Molecular design of novel pH/temperature-sensitive hydrogels

A series of poly(β-amino ester)-poly(ɛ-caprolactone)-poly(ethylene glycol)-poly(ɛ-caprolactone)-poly(β-amino ester) pentablock copolymers (PAE-PCL-PEG-PCL-PAE) were designed and prepared to examine factors affecting sol-gel phase transition behavior. First, the composition of a series of PCL-PEG-PCL copolymers was controlled by changing the feed ratios of PCL/PEG and the molecular weight of PEG. Second, the composition of pentablock copolymers was varied using different PCL-PEG-PCL copolymers and several feed ratios of PAE monomers. The physicochemical properties of triblock and pentablock copolymers were characterized by 1H NMR and gel permeation spectroscopy. The PAE-PCL-PEG-PCL-PAE copolymers in aqueous solution (20-30 wt%) underwent sol-gel transitions with changes in both pH change and temperature. With increasing molecular weight of PAE, the sol-gel transition zone became narrower because the hydrophobic character of the copolymers decreased. Also, with increases in PCL/PEG ratio and PEG molecular weight, changes in the hydrophobic/hydrophilic balance within copolymers resulted in alterations in sol-gel phase transitions.