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).