Hairpin Rubber Elasticity: Molecular Basis for Cold Drawing in Smectic Elastomers

发夹橡胶弹性：近晶弹性体冷拔的分子基础

• 批准号: 1006815
• 负责人: Ronald Hedden
• 金额: $ 19万
• 依托单位: Texas Tech University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2010
• 资助国家: 美国
• 起止时间: 2010-06-01 至 2013-04-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1006815&HistoricalAwards=false
• 关键词: Hairpin; Rubber; Elasticity; Molecular; Basis

TECHNICAL SUMMARY:Cold-drawing is a well-known phenomenon in both glassy amorphous and semicrystalline polymers, which is seldom observed in rubber-like networks above the glass transition temperature. Cold-drawing and neck formation have recently been reported in smectic main-chain liquid crystalline (LC) elastomers under certain conditions of temperature and elongation rate, however. Cold-drawing in LC elastomers, glassy amorphous polymers, and semicrystalline polymers clearly cannot be attributed to common morphological features, but it may have common origins in conformational transitions at the chain level. This investigation is aimed at elucidating the underlying molecular basis for cold-drawing in polydomain smectic LC elastomers. The necking instability is proposed to arise from strong energetic contributions to the elastic free-energy upon elongation, in contrast to the classical picture of entropy-driven rubber elasticity. Elastomers containing larger, more stable domains are hypothesized to exhibit a larger yield stress and to be more prone to mechanical instability due to an increased energetic penalty for disrupting smectic ordering during elongation. Macroscopic mechanical response of polydomain smectic LCE will be linked with conformational instability at the chain level by X-ray measurements of domain size and small-angle neutron scattering (SANS) measurements of chain dimensions. A pressing fundamental issue underlying all of the work is the concept of hairpins, chain folds by which the backbone reverses direction upon itself. The number of hairpins per elastic chain affects the domain size along the layer normal, which will be probed by X-ray lineshape analysis. Using SANS, the number of hairpins per chain will be measured as a function of chain length, temperature, and deformation history. The combined results of X-ray diffraction and SANS will provide potentially transformative insights regarding the role of hairpinned chain statistics in promoting cold-drawing and necking in elastomers, and possibly more broadly in other polymers.NON-TECHNICAL SUMMARY:Smectic liquid crystalline elastomers (LCE) are rubber-like materials that possess the flexibility and toughness of a rubber-like polymer, but have layered molecular ordering at the nanometer scale. Smectic LCE have unique mechanical properties that potentially make them useful as vibration damping or impact-absorbing rubber coatings, or as soft actuators with properties similar to muscle tissue. One of their unusual features is cold-drawing, a process by which the material yields and elongates drastically when placed under tension, while forming a contraction or "neck." Most rubber-like polymers do not undergo cold-drawing or necking. To better understand how molecular structure in smectic LCE leads to cold-drawing, experimental methods of small-angle neutron scattering and X-ray diffraction will be applied to characterize structural changes at the molecular level due to mechanical deformation. The results of our study will broaden understanding of the mechanical behavior of rubber-like polymers, and possibly uncover broader insights regarding mechanical instability in polymers. This project supports valuable educational activities at Texas Tech University, including graduate and undergraduate education, ethics training for all researchers involved, and mentoring of postdoctoral researchers. The students and postdocs will actively participate in outreach programs that introduce honors students (including women and minorities) to polymers and Chemical Engineering fundamentals, fostering diversity among future researchers in scientific and engineering disciplines.

冷拉伸是玻璃态无定形和半结晶聚合物中的一种众所周知的现象，在高于玻璃化转变温度的橡胶状网络中很少观察到这种现象。 然而，近晶主链液晶（LC）弹性体在一定的温度和伸长率条件下的冷拉伸和颈缩形成最近有报道。 在LC弹性体，玻璃态无定形聚合物，和半结晶聚合物的冷拉伸显然不能归因于共同的形态特征，但它可能有共同的起源在链水平上的构象转变。 本研究旨在阐明多畴近晶液晶弹性体冷拉伸的分子基础。 颈缩的不稳定性，提出了从强有力的贡献，弹性自由能伸长时，相反，熵驱动的橡胶弹性的经典图片。 假设含有更大、更稳定的域的弹性体表现出更大的屈服应力，并且由于在伸长期间破坏近晶有序的能量损失增加而更易于机械不稳定。 宏观力学响应的多域近晶LCE将与构象不稳定性链水平的域尺寸和小角中子散射（SANS）测量的链尺寸的X射线测量。 一个紧迫的基本问题，所有的工作是发夹的概念，链折叠的骨干扭转方向对自己。 每个弹性链的发夹的数量影响沿着层法线的域大小，这将通过X射线线形分析来探测。 使用SANS，每条链的发夹数将被测量为链长、温度和变形历史的函数。 结合X射线衍射和SANS的结果将提供潜在的变革性的见解，在促进弹性体中的冷拉伸和颈缩的发夹链统计的作用，并可能更广泛地在其他polymers.NON-TECHNICAL摘要：近晶液晶弹性体（LCE）是橡胶类材料，具有弹性和韧性的橡胶类聚合物，但在纳米级的分层分子有序。 近晶LCE具有独特的机械性能，这可能使它们可用作振动阻尼或冲击吸收橡胶涂层，或用作具有类似于肌肉组织特性的软致动器。 它们的一个不寻常的特征是冷拉，这是一个材料在张力下屈服和伸长的过程，同时形成收缩或“颈部”。"大多数橡胶类聚合物不经历冷拉伸或颈缩。 为了更好地理解近晶LCE中的分子结构如何导致冷拉伸，小角中子散射和X射线衍射的实验方法将被应用于表征由于机械变形而在分子水平上的结构变化。 我们的研究结果将拓宽对橡胶类聚合物机械行为的理解，并可能揭示有关聚合物机械不稳定性的更广泛见解。 该项目支持德克萨斯理工大学的重要教育活动，包括研究生和本科生教育，所有相关研究人员的道德培训，以及博士后研究人员的指导。 学生和博士后将积极参与外展计划，向荣誉学生（包括妇女和少数民族）介绍聚合物和化学工程基础知识，促进科学和工程学科未来研究人员的多样性。