Selective Mechanical Reinforcement of Thermoplastic Polyurethane by Targeted Insertion of Functionalized SWCNTs

We have prepared composites from a thermoplastic polyurethane reinforced with functionalized single walled nanotubes. Nanotubes with two types of functional groups were used: water-soluble tubes functionalized with polyethyleneglycol or poly(amino benzene sulfonic acid) and tetrahydrafuran-soluble tubes functionalized with octadecylamine. Composites prepared with water- or tetrahydrafuran-soluble tubes showed markedly different properties. Addition of water-soluble tubes tended to result in crystallization of the polyurethane soft segments, whereas addition of the tetrahydrafuran-soluble tubes promoted crystallization of the polyurethane hard segments. We interpret this as evidence of selective insertion of tubes in either hard or soft segments depending on the surface chemistry of the (functionalized) nanotube and the chemical structure of the segment. This interpretation is supported by differences in the mechanical properties of the composites. The water-based composites tend to be stiffer and display higher plateau stress, consistent with reinforcement of the soft segments. However, the tetrahydrafuran cast composites tend to maintain their strength and ductility at higher nanotube loading levels, whereas the water-based composites become weak and brittle above ∼10 vol % nanotubes. This is consistent with the water-based nanotubes impeding the extension and motion of the soft segments, resulting in loss of ductility. In contrast, the tetrahydrafuran-soluble nanotubes become segregated in the hard segments and so do not negatively impact on the mechanical properties at high nanotube content. This controlled reinforcement has allowed us to prepare composites with modulus, plateau stress, strength, and ductility of up to 250 MPa, 8 MPa, 60 MPa and 750%, respectively, significantly better than neat polyurethane.