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 (Tg) and stimuli-responsive (TSR) transitions. These studies show that the relationship between the newly discovered TSR and known Tg relaxations can be predicted by the following formula: 1/TSR = [Tg1 × Tg2 × (Tbinary − T)]/[Tbinary × T × (Tg1 − Tg2) × Tg] + (Tg1 × T − Tbinary × Tg2)/[Tbinary × T × (Tg1 − Tg2)], where TSR is the stimuli-responsive transition temperature, Tg is the glass transition temperature of the copolymer; Tbinary is the temperature of stimuli-responsive homopolymer in a binary polymer–water equilibrium, Tg1 and Tg2 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 TSR for a non-protonated state, and the shift of the TSR 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 TSR. The total energies (ΔEtotal) of the TSR 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).