2.1. Materials

A novel thermoplastic acrylic resin Elium^®188O (Arkema, Colombes, France), which has a low viscosity of 100 mPa*s and a density of 1.01 g cm⁻³, was used as the polymer matrix for the fabricated nanocomposites. MWCNTs, with the trade name NC7000 from Nanocyl (Sambreville, Belgium) and the following properties: average diameter—9.5 nm, average length—1.5 µm, and purity—90%, were used as nanofillers after chemical modification. Materials and chemicals were purchased from the following sources: potassium bromide (KBr), dimethylformamide (DMF), isoamyl nitrite, and all the aniline compounds (aniline, p-nitroaniline, anthranilic acid) were purchased from Merck (Poznań, Poland); calcium oxide (CaO), isopropanol, toluene, methylene chloride, and tetrahydrofuran (THF) were purchased from Avantor Performance Materials (Gliwice, Poland). DMF was dried and purified by distilling it from CaO and was kept over 3A molecular sieves. KBr was dried at 105 °C overnight and kept in an airtight glass container. The other reagents were used as received.

The surface functionalization of MWCNTs was carried out by means of aryl groups grafting via the diazotization reaction of the selected aniline derivatives with isoamyl nitrite, under nonaqueous conditions, based on the methodology as described in the literature [19,20]. The procedure was as follows:

(1) The dispersion 1 g of MWCNTs for 1 h in 400 mL of dry (DMF) at 50 °C in a two-necked round-bottom flask, carried out in an ultrasonic bath; (2) the addition of a solution of 100 mmol of the selected aniline derivative in 200 mL DMF to the MWCNTs suspension; (3) placement of the solution in a reaction setup equipped with a reflux condenser and a magnetic stirrer and stabilization of the temperature at 60 °C; (4) the dropwise addition of isoamyl nitrite over a time of 1 h; (5) heating of the system for 20 h; (6) the addition of 2 mL of anhydrous acetic acid in 20 mL of DMF and an increase in the temperature to 100 °C to remove any possible unreacted residues of diazonium salts; (7) the hot filtering of carbon nanotubes through filter paper; (8) thorough washing of the nanotubes successively with DMF, isopropanol, toluene, methylene chloride, and THF and again with isopropanol in order to remove excess unreacted modifier and its decomposition products; (9) a final washing of the nanotubes with isopropanol, performed in an ultrasonic bath; (10) drying the modified nanotubes in a drying cabinet for 24 h at 40 °C; and (11) grinding the modified nanotubes with a pestle and mortar until a fine powder was obtained.

The scheme of the grafting reaction is shown in Figure 1, and Table 1 presents the modifications used.

Reaction scheme for grafting MWCNTs with the selected aniline derivatives as performed in this work.

The aniline modifications used in this work.

Elium^®/MWCNTs mixtures were prepared by mixing resin with the following MWCNTs concentrations: 0.02 wt%, 0.10 wt%, and 0.15 wt%. For each MWCNT concentration, four dispersions were prepared. One was prepared with neat MWCNTs and three were prepared with MWCNTs after surface modifications: MWCNT-AN, MWCNT-NAN, and MWCNT-ABA. After mechanical mixing, Elium^®/MWCNT dispersions were treated with ultrasonic waves, which is a commonly used method in the case of CNTs [28,29]. The ultrasonication was performed using a VCX1500 (Sonics & Materials, Newtown, CT, USA) ultrasonic processor with a maximum frequency of 20 kH for 1 h at an amplitude of 40%. Ultrasonic waves were on for 9 s out of every 14 s of the cycle to prevent the temperature from increasing over 45 °C. After ultrasonication, 2.0 wt% of the initiator (Dibenzoyl peroxide, Acros Organics, Antwerp, Belgium) was added to the prepared mixtures, which were then placed in a mould for 24 h at room temperature. Finally, manufactured composites were post-cured at 80 °C for 24 h.