Thermal properties of poly(N-isopropylacrylamide)-g-poly(ethyl oxide) in aqueous solutions: Influence of the number and distribution of the grafts

Thermal properties of copolymers, poly(N-isopropylacrylamide) (PNIPA) graft ed with different amounts of poly(ethylene oxide) (PEO), were studied in aq ueous solutions. A functional backbone copolymer with M-w 1.8 x 10(5) was g rafted by amino-terminated PEO (M-w 6000) in water at two different tempera tures, 15 and 29 degrees 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 sm all polymer spheres were detected. EPR experiments showed that with increas ing temperature an amphiphilic spin probe, 5-doxylstearic acid, diffused mo re 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 c ollapsed copolymer. A polymer grafted in dioxane, which contained 10 PEO gr afts, was used as a reference. A comparison of all the three graft copolyme rs suggests that the conformation of PNIPA during the grafting affects the properties of the product polymers.