Investigating Crystallization and Morphology of PLLA/PTMC Triblock Copolymer Solid Electrolytes

Abstract

ABA-type block copolymers (BCPs) of poly(trimethylene carbonate-co-trimethylene ether) (PTMC-co-PTME) and poly-l-lactic acid (PLLA) were synthesized through ring-opening polymerization. The BCP was blended with varying concentrations from 10 to 30 wt % lithium bis(trifluoro methylsulfonate) (LiTFSI) to form solid polymer electrolytes (SPEs). Electrochemical impedance spectroscopy was used to study the ionic conductivity of the SPEs in the temperature interval from 30 to 150 °C. Simultaneous small-angle X-ray scattering and wide-angle X-ray scattering were used to study the kinetics of crystallization, together with the phase behavior of those BCPs. It was found that PLLA-b-PTMC-co-PTME-b-PLLA exhibits hierarchical organization at the micro- and nanoscale. More specifically, ABA BCP formed spherulitic superstructures, composed of alternating layers of primarily α′-form crystalline PLLA and amorphous regions. Interestingly, the amorphous phase consisted of phase-separated nanodomains of PLLA, embedded in the PTMC-co-PTME matrix. The investigation into the interplay between crystallization and phase separation via time-resolved scattering methods revealed that the crystallization of PLLA is the driving force for self-assembly at two distinct scales. The microstructure was similar, independent of the salt content; however, elevated doping of LiTFSI slowed the crystallization rate of PLLA and affected the crystalline phase composition. The presence of a crystalline phase slightly lowers the observed ionic conductivity. The best-performing electrolyte, with 20 wt % of LiTFSI, showed a conductivity of 1.2 × 10–6S cm–1at 60 °C.