## 3 Results and Discussion  
### Rheological Behavior  
Rheology tests can provide useful information on the processability of the polymer and nanocomposites in the molten state. The complex viscosity at two different temperatures obtained from oscillatory rheometer is reported in Figure 4. At both 250 and 280 \({}^{\circ}\)C, ABS exhibited a pseudo-Newtonian behavior with a viscosity plateau at low frequency. The presence of nanofillers increased the viscosity in the low frequency region for both temperatures. A significant increase in viscosity was observed by the addition of carbon nanotubes. ABS/GNP nanocomposite showed a viscosity smaller than that of neat ABS at high frequencies for 250 \({}^{\circ}\)C. This behavior could be explained by the fact that the particle-particle interaction was not pronounced, and viscosity of ABS/GNP nanocomposites was decreased by a lubricating effect played by graphene nanoplatelets [23]. Similar behavior was also observed in a prior study on ABS composites containing carbon black (1-3 wt.%) or carbon nanotubes (up to 1 wt.%) [17]. Additionally, details about the melt flow index values of ABS, ABS/GNP and ABS/CNT at the same temperature were reported in our previous paper [21]. The effect of graphene and carbon nanotubes on the storage modulus and loss modulus vs. frequency of the ABS can be observed in Figure 5. Both storage and loss of moduli increased significantly with graphene and carbon nanotubes at low frequencies. The dependence of storage modulus on frequency (slope of G' vs. frequency at low frequencies) decreased with respect to the viscosity of materials for both temperatures. For instance, the slope of neat ABS and its nanocomposite decreased, respectively. The storage modulus of ABS/CNT nanocomposite was almost independent of frequency. This pseudo-solid-like behavior is attributed to the formation of a space-filling interconnected network of nanofillers [23]. A typical response for a polymer melt is that elastic behavior (storage modulus) dominates at high frequencies while viscous behavior (loss modulus) dominates at low frequencies. The cross-over point between storage and loss modulus can be defined depending on the molecular weight and molecular weight distribution of some linear polymers. Our result showed that neat ABS seemed to have a cross-over point at a frequency of about 100 Hz at 250 \({}^{\circ}\)C and at higher frequencies for 280 \({}^{\circ}\)C. ABS/GNP nanocomposite was likely to have cross-over point at frequencies of 100 Hz for both temperatures. No cross-over point for ABS/CNT suggested the presence of a rheological percolated network, and the effect of increasing temperature significantly reduced the storage modulus and loss modulus of the nanocomposite. Figure 4: Complex viscosity as a function of frequency at temperature of 250 and 280 \({}^{\circ}\)C; Acrylonitrile–butadiene–styrene; ABS/graphene nanoplatelets (GNP); ABS/carbon nanotubes (CNT)  
Figure 5: Storage modulus (full symbols) and loss modulus (open symbols) of ABS, ABS/GNP and ABS/CNT nanocomposites at 250 and 280 \({}^{\circ}\)C.  
Analogous Cole-Cole plots are used to describe the viscoelastic properties of materials with a relaxation time distribution such as heterogeneous polymeric systems [24; 25]. The real and imaginary components of viscoelastic properties are presented. In this study, we used Cole-Cole plots to investigate temperature-induced changes in the microstructure of nanocomposites and the results are reported in Figure 6 at two different temperatures (e.g., 250 and 280 \({}^{\circ}\)C) which were the two selected temperatures for the 3D printing process. It can be clearly observed that at 250 \({}^{\circ}\)C, the neat ABS presents a single relaxation arc. Due to a relatively higher amount of nanofillers, the linear region was evident for ABS/GNP and ABS/CNT nanocomposites. This suggests a restraint of long-range motion of polymer chains. In addition, our results do not show a significant effect of temperature in Cole-Cole plots.