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Ultra-tough inverse artificial nacre based on epoxy-graphene by freeze-casting, Huang, Peng, Wan, Du, Dou, Wagner, Tomsia, Jiang, Cheng, Angewandte Chemie International Edition, 2019

Novelty/impact/significance:

In contrast to natural nacre and artificial nacre-inspired composites that are ceramic-based (inorganic component 96wt% for nacre) but toughen through the brick-and-mortar structure, here a nacre-like epoxy-graphene layered nanocomposite (organic component 99wt%) is created with ultrahigh toughness (4.2 times higher than pure epoxy) and interesting electrical properties correlated with temperature-sensing behaviors. This provides new ideas in and promote the fabrication of high-performance epoxy composites with functional properties.

Scientific question:

How to effectively toughen the widely used epoxy nanocomposites?

Key of how:

Inspired by the brick-and-mortar structure toughening the natural nacre, a nacre-like epoxy nanocomposite (called inverse artificial nacre here) is created through freezing casting (avoiding nanofiller agglomeration) of 99wt% epoxy and graphene. The fabricated inverse artificial nacre composites show ultrahigh toughness due to effective toughening mechanisms such as crack deflection, crack branching, interfacial friction and crack bridging. The anisotropic conductivity is ascribed to the lamellar graphene arrangement.

Major points:

1. Ultratough and functional epoxy composites are importantly used in areas including aerospace, transportation, electronics, etc., while traditional toughening methods are not amenable to tailor and optimize the performances. New approaches for design and manufacturing high-performance epoxy nanocomposites are needed.

2. Natural nacre shows effective toughening resulted from its brick-and-mortar structure, and freeze casting is a promising technique for accurate control of structural details. Thus the structural design from nacre and the freeze casting to obtain this structure are used in this work: (1) graphene oxide (GO) with alginic acid sodium as a colloidal slurry is prepared, (2) the slurry is freeze-cast (bidirectional) into lamellar scaffolds/nanosheets, (3) partially reduce the scaffolds by annealing, (4) infiltrate with epoxy and cure, and l-ascorbic acid is added for reduction. Control groups are homogeneous epoxy graphene nanocomposites via solution mixing.

3. The fabricated lamellar GO nanosheets show decreasing lamella thickness and decreasing space between lamellae with increasing freezing rate. The lamellar structure is retained after thermal annealing. PS: why reducing the GO through annealing is needed? What happens in this process and is useful for the performance?

4. The inverse artificial nacre sustain much higher forces than the control groups (epoxy-graphene nanocomposites) followed by pure epoxy (notched samples, three-point bend). All inverse artificial nacre nanocomposites show a stair-like behavior after the maximum load, indicating the prevention of catastrophic failure (higher freezing rates seem generate stronger nanocomposites).

5. The initial fracture toughness (KIC) of inverse artificial nacre composites increases with increasing freezing rate, all higher (1.9 times higher) than the control groups and pure epoxy. The critical strain energy release rate is 3.5 times higher than that of pure epoxy, and the fracture toughness (KJC, 2.9 MPa m1/2) is 4.2 times higher than that of pure epoxy.

PS: it might be useful to compare these numbers with those of the natural nacre.

Compared with the smooth and stripes-like fracture morphologies of pure epoxy and epoxy added with graphene, the inverse artificial nacre composites show layered structures that elongate crack propagation and alleviate high local stress to dissipate more energy.

Compared with other epoxy nanocomposites reinforced with nanofillers such as GO, graphene, carbon nanofibers, carbon nanotubes, and clay, the inverse artificial nacre composites show much higher fracture toughness (the ratios of fracture toughness of composite over epoxy, 1.0-2.5 versus 4.2) and work of fracture (1.0-4.0 versus 17.5). This is correlated to the success in avoiding the nanofiller agglomeration and improving the nanofiller toughening efficiency.

6. In-situ observation reveals similar toughening mechanisms to nacre for inverse artificial nacre composites: crack propagation follows a tortuous path with crack deflection, interfacial delamination/friction, crack branching, and crack bridging.

7. The inverse artificial nacre composites also show interesting electrical properties: anisotropic conductivity due to the lamellar rGO structure (100 times higher in parallel direction than in perpendicular direction), decreasing electrical resistance with increasing temperature, and stable conductivity with varying humidities (30% -80%), and quick respond in electrical resistance to temperature changes, which are useful as functional/sensing devices.

This work is interesting and thought-provoking.

Here is the link of the fulltext: https://onlinelibrary.wiley.com/doi/full/10.1002/anie.201902410

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