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

技术概述：液晶弹性体（LCE）是一种类似橡胶的聚合物网络，由于介形有序的存在，它表现出与普通橡胶弹性的强烈偏差。“主链”近晶LCE由于嵌入在网络链骨干中的介质而形成片层状介相，表现出复杂的动态力学行为和广泛的弛豫时间谱。近年来对非线性缩聚法制备主链近晶弹性体的研究在了解其动态力学响应方面取得了一定进展，但进一步的进展取决于能否更好地控制交联间弹性链的摩尔质量分布。这种SGER涉及到通过末端链接对体系结构上定义良好的smic LCE进行具有挑战性的综合，从而允许对Mc进行控制。具有最小缺陷的“模型”末端连接网络和故意引入垂链或自由链的“不完美”网络将被合成。端链MCLCE的力学性能表现为小应变振荡变形和拉应力松弛。x射线衍射将表征晶型和非晶型材料的畴大小和体积分数。比较模型弹性体和不完美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梳理近晶LCE中复杂的弛缓过程，为理解液晶聚合物网络的物理特性提供了催化快速创新进展的潜力。非技术概述：主链液晶弹性体（MCLCE）是一种结合了橡胶类聚合物的柔韧性和液晶分子有序性的材料。MCLCE具有独特的机械性能，这可能使其成为阻尼或吸收冲击的橡胶涂层，或具有类似肌肉组织性能的软致动器。为了更好地理解分子结构与宏观性质之间的联系，必须研究具有可控化学结构和最小缺陷的“模型”MCLCE。该SGER基金支持通过末端连接技术制备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smectic MCLCE的基本理解，该MCLCE在纳米尺度上具有分层排序。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的研究，支持积极参与向女性和高中生介绍聚合物和材料科学的推广项目的学生的工作。

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.