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Materials 2019, 12, 1281 5 of 14 studied using SEM analyses as shown in Figure 3. Tukey’s ANOVA analysis shows that statistically significant increases of the fibre diameter were noted by the increase in the amount of CTP from 0% to 50%. Moreover, as presented in Table 1, U2 sample possesses a lower standard deviation compared to the U4 sample. In other words, more uniformity in fibres containing 25% CTP was observed and incorporation of 50% CTP reduces the uniformity of the produced fibres. As can be seen from Figure 2a, the viscosity of all three solutions decreased with an increase of the shear rate, which denotes that PAN/CTP solutions behave as non-Newtonian fluids. For non-Newtonian fluids, various mathematical models can be used to fit the relationship between shear stress and shear rate. Among them, the Power law model is the most commonly used method to describe this behaviour according to the following equation [31,32]: T = K × Yn, (3) where T is the shear stress, K is the flow consistency coefficient, Y is the shear rate, and n is the flow behaviour index. With the power law model, the flow consistency coefficient and flow behaviour index can be obtained. Flow behaviour index indicates whether the solution is Newtonian or non-Newtonian. Power law model parameters were extracted which are presented in Table 1. As shown, the addition of 25% CTP to PAN solution (U2 sample) increases the viscosity. Such increment in viscosity can lead to an increase in fibre diameter, and negatively affect the spinnability, as seen in Figure 3. A similar trend can be also seen for PAN solution containing 50% CTP (U4 sample). Additionally, the shear thinning phenomena can be detected in viscosity behaviour of both PAN solutions containing 25% and 50% CTP. To study the shear thinning properties, flow behaviour index (n) obtained from the Power law equation is reliable. Accordingly, the decreases of n from 0.99 for pure PAN solution to 0.95 and 0.92, respectively for U2 and U4 samples, can be evident for such deduction. The presence of CTP in PAN solution, on one hand, can result in local interactions between these two ingredients (e.g., dipole-dipole interactions); therefore, the solution shows initial resistance towards shear rate. On the other hand, in the presence of CTP, the PAN polymer chains can be aligned easier, which in turn can lead to shear thinning behaviour. The reason behind this can possibly be attributed to the increases in the solution inertia [33]. Generally, it is hypothesised that the addition of CTP to PAN solution can result in an adverse effect on the electrospinning process. Moreover, the inclusion of CTP can lead to the drop in charge density of PAN solution and consequently decrease in repulsion forces, as evidenced by a decrease in solution electrical conductivity. This effect can be detrimental to achieving dense and smooth fibres [34]. To investigate the potential of fibre to carry the charges, electrical conductivity measurement was performed. As shown in Table 1, compared to U0, the reduction in electrical conductivity of the U4 sample doubled compared to the U2 sample. Therefore, as discussed above, the non-uniformity of fibres is much more profound for U4 sample, compared to U2 sample. This non-uniformity can be evaluated by comparing the standard deviation of calculated average diameters for different samples, denoting diameter distribution for U4 sample is significantly non-uniform. On the other hand, generally, the reduction of the n value is considered as the indication of PAN chains entanglement, which is a vital aspect for fibres deposition during the electrospinning process [35]. Therefore, although the increase in fibre diameter is observed (U2 sample), the decrease of standard deviation in the diameter of U2 sample suggests the electrospun nanofibers have uniform fibre diameters, compared to the U0 sample.PDF Image | Low-Cost Carbon Fibre Derived from Sustainable Coal Tar Pitch and Polyacrylonitrile
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