Formation Mechanism of Extended-Chain Crystal of Polyethylene

在我们的研究项目“聚乙烯延伸链晶体的形成机理”中得到了以下结果。制备了超高分子量聚乙烯（PE）溶液结晶样品。采用高压差热分析方法研究了其在高压下的熔融过程，并通过差热分析曲线的峰值熔化温度得到了其相图。在600 MPa等高压区域，加热过程中迅速形成类似于延伸链晶体（ECC）的厚片层。薄层的快速增厚可能是由于缠结减少所致。制备了稀溶液结晶的PE单晶垫样。采用高压差热分析研究了其熔融和结晶过程。采用小广角x射线分析研究了熔融过程中片层增厚和分子链取向的变化。(1)在低于300 MPa的压力区，层状增厚一直持续到DTA曲线的峰值熔化温度。在它上面，晶体开始熔化，因此不会形成ECC。(2)在高于300 MPa的压力区，即使在六方相也会发生层状增厚，因此在熔融过程中会形成像ECC这样非常厚的层状。(3)在高压区，熔融后分子链的原有取向不发生改变，这种分子取向的持续存在对ECC的生长有影响。

Following results were obtained in our research project "Formation Mechanism of Extended-Chain Crystal of Polyethylene"1. A solution crystallized sample of ultra-high molecular weight polyethylene (PE) was made. Melting process of it under high pressure was studied by high pressure DTA, and a phase diagram of it was obtained from peak melting temperature of DTA curves.In the high pressure region such as 600 MPa, thick lamellae similar to extended-chain crystal (ECC) are formed very rapidly in the heating process. The rapid lamellae thickening is suggested to be due to the decrease of entanglements.2. A single-crystal mat sample of PE crystallized from dilute solution was made. The melting and crystallization processes were studied by high pressure DTA. Lamellar thickening and change of orientation of molecular chain in the melting process were studied by small and wide angle X-ray analysis.(1) In the pressure region lower than 300 MPa, lamellar thickening proceeds up to the peak melting temperature in the DTA curve. Above it, melting of the crystal starts, therefore ECC does not form.(2) In the pressure region higher than 300 MPa, the lamellar thickening proceeds even in the hexagonal phase, therefore very thick lamellae like as ECC is formed in the melting process.(3) In the high pressure region, original orientation of the molecular chain does not change after the melt, then this persistence of the molecular orientation have an effect on the growth of ECC.