An investigation on phase behavior and orientation factor of electrospun nanofibers

by 1980- Feng, Kai

Electrospinning is a straightforward method to produce polymer nanofibers (10-500nm in diameter) from polymer solutions and melts. When the applied electrical force at the surface of polymer solution or polymer melt overcomes its surface tension, a charged jet is ejected, which travels towards a grounded target. Solvent evaporates and nanofibers form on the target surface. In this thesis, an investigation on phase behavior of the nanofibers electrospun from polymer blends and orientation factors within nanofibers electrospun from both single polymer solution and polymer blends were conducted. Single-phase nanofibers were produced by electrospinning blends of polycarbonate (PC) and polyvinylchloride (PVC) dissolved in a mixture of tetrahydrofuran (THF) and N,Ndimethylformamide (DMF). The phase behavior of the as-spun fibers was determined by a dynamic mechanical analyzer (DMA). Only one glass transition temperature was obtained which indicates single-phase structure. The surface morphology of the as-spun fibers was observed by scanning electron microscopy (SEM). The as-spun fibers were annealed for different times and same type of DMA test was performed on the fibers and there appeared two separate glass transition temperatures, which demonstrated phase separation. The annealed fibers were also stained by ruthenium tetroxide (RuO4). The resulting phase morphology of the fiber was examined by transmission electron microscopy (TEM). The TEM images demonstrated that by controlling the annealing of the as-spun fibers, a specific fiber surface morphology could be accessed. iv Nanofiber mats were produced by directly electrospinning nylon 66 dissolved in formic acid and PVC and PVC blend dissolved in a solvent mixture of THF and DMF onto a rotating metal wheel. SEM and FTIR were used to investigate the orientation of the sample. In order to determine molecular orientation within the fibers with respect to the fiber axis, molecular orientation with respect to fiber winding direction obtained by FTIR was divided by fiber orientation measured manually from the SEM pictures. The relationship between the molecular orientation of the electrospun nanofibers and three electrospinning process parameters were determined. The molecular orientation will increase if the wheel speed increases due to the increase of the mechanical tensile drawing force when the nanofibers hit the rotating wheel. The molecular orientation decreases if the applied voltage increases due to the decrease of the same mechanical tensile drawing force. High molecular weight polymers had higher molecular orientation when electrospun into nanofibers because the higher molecular polymers have greater relaxation time to preserve more orientation obtained. The mechanical properties of the oriented non-woven nanofibers mats were studied by DMA. It seems that the mechanical properties follows the same trend of the overall orientation with respect to fiber winding direction, not just the molecular orientation within the fiber and with respect to fiber axis. The information available in the literature was considered together with the experimental results to explain the phase behavior of the nanofiber electrospun from polymer blends and molecular orientation factors within the nanofibers. v