Interface-Mediated Nanophase Network Formation of Poly(Urethane-Urea) in Aligned Carbon Nanotube Nanocomposites: Implications for Advanced Structural Composites

Poly(urethane-urea) (PUU) is a biphasic, structural polymer that allows for tuning of mechanical properties through tailoring of stoichiometries and interactions of hard and soft segments. In this work, we demonstrate that PUU self-assembles into nanophase networks when polymerized in situ in the presence of high-density aligned carbon nanotube (A-CNT) arrays, where spacing between CNT fibers is 10s of nanometers, revealing strong process-structure–property relations. PUU with two stoichiometric variations were investigated: a 2:1:1 (PUU211) and a 5:4:1 (PUU541) ratio of isocyanate:diamine:polyol at three A-CNT volume percents (0, 1, and 10 vol % A-CNTs). Along the A-CNT long axis, both PUU stoichiometries form elongated hard-segment structures, attributed to the preferential bonding of the urea moieties to the surfaces of the CNTs. The CNTs influence the nanophase self-assembly prior to polymerization, which leads to oriented nanophases in the cured matrix. The more stiff PUU541, with a greater extent of urea moieties, more readily patterns hard segments off the A-CNTs. Longitudinal stiffness of PUU541 in the CNT axial direction increased from 1.35 GPa in the neat polymer to 3.20 GPa in the polymer nanocomposite (PNC) with 10 vol % A-CNTs, significantly higher than 1.30 GPa for PUU211 PNC with 10 vol % A-CNTs. This represents a stiffness increase of 2800% and 137% in the PUU211 and PUU541, respectively, as a result of 10 vol % A-CNTs, a mechanical enhancement 3× higher than expected via rule of mixtures, and attributed to PUU biphasic network morphology change. Additionally, the mechanical anisotropy observed from nanoindentation correlates well with the PUU network morphology of the nanocomposites revealed by atomic force microscopy (AFM) and the changes in phase size found from wide- and small-angle X-ray spectroscopy (WAXS and SAXS) measurements. These strong property-structure relations between biphasic PUU and densely packed A-CNTs provide additional routes toward tuning of properties for PUU nanocomposites for advanced structural composites.