Optical microscopy study on the crystallization in PEO-salt polymer electrolytes

The ionic conductivity of polyethylene oxide (PEO)-salt electrolytes is complicated due to the coexistence of crystalline and amorphous phase and, furthermore, due to the slow phase equilibration processes. In the present study, thermal, optical and electrical data have been measured with similar experimental conditions to compare the effect of slow crystallization kinetics. The differential scanning calorimetry (DSC) data and optical micrographs show that the onset of crystalline to amorphous transition occurs from far below the melting peak temperature, more seriously for higher salt content. The pre-melting, hence, varying degree of crystallinity, further complicates the ionic conductivity. The primary crystallization, corresponding to radial growth of spherulites, occurs very fast, within a few minutes except just within the vicinity of the melting point. However, the spherulitic texture once impinged is gradually perfected for a long time. It corresponds to a secondary crystallization, by thickening slowly through a development of subsidiary lamellae. In the case of higher salt content (e.g., PEO10LiClO4), a non-volume filling pattern of spherulites appears and the isolated spherulitic aggregates grow very slowly over 10 days.