Dynamics of Associative Polymers Revealed by Self-Diffusion

非技术综述：缔合聚合物是含有“粘性”官能团的大分子，通过可逆的物理键将它们结合在一起。一些最广泛使用的聚合物是缔合聚合物，包括用于制造高尔夫球的聚合物、尿布中的吸收凝胶、洗发水或护发素的流动改进剂，以及正在开发的用于生物医学和提高石油采收率的新材料。这些现有材料的使用和新系统的开发依赖于了解分子是如何运动的：它们如何被加工成最终物体的形状，以及它们如何在机械力下变形。归根结底，这些性质取决于分子在微观尺度上的运动方式。最近，研究人员发现，缔合聚合物中的分子运动显示出分子运动速度与其运动距离之间出人意料的关系，这促使人们重新考虑聚合物性质设计背后的一些科学理解。这个项目将使用专门的X射线和激光技术来研究微米和纳米尺度的缔合聚合物运动，使我们能够进一步了解它们移动的机制。这些系统的理论和模拟将应用于将分子特性与新聚合物的化学设计联系起来，提供基本的见解，使人们能够更快地发现改进的聚合物材料。一群不同的研究生和本科生研究人员将通过这个项目进行见习，开发将为美国经济做出贡献的科学专业知识。这个项目的发现将通过YouTube和科学出版物和演示文稿与公众分享，还将与高中教师合作开发新材料，以激发年轻学生的创造力和创造力。技术摘要：缔合聚合物的研究已导致基于遥距聚合物的瞬时网络理论的坚实的理论理解，该理论基于变形过程中的键缔合和解离的动力学方程对网络的动力学进行建模。这些概念还导致了粘性Rouse和粘性爬行理论的发展，这些理论植根于相同的概念框架，但适用于具有沿主链间隔的缔合基团的聚合物。对各种缔合聚合物体系的流变学测量已经确立了这些方法的有效性，至少在定性意义上是如此。然而，这些理论并没有用扩散动力学的实验测量进行类似的检验。最近使用强迫瑞利散射(FRS)进行的扩散测量显示了任何理论都没有预料到的东西：在长度尺度上的明显超扩散区域比Rg大10-1000倍，即使在这个尺度上凝胶没有显示出结构的迹象。现在已经在四个不同的系统中观察到了类似的行为，这表明它在许多聚合物中都是常见的。假设这种超扩散机制是由于分子的“跳跃”或“跳跃”在长尺度上作为主要的扩散机制，这在现有的理论中是没有预料到的。在简化的缔合聚合物质心扩散模型上进行的初步布朗动力学模拟表明，该假设产生的扩散结果与FRS测量结果定性匹配。在整个提案中，将使用FRS和X射线光子相关光谱(XPCS)在实验和简化的粗粒度模型中系统地探索分子设计的影响。由于缔合聚合物在应用中的无处不在的性质以及这些材料中动力学对其最终使用的关键重要性，拟议的工作将在许多不同的行业和技术领域提供巨大的基础科学见解的潜力，从石油回收到润滑剂到医药到食品。为了与更广泛的社区分享这些日常材料的有趣物理的兴奋，将为YouTube制作一系列教育视频，解释实验和理论的不同方面，随着它们的发展。在该项目的最后一年，将与一名高中教师合作，开发一种教育模式，将视频与简单的实验室流变学实验相结合，向学生介绍缔合聚合物的概念。

NON-TECHNICAL SUMMARY: Associative polymers are giant molecules that contain "sticky" functional groups, binding them together with reversible physical bonds. Some of the most widely used polymers are associative polymers, including those used to make golf balls, the absorbing gels in diapers, flow modifiers for shampoos or conditioners, and new materials being developed for biomedical uses and enhanced oil recovery. The use of these existing materials and the development of new systems relies on understanding how the molecules move: how they are processed into the shape of a final object and how they deform under mechanical force. Ultimately, these properties are governed by the way that the molecules move on microscopic scales. Recently researchers have discovered that molecular motion in associative polymers shows unexpected relationships between how fast a molecule moves and the distance that it moves, prompting a reconsideration of some of the scientific understanding behind the design of the polymer properties. This project will use specialized X-ray and laser light techniques to study associative polymer motion at the micro and nanoscale, allowing us to further understand the mechanisms by which they move. Theory and simulation of these systems will be applied to relate molecular properties to the chemical design of new polymers, providing fundamental insight that allows faster discovery of improved polymeric materials. A diverse group of graduate and undergraduate researchers will apprentice through this project, developing scientific expertise that will contribute to the U.S. economy. The discoveries in this project will be shared with the public through YouTube and scientific publications and presentations and will also be developed in collaboration with high-school teachers into new materials to inspire the creativity and inventiveness of young students.TECHNICAL SUMMARY: The study of associative polymers has led to a solid theoretical understanding based on transient network theory for telechelic polymers, which models the dynamics of the networks based upon kinetic equations that model bond association and dissociation during deformation. These concepts have also led to the development of sticky Rouse and sticky reptation theories, grounded in the same conceptual framework but applicable to polymers with associating groups spaced along the backbone. Rheological measurements on a huge variety of associative polymer systems have established the validity of these approaches, at minimum in a qualitative sense. However, these theories have not been similarly tested using experimental measurements of diffusion dynamics. Recent diffusion measurements performed using forced Rayleigh scattering (FRS) show something that is not anticipated by any of the theories: an apparent super-diffusive regime on length scales 10-1000 times greater than Rg, even though the gels show no signs of structure at this scale. Similar behavior has now been observed in four separate systems, suggesting it is common across many polymers. It is hypothesized that this super-diffusive regime is due to molecular "jumping" or "hopping" as the dominant diffusion mechanism at long length scales, something that is not anticipated in existing theories. Preliminary Brownian dynamics simulations performed on a simplified associative polymer model for center-of-mass diffusion show that this hypothesis produces diffusion results that qualitatively match the FRS measurements. Throughout the proposal, effects of molecular design will be systematically explored using FRS and X-ray photon correlation spectroscopy (XPCS), both in experiments and using simplified coarse-grained models. Because of the ubiquitous nature of associative polymers in applications and the key importance of dynamics in these materials for their end use, the potential for fundamental scientific insight provided by the proposed work will be large across many different industries and technology areas, ranging from oil recovery to lubricants to medicine to food. To share with the broader community excitement about the interesting physics of these everyday materials, a series of educational videos will be made for YouTube that explain different aspects of the experiments and theory as they develop. In the final year of the project, a collaboration will start with a high-school teacher to develop an educational model that combines the videos with simple lab rheology experiments to introduce students to the concepts of associative polymers.