| dc.description.abstract | In this study, I have investigated the micro-phase separated structures of solvent annealed PS-b-PEO thin films in mixed vapors of binary nonpolar/polar solvents. Before solvent annealing, the surface morphology of as-spun films was dominated by disordered spheres. First, polar-solvents were used to preferentially swell PEO chains, by which the segregation strength between the two segments can be enhanced. This stage is called as a “pre-annealing” step. Then the vapor of nonpolar solvents (benzene or THF) was used for annealing thin films at 17 °C. After solvent annealing, the surface morphology was dominated by hexagonal arrays of spheres. By contrast, if thin films were directly exposed to the vapor of benzene or THF without undergoing the pre-annealing step, the surface morphology was dominated by disordered spheres. The reason is that benzene or THF vapor also swelled the PEO domain in addition to swelling the PS domain. This reduced the segregation strength between the two segments.
Next, thin films were exposed to non-polar/polar co-solvent vapors at 17 °C. The non-polar solvents used were, respectively, toluene, benzene and THF, and polar-solvents were water, methanol, ethanol, propanol, butanol, and hexanol, respectively. In systems of solvent annealing in THF/alcohol co-solvent vapors, both methanol and ethanol have high vapor pressure. Upon exposing to the vapor of THF/methanol and THF/ethanol, the surface morphology of solvent-annealed films revealed disordered nanocylinders. By contrast, upon exposing to solvent vapors of THF mixed with propanol, butanol or hexanol which have lower vapor pressure than that of methanol and ethanol, parallel-oriented nanocylinders with little density of defects can be obtained through transitions from disordered spheres to hexagonal packed ones and then to parallel nanocylinders with long-range order. The reason is that the vapor pressure of propanol, butanol and hexanol is lower than that of methanol and ethanol. As a result, the energy barrier for the transformation directly from disordered spheres to lying cylinders with long-range order was high. I speculate that the disordered spheres transform into lying nanocylinders with long-range order proceeding through an intermediate stage of hexagonal-packed spheres.
Furthermore, only spheres were present in thin films with solvent annealing in vapor of THF/water at 17 °C. As the temperature was decreased to 12 °C, the ordering of nanospheres can be improved. Such morphology was also obtained for solvent annealing in vapor of THF/propanol at 20 °C (5h). In the final part of the thesis, I demonstrate that switchable phase transitions can be induced upon solvent annealing at different temperatures or in vapors of different co-solvent mixtures. | en_US |