Functionalization of multi-walled carbon nanotubes (MWCNTs) with pimelic acid molecules: effect of linkage on β-crystal formation in an isotactic polypropylene (iPP) matrix

This work proposes an alternative method for the functionalization of MWCNT with molecules of pimelic acid (PA) using an ionic bridging linkage. This bridged linkage increases the amount of β-crystal in isotactic polypropylene (iPP) matrix compared to that obtained with chelating linkages of the same molecule. Evidence of a lateral bridge between the PA and MWCNT components was obtained from infrared spectra of the functionalized carbon nanotubes (MWCNT-f). This fact was confirmed by the absence of a characteristic infrared band at 1540 cm⁻¹, which was attributed to a particular chelating form of the PA, known as calcium pimelate (MWCNT-PS). Furthermore, an increase in the thermal stability of the attached PA due to ionic linkage was observed using differential scanning calorimetry (DSC) and thermo-gravimetric analysis. iPP nanocomposites were prepared with these MWCNT-f, yielding an improvement in the induction of β-phase within the nanocomposites; this finding was further corroborated by DSC and wide-angle X-ray diffraction analysis (WAXD). The relative content of β-crystals reaches a value as high as 85.7 % at a loading of 0.45 w/w % MWCNT-f, resulting in an increase in impact strength and the glass transition temperature (Tg), while the storage modulus decreased. In addition, the evolution of the crystallization activation energy of the resulting nanocomposites was investigated. We correlate the energy requirements of the interactions between nucleating agents and the segments of iPP. The bridged form of the molecule was associated with an increased energy barrier during the crystallization process due to both the thermodynamic instability of the β-crystal and the higher amount of induced β-crystal relative to the amount promoted by the chelated form. In this article, we demonstrate how the linkage type between MWCNT and PA components can strongly influence the ability of this organic molecule to nucleate β-crystal and can impact the crystallization behavior in iPP nanocomposites.