Properties of Polymer Brushes in Confined Geometries

聚合物刷在受限几何条件下的性质。Kuhl / CTS-0625594 / U。加州DavisPolymer分子在固体或流体界面上具有广泛的应用，从机械表面的润滑到生物相容性界面的合成。在过去的十年中，中子反射率测量已经成为一个强大的工具，用于测量超薄聚合物层的界面处的构象，然而，聚合物薄膜的结构约束和/或剪切下仍然难以捉摸，由于固有的困难，在测量薄膜性能。基于主要研究者最近的开创性工作，从掩埋界面的镜面中子散射，该项目是一个实验计划，直接测量和表征聚合物刷的结构下限制和剪切使用新的实验和分析工具。他们建立在他们的初步努力对原位表征聚合物刷和分子厚膜的结构和相互作用力的约束和剪切的功能，通过实验，使用中子散射，利用对比度匹配技术，并增强与相互作用力剖面测量。研究人员开发了一种装置，中子约束剪切单元（NCSC），它使对齐的基板之间的表面分离（100 nm）进行控制，同时进行原位结构表征的介入材料与中子反射率测量在静态和剪切条件下。还将开发第二代NCSC，增强动态剪切测量能力。他们将能够探索丰富的参数空间，包括聚合物特性（膜厚度，分子量，接枝模态，接枝密度）和溶剂质量，为聚合物刷的合理设计提供洞察力和预测能力。实验结果将用于区分有效的理论模型和模拟，并可能影响这些努力中使用的假设。初步研究结果表明，实验结果和理论预测之间的差异，特别是关于刷刷互穿的水平。阐明和量化受限几何形状中接枝聚合物的行为和性质将有助于它们预测是什么使得一种化学结构和接枝架构的链在降低摩擦力方面比另一种更有效，并开发响应溶剂质量或流动变化的智能聚合物刷层。为了这些努力，研究人员还将开发用于无模型镜面散射数据分析的创新工具，这将使人们能够使用新的方法来准确地表征散射长度密度分布，以模拟独立的1D轮廓拟合和2D表面模型，更适合于表示薄膜结构的横向变化。这些程序将提供给一般的中子和X射线散射用户社区。本项目中开发的技术和发现将大大有助于对受限几何形状中聚合物刷的结构-性能关系的基本理解，以及它们作为超低摩擦润滑剂、粘附改性剂、控制微流体装置和膜技术中的流动的利用，更广泛的影响拟议的工作将有助于支持和发展中子散射社区，研究所将开发和传播新技术和分析工具，用于研究界面处和受约束的材料，这将有利于中子散射和纳米科学。学生和博士后研究人员将接受利用国家首屈一指的中子散射设施使用先进研究方法的培训。这些活动与一门新课程的开发密切相关，该课程名为“材料科学中的散射技术”，将提供给研究生和高年级本科生。该项目还将影响代表性不足的少数群体参与科学和工程。这项赠款提案的PI是一名妇女，在指导女研究生和本科生方面有着良好的记录。

AbstractProperties of Polymer Brushes in Confined GeometriesTonya L. Kuhl / CTS-0625594 / U. Calif. DavisPolymer molecules at solid or fluid interfaces have an enormous spectrum of applications in a wide variety of technologies from lubrication of mechanical surfaces to the synthesis of biocompatible interfaces. Over the past decade, neutron reflectivity measurements have become a powerful tool for measuring the conformations of ultra-thin polymer layers at interfaces, however the structure of polymer thin-films under confinement and/or shear has remained elusive due to the inherent difficulties in measuring thin-film properties. Building on the principle investigator's recent pioneering work with specular neutron scattering from buried interfaces, the project is an experimental program to directly measure and characterize the structure of polymer brushes under confinement and shear using new experimental and analysis tools. They build upon their preliminary effort towards the in-situ characterization of the structure and interaction forces of polymer brushes and molecularly thick films as a function of confinement and shear by experiments that use neutron scattering, exploit contrast matching techniques, and are augmented with interaction force profile measurements. The investigator has developed an apparatus, Neutron Confinement Shear Cell (NCSC), which enables the surface separation (100nm) between aligned substrates to be controlled while simultaneously conducting in-situ structural characterization of the intervening material with neutron reflectivity measurements under both static and shear conditions. A second generation NCSC will also be developed with enhanced capabilities for dynamic shear measurements. They will be able to explore a rich parameter space including polymer characteristics (film thickness, molecular weight, grafting modality, grafting density), and solvent quality to provide insight and predictive capabilities for rational design of polymer brushes. The experimental results will be used to discriminate valid theoretical models and simulations and will likely affect the assumptions used in such efforts. Preliminary findings indicate discrepancies between experimental results and theoretical predictions, particularly regarding the level of brush-brush interpenetration. Elucidating and quantifying the behavior and properties of grafted polymers in restricted geometries will aid their ability to predict what makes chains of one chemical structure and grafted architecture more effective in lowering friction forces than those of another and to develop smart, polymer brush layers that respond to changes in solvent quality or flow. Towards these efforts, the investigator will also develop innovative tools for model-free specular scattering data analysis that will make it possible to accurately characterize the scattering length density profile using new approaches to model independent 1D profile fitting and 2D surface models that are more suited for representing lateral variations in film structure. These programs will be made available to the general neutron and x-ray scattering user community. The techniques and findings developed in this project will contribute significantly to the fundamental understanding of structure-property relationships of polymer brushes in restricted geometries and their utilization as ultra-low friction lubricants, adhesion modifiers, to control flow in microfluidic devices and membrane technologies, and colloidal dispersion stabilization and rheology.Broader ImpactsThe proposed work will serve to support and grow the neutron scattering community in soft condensed matter and will develop and disseminate new techniques and analysis tools for studying materials at interfaces and under confinement, which will benefit both neutron scattering and nanoscience. Students and postdoctoral researchers will be trained in the use of advanced research methods utilizing the nation's premier neutron scattering facilities. These activities are strongly coupled to the development of a new course, titled Scattering Techniques in Materials Science" which will be offered to graduate and senior undergraduate students. The project will also impact the participation of underrepresented minorities in science and engineering. The PI on this grant proposal is a woman and has an established track record in mentoring female graduate and undergraduate students.