Strong Dependence of Mechanical Properties on Fiber Diameter for Polymer−Nanotube Composite Fibers: Differentiating Defect from Orientation Effects

We have prepared polyvinylalcohol−SWNT fibers with diameters from ∼1 to 15 μm by coagulation spinning. When normalized to nanotube volume fraction, V f, both fiber modulus, Y, and strength, σB, scale strongly with fiber diameter, D: Y/V f ∝ D −1.55 and σB/V f ∝ D −1.75. We show that much of this dependence is attributable to correlation between V f and D due to details of the spinning process: V f ∝ D 0.93. However, by carrying out Weibull failure analysis and measuring the orientation distribution of the nanotubes, we show that the rest of the diameter dependence is due to a combination of defect and orientation effects. For a given nanotube volume fraction, the fiber strength scales as σB ∝ D −0.29 D −0.64, with the first and second terms representing the defect and orientation contributions, respectively. The orientation term is present and dominates for fibers of diameter between 4 and 50 μm. By preparing fibers with low diameter (1−2 μm), we have obtained mean mechanical properties as high as Y = 244 GPa and σB = 2.9 GPa.