(383c) Probing Multiscale Structure in Aligned Nanotube-Polymer Composites

Composites made from vertically aligned carbon nanotube (CNT) forests are important for a broad range of applications, such as for thermal management, high-flux membranes, protective fabrics, mechanical reinforcement, and energy dissipation. Despite progress, there remains a need for quantitative understanding of how the multiscale structure of CNT forests dictates both the fabrication of advanced composites and ultimately their performance. Drawing such quantitative relationships is challenging because measuring the morphological structure and order in CNT forests is difficult for a) multi-component systems, b) over large range of length scales, and c) without destroying the material.

To nondestructively investigate salient CNT forest characteristics, we built upon a suite of complementary soft and hard X-ray scattering techniques including polarized resonant soft X-ray scattering (R-SoXS â?? micro/meso/nano), small- and wide-angle X-ray scattering (SAXS/WAXS â?? nano/atomic) and X-ray attenuation. R-SoXS is a novel technique at the Advanced Light Source (ALS) ideally suited for composite characterization because it enables selective probing of the CNTs and their polymer matrix viascattering with chemical specificity by tuning of incident photon energy. Using our combined synchrotron techniques, we analyzed high-performance nanocomposite membranes fabricated by: 1) synthesizing self-aligned CNT forests by chemical vapor deposition; 2) infiltrating the void space between CNTs with a polymeric material; and 3) then etching open the ends of a large number of CNTs to expose their core volume to fluid transport.

In particular, we mapped structural characteristics in self-aligned CNT forests/composites with chemical, spatial, and length-scale resolution. We developed a unified analytical model to quantitatively describe the structural hierarchy of aligned CNTs and to extract key structural parameters via curve-fitting of our X-ray scattering data. From the bottom up, our model parameters define the graphitic lattice and wall number (atomic), CNT diameter (nano), CNT bundling and spacing (meso), regular corrugations (micro), and number density (macro). Comparison with electron microscopy reveals good agreement for forests with a wide range of characteristics (e.g., 1-11 walls, 1.5-15 nm diameter, 10¹⁰-10¹² cm^-2density). With R-SoXS as our complementary chemical probe, we also analyzed the structure of the polymer matrix around the outside of the CNTs and ions inside the CNT nanochannels.

Finally, we show how the structural characteristics of our CNT forests inform the fabrication of large-area, flexible polymer composites toward their application as ultrabreathable and protective membranes.¹ We leveraged composite structural information to precisely quantify the fluid flow enhancement measured experimentally through the CNT nanochannels in our membranes. This is an exciting advancement because insufficient information regarding the CNT membrane structural parameters is thought to be the primary source of orders-of-magnitude discrepancies in the ultrafast fluid transport inside CNT nanochannels reported in the literature.² More precise understanding of the transport in these bulk materials may help elucidate open controversies regarding transport through individual CNT nanochannels.³ While we focused here on fluidic properties, our methodology is broadly applicable for drawing quantitative, multiscale relationships between structure and other (mechanical, thermal, electrical, etc.) properties in nanocomposite systems.

This work is supported by the Defense Threat Reduction Agency (DTRA) D[MS]2 project under Contract No. BA12PHM123 and was performed under the auspices of U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

1. Bui, N.; Meshot, E. R.; Kim, S.; Peña, J.; Gibson, P. W.; Wu, K. J.; Fornasiero, F., Ultrabreathable and Protective Membranes with Sub-5 nm Carbon Nanotube Pores. Advanced Materials 2016.

2. Kim, S.; Fornasiero, F.; Park, H. G.; In, J. B.; Meshot, E.; Giraldo, G.; Stadermann, M.; Fireman, M.; Shan, J.; Grigoropoulos, C. P.; Bakajin, O., Fabrication of Flexible, Aligned Carbon Nanotube/Polymer Composite Membranes by in-Situ Polymerization. Journal of Membrane Science 2014, 460(0), 91-98.

3. Guo, S.; Meshot, E. R.; Kuykendall, T.; Cabrini, S.; Fornasiero, F., Nanofluidic Transport through Isolated Carbon Nanotube Channels: Advances, Controversies, and Challenges. Advanced Materials 2015, 27 (38), 5726-5737.