| The microdomain morphologies and phase behavior of block copolymers were examined in both the melt and in concentrated solution. Polymer morphology was investigated on a size scale of tens of nanometers at varying temperatures, by employing small-angle X-ray scattering (SAXS). First a series of polystyrene-polyisoprene (PS-PI) diblocks and their hydrogenated derivatives were used to explore the phase boundaries between the lamellar, gyroid, and cylindrical mesophases. The phase diagrams of these materials differ subtly both from each other and from recent self-consistent field (SCF) calculations for systems of varying conformational asymmetry. We observed an interesting modulated-lamellae structure (denoted as the L' phase) in several polystyrene-poly(ethylene- alt-propylene) (PS-PEP) diblocks and their blends. One particular PS-PEP sample exhibits a direct lamella-cylinder (L-C) transition which was characterized by time-resolved SAXS. The rate of the C → L' transformation goes through a maximum with temperature, due to the competition between the thermodynamic driving force and the chain mobility. The reverse L' → C transformation monitored by synchrotron SAXS is orders of magnitude faster than the C → L' transformation, implying a complex mechanism for this phase transformation.;Solutions of PS-PI diblocks with varying compositions were prepared in neutral and selective solvents, and their phase behavior was investigated as a function of temperature and polymer concentration in solution. The use of selective solvents provides a way to tune the morphology of a block copolymer over a wide range, while keeping it in the ordered state. To a good approximation, the morphology obtained may be predicted from the bulk phase diagram if the volume fraction of the insoluble block is used as the correlating parameter. Results in three selective solvents (tetradecane, tribulation, and squalane) show modest quantitative differences in the location of the mesophase boundaries, and significantly different order-disorder transition temperatures, suggesting a crucial effect of solvent selectivity. An independent measurement of solvent selectivity was obtained by measuring the glass transition temperature of the PS microdomains in solution. The domain spacing of the body-centered cubic (BCC) spherical phase in a series of asymmetric PS-PI diblock copolymer solutions showed only a modest change upon dilution. Theoretical calculations of the free energy contributions from the core, interface, and corona in these concentrated micelles indicate that the interfacial energy is the principal factor in determining the equilibrium aggregation number. Qualitative agreement for the decrease in aggregation number on dilution was obtained between experimental values and calculations based on theories available for similar systems. |
