Confinement effects in microphase separated block copolymer electrolytes - conductivity and crystallinity

A series of strongly microphase separated block copolymers of polystyrene (PS) and poly(ethylene oxide) (PEO) were studied with morphologies of spheres, cylinders, and lamellae. Each morphology comprised two samples, one with glassy PS as the majority component and one with semicrystalline PEO as the majority block. Complementary block copolymer (BCP) electrolytes were formed by mixing each BCP with lithium bis(trifluoromethanesulfonylimide) (LiTFSI) salt. The thermal and ion transport properties of these neat and electrolyte BCPs did not exhibit clear trends with respect to either PEO volume fraction or effective medium theory predictions, which prompted an examination of the confinement of the PEO conductive phase. The domain spacing of each morphology was measured with X-ray scattering, and a confinement length was defined as the characteristic PEO domain size in PEO-minority BCPs or the minimum distance between two non-conductive PS microdomains in PEO-majority BCPs. Confinement was found to impede crystallization in an exponentially decaying fashion with an exponential constant of 15 nm, which is similar to PEO crystal lamella size. The dependence of conductivity on confinement was found to be more nuanced. Confinement was detrimental to ionic conductivity in lamellar morphologies, approaching the effective-medium prediction with increasing confinement length (i.e. less confined). Conversely, in morphologies with curvature, conductivity was enhanced, increasing strongly with decreasing confinement length. This remarkable result could be leveraged to design more conductive polymer electrolytes for solid state batteries.