Effect of Composition and Molecular Structure of Poly(L-lactic acid)/Poly(ethylene oxide) Block Copolymers on Micellar Morphology in Aqueous Solution

The effect of the hydrophobic block length in diblock (PLLA(x)-b-PEO113, x = 64, 166, 418) and triblock (PLLA(y)-b-PEO91-b-PLLA(y), y = 30, 52, 120) copolymers of L-lactic acid and ethylene oxide on the structure of micelles prepared by dialysis was studied by wide- and small-angle X-ray scattering in dilute aqueous solution, dynamic light scattering, transmission electron microscopy, atomic force microscopy, and force spectroscopy. It was found that the size of the crystalline PLLA core is weakly dependent on the PLLA block length. In addition to individual micelles, a number of their micellar clusters were detected with characteristic distance between adjacent micelle cores decreasing with an increase in PLLA block length. This effect was explained by the change in the conformation of PEO chains forming the micellar corona because of their overcrowding. Force spectroscopy experiments also reveal a more stretched conformation of the PEO chains for the block copolymers with a shorter PLLA block. A model describing the structure of the individual micelles and their clusters was proposed.