Toughness and Friction of Model Polyelectrolyte Gels

非技术总结：聚合物凝胶是一种柔性固体材料，含有高比例的小分子组分，如水。 该项目的目的是开发非常坚固的聚合物凝胶，也可用于生产超低摩擦的特别光滑的表面。 这种可能性已经被自然界所说明，以某些类型的软骨组织的形式，这些软骨组织负责润滑身体中的承重关节。 这些组织的合成版本的可用性将改变整形外科实践，延长髋关节和膝关节置换术的寿命，甚至使这些置换手术变得不必要。此外，具有所追求的特性组合的材料可用作各种制造环境中的接触表面，通过减少由于摩擦加热而损失的能量来提高能源效率。 过去生产高强度、低摩擦凝胶的努力受到以下事实的限制，即增强强度的相同因素通常也会增加摩擦。 为实现项目的总体目标，将开展项目的两个组成部分。 第一个是合成和测试新的凝胶，旨在打破凝胶强度和凝胶摩擦之间的联系。 这些凝胶具有与其天然对应物（如软骨组织）相同的特征，但可以通过改变凝胶结构的某些细节进行调整。 该项目的第二个要素是概念性的，涉及一个通用设计策略的发展，以指导未来的高强度，低摩擦凝胶的合成。该项目的目标是开发具有超低摩擦力的高韧性凝胶，并开发生产这些类型材料的通用设计策略。 这项工作的基础上合成的混合凝胶组成的一个带电的，多价阳离子已被添加到网络。 这些材料可以具有非常高的机械韧性，这是由于在同一材料中存在弱键和强键而产生的能量耗散机制。 由于高润滑、低摩擦材料的开发要求能量耗散最小化，因此在保持材料韧性的同时减少摩擦是特别具有挑战性的。 该项目的工作假设是，摩擦和韧性是由能量耗散机制，在不同的时间尺度上运作。 该项目的目的是阐明这些影响，在一套模型凝胶系统，使相关的时间尺度得到控制。 最重要的时间尺度是弱键的力依赖寿命，以及这些弱键在断裂后能够改革的速率。 实验模型系统是基于凝胶形成的控制组装的高分子量的三嵌段共聚物与聚（甲基丙烯酸）（PMAA）的中间嵌段和聚（甲基丙烯酸甲酯）（PMMA）端块。这些聚合物的组装导致形成具有由玻璃状PMMA聚集体组成的强键的聚合物凝胶。 弱键通过中间嵌段羧酸盐与原子或聚合阳离子的络合引入，其中键形成时间和键寿命由所选择的特定阳离子控制。 在这些系统中的弱键和强键的可调和可量化的性质将使广泛适用的设计原则的发展，用于生产具有所需摩擦性能的高韧性材料。

NON-TECHNICAL SUMMARY:Polymer gels are flexible, solid materials containing a high proportion of a small-molecule component like water. The aim of the project is to develop polymer gels that are very strong, and that can also be used to produce exceptionally slippery surfaces with ultra-low friction. The possibilities have been illustrated by nature, in the form of certain types of cartilage tissue that are responsible for lubricating load-bearing joints in the body. The availability of synthetic versions of these tissues would transform orthopedic practice, extending the life of hip and knee replacements or even making these replacement procedures unnecessary. In addition, materials with the sought after combination of properties can be used as contact surfaces in a variety of manufacturing settings, enhancing energy efficiency by reducing the energy lost due to frictional heating. Past efforts to produce high-strength, low-friction gels have been limited by the fact that the same factors that enhance strength generally increase the friction as well. Two elements of the project will be undertaken in pursuit of the overall project aim. The first of these is the synthesis and testing of new gels designed to break the connection between gel strength and gel friction. These gels have features in common with their natural counterparts (like cartilage tissue), but can be tuned by altering certain details of the gel structure. The second element of the project is more conceptual, and involves the development of a general design strategy to guide future synthesis of high-strength, low-friction gels. TECHNICAL SUMMARY:The goal of the project is to develop high-toughness gels with ultralow friction, and to develop a generalized design strategy for producing these types of materials. The work is based on the synthesis of hybrid gels consisting of a charged, polyelectrolyte network to which multivalent cations have been added. These materials can have an extraordinarily high mechanical toughness, resulting from energy dissipation mechanisms arising from the presence of both weak and strong bonds in the same material. Because the development of a highly lubricious, low-friction material requires that energy dissipation be minimized, it is particularly challenging to maintain material toughness while simultaneously reducing the friction. The working hypothesis for the project is that friction and toughness are controlled by energy dissipation mechanisms that are operative on different time scales. The project is designed to elucidate these effects in a set of model gel systems that enables the relevant time scales to be controlled. The most important time scales are the force-dependent lifetimes of the weaker bonds, and the rate at which these weak bonds are able to reform after they are broken. The experimental model systems are based on gels formed from the controlled assembly of high molecular weight triblock copolymers with a poly(methacrylic acid) (PMAA) midblock and poly(methyl methacrylate) (PMMA) end blocks. Assembly of these polymers results in the formation of polymer gels with the strong bonds consisting of glassy PMMA aggregates. The weak bonds are introduced by complexation of midblock carboxylates with atomic or polymeric cations, with bond formation times and bond lifetimes controlled by the specific cations that are chosen. The tunable and quantifiable nature of the weak and strong bonds in these systems will enable the development of widely applicable design principles for producing high-toughness materials with the desired frictional properties.