Lithium Ion Induced Nanophase Ordering and Ion Mobility in Ionic Block Copolymers

Abstract

Diblock copolymers of poly(styrene-b-methacrylic acid) (PS−PMAA) of various compositions have been synthesized by anionic polymerization and were employed as templates for the development of “single-ion” polymer electrolyte systems. Lithium ions were introduced as the effective component that alters the phase state and contributes to ionic conductivity of these block polyelectrolytes. The local structure, microdomain morphology, and ion dynamics of the pure block copolymers and of the block polyelectrolytes were investigated by infrared spectroscopy, small-angle X-ray scattering, differential scanning calorimetry, and dielectric spectroscopy. Investigation of the microdomain structure revealed complexation of lithium ions with the carboxylate units of the PMAA phase in two types of coordination (chelating vs bringing) that alters the phase state of the copolymers and results in a change from the weak- to strong-segregation limit. The investigation of dynamics revealed a new dielectrically active process associated with a local relaxation of lithium ions coupled to the MAA segments via the two types of coordination. The ionic conductivity of the block polyelectrolytes was found enhanced by about 3 orders of magnitude relative to that of the parent copolymers. These results contribute to a better understanding of the changes in microdomain morphology as well as of the ion dynamics in systems resulting from the association of lithium ions with the charged PMAA block.