Aligned carbon nanotube morphogenesis predicts physical properties of their polymer nanocomposites

Abstract

Tomography derived nanoscale 3D morphological information is combined with modeling and simulation to explain anisotropy and scaling of experimental mechanical, thermal, and electrical properties of aligned carbon nanotube polymer composites.

High-Volume-Fraction Textured Carbon Nanotube–Bis(maleimide) and −Epoxy Matrix Polymer Nanocomposites: Implications for High-Performance Structural Composites

“…9,12,15 In addition, this axial indentation modulus scales nonlinearly with CNT V f in both PNCs, similar to the nonlinear scaling of the axial indentation modulus with increasing CNT V f that was previously observed for A-CNT arrays 11 and A-CNT−epoxy PNCs measured up to 20 vol %. 12,15,61 This was attributed to w decreasing at increasing CNT V f by up to a factor of 2 8,9,61 (consistent with the Raman spectroscopy results discussed in section S4 in the Supporting Information) during the biaxial mechanical densification process, thereby yielding a higher contribution of stiff longitudinal CNT reinforcement to the PNC mechanical behavior with high CNT V f .…”

“…Figure 8 shows the scaling of the axial and transverse (i.e., parallel/longitudinal and perpendicular) indentation moduli of the BMI and epoxy PNCs after fabrication, curing, and microtoming (see Methods). For both the BMI and epoxy PNCs, the transverse force−displacement curves (see section S5 of the Supporting Information for additional details and data sets) exhibit a similar mechanical response at all CNT V f because of polymer matrix domination in this orientation, 9,61 while the axial curves show a significantly enhanced mechanical response because CNT V f increases from 0 to 30 vol % (i.e., a smaller maximum indentation depth with a larger slope at the point of unloading) because of alignment with the CNT axial direction. As shown in Figure 8, this results in a ∼2× increase in the axial indentation modulus and anisotropy ratio (axial/ transverse) for 30 vol % CNTs compared to the cured neat resin: ∼5.8−13.43 GPa in BMI PNCs with an anisotropy ratio up to 2.23 compared to ∼5.0−10.67 GPa in epoxy PNCs with an anisotropy ratio of up to 2.04.…”

“…Although the axial moduli do increase with CNT V f , it is observed that, even at 30 vol % CNTs, they are still less than estimates via the rule of mixtures (>100 GPa for 1 TPa stiffness CNT walls) 63 because of CNT waviness (w) governing the mechanical behavior, which can reduce the effective elastic axial modulus of long-fiber reinforced composites, including A-CNT arrays, by several orders of magnitude. 8,9,11,12,15,61 This highlights the importance of CNT waviness and the alignment direction that govern the resulting mechanical properties, because CNTs that are oriented either parallel 64 or perpendicular 65 to the film direction can exhibit different tensile or compressive behaviors, respectively, with the highest mechanical properties observed in the tensile direction for composites with uniform CNT distribution and high alignment in the film direction. 64 Further, because of the wavy CNTs, the compressive versus tensile behavior (including to failure) may be different in these nanocomposites compared to traditional aligned microfiber composites, and this will be explored in future work with scaled specimens, discussed in the next section.…”

“…To more accurately describe the scaling of axial indentation moduli that accounts for CNT waviness in the PNCs tested here, a previously developed simulation framework 9,61 for PNCs with 10 4 CNTs within the matrix was applied to this nanoindentation analysis. This model simulated the evolution of w with CNT V f to evaluate the scaling of the effective CNT elastic modulus with w by incorporating the extension, shear, and bending contributions to the effective CNT compliance in the A-CNT PNC.…”

“…This model simulated the evolution of w with CNT V f to evaluate the scaling of the effective CNT elastic modulus with w by incorporating the extension, shear, and bending contributions to the effective CNT compliance in the A-CNT PNC. 9,61 For this nanoindentation analysis, the reinforcement modulus of the CNTs (E CNT ) 8,9 at various V f values was determined using the simulation technique and used in the axial rule-of-mixtures formula, 9,61 along with w and the polymer matrix elastic modulus (E m ), to estimate the composite elastic modulus, E PNC (V f ):…”