Laser Processing of Aligned Polylactic Acid Nanofibers—Process, Structure and Function

Problem:<br/>Electrospun nanofibers should have higher strength then their larger scale counterparts, however, due to the difficulty in drawing the fibers, a critical processing step that creates high molecular alignment in the fiber, their promise of high strength remains unkept. Individual nanofibers have been drawn using a parallel based track collection and drawing system, but this has only been done at ambient temperatures. The proper application of heat will allow for the fibers to be drawn to much higher draw ratios, and allow for the release of chain entanglements and imperfect crystal structures so that the resultant fibers can have optimal chain alignment and straight chain crystals.<br/>Solution:<br/>We aim to apply the technique of laser zone drawing to track electrospun nanofibers. Laser zone drawing heats up just a small portion of the fiber at a time, allowing for precise thermal control over the fiber, both spatially and temporally. This technique has been applied to larger fibers with incredible results but has yet to be applied to nanofibers. We believe that by combining the techniques of track electrospinning and laser zone drawing, we can create nanofibers with near ideal macromolecular properties, which may result in polymer nanofibers with the highest tensile strength to date. Additionally, this method is scalable and flexible, and should be applicable to any thermoplastic polymer that can be spun into nanofibers.<br/>Methods:<br/>Track electrospinning and Laser Processing: Polylactic acid (PLA) nanofibers are electrospun onto a parallel track collection system, which allows to the collection of aligned nanofibers. The fibers are then transferred onto a robotic stage which passes the fibers through a 9.3 micron wavelength laser beam.<br/>Raman Spectroscopy:<br/>Single fiber Raman spectroscopy is used to measure changes in the molecular alignment and crystallinity of the fibers.<br/>Scanning Electron Microscopy (SEM):<br/>SEM is used to evaluate the diameters and morphology of the fibers.<br/>Mechanical Properties:<br/>Dynamic Mechanical Analysis is used to determine the modulus off fiber mats, because individual fibers do not produce a large enough load.<br/>Results:<br/>We have demonstrated that PLA fibers of different diameters can be predictably thinned to diameters of less than 20% of the original diameter with low variability simply by increasing the power density of the processing laser. This allows for the nanofibers’ diameters to be precisely controlled after the spinning process. We have also established that this effect is not dependent on the duration of laser exposure, indicating that the fibers reach thermal equilibrium during the process, despite being exposed to very high laser power densities. This is explained by their enormous surface area to volume ratio, which only increases as they thin, which keeps them from melting even under extreme laser exposure. We have also demonstrated that the fibers can be thinned in a single step, as there was no difference in diameter compared to fibers that were treated multiple times at incrementally increasing powers. By comparing the power density of the laser to the diameter, using Newtons law of cooling, and assuming thermal equilibrium, we can calculate the convective heat transfer coefficient of the nanofibers. Knowing this will allow us to predictably raise the fibers’ temperature using the laser, which will aid in the zone drawing process, by allowing us to heat just above Tg, which has been shown to be the optimal temperature for zone-drawing.