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mechanical strength in Figure 3.2 were twice as high (f = 0.52) compared to those for fibers with small tensile strengths. Equally high, or even higher, orientation factors were reported before from X-ray measurements for macroscale PAN fibers used as precursors for carbon fibers [60]. This improved molecular structure could be understood in terms of the distance between the polymer source and the collector: short electrospinning distances (15 cm) limited the number and order of molecule-stretching bending instabilities during electrospinning [36,61], while long electrospinning distances permitted multiple bending instabilities to take place and guaranteed the evaporation of the majority of the solvent whose presence could have also promoted (undesirable) molecular relaxations at short spinning distances. Therefore, the PAN nanofibers used in this work were those fabricated at 25 kV and 25 cm distance from the collector because they resulted in highest tensile strength and Young’s modulus. 3.3. Optimization of Nanofiber Stabilization Conditions Stabilization is important for the formation of a three-dimensional carbon network which is thermally stable for subsequent carbonization and graphitization treatments. In this regard, several researchers have pointed out to the importance of process optimization [16,18,28,31,32]. In the present experiments, the PAN nanofibers were collected from the metal target on an open metal clip so that they were in tension during stabilization and carbonization, in order to obtain high mechanical strength and modulus [23]. The optimum temperature and time of stabilization were determined by DSC. Sample curves are shown in Figure 3.3, where three sets of PAN nanofibers were heated at 5°C/min to 250°C, 275°C and 300°C and were held at peak temperature for 1 hr. Stabilization of PAN is an exothermic process and, therefore, a DSC scan shows the amount of heat released as a function of time and, hence, the degree of completion of the reaction. As shown in Figure 3.3, for temperatures 250°C and 275°C the exothermic reaction was not completed and the samples continued to release heat even after 1 hr. However, the reaction was completed after 1 hr at 300°C and the released heat was dramatically more than at 250°C and 275°C. A second DSC scan was done at 300°C but no further heat was released which confirmed that stabilization was completed during the 25PDF Image | HIGH STRENGTH CARBON NANOFIBERS DERIVED FROM ELECTROSPUN POLYACRYLONITRILE
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