Interaction parameters governing self-assembly of ion-containing block copolymers

Block copolymers have been studied for use in lithium metal solid-state batteries due to their ability to decouple ion transport and mechanical properties. It is not surprising that several theoretical models have been developed to describe the effect of salt on self-assembly including the counter-ion entropy model of Rabin, the Born solvation approach of Wang, the ionic self-consistent field theory (SCFT) of Sing and de la Cruz, the polarizable field theory of Fredrickson, and the free ion SCFT of Qin. In order to discriminate between theories, we have determined the morphology of a series of block copolymer/salt mixtures as a function of composition, chain length, salt concentration and temperature. The effect of salt on chain dimensions is determined independently from neutron scattering experiments on homopolymer/salt mixtures. The effective interactions between the blocks are determined by two methods: from characterization of disordered concentration fluctuations and from the locations of phase boundaries. Comparisons between experiment and theory will be presented. The experiments show the presence of coexisting phases at some phase boundaries (e.g. two BCC phases with different lattice constants) that have not been seen in salt-free systems nor captured by any theory.