Block copolymers in ionic liquids.

Abstract

In this thesis the self-assembly behavior of block copolymers diluted with ionic liquids has been investigated. Initial experiments involved characterizing the selfassembly of poly(styrene-b-methyl methacrylate) (PS–PMMA) and poly(butadiene-bethylene oxide) (PB–PEO) copolymers at dilute concentrations (~1 wt%) in the ionic liquids 1-butyl-3-methylimidazolium hexafluorophosphate ([BMI][PF6]) and 1-ethyl-3- methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]). Dynamic light scattering and cryogenic transmission electron microscopy results showed that the ionic liquids behave as selective solvents for the PMMA and PEO blocks of the copolymers, and that the micelle morphology and self-assembly behavior of the block copolymers in the ionic liquids was analogous to that observed in conventional solvents. At increased solution concentrations (≥ 20 wt%) the lyotropic mesophase behavior for PB–PEO diluted with [BMI][PF6] and [EMI][TFSI], and poly(styrene-bethylene oxide) (PS–PEO) diluted with [EMI][TFSI] was investigated via small angle X-ray scattering. These experiments showed a microstructure phase progression with addition of ionic liquid that was analogous to that expected for an increase in the PEO volume fraction of the bulk copolymers. Additionally, an increase in the lamellar microstructure domain spacing with ionic liquid content indicated that both ionic liquids behave as strongly selective solvents for the PEO blocks of the copolymers. The ionic conductivity of the concentrated PS–PEO/[EMI][TFSI] solutions was measured via impedance spectroscopy, and found to be in the range of 10−3 S/cm at elevated temperatures (~100 °C). Additionally, the ionic conductivity of the solutions was observed to increase with both ionic liquid content and molecular weight of the PEO blocks of the copolymer. Finally, preliminary investigations of the microstructure orientation in thin films of a concentrated PS–PEO/[EMI][TFSI] solution were conducted. The copolymer microstructure was observed to align perpendicular to the film surface with short term (≤ 2 hours) thermal annealing. Longer term thermal annealing resulted in a transition to parallel alignment of the copolymer microstructure relative to the film surface.