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

University Of Delaware, Newark DE

Investigators

Abstract

TECHNICAL SUMMARY The 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 SUMMARY One 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.