New Thermal Transitions in Stimuli-Responsive Copolymer Films

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 (T-g), which follows the Fox equation for random copolymers, a new composition-sensitive endothermic stimuli-responsive transition (T-SR) 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: I/T-SR = w(1)/T-binary + w(2)/T or 1/T-SR = w(1)(1/T-binary - I/T) + 1/T, where T-SR is the temperature of the stimuli-responsive transition, T-binary is the temperature of the stimuli-responsive homopolymer in a binary polymer-water equilibrium, w(1), and w(2) (w(2) = 1 - w(1)) 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 (Delta H) components of the T-g and T-SR transitions determined from DSC measurements are 122 kcal/mol for T-g and 199 kcal/mol for T-SR, which are part of the total energy, Delta E-tot, of the system. The calculated values of the Delta E-tot obtained using computer modeling simulations (168 kcal/mol for T-g 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 Delta E-tot.