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 (Mw=5000 g/mol)(Mw=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 (C0)(C0) in semidilute aqueous solutions: the reduced viscosities increased sharply (namely, thermothickening behavior) at LCST when concentrations were higher than C0C0, or decreased sharply at LCST when concentrations lower than C0C0. A plateau was also found near C0C0 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 (>∼3 g/L)(>∼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 (<∼3 g/L)(<∼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.

The formation of physical “cross-linking points” causes the thermothickening phenomenon or gelation in higher concentration at human body temperature, while the formation of independent globule causes the thermothinning phenomenon in lower concentration at the temperature higher than LCST of PNIPAM-g-PEO copolymer.Figure optionsDownload as PowerPoint slide