Tuning the mechanical properties of composites from elastomeric to rigid thermoplastic by controlled addition of carbon nanotubes

A commercial thermoplastic polyurethane is identified for which the addition of nanotubes dramatically improves its mechanical properties. Increasing the nanotube content from 0% to 40% results in an increase in modulus, Y, (0.4-2.2 GPa) and stress at 3% strain, Δ_ε = 3%, (10-50 MPa), no significant change in ultimate tensile strength, Δ_B, (≈50 MPa) and decreases in strain at break, ε_B, (555-3%) and toughness, T, (177-1 MJ m^-3). This variation in properties spans the range from compliant and ductile, like an elastomer, at low mass fractions to stiff and brittle, like a rigid thermoplastic, at high nanotube content. For mid-range nanotube contents (≈15%) the material behaves like a rigid thermoplastic with large ductility: Y = 1.5 GPa, Δ_{ε = 3%} = 36 MPa, Δ_B = 55 MPa, ε_B = 100% and T = 50 MJ m^-3. Analysis suggests that soft polyurethane segments are immobilized by adsorption onto the nanotubes, resulting in large changes in mechanical properties. The yield stress of thermoplastic polyurethane can be dramatically increased by the addition of nanotubes, while the strain at break degrades significantly. The combination of yield stress and strain at break varies with nanotube content, resembling an elastomer at low content but a rigid thermoplastic at high content. By tuning the nanotube content the mechanical properties of a range of commodity polymers can be mimicked.