Block length dependence of morphological phase diagrams of the ternary system of PS-b-PAA/dioxane/H2O

The block length dependence of the morphological phase diagram was investig ated for PS-b-PAA copolymers in dioxane/water using transmission electron m icroscopy and light scattering techniques. The study was performed on copol ymers with PS block lengths ranging from 49 to 310 repeat units and PAA blo ck lengths from 7.2 to 26 repeat units. The water content ranged from 0 to 50 wt % and the copolymer concentration from 0 to; 10 wt %. For all the cop olymers with increasing water content, the sequence of morphologies, in gen eral, follows the order of single chains, spheres, sphere and rod mixtures, rods, rod and bilayer mixtures, pure bilayers, and finally mixtures of bil ayers and inverted structures. The bilayers here include vesicles, lamellae , and more complicated structures. It was found that the boundaries of vari ous morphological transitions generally shift to lower water contents with increasing total block length and with decreasing PAA block length. Most im portantly, it was found that long core-forming blocks and high water conten ts favor the formation of vesicles and that short core-forming blocks and l ow water contents favor open bilayers (e.g., lamellae). This finding sugges ts that in block copolymer systems the increase of bending modulus favors v esicle formation, which reinforces the conclusion of a theoretical study fo r mixed small molecule surfactant systems. The Gibbs free energies for the single chain to sphere transition were also estimated from the morphologica l phase diagram. It was found that at the same water content the free energ y becomes less negative both with decreasing total block length and with in creasing PAA block length.