Tie Chains in Semicrystalline Homopolymers and Copolymers

第1部分：非技术总结为什么石蜡易碎，而髋关节置换术中使用的聚合物坚韧耐用？ 两者都是碳氢化合物链分子--“聚乙烯”的变种。 关键的区别在于，髋关节置换术中使用的材料（沿着用于耐用煤气和水管的其他品种的聚乙烯）含有足够长的分子链，可以跨越材料的微观晶体之间，形成将材料固定在一起的“系链”。 虽然这个例子清楚地表明，聚合物需要超过一定的链长，以拥有韧性和延展性，令人惊讶的是，它是不知道“多长时间是足够长？“--阈值也取决于材料的微观结构特征，例如这些微观晶体之间的距离。 该项目将通过合成严格控制链长和分子结构的聚合物，建立半结晶聚合物延展性的定量标准，并研究当材料变形时这些连接分子如何改变形状。 这些定量标准将有助于优化的半结晶聚合物（聚乙烯只是其中一个例子）的设计和加工：例如，其中聚合物链足够长，使得材料在使用过程中不会断裂或碎裂，但是足够短，使得材料可以使用最小的能量进行处理。 该项目将为研究生，本科生和高中生，包括代表性不足的群体的成员提供综合研究和教育经验。 首席研究员和学生还将通过在校园内外广泛宣传的活动中进行科学宣传，吸引公众参与。 第二部分：半结晶聚合物的延展性和韧性来自于连接链：分子足够长，可以桥接两个或多个晶体。 尽管有这种定性的理解，今天我们不知道先验的最小大分子尺寸需要赋予延展性的半结晶聚合物，即使其固态结构的所有方面（如微晶和无定形层厚度）是已知的。 该项目将使用最近开发的开环易位聚合路线来合成基于线性聚乙烯和氢化聚异戊二烯的定义明确的半结晶均聚物、统计共聚物和嵌段共聚物。 在一个推力，检查线性均聚物的小角中子散射将允许的大分子（单链）的变形发生在结链超过屈服点的评估。 在第二个推力，系统检查的均聚物和统计共聚物的分子量和noncrystallizable共聚单体含量进行调整，将使我们能够阐明所需的延展性的结构特征。 最后，我们将在此基础上，通过引入短玻璃态嵌段，同时将这些聚合物的屈服强度提高到结构（工程）塑料的范围内，同时保持延展性。学生将在项目的所有阶段工作，从合成到表征到属性测量，获得一个大的前景，这将为他们提供一个未来的基础。 项目参与者将在广泛宣传的公共外联活动中吸引公众参与，重点是聚合物材料科学和技术。该奖项反映了NSF的法定使命，并被认为是值得通过使用基金会的知识价值和更广泛的影响审查标准进行评估的支持。

PART 1: NON-TECHNICAL SUMMARYWhy is paraffin wax brittle, while the polymer used in hip replacements is tough and durable? Both are hydrocarbon chain molecules -- varieties of "polyethylene". The key difference is that the material used in hip replacements (along with other varieties of polyethylene used in durable gas and water pipes) contains molecular chains which are long enough to span between the microscopic crystals of the material, creating “tie chains” that hold the material together. While this example shows clearly that polymers need to be above a certain chain length to possess toughness and ductility, surprisingly it is not known "how long is long enough?" -- a threshold which also depends on microstructural features of the material, such as the distance separating those microscopic crystals. This project will establish quantitative criteria for ductility in semicrystalline polymers, by synthesizing polymers of tightly-controlled chain length and molecular structure, and will also investigate how these tie molecules change shape when the material deforms. These quantitative criteria will aid the design and processing of optimized semicrystalline polymers (of which polyethylene is just one example): e.g., where the polymer chains are long enough that the material does not fracture or fragment during use, but short enough that the material can be processed using a minimum of energy. The project will provide an integrated research and educational experience for graduate students, undergraduate students, and high-school students, including members of underrepresented groups. The principal investigator and students will also engage the general public through science outreach at broadly-advertised events, both on campus and off. PART 2: TECHNICAL SUMMARYSemicrystalline polymers derive their ductility and toughness from tie chains: molecules sufficiently long that they can bridge two or more crystals. Despite this qualitative understanding, today we do not know a priori the minimum macromolecular size needed to impart ductility to a semicrystalline polymer, even if all aspects of its solid-state structure (such as the crystallite and amorphous layer thicknesses) are known. This project will use recently-developed ring-opening metathesis polymerization routes to synthesize well-defined semicrystalline homopolymers, statistical copolymers, and block copolymers, based on linear polyethylene and hydrogenated polynorbornene. In one thrust, examination of linear homopolymers by small-angle neutron scattering will allow an assessment of the macromolecular (single-chain) deformation which occurs in tie chains beyond the yield point. In a second thrust, systematic examination of homopolymers and statistical copolymers as molecular weight and noncrystallizable comonomer content are tuned will allow us to elucidate the structural features required for ductility. Finally, we will build on this knowledge to simultaneously raise these polymers' yield strengths into the range of structural (engineering) plastics via incorporation of a short glassy block, while retaining ductility. Students will work across all phases of the project, from synthesis to characterization to property measurement, gaining a big-picture outlook which will serve them well as a future foundation. Project participants will engage the general public at broadly-advertised public outreach events, with a focus on polymeric materials science and technology..This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.