Phase Behavior and Physical Properties of Thermotropic and Lyotropic Liquid Crystal Oligomers

摘要：液晶以其在智能手机屏幕和平板电视等信息显示中的应用而闻名。它们也是一种迷人的物质状态，其基本理解和技术潜力继续构成有趣的挑战和机遇。虽然人们对哪种分子或分子组合倾向于形成液晶已经了解得很多，但关于这些成分的特定结构如何决定它们所表现出的液晶状态的性质和性质，还有一些重要的开放性问题。该项目探索了由“低聚物”形成的液晶状态，这些“低聚物”是通过柔性连接将少量细长的刚性部件连接在一起而形成的。该项目的一个重点是由柔性单链连接的双链DNA（双链DNA）的短刚性片段组成的低聚物。这里的目的是阐明在这些结构的浓水溶液中形成的液晶状态，包括在完全配对的双链溶液中未观察到的层状结构，这可能为模拟拥挤的DNA（生理）环境提供一种新的手段。第二个重点是研究更小的“刚性+柔性”低聚物的液晶性质，这些低聚物是由类似于显示器中使用的分子组成的。这些材料为调整某些粘弹性特性提供了新的可能性，这可能会改善现有的液晶装置或激发全新的液晶装置。在不同的微观长度尺度上表达相似结构的两种低聚体系的平行研究应该加强对这些结构如何决定宏观材料性质的见解。该项目培养研究生和本科生，跨学科（物理，生物化学，材料科学），在21世纪的科学劳动力生产的职业生涯。学生有充足的机会掌握广泛的实验技术，无论是在“台式”实验室设置和大型美国国家实验室。技术摘要：本项目通过实验研究了两种不同的分子体系，其中将少量刚性亚基与柔性连接器连接可以促进新的液晶（LC）相行为或显着改变熟悉的LC相的宏观物理性质。热致性LC低聚物由两个或多个由柔性间隔物连接的小而刚性的分子元素组成，其特定的排列有利于弯曲或螺旋构象。这些低聚物在主要由焓变化驱动的LC态之间发生转变。溶性核酸基lc由单链“间隙”连接的DNA双链或含有额外的、未配对的碱基内含物（“结”）组成，也具有“刚性+柔性”的低聚物样结构，其长度约为热致性材料的10倍。DNA结构表现出主要受熵影响的浓度依赖的中间阶段。该项目的主要目标是：(1)测量热致性LC低聚物的取向粘弹性参数，重点是将结果从二聚体扩展到更高n-聚合物，并将其相对大小和温度依赖性与这些低聚物形成的纳米级调制相（如“扭转弯曲”相）的性质联系起来；(2)研究低聚物在LC液滴和准二维独立LC膜自由表面的定向锚定作用；(3)确定由于“间隙”和“结”的选择性放置而形成寡聚物样结构的DNA双链在浓缩溶液中所表现出的中间相结构；(4)有选择地探索某些潜在的技术应用领域，从响应式光学设备到模拟拥挤的生理DNA环境的新方法，再到病毒DNA/RNA检测的概念。实验在PI和合作PI的实验室以及美国国家实验室进行，并利用包括激光散射、小角和广角x射线散射、高灵敏度荧光检测、高场磁光学和“点击化学”方法来组装DNA结构。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 Abstract:Liquid crystals are best known for their use in information displays like smartphone screens and flat-panel televisions. They are also a fascinating state of matter whose fundamental understanding and technological potential continue to pose interesting challenges and opportunities. While much is known about what kinds of molecules or molecular assemblies tend to form liquid crystals, there are important open questions about how the specific architecture of these constituents determines the nature and properties of the liquid crystalline states they exhibit. This project explores liquid crystalline states formed by "oligomers" that are constructed by connecting together a small number of elongated, rigid components via flexible linkages. One thrust of the project focuses on oligomers composed of short, rigid segments of double-stranded DNA (duplex DNA) linked by flexible, single strands. Here the objective is to elucidate the liquid crystalline states formed in concentrated aqueous solutions of these constructs, including layered structures that are not observed in solutions of fully paired duplexes and that may offer a new means to simulate crowded DNA (physiological) environments. A second thrust investigates the liquid crystalline properties of much smaller "rigid+flexible" oligomers built up from molecules similar to the types used in displays. These materials afford new possibilities to tune certain viscoelastic properties, which could improve existing liquid crystal devices or motivate entirely new ones. The parallel study of the two oligomeric systems with similar architectures expressed on different microscopic length scales should enhance insights into how these architectures determine macroscopic material properties. The project trains students at graduate and undergraduate levels, across disciplines (physics, biochemistry, materials science), for productive careers in the 21st century scientific workforce. The students have ample opportunities to master a broad spectrum of experimental techniques both in the "benchtop" laboratory setting and at large US national laboratories.Technical Abstract:This project experimentally investigates two distinct molecular systems where linking a small number of rigid subunits with flexible connectors promotes new liquid crystalline (LC) phase behavior or significantly alters macroscopic physical properties of familiar LC phases. Thermotropic LC oligomers consist of two or more small, rigid molecular elements connected by flexible spacers, whose specific arrangement can favor bent or helical conformations. These oligomers undergo transitions between LC states driven primarily by changes in enthalpy. Lyotropic, nucleic acid-based LCs, composed of DNA duplexes linked by single-stranded "gaps" or containing extra, unpaired base inclusions ("kinks"), also possess a "rigid+flexible" oligomer-like architecture, at ~10 times the length scale of the thermotropic materials. The DNA constructs exhibit concentration-dependent mesophases influenced primarily by entropy. The major objectives of the project are: (1) To measure orientational viscoelastic parameters of thermotropic LC oligomers, with emphasis on extending the results from dimers to higher n-mers, and to correlate their relative magnitudes and temperature dependencies with the nature of nanoscale modulated phases, such as the "twist-bend" phase, these oligomers form; (2) To examine the orientational anchoring of oligomers at the free surfaces of LC droplets and quasi-two-dimensional, freestanding LC films drawn from n-mer/monomer mixtures; (3) To determine the structure of mesophases exhibited in concentrated solutions of DNA duplexes that form oligomer-like constructs due to the selective placement of "gaps" and "kinks"; and (4) To explore, selectively, certain potential technological applications in areas ranging from responsive optical devices to new approaches for simulating crowded, physiological DNA environments to a concept for viral DNA/RNA detection. Experiments are performed in the PI and co-PIs' laboratories and at US National Labs, and utilize techniques including laser light scattering, small- and wide-angle X-ray scattering, high sensitivity fluorescence detection, magneto-optics in high fields, and "click chemistry" methods to assemble the DNA constructs.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.