A synthetic hydrogel with thermoreversible gelation, III: an NMR study of the sol–gel transition

Abstract The sol–gel transition mechanism of a thermoreversible hydrogel composed of a copolymer comprising poly(N‐isopropylacrylamide) and poly(ethylene glycol) (PNIPAAm–PEG) was studied by NMR. The 1 H– and 13 C–NMR spectra measured on a PNIPAAm–PEG solution in 99.9% D 2 O showed a remarkable line width broadening of the PNIPAAm block of more than that of the PEG block, during thermally induced hydrogel formation. This result suggested that the mobility of the PNIPAAm block is more restricted than that of the PEG block during gelation. A crosslinked polymer network formation was ascertained by a sudden reduction in the spin‐lattice relaxation time (T 1 ) of the residual HDO proton during gelation. The temperature dependency of the T 1 values for the PNIPAAm and PEG blocks revealed that the microscopic condition of the PNIPAAm block in water was drastically changed during gelation, while that of the PEG block was unchanged. The experimental results from NMR supported the following gelation mechanism; that an aggregation of PNIPAAm blocks in the separate copolymers caused by hydrophobic interaction forms crosslinking points to give an infinite three‐dimensional network structure. The hydrated PEG chains in the copolymers provide the network with a swelling property in water, and prevent the aggregation from causing a macroscopic phase separation.