Learning from Nature: Molecular Rearrangement in the Bismaleimide System Leading to Dramatic Increase in Impact Strength

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.