Ionic solvation and dielectric screening in salt-doped block polymers
Keywords: Chemical Engineering.

Salt-doped block polymers are materials with promising applications as electrolytes in lithium batteries. These plastics self-assemble into ordered microstructures which exhibit both necessary mechanical strength and desirable ionic conductivity. It is well known that salt-doping changes the morphological behavior of block polymers. Variations in the self-assembled morphologies of these materials are extensively documented, but remain poorly understood. This is largely due to the challenges of modeling electrostatic interactions in heterogeneous media with low average permittivity. I seek to address these challenges using a multi-scale approach to study ionic solvation and dielectric screening in salt-doped block polymers. Self-consistent field theory (SCFT) is the standard theoretical tool for investigating the morphological behavior of neutral block polymers. I have extended the SCFT framework to salt-doped polymers by explicitly modeling salts as fully mobile ions immersed in the heterogeneous polymeric medium. This model accounts for dielectric heterogeneity, ionic solvation using the Born model, and two-body electrostatic interactions at the Poisson-Boltzmann level. Dielectric permittivity is coupled to the polymer density fields of self-assembled structures, which are constrained by space group symmetry. To efficiently resolve the electrostatic potential in the resulting heterogeneous medium, I have implemented a symmetry-adaptive algorithm which exploits the symmetry of self-assembled structures. This model and algorithm have been incorporated into a software package, iPSCF (ionic polymer self-consistent field theory), which enables efficient free energy calculations for self-assembled morphologies, and the generation of full phase diagrams for salt-doped block polymers. This model identifies two distinct thermodynamic regimes in salt-doped block polymers: an `entropic' regime where the addition of salts destabilizes ordering, and a `solvation' regime where the addition of salts enhances ordering and promotes self-assembly. These regimes are governed by the relative magnitudes of ion translational entropy and solvation free energy. To validate this theory, I have conducted an extensive comparison with compiled experimental data for polystyrene-block-polyethylene oxide (PS-b-PEO) doped with lithium salts, using only a single free parameter which defines the magnitude of the ionic solvation energy. This comparison confirms that ionic solvation is the principal driver of thermodynamics in these systems, but suggests that the solvation free energy prescribed by the Born model is too strong. Two complementary effects which may explain this discrepancy have been identified. The first is the neglect of composition fluctuations in polymer field theory. Fluctuations stabilize the disordered phase, and are expected to weaken the degree of ordering induced by ion solvation. The second is the neglect of molecular-scale solvation structure in the Born model. To further investigate this second effect, I have developed polarizable atomistic simulations to elucidate dielectric mixing rules and resolve the solvation structure in salt-doped block polymers. Altogether, this work establishes a foundation for future studies of ionic polymers which bridges the atomistic and mesoscopic (~10 nm) length scales