Aqueous-only exfoliation of pristine graphite to graphene : towards sustainable, multifunctional polymer nanocomposites
Seyed Ismail Seyed Shahabadi
Date of Issue2018
School of Materials Science and Engineering
The conventional, linear, take-make-waste economy which was a by-product of the industrial revolution is not a viable option anymore. Environmental concerns are driving the development of a new circular model in which reduce, reuse, and recycle are key. Since this circular model would keep materials in commerce for longer, safer product designs that would reduce the amount of hazardous materials are vital. In this work, the aim is to implement this design philosophy in producing polymer/graphene nanocomposites as a rapidly emerging class of materials. The design philosophy is based on two strategies: first, to substitute conventional, synthetic components by their naturally-occurring counterparts and, second, by eliminating chemicