Effect of crystallization temperature and propylene sequence length on the crystalline structure of propylene-ethylene random copolymers

Crystallization behavior and resultant crystalline structure of a series of temperature-rising elution-fractionated specimen of a Ziegler-Natta catalyst-synthesized propylene-ethylene random copolymer were studied by DSC, WAXD and AFM. The experimental results indicate that both crystallization temperature and propylene sequence length exhibit great influence on the crystallization behavior and crystalline structure of the copolymer. It was found that the ethylene co-monomers acting as point defects inserted into the polypropylene chains play an important role in the formation of y-iPP. As the co-monomer content increases, the crystallizable sequence length of iPP decreases, which produces an appropriate condition for its y crystallization. At the same time, the existence of chain defects leads to a lower crystallinity of the copolymer and imperfection of the resultant crystals. For each individual sample with certain propylene sequence length or ethylene content, the increment of y-iPP crystal content with increasing crystallization temperature demonstrates that higher crystallization temperature is in favor of the y-iPP crystallization. Pure y-iPP crystals have been got in samples with propylene sequence length lower than 21 under suitable crystallization conditions.     

同步辐射X射线衍射原位研究等规聚丙烯取向中间相的相转变

利用同步辐射宽角X射线衍射原位研究了淬冷得到的中间相和由其被拉伸得到的取向中间相在升温和等温退火下向晶体的转变过程.实验结果表明,虽然两者开始相转变形成晶体的温度是相同的,但发生大量相转变所需的温度,取向的中间相要明显高于淬冷直接得到的中间相,这说明前者具有更高的稳定性.通过对比两种不同中间相的初始结构,发现取向后的中间相在沿拉伸方向31螺旋结构具有更大的长程有序,从而导致了其具有更高的熔点.