Thermoassociating poly(N-isopropylacrylamide) A-B-A stereoblock copolymers

Properties of aqueous solutions of A-B-A stereoblock copolymers with an isotactic poly(N-isopropylacrylamide) (PNIPAM) as the other block (either A or B) and atactic PNIPAM as the other block were studied. Introduction of isotacticity lowered the solubility of the polymers in water at room temperature, but the copolymers were found to dissolve,easily at 5 degrees C. Calorimetric scans of the polymer solutions did not reveal other transitions than the typical endotherm close to the cloud point of PNIPAM (32 degrees C). The transition temperature did not change with polymers with different lengths of the isotactic blocks, but the enthalpy change was significantly lowered compared with atactic PNIPAM. Polymers containing isotactic A blocks were found to exhibit thermothickening property, with enhanced viscosity at room temperature. The polymers formed hydrogels at room temperature at polymer concentrations above 45 g/L, while solutions of polymer containing an isotactic B block did not exhibit enhanced viscosity. Thermothickening was found to occur at lower temperature and lead to more elastic hydrogels with polymers with longer isotactic A block.