Role of Chain Structure and Melt Topology in Polymer Crystallization

非技术摘要：大多数商业聚合物通过从高温冷却而加工成有用的材料。在这个过程中，一部分长的聚合物分子在有序的结晶区域中自组装，这些区域由分子保持无序的区域连接。固化过程的速度取决于熔体的液相结构和长聚合物分子在加热期间采用的缠结（拓扑结构）。在熔体中保持有序状态记忆的分子充当种子以加速结晶。本项目主要研究短支链或长支链、极性支链或非极性支链的聚合物中，晶种熔体结构的变化及其对结晶的影响。对模型共混物和具有侧基的聚乙烯的研究，无论是随机的还是精确的距离，都将能够建立分子结构和性能之间的基本关系，以预测商业复杂聚乙烯材料的行为。这些研究意义重大，还因为页岩气开采的巨大扩张使美国成为聚乙烯等衍生物的极具竞争力的供应商。因此，在理解聚乙烯的熔融和结晶状态方面的进展具有重要意义，因为它们可以导致改进加工技术并降低成本和能源利用。拟议的研究还旨在为研究生和本科生提供学习和培训的机会，其中许多人代表性不足的少数民族。技术摘要：拟议的研究计划建立在最近的发展上的强熔体记忆的无规乙烯共聚物的结晶，和独特的精密层状结晶卤素取代聚乙烯，以进一步解决链结构的作用，在进化和最终的晶体形态的半结晶聚合物与各种类型的短链和长链支化。无规共聚物的模型将用于解决：（a）分支长度的影响（乙基至己基），链结构（B）共聚单体含量分布对模型共混物中共结晶的影响，以及序列分离和LLPS之间的相互作用;（c）晶体熔体记忆中序列选择作用的一般性。第二个推力的目的是推进理解的聚乙烯的结晶状态的演变与侧基放置在相同的等距长度沿着骨干。利用模型系统的侧基与控制立构规整度，起源和驱动器的形成的层状晶体从全同立构以及从无规立构系统将寻求。亚层状结构和结晶动力学的差异是特别感兴趣的，因为它们涉及到新的微晶形成的乙烯类共聚物与控制的立构规整度，是不可访问的经典无规乙烯共聚物。晶体生长前沿的分子细节将从生长速率动力学的独特温度梯度中提取。

NON-TECHNICAL SUMMARY:Most commercial polymers are processed into useful materials by cooling them from a high temperature. During this process, a fraction of the long polymer molecules self-assemble in ordered crystalline regions connected by regions where the molecules remain disordered. The speed of the solidification process depends on the liquid phase structure of the melt and the entanglements (topology) that long polymer molecules adopt during heating. Molecules that retain memory of the ordered state in the melt act as seeds to speed up the crystallization. This project addresses how the seeded melt structure changes in polymers with short or long branches, or with polar or non-polar branches and their role on crystallization. Studies of model blends, and polyethylenes with pendant groups, either random or at a precise distance will enable building of fundamental relations between molecular structure and properties to predict the behavior of commercial complex polyethylene materials. These studies are significant also because the great expansion of shale gas extraction has placed the US as a very highly competitive provider of derivatives such as polyethylenes. Hence, advances in the understanding of the state of the melt and the crystallization of polyethylenes are of major relevance as they could lead to improved processing technologies and to reduced costs and energy utilization. The proposed research is also aimed at providing learning and training opportunities for graduate and undergraduate students, many of them underrepresented minorities.TECHNICAL SUMMARY:The proposed research program builds on recent developments on the strong melt memory of crystallization of random ethylene copolymers, and the unique layered crystallization of precision halogen substituted polyethylenes to further address the role of chain structure in the evolution and final crystal morphology of semicrystalline polymers with various types of short and long-chain branching. Models for random copolymers will be used to address: (a) the effect of branch length (ethyl to hexyl), chain architecture (stars, H-type, comb, pom-pom), and branch polarity (acetoxy) on melt diffusion, and self-nucleation; (b) the effect of comonomer content distribution on co-crystallization in model blends, and the interplay between sequence segregation and LLPS; (c) the generality of the role of sequence selection in crystalline melt memory. A second thrust aims to advance understanding of the evolution of the crystalline state of polyethylenes with pendant groups placed at the same equidistant length along the backbone. Taking advantage of model systems with pendant groups with controlled tacticity, the origin and drive for the formation of lamellar crystallites from isotactic as well as from atactic systems will be sought. Differences in sublamellar structure, and crystallization kinetics are of special interest as they relate to novel crystallites formed from ethylene-like copolymers with controlled tacticity that are not accessible in classical random ethylene copolymers. Molecular details at the crystal growth front will be extracted from the unique temperature gradient of growth rate kinetics.