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By means of a die-drawing technique in the rubbery state, the effect of the orientation of the microstructure on the dielectric properties of polypropylene (PP)/multiwalled carbon nanotube (MWCNT) nanocomposites was examined in this study. The viscoelastic behavior of the PP/MWCNT nanocomposites with MWCNT weight loadings ranging from 0.25 to 5 wt % and the dielectric performance of the stretched PP/MWCNT nanocomposites at different drawing speeds and drawing ratios were studied to obtain insight into the influences of the dispersion and orientation state of the MWCNTs and matrix molecular chains. A viscosity decrease (ca. 30%) of the PP/MWCNT-0.25 wt % (weight loading) melt was obviously due to the free volume effect. Differential scanning calorimetry (DSC) and wide-angle X-ray diffraction were adopted to detect the orientation structure and the variation of crystal morphology of the PP/MWCNTs. Melting plateau regions, which indicated the mixed crystallization morphology for the stretched samples, were found in the DSC patterns instead of a single-peak for the unstretched samples. We found that the uniaxial stretching process broke the conductive MWCNT networks and consequently increased the orientation of MWCNTs and molecular chains along the tensile force direction; this led to an improvement in the dielectric performance.

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In this study, the effect of orientation on the indentation hardness and energy absorption of an oriented transparent Trogamid polyamide was investigated with a spherical indentation methodology. It was found that the orientation significantly improved the indentation hardness and energy absorbed by plastic deformation. From the indentation hardness measurement, the elastic modulus, yield stress, and strain hardening exponent were derived from both the elastic and plastic regions of the indentation load-displacement curves. The elastic modulus was found to remain the same with orientation; the yield stress and the strain hardening exponent increased with orientation. The increase in the strain hardening exponent was the primary reason for the improved indentation hardness and energy absorption in the oriented samples. The mechanical properties from indentation measurements were compared to values obtained from tension true stress/true strain measurements. Good agreement was observed between the results from the indentation and tension tests. The effect of temperature on the mechanical properties was also studied. It was found that the modulus and yield stress were higher at a lower temperature; however, the strain hardening exponent remained unaffected.