Vinyl Polymers Containing L-Valine and L-Leucine Residues: Thermodynamic Behavior of Homopolymers and Copolymers with N-Isopropylacrylamide

Abstract

Vinyl polymers carrying carboxyl groups and the groups (amido and isopropyl) present in the well-known thermosensitive poly(N-isopropylacrylamide) (PNIPAAm) were studied for protonation reaction thermodynamics at different temperatures (25, 30, and 35 °C). The study was performed in aqueous solution (0.1 M NaCl) to elucidate the mutual influence of temperatures and pH on the mechanism responsible for phase separation of polymers having a lower critical solution temperature (LCST). The thermodynamic data (viscometry, basicity constants, enthalpy changes) for the protonation of carboxylate groups in homopolymers and copolymers with N-isopropylacrylamide (NIPAAm) showed that subtle conformational changes occurred at a critical degree of protonation (α). Beyond this critical α value, a larger endothermic effect was superimposed on that of the protonation of the COO - group. The phenomenon was ascribed to hydrophobic forces between isopropyl groups outweighing the repulsive electrostatic interactions of the polymer in the ionized, unfolded state. The enthalpy changes (-ΔH°) became larger as the NIPAAm content increased in the copolymers, and at higher temperatures the magnitude of this change dropped sharply. The critical α shifted to a lower value because higher temperatures enhanced hydrophobic interactions. The reduced amount of structured water molecules on the polymer, responsible for the LCST phenomenon, was revealed by the lower entropy change (AS°), that confirmed that the process is entropy-driven and based on a critical balance between hydrophobic and electrostatic forces.