Cross-Linked Block Copolymer/Ionic Liquid Self-Assembled
Blends for Polymer Gel Electrolytes with High Ionic Conductivity and
Mechanical Strength
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Abstract
Poly(propylene
oxide)–poly(ethylene oxide)–poly(propylene oxide) (PPO–PEO–PPO)
block copolymers (BCPs) with cross-linkable end groups were synthesized,
blended with an ionic liquid (IL) diluent, and cross-linked to form
polymer gel electrolytes. The IL prevented crystallization of PEO
at high concentrations, enabling fast ion transport. In addition,
the IL was selective for the PEO block, inducing strong microphase
separation in what are otherwise disordered or weakly ordered BCP
melts. Cross-linking the BCPs in the presence of the IL resulted in
the formation of solid, elastic gels with high ionic conductivitiesgreater
than 1.0 mS/cm at 25 °C for some compositions. However, it was
found that neither the presence or absence of microphase separation
nor the BCP composition of the microphase separated gels substantially
influenced ionic conductivity. Increasing the cross-link density through
the use of phase-selective PEO- and PPO-based cross-linking reagents
was also evaluated. It was revealed that confinement of cross-links
to the PPO rich domains through the use of PPO-based diacrylates enhanced
the mechanical strength of the gels without detriment to the ionic
conductivity. Conversely, cross-linking in the PEO-rich domains through
the use of PEO-based acrylates significantly reduced conductivity.
Isolation of cross-links within a minor nonconducting domain in a
microphase separated gel is a viable strategy for mechanical property
enhancement without a large sacrifice in conductivity, effectively
decoupling ionic conductivity and mechanical strength. This approach
yielded solid-like gel electrolytes fabricated from BCPs that can
be produced inexpensively, with ionic conductivities of 0.64 mS/cm
at 25 °C and a frequency independent storage modulus of approximately
400 kPa