Tie Chains in Semicrystalline Homopolymers and Copolymers

为什么石蜡易碎，而用于髋关节置换术的聚合物坚韧耐用？两者都是碳氢链分子——“聚乙烯”的变种。关键的区别在于，用于髋关节置换的材料（以及用于耐用气体和水管的其他种类的聚乙烯）包含足够长的分子链，可以在材料的微观晶体之间跨越，形成“领带链”，将材料连接在一起。虽然这个例子清楚地表明，聚合物需要超过一定的链长才能具有韧性和延展性，但令人惊讶的是，人们并不知道“多长才足够长？”这个阈值也取决于材料的微观结构特征，比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，并将研究这些结合分子在材料变形时如何改变形状。这些定量标准将有助于优化半结晶聚合物的设计和加工（聚乙烯只是其中一个例子）：例如，聚合物链足够长，材料在使用过程中不会断裂或破碎，但足够短，材料可以使用最少的能量进行加工。该项目将为研究生、本科生和高中生提供综合研究和教育经验，包括代表性不足的群体的成员。首席研究员和学生还将通过在校内外广泛宣传的科学推广活动与公众接触。半晶聚合物的延展性和韧性来自于链：足够长的分子，它们可以桥接两个或更多的晶体。尽管有了这种定性的理解，今天我们仍然不能先验地知道赋予半结晶聚合物延展性所需的最小大分子尺寸，即使它的固态结构的所有方面（如晶体和非晶态层厚度）都是已知的。该项目将利用最近开发的开环复分解聚合路线，以线性聚乙烯和氢化聚降冰片烯为基础，合成定义明确的半结晶均聚物、统计共聚物和嵌段共聚物。在一次推力中，通过小角度中子散射对线性均聚物的检查将允许评估在超过屈服点的链中发生的大分子（单链）变形。在第二个推力中，系统地检查均聚物和统计共聚物的分子量和非结晶共聚物的含量将使我们能够阐明延性所需的结构特征。最后，我们将建立在这方面的知识，同时提高这些聚合物的屈服强度到结构（工程）塑料的范围内，通过结合短玻璃块，同时保持延展性。学生将在项目的所有阶段工作，从合成到表征再到属性测量，获得一个大的前景，这将为他们未来的基础服务。项目参与者将在广泛宣传的公众宣传活动中与公众接触，重点关注聚合物材料科学和技术。该奖项反映了美国国家科学基金会的法定使命，并通过基金会的智力价值和更广泛的影响审查标准进行评估，认为值得支持。

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.