Formation Mechanism of Extended-Chain Crystal of Polyethylene

本课题“聚乙烯扩链晶体的形成机理”1.制备了超高相对分子质量聚乙烯(PE)的溶液结晶样品。用高压DTA研究了它在高压下的熔融过程，从DTA曲线的峰值熔化温度得到了它的相图。在600 Mpa的高压区，加热过程中迅速形成了类似于扩链晶体(ECC)的厚片晶。片层的快速增厚可能是由于缠结减少所致。制备了从稀溶液中结晶的PE单晶垫样品。用高压DTA研究了聚合物的熔融结晶过程。用小角度和广角X射线分析研究了熔融过程中的片层增厚和分子链取向的变化：(1)在低于300 Mpa的压力区，片层增厚一直持续到DTA曲线上的熔融温度峰值。在其上，晶体开始熔化，因此不形成ECC。(2)在高于300 Mpa的压力区，即使在六方相中也会发生片层增厚，从而在熔化过程中形成非常厚的片层，如ECC。(3)在高压区，熔化后分子链的原始取向没有改变，这种分子取向的持续对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.