| For semi-crystalline polymers ultra high molecular weight polyethylene (UHMWPE), its properties and applications are determined by the crystalline microstructure and morphology. However the crystalline microstructure and morphology can be regulated by crystallization processes, meanwhile, the molecular weight of UHMWPE is also an important factor in the crystallization behavior. Therefore, reveal the effect of molecular weight of UHMWPE on the crystallization behavior, and establish structure-property relationship between condensed structure, chain structure and a variety of hierarchy aggregation structure of UHMWPE is also necessary and even more important guding significance for its high performance.Firstly isothermal, non-isothermal crystallization behavior and their kinetics for UHMWPE with different molecular weight are studied by Avrami equation and Caze method, respectively. It shows that crystallization peak temperature (Tc) and the half crystallization time (t1/2) increases with the UHMWPE molecular weight increases. Meanwhile, non-isothermal crystallization kinetics of UHMWPE in different range of crystallinity are analysed by Caze method, the result shows that with the increase of crystallinity (Xc), the n value gradually reduced from about 3 (Xc=0～30%) to 0.5 (Xc=60～80%), which indicates that the crystal growth restricted gradually. The effect of molecular weight on the mechanical properties of UHMWPE is also studied. The result shows that the impact strength increase with the molecular weight increase, then decrease; and for elongation at break, the higher molecular weight, the lower elongation at break; but the molecular weight influence on tensile little strength, flexural modulus and yield strength are minimal.Secondly we examins two similar molecular weight UHMWPE, namely SLL-5 producted by Shanghai Alliance Lok Chemical Technology Co., Ltd. and GUR 4150 from Germany Ticona. The difference of two kind materials between in the mechanical properties and wear resistance, and the results show that both of the tensile strength, flexural strength, modulus, yield strength and wear resistance are insignificant, but charpy impact strength of GUR 4150 was 3.3 times more than SLL-5. In order to reveal the relationship between macro-properties and micro-structure, molecular weight distribution, lamellar thickness and inter-phase content of UHMWPE are determined by DSC, Rehology and Raman spectrum, respectively. The result of DSC analysis shows that GUR 4150 has large lamellar thickness distribution owing to higher molecular weight distribution, which facilitate impact strength. Rheological studies show that GUR 4150 balance storage modulus (G0’) smaller nearly an order of magnitude than SLL-5. Smaller G0’ shows that smaller degree of entanglement for GUR 4150, which promote the fusion between different particles and reduce the interface defects. Morever, whereby the thickness of lamellae and the grain size are smaller during crystallization, which improve the performance of impact owing to the more grain boundaries. Raman study result shows that mesophase content of GUR 4150 is significantly greater than SLL-5, it means that tie-molecular existed in intermediate phase greatly improved impact performance.Finally, the molding conditions and the impact of adding a nucleating agent on SLL-5 impact properties are also studied. Different molding conditions show that with increasing molding temperature, the impact strength of SLL-5 decreases and then increases; with the increasing pressure, impact strength SLL-5 shows a gradual upward trend; larger cooling rate in favor of improved SLL-5 impact strength. This shows that impact properties of UHMWPE can be improved by optimizing the moulding conditions. The Tc of SLL-5 increased by 1℃ when HPN-20E (0.2%wt) nucleating agent of polyethylene is added, and the impact properties of SLL-5 does not improve, but decreases by nearly 20%. It means that adding the nucleating agent does not help to improve the impact properties of UHMWPE. |
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