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

技术概述：冷拔是玻璃非晶和半晶聚合物中众所周知的现象，在玻璃化转变温度以上的类橡胶网络中很少观察到这种现象。然而，在一定的温度和延伸率条件下，近晶主链液晶（LC）弹性体出现了冷拔和颈状形成的现象。LC弹性体、玻璃状非晶聚合物和半结晶聚合物的冷拔显然不能归因于共同的形态特征，但它可能在链水平的构象转变中有共同的起源。本研究的目的是阐明冷拔多晶LC弹性体的潜在分子基础。颈缩不稳定性是由于弹性自由能在伸长时的强大能量贡献而产生的，这与经典的熵驱动橡胶弹性的图像相反。假设弹性体含有更大、更稳定的结构域，表现出更大的屈服应力，并且由于在延伸过程中破坏近晶有序而增加的能量惩罚，更容易产生机械不稳定性。通过x射线的畴尺寸测量和小角中子散射（SANS）的链尺寸测量，将多畴近晶LCE的宏观力学响应与链水平上的构象不稳定性联系起来。所有工作的一个紧迫的基本问题是发夹的概念，链折叠的骨干倒转方向上自己。每个弹性链的发卡数会影响沿层法向的畴尺寸，这将通过x射线线形分析来探讨。使用SANS，每条链的发夹数量将作为链长、温度和变形历史的函数来测量。x射线衍射和SANS的综合结果将为发夹链统计在促进弹性体冷拔和颈缩方面的作用提供潜在的革命性见解，并可能更广泛地应用于其他聚合物。非技术概述：smetic液晶弹性体（LCE）是一种类橡胶材料，具有类橡胶聚合物的柔韧性和韧性，但在纳米尺度上具有层状分子有序。Smectic LCE具有独特的机械性能，可用于减振或吸收冲击的橡胶涂层，或作为具有类似肌肉组织性能的软致动器。它们的一个不寻常的特点是冷拔，这是一种材料在受到张力作用时急剧屈服和伸长的过程，同时形成收缩或“颈部”。大多数类橡胶聚合物不进行冷拔或颈缩。为了更好地了解近晶LCE中的分子结构是如何导致冷拉的，我们将采用小角中子散射和x射线衍射的实验方法来表征由于机械变形导致的分子水平的结构变化。我们的研究结果将扩大对类橡胶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有价值的教育活动，包括研究生和本科生教育，所有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向优等生（包括女性和少数族裔）介绍聚合物和化学工程基础知识的推广项目，促进科学和工程学科未来研究人员的多样性。

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.