Electric Field Effects on the Conformation, Crystal Structure, and Molecular Orientation of Polymer Micro- and Nanofibers Electrospun from Solution

本研究将深入研究中等浓度聚合物溶液在溶剂蒸发过程中电场的动态效应。先前的工作已经证明，静电纺丝过程可以导致聚合物的构象和晶体结构的变化，使其具有不同的结构，有时甚至比观察到的块状材料或传统加工的薄膜或纺丝纤维更不稳定。此外，电场和不同集热器几何形状的使用可以导致纤维的宏观排列和纤维内聚合物链的微观排列。这强烈表明，静电纺丝和/或收集过程中使用的电场与蒸发聚合物溶液的相互作用是重要的。不幸的是，由于缺乏具有时间分辨率和化学特异性的光谱技术，无法在适当的时间尺度上提供详细的分子水平信息，因此对材料在电场作用下经历小尺度和大尺度取向的分子动力学的详细理解受到了严重影响。最近，在美国国家科学基金会DMR（#0315461）和CHEM （SGER #0346454）项目的前期支持下，我们构建了一个基于焦平面阵列（FPA）检测的有限带宽（1200 cm-1）无移动部件红外光谱仪的原型。该仪器具有监测从亚毫秒时间尺度到几个小时时间尺度的动态事件的能力。因此，它具有跨越近70年频率的带宽，可用于研究聚合物在直流和交流电场作用下的重定向动力学。首先，将研究电场对溶剂损失薄膜构象和晶体结构发展的影响。这些结果将在静电纺丝过程和收集阶段扩展到纤维。这项工作的智力价值有三个方面：1)它将提供在时间尺度上电场作用下聚合物结构重定向和重组的基本分子信息，这是任何其他表征技术难以获得的不可重复过程；2)进一步了解高电场作用下材料分子取向和极化的机理；3)它将提供分子结构与介电性能之间的相关性，可作为具有增强性能的工程材料的模板。此外，本研究的知识将直接应用于聚合物静电纺丝的加工参数的操纵，从而优化聚合物微纳米纤维的结构/加工/性能关系。当前纳米技术发展的一个特别重要的方面涉及纳米级直径纤维（直径小于1微米的纤维（人的头发直径为75微米））的生产和使用机械或电气手段来改善其最终性能。这些纤维将对工业过程产生关键影响，如空气和水过滤、复合材料、生物医学植入物、膜和燃料电池分离器等。对结构发展、加工历史、介电性能和机械性能之间的相关性的理解将提供一个模板，通过先进的加工技术可以为商品材料增加“价值”，这种影响将遍及许多工业部门。拟议的研究活动的教育影响将扩展到学生和博士后研究员，他们将接受培训，建立和使用最先进的光谱仪器，以获得关于分子取向和介电弛豫的分子动力学的时间分辨信息。此外，构建的仪器将被纳入高级本科-研究生课程MSEG 602材料科学分析方法，以便与更传统的仪器（例如介电光谱）相比，可以评估其优点，用于研究材料性质。本课程平均每年招收25名学生，其中包括几名来自行业的“归国专业人士”。

TECHNICAL SUMMARYThe proposed research will involve a thorough study of the dynamic effects of electric fields on moderately concentrated polymer solutions as the solvents evaporate. Previous work has demonstrated that the electrospinning process can lead to a change in conformation and crystal structure of a polymer to a different, and sometimes less stable structure than that observed for either the bulk material or conventionally processed films or spun fibers. In addition, the use of electric fields and different collector geometries can lead to both macroscopic alignment of the fibers and microscopic alignment of the polymer chains within the fibers. This strongly suggests that the interaction of the electric field, used in the electrospinning and/or collection process, with the evaporating polymer solution is important. Unfortunately, a detailed understanding of the molecular dynamics involved in materials undergoing small and large scale orientation when subjected to electric fields has been severely impacted by the lack of spectroscopic techniques with the time resolution and chemical specificity to provide detailed molecular level information on an appropriate time scale. Recently, under prior NSF support from the DMR (#0315461) and CHEM (SGER #0346454) program, we have constructed a prototype, limited bandwidth (1200 cm-1), no-moving parts infrared spectrograph based on focal plane array (FPA) detection. This instrument has the capability of monitoring dynamic events from the sub millisecond time scale up to time scales of several hours. Thus, it has a bandwidth spanning almost seven decades of frequency and can be used to investigate the reorientation dynamics of polymers subjected to both DC and AC electric fields. Initially, the effects of electric field on the development of both conformation and crystal structure in films undergoing solvent loss will be studied. These results will then be extended to fibers during the electrospinning process and during the collection stage. The intellectual merit of this work is three-fold: 1) it will provide fundamental molecular information on structural reorientation and reorganization in polymers subjected to an electric field on a time scale which has been difficult to access for non-repeatable processes by any other characterization technique; 2) it will advance our understanding of the mechanism of molecular reorientation and polarizability in materials subjected to high electric fields; and 3) it will provide a correlation between molecular architecture and dielectric properties that can be used as a template for engineering materials with enhanced properties. In addition, the knowledge from this study will be directly applicable to the manipulation of the processing parameters used for electrospinning of polymers so as to optimize structure/processing/property relationships in polymer micro- and nanofibers.NON TECHNICAL SUMMARYOne particularly important aspect of current developments in nanotechnology relates to the production of nanoscale diameter fibers (fibers with diameters less than 1 micron (a human hair is 75 microns in diameter)) and the use of mechanical or electrical means to improve their ultimate properties. These fibers will have a critical impact on industrial processes such as air and water filtration, composite materials, biomedical implants, membranes, and fuel cell separators, to name a few. An understanding of the correlation between structure development, processing history, dielectric properties and mechanical properties would provide a template by which "value" can be added to commodity materials through advanced processing techniques, an impact that would be pervasive across many industrial sectors. The educational impact of the proposed research activities extends to the students and postdoctoral fellows that will be trained to build and use state-of-the-art spectroscopic instruments to obtain time-resolved information on the molecular dynamics of molecular orientation and dielectric relaxation. In addition, the instruments constructed will be incorporated into a senior undergraduate-graduate course, MSEG 602 Analytical Methods in Materials Science, so that its merits can be evaluated in comparison to more traditional instruments (e.g., dielectric spectroscopy) for studying materials properties. Student enrollment in this course averages 25 students per year including several "Returning Professionals" from industry.