The effect of acid treatment on CNT was assessed through HRTEM studies. The sidewall structures of pristine and chemically functionalized individual nanotubes are presented in Figure 1. The nanotubes have an outer diameter of ~30 nm and are made up of several shells, as indicated by diffraction lines produced from coaxial shell configuration. The attachment of a functional group on the nanotube sidewalls is evident from discontinuity in the shell structure [17]. While HCl-treatment leads to the removal of metal catalyst nanoparticles (Figure 1b), further treatment for amine-functionalization causes formation of crystallographic defects beside covalent attachment of amine groups with acidic sites having hydroxyl, carbonyl, and carboxylic groups. The disruption of outer and near-surface shell structure, as manifested in Figure 1c, is indicative of strong interaction between chemical reagents and the MWCNT surface.

After neat and composite PA6 samples were cryogenically fractured in liquid nitrogen, microstructural examination was performed under scanning electron microscope (SEM) to investigate surface morphology and the effect of CNT incorporation. Low and high magnification images of the neat and composite (reinforced with pristine or amine-functionalized CNTs) PA6 surfaces after fracture are presented in Figure 2. SEM examination at low magnifications revealed that whereas neat PA6 underwent brittle fracture, nanocomposite samples had rough, grainy surface indicative of extensive deformation prior to failure. At high magnification (Figure 2c), distribution of P-CNT in PA6 matrix is clearly evident from filament-like features that appear to be protruding out of the polymer matrix. In case of PA6/0.5P-CNT, the width and length of these filaments are in the range of ~110 to 193 nm and few hundreds of nanometers, respectively. Similar features were observed in case of PA6/0.5N-CNT samples with width varying from 52 to 140 nm. For same loading level, the extent of chemical functionalization dictates dispersion as well as interfacial strength with the PA6 matrix. Qualitatively, better CNT distribution was inferred from overall appearance of the composites that were of grey-black color in case of PA6/N-CNT composition, whereas PA6/P-CNT samples appeared matte black with inhomogeneous color. Functionalized CNTs usually exhibit better dispersion characteristics than P-CNT in both formic acid solution and the resulting nanocomposites [18,19]. A more comprehensive and quantitative information regarding effect of MWCNT incorporation can be derived from mechanical testing and dynamic thermal mechanical analysis of such nanocomposites samples.

X-ray diffraction spectra of the neat PA6 and nanocomposite samples are shown in Figure 3. As shown in Figure 3, while melt extrusion process produces PA6 with both α- and γ-form crystals present, the solution mixing route leads to α-phase formation only, as indicated by the two peaks located at 2θ values of 19.8° and 23.7° with corresponding d-spacing of 4.43 and 3.7 Å for (200) and [(002), (220)] planes, respectively. It is noteworthy that α-phase is the thermodynamically stable phase consisting of sheets of hydrogen-bonded chains that are packed in an anti-parallel fashion whereas the γ-phase is the least stable phase formed due to random hydrogen bonding between parallel chains [7]. Little difference is observed between XRD patterns for neat and PA6/P-CNT composite sample. In the case of N-CNT incorporation, however, the enhanced degree of crystallanity is manifested by more intense peaks. Due to low CNT content, however, peaks characteristic of CNT do not appear on the XRD patterns. Thus, both the solution mixing processing route with formic acid as the solvent and the presence of pristine or functionalized CNT as potential sites for heterogeneous nucleation promote preferential growth of α-form crystals in PA6/CNT composites. The crystallite size (t) was determined using the Scherrer equation and was found to be ~8.4 nm for the neat PA6 sample. While incorporation of N-CNT caused an increase in t to 10.5 nm, addition of P-CNT by the same amount led to a small drop in the t value to ~8 nm. Thus, CNT incorporation into PA6 improves crystalline quality by offering more nucleation sites for crystallization.

Using the attenuated total reflectance Fourier transform infra-red spectroscope (ATR-FTIR), the spectra for neat PA6 as well as PA6 composites containing 0.5 wt % of amine-functionalized nanotubes were obtained, as shown in Figure 4. For the neat PA6 sample, absorption bands positioned at 1625, 1530 and 1364 cm⁻¹ can be assigned to the amide I, amide II and/or H–C–H asymmetric deformation and amide III band and/or H–C–H waging, respectively. Despite very small CNT content in composite samples, additional absorption bands characteristics of carboxylic, acyl and amine groups were noticed in FTIR spectra. The location of all the bands and their respective chemical bond assignments are given in Table 1. There was not much difference between spectra obtained for PA6 composites containing pristine or amine-functionalized nanotubes. Also, the presence of a weak absorption band at 1203 cm⁻¹ confirmed formation of α-form crystals [18], which corroborates the finding of the XRD studies.

