New Thermal Transitions in Stimuli-Responsive Copolymer Films
Originally published in Macromolecules, 2009, 42, 2161-2167
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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 stimuliresponsive 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.