Development and Study of Structurally-Dynamic Covalent Polymers

传统聚合物或塑料的设计是为了尽量减少降解（例如，通过破坏共价键），从而在其使用寿命内保持其机械性能。这导致了各种非常有用的材料（如纤维、塑料和粘合剂）在我们的日常生活中无处不在。这类材料的一个问题是，当它们破裂或降解时，修复或回收它们可能很困难，而且/或没有成本效益。如果塑料可以被使用，那么它们就可以治愈划痕或变形，或者更有效地回收利用。实现这一目标的一种方法是在聚合物结构中设计可逆键，这种键可以在相对少量的热或光的作用下被破坏和重建。在美国国家科学基金会的资助下，罗文小组正在研究一系列不同的刺激响应可逆键，这些键将被纳入聚合物中，并用于获得新型的响应/自适应材料。这些系统的一个关键组成部分是能够系统地控制所需的刺激，以获得键的可逆特性，从而使它们能够针对不同的应用进行定制。有了这些材料，Rowan集团将专注于开发(1)新型软致动器，其作用类似于“聚合肌肉”，并为（软）机器人技术提供应用；(2)具有划伤愈合特性和增强韧性的材料，以延长其使用寿命；(3)按需可逆胶水和粘合剂。该项目涉及研究生和本科生，来自当地高中的学生，包括来自芝加哥服务不足和主要少数族裔社区的学生。这个项目的综合方法为各个层次的学生提供了一个令人兴奋的学习环境和广泛的研究经验。此外，罗文教授和他的研究小组将为克利夫兰自然历史博物馆在马丁·路德·金博士纪念日举办的“冬季发现日”活动，设计一个名为“自然材料”的博物馆外展项目，进行新的实践演示。该项目旨在(i)让当地社区了解聚合物，以及大自然的材料如何帮助我们创造一个可持续发展的星球，（ii）培训当前的研究生如何与公众和年轻学生进行科学技术交流和教育。在聚合物网络中引入动态键（可以进行可逆交换）赋予材料新的自适应特性。自适应特性来自于网络通过动态键交换改变其结构（和/或组成）的能力，因此被称为结构动态聚合物。根据结合到网络中的动态键的具体类型、位置和数量，所得到的薄膜将具有可再加工/可再成型的能力，表现出愈合和/或形状记忆特性，甚至为具有增强韧性、应力松弛和/或自适应粘附能力的材料打开了大门。本提案概述了三种不同类型的结构动态聚合物的合成以及结构（通过NMR， MALDI-MS, FT-IR， UV， POM和WAXS/SAXS）和机械（流变学，拉伸测试和动态机械热分析）研究，重点不仅是研究这些系统的基础科学，而且针对适合薄膜化学特定动态行为的特定应用。具体来说，Rowan小组将专注于(1)聚二硫化物，(2)含thia-Michael加合物聚合物和(3)聚烷基脲的合成，表征和研究。虽然聚（二硫化物）网络是已知的，并且已经作为可治愈的材料进行了研究，但它们用于获取光适应液晶弹性体是新的。具体的兴趣是在访问3D驱动电影与这类材料。thia-Michael反应在室温下是动态的，但迄今为止，这类动态键在聚合物领域受到的关注很少。这种键的优点是，它的交换热力学和动力学都可以通过改变烯烃（迈克尔受体）的电子来系统地改变。PI将研究的最后一类动态键是最具商业相关性的，它基于大体积的烷基脲。烷基脲在聚氨酯工业中用作阻塞（保护）异氰酸酯，在100°C的温度下阻塞。PI将针对/开发可在较低温度下堵塞的烷基脲及其衍生物。后两类材料的目标是发展结构/性能关系，重点关注其固态力学和自适应性能。

PART 1: NON-TECHNICAL SUMMARYTraditional polymers or plastics have been designed to minimize degradation (e.g. by breaking of covalent bonds) and as such maintain their mechanical properties over their lifetime. This has led to a wide range of very useful materials (such as fibers, plastics and adhesives) that are ubiquitous in our daily lives. One issue with such materials is that when they break or degrade it can be difficult and/or not cost effective to repair or recycle them. What if plastics could be accessed that would allow them to either heal scratches or deformations or to be more efficiently recycled. One way to achieve this is to design into the polymer structure reversible bonds that can be broken and remade upon application of a relatively small amount of heat or light. With this NSF funding the Rowan group is working on a range of different stimuli-responsive reversible bonds that will be incorporated into polymers and used to access new classes of responsive/adaptive materials. A key component of these systems is the ability to systematically control the stimulus required to access the reversible character of the bond which in turn allows them to be tailored for different applications. With these materials the Rowan group will focus on the development of (1) new soft actuators that act like 'polymeric muscles' and offer applications to (soft) robotics, (2) materials that exhibit both scratch-healing characteristics as well as enhanced toughness to extend their useful operational lifetime, and (3) on-demand reversible glues and adhesives. This project involves graduate and undergraduate students, students from local high schools, including students from underserved and predominately minority neighborhoods of Chicago. The integrated approach of this project provides students at all levels with an exciting learning environment and broad research experiences. In addition, Prof. Rowan and his research group will design new hands-on demonstrations for a Museum outreach program entitled "Nature's Materials", which is part of the Cleveland Museum of Natural History's "Winter Discovery Day" on Dr. Martin Luther King Jr. Day. This program aims (i) to expose the local community to polymers and how Nature's materials can help us create a sustainable planet, and (ii) to train current graduate students on how to communicate to and educate the general public and younger students about science and technology.PART 2: TECHNICAL SUMMARYThe introduction of dynamic bonds (that can undergo reversible exchange) into a polymer network imparts new adaptive properties onto the materials. The adaptive properties come from the network's ability to alter its architecture (and/or composition) through dynamic bond exchange and as such have been termed structurally-dynamic polymers. Depending on the specific type, placement and amount of the dynamic bond incorporated into the network the resulting films will have the ability to be re-processable/re-moldable, exhibit healing and/or shape-memory properties, and even open the door to materials that have enhanced toughness, stress relaxation, and/or adaptive adhesion capabilities. This proposal outlines the synthetic as well as structural (via NMR, MALDI-MS, FT-IR, UV, POM and WAXS/SAXS) and mechanical (rheology, tensile testing, and dynamic mechanical thermal analysis) studies on three different classes of structurally-dynamic polymers, focusing not only on investigating the basic science of these systems but also on targeting specific applications that suit the specific dynamic behavior of the film's chemistry. Specifically, the Rowan group will focus on the synthesis, characterization, and investigation of (1) poly(disulfides), (2) thia-Michael adduct-containing polymers and (3) poly(alkylureas). While poly(disulfides) networks are known and have been investigated as healable materials, their use to access photo-adaptive liquid crystalline elastomers is new. Specific interest is in accessing 3D actuating films with this class of material. The thia-Michael reaction can be dynamic at room temperature, but to date, this class of dynamic bond has received little attention in the polymer field. The advantage of this bond is that both its exchange thermodynamics and kinetics can be systematically altered by changing the electronics of the alkene (Michael acceptor). The final class of dynamic bond the PI will investigate is the most commercially relevant and is based on bulky alkylureas. Alkylureas are used as blocked (protected) isocyanates in the polyurethane industry that deblock at temperatures 100°C. The PI will target/develop alkylureas and derivatives that can deblock at lower temperatures. The goal for these last two classes of materials is to develop structure/property relationships focusing on their solid state mechanical and adaptive properties.