| The method of calculating the crystallinity of polyethylene (PE) was studied by DSC. The macromolecular structures of PEs were characterized via MDSC, FTIR, GPC and WAXD. The chain branching and melting of PEs were also studied by thermal fractionation (TF). It was found that the melting process of PEs can respond relatively to the modulated frequency of temperature of MDSC. In the mode of quasi-isothermal process near melting peak, the declining speed of the amplitude of modulated heat-flow is in the order of HOPE > LLDPE > LDPE. When quasi-isothermal process was introduced into TF, the higher temperature fraction can be separated more easily, moreover, quasi-isothermal process can't be equal to the conventional isothermal process at the same average experiment temperature.The paper also studied the relationship of PEs' crystallinities with external experimental conditions. It was found that annealing can improve the crystallinity remarkably, the highest value can reach to 3%, however, there is some differences between LDPE and HOPE. The orientation can increase PEs' crystallinities. Following orientation, the crystallinities can increase up to 7.0% for HDPE, 3.0% for LDPE respectively. The treatment by ultra-violet-ray (UV) can destroy PEs' inherent crystallinities dramatically. Compared with HDPE, the effect of UV is more serious on LDPE. The result also indicated that there exist some cross-linking structures and new polar groups after exposing toUV.Finally, the solution crystallization method was employed to prepare higher crystallinity samples of PE. It was found that the solution DSC analysis can be used to investigate the detailed dissolving process of PEs. When EGME(ethylene glycol mono etyl ether) was added to the PE/xylene system, the solution crystallizing temperature and peak temperature of PEs' changed accordingly. By adding EGME to PEs' xylene solution with content of 1.0wt% at 110C, a critical point of crystallization of PE can be found at 31.6wt%(EGME).
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