No abstract

Materials in nature such as nacre that are made of mechanically inferior building blocks exhibit extreme toughness at the macro scale because of the geometry and arrangement of their constituents. Taking a cue from these systems, we have investigated whether the molecular rearrangement in a heterogeneous polymeric system can alter toughness at the macro scale. To this end, a multicomponent bismaleimide system is employed and processed by using (a) a melt and cast (termed Melt) approach and (b) a dual asymmetric centrifuge based high-speed shear mixing (termed HSSM) approach to enforce molecular rearrangement. Cured HSSM BMI exhibited an extraordinary increase of 393% in impact strength compared to the cured Melt BMI (an increase from 14 ± 6 to 69 ± 13 kJ/m2), without high shear mixing. Prior studies in the literature have reported a maximum impact strength of only 19 kJ/m2 for any BMI system. FTIR, Raman, and NMR spectroscopies provide evidence of molecular rearrangement upon HSSM processing. This molecular rearrangement also enhances the glass transition temperature in cured HSSM BMI by 16 °C compared to the cured Melt BMI. Small-angle X-ray scattering shows electron density heterogeneity at the scale of ∼16 nm in cured HSSM BMI. A direct relationship between the domain size calculated from the SAXS peak position, its intensity, and the impact strength is observed. Fractographs of cured HSSM BMI show unique, near-spherical micronodular features of ∼10–20 μm diameter, not observed in cured Melt BMI specimens. This study provides a pathway for designing and manufacturing of materials with extreme fracture toughness.

One of the factors limiting the performance of polymer–carbon nanotube (polymer–CNT) composites is the insufficient load transfer across their interface. In this study, interfacial strain in polyurea–CNT systems with two types of CNT sheet has been studied. These are (a) as manufactured CNT sheets (termed unbaked CNT sheets) containing amorphous carbon and (b) thermally treated CNT sheets (termed baked CNT sheets) with amorphous carbon eliminated. The process of baking not only eliminates amorphous carbon but also creates chemically active defects in CNTs as well as sp3 carbon. These factors serve to enhance interfacial strain in the polyurea–CNT system. The shift in the Raman G′ band, while the sheets were mechanically strained, has been used to estimate the polyurea–CNT interfacial strain. The maximum Raman G′ downshift in the range of 33–39 cm–1 has been observed in composites of baked CNT sheets with 53 wt % polyurea. This is more than a factor of 2 higher than the downshift reported for any polymer–CNT system to date and represents a local CNT strain of ∼0.9–1% or a local CNT stress of 9–10 GPa. This study opens a window toward achieving the mechanical properties potential of bulk polymer nanocomposite for applications including, but not limited to, structural materials.

Polyacrylonitrile (PAN) is among the most promising precursor polymers to produce strong and lightweight carbon fiber. Conformations in solution and the extent of binding to carbon nanotubes (CNTs) are critical during gel spinning and for alignment of graphitic layers upon carbonization. First quantitative insights into these processes are reported using molecular dynamics simulations from the atomic scale including virtual π electrons and comparisons to experimental data. Common solvents for fiber spinning induce significant differences in PAN conformations in dilute solution at 25 C with persistence lengths between 0.5 and 2 nm.Variations in conformation become smaller at 75 C, in the presence of CNTs, and at higher PAN concentration. "Aging" of PAN conformations in dimethylformamide and dimethylsulfoxide at higher temperature is explained and a correlation between extended polymer conformations and increased binding to CNTs identified in dilute solutions. PAN is overall barely attracted to CNTs under common solution conditions and enters significant surface contact only at higher concentration as solvent is physically removed. The impact of temperature is small, whereby binding increases at lower temperature. The results provide first guidance to control interactions of polymers with CNTs to induce distinct conformations and specific binding at the early stages of assembly.

While the addition of well‐dispersed carbon nanotubes (CNTs) in polymer nanocomposites typically improves the polymers' impact strength, herein, we report that good CNT dispersion alone is not a sufficient condition to improve the impact strength of the nanocomposites. Our results demonstrate that the impact strength of the nanocomposite also depends on the CNT type and the nanocomposite processing conditions. Depending on these factors, CNTs can either disrupt the crosslinking network of bismaleimide (BMI), or the CNTs themselves can be compressed by up to 2.9 GPa upon the curing of BMI or both. The effect of these factors on the impact strength is discussed. Overall, impact strength of up to 54 kJ/m2 has been achieved, which is 80% higher than the best impact strength reported for any CNT‐BMI system in the literature.