Investigation Of Microstructure And Properties For Polymer By Positron Probe

In this dissertation, the microstructure and properties of two kinds of materials, including (PEO)-based nanocomposite polymer electrolytes and epoxy resins, were systematically investigated by positron annihilation analysis, combining with the other experimental methods. The main results are as follows.1. Investigation of microstructure and ionic conductivity on (PEO)8-ZnO-LiClO4 nanocomposite polymer electrolytesOwing to high energy density, good cyclability, reliability and safety, lithium-ion rechargeable batteries are important for a wide variety of applications including consumer electronics, transportation and large-scale energy production. The effects of nanosized ZnO on the microstructure, the free volume and ionic conductivity of poly (ethylene oxide) nanocomposite electrolytes ((PEO)8-ZnO-LiClO4) were systematically studied by Scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and positron annihilation lifetime spectroscopy (PALS), respectively.a) nano-ZnO powder has a significant effect on the ionic conductivity of the nanocomposites polymer electrolytes (NCPEs). The ionic conductivity increases significantly with the addition of the ZnO powder up to a maximum value 1.82×10-4 S cm-1 when the ZnO concentration is at 6 wt%.b) From XRD patterns of pure PEO and the NCPEs of (PEO)8-ZnO-LiClO4, we can see that the characteristic diffraction peaks of crystalline PEO become broader and less prominent after the addition of LiC104, and then the intensities of these diffraction peaks further decrease when nano-ZnO is added. This fact suggests the crystallinity of the PEO decreases with the addition of LiC104 and nano-ZnO.c) The results of FTIR show that there are the Lewis-based interactions between lihthium ions and PEO segments or nano-ZnO. The experimental results indicate that nanosized ZnO filled reduces the crystallinity of PEO effectively through the strong Lewis-based interactions with PEO chains and improve ion transport properties, leading to obvious enhancement of ionic conductivity of the (PEO)8-ZnO-LiClO4.d) Both the finite analysis PATFIT and the continuous analysis MELT show an increase of the free volume with the addition nanosized ZnO into (PEO)g-ZnO-LiClO4 polymer electrolytes. Based on the experimental results, a direct correlation between the free volume and the ionic conductivity was observed, which suggests that the higher the free volume, the higher the ion conductivity. We studied the free volume theory of the ion conductive polymer electrolytes from the atomic scale. It is interesting to observe the occurrence of peaks of the free volume distribution split after nano-ZnO filled, which indicated that the ZnO added into (PEO)g-LiClO4 has a prominent effect on the microstructure for NCPEs. This phenomenon has not been reported before.2. Effect of curing agent polarity on water absorption and free volume in epoxy resinsMicrostructure and water absorption were systematically studied by positron annihilation lifetime spectroscopy (PALS), gravimetric measurements and Differential Scanning Calorimetry (DSC) for epoxy resins DER331 (E51) cured with three different kinds of amine curing agents DDS, DDM and MOCA.a) Experimental results indicated that the water absorption as a function of immersed time could be well fitted to Fick’s second law. Based on the gravimetric measurement, we found that the equilibrium water sorption M∞and the diffusion coefficients D of the epoxy resins have an order: E51-DDS> E51-DDM> E51-MOCA, which indicated that the curing agent plays an important role in determining the content of the water absorbed.b) Positron experimental results showed that the o-Ps lifetime dramatically decreased with the immersed time from 0 to 6 h, demonstrating the evidence of the antiplasticization effect due to the water sorption, which suggested that water molecules were filled into free volume holes and the forming of strong water-polymer interactions decreased the mobility of molecular chains.c) In order to deeply discern the antiplasticization influence upon the free volume, the continuous lifetime analysis (the maximum entropy lifetime method (MELT)) was employed to obtain the ortho-positronium lifetime (o-Ps) and the free volume holes distributions. Compared to dry sample, From MELT analyses, we found the existences of two the long-lived components (τ3 andτ4). An unusual phenomenon was observed that when the temperature was increased, the peak values ofτ3 andτ4 all shifted to lower values, especially at 75℃, which was attributed to the antiplasticization effect. Research in this regard has not been reported. This fact indicated that, compared to the antiplasticization induced by the water absorbed, the swelling of the free volume hole can be neglected in the glassy state due to the strong interaction between the water absorbed and the epoxy matrix restricting the motion of the segmental chains. Our experimental results gave valuable information for understanding the antiplasticization mechanism.