Role of Chain Structure and Melt Topology in Polymer Crystallization

非技术总结：大多数商业聚合物是通过高温冷却加工成有用材料的。在这个过程中，一小部分长聚合物分子在有序的晶体区域中自组装，这些区域由分子保持无序的区域连接。凝固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过程中所采用的缠结（拓扑）。在熔体中保留有序状态记忆的分子就像种子一样加速结晶。本项目研究了短分支或长分支、极性分支或非极性分支聚合物中种子熔体结构的变化及其在结晶中的作用。对模型共混物和具有垂坠基团的聚乙烯的研究，无论是随机的还是精确的距离，都将有助于建立分子结构和性能之间的基本关系，从而预测商业复杂聚乙烯材料的行为。这些研究意义重大的另一个原因是，页岩气开采的大规模扩张，使美国成为聚乙烯等衍生品的极具竞争力的供应国。因此，在了解聚乙烯熔体和结晶状态方面取得进展具有重大意义，因为它们可能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，其中许多是代表性不足的少数民族。技术概述：该研究计划建立在随机乙烯共聚物结晶的强熔体记忆和精密卤素取代聚乙烯的独特分层结晶的最新发展基础上，进一步解决链结构在具有各种类型的短链和长链分支的半结晶聚合物的演变和最终晶体形态中的作用。随机共聚物的模型将用于解决：(a)分支长度（乙基到己基），链结构（星形，h型，梳状，绒球状）和分支极性（乙氧基）对熔体扩散和自成核的影响；(b)共聚单体含量分布对模型共结晶的影响，以及层序偏析与LLPS的相互作用；(c)序列选择在结晶熔体记忆中的作用的普遍性。第二个重点是促进对沿主链具有相同等距长度的垂基团的聚乙烯晶体状态演变的理解。利用具有可控制取向的悬垂基团的模式体系，探索等规体系和无规体系形成片层晶的起源和驱动。层下结构和结晶动力学的差异是特别有趣的，因为它们涉及到由具有控制战术的类乙烯共聚物形成的新型晶体，这在经典的无规乙烯共聚物中是无法获得的。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出来。

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.