Structural studies of complex fluids: Block copolymers and liquid crystals

Complex fluids, often referred to as soft matter, are materials in which inter particle interactions are weak in comparison to the inter-particle forces in solids. In this study, structural and dynamics studies have been carried out on two classes of complex fluids, including diblock copolymers and liquid crystals, with the aim of determining the factors that control association of molecules in these complex fluids. Diblock copolymers consist of two chemically distinct groups bound by a covalent bond. The incompatibility drives ordering in the bulk, association in solvents and unique interfacial characteristics. Liquid crystals are molecules that exhibit orientational order and only limited positional order. These two groups represent two large classes of soft matter in which intermolecular interactions are of the order of 1-10 kBT. The structure and dynamics of these systems were studied using neutron scattering, NMR and microscopy.; Diblock copolymers self associate into micelles in a selective solvent, where the structure and dynamics of the micelles are affected by many factors including temperature and pressure. The responses of diblock copolymer micelles formed by polystyrene-b-polyisoprene (PS-PI) in decane, polystyrene-b-poly(ethylene oxide) (PS-PEO) in cyclohexane, to temperature and pressure were studied by Small Angle Neutron Scattering(SANS). The dynamics of these micelles were investigated by Nuclear Magnetic Resonance (NMR) spin lattice relaxation time measurements. At 1atm, the diblock copolymers form spherical micelles at low temperatures and dissociate with increasing temperature. At high pressures, the spherical micelles transform into sheet-like aggregates and grow with increasing temperature.; When diblock copolymers are confined in thin films, with the film thickness on the order of several radii of gyration of the molecules, the dynamics and stability of these thin films are influenced by the interactions between polymers and interfaces, and between the two blocks. The dewetting kinetics of thin films made from a low molecular weight symmetric PS-PI, with thickness ca. 80A and ca. 140A were studied by Atomic Force Microscope (AFM). Results have shown that the molecular motion as well as the connectivity of the two blocks controls the dewetting kinetics.; The second class of complex fluids studied is liquid crystals where a liquid crystal, 4, 4'-octylcyanobiphenyl (8CB) has been studied as a model system. Small rigid biphenyl derivatives with different fluorination levels were used to modify the intermolecular interactions. (Abstract shortened by UMI.)...