Effects of Molecular Weight and Its Distribution of PEG Block on Micellization and Thermogellability of PLGA-PEG-PLGA Copolymer Aqueous Solutions

While amphiphilic block copolymers have been extensively investigated, little is known about the effect of molecular weight distribution (MWD) of the hydrophilic blocks on corresponding physical gelation behaviors. Herein, we employed thermogelling poly(d,l-lactide-co-glycolide)-b-poly(ethylene glycol)-b-poly(d,l-lactide-co-glycolide) (PLGAPEGPLGA) as the model system. We synthesized the block copolymer and prepared a series of copolymers with similar PLGA blocks but varied lengths and distributions of the PEG block. The micellization in dilute solutions was detected by light scattering and fluorescence spectroscopy using the probe 1-anilino-8-naphthalenesulfonate (ANS) specifically for the micelle coronae. Vial-inverting observations and rheological measurements were carried out to judge sol or gel states for the concentrated aqueous systems. The gel-to-sol transition or sol-to-gel transition occurred only with appropriate molecular weight (MW) and molar mass dispersity (D-M). In this study, we denote the normal hydrogel with a sol-to-gel transition upon cooling as gel-1, and gel-2 refers to the thermogel with a reversed physical gelation in a sol-to-gel transition upon heating. We found that wider MWD of PEG block sometimes even led to coexistence of normal gel (gel-1) and reversed gel (gel-2). The corresponding concentrated aqueous system of copolymers underwent gel-1-to-sol-to-gel-2 transitions with an increase of temperature. The macroscopic physical gelation was further discussed based upon the mesoscopic micellization and the micellar aggregation.