Thermal Properties of Poly(N-isopropylacrylamide)-g-poly(ethylene oxide) in Aqueous Solutions:  Influence of the Number and Distribution of the Grafts

Thermal properties of copolymers, poly(N-isopropylacrylamide) (PNIPA) grafted with different amounts of poly(ethylene oxide) (PEO), were studied in aqueous solutions. A functional backbone copolymer with M w 1.8 × 105 was grafted by amino-terminated PEO (M w 6000) in water at two different temperatures, 15 and 29 °C. The average number of PEO side chains increased from 6 to 7 with increasing reaction temperature. Above the lower critical solution temperature (LCST), the polymers form stable spherical aggregates observable by light scattering. In a viscous flow, however, the formation of aggregates during the collapse of the PNIPA chains was prevented, and small polymer spheres were detected. EPR experiments showed that with increasing temperature an amphiphilic spin probe, 5-doxylstearic acid, diffused more easily out from the copolymer with more PEO grafts, this indicating that the hydrophilic side chains affect the size of the hydrophobic core in a collapsed copolymer. A polymer grafted in dioxane, which contained 10 PEO grafts, was used as a reference. A comparison of all the three graft copolymers suggests that the conformation of PNIPA during the grafting affects the properties of the product polymers.