Centrifugal jet spinning of polymer nanofiber assembly : process characterization and engineering applications

Abstract

High surface area together with unique electrical, thermal, and mechanical properties makes polymer nanofibers suitable in various commercial sectors, including but not limited to electronics, energy, mechanical/chemical, consumer, and life sciences. The application of polymer nanofibers are expanding rapidly and playing invaluable roles in a range of advances in nanoscience, bioscience, engineering and technology. However, the low cost-versus-yield efficiency of the current nanofiber fabrication technologies hinders the further integration of nanofibers into a wider range of practical large-scale applications and limits market applications driven by the balance of materials cost and performance. This substantial gap between supply and demand calls for the scaling-up of nanofiber production methods with high cost-versus-yield efficiency and versatility. Increasing attention has been paid to fundamental principles of nanofiber fabrication methods. Despite the scaling-up capability of currently popular nanofiber fabrication methods, their wide applications are limited by the high cost of spinning equipment and the low efficiency of production rate versus cost competitiveness. Meanwhile, centrifugal spinning is a high-production-rate and low-cost fiber fabrication method, however, the flexibility and high efficiency of centrifugal spinning is not fully developed as a nanofiber fabrication method compared to the wide application of electro-spinning because the fundamental mechanics involved in centrifugal spinning has not been fully investigated.

The driving mechanism behind the CJS process harnesses the centrifugal forces, the viscoelastic properties and the mass transfer characteristics of spinning solutions to promote the controlled thinning of a macro-scale polymer solution filament into desired nanofibers. Three different spinning stages, jet initiation, jet extension, and fiber formation are investigated from the role of physics involved in different spinning stages including fluid viscoelasticity, centrifugal forces and solvent mass transfer. Four different polymer solution systems with a wide range of fluid viscoelasticity properties and solvent mass transfer properties are characterized and used to fabricate polymer fibers under different rotational speeds. The correlations between spinning product morphology and spinning solution rheological properties are studied based on experimental data and observations. Dimensional analysis of the fiber formation process physics is used to adapt the experimental data into an empirical relationship describing fiber formation from spinning solution viscoelastic properties. A semi-empirical relationship is developed to predict fiber diameter as a function of spinning solution dimensionless properties without lengthy trial-error method. A 2-dimensional computational fluid mechanics model of CJS is also developed to provide the understanding of nanofiber formation with controlled air flow inside the CJS system.

In this work, a new configuration of the centrifugal jet spinning (CJS) set-up is developed to facilitate the consistent fabrication of polymeric nanofibers with controlled alignment and morphology. Polymer nanofibers including polyvinylpyrrolidone (PVP), polycaprolactone (PCL), polylactic acid (PLA), polyethylene oxide (PEO) and polyvinylalcohol (PVA) are fabricated down to the smallest average diameter of 80 nm. The quality of the resulting nanofibers, with regard to fiber production consistency, fiber diameter distribution, fiber alignment control, fiber morphology control and fiber collection efficiency, are carefully examined. CJS has been proven to be a viable alternative for mass production of both polymer nanofibers as well as ceramic nanofibers with 500 times higher production rate compared to traditional electrospinning technique.

It is proved that the air foil played important roles for the formation of fibers with diameters below 100 nm. Application of the CJS process as a highly efficient method of fabricating porous PLLA nanofiber is described for potential large-scale application for biodegradable tissue regeneration scaffolds. PLLA nanofibers with controlled porosity are fabricated to illustrate the versatility of the CJS technique. It is proven that PLLA fibers with increasing surface roughness and porosity demonstrated higher cell attachment and proliferation.