Thermoreversible gelation of biodegradable poly(epsilon-caprolactone) and poly(ethylene glycol) multiblock copolymers in aqueous solutions

The multiblock copolymers composed of poly(ethylene glycol)s (PEGs) and bio degradable poly(epsilon -caprolactone)s (PCLs) were synthesized through one -step condensation copolymerization with hexamethylene diisocyanate (HDI) a s a coupling agent. The typical phase diagram of these multiblock copolymer s in aqueous solution displayed a critical gel concentration (CGC) and an u pper phase-transition temperature, which were mainly determined by the PEG/ PCL block ratio, the PEG or PCL block lengths and the molecular weight. Wit h decreasing PEG/PCL block ratio, the CGC decreased with an elevated sol-ge l transition temperature on account of the enhanced hydrophobicity. The HDI /Diols ratio was used to control the molecular weight. At high molecular we ights. the CGC decreased, related to the enhanced aggregation of PCL blocks and physical crosslinkage between PCL block domains due to the increased n umber of PCL blocks in each molecule. For the sample containing the long PC L2000 block (M-n, 2000), the CGC dropped dramatically due to the high hydro phobicity and the poor compatibility between PCL and PEG. The dynamic phase transition process was observed by combining optical microscopy (OM) and d ifferential scanning calorimetry (DSC) in a certain heating/cooling rate. F inally, a possible phase separation-induced gelation mechanism is suggested . (C) 2001 Elsevier Science B.V. All rights reserved.