VINYL-POLYMERS CONTAINING L-VALINE AND L-LEUCINE RESIDUES - THERMODYNAMIC BEHAVIOR OF HOMOPOLYMERS AND COPOLYMERS WITH N-ISOPROPYLACRYLAMIDE

Vinyl polymers carrying carboxyl groups and the groups (amido and isop ropyl) present in the well-known thermosensitive poly(N-isopropylacryl amide) (PNIPAAm) were studied for protonation reaction thermodynamics at different temperatures (25, 30, and 35 degrees C). The study was pe rformed in aqueous solution (0.1 M NaCl) to elucidate the mutual influ ence of temperatures and pH on the mechanism responsible for phase sep aration of polymers having a lower critical solution temperature (LCST ). The thermodynamic data (viscometry, basicity constants, enthalpy ch anges) for the protonation of carboxylate groups in homopolymers and c opolymers with N-isopropylacrylamide (NIPAAm) showed that subtle confo rmational changes occurred at a critical degree of protonation (alpha) . Beyond this critical a value, a larger endothermic effect was superi mposed on that of the protonation of the COO- group. The phenomenon wa s ascribed to hydrophobic forces between isopropyl groups outweighing the repulsive electrostatic interactions of the polymer in the ionized , unfolded state. The enthalpy changes (-Delta H degrees) became large r as the NIPAAm content increased in the copolymers, and at higher tem peratures the magnitude of this change dropped sharply. The critical a lpha shifted to a lower value because higher temperatures enhanced hyd rophobic interactions. The reduced amount of structured water molecule s on the polymer, responsible for the LCST phenomenon, was revealed by the lower entropy change (Delta S degrees), that confirmed that the p rocess is entropy-driven and based on a critical balance between hydro phobic and electrostatic forces.