Effect of multi-walled carbon nanotubes size on the performances of polypropylene

Abstract

Isotactic polypropylene (PP) nanocomposites have been prepared with multi-walled carbon nanotubes (MWCNTs) of three different sizes, namely Particle 1, Particle 2 and Particle 3, having the aspect ratios of 1000, 444 and 200, respectively. The Particles 1, 2 and 3 respectively have the diameters ranging 5–15 nm, 15–30 nm and 40–60 nm and have the same length ranging 5–15 μm. Nanocomposites of PP and MWCNTs have been fabricated with 0.05, 0.5 and 2.0 wt% nanotube contents using extrusion followed compression-moulding techniques. The virgin PP and the nanocomposites have been characterized by X-ray diffraction (XRD) measurements, differential scanning calorimetry (DSC), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), mechanical test, micromechanical test, differential thermal analyses (DTA) and thermogravimetric analyses (TGA). XRD studies reveal that the pure PP exhibits monoclinic form, known as -crystal with the a- and the b-axes orientations. The incorporation of MWCNTs in PP also develops the same -crystal along with the formation of minor -crystal (triclinic form). The analyzed crystallite size is found to decrease with increasing both the filler concentration and size. Long period of lamellar structure of PP has changed from 15 nm to 16 nm by MWCNTs inclusion. DSC investigation shows a slight cold crystallization of compression molded virgin PP and no cold crystallization of the nanocomposites. Crystallinity values as evaluated from both XRD and DSC apparently show the maximum increase of about 20% for Particle 2 and 0.05 wt% MWCNTs content. On the other hand, the melt crystallization temperature (Tmc) observed by DSC is found different for pure PP and nanocomposites and the maximum deviation of Tmc obtained between them is Tmc = 13°C. The FE-SEM micrographs of the un-deformed samples show a smooth surface morphology, aggregates of MWCNTs and pull-out of carbon nanotubes from the nanocomposites. TEM clearly displays the nanotubes of different diameters and lengths, boundles of nanotubes as well as MWCNT aggregates. Tensile strength (TS), tensile modulus (TM) and microhardness (H) of the samples considerably change by different loadings and sizes of MWCNTs. The TS values of PP and naonocompoistes lie in the range 25–30 MPa. The TM value for them shows the range 1074–1560 MPa. The H value of the samples has been found to increase with both the increase of filler amount and particle size. Besides, the estimated values of H/TM lie in the range 0.09–0.10 for all samples, approaching the predicted value of 0.10 for polymers. The melting temperature, Tm, found by DTA is almost consistent to that found by DSC. The TGA thermograms show the thermal degradation temperature for PP of 448 °C, which increases for nanocomposites depending on the amount and particle size of MWCNT, giving rise to the maximum value of 471 °C.