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

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^-6 S cm^-1 at 60 °C.