Aqueous mixtures of block copolymer surfactants
Amphiphilic compounds such as lipids and surfactants, are fundamental building blocks of soft matter. Amphiphilic Block Copolymers represent the high molecular weight analogs of conventional surfactants. This thesis describes experiments with poly(1,2-butadiene-b-ethylene oxide) (PB-PEO) diblock copolymers, which self-assemble forming a cornucopia of structures when dissolved in water. A set of 39 PB-PEO diblock copolymers constituting three series of copolymers each with different core molecular weight were synthesized. Micellar morphologies were determined using cryogenic transmission electron microscopy (cryo-TEM) and small angle X-ray scattering (SAXS). Morphological studies show that the high molecular weight OB copolymers self-assemble forming Y-junctions and three-dimensional networks in water at weight fractions of PEO intermediate to those associated with vesicle and wormlike micelle morphologies. Data obtained with three sets of PB-PEO compounds indicate that this type of self-assembly appears above a critical molecular weight.
Cryo-TEM results obtained from copolymer mixtures formed by different blending protocols establish there is no perceptible exchange of macromolecules between aggregates resulting in a nonergodic state, where overall equilibrium is never achieved. However, analysis of many microscopic images leads to the conclusion that these nonergodic micelles relax to a state of local equilibrium through the redistribution of block copolymer within the topological framework established during dispersion. At compositions where the monomodal and nearly monodisperse amphiphiles produce branched wormlike micelles and a network, bimodal mixtures displayed intramicellar segregation leading to cylindrical undulations and octopus-like aggregates with cylindrical micelles emanating from a single bilayer core.
Further, the melt and lyotropic phase behavior of PB-PEO block copolymers was studied using small-angle x-ray scattering (SAXS) and cryogenic scanning electron microscopy (cryo-SEM). A series of sample solutions with varying copolymer concentration were investigated for a set of block copolymers spanning a range of compositions and molecular weights. A sequence of lyotropic liquid crystalline morphologies was documented upon gradual variation of water in block copolymer solutions. The most important result is the formation of a tubular sponge morphology in the binary mixtures above a certain critical molecular weight between compositions/concentrations associated with the cylindrical and lamellar phases.