Influence of multi - wall carbon nanotubes on the mechanical , thermal and electrical properties of isotactic polypropylene nanocomposites

Abstract

Nanocomposites of isotactic polypropylene (iPP) and multi-walled carbon nanotubes (MWCNTs) with different wt% of MWCNTs were prepared by multiple extrusions, followed by injection molding technique. Surface morphology of the composites was observed by a scanning electron microscope (SEM). Simultaneous wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS) experiments were performed for structural analyses of the neat iPP and the nanocomposites. SAXS and WAXS measurements were also carried out at various positions i.e. at 1 cm apart from the mold-line of the injection molded dumbbellshaped samples to study the structural anisotropy occurring in the samples due to the meltflow disturbances. Mechanical properties such as tensile strength (TS), flexural strength (FS), percentage of eleongation-at-break [EB(%)], flexural strain [FS(%)], Young’s modulus (Y) and tangent modulus (G) were investigated by a universal testing machine. Micromechanical properties such as microhardness (H) and microhardness anisotropy (DH) were measured by a Vicker’s square based diamond indenter. Thermal properties such as melting temperature (Tm), degradation temperature (Td) and percentage of weight loss of the composites were monitored by a coupled differential thermal analyzer (DTA) and thermogravimetric analyzer (TGA). AC electrical measurements were also performed by a low impedance analyzer. SEM micrographs of the neat iPP and the nanocomposites show that with increasing MWCNTs content the surfaces become blackish appearance with good adhesion between iPP and MWCNTs. WAXS measurements reveal that an a–crystal is developed in the neat iPP along with lamellar structure having a long period of lamellar stacks of 150 Å, which is observed by SAXS measurements. Inclusion of MWCNTs increases this long period and the intensity of SAXS patterns. The long period increases to 165 Å in 2 wt% MWCNTs-loaded composite, indicating that MWCNTs enhance crystallization of iPP as a nucleating factor. Analyses of SAXS and WAXS indicate a development of a*-axis oriented A-lamellae with epitaxial Blamellae. In addition to these, the SAXS shows a development of the MWCNTs aggregates, which are due to nanoparticle-nanoparticle interaction and isotropically distributed in the iPP matrix with an average distance greater than 330 Å. SAXS results at different positions have clearly displayed anisotropic structure near the mold-line and amorphous structure at about 4 cm away from the mold-line. TS, FS, Y, G and H are found to increase with increasing MWCNTs content. This increase can be attributed to the increased crystallinity of the composites. On the other hand, a decrease of EB(%) and FS(%) occurs with respect to the increase of MWCNTs content. This fact can be explained on the basis of the restriction of chain mobility due to the increased crystallinity emanated by the presence of MWCNTs in the sample. DH was measured at 0.5 cm apart from the mold-line is found to vary showing the anisotropic nature of microhardness and strongly supporting the anisotropy in the structure obtained by SAXS and WAXS. Thermal analysis represents an increase of melting temperatures (Tm) and a decrease of degradation temperature (Td) of the composites with increasing MWCNTs. The increase of Tm can be assigned to the increase of lamellar thickness, whereas the Td decrease is due to the higher thermal conductivity of nanotubes that cause the iPP molecules to degrade at a relatively lower temperature. AC electrical analysis shows a slight increase in both the dielectric constant and the conductivity of the sample with increasing MWCNTs content. The slight increase in dielectric constant is probably due to the increase in degree of crystallinity as is evident from the X-ray diffraction analysis.