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

稀疏的长链分支，LCB（侧链附在聚合物主链上），即分支水平通常小于每1000个主链碳原子一个分支，以及臂分子量，Ma，显著大于缠结的临界分子量，Me，已被报道对聚烯烃的流变学和加工行为有显著影响。例如，平均每三个链中有一个分支，对聚乙烯（PE）的流变性能产生与具有不同类型（长和短）的多个分支、高分子量和宽分子量分布相似的影响。本研究的目的是为已知聚合动力学的熔体的流变反应和相关处理性能开发一个定量理论。换句话说，需要预测聚合条件和催化剂结构，这将导致产生所需的流变性、加工性能和性能所需的分子结构。此外，期望评估LCB在使超高分子量树脂熔融可加工方面的使用，为生产具有许多应用的高性能材料打开大门。假肢、弹道防护、涂料等)。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，将需要高度跨学科的努力，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（这些专业知识无法在任何一个地方找到）。来自美国两所大学（弗吉尼亚理工大学和田纳西大学）的科学家们在拉伸和非线性流变学、流动双折射、聚合物加工、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)大学的科学家们合作。研究工作将利用基于利兹的微尺度聚合物加工（MuPP）联盟，该联盟的贡献来自利兹（分子流变学，反应动力学），达勒姆和帝国理工学院（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）。伦敦帝国理工学院（Imperial College, London）的研究小组正在加入这一合作项目，他们在聚合催化剂开发方面具有专门知识，可用于定制分子结构。利兹大学的科学家们在分支聚合物分子理论的发展方面处于世界领先地位。一般的方法是使用模型系统来建立流变标准，通过该标准来识别商业生产的pe中存在的结构（标准分析技术无法提供所需的信息），然后开发聚合动力学，分子结构和处理性能之间的相关性。为设计用于加工性能的聚烯烃和其他树脂的分支拓扑结构以及建立生成该范围拓扑结构的聚合条件奠定了基础，这在以前的分支聚合物中是没有做过的。此外，该计划将为正在发展的支化聚合物动力学基础研究计划提供资金。分支聚合物熔体流变性的分子理论的改进也将是这一合作研究努力的结果。这项研究将具有实际意义，因为它将允许工业优化分子结构的加工性能和理论上的性质，从而缩短时间密集的实验程序。研究生将直接学习分子流变学的最新发展及其在设计分子加工性能方面的应用。小组之间的交流将加速本项目中使用的理论和实验技术的学习。从代表性不足的群体中精心挑选的本科生将被带入该计划，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.