Micellization and gelation of aqueous solutions of star-shaped PLLA-PEO block copolymers

Nontoxic and biodegradable star-shaped poly((L)-lactide-b-ethylene oxide) (star PLLA-PEO) block copolymers were synthesized by the coupling reaction of two reactive precursors, a hydroxy-terminated 3-armed poly((L)-lactide) (star PLLA) and alpha-monocarboxy-omega-monomethoxypoly(ethylene oxide) (CMPEO). The chemical structure and physical properties of the resulting star-block copolymers were characterized. In the dilute aqueous solutions of star-block copolymers, micellization behavior was investigated, and over specific concentration and specific temperature, the unimer to micelle transition occurred. 1,6-Diphenyl-1,3,5-hexatriene (DPH) was solubilized in the micelle core, and the absorbance of DPH at 356 nm increased when micelle was formed. In addition, the effective diameter was highly affected by temperature. With increasing temperature, micelle size decreased dramatically. In high-concentration solution, star-block copolymer showed temperature-sensitive sol-gel transition behavior. Over specific concentration, the whole system cannot flow by the packing of micelles. However, with increasing temperature, packing structure was destroyed by the decrease of the micelle volume due to the dehydration and contraction of the PEO chain, and the system flew. In the sol-gel transition phase diagram obtained by the vial tilting method, the critical gel concentration decreased, the boundary curve shifted to the left, and the gel regions were expanded with increasing the molecular weight of PLLA block and PEO block.