Crystallization kinetics and the fine morphological evolution of poly(ethylene oxide)/ionic liquid mixtures

Abstract

The overall crystallization kinetics and spherulite morphologies of miscible poly(ethylene oxide) (PEO)/1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) mixtures were studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM) and rheological measurements. The finer crystal structures were further detected by wide angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS). Crystallization of PEO is largely suppressed by [BMIM][PF6] addition especially at higher ionic liquid (IL) concentrations above 20 wt%. Both the overall crystallization rate and the spherulite growth decrease with the increase of IL content and crystallization temperature; however, the crystallization mechanism keeps unchanged as evidenced by the similar Avrami exponent n and WAXD results. The addition of [BMIM][PF6] could induce more nuclei to some extent, but the induction time of crystallization is evidently prolonged, and a linear to non-linear transition of the spherulite growth (R ∝ t to R ∝ t 1/2) can be observed. At higher IL concentration, the spherulite texture changes apparently from particular serrated to branch surface due to the diffusion-controlled growth and the dilution effect, which also as a main factor contributes to the increasing trend of the long period of crystals.