Origins of Mechanical Strength and Elasticity in Thermally Reversible, Acrylic Triblock Copolymer Gels

Thermoreversible gels were formed by dissolving a poly(methyl methacrylate)−poly(tert-butyl acrylate)−poly(methyl methacrylate) triblock copolymer in a variety of alcohols, including ethanol, 1-butanol, 2-ethylhexanol, and 1-octanol. The gels exhibit an ideal and reversible solid/liquid transition in each of these solvents, behaving as strong elastic solids at room temperature and as freely flowing liquids above the gel transition. The time-dependent elastic properties of the gels are governed by two transition temperatures. The first transition is the critical micelle temperature (cmt) near 60 °C, below which the PMMA blocks aggregate to form a physically cross-linked network. As the gels are cooled to room temperature, differential scanning calorimetry (DSC) reveals a second transition where the PMMA domains undergo a glass transition. The glass transition temperature of the PMMA domains increases when the gels are aged at room temperature. Time−temperature superposition can be applied below the cmt to give master curves describing the relaxation behavior of the gels in the vicinity of the glass transition of the PMMA domains. These relaxation times increase by 1 decade for every 8 K decrease in temperature, a result that is consistent with previous measurements of polymer relaxations in the vicinity of the glass transition temperature.