Chen Yan-Hui, Fang Du-Fei, Lei Jun, Li Liang-Bin, Hsiao Benjamin S, Li Zhong-Ming
†Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi'an 710129, China.
‡College of Polymer Science and Engineering and State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China.
J Phys Chem B. 2015 Apr 30;119(17):5716-27. doi: 10.1021/acs.jpcb.5b01480. Epub 2015 Apr 15.
Although a shear flow field and β-nucleating agents (β-NAs) can separately induce the formation of β-crystals in isotactic polypropylene (iPP) in an efficient manner, we previously encountered difficulty in obtaining abundant β-crystals when these two factors were applied due to the competitive growth of α- and β-crystals. In the current study, to induce the formation of a high fraction of β-crystals, a strategy that introduces a relaxation process after applying a shear flow field but before cooling to crystallize β-nucleated iPP was proposed. Depending on the relaxation state of the shear-induced oriented precursors, abundant β-crystals with a refined orientation morphology were indeed formed. The key to producing these crystals lay in the partially dissolved shear-induced oriented precursors as a result of the relaxation process's ability to generate β-crystals by inducing the formation of needlelike β-NAs. Therefore, the content of β-crystals gradually increased with relaxation time, whereas the overall crystallization kinetics progressively decreased. Moreover, more time was required for the content of the β-phase to increase to the (maximum) value observed in quiescent crystallization than for the effect of flow on crystallization kinetics to be completely eliminated. The c-axis of the oriented β-lamellae was observed to be perpendicular, rather than parallel, to the fiber axis of the needlelike β-NAs, as first evidenced by the unique small-angle X-ray scattering patterns obtained. The significance of the relaxation process was manifested in regulating the content and morphology of oriented β-crystals in sheared, β-nucleated iPP and thus in the structure and property manipulation of iPP.
虽然剪切流场和β成核剂（β-NA）能够分别有效地诱导等规聚丙烯（iPP）中β晶体的形成，但由于α晶体和β晶体的竞争性生长，我们之前在同时应用这两个因素时难以获得大量的β晶体。在当前的研究中，为了诱导形成高比例的β晶体，提出了一种策略，即在施加剪切流场之后、冷却使β成核的iPP结晶之前引入一个松弛过程。根据剪切诱导的取向前驱体的松弛状态，确实形成了具有精细取向形态的大量β晶体。产生这些晶体的关键在于剪切诱导的取向前驱体部分溶解，这是由于松弛过程能够通过诱导针状β-NA的形成来生成β晶体。因此，β晶体的含量随着松弛时间逐渐增加，而整体结晶动力学则逐渐降低。此外，与完全消除流动对结晶动力学的影响相比，β相含量增加到静态结晶中观察到的（最大）值所需的时间更长。观察到取向β片晶的c轴垂直于而不是平行于针状β-NA的纤维轴，这首先由获得的独特小角X射线散射图案所证明。松弛过程的重要性体现在调节剪切、β成核的iPP中取向β晶体的含量和形态，从而体现在iPP的结构和性能调控方面。
