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

## TL;DR

This paper studies how the structure of a specific polymer changes when combined with a salt, affecting its ability to conduct electricity.

## Contribution

The study reveals how PLLA crystallization drives self-assembly at multiple scales in the presence of LiTFSI.

## Key findings

- PLLA crystallization drives hierarchical self-assembly at micro- and nanoscales.
- PLLA crystalline layers alternate with amorphous regions containing nanodomains.
- Higher LiTFSI content slows PLLA crystallization and reduces ionic conductivity.

## 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 × 10–6 S cm–1 at 60 °C.