高分子材料在当今社会应用广泛，其中超过三分之二是可结晶性的。可结晶性高分子的工业加工中普遍存在流动诱导结晶（FIC）现象。流动场会数量级地加速结晶动力学，并诱导产生取向结构的晶体。串晶是典型的FIC结构，预期能改善高分子材料的机械性能和热稳定性，其结构上由中心shish晶核与其上附生的片晶两部分构成。揭示串晶的微观形态与形成机理有助于推动FIC分子理论发展，对研究与改善高分子结晶产品性能具有指导意义。 本论文旨在从分子链行为的角度探究串晶结构控制机理。样品为系列轻度交联高密度聚乙烯（XL-HDPE），其在熔体状态被拉伸以诱导结晶，测量方法主要是原位小角X射线散射（SAXS）。我们首先建立串晶结构的定量分析方法，然后以此研究链构象对shish成核的影响，以及链扩散与消耗对片晶生长动力学控制机制的影响。 分析方法研究方面，我们利用球谐函数展开分解SAXS谱，构建综合的几何模型来定量分析FIC的串晶结构。基于该方法，我们发现在高应变下半结晶结构存在不均匀性。此外，随应变增加，多分散度减小的现象标志着成核过程从散现成核向瞬时成核的转变。 shish成核研究方面，我们探究了网络与自由链构象对shish成核的影响及“聚合物变形-shish成核-最终串晶结构”之间定量的链式关系。结果揭示了网络拉伸对shish形成的诱导作用，网络变形对shish生长的几何约束，以及自由链构象对成核类型与shish长度的显著影响。特别地，shish的横向有序性可以诱导互锁的串晶结构，该结构预期具有超高模量。 片晶生长研究方面，我们发现其依次经历界面控制、扩散控制和奥氏熟化机制主导的晶体完善三个阶段。进一步的分析表明链扩散与消耗在不同片晶生长阶段的转变方面发挥了关键作用。
Polymeric materials have been applied in a wide range of industrial fields, and more than two-thirds of them are semicrystalline. It is known that the flow-induced crystallization (FIC) phenomenon is ubiquitous in polymer processing methods. The external flow can enhance crystallization kinetics of polymers in orders of magnitude and induce oriented structures. Among various studies of FIC, the shish-kebab superstructure gains the most attention, which is expected to significantly improve the mechanical properties and thermal stability of polymeric materials. A shish-kebab unit consists of a long central fibrillar backbone (shish) surrounded by chain-folded lamellar crystals (kebabs). The research on the microscopic morphology and control mechanism of shish-kebab structure, on the one hand, is crucial for unveiling the molecular mechanism of FIC and developing relevant theories. On the other hand, it is meaningful for the rational design and adjustment of the semicrystalline structure, which is important in producing high-performance polymer products. In this work, we systematically study the FIC of lightly crosslinked high-density polyethylene melts. In situ synchrotron-radiation small-angle X-ray scattering (SAXS) complemented by rheometry is employed as the major experimental tool. To understand the microscopic control mechanism of shish-kebab structure from the perspective of chain behaviors, we first establish a method to quantify the shish-kebab structure, and then investigate the effect of chain conformation on shish nucleation and the effect of chain diffusion and consumption on the kinetic control mechanism of lamellar growth. For methodological research, we systematically analyze the detailed structure in the FIC by combining the SAXS measurement and the spherical harmonic expansion (SHE) method. With a comprehensive geometrical model, we determine the morphological parameters of shish-kebab structures. Particularly, we observe the existence of heterogeneity in semicrystalline structure at high strains. Moreover, the lamellar polydispersities show a diminishing behavior as strain increases, indicating a transition of nucleation process from the sporadic way to the instantaneous way. For shish nucleation, we investigate the role of chain conformation in flow-induced shish nucleation and the quantitative relation of "polymer deformation ? shish nucleation ? final shish-kebab structure". The results reveal the induction of network stretching on shish formation, the constraint of network geometry on shish growth, and the effect of free chain conformation on nucleation type and shish length. In addition, it is found that the lateral ordering of shish induces the interlocked shish-kebab structure, which is a candidate for polymeric materials with ultra-high modulus. For lamellar growth, we find that it sequentially undergoes three stages, i.e., the interface-controlled stage, the diffusion-controlled stage, and the crystal perfection stage through Ostwald ripening mechanism. Further analyses suggest that the chain diffusion and consumption play a critical role in determining the transitions between different growth stages.
