Materials World Network: Molecular Engineering of Polymers for Processing Performance and Properties

稀疏长链支化，LCB（连接到聚合物主链的侧链），即，据报道，通常小于每1000个主链碳原子一个分支的支化水平和显著大于缠结的临界分子量（MW）Me的臂分子量Ma对聚烯烃的流变学和加工行为具有显著影响。例如，被认为平均每三个链中有一个分支的分支可以对聚乙烯（PE）的流变学具有与具有各种类型（长和短）的多个分支、高分子量和宽分子量分布类似的影响。 本研究的目的是发展一个定量的理论流变学响应和相关的加工性能的熔体的聚合动力学。 换句话说，期望预测聚合条件和催化剂结构，其将导致产生期望的流变性、加工性能和性质所需的分子结构。此外，期望评估使用LCB以使超高MW树脂可熔融加工，从而打开生产具有许多应用（例如，修复术、弹道防护、涂层等）的高性能材料的大门。为了从理论上设计聚合物链的分子结构以产生所需的加工性能，需要高度跨学科的努力，其中包括实验和理论流变学、聚合物加工、聚合动力学和催化剂以及聚合物合成和表征方面的专家（这种专业知识在任何一个地方都找不到）。来自美国两所大学（弗吉尼亚理工大学和田纳西大学）的科学家在拉伸和非线性流变学、流动双折射、聚合物加工以及聚合物合成和表征方面具有专业知识，他们将与来自英国（7）、荷兰（1）和希腊（1）大学的科学家联手。这项研究工作将利用利兹的微尺度聚合物加工（MuPP）联盟，来自利兹（分子流变学，反应动力学），达勒姆和伦敦帝国理工学院（化学），谢菲尔德（化学和结晶），剑桥和布拉德福德（小规模加工），牛津和埃因霍温（固态）的贡献。伦敦帝国理工学院的研究小组将加入这一合作项目，他们在聚合催化剂开发方面具有专门知识，可定制分子结构。 利兹的科学家们是支化聚合物分子理论发展的世界领导者。一般的方法是使用模型系统来建立流变学标准，通过该标准来识别商业生产的PE中存在的结构（标准分析技术不能提供所需的信息），然后开发聚合动力学之间的相关性，分子结构和加工性能。设计聚烯烃和其他树脂的支化拓扑结构以获得加工性能和建立聚合条件的基础用于产生这一拓扑范围，这在以前对于支化聚合物还没有进行过。 此外，该计划还将纳入支化聚合物动力学的基础研究计划。 支化聚合物熔体流变学的分子理论的改进也将是这一合作研究努力的成果。 这项研究将具有实际意义，因为它将使工业界能够从理论上优化分子结构以获得加工性能和性能，从而缩短时间密集型实验项目。研究生将直接学习分子流变学的最新发展及其在设计加工性能分子中的应用。小组之间的交流将加速学习该计划中使用的理论和实验技术。从代表性不足的群体中精心挑选的本科生将被纳入该计划，以提高他们对聚合物科学和工程的兴趣。

Sparse long chain branching, LCB(side chains attached to the main polymer backbone), i.e., branching levels typically less than one branch per 1000 backbone carbon atoms, and arm molecular weights, Ma, significantly greater than the critical molecular weight (MW) for entanglements, Me, have been reported to have remarkable effects on the rheology and processing behavior of polyolefins. For example, what is believed to be one branch in every three chains on average can have similar effects on the rheology of a polyethylene(PE) as having multiple branches of various types(long and short), high molecular weight, and a broad molecular weight distribution. The goal of this research is to develop a quantitative theory for the rheological response and associated processing performance of a melt of known polymerization kinetics. In other words, it is desired to predict polymerization conditions and catalyst structure which will lead to the molecular architecture needed to produce the desired rheology, processing performance, and properties. Furthermore, it is desired to evaluate the use of LCB to render ultra-high MW resins melt processable opening the door for producing high performance materials with many applications(e.g. prosthetics, ballistics protection, coatings, etc.).In order to theoretically design the molecular architecture of polymer chains for generating the desired processing performance, a highly interdisciplinary effort will be required which incorporates experts in experimental and theoretical rheology, polymer processing, polymerization kinetics and catalysts, and polymer synthesis and characterization (this expertise cannot be found in any one location). Scientists from two U.S. universities (Virginia Tech and the University of Tennessee) with expertise in extensional and non-linear rheology, flow birefringence, polymer processing and polymer synthesis and characterization will join forces with scientists from English(7), Dutch(1) and Greek(1) universities. The research effort will capitalize on the Leeds-based Microscale Polymer Processing (MuPP) consortium with contributions from Leeds (molecular rheology, reaction kinetics), Durham and Imperial College-London (chemistry), Sheffield (chemistry and crystallization), Cambridge and Bradford (small-scale processing), and Oxford and Eindhoven (solid state). The group at Imperial College, London is joining this co-operative program with expertise in polymerization catalyst development for tailored molecular structure. Scientists at Leeds are the world leaders in the development of molecular theories for branched polymers. The general approach is to use model systems to establish a rheological standard by which to identify the structures present in commercially produced PEs (standard analytical techniques cannot provide the required information) and then develop correlations between polymerization kinetics, molecular architecture and processing performance.The basis for designing the branching topology of polyolefins and other resins for processing performance and establishing polymerization conditions for generating this range of topologies will be established which has not been done before for branched polymers. In addition, this program will feed into the developing fundamental research program on the dynamics of branched polymers. Improvements in the molecular theory for the melt rheology of branched polymers will also be an outcome of this cooperative research effort. This research will have practical implications as it will allow industry to optimize the molecular architecture for processing performance and properties theoretically and thereby shorten time-intensive experimental programs. Graduate students will learn first hand the latest developments in molecular rheology and its use in designing molecules for processing performance. Exchanges between groups will accelerate the learning of both the theory and experimental techniques used in this program. Carefully selected undergraduates from underrepresented groups will be brought into the program to enhance their interest in polymer science and engineering.