Study On Crystallization Behavior And Structure Of PEO/LiClO₄ Blends

The high concentration of electron pairs and polarizability of the ether groups in Poly(ethylene oxide) (PEO) make it one of the most promising polymer electrolytes. Lithium is the most electropositive as well as the lightest metal, thus facilitating the design of storage systems with high energy density. Therefore, PEO-LiX based solid polymer electrolytes (SPEs) have been extensively studied and there was a profound lack of information concerning the structure of polymer electrolytes. Yet knowledge of the crystal structures was vital if we were to explore ion transport and improve the conductivity of polymer electrolytes.The crystallization structure, crystallization behavior and structure transition of PEO were investigated using POM, FT-IR, DSC and SR-SAXS techniques. The influence of LiClO₄ on the crystallization structure, crystallization behavior and structure transition of LiClO₄/PEO systems also have been investigated with the same techniques.PEO shows typical spherulites with Maltese-cross pattern and have fine crystallization texture. The spherulites show the patterns of feather-like lamellar bundles in salt-doped PEO, and the crystalline morphology differs significantly from that of neat PEO. Spherulite growth rate for neat PEO are obtained and compared with those for LiClO₄-doped PEO. All of the spherulite growth rate based results exhibit a similarly linear growth mechanism with respect to time (within the time frame studied), but the slopes vary with the isothermal temperature held for crystallization. By comparison, the growth rates for LiClO₄-doped PEO are almost order-of-magnitude slower than those for salt-free neat PEO. That is, LiClO₄ doping in PEO induced a significant retardation on PEO crystal growth.The structure parameters such as long period (L) and crystalline lamellar thickness (Lc) or amorphous layer thickness (La) can be obtained from the SAXS profiles calculated according to the one-dimensional electron density correlation function. The results show that the long period is almost constant during the isomal crystallization process. The amorphous layer is reduced in course of the secondary crystallization, which results in the decrease of amorphous layer thickness(La) and the increase of average crystalline lamellar thickness (Lc).The occurrence of contact ion pairs in PEO/LiClO₄ has been suggested by infrared Spectroscopy. The IR peak of the ether group of PEO slightly down-shifts to a lower frequency, which is indicative of the existence of interactions between Li+ ions and ether group of PEO in the case of PEO/LiClO₄ polymer system. This implies that LiClO₄ is capable of reducing the PEO crystalline phase.