer thermosets as photochromic dye hosts. These thermosets incorporate low glass transition temperature (Tg) aliphatic polyester domains that facilitate rapid dye decoloration in a rigid poly(methyl methacrylate) matrix by using a polymerization-induced microphase separation (PIMS) strategy. The spontaneous ring closing of SP was monitored in PIMS materials by UV–vis spectroscopy after irradiation with ultraviolet light. The rate of decoloration was found to be sensitive to a variety of molecular parameters including the molar mass and mass fraction of the polyester block, and the cross-link density of the poly(methyl methacrylate) block allowing for tunability of the dye performance without changing the chemical identity of the components. Dye decoloration in the PIMS system could be controlled from two extremes: low Tg homopolymer (t1/2 = 170 s) behavior to bulk rigid polymer (t1/2 = 1400 s) behavior. Through control over a variety of molecular parameters, we have been able to achieve fast decoloration rates in rigid and optically clear thermosets.