Poly(D,L-lactic acid-co-glycolic acid)-b-poly(ethylene glycol)-b-poly (D,L-lactic acid-co-glycolic acid) triblock copolymer and thermoreversible phase transition in water

Novel thermoreversible gelation behavior of aqueous solutions of ABA-type triblock copolymers composed of the central polyethylene oxide (PEG) block and two poly(D,L-lactic acid-co-glycolic acid) side blocks was found. Phase transition characteristics, such as critical gel concentration (CGC) and lower and upper critical gel temperature (CGT), are closely related to the molecular structure of the triblock copolymers. The CGC and the lower CGT both increases with increasing PEG/PLGA molecular weight ratio. Increasing the GA content in PLGA block induces a somewhat higher CGC. The copolymer forms micelles with a PLGA loop core and a PEG shell in water. Also grouped micelles are identified seemingly due to the bridging of two micelles sharing two PLGA blocks of a block copolymer chain. As the temperature increases the association of micelles increases, which results in g-elation. The ABA-type copolymers exhibit a relatively low CGC (<10'%) and low sol-gel transition temperatures compared to BAB-type copolymers. As the temperature increases further gel-sot transition is observed, which would result from the shrinkage of micelles with temperature increase. The hydrodynamic size of the micelles is monitored by dynamic laser scattering, and a possible gelation mechanism was suggested. (C) 2002 Wiley Periodicals, Inc.