Poly(lactic acid)/poly(ethylene glycol) stereocomplexed physical hydrogels showing thermally-induced gel-sol-gel multiple phase transitions

Abstract

Stereocomplexation of complementary chiral blocks has been a feasible way to prepare thermoresponsive physical gels (or thermogels). Understanding the thermoresponsive phase behavior of stereocomplexable amphiphilic copolymers in water is essential to prepare thermogels with tunable physical properties. Herein, we use an enantiomeric mixture of the poly(D-lactic acid) (PDLA)/ poly(ethylene glycol) (PEG) diblock copolymer and the poly(L-lactic acid) (PLLA)/PEG triblock copolymer as a stereocomplexable system and report a novel thermogel that exhibits unique gel-sol-gel multiple transitions upon heating. The thermally-induced phase transition of the PEG-PDLA/PLLA-PEG-PLLA thermogel was governed by the copolymer composition and the stereocomplexation of PLLA/PDLA segments. The molecular motion of PEG segments enhances during the gel-sol transition but slows down in the subsequent sol-gel transition. The gel-sol and sol-gel transitions that occurred at low and high temperatures are driven by the increased mobility of PEG segments and micelles and the promoted PLLA/PDLA stereocomplexation, respectively. PEG-PDLA/PLLA-PEG-PLLA gels can be used to capsulate and release a hydrophobic drug; the drug release rate increases as the content of hydrophilic blocks increases.