CAREER: Solid-state molecular motion, reversible covalent-bond formation, and self-assembly for controlling thermal expansion behavior

在实际环境中使用的材料经常暴露在温度变化中。对于户外应用中使用的材料，如混凝土，这通常是由于天气或季节变化。对于用于诸如计算机或电子设备的材料，这是由于产生热量的多余能量。热膨胀是材料对温度变化的反应。材料对温度的反应方式会影响其功能。如果不了解和控制材料的热膨胀行为，则很可能由于温度波动而发生失效或断裂。分子的化学结构和将固体材料结合在一起的键通常决定了热膨胀行为。像混凝土这样的材料的行为是很容易理解的；然而，有机（碳）基材料的类似行为预测和设计更具挑战性，因为这些材料通过较弱的力结合在一起。有机材料正越来越广泛地应用于电子等各个领域。平衡高材料性能与理想的热膨胀是这类应用的关键。该CAREER项目由材料研究部固态与材料化学项目支持，旨在开发控制有机材料热膨胀行为的基本策略。具体来说，热膨胀是通过使用动态基团来影响的，动态基团通过运动或通过建立和破坏将材料结合在一起的键来响应温度变化。本项目制定的战略预计将影响具有可预测热膨胀性能的新型材料的设计和制备，用于促进国家繁荣的技术应用。与研究计划相结合的是一个全面的教育、指导和推广计划，涉及从中学到研究生院的每个教育阶段的弱势群体。这些活动包括(1)在Tech Savvy上开发和实施关于“我们周围的热膨胀”的年度报告——一个面向中学女生的STEM研讨会，(2)一个针对西德克萨斯州高中生的关于分子结构和固态特性的旅行实验室实验，以及(3)一个面向高年级本科生和研究生的STEM职业准备研讨会。该职业项目由材料研究部固态和材料化学项目支持，通过合成具有可控和可预测热膨胀行为的新型动态固态材料，对有机结晶固体中的热膨胀（TE）行为有了基本的了解。TE是材料对温度变化的响应。分子的化学结构和将固体结合在一起的相互作用通常决定了TE的行为。然而，其他机制，如结构灵活性或运动可以引起意想不到的或独特的TE。对于无机或共价网络固体，分子间的作用力很强，结构组装在三维空间中得到很好的控制，并且TE通常是可预测的。另一方面，纯有机分子固体通过较弱的非共价相互作用在三维空间中结合在一起。指导单个有机分子的自组装成固体结构，并完全控制包括所有晶体尺寸的非共价相互作用是具有挑战性的。非共价力、运动和柔韧性都影响有机分子固体中的TE。可靠地指导、实现和控制固态运动、自组装以及预测它们对TE的影响仍然具有挑战性。本CAREER项目通过以下途径开发控制和调整有机分子固体TE的基础知识和系统策略：(1)安装能够进行固态分子运动并可靠地打开和关闭运动的官能团；(2)使用可逆的固态共价键形成反应在单个固体中在大TE行为和接近零TE行为之间切换。(3)利用正交非共价相互作用控制有机分子在所有三个晶体维度上的自组装。这项工作有望推进固态运动、反应性、自组装和TE的基础知识，并改变具有动态特性的功能固态材料的设计。教育和推广活动强调通过让学生在从中学到研究生院的每个教育阶段参与STEM活动，以STEM为动力的教育和职业准备方法。这是通过以下方式实现的：(1)在Tech Savvy上开发和实施关于“我们周围的热膨胀”的年度报告——一个针对中学女生的STEM研讨会，(2)一个针对西德克萨斯州高中生的关于分子结构和固态特性的旅行实验室实验，以及(3)一个针对高年级本科生和研究生的STEM职业准备研讨会。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。

Non-Technical SummaryMaterials used in real-world settings are frequently exposed to changes in temperature. For materials used in outdoor applications such as concrete, this is typically due to weather or seasonal changes. For materials used in devices such as computers or electronics, this is due to excess energy that results in heat. Thermal expansion is the response of a material to any change in temperature. The way a material responds to temperature impacts its ability to function. If the thermal expansion behavior of a material is not understood and controlled, failure or fracture is likely to occur as a result of temperature fluctuations. The chemical structures of the molecules and the bonds that hold a solid material together typically dictate the thermal expansion behaviors. The behaviors of materials like concrete are well-understood; however, analogous behaviors for organic (carbon)-based materials are more challenging to predict and design because these materials are held together by weaker forces. Organic materials are becoming more widely used in a variety of fields such as electronics. Balancing high material performance with ideal thermal expansion is critical to such applications. This CAREER project, supported by the Solid State and Materials Chemistry Program in the Division of Materials Research, develops fundamental strategies for controlling the thermal expansion behaviors of organic materials. Specifically, thermal expansion is influenced through the use of dynamic groups, which respond to temperature changes by undergoing motion or by making and breaking the bonds that hold the material together. The strategies developed in this project are expected to influence the design and preparation of novel materials with predictable thermal expansion properties for use in technological applications that advance national prosperity. Integrated with the research plan is a holistic education, mentorship, and outreach program involving underrepresented groups at each education stage from middle school through graduate school. The activities include (1) development and implementation of an annual presentation on 'Thermal Expansion Around Us' at Tech Savvy – a STEM workshop for middle school girls, (2) a traveling lab experiment on molecular structures and solid-state properties for high school students in West Texas, and (3) a STEM career preparation workshop for upper-level undergraduate and graduate students.Technical SummaryThis CAREER project, supported by the Solid State and Materials Chemistry Program in the Division of Materials Research, develops a fundamental understanding of thermal expansion (TE) behaviors in organic crystalline solids through synthesis of novel, dynamic solid-state materials with controllable and predictable TE behaviors. TE is the response of a material to a change in temperature. The chemical structures of the molecules and the interactions that hold the solid together typically dictate TE behavior. However, other mechanisms such as structural flexibility or motion can give rise to unexpected or unique TE. For inorganic or covalent network solids, intermolecular forces are strong, structural assembly is well-controlled in three dimensions, and TE is often predictable. On the other hand, purely organic molecular solids are held together in three dimensions by weaker, noncovalent interactions. Directing self-assembly of individual organic molecules into a solid structure with full control over the noncovalent interactions comprising all crystallographic dimensions is challenging. Noncovalent forces, motion, and flexibility all affect TE in organic molecular solids. Reliably directing, achieving, and controlling solid-state motion, self-assembly, and predicting their influence on TE remains challenging. This CAREER project develops fundamental knowledge and systematic strategies for controlling and tuning TE in organic molecular solids through (1) installation of functional groups capable of undergoing solid-state molecular motion and reliably turning motion on and off, (2) use of reversible solid-state covalent-bond-forming reactions to switch between large and near zero TE behaviors within a single solid, and (3) control over self-assembly of organic molecules in all three crystallographic dimensions using orthogonal noncovalent interactions. The work is expected to advance fundamental knowledge of solid-state motion, reactivity, self-assembly, and TE, and transform the design of functional solid-state materials that exhibit dynamic properties. The educational and outreach activities emphasize a STEM-powered approach to education and career preparation by engaging students in STEM activities at each stage of education from middle school through graduate school. This is achieved through (1) development and implementation of an annual presentation on 'Thermal Expansion Around Us' at Tech Savvy – a STEM workshop for middle school girls, (2) a traveling lab experiment on molecular structures and solid-state properties for high school students in West Texas, and (3) a STEM career preparation workshop for upper-level undergraduate and graduate students.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.