Structure Formation In Nanofibers: Modeling, Experiments and Applications

Monday, October 17, 2011: 9:20 AM
L100 B (Minneapolis Convention Center)
Yong Lak Joo, Chemical and Biomolecular Engineering, Cornell University, Ithaca, NY

Electrostatic fiber spinning or ‘electrospinning’ is a unique process for forming fibers with submicron scale diameters through the action of electrostatic force. The resulting nanofibers are collected as non-woven mats with extremely large surface to mass ratios which can be used in filtration, catalysis, and biomedical applications. We will present our recent work on the structure formation in nanofibers: i) modeling of electrically driven polymeric jets and structures in nanofibers, ii) experiments on the creation of hierarchical nanostructures in nanofibers, and iii) applications of nanostructured nanofibers. In the first part of the talk, we will present recent modeling efforts including non-isothermal modeling of polymer melts during electrospinning. Our recent in-situ infrared thermal measurements on polymer melt jets during electrospinning have revealed rapid quenching by ambient air, an order of magnitude faster than predicted by classical correlations. This drastic heat transfer enhancement which causes premature solidification of the melt jet has been described by electrohydrodynamic (EHD) effects. Analysis of EHD-driven air flow was performed and included into a comprehensive model for polymer melt electrospinning. The analysis was validated by excellent agreement of both predicted jet radius and temperature profiles with experimental results for electrospinning of Nylon-6, PP, and PLA melts. Based on this analysis, we present the concept of gas-assisted melt electrospinning which alleviates the undesirable quenching process and thus can increase the attenuation rate and resulting in production of sub-micron scale fibers. The second part of the talk deals with the formation of self-assembling structures of block copolymers such as PS-b-PI during electrospinning and the utilization of such confined assembly to control the distribution and location of nanoparticles in nanofibers. Transmission electron microscopy images of microtomed electrospun fibers reveal that magnetite nanoparticles were uniformly dispersed only in one of the domains in block copolymer nanofibers. We also present a coarse-grained Molecular Dynamics simulation study to elucidate the effect of nanoparticle inclusion and strong deformation on self assembly in block copolymer/nanoparticle composite nanofibers. The results confirm our electrospinning experiments that deformation can be used as a parameter to effectively control nanoparticle location and distribution. Finally, we have applied nanostructured nanofibers as an effective catalyst system for the hydrogen production via the alkaline hydrolysis of glucose. When coaxial nanofibers that consist of pure ceramic (silica, alumina, zirconia) nanofibers in the core and ceramic nanofibers containing metal (iron or nickel) nanocrystals in the sheath layer were compared with monoaxial inorganic nanofibers metal nanoparticles distributed throughout the nanofiber, a four-fold decrease in the catalyst for greater than 90% conversion was achieved by tuning the catalyst location to the surface of the nanofiber. Placing the catalyst selectively into the sheath layer also delayed the onset of side reactions such as thermal decomposition to form CH4, CO and CO2, and thus the maximum reaction temperature for pure hydrogen production could be elevated from 140ºC to 200ºC. The increased reaction temperature while maintaining the hydrogen selectivity in turn resulted in a substantial increase in the production rate of pure hydrogen.

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