Vesicle Formation of PLA_x−PEG₄₄Diblock Copolymers

Three PLA(A)-PEG(44) diblock copolymers with fixed hydrophilic PEG block and various lengths of hydrophobic PLA block were synthesized via Iing-opening polymerization. The micelle formation and micelle mogphologies of the PLA-PEG(44) copolymers in selective solvents were investigated by using cryogenic transmission electron microscopy (cryo-TEM) and light scattering techniques PLA(x)-PEG(44) diblock copolymers in aqueous solution form various micelle structures when increasing A from 56 to 212 m order to minimize the overall free energy of the systems. The micelles transform from wormlike micelles for PLA(36)-PEG(44) in to vescles for PLA(212)-PEG(44). Interestingly, vesicular structures with various morphologies, such as large polydisperse vesicles, entrapped vesicles, hollow concentric vesicles, ellipsoidal vesicles, open bending lamellae, vesicles with irregular shapes, etc., were found to be coexisting in PLA(212)-PEG(44) THF/H2O and PLA(212)-PEG(44) dioxane/H2O mixtures with 30 and 40 wt % water contents. Toroid micelles with new morphologies were also observed. These observations indicate that the vesicular micelles of amphiphilic block copolymers in mixed solvents fluctuate from time to time and are able to kinetically form different shapes of morphologies in the solutions. The membrane fluctuation of PLA(212)-PEG(44) vesicles in mixed solvent was verified by dynamic light scattering.