Researchers from Nanjing Forestry University and the University of Maryland have designed high-performance microfibers by hybridizing two-dimensional (2D) graphene oxide (GO) nanosheets and one-dimensional (1D) nanofibrillated cellulose (NFC) fibers. The resulting well-aligned, strong microfibers have the potential to supersede carbon fibers due to their low cost, the team suggests in an open access paper published in the journal NPG Asia Materials.
The hybrid microfibers are much stronger than microfibers composed of 1D NFC or 2D GO alone. In their paper, they reported that experimental results and molecular dynamics simulations reveal the synergistic effect between GO and NFC: the bonding between neighboring GO nanosheets is enhanced by NFC because the introduction of NFC provides the extra bonding options available between the nanosheets.
In addition, 1D NFC fibers can act as ‘lines’ to ‘weave and wrap’ 2D nanosheets together. A 2D GO nanosheet can also bridge several 1D NFC fibers together, providing extra bonding sites between 1D NFC fibers over a long distance. The design rule investigated in this study can be universally applied to other structure designs where a synergistic effect is preferred.—Li et al.
NFC, derived mainly from wood, is an inexhaustible one-dimensional (1D) material with a diameter in nanoscale and a length in microscale. It has an elastic modulus of ~140 GPa. It also possesses a high specific area and strong interacting surface hydroxyls, and can thus act as an excellent reinforcement/binder.
Chemically exfoliated two-dimensional (2D) graphene oxide (GO) nanosheets exhibit excellent mechanical properties, a high aspect ratio and good processability, making the nanosheets another attractive building block to produce strong microfibers. GO microfibers exhibiting a tensile strength of 442 MPa and an elastic modulus of 47 GPa have been reported.
In addition to the excellent mechanical properties of the individual building blocks themselves, the alignment and interaction between the building blocks are crucial to achieve superior mechanical properties. Alignment is a proven strategy to achieve high-strength nanocomposites by providing the maximum packing density with the minimum amount of defects for crack formation and propagation. Both concentrated NFC and GO suspensions can self-assemble into ordered LC arrangements; this methodology has been demonstrated to create strong and well-aligned fibers.—Li et al.
When the team uniformly mixed together the GO and NFC, the solution yielded a uniform parallel-banded texture. They then prepared the microfibers by extruding the GO–NFC spinning solution directly into a coagulation bath of ethanol. From 1 ml of spinning solution, fibers that are more than 10 m long can be produced. Diameters between 10–40 μm can be created and tuned by using needles with different diameters. The hybrid microfibers were strong and flexible and could be readily twisted into yarn by hand.
On testing they found that the average elastic modulus and ultimate tensile strength (UTS) of the GO–NFC hybrid microfibers were 20.6±0.9 GPa and 274.6±22.4 MPa, respectively—higher than those of both the NFC microfibers (15.5±4.5 GPa, 139.1±28.7 MPa) and the GO microfibers (2.3±2 GPa, 84.0±2.8 MPa), but not superior to that reported in the literature.
To improve the mechanical properties of the GO–NFC hybrid microfibers, the researchers used metal ions (Ca2+) to introduce ionic bonding between the GO and the NFC by taking advantage of the oxygen and carboxylate groups from these two building blocks.
|Typical stress–strain curves of GO microfibers, NFC microfibers, and GO–NFC hybrid microfibers before (a) and after (b) metal ion (Ca2+) infiltration. GO, graphene oxide; NFC, nanofibrillated cellulose. Li et al. Click to enlarge.|
The resulting GO–NFC hybrid microfibers possessed an elastic modulus of 31.6±2.5 GPa and a UTS of 416.6±25.8 MPa. The best wet-spun infiltrated GO–NFC hybrid microfibers exhibited an elastic modulus of 34.1 GPa, a UTS of 442.4 MPa, and a tensile failure strain of 2.0%.
The major finding is that well-aligned hybrid microfibers are much stronger than the microfibers composed of 1D NFC or 2D GO alone. Molecular dynamics simulations reveal that the synergetic interaction between the 2D GO and 1D NFC is the key factor contributing to the enhanced mechanical performance. 1D flexible NFC fibers have good interfacial contact with 2D GO nanosheets, and can provide extra bonding options between the GO nanosheets. In addition, 1D NFC flexible fibers can act as ‘lines’ to ‘weave and wrap’ 2D nanosheets together. 2D GO nanosheets create a bridge between the neighboring NFC fibers, providing extra bonding sites between 1D NFC fibers over a long distance.
… Note that the infiltrated GO–NFC microfibers were also lightweight in nature. The low-cost, earth-abundant building blocks used in this study satisfy the economic requirement to produce mechanically strong fibers for potential commercial use. We expect that further studies using high-quality building materials, for example, larger GO nanosheets, an improved fabrication process and infiltration of other metal ions may enable us to create even stronger and tougher microfibers. Additionally, the mechanistic finding of the synergetic interactions between 2D and 1D building blocks can be potentially applicable to the design of high-performance microfibers composed of other materials.Li et al.
Yuanyuan Li, Hongli Zhu, Shuze Zhu, Jiayu Wan, Zhen Liu, Oeyvind Vaaland, Steven Lacey, Zhiqiang Fang, Hongqi Dai, Teng Li and Liangbing Hu (2015) “Hybridizing wood cellulose and graphene oxide toward high-performance fibers” NPG Asia Materials 7, e150; doi: 10.1038/am.2014.111 Published online 9 January 2015