Structure, processing, and properties of carbon nanotube composite with polypropylene
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Having high aspect ratio and structural similarity to the macromolecular building blocks, carbon nanotubes (CNTs) have demonstrated their great potential in tailoring the physical properties, e.g. conformation, crystallization, rheological, electrical and thermal characteristics etc., of the polymers. However, achieving good CNT dispersion, while also achieving good interfacial properties, remains a challenge, especially in non-polar polymers such as polyethylene (PE) and polypropylene (PP). In this study, the CNT modified PP with an engineered interphase was manufactured using a co-solvent process where a homogeneous PP layer was bonded non-covalently on acid functionalized multiwall carbon nanotubes (f-MWNTs). Unlike conventional melt blending, which simply compounds the neat polymer and pristine CNTs, or preparing the nanocomposite with covalently modified CNTs, e.g. via in-situ polymerization of macromolecules, this approach provides a practical way not only to obtain good CNT dispersion but also to alter the polymer physical properties by using small amount of CNTs (less than 1 wt%). This research systematically investigates the polymer-CNT interaction when the interphase is tailored. Three types of interphase were studied: (i) The PP/f-MWNT system using co-solvent process. (ii) The maleic anhydride grafted polypropylene (MA-g-PP)/f-MWNT system using co-solvent process. (iii) The PP/pristine MWNT (p-MWNT) system using melt process. The f-MWNT based systems, termed as master batches, were further melt blended with neat polymers to the target f-MWNT concentrations (0.001 wt% to 1 wt%) followed by injection molding. Mechanical properties, including the tensile and impact testing, as well as rheological, crystallization and the structure of the interphase were studied. It was shown that CNTs serve as a strong nucleating agent for templated polymer crystal growth. With addition of 1 wt % f-MWNT, an 152 % increase in PP impact strength was achieved in the PP/f-MWNT system. On the other hand, addition of pristine (unmodified) MWNTs yielded no statistical difference in the impact strength between the nanocomposite and neat PP. Strong adhesion between PP and f-MWNTs owing to the presence of interphase was verified by Raman spectroscopy as well as SEM and was shown to be absent between PP and p-MWNTs. Also, the structure of this interphase was characterized by thermal analysis and wide-angle X-ray diffraction. It was found that the polymer at the interphase exhibits higher melting temperature, suggesting higher crystal perfection and/or larger crystals, with respect to the bulk polymer. With a designed thermal treatment, the span of this interphase can be further increased as demonstrated by the formation of columnar crystals surrounding CNTs. Abnormal temperature dependence of shear viscosity was observed when temperature of the polymer melt was increased from 190 °C to 220 °C. Overall, this research demonstrates that tailored CNT/polymer interphase is needed for achieving high performance nanocomposites and improving the polymer-CNT interaction.