Tailoring Crystallization Behavior of PEO-Based Liquid Crystalline Block Copolymers through Variation in Liquid Crystalline Content

Abstract

A series of liquid crystalline–semicrystalline–liquid crystalline triblock copolymers, with poly(ethylene oxide) (PEO) as the semicrystalline central block and polymethacrylate bearing side-chain cholesteryl mesogens as the liquid crystalline (LC) end blocks, are prepared using reversible addition–fragmentation chain transfer (RAFT) polymerization. Starting with 20 kg/mol PEO, the weight fractions of the LC blocks in the triblock copolymers are varied from 21 to 86 wt %. The wide-angle and small-angle X-ray scattering (WAXS and SAXS) as well as transmission electron microscopy (TEM) studies show that with the increased LC content in the triblock copolymers different hierarchical structures including “LC lamellae in PEO lamellae” and “PEO cylinders in LC matrix” are observed sequentially. Differential scanning calorimetry (DSC) study shows that the triblock copolymers with “LC lamellae in PEO lamellae” crystallize at normal undercooling conditions (crystallization temperature Tc observed at 31.0–36.4 °C, which is close to that of homopolymer PEO), while those with “PEO cylinders in LC matrix” crystallize at very large undercooling (Tc drops to −23.5 to −27.8 °C). The large variation of the undercooling conditions required for PEO crystallization is attributed to the nanoconfinement effect from different hierarchical structures at varied LC contents. Avrami analysis has been performed to understand the PEO crystallization mechanism. In “LC lamellae in PEO lamellae”, the PEO crystallization is confined within 2D microdomains between LC lamellae and follows heterogeneous nucleation mechanism with subsequent long-range crystal growth. In “PEO cylinders in LC matrix”, the PEO crystallization is confined within 1D cylindrical microdomains and dominated by homogeneous nucleation, and long-range crystal growth is prohibited by the surrounding LC matrix. This study demonstrates that microsegregated LC domains can provide efficient confinement of the PEO crystallization, and by simply increasing the LC content, amorphous PEO can be obtained at room temperature. These LC–semicrystalline–LC triblock copolymers, with room temperature amorphous PEO confined in microsegregated nanodomains, may be used as scaffolds for lithium ion batteries and solid-state electrolytes.