Producing Melt Blown Nano-/Micro-fibers with Unique Surface Wetting Properties

Abstract

Melt blowing is a one-step process producing microfibers with an average fiber diameter (dav) of 1 – 5 μm. One application of melt blown fibers is filter media of water-in-diesel engine fuel filtration. In order to meet the increasingly higher requirements on fuel cleanliness, enhancing the filter efficiency has been desired. One attractive strategy is applying nanofibers (dav < 1 μm). The other is modifying the fiber surface wetting properties, such as superhydrophobic. The goal of this work is to produce melt blown nano-/micro-fibers with superhydrophilic and/or superhydrophobic surfaces for water-in-diesel filtration. In this thesis, we advanced the new technique of producing nanofibers from melt blown fiber-in-fiber polymer blends by understanding the nanofiber formation and developing water-extractable nanofibers. Firstly, poly(styrene) (PS)/poly(butylene terephthalate) (PBT) and PS/poly(ethylene-co-chlorotrifluoroethylene) (ECTFE) binary blends were melt blown followed by tetrahydrofuran (THF) extraction. The control extrusions demonstrated that the core nanofibers were primarily generated during the fiber blowing process. We speculate that both aerodynamic and fiber-fiber drag influence the nanofiber formation. Varying the minor phase volume fraction revealed the nanofiber breakup. We proposed that both the single-droplet-elongation and dumbbell-formation contribute to the formation of nanofibers. In addition, we hypothesize that the coalescence leads to the formation of non-uniform nano-/micro-fibers. Following the discussion on nanofiber formation, we showed nanofiber fabrication from water-extractable melt blown fiber-in-fiber polymer blends containing a water-dispersible sulfopolyester (SP). This new method eliminates issues associated with organic solvents. Also, it provides another route to prepare multilayer nano-/micro-fiber composites. Surface wetting modifications of melt blown PBT fibers were achieved by alkaline hydrolysis and subsequent fluorination. Application of alkaline hydrolysis generates sponge-like PBT fibers decorated with hydroxyl and carboxyl functional groups, resulting in a superhydrophilic fiber mat surface. The subsequent fluorination creates a sticky-superhydrophobic surface. An alternative achieving enhanced or even superhydrophobic PBT fiber surfaces is incorporation of a random perfluorinated multiblock copolyester (PFCE). Adding only 5 wt% of PFCE into PBT leads to over 20 wt% of PFCE on PBT fiber surfaces. The blooming of fluorine and the overall mat roughness increased the water contact angle from about 128° to above 150° and lowered the fiber mat surface adhesion energy by 25%. Finally, the appendices show some preliminary works on: 1) synthesis and characterizations of poly(L-lactide) and perfluoropolyether block copolymer; 2) the effects of alkaline hydrolysis on surface topography and mechanical properties of melt blown PBT fiber mats; 3) compatibilized polyamide 6 and poly(ethene-co-tetrafluoroethene) (PETFE) blends, which, however, were not suitable for melt blowing due to high viscosity; 4) collection of individual melt blown nanofibers.