By: Akanksha Urade
(Ph.D. Scholar at IIT Roorkee and Graphene & 2D Materials Science Writer)
The lateral size and dispersibility of graphene are essential to the performance of graphene-polymer composite materials. For utilizing the high mechanical properties of graphene in polymer composites, the formation of large-size and low structural defects graphene is favored. This article aims to shed light on why large size graphene flakes (GFs) are crucial for robust mechanical reinforcement.
Role of Lateral Size of Graphene Flakes
Graphene filler has seen significant use in recent years to mechanically reinforce raw polymers owing to their high aspect ratio and exceptional mechanical strength (i.e., the tensile strength of 130 GPa and elastic modulus of 1 TPa).
Nevertheless, while designing graphene-based composite materials, it’s crucial to consider the impact of the lateral size of graphene flakes (GFs) on the mechanical reinforcement of graphene-polymer composites. It is reported that the larger graphene flakes (typically > 30 μm) improve the filler-to-matrix contact area, shows good stress transfer efficiency at the filler-matrix interface, and most importantly, better crack bridging.
To put it simply, when graphene is added as a filler in polymer composites, the resulting polymer network becomes progressively interconnected as the amount of graphene gradually increases. When it reaches a critical concentration, known as the percolation threshold, a mechanically effective network develops between the graphene and the polymer. So, the larger lateral size leads to much lower values for the percolation threshold because of the significant increase in the contact area between graphene and matrices. As a result, substantially less graphene (<10 wt%) is required to establish a percolative pathway across the matrix than traditional fillers such as carbon nanotubes and carbon black.
It is crucial to note that increasing the graphene content above the percolation threshold will result in a weakening efficiency of mechanical enhancement because larger GFs form more folded morphologies at higher loading percentages, which can also lead to the easier formation of agglomerates due to stacking. This means that only a tiny amount of large, thin and defect free graphene flakes are needed to achieve high mechanical strength compared to other graphene derivative materials.
Research Examples of Enhancement in Mechanical Reinforcement Using Large GFs
To give you some research examples, experts from the Laboratory for Functional Polymers, Switzerland, systematically evaluated the effect of lateral dimension with flakes size 5 μm and 25 μm on the strengthening mechanism of graphene in an epoxy polymer. According to their findings, a graphene-epoxy composite containing 2wt% graphene nanoplatelets with an average size greater than 25 μm outperformed pure epoxy by 82% in terms of fractural toughness. GFs with 5 μm lateral diameters, on the other hand, demonstrated just 36% enhancement.
In another study, Dalian University of Technology researchers reported that for a given loading of graphene, the reinforcing effect reduces as the graphene flake size diminishes. They demonstrated that incorporating graphene with a lateral size of > 50 µm in cement composite improves the compressive toughness and compressive strength by 95.7% and 43.5%, respectively, for <0.5 % graphene dosage when compared to smaller flake size (< 5 μm).
These findings are in complete agreement with a paper published in Advance Materials by Prof. Konstantin Novoselov and colleagues. They predicted that very large pristine graphene flakes (> 30 μm) would be required before efficient reinforcing can occur.
Can a graphene flake with less lateral size have high mechanical strength?
I’m sure you’re probably wondering what happens if we have GFs with lateral sizes less than 30 μm. Well, with reference to above-mentioned work by Nobel Laureate Prof. Novoselov, the researchers showed that the critical lateral size for reinforcement of polymethylmethacrylate (PMMA) by pristine graphene starts at 3 μm and reaches a critical “tipping point” at more than 30 μm to achieve efficient reinforcement.
There are, however, many open questions about the mechanisms and extent to which large area GFs can enhance mechanical qualities. Clearly, a more comprehensive analysis is required to determine the connection between the GFs’ size and their mechanical properties.
When integrated into polymers, the structural defects inherent to defective graphene flakes limit their reinforcing capacity and diminish their mechanical characteristics. As a result, pristine graphene flakes, a defect-free type of graphene, have recently garnered great interest. However, excessive sonication and mechanical exfoliation inevitably lead to graphene fragmentation, limiting the production of large-size pristine flakes. In response to these issues, Avadain has established a cost-effective and sustainable method of producing large (average μm2 55 up to 100 μm 2), thin (average 1 nm), and practically defect-free graphene flakes. This opens the door for graphene’s wider use in applications requiring demanding mechanical strength.
- Liu, Mufeng, et al. “Modelling mechanical percolation in graphene-reinforced elastomer nanocomposites.” Composites Part B: Engineering178 (2019): 107506.
- Chatterjee, S., et al. “Size and synergy effects of nanofiller hybrids including graphene nanoplatelets and carbon nanotubes in mechanical properties of epoxy composites.” Carbon50.15 (2012): 5380-5386.
- Dong, Sufen, et al. “Nano/micro-structures and mechanical properties of ultra-high performance concrete incorporating graphene with different lateral sizes.” Composites Part A: Applied Science and Manufacturing137 (2020): 106011.
- Gong, Lei, et al. “Interfacial stress transfer in a graphene monolayer nanocomposite.” Advanced Materials22.24 (2010): 2694-2697.