The efficacy of defective sites in MWCNTs as free radical scavengers was investigated on the basis of thermal analysis of ultra-high molecular weight polyethylene (UHMWPE)/MWCNTs and UHMWPE/γ-MWCNTs nanocomposites. Homogenized composites of polyethylene (PE) with MWCNTs and γ-MWCNTs (0.5% by weight) were prepared by the ball milling process for 2 h at a speed of 200 rpm. Thereafter, micron-sized sheets were prepared (by hot pressing) and subjected to γ-dose ranging from 25 to 150 kGy. To investigate the thermal properties and thermal stability, differential scanning calorimetric (DSC) and thermo-gravimetric analysis (TGA) measurements were performed in an inert atmosphere. The results showed that peak melting temperature (T m) and lamellae thickness (L c) remained unaltered for UHMWPE/MWCNTs composites; however, for UHMWPE/γ-MWCNTs composites the values varied from 138 to 141 • C. This behaviour was attributed to chain scission close to crystalline lamellae due to radiation-induced free radicals, thus reducing the percentage of inter-phase contents within the polymer matrix and enhancing the percentage crystallinity for composites. Furthermore, TGA revealed the higher thermal stability of composites as compared with pristine ones and significant increase in residues percentage for composites, i.e., from ∼ 3 to 7%. These results confirmed the importance of defective sites within the MWCNTs as free radical quenchers, and stopping the chain scission, particularly close to crystalline lamellae, thus preventing loss of the important interphase region of UHMWPE.

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A multifunctional designing approach is of great importance for advanced composite applications. This study assessed the use of ionic liquids (ILs) to modify the surface of carbon fiber (CF) and impart multifunctional characteristics to it. For that, ethanolic solutions of different ILs, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-butyl-3-methylimidazolium chloride and 1-(2-hydroxyethyl)-3-methylimidazolium chloride, at different concentrations, were used to treat the CF. Fourier-transform infrared spectroscopy confirmed the presence of IL on the CF surface. The contact angle for 1% w/v IL-treated CF and DGEBA epoxy decreased by up to 35%, corresponding to an increase in surface energy of fiber, accompanied by an increase of 91% in interfacial shear strength. These enhancements were achieved with the hydroxy-functionalized IL, showing the tunability of CF properties through the N-imidazolium substituent. An increase in crystallite size along the basal plane was also found due to the ordering of the graphitic structure on the surface. Moreover, there was a decrease in electrical resistivity of 77%. In all, the imidazolium ILs were considered a promising approach to induce multifunctional characteristics, namely enhanced interfacial strength and electrical conductivity, to unsized CF, which can also be beneficial for recycled fibers without deteriorating their inherent surface properties.

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Hybrid composite laminates with nanofillers are high performance materials for thermal and structural applications. In this work, multiwall carbon nanotubes (MWCNT) were non-covalently modified by treating them with 1 wt% or 5 wt% of ionic liquid (IL), 1-butyl-3-methylimidazolium chloride. The treated MWCNT were dispersed in epoxy resin (weight content: 0.2–2 wt% or 1 to 3 wt%, respectively) and used to obtain hybrid composite laminates based on carbon fibers (CF) molded by hand layup and vacuum bagging techniques. The hybrid composite laminate containing 1 wt% of MWCNT modified with 1 wt% of IL showed the best mechanical and thermomechanical properties, including an increase of 210% and 151% in flexural strength and modulus, and an increase of 101%, 116% and 29.3% in storage modulus, loss modulus and damping, respectively. Scanning and transmission electron micrographs showed the enhanced MWCNT dispersion and network at low IL content, justifying its improved mechanical properties. The application of a low amount of IL dispersant was found a promising approach to prepare MWCNT/CF/epoxy composites with enhanced properties.

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