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Electrospinning and Electrospraying - Techniques and Applications Figure 2. (a-f) SEM images of the CNFs synthesized from the differently regenerated cellulose with additional NH4Cl impregnation [38]. Figure 3. Schematic diagram for the preparation of porous CNFs with hollow cores. (a) Preparation of stable polymer solutions from two separate phases; nanoscale phase separation occurs due to their different molecular weights; PMMA forms the discontinuous phase and PAN forms the continuous phase; (b) nanofiber formation (with two phases) by electrospinning; (c) removal of the PMMA phase at elevated temperatures [39]. the low-surface-tension polymer (PAN) would occupy the continuous phase of the solution (sea), while the high-surface-tension polymer (PMMA) forms the discontinuous phase (islands). In fact the two separate phases are due to the intrinsic properties (e.g., interfacial tension, viscosity, elasticity) of the polymers [40]. The electrospinning technique was used to obtain the PAN/PMMA nanofibers contain- ing two separate phases. The thermal treatment of PAN/PAMM nanofibers at over 1000°C would eventually form the porous carbon nanofibers with hollow cores. The complete removal of PMMA phase and the transformation continuous PAN phase would result in the formation of porous CNFs with hollow cores (Figure 4). Highly flexible N- and O-containing porous ultrafine CNFs were prepared by Wei and co-workers [41]. The ultrafine porous CNFs were obtained by simply vary- ing the PAN/PMMA ratios (10/0, 7:3, 5:5, and 3:7). Briefly, PAN/PMMA solutions with different ratios (10:0, 7:3, 5:5, and 3:7) are prepared in DMF. For better disper- sion, the PAN/PMMA solution was sonicated followed by stirring at 60°C for 2 h. The polymer blend was electrospun under an electric field of 9 kV at a tip-to-collector distance of 15 cm. The resultant electrospun PAN/PMMA nanofibers were stabilized under air flow at 300°C with the heating rate of 1°C/min for 1 h. Subsequently, the stabilized nanofibers were carbonized under N2 atmosphere at 900°C with heating rate of 5°C/min for 1 h. It was proven that the increasing the ratio of PMMA would result in the formation of ultrafine CNFs. Figure 5 shows the FE-SEM images of CNFs; CNFs, 7:3; CNFs, 5:5; and u-CNFs, 7:3. The FE-SEM images show that the morphology of CNFs is homogeneous, continuous, and a typical cylindrical shape. 4PDF Image | Preparation, Characterization, and Applications of Electrospun Carbon Nanofibers
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