Temperature sensitive sol-gel transition behavior of star-shaped PLGA-PEG block copolymer

Abstract

Star-shaped poly((DL-lactide-co-glycolide)-b-ethylene glycol) (star PLGA-PEG) copolymers were synthesized by the coupling reaction of two reactive precursors, a hydroxy terminated 3-armed star PLGA with a-monocarboxy-w-monomethoxy poly(ethylene glycol) (CMPEG). The resulting star-block copolymers were characterized by $^1H$ and $^13C$ nuclear magnetic resonance, gel permeation chromatography, and differential scanning calorimeter. Micellization behavior of star block copolymer in dilute solution was investigated by using UV/Visible spectrometer and dynamic light scattering (DLS). In the dilute aqueous solutions of star-block copolymers, Critical micellization temperature was determined by UV/Visible spectroscopy using DPH probe. Over specific concentration and temperature, unimer to micelle transition occurred. The hydrophobic PLGA block aggregated by hydrophobic interaction, and hydrophilic PEG block surrounded the PLGA core. DPH can be solubilized in the micelle core and the absorbance of DPH at 356 nm increases when micelle is formed. The effective diameter of micelle was measured by DLS. The effective diameter was highly affected by temperature. With increasing temperature, effective diameter increased dramatically. In high concentration solution, star-block copolymer showed temperature sensitive sol-gel transition behavior. The sol-gel transition phase diagram is obtained by tube tilting method. Over specific concentration and temperature, the whole system could not flow (gel state). However, with further increase of temperature, the system flowed (sol state). The obtained transition curve shifted to the lower left when the molecular weight of PLGA block increased.