Covalent Polymer Mechanochemistry

有机聚合物材料，或塑料，由于它们在使用周期中经历的机械力而分解。聚合物的机械降解限制了它们在轻质结构材料、消费品和生物医学应用中的应用。杜克大学（Duke University）的斯蒂芬·克雷格（Stephen Craig）教授正在研究如何控制被施加的机械力加速的化学反应的速率和结果。这些反应影响了材料设计的多个方面，包括当前聚合物材料的宏观失效和机械限制。此外，机械响应官能团可能成为新型应力响应和自修复聚合物材料的关键元素。Craig教授的研究旨在深入了解聚合物在使用过程中所经历的宏观机械力如何有效地转化为所需的化学反应，为新型聚合物提供基础。该项目的更广泛影响包括：(1)通过一个新的区域技术培训计划，支持和丰富主要由本科院校、社区学院、高级高中和小型研究型大学提供的研究活动和科学培训经验，将研究和教育结合起来；(2)通过本科和高中的研究经验，在化学入门课程中主动学习模块和相关的实验室经验；(3)在科学领域出现不成比例的人员流失之前，通过在年轻科学家的科学职业生涯早期吸引和招募他们，扩大代表性不足群体的参与；(4)广泛传播研究成果；(5)解决材料分子行为的基本问题，这些问题将对包括聚合物化学、物理有机化学、自愈和应力响应材料在内的广泛领域产生影响。总体技术目标是通过采用最先进的物理测量和开发新的定量方法，为机械化学动力学奠定定量基础。具体的实验包括直接的、实验的表征和定量的机械力对沿着拉伸聚合物骨架引发的共价反应的影响。因为机械力不像传统形式的能量输入，如热或光，是定向的，机械力和反应性之间的耦合提供了对过渡态结构和反应势能面形状的见解。然而，尽管力对化学反应的影响很重要，但对它的定量测量却很少。本项目旨在开发两种量化机械化学反应性的新方法：用原子力显微镜拉多机械团不可剪聚合物的单分子，以及多机械团不可剪聚合物的脉冲超声和分子量降解。观察到的机械化学活性的模型可以定量评估这两类机械团的反应性。该奖项反映了美国国家科学基金会的法定使命，并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，被认为值得支持。

Organic polymeric materials, or plastics, break down due to the mechanical forces they experience during their use cycles. The mechanical degradation of polymers limits their use in lightweight structural materials, consumer products, and biomedical applications. Prof. Stephen Craig at Duke University is learning how to dictate the rates and outcomes of chemical reactions that are accelerated by an applied mechanical force. These reactions impact multiple aspects of materials design, including the macroscopic failure and mechanical limitations of current polymeric materials. In addition, mechanically responsive functional groups might serve as the critical elements in new stress-responsive and self-healing polymeric materials. Prof. Craig's studies aim to provide insight into how the macroscopic mechanical forces experienced by polymers during use can be effectively channeled into desired chemical responses, providing a foundation for new classes of polymers. Broader impacts of the project include: (1) integration of research and education through a new, regional technical training program that supports and enriches the research activities and scientific training experiences provided by primarily undergraduate institutions, community colleges, advanced high schools, and smaller research universities; (2) active learning modules and associated laboratory experiences in introductory chemistry and through coupled undergraduate and high school research experiences; (3) broadening the participation of underrepresented groups by engaging and recruiting young scientists early in their scientific careers, before the onset of disproportionate attrition from the sciences; (4) disseminating the results of the research broadly; and, (5) addressing fundamental questions of molecular behavior in materials in a manner that will have an impact on a broad range of fields including polymer chemistry, physical organic chemistry, and self-healing and stress-responsive materials. The overarching technical objective is to lay a quantitative foundation for mechanochemical kinetics by employing state-of-the-art physical measurements and developing new methods for quantitation. Specific experiments include the direct, experimental characterization and quantification of the effect of mechanical forces on covalent reactions triggered along overstretched polymer backbones. Because mechanical force, unlike conventional forms of energy input such as heat or light, is directional, the coupling between mechanical force and reactivity provides insights into the structure of transition states and the shapes of reaction potential energy surfaces. Despite its importance, however, quantitative measures of the effect of force on chemical reactions are rare. This project aims to develop two novel approaches to quantifying mechanochemical reactivity: pulling on single molecules of multi-mechanophore, non-scissile polymers with an atomic force microscope, and the pulsed sonication and molecular weight degradation of multi-mechanophore, scissile polymers. Models for the observed mechanochemical activity enable quantitative assessment of reactivity in both classes of mechanophores.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.