The effect of CNT incorporation on thermal stability of PA6 was assessed by heating neat and composite PA6 samples to ~850 °C under air atmosphere. As shown in the thermographs presented in Figure 5, neat and composite PA6 exhibited similar behavior until 275 °C, after which thermal degradation occurred in a manner that was dependent upon sample composition. For neat PA6, samples prepared using solution processing route indicated a higher temperature of onset of decomposition than those produced via in situ polymerization [19] and melt blending process [7] which is characteristic of more stable α-phase formed. Thus, our synthesis route involving extensive magnetic stirring at mildly high temperature of 100 °C produced more stable PA6 samples unlike those produced at higher temperatures, pressures, and/or sonication that may cause polymer chain fragmentation, formation of less stable γ-phase, or even reduction in CNT length.

Comparison of neat and composite PA6 samples revealed improvement in thermal stability upon CNT incorporation of any type (pristine or functionalized) with maximum increase in onset of thermal degradation observed in case of PA6/N-CNT composition. The zoomed in view of the thermograph (Figure 5 inset), depicting thermal degradation behavior in the range of 430 to 630 °C, clearly demonstrates that temperature indicative of initial decomposition increased by as much as 70 °C. Whereas neat PA6 sample started to undergo thermal degradation at ~450 °C, PA6/N-CNT exhibited similar behavior once heated to 520 °C. Thermal decomposition resulted in ~90 percent loss of initial sample weight at temperatures in the range of 700–730 °C, depending on sample composition. Another dip in the thermograph beyond a small plateau is anticipated to represent CNT decomposition.

Since thermo-oxidation is a well-known factor causing aging and degradation of PA6 in air, it is believed that CNT incorporation with homogeneous dispersion and strong PA6/CNT interfacial bonding reduced the extent of PA6 thermo-oxidation.

The effect of P- or N-CNT addition on crystallization and melting behavior of PA6 was assessed using DSC. Thermograms showing the crystallization behavior of neat PA6 and PA6/CNT composites containing 0.5 weight percent of pristine- or amine-functionalized CNT are presented in Figure 6a. While neat PA6 exhibits crystallization peak at 177.8 °C, an increase in crystallization temperature (T_C) by as much as 34 °C is observed in case of PA6/N-CNT sample. The T_C values for PA6/P-CNT and PA6/N-CNT composite samples were estimated to be 194.7 and 211.7 °C, respectively. The increase in T_C value upon MWCNT incorporation can be attributed to their role as a nucleating agent for the PA6 matrix. During melting of the PA6 and PA6/CNT nanocomposites, it is noticed that MWCNT incorporation increases the melting temperature (T_m) of the nanocomposite (Figure 6b). Furthermore, it is found that degree of increase in T_m is greater upon incorporation of amine functionalized MWCNT as compared to P-CNT. To varying degrees, the presence of MWCNT imposes restrictions on the motion of the PA6 chains upon heating, thus causing an increase in T_m by as much as ~17 °C for PA6/0.5N-CNT nanocomposite.

The results suggest that, in comparison with the melt extrusion process, solution mixing is a better technique since it promotes nucleation and growth of α-form crystals (instead of less stable γ-phase) and causes little or no damage to polymer structure during processing , thus yielding higher T_C values. Secondly, carbon nanotubes, whether unmodified or modified through chemical functionalization, improve the crystalline nature of PA6 by offering nucleation sites on nanotubes sidewalls. In our study, the T_C value obtained from addition of 0.5 wt % amine-functionalized CNTs is in the same range as that reported for incorporation of 2 wt % pristine-CNTs using melt mixing [22] and higher than those reported for PA6 composites containing pristine or COOH-functionalized nanotubes produced via the in situ polymerization process [23]. Also, functionalized nanotubes are better than pristine CNTs due to their better dispersion characteristics and strong PA6/CNT interfacial adhesion.