Thermo-induced formation of physical "cross-linking points" of PNIPAM-g-PEO in semidilute aqueous solutions

Linear poly(N-isopropylacrylamide) chains grafted with short poly(ethylene oxide) chains (PNIPAM-g-PEO) were prepared by free radical copolymerization of NIPAM and PEO macromonomers (M-W = 5000 g/mol) end-capped with methacrylate in water. Temperature effects on the solution viscosity of thermally sensitive copolymer were studied in different aqueous concentrations. A specific transition was observed during the measurement of the reduced viscosities of PNIPAM-g-PEO copolymer at a certain concentration (CO) in semidilute aqueous solutions: the reduced viscosities increased sharply (namely, thermothickening behavior) at LCST when concentrations were higher than C-0, or decreased sharply at LCST when concentrations lower than C-0. A plateau was also found near C-0 when temperature was closing to LCST from low temperature, showing there is no change in reduced viscosity under this circumstance. The inverse increase of the viscosities at higher temperatures in higher concentration (> similar to 3 g/L) is attributed to the forming of physical "cross-linking points" composed of collapsed PNIPAM core and expanded PEO shell, The sharp decrease of the viscosities at higher temperatures in lower concentration (< similar to 3 g/L) is attributed to the forming of independent globules. The plateau could be attributed to the equilibrium competition between forming of physical "cross-linking points" and independent globules depending on the copolymer solution concentrations. (c) 2006 Elsevier Inc. All rights reserved.