Novel crosslinked thermoresponsive hydrogels with controlled poly(ethylene glycol)—poly(propylene glycol) multiblock copolymer structure

New thermoresponsive crosslinked hydrogels with controlled multiblock copolymer structure were prepared from equimolar amounts of α,ω-diamino poly(propylene glycol)s with molecular weights (MW) 230, 400, and 2,000 gmol−1 and diepoxy-terminated poly(ethylene glycol)s of approximate MW 1,000; 2,000; and 4,000 g mol−1. Their thermoresponsive character was investigated on the 10–70 °C interval, while the swelling behavior was tested at 21, 37, and 50 °C. All hydrogels displayed temperature sensitivity, but a volume
phase transition was noticed only in the case of poly(propylene glycol) (PPG)2000-containing hydrogels. The volume phase transition temperature (TVPT) depended on the MW of the hydrophilic poly(ethylene glycol) (PEG) chains attached to the PPG2000 block, as well as on the added salts. Longer PEG blocks determined a shift of TVPT towards higher values, while the influence of the salt added was in agreement with the Hofmeister series, except for NaH2PO4 which determined the destruction of the hydrogel network. The equilibrium swelling degree depended on the MW of both PEG and PPG blocks, as well as on temperature. The analysis of the swelling process indicated a modification of the gel characteristics with temperature and second-order kinetics for the water penetration into the hydrogel.