Improved Route to Linear Triblock Copolymers by Coupling with Glycidyl Ether-Activated Poly(ethylene oxide) Chains

Abstract

Poly(ethylene oxide) block copolymers (PEO$_z$ BCP) have been demonstrated to exhibit remarkably high lithium ion (Li$^+$) conductivity for Li$^+$ batteries applications. For linear poly(isoprene)-b-poly(styrene)-b-poly(ethylene oxide) triblock copolymers (PI$_x$PS$_y$PEO$_z$), a pronounced maximum ion conductivity was reported for short PEO$_z$ molecular weights around 2 kg mol$^{−1}$. To later enable a systematic exploration of the influence of the PI$_x$ and PS$_y$ block lengths and related morphologies on the ion conductivity, a synthetic method is needed where the short PEO$_z$ block length can be kept constant, while the PI$_x$ and PS$_y$ block lengths could be systematically and independently varied. Here, we introduce a glycidyl ether route that allows covalent attachment of pre-synthesized glycidyl-end functionalized PEO$_z$ chains to terminate PI$_x$PS$_y$ BCPs. The attachment proceeds to full conversion in a simplified and reproducible one-pot polymerization such that PI$_x$PS$_y$PEO$_z$ with narrow chain length distribution and a fixed PEO$_z$ block length of z = 1.9 kg mol$^{−1}$ and a Đ = 1.03 are obtained. The successful quantitative end group modification of the PEO$_z$ block was verified by nuclear magnetic resonance (NMR) spectroscopy, gel permeation chromatography (GPC) and differential scanning calorimetry (DSC). We demonstrate further that with a controlled casting process, ordered microphases with macroscopic long-range directional order can be fabricated, as demonstrated by small-angle X-ray scattering (SAXS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It has already been shown in a patent, published by us, that BCPs from the synthesis method presented here exhibit comparable or even higher ionic conductivities than those previously published. Therefore, this PEO$_z$ BCP system is ideally suitable to relate BCP morphology, order and orientation to macroscopic Li$^+$ conductivity in Li$^+$ batteries