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

Abstract

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 T_SR and known T_g relaxations can be predicted by the following formula: 1/T_SR = [T_g1 × T_g2 × (T_binary − T)]/[T_binary × T × (T_g1 − T_g2) × T_g] + (T_g1 × T − T_binary × T_g2)/[T_binary × T × (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 (ΔE_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).