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.

A B S T R A C TOrdered polyacrylonitrile (PAN) interphase structures were formed in solution-cast PAN/carbon nanotube (CNT) composite films by enhancing polymer crystallization conditions and processing parameters for five types of CNTs. All film samples were heat-treated using similar stabilization and carbonization (up to 1100°C) processes. Both the precursor and carbonized materials were characterized by electron microscopy and X-ray spectroscopy. Highly ordered graphitic structure was formed predominantly in the carbonized materials at 1100°C (i.e., 1500°C lower than the temperature used in a commercial graphitization process). The ordering of the graphite structure formed at 1100°C was further improved by heat treatment up to 2100°C. Multiple characterization results indicate that the early onset of PAN conversion to graphite is directly related to the polymer interphase formation as well as the CNT type. Based on the stabilization and carbonization parameters used in this study, PAN/single-wall carbon nanotube (SWNT) samples showed more prevalent graphite formation at 1100°C. This work demonstrates the influence of CNT type regarding interfacial confinement toward this low-temperature polymer-to-graphite conversion process.

A comparative study on interfacial crystallization of isotactic polypropylene (iPP) surrounding macroscopic carbon nanotube and graphene fibers has been carried out in single fiber polymer composites by means of in situ polarized optical microscope, scanning electron microscope and X-ray diffraction. Ordered interfacial microstructures of iPP nucleate on both nanocarbon fibers in the form of a transcrystalline interphase. Nanotube fibers tend to promote negative birefringence transcrystals whereas graphene fibers induce positive birefringence transcrystals. The microstructures of transcrystals are strongly dependent on the thermal history and the double-layered transcrystals consisting of a negative inner layer and a positive outer layer occur under certain conditions. Transcrystallization kinetics has been studied and the Lauritzen-Hoffman theory of heterogeneous nucleation used to analyze the dynamic crystallization process. While the fold surface energy of iPP transcrystals surrounding both nanocarbon fibers shows little difference, the nanotube fiber promotes shorter induction time than the graphene fiber. Thermal resistance test demonstrates that the ordered interfacial microstructures possess higher melting temperature in the nanotube fiber composites than those in the graphene fiber composites. Under appropriate conditions, the-form transcrystals of iPP are observed. The amount of the-form iPP surrounding the nanotube fiber is much higher than that surrounding the graphene fiber. A theoretical model is proposed to interpret the difference between the nanotube and graphene fiber composites and the mechanisms behind its influence on interfacial crystallization.

A joint experimental and computational approach is used to explore the exfoliation mechanism for hexagonal boron nitride (h-BN) in polyethylene (PE)/h-BN composite fi bers during hot-drawing. A shear-fl ow gel-spinning apparatus is utilized to fabricate PE/h-BN composite fi bers with 11 wt% h-BN loading. Different exfoliation states of the h-BN platelets before and after hot-drawing are experimentally examined using wide-angle X-ray diffraction and Raman spectroscopy. Compared with the undrawn (as-spun) fi bers, both analyses show that the intensity of the major h-BN peaks attributed to interlayer interaction signifi cantly decreased for the drawn fi bers, suggesting exfoliation of the h-BN. A full atomistic steered molecular dynamics approach is used to obtain baseline force and work required for h-BN layer separation, as well as to simulate the h-BN exfoliation behavior as a result of the PE matrix shearing effect in the composite. Computational results indicate that a large interactive area between the polymer and the fi llers is required to induce enough stress transfer to exceed the h-BN exfoliation force/energy threshold. Once this threshold is achieved, complete exfoliation of the platelets to monolayer h-BN is demonstrated. By understanding the relationship between interfacial area and interaction strength between polymer matrix and fi llers, this work provides new insight toward use of polymers for producing mono-and few-layered h-BN.

Hexagonal boron nitride (BN) platelets are used for fabrication of ultrahigh molecular weight polyethylene (UHMWPE)/BN composite fibers using a flow‐crystallization process. As‐spun fibers are subsequently drawn near the UHMWPE melting temperature. Interfacial crystallization of UHMWPE is observed on the surfaces of the BN platelets. Both X‐ray and Raman spectroscopy analysis of the drawn composite fibers show a significant decrease in the intensity of the signature BN peaks associated with the platelet interlayer spacing, as compared to undrawn samples. This suggests exfoliation of the BN platelets occurs during the fiber hot‐drawing process. Exfoliation of BN platelets is induced due to interfacial stress transfer between both components (i.e., UHMPWE and BN) within the composites. This stress transfer is facilitated by molecular rearrangement of polymer matrix during drawing (i.e., unfolding of the polymer chain). The changes in micro‐ and nanoscale morphologies due to unfolding of the polymer chains as well as simultaneous exfoliation of the BN platelets are fully characterized and show that these structural changes provide insight for understanding the property trends of the fibers.