Rheology-tailored stable aramid nanofiber suspensions for fabricating ultra-strong and electrically insulated additive-free nanopapers

Although aramid nanofiber paper (ANF-P) is a promising alternative to conventional electrical insulation paper, its performance requires further optimization. This study aimed to establish an optimal nanopaper-fabrication process by using rheology-controlled suspensions to achieve remarkably strengthened pure ANF-Ps. The ANF-P was fabricated in two steps: 1) preparing ANF suspension (ANF-SH???O) by precipitating ANF/dimethyl sulfoxide (DMSO) (ANF-DDMSO), and 2) preparing ANF-P by vacuum-filtrating ANF-SH2O. Under an in-situ homogenization-assisted precipitation in step 1, the concentration of ANF-DDMSO predominantly affected both the suspension stability and nanopaper performance. Semi-dilute ANF-DDMSOs (0.1???0.7 wt%) produced stable suspensions and strong ANF-Ps (mechanical modulus and strength of 4.5???5.1 GPa and 221.4???243.4 MPa, respectively), while concentrated ANF-DDMSOs (1.0???2.0 wt%) yielded unstable suspensions and weak ANF-Ps (0.3???3.2 GPa and 12.6???139.0 MPa, respectively). The former ANF-SH2Os comprised branched or sheet-like precipitated particles that were favorable for structuring the paper, whereas the latter ones consisted of irregular particles. Particularly at a thickness of 17 ??m, ANF-Ps from 0.3 wt% ANF-DDMSO exhibited record-high mechanical performances (modulus, strength, and toughness of 7.4 GPa, 382.3 MPa, and 32.5 MJ???m???3, respectively) compared to previously reported pure ANF-Ps. In addition, ANF-Ps exhibited a remarkable dielectric breakdown strength of ???200.3 kV???mm???1. Rheologically, ANF-SH2Os from semi-dilute ANF-DDMSOs provided a higher scaling exponent of elastic modulus, indicating a higher degree of particle entanglement. Moreover, the strain-induced modulus overshoot phenomena revealed a highly structured suspension network. Therefore, linear- and nonlinear-suspension rheology provide a fundamental guideline for fortifying the foundation of industrial production of high-performance nanopapers