Among the many potential applications of carbon nanotubes (CNT), its usage to strengthen polymers has been paid considerable attention due to the exceptional stiffness, excellent strength, and the low density of CNT. This has provided numerous opportunities for the invention of new material systems for applications requiring high strength and high modulus. Precise control over processing factors, including preserving intact CNT structure, uniform dispersion of CNT within the polymer matrix, effective filler–matrix interfacial interactions, and alignment/orientation of polymer chains/CNT, contribute to the composite fibers’ superior properties. For this reason, fabrication methods play an important role in determining the composite fibers’ microstructure and ultimate mechanical behavior. The current state-of-the-art polymer/CNT high-performance composite fibers, especially in regards to processing–structure–performance, are reviewed in this contribution. Future needs for material by design approaches for processing these nano-composite systems are also discussed.

In order to improve stress transfer between polymer matrixes and nanofillers, controlling the structure development in the interphase region during composite processing is a necessity. For polyacrylonitrile (PAN)/single-wall carbon nanotubes (SWNT) composites, the formation of the PAN interphase in the presence of the SWNT as a function of processing conditions is studied. Under these conditions, three distinct interfacial coating morphologies of PAN are observed on SWNT. In the semidilute polymer concentration regime subjected to shearing, PAN extended-chain tubular coatings are formed on SWNT. Dilute PAN/SWNT quiescent solutions subjected to cooling yields hybrid periodic shish-kebab structures (first observation for PAN polymer), and dilute PAN/SWNT quiescent solutions subjected to rapid cooling results in the formation of an irregular PAN crystalline coating on the SWNT.

The extraordinary structural properties of graphene and carbon nanotube materials motivate the development of practical methods for their use in fabricating continuous, strong, and tough composite fibers. Poly(vinyl alcohol) (PVA)/carbon nano-chip fiber (CNCF) composite tapes with 0.5 wt % loading show that Young's modulus, tensile strength, and toughness are increased by 585%, 653%, and 20%, respectively as compared to the control (PVA) tapes. Nano-chips exfoliated from the CNCF during processing, lubricate polymer chain alignment, and orientation during drawing, where composite tapes could be drawn to higher draw ratios compared with the control tapes. As a result, the Herman's orientation factor (f) increased from 0.5 (control tape) to 0.8 (composite tape). Theoretical analysis shows $ 16 vol % of the composite tapes consists of fully oriented PVA chains, which contributes to its exceptional mechanical performance.

PVA/single-walled nanotube (SWNT) composite fibers are fabricated using a steady shear-flow gel-spinning method. The resultant fibers show excellent tensile strength, modulus, and toughness of 4.9 GPa, 128 GPa, and 202 J Á g À1 , respectively. Templated interfacial crystallization of PVA in the vicinity of SWNT is controlled by tailoring the degree of undercooling of PVA during the composite solution preparation. WAXD shows that the templated crystallization behavior of the PVA at the SWNT interfacial region is new. PVA/SWNT fibers that exhibit interfacial structure show a predominant crystallization plane of (001) as compared to the (101) plane seen in PVA/SWNT fibers without a distinct interfacial structure. This demonstrates that the PVA interfacial region around SWNT has denser crystalline chain-packing.

A combination of computational and experimental methods was implemented to understand and confirm that conformational changes of a polymer [specifically polyacrylonitrile (PAN)] vary with the dispersion quality and confinement between single-wall carbon nanotubes (SWNT) in the composite fibers. A shear-flow gel-spinning approach was utilized to produce PAN-based composite fibers with high concentration (i.e., loading of 10 wt %) of SWNT. Dispersion qualities of SWNT ranging from low to high were identified in the fibers, and their effects on the structural morphologies and mechanical properties of the composites were examined. These results show that, as the SWNT dispersion quality in terms of distribution in the fiber and exfoliation increases, PAN conformations were confined to the extended-chain form. Full atomistic computational results show that the surface interaction energy between isolated PAN and SWNT was not preferred, leading to the self-agglomeration of PAN. However, confinement of the polymer chains between SWNT bundles or individual tubes (i.e., molecular crowding) resulted in large increases in the PAN-SWNT interaction energy. In other words, the crowding of polymer chains by the SWNT at high concentrations can promote extended-chain conformational development during fiber spinning. This was also evidenced experimentally by the observance of significantly improved PAN orientation and crystallization in the composite. Ultimately this work provides fundamental insight toward the specific structural changes capable at the polymer/nanotube interface which are important toward improvement of the effective contribution of the SWNT to the mechanical performance of the composite.