Synergistic temperature and pH effects on glass (Tg) and stimuli-responsive (TSR) transitions in poly(N-acryloyl-N′-propylpiperazine-co-2-ethoxyethyl methacrylate) copolymers

N-Acryloyl-N'-propylpiperazine (AcrNPP) and 2-ethoxyethyl methacrylate (EEMA) monomers were copolymerized to form random stimuli-responsive p(AcrNPP/EEMA) copolymers in a form of colloidal dispersions which upon coalesce form uniform films. The presence of AcrNPP units facilitates temperature and pH responsiveness, thus resulting in composition-dependent and pH-temperature sensitive endothermic transitions: the glass (T-g) and stimuli-responsive (T-SR) transitions. These studies show that the relationship between the newly discovered TSR and known Tg relaxations can be predicted by the following formula: 1/T-SR [Tg1 x Tg2 x (T-binary - T)]/[T-binary x T x (T-g1 - T-g2) x T-g] + (T-g1 x T - T-binary x T-g2)/[T-binary x T x (T-g1 - T-g2)], where T-SR is the stimuli-responsive transition temperature, T-g is the glass transition temperature of the copolymer; T-binary is the temperature of stimuli-responsive homopolymer in a binary polymer-water equilibrium, T-g1 and T-g2 are the glass transition temperatures of stimuli-responsive and non-stimuli-responsive homopolymers, respectively, and T is the film formation temperature. Experimental spectroscopic and differential scanning calorimetry (DSC) evidence showed that dipole-dipole interactions are responsible for the molecular changes at the T-SR for a non-protonated state, and the shift of the T-SR under protonated conditions is attributed to the synergistic pH and temperature effects associated with H-bonding and conformational backbone and side chain rearrangements. Computer simulations also showed that the buckling of the copolymer backbone and collapse of propylpiperazine groups occur above T-SR. The total energies (Delta E-total) of the T-SR transitions for protonated and non-protonated states are 159 and 132 kcal mol(-1), respectively, and are in good agreement with the energy values determined experimentally (DSC).