A persistent lack of detailed and quantitative structural
analysis
of these hierarchical carbon nanotube (CNT) ensembles precludes establishing
processing-structure-property relationships that are essential to
enhance macroscale performance (e.g., in mechanical, electrical, thermal
applications). Here, we use scanning transmission X-ray microscopy
(STXM) to analyze the hierarchical, twisted morphology of dry-spun
CNT yarns and their composites, quantifying key structural characteristics
such as density, porosity, alignment, and polymer loading. As the
yarn twist density increases (15,000 to 150,000 turns per meter),
the yarn diameter decreased (4.4–1.4 μm) and density
increased (0.55–1.26 g·cm–3), as intuitively
expected. Yarn density, ρ, ubiquitously scaled
with diameter d according to ρ ∼ d
–2 for all parameters studied here. Spectromicroscopy
probes with 30 nm resolution and elemental specificity were employed
to analyze the radial and longitudinal distribution of the oxygen-containing
polymer content (∼30% weight fraction), demonstrating nearly
perfect filling of the voids between CNTs with a vapor-phase polymer
coating and cross-linking process. These quantitative correlations
highlight the intimate connections between processing conditions and
yarn structure with important implications for translating the nanoscale
properties of CNTs to the macroscale.