Bioinspired design and macroscopic assembly of poly(vinyl alcohol)-coated graphene into kilometers-long fibers.

Nacre is characterized by its excellent mechanical performance due to the well-recognized "brick and mortar" structure. Many efforts have been applied to make nacre-mimicking materials, but it is still a big challenge to realize their continuous production. Here, we prepared sandwich-like building blocks of poly(vinyl alcohol) (PVA)-coated graphene, and achieved high-nanofiller-content kilometers-long fibers by continuous wet-spinning assembly technology. The fibers have a strict "brick and mortar" layered structure, with graphene sheet as rigid brick and PVA as soft mortar. The mortar thickness can be precisely tuned from 2.01 to 3.31 nm by the weight feed ratio of PVA to graphene, as demonstrated by both atomic force microscopy and X-ray diffraction measurements. The mechanical strength of the nacre-mimicking fibers increases with increasing the content of PVA, and it rises gradually from 81 MPa for the fiber with 53.1 wt% PVA to 161 MPa for the fiber with 65.8 wt% PVA. The mechanical performance of our fibers was independent of the molecular weight (MW) of PVA in the wide range of 2-100 kDa, indicating that low MW polymers can also be used to make strong nanocomposites. The tensile stress of fibers immersed in PVA 5 wt% solution reached ca. 200 MPa, surpassing the values of nacre and most of other nacre-mimicking materials. The nacre-mimicking fibers are highly electrically conductive (∼350 S m(-1)) after immersing in hydroiodic acid, enabling them to connect a circuit to illuminate an LED lamp.