Abstract
Ion-conductive unsaturated polyesters (UP) were synthesised from poly(ethylene oxide) (Xn = 9, 13, 22, 90) or poly(propylene oxide) (Xn = 7, 13, 20, 34, 68) and maleic anhydride. Subsequently, the polyesters were doped with LiClO4 and cross-linked with styrene using a redox initiator. For PEO-based polyesters, the minimum resistivity is found at an O/Li+ molar ratio of 50/1. In contrast, more lithium is required to reach the minimum when using PPO (O/Li+ = 10/1). Unlike the PEO-based polyesters, cross-linking of the PPO types gives rise to decreasing resistivities at increasing molecular weight. This correlates well with the transverse proton relaxation time determined by single-sided NMR, which is an indicator of the chain mobility. The cross-linking reaction of these UP with styrene exactly follows the predictions based on the copolymerisation parameters and is, therefore, not dependent on the ratio of styrene to UP double bonds as previously reported. Due to the opposing effects of the molecular weight on the ion conductivity of PEO- and PPO-based UP, 1:1 blends of short-chain PPO and long-chain PEO polyesters were cross-linked with styrene. The resulting networks showed a resistivity of 4 kΩ m (σ = 2.5∙10−4 S∙m−1), which is 5 times lower than the pure PEO and 3 times lower than the pure PPO materials.
Highlights
The incorporation of PPO inhibits the crystallisation of poly(ethylene oxide) (PEO) and leads to highly conductive polymer electrolytes. [1, 33] Khan et al found that the conductivity more than doubles for blends of PEO (Mw = 600,000) with PPO (Mw = 1025) containing LiClO4, from 1.4 10−6 to 3.55 10−6 S∙cm−1 in comparison to pure PEO
There are some reports in literature on cross-linked networks based on unsaturated polyesters of PEO/PPO as well as blends of commercial UP with PEO and/or PPO, but mainly with the focus on the morphology, miscibility, and thermal properties. [56,57,58,59,60] In our previous paper, we studied the curing kinetics and impedance behavior of PEO-based unsaturated polyesters
For unsaturated polyesters that are not cross-linked with styrene, the identifier refers to the amount of the repeat units; e.g. E9 refers to a polyester made from PEO with 9 EO repeat units and MSA
Summary
All types of polyesters were doped with lithium perchlorate to achieve ion conductivity and cross-linked (cured) by radical copolymerisation with styrene using methyl ethyl ketone peroxide (MEKP) and cobalt(II) 2-ethylhexanoate (Co-Oct) as redox initiator to increase the mechanical stability [40]. The unsaturated polyesters are doped with LiClO4 to cause ion conductivity and, with regard to future applications as sensor materials, cross-linked with styrene in order to increase the mechanical stability [40]. The significant increase in resistivity of the cross-linked UP 50E45 is, assigned to crystallisation of the higher molecular weight polyesters, which hampers the movement of ions.
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