Effect of surface-active substances on the macrodispersion of thermal stress modified multi-wall carbon nanotubes under cryogenic conditions

In this study, the effect of surfactants on the macrodispersion of thermal stress-modified multi-walled carbon nanotubes (MWCNT) under cryogenic conditions, especially for use in the preparation of polymer films, was investigated. Firstly, optimum conditions for the modification of MWCNT by thermal stress in cryogenic conditions were determined using liquid nitrogen. For this purpose, the number of cycles (2, 4 and 6), the waiting time in liquid nitrogen (10, 20 and 30 min) and the waiting time at room temperature (5, 12 and 20 min) were determined as variable parameters. According to the experimental results, the highest surface area was reached under conditions of 2 cycles, 20 min in liquid nitrogen and 5 min at room temperature. Brunauer–Emmett–Teller (BET) surface area analysis, X-Ray Diffraction analysis (XRD), Raman spectroscopy and High Contrast Transmittance Electron Microscopy (CTEM) were used to examine the effect of the functionalization process on the internal graphitic structure and thickness distribution of MWCNTs. With the results obtained, it was investigated how the closed, graphitic end caps typical for carbon nanotubes are affected by thermal stress functionalized long MWCNTs in liquid nitrogen, how the tubes can be shortened and how the surface area changes. CTEM micrographs showed that long MWCNTs functionalized by thermal stress under cryogenic conditions could be shortened in length, while Raman analysis results showed that the average graphite quality was increased by functionalization. While these results are supported by the increase in the BET surface area from 297,551 m2g-1 to 397,295 m2g-1, it was determined that the modification process was carried out with 100% yield without sample loss. The effects of surfactants (Tween 80: Polyethylene glycol sorbitan monooleate, SDS: Sodium Dodecyl Sulfate and CTAB: Cetyltrimethyl Ammonium Bromide) on the macrodispersion degrees (DM%) and energy band gaps of MWCNTs were investigated using UV-Vis absorption spectroscopy. As a result of the analysis, it was determined that MWCNTs reached the highest and stable macrodispersion degree in CTAB solution. The use of surfactants allowed the energy band gaps of MWCNTs to decrease from 5.65-5.75 eV to 3.53-3.60 eV. Among the surfactants used, it was determined that the best colloidal stability was achieved with CTAB cationic surfactant with a zeta potential of 44.2 mV and SDS anionic surfactant with -49.9 mV. According to these results, PVA polymer films were prepared with MWCNT solutions with the highest and stable macrodispersion degree. In order to understand the effect of the MWCNT dispersion process on the physical properties of the PVA/MWCNT film, the characterization of the polymer films was performed by SEM, FTIR and DSC analyses. As a result, homogeneous incorporation of MWCNT into PVA films has been achieved successfully. It has been shown that the technique developed in this study provides an easy and simple way for thermal modification of MWCNT and can lead to the creation of MWCNT solution media with higher dispersion degrees and stability for use in various polymer films with better dispersion.