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Processing, structure and properties of polyacrylonitrile fibers with 15 weight percent single wall carbon nanotubes, Arias-Monje, Lu, Ramachandran, Kirmani, Kumar, Polymer, 2020


Polyacrylonitrile (PAN) fibers containing as high as 15 wt% of single-wall carbon nanotubes (SWNTs) with good dispersion are produced for the first time. The properties (modulus 32.1 GPa, strength 0.8 GPa, electrical conductivity 2.2 S/m) are substantially higher to prior reported PAN fibers containing SWNTs, and the amounts of solvent and time needed for processing spinning solution are significantly reduced, thus being more appropriate for industrial mass production.

Scientific question:

How to make CNT-containing PAN fibers with high filler content, high mechanical reinforcement and easy/fast processing?

Key of how:

The SWNTs are non-covalently wrapped with poly(methyl methacrylate) (PMMA), and then the SWNT dispersion concentration is increased (7 to 200mg/dl) by solvent exchange and phase separation. This PAN-SWNT interface modification becomes part of the filler-matrix interphase, and significantly increases the content of well-dispersed SWNTs (for effective stress&strain transfer) and reduces the time for processing, thus enhancing the mechanical and electrical properties.   

Major points:

1. PAN-CNT fibers are of great interests as they can be converted to CNT-reinforced carbon fibers with further exceptional performance at industrial manufacturing level (spun from solution) for broad engineering applications.

2. There are several limitations: (1) the achievable filler (CNTs) content is low (<10wt%) due to the filler limiting the rheology and spinnability of the solution, (2) the mechanical reinforcement is low because the CNTs are not fully used with low stress transfer, (3) the processing is time-consuming and energy-expensive since dilute concentrations to guarantee CNT dispersion for solvent removal are mostly used.

3. Here present a methodology that can address the above issues. The high CNT content (up to 15wt%) and low processing time are achieved by firstly using PMMA to modify the SWNT surface and then processing spinning. Briefly, (1) obtain the PMMA-wrapped-SWNTs (the sonicated PMMA-SWNTs in DMF with free/excessive PMMA, the PMMA-SWNTs in ethyl acetate without free PMMA ~200 mg/dl), (2) prepare the spinning solutions: both solutions (PMMA/SWNTs in DMF and PMMA/SWNTs in ethyl acetate) are added to the PAN solution with excess solvent removed for desired compositions (1, 5-A, 5-B, 15wt%), (3) spin PAN fibers with PMMA-wrapped-SWNTs: dry-jet wet-spinning on a single filament system, by extruding the spinning solutions at certain rates, being coagulated in the methanol bath, being treated by a two-stage drawing, (4) test and characterize the fibers: tensile testing, DMA, WAXD, SAXS, Raman (custom-designed), SEM, Keithley 2400 source-meter, etc., to examine the mechanical properties, the SWNT strain, orientation and electrical conductivity, the fibrils morphology, the quantitative SWNT dispersion, etc.    

3. The PMMA wrapping around SWNTs is in an ordered helical fashion with non-covalent nature, which remains after solvent exchange, evaporation, and stirring.

By analyzing results and related literature, it is deduced that the crystalline PAN forms an orthorhombic structure as a coating along the SWNT, as the PMMA-SWNTs have good interaction with the PAN molecules and only ~15% of the SWNT length is covered by the PMMA.

4. Dilute solutions (5 mg/dl v.s. 110 mg/dl) show better dispersed SWNTs at 5wt% (less and smaller agglomerates, larger viscosity and storage modulus, higher homogeneity), but significantly increases the solvent-evaporation time (2 to 15 days).

Introducing the PMMA-SWNTs in ethyl acetate with high concentration (~200 mg/dl) into the PAN solution leads to substantially enhanced efficiency, 15wt% SWNTs, with 3 days evaporation time and the similar agglomerate sizes as the 5wt%.

5. Increasing the SWNT content increases the viscosity and elastic component of the polymer dispersions, while reducing the spinnability and lowering the stretch ratio and damping factor. Thus the content cannot be increased infinitely.

6. The fabricated PAN fiber (diameter 11-13 µm) show increasing tensile modulus and strength with increasing SWNTs content.

The 5-A and 5-B wt% mechanical properties show that better dispersed/individualized SWNTs in dilute solutions lead to higher strength and modulus, and higher draw ratio, explaining why the low filler concentration solutions have been used in literature.

7. Despite increasing the content significantly, the 15wt% SWNT-PAN fiber shows good dispersion (Raman, estimated SWNT bundle size, SEM) almost similar as the 5-B wt% one.

The PMMA is still helically wrapping the SWNTs, becoming part of the SWNT-PAN interphase.

The PAN phase has lowest crystallinity and smaller crystal size than the 5-B wt% one, as higher content of well-dispersed SWNTs restrict the space and movement of PAN molecules.

8. The fibrils shown from fractured and dissolved 15wt%SWNT-PAN fiber exhibit similar diameter distribution, 10.3 to 3.8 nm. These fibrils are believed to be formed by PMMA-wrapped SWNT bundles covered by PAN with varying thicknesses; the different fibril diameter and PAN coating thickness are analyzed from measurements in filler strain (PMMA increases strain transfer from the matrix to the SWNTs), G’ shift rate and maximum G’ shift.  

9. The 15wt% SWNT-PAN fiber show significant higher modulus, strength and elongation at strain, in spite of lowest draw ratio and PAN crystallinity, demonstrating the reinforcement provided by the SWNTs.

10. DMA results reveal the interactions between the PMMA-wrapped-SWNTs and the PAN matrix, as SWNTs affect the mobility of the amorphous PAN molecules.

Thermal behavior analyses indicate that the increase in tanδ beyond βc of the SWNT-PAN fibers originates by the presence of PAN interactions with the PMMA-wrapped-SWNTs that restricts chain mobility or altogether chain repetition (and increase amorphous contribution for 15wt% fiber).

11. The 15wt%SWNT-PAN fibers show electrical conductivity and modulus that are 5 orders of magnitude and 3 times higher than other reported PAN fibers with high carbon nanotube concentrations, respectively.

PS: can the PMMA sustain high temperature or how will the as-fabricated fibers be after oxidization, carbonization and graphitization?

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