SGER: Stress Relaxation Mechanisms in End-Linked Main-Chain Smectic Elastomers

技术综述：液晶弹性体(LCE)是一类橡胶状聚合物网络，由于介晶有序的存在而表现出与普通橡胶弹性的强烈偏差。“主链”近晶LCE由于介元嵌入到网络链的骨架中而形成层状中间相，表现出复杂的动态力学行为和宽弛豫时间谱。近年来，用非线性缩聚法制备的主链近晶弹性体在动态力学响应方面取得了一些进展，但进一步的进展取决于能否更好地控制弹性链在交联物之间的摩尔质量分布(Mc)。这种SGER涉及一种具有挑战性的合成方法，即通过末端连接来合成在结构上定义良好的近晶LCE，这允许控制Mc。具有最少量缺陷的“模型”末端连接网络和故意引入侧链或自由链的“不完美”网络将被合成。端联MCLCE的力学性能将以小应变振荡变形和拉应力松弛为特征。近晶态与非晶态的微区尺寸和体积分数将通过X射线衍射进行表征。比较模型和不完善的弹性体，将揭示节段级或域级松弛过程如何与宏观动态力学响应相关联。这项研究将首次使用末端连接网络来分类近晶LCE中复杂的弛豫过程，为在理解液晶聚合物网络的物理方面取得快速和创新的进展提供了潜在的催化作用。非技术摘要：主链液晶弹性体(MCLCE)是将橡胶类聚合物的柔性和韧性与液晶的分子有序性相结合的材料。MCLCE具有独特的机械性能，可能使其用作减振或吸收冲击的橡胶涂层，或用作具有类似肌肉组织特性的软执行器。为了更好地理解分子结构和宏观性质之间的联系，必须研究具有受控化学结构和最少缺陷的“模型”MCLCE。这笔SGER赠款支持通过一种名为末端连接的技术来制备MCLCE模型，随后将进行旨在表征其物理性质的实验研究。该项目将促进对近晶MCLCE的基本理解，近晶MCLCE在纳米尺度上具有分层有序性。这项研究支持宾夕法尼亚州立大学有价值的教育活动，包括研究生和本科生研究，支持积极参与向女性和高中生介绍聚合物和材料科学的外联计划的学生的工作。

TECHNICAL SUMMARY:Liquid crystalline elastomers (LCE) are rubber-like polymer networks that exhibit strong deviations from ordinary rubber elasticity due to the presence of mesomorphic ordering. "Main-chain" smectic LCE, which form lamellar mesophases due to mesogens embedded in the backbones of the network chains, exhibit complex dynamic mechanical behavior with broad relaxation time spectra. Recent studies of main-chain smectic elastomers prepared by non-linear polycondensation made some progress in understanding their dynamic mechanical response, but further progress depends on achieving better control over the molar mass distribution of elastic chains between crosslinks (Mc). This SGER involves a challenging synthesis of architecturally well-defined smectic LCE by end-linking, which allows control of Mc. "Model" end-linked networks having minimal amounts of defects and "imperfect" networks having deliberately introduced pendant or free chains will be synthesized. Mechanical properties of end-linked MCLCE will be characterized by small-strain oscillatory deformation and tensile stress relaxation. Domain size and the volume fraction of smectic vs. amorphous material will be characterized by X-ray diffraction. Comparing model and imperfect elastomers will reveal how segment-level or domain-level relaxation processes are tied to macroscopic dynamic mechanical response. This study will be the first to use end-linked networks to sort out the complex array of relaxation processes in smectic LCE, offering the potential to catalyze rapid and innovative advances in understanding the physics of liquid crystalline polymer networks.NON-TECHNICAL SUMMARY:Main-chain liquid crystalline elastomers (MCLCE) are materials that combine the flexibility and toughness of a rubber-like polymer with the molecular ordering of liquid crystals. MCLCE 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. To better understand the connections between molecular structure and macroscopic properties, "model" MCLCE with controlled chemical structures and minimal amounts of defects must be studied. This SGER grant supports preparation of model MCLCE by a technique called end-linking, which will be followed by experimental research aimed at characterizing their physical properties. The project will advance basic understanding of smectic MCLCE, which have layered ordering at the nanometer scale. The research supports valuable educational activities at Penn State University including graduate and undergraduate research, supporting the work of students who actively participate in outreach programs that introduce women and high-school students to polymers and materials science.