The effect of nanotube content and orientation on the mechanical properties of polymer-nanotube composite fibers: Separating intrinsic reinforcement from orientational effects

We have measured the mechanical properties of coagulation-spun polymer-nanotube composite fibers. Both the fiber modulus, Y, and strength, σB, scale linearly with volume fraction, Vf, up to Vf ∼10%, after which these properties remain constant. We measured dY/dVf = 254 GPa and dσB/dVf = 2.8 GPa in the linear region. By drawing fibers with Vf<10% to a draw ratio of ∼60%, we can increase these values to dY/dVf = 600 GPa and dσB/dVf = 7 GPa. Raman measurements show the Herman's orientation parameter, S, to increase with drawing, indicating that significant nanotube alignment occurs. Raman spectroscopy also shows that the nanotube effective modulus, YEff, also increases with drawing. We have calculated an empirical relationship between the nanotube orientation efficiency factor, ηo, and S. This allows us to fit the data for YEff versus ηo, showing that the fiber modulus scales linearly with ηo, as predicted theoretically by Krenchel. From the fit, we estimate the nanotube modulus to be; YNT = 480 GPa. Finally, we show that the fiber strength also scales linearly with ηo, giving an effective interfacial stress transfer of τ = 40 MPa and a nanotube critical length of l c=1250 nm. This work demonstrates the validity of the Cox-Krenchel rule of mixtures and shows that continuum theory still applies at the near-molecular level. The strength and stiffness of polymer-nanotube fibers as a function of nanotube content and orientation is presented. The rule of mixtures, including Krenchel's orientation dependence, applies extremely well to such nanocomposites. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.