Date of Award

Spring 2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Materials Science and Engineering - (Ph.D.)

Department

Committee for the Interdisciplinary Program in Materials Science and Engineering

First Advisor

N. M. Ravindra

Second Advisor

Michael Jaffe

Third Advisor

Norman W. Loney

Fourth Advisor

Treena Livingston Arinzeh

Fifth Advisor

George Collins

Sixth Advisor

Leslie LaNieve

Abstract

The objective of this research was the development of a mathematical model of the electrospinning process using dry spinning modeling principles as a basis. This model is directed at the identification of parameters which influence final fiber characteristics, e.g., solvent concentration, temperature, spin line tension, and electric field. Preliminary computer simulations were performed; however, the generated data was inconclusive and was determined to be due in part to the complexity of the modeled system and the subsequent computational difficulties encountered. Although a comprehensive computational model of the electrospinning process has not yet been demonstrated, the theoretical development that was undertaken provides a firm foundation for understanding and evaluating the electrospinning process. This development also provides a basis for the future development of a computational model based on this novel approach to electrospinning.

Electrospinning is a method of spinning nanoscale fibers that employs an electric field to propel a stream of polymer solution to create the sub-micron diameter fiber. Although much research has been done on the process itself, its wide-scale adoption has been inhibited by a lack of predictive control on the fiber properties. By developing an accurate computational model, enhanced process control and the production of fibers with desired properties can be attained. A mathematical model of electrospinning was developed that incorporates dry spinning and electrohydrodynamics principles. The model was based on the premise that the electrospinning of polymer solutions is, in many respects, an extension of dry spinning. Dry spinning is the fiber spinning process where a polymer solution is extruded through a spinneret into a body of circulating air. The air forces the solvent component to vaporize, forming a solid polymer fiber. The model was constructed by incorporating modified components of published 1-dimensional dry spinning and electrospinning models for their treatments of the mass, energy, and electrostatic transport equations. The momentum transport equation was derived independently in order to accurately describe the dynamic conditions unique to the electrospinning regime. This equation also includes terms for electrostatic stresses to account for the electrohydrodynamic interactions between the electrical charges residing on the filament surface and the electrical field. Initial modeling attempts were plagued with issues involving programming and the non-convergence of solutions. The challenge was to properly adapt the aspects of dry spinning to the electrospinning regime. In relation to dry spinning, electrospinning is characterized by high spin line velocities, high strain rates, increased solvent loss rates, and high air drag forces. The extreme changes these quantities undergo within a small length of space, particularly in the initial region just beyond the jet origin, may be a factor in contributing to the numerical instability of the model. Reevaluating the material property formulations and a more robust computational scheme will be considered. The novel incorporation of the principles of electrohydrodynamics (as a mechanism for fluid movement) coupled with very high solvent evaporation rate behavior contributed to a new and representative description of the extreme case of filament diameter reduction inherent in the electrospinning process.

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