Structural Characterizations and Analysis of Crystal Points in Polyethylene Pipes
The crystal points on the surface of the polyethylene pipes were characterized and analyzed in detail by the optical microscope,scanning electron microscopy,molecular weight distribution,differential scanning calorimetry,Fourier transform infrared spectroscopy and successive self-nucleation and annealing fractionation.The compositions and the molecular chain structures of the crystalline points were investigated,and the reasons for the formation of the crystalline points were also analyzed.The results indicate that the crystal points on the surface of the polyethylene pipes present a lamellar stacking structure in the localized state,along with the generation of some cracks.The raised crystal point regions consist of polyethylene chain segments with high molecular weight.In the process of fabrication,the high molecular weight chain segments move slowly and tend to entangle,forming the crystal points.Compared to the regions of the samples without crystalline points,the regions with crystalline points have lower enthalpy of crystallization,lower crystallization temperature,lower melting enthalpy and lower crystallinity.The results of the successive self-nucleation and annealing fractionation indicate that the major differences between the samples with and without crystal points are in the high temperature melting regions.The high-temperature melting peaks in the regions of the samples with crystal points are lower,which indicates that the molecular chain structures in the crystal point regions are less regular,and the thickness of the lamellar crystals formed is smaller.Comparing the differences between the crystal points on the surface of the pipes made from different raw materials,the longer chain structures of the copolymers result in larger crystal points,and the higher content of the high-molecular-weight portion results in larger thickness of the lamellar crystals.
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