Materials formed by combining block copolymers with ionic liquids that can selectively solvate one of the constituent blocks are being actively explored for use in a diverse range of applications. However, in many cases the physical properties and thus performance of these materials are inherently linked to the self-assembled morphology, which can be difficult to anticipate in the presence of ionic liquids because factors such as the domain swelling and compatibility must be considered. This thesis therefore aims to systematically and comprehensively investigate the influence of ionic liquids on the lyotropic phase behaviour of a model block copolymer across a wide concentration range. The results of these studies could then be summarised in the form of an experimental phase diagram, towards the creation of a universally applicable model which can be employed in future studies of these materials that require a specific self-assembled morphology to suit a desired application. From this knowledge, a series of block copolymer samples with a variety of morphologies was created so that further insights into the structure-property relationships in these materials could be gained.
To this end we studied the ionic liquid induced order-disorder transition of a series of low molecular weight polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) block copolymers (χN < 10.5 at 298 K) in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide (EMIM Tf2N), allowing estimation of the dependence of χeff with ionic liquid concentration. Higher concentrations of ionic liquids resulted in a series of lyotropic phase transitions. Using results from these experiments, an experimental χeffN versus ƒ'PS phase diagram was constructed. Importantly, we observed distortions of the phase diagram as a function of ionic liquid concentration that were significantly different from the phase diagrams of neat block copolymers or block copolymers swollen with selective solvents.
We then sought to expand the applicability of this phase diagram by mapping the phase behaviour of the same five PS-b-PMMA block copolymers in four additional ionic liquids with various cation structures: 1-butyl-3-methylimidazolium (BMIM) Tf2N, 1-octyl-3-methylimidazolium (OMIM) TF2N, 1-butyl-1-methylpyrrolidinium (BMP) Tf2N and trioctyl(tetradecyl) phosphonium (TOTDP) Tf2N. This enabled the effect of ionic liquid structure on χeff to be quantified, towards the development of a more robust model which can better account for these variations, thus making it more experimentally relevant and reliable.
During our investigations of the phase behaviour of the lowest molecular weight block copolymers with a high ionic liquid content, on several occasions we obtained SAXS profiles which were consistent with the Frank-Kasper σ-phase, a quasicrystal approximant structure only recently identified as a stable phase in diblock copolymer systems. This finding provides additional systems for the study of space filling and crystallisation in other hard and soft materials which form complex, low symmetry structures; and also offers the potential for a number of exciting applications which require complex polymer morphologies and can also exploit the interesting properties of ionic liquids.
Our extensive mapping of the phase behaviour of these systems provided us with a convenient model with which to explore the structure-property relationships in these materials, including for the highly sought after gyroid phase. We therefore measured the dielectric and dynamic shear properties of a series of PS-b-PMMA (10.0 kDa-b-10.0 kDa) block copolymer mixtures with a variety of concentration dependent morphologies in the ionic liquid EMIM Tf2N. Normalising the dc conductivity (σ0) of each sample to the effects of concentration revealed that the sample possessing the gyroid morphology did not exhibit an observable increase in normalised conductivity compared with the other microstructures that were examined. Interestingly, the dynamic shear modulus (G′) of the glassy plateau regime for of all the ionic liquid containing samples between ϕP = 0.90 – 0.70 was greater than that of the pure block copolymer when below the Tg of the PMMA/EMIM Tf2N phase. Furthermore, G′ counterintuitively increased quite significantly as a function of ionic liquid concentration for the samples which possessed the gyroid morphology, culminating in G′ values over two fold higher than the original block copolymer. This phenomenon was concluded to arise from confining the glassy PMMA/EMIM Tf2N phase within the 3-D interconnected, nano-sized domains of the gyroid morphology, though additional experiments are still required to discern the mechanism(s) by which this occurs.
Finally, we explored the feasibility of using EMIM Tf2N to give PS-b-PMMA block copolymers the properties of a “high χ-low N” block copolymer and thus extend the scope of this system for directed self-assembly. These studies demonstrated that in thin films it was possible to tune the lamellae long period (pitch), or induce transitions from one morphology to another by varying the concentration of EMIM Tf2N, as observed in the bulk systems. Importantly, for the thin films formulated with 0 vol % – 25 vol % EMIM Tf2N, it was demonstrated that lamellae oriented perpendicular to the substrate could be obtained using the same underlayer, i.e., a neutral underlayer optimised for PS-b-PMMA with no added EMIM Tf2N. As a result this can all be achieved without conducting additional processing steps and does not require additional infrastructure in the laboratory, thus making this approach amenable to integration into current preproduction processing steps that have been developed by industry.