Effect of nanoscale morphology on the conductivity of polymerized ionic liquid block copolymers

Polymerized ionic liquid (POIL) block copolymers represent a unique class of materials for fundamental studies of single ion conduction as a function of morphology in microphase-separated polymer electrolytes for energy storage and conversion applications. We describe the synthesis of a series of poly(styrene-b-4-vinylbenzylalkylimidazolium bis(trifluoromethanesulfonyl)imide) (PS-b-PVBn(alkyl)ImTFSI; alkyl = CH₃ (Me), n-C₄H ₉ (Bu), n-C₆H₁₃ (Hex)) diblock copolymers (2.7-17.0 mol % POIL) via exhaustive functionalization and ion exchange of relatively narrow molecular weight dispersity poly(styrene-b-4-vinylbenzyl chloride) precursors derived from nitroxide-mediated block copolymerizations. The solid-state morphology of these PS-b-PVBn(alkyl)ImTFSI copolymers were studied using a combination of temperature-dependent synchrotron small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). From electrochemical impedance spectroscopy measurements, we observe that lamellar samples having similar compositions exhibit comparable values of conductivity (0.1 mS cm^-1 at 150 °C) regardless of imidazolium alkyl substituent. The ionic conductivity of a compositionally varied series of PS-b-PVBnHexImTFSI diblocks depends nonlinearly on POIL composition (0.01 mS cm^-1 for 8.6 mol % POIL and 0.1 mS cm^-1 for 17.0 mol % POIL at 150 °C), thus highlighting the influence of morphology on the observed ionic conductivity of POIL block copolymers for the first time. By using different polymer processing strategies, we further demonstrate that the ionic conductivity of a single sample (8.6 mol % POIL) may vary by more than one order of magnitude depending on the long-range ordering of the microphase separated morphology. These studies indicate that macroscopic connectivity and morphological defects strongly affect the observed conductivity in these materials.