New Thermal Transitions in Stimuli-Responsive Copolymer Films
Chemistry and Biochemistry
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).
Urban, M. W.
(2009). New Thermal Transitions in Stimuli-Responsive Copolymer Films. Macromolecules, 42(6), 2161-2167.
Available at: http://aquila.usm.edu/fac_pubs/12713