Ternary Block Copolymer Systems - Phase Behaviour and Phase Structure

Abstract: Ternary isothermal (25°C) polyoxyalkylene triblock block copolymer ((EO)x(PO)y(EO)x where x is the number of ethylene oxide, EO, segments and y is the number of propylene oxide, PO, segments) systems with water and p-xylene as selective solvents for the PEO and PPO blocks, respectively, were chosen as study systems in this thesis as these ternary systems are good model systems for the self-assembly of block copolymers. The phase behaviour has been studied at varied block copolymer molecular weight and relative PEO/PPO block ratio. Symmetric copolymers show a ternary phase behaviour, which is symmetric with respect to the interchange of water and oil volume fractions. Increased molecular weight at constant PEO/PPO block ratio leads to increased ordering of the system, i.e. the number of phases formed increases and ordered liquid crystalline phases are formed at lower copolymer concentration. The liquid crystalline phases are in equilibrium with micellar solutions and the copolymer blocks are unsaturated with solvents. Therefore, the preferred curvature depends on the composition in the system, as in the block copolymer melts. With the asymmetric copolymers the symmetry in the phase diagram is no longer present. The copolymer rich in EO form predominantly normal (oil-in-water) structures while the copolymer rich in PO form predominantly reverse (water-in-oil) structures. In the liquid crystalline phases the short copolymer block(s) are saturated with solvent, and a spontaneous curvature can be defined. PEO/PPO block copolymers are soluble in p-xylene although without forming aggregates. However, the effective block segregation is dramatically increased upon addition of small amounts of water, leading to micelle formation, i.e. the reverse "cmc" is water induced. The micelles are approximately spherical and the micellar radius depends slightly on the copolymer molecular weight, the temperature, and the water content, but is approximately constant with copolymer concentration. Reverse micelle formation begins at a lower polymer concentration when the polarity of the oil decreases (from toluene to p–xylene to 1,3,5–trimethyl benzene). The solvent free polymer melt is non-segregated, although the PEO blocks crystallise at lower temperatures leading to formation of a lamellar structure. The lamellar structure has a one-dimensional swelling with increasing p-xylene concentration. The amount of crystallinity and the degree of long-range and short-range order decrease gradually with increasing temperature and decreasing copolymer concentration.