A novel pH-sensitive PEG-PPG-PEG copolymer displaying a closed-loop sol-gel-sol transition

A thermogelling PEG-PPG-PEG triblock copolymer, Pluronic F127, was modified with diamines containing different numbers of secondary amino groups in order to produce a novel pH-sensitive gelling material. The degree of end group modification was over 95% without any noticeable multi-block formation. Modified F127 displayed a closed-loop sol-gel-sol transition in 16 wt% PBS solution and the loop was significantly shifted with pH depending on the nature of end-capping diamines. Temperature- and pH-sensitive phase transition was caused by a subtle change in hydrophilicity and hydrophobicity balance derived from the pK_a values of the diamine groups at the distal PEG blocks. ¹H-NMR, GPC and dynamic mechanical analysis were employed for the characterization of modified F127 and its gelation behavior.