New Thermal Transitions in Stimuli-Responsive Copolymer Films

Abstract

These studies report for the first time new thermal relaxations in stimuli-responsive solid-phase copolymers detected by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). When 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA) and n-butyl acrylate (nBA) monomers were copolymerized into colloidal dispersions and allowed to coalesce to form solid continuous films, in addition to the glass-transition temperature (Tg), which follows the Fox equation for random copolymers, a new composition-sensitive endothermic stimuli-responsive transition (TSR) was observed. The TSR transition changes with the composition of the stimuli-responsive component of the copolymer, the temperature, and the rate of temperature change. On the basis of the experimental data, the following relationship was established: 1/TSR = w1/Tbinary + w2/T or 1/TSR = w1(1/Tbinary − 1/T) + 1/T, where TSR is the temperature of the stimuli-responsive transition, Tbinary is the temperature of the stimuli-responsive homopolymer in a binary polymer−water equilibrium, w1 and w2 (w2 = 1 − w1) are weight fractions of each component of the copolymer, and T is the film-formation temperature. This relationship allows us to predict TSR transitions in stimuli-responsive solid copolymers. The enthalpic (ΔH) components of the Tg and TSR transitions determined from DSC measurements are 122 kcal/mol for Tg and 199 kcal/mol for TSR, which are part of the total energy, ΔEtot, of the system. The calculated values of the ΔEtot obtained using computer modeling simulations (168 kcal/mol for Tg and 223 kcal/mol for TSR, respectively) are in good agreement with the experimental data, and the energy difference is attributed to the inclusion of the entropic components in ΔEtot calculations.