Monday, September 21, 2015

PLGA-PEG-PLGA thermogel mechanism and molecular weight effects detailed

PolySciTech division of Akina, Inc. (www.polyscitech.com) provides a wide array of biodegradable block copolymers including thermogelling PLGA-PEG-PLGA polymers with varying molecular weights and block constituents (PolyVivo AK012, AK024, AK097, etc.). Unlike normal materials, these polymers when dissolved in aqueous solution for a liquid at cold temperature and solidify when the temperature is increased. The primary mechanism for this lies in the competing forces between water-molecule binding attraction to the PEG block and entropic forces encouraging water to be free of the PEG block. Recently a paper was published detailing the effects of polymer block sizes, concentration, and temperature on the gelation and precipitation behavior of these types of polymers. Read more: Chen, Liang, Tianyuan Ci, Lin Yu, and Jiandong Ding. "Effects of Molecular Weight and Its Distribution of PEG Block on Micellization and Thermogellability of PLGA–PEG–PLGA Copolymer Aqueous Solutions." Macromolecules (2015). http://pubs.acs.org/doi/abs/10.1021/acs.macromol.5b00168

“Abstract: 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) (PLGA–PEG–PLGA) 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 (─ÉM). 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.”




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