Committee Members:     

• Prof. Blair Brettmann, Advisor, ChBE/MSE  
• Prof. Mary Lynn Realff, MSE  
• Prof. Karl Jacob, MSE/ME  
• Prof. Donggang Yao, MSE 
• Prof. Saad Bhamla, ChBE  

Advancing Product Development of Ultrafine Fibers: From Formulation Considerations to Thermoelectric Textiles 

Abstract:  

Ultrafine fibers are of great interest in new product development in the fiber and textile industry especially for complex, high value-add materials. Due to their unique inherent properties of fine diameter, high surface area to volume ratio, high porosity, and molecular orientation along the fiber length they are highly desired for applications in biomedical fields, filtration, and wearable electronics. Electrospinning is the most common method used for ultrafine fiber production, but it classically has many material limitations such as requiring a high molecular weight linear polymer dissolved in a conductive and volatile solvent at concentrations above the entanglement concentration. These limitations hinder the development of more functional materials. There are significant knowledge gaps in how complex formulations needed for high value-add materials influence spinning solutions and the manufacturing space. This thesis explores methods of circumventing the material limitations for ultrafine fibers in order to advance future product development.  

Poor fiber formation is often the result of jet breakup when the elasticity of the polymer solution is insufficient to suppress instabilities. The primary instability driving jet breakup is capillary force driven instabilities, where low elasticity in solutions is insufficient to suppress Rayleigh instabilities. A decrease in capillary force should allow for less elastic solutions to form smooth fibers. This was studied through the effect of surface tension on the electrospinnability with the use of extensional rheology. PVP was tested with methanol, and water, and water with surfactant to directly study the impact of surface tension on electrospinnability. Low surface tension solutions readily formed fibers at lower concentrations than high surface tension solutions, supporting the theory that less elasticity is needed to stabilize the jet from breakup. Through this study I provide a deeper understanding of the connection between solvent characteristics, viscoelasticity, and electrospinnability, which enables the rational preparation for more complex spinning solutions being explored.  

Electrospun fibers are typically composed of solely high molecular weight polymers, but including hard particles is also of interest in ultrafine fibers for drug delivery, active filtration, wearable electronics, etc. In this thesis, I show that large particles (10x the fiber diameter) of varying shapes, densities, and chemistries can be incorporated in fibers at loadings exceeding the polymer concertation without disrupting fiber formation. Additionally, the large particles increase the mechanical strength of the fibers in the same manner as fiber reinforced composites. Fiber mats were found not to fracture when particles are included, indicating they are not creating weak points in the fibers. Hence, large functional particles such as active pharmaceutical ingredients can be encapsulated in fibers at high quantities without damaging the fiber integrity.  

Conducting polymers are typically low molecular weight or are particle dispersions which fall outside the realm of electrospinnable materials, yet they are of great interest in ultrafine fiber applications. Developing methods of creating conducting ultrafine fibers enables their use in high performing thermoelectric textiles for wearable electronics. Ultrafine fibers with their inherent porosity and high surface area are of ideal use in wearable electronics, as they are breathable and provide a good platform for charge transport. With the knowledge gained in the early work in this thesis, particle electrospinning and surface tension effects on fiber formation, fibers with a high concentration of PEDOT:PSS were created that exhibit thermoelectric properties similar to equivalent thin films while maintaining porosity and flexibility. Additionally ultrafine poly(NiETT) fibers were synthesized for the first time, providing a novel n-type textile with conductivities rivaling other n-type textiles.                

Through the deeper understanding of the connection between solvents, polymers, and additives in formulations to viscoelasticity and electrospinnability contributes to greater knowledge of ultrafine fiber formation. The results of this thesis show how better understanding of fiber formation and proper formulations expand the realm of electrospinnable materials and give insights into future product development of novel and advanced textile based materials.