聚丙烯取向β晶生成与工程实现研究

β-crystal is a thermodynamic modification of polypropylene (PP), which exhibits excellent toughness, elongation at break and pore-forming property. β-crystal can only be obtained by specific methods. Among those methods, adding β-nucleating agent is the most efficient way. Since β-crystal will transform into stable α-crystal under stress, it is difficult to prepare oriented β-crystal by orienting the molecular chains upon stretching. Until now, there are no reports about production of plenty oriented β-crystals upon processing. In this project, we plan to design the die tunnel of extruder which can manipulate the crystallization process of PP melt with β-nucleating agent during extrusion in two stages, in order to realize the engineering production of oriented β-crystals. After β-nucleating agent demonstates the needlike state, the first stage is utilizing strong flow field to orient needlike β-nucleating agent crystals and PP molecular chains; while the second stage is making α-oriented row nuclei induced by the oriented PP molecular chains relax under high temperature and weak flow field, but maintain the orientation of needlike β-nucleating agent crystals. Thus, the negative effect of α-oriented row nuclei on the content of oriented β-crystals can be avoided. At the meantime, abundant oriented β-crystals epitaxially grow on the surface of well-oriented needlike β-nucleating agent crystals. In this project, we attempt to systematically investigate the formation of oriented β-crystals under flow field, the engineering fabrication of oriented β-crystals, and relationship between structure and properties of oriented β-crystals. The results are promising to give some valuable theory guidelines and technical supports for manipulating the structures of PP and realizing the application in microporous membrane.

β晶是聚丙烯(PP)热动力学亚稳晶型，具有优异韧性、断裂伸长率和成孔特性，只在特定方法下才能生成，其中添加β成核剂是获得大量β晶最有效方法。因β晶在拉伸应力下会向稳定晶型α晶转变，无法使用分子链取向方法获得取向β晶。迄今为止，尚未有实际成型加工下制备大量取向β晶的报道。本项目拟对挤出机口模流道进行设计分两阶段控制含β成核剂的PP结晶过程，实现取向β晶的工程制备，即在β成核剂形成针状晶体后，首先在较高温度下施加强流动场使针状β成核剂晶体取向排列，此时PP分子链也发生取向，然后降低流动场强度且保持较高温度，保存针状β成核剂晶体取向的同时使PP取向分子链诱导生成的α取向晶核解取向，从而可消除α取向晶核对β晶含量的不利影响并诱导大量高度取向β晶生成。本项目拟系统研究流动场下取向β晶的形成，取向β晶工程实现及取向β晶结构与性能关系，为PP制品结构形态调控及在微孔薄膜中的应用提供理论依据与技术指导。

聚丙烯(PP)的β晶型可赋予PP优异的韧性、断裂伸长率和成孔特性，但由于β晶是热动力学亚稳晶型，需在特定条件下才能获得。因此，β晶的形成调控一直是聚丙烯高性能化、多功能化与合金化研究的热点与难点。其中，取向β晶的制备则是难点中的难点。由于β晶的不稳定性，常用的拉伸分子链获得取向晶体的方法会使β晶向α晶转变，无法获得取向β晶。.本项目提出了“同时取向-选择松弛”的两步法，成功实现了成型加工中聚丙烯取向β晶的大量制备，即：先采用适宜的加工温度在聚丙烯熔体中获得针状β成核剂，再施加强流动场使针状β成核剂晶体与聚丙烯分子链取向排列，然后降低流动场强度且保持高温，目的是保存针状β成核剂晶体取向的同时使取向的聚丙烯分子链解取向，消除取向分子链对于取向β晶生成的抑制作用，从而获得大量高度取向β晶。本项目研究了温度场、流动场及时间场等对于取向β晶形成的影响，并调控注塑成型工艺制备出含大量高度取向β晶的聚丙烯注塑制品，并对该制品在拉伸应力下的结构演变进行了系列研究。通过上述工作，揭示了流动场下取向β晶形成规律；协调流动场与β成核剂的共同作用诱导大量取向β晶生成；在注塑成型过程中实现了取向β晶的工程制备，获得了即增强又增韧的注塑制品；阐明了拉伸应力下取向β晶形变机理，即“应力诱导晶体破碎-重结晶”，且重结晶起始点为屈服点。本项目为在聚丙烯中制备大量取向β晶提供新思路和新方法，为聚丙烯制品的结构形态控制提供理论依据，为聚丙烯材料高性能化奠定理论基础，也可为制备锂离子电池用高性能聚丙烯多孔隔膜提供技术指导。项目执行期间在Macromolecules, The Journal of Physical Chemistry B，Polymer等国内外核心期刊及会议上发表学术论文17余篇，SCI收录17篇，EI收录16篇，准备申请发明专利2项。