Fluorescence and EPR studies on the collapse of poly( N-isopropyl acrylamide)- g-poly(ethylene oxide) in water

Abstract

Studies on the thermal behaviour of copolymers, poly( N-isopropyl acrylamide), PNIPA, grafted with different amounts of poly(ethylene oxide), PEO, were conducted in aqueous solutions by fluorescence and EPR spectroscopies. The grafting reaction had been conducted under various reaction conditions and the number of PEO grafts per a PNIPA chain varied from 6 to 10. A fluorescence probe, 4-(dicyano methylene)-2-methyl-6-( p-dimethyl amino styryl)-4H-pyran (4HP) was observed to localise in the hydrophobic parts of the polymers. The changes in the polarity and microviscosity in the surroundings of 4HP upon the collapse of PNIPA depended in general on the amount of PEO in the polymer, polymer concentration, and sample-heating rate. However, fluorescence data revealed differences between the polymers, owing to the varying distribution of the PEO grafts along the PNIPA main chain. The polymer grafted in water at an elevated temperature ( T∼LCST) differed clearly from the others, those synthesised either in cold water or in an organic solvent. This particular polymer formed dense local hydrophobic domains inside the collapsed globule. Two ways of preparing the samples for EPR measurements resulted in a different localisation of a spin probe 5-doxyl stearic acid in the polymers. The probe resided either in the outer PEO shell close to the aqueous phase or in a less polar phase close to the PNIPA core of the polymer. The rate of the rotational diffusion of the radical increased discontinuously during the polymer collapse in the former case but in the latter, a typical linear activation of the thermal motion was observed. The results strongly suggest that the distribution of the PEO grafts along the PNIPA chains depends on the conformation of the functionalised PNIPA during the grafting, and this affects the behaviour of the graft copolymers.