Effects of Reflux Temperature and Molarity of Acidic Solution on Chemical Functionalization of Helical Carbon Nanotubes

The use of nanomaterials and nanostructures have been revolutionizing the advancements of science and technology in various engineering and medical fields. As an example, Carbon Nanotubes (CNTs) have been extensively used for the improvement of mechanical, thermal, electrical, magnetic, and deteriorative properties of traditional composite materials for applications in high-performance structures. The exceptional materials properties of CNTs (i.e., mechanical, magnetic, thermal, and electrical) have introduced them as promising candidates for reinforcement of traditional composites. Most structural configurations of CNTs provide superior material properties; however, their geometrical shapes can deliver different features and characteristics. As one of the unique geometrical configurations, helical CNTs have a great potential for improvement of mechanical, thermal, and electrical properties of polymeric resin composites. The coil spring shape of these CNTs give them the capability to withstand high loads and considerable deformations that can be recovered, upon unloading. In addition, due to their helical shape, the entanglement of these CNTs with each other and the polymeric crosslinked molecular chains is much higher compared to the straight CNTs. Therefore, they can potentially absorb more impact loads and deliver higher resilience and flexibility in composites. One of the main issues for using these nanomaterials in composites is the uniform dispersion of these nanoparticles in the matrix system, besides their poor interface properties. This research reports a method for functionalization of helical CNTs (HCNTs) and investigates the effects of processing parameters, which can influence the effectiveness of the proposed chemical functionalization methods. The proposed method uses a refluxing process with a strong acidic solution; here, the effects of molarity and temperature, as two influential parameters, were investigated. For characterization of the functionalized helical CNTs, X-ray diffraction spectroscopy (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), visual dispersion inspection, and Scanning Electron Microscopy (SEM) methods were used. According to the results, all functionalization processes were successful in increasing the attachment of functional groups to the surface of helical CNTs that resulted in better dispersion of helical CNTs in DI water. The increase in molarity (M) of the acidic solution beyond 3M resulted a higher dispersion rate and a lower crystallinity. In the case of using a high-reflux temperature, helical CNTs which were processed with a 3M or 16M acids displayed a substantial increase in the solubility compared to the ones which were refluxed at a low temperature.