Super-Tough and Environmentally Stable Aramid. Nanofiber@MXene Coaxial Fibers with Outstanding Electromagnetic Interference Shielding Efficiency

Highlights Super-tough and ultra-strong ANF@MXene fibers are wet-spun by a coaxial technique. High toughness of ~ 48.1 MJ m−3 and strength of ~ 502.9 MPa are achieved. The fibers exhibit superb chemical stability under extreme environmental conditions. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-022-00853-1.


Fig. S1
Schematic illustrating the deprotonation of PPTA fibers with KOH to form an ANF solution        Differently, the f value of the 17-0.9 fiber can reach 0.84, and its Π value becomes 0.86 (Fig.  S11). In particular, the ANF@M fibers have significantly higher orientation than pure MXene fibers, highlighting the spatial confinement effect of the ANF shell on the improvement of orientation and arrangement of the MXene core. In addition to the drawing effect, the fluid drafting also provides favorable confinement in radial fiber direction [S1]. With the increase of R value, the overall fiber structure steadily becomes waist-shaped, and the tightly arranged origami-like MXene core indicates a higher order arrangement of MXene sheets during the spinning process (Figs. 1d and S11). As reported, the orientation degree of the fibers depends on the R, and the appropriate R can maximize the order arrangement of the microfibrils and nanosheets along the fiber axis [S2, S3]. Interestingly, the 17-1.1 fiber shows the highest f value of 0.90 as compared to those of 17-0.9 (0.84) and 17-1.3 (0.88) fibers. Consistently, the 17-1.1 fiber has the smallest half-maximum (fwhm) of 23.9° and the highest orientation degree (Π) of 0.87, validating the largest crystal plane orientation [S1]. To explain these results, the weak tensile force at low R only cannot fully stretch the sheets and microfibrils; whereas, a too high R also causes incompatible orientation of the inner and outer layers due to their different rheological properties and fluidities    The rheological properties of the spinning dopes are critical for the spinnability and fiber morphology. The zero-shear viscosity of the MXene suspension can reach 100 Pas at 50 mg mL -1 (M50) (Fig. S14a). The reduction of MXene content from 50 to 20 mg mL -1 (M20) obviously decreases the viscosity of the spinning dope. Note that the viscosity of the ANF dispersion is far higher than that of the MXene spinning dopes (M50, M20), as evidenced by the almost frequency-independent modulus within most frequency range explored (Fig. S14b). In contrast, the elastic modulus (G') and viscous modulus (G'') of M20 and M50 are more dependent on the frequency. However, the G'/G'' values of the three spinning dopes are all greater than 1. Their viscoelastic gel properties make them suitable for wet spinning (Fig. S15) [S4].   The shell thickness also plays a crucial role in determining the mechanical properties of ANF@M fibers. When the shell thickness is reduced, the tensile strength and elongation at break are reduced to 230.5 MPa and 3.4% (18-1.1 fiber), respectively ( Fig. S18-S19). The decrease in the thickness of the ANF shell results in a decrease in the comprehensive mechanical properties of the fiber, which proves the importance of the high-performance shell [S5]. However, when the shell thickness increases, the elongation at break of the 16-1.1 fiber are 13.1%, higher than that of 17-1.1 fiber; whereas the toughness and tensile strength of 16-1.1 fiber are 34.2 MJ m -3 and 329.2 MPa, similar to those of 17-1.1 fiber. The reason is that when the core layer is unchanged and the thickness of the shell layer is increased, the overall diameter of the fiber increases, and the interface defects also increase accordingly.

Fig. S26 Digital photo of ANF@M fibers woven into a net to bounce a table tennis
Nano-Micro Letters S11/S17   Different from the tabular core-shell ANF@M fiber, the nearly circular ANF fiber is composed of numerous closely stacked microfibers encapsulated by a smooth skin (Fig. S29). The obvious plastic deformation zone in the stress-strain curve of the neat ANF fiber indicates its supertough feature (Fig. S27). After fracturing, the microfibrils are highly stretched and orientated along the tensile direction (Fig. S29b), providing the ANF fiber with outstanding tensile strength and toughness.  The results of the reflection loss (SER), absorption loss (SEA), and total shielding value (SETotal) indicate the absorption-dominated EMI shielding mechanism (Fig. S32). When the mesh grid or the thickness of the conductive layer varies, the SER value is almost unchanged, while the SEA increases linearly with the SETotal, indicating that the decrease in the pore size or the increase in the thickness of the conductive layer mainly affects the SEA of the textile [S6]. The SER is mainly related to the electrical conductivity of the textile, while the SEA is related to the structure of the textile. Small pores or multiple layers can effectively increase the SEA, thereby increasing the EMI SE of the textile [S7, S8].     * rGO: reduced graphene oxide; PEDOT: poly(3,4-ethylenedioxythiophene); ANF: aramid nanofiber; TOCNFs: TEMPO (2,2,6,6-tetramethylpiperidine-1-oxylradi-cal)-mediated oxidized cellulose nanofibrils.
# The concentration of MXene solution is 50 mg mL -1 ; ## The concentration of MXene solution is 20 mg mL -1 .

Video S1
The circular motion process.

Video S2
The ANF@M textile can withstand vigorous bending and kneading.

Video S3
A ball can be bounced on the net of the fibers.

Video S4
The ANF@M textile can bear the falling impact of a ball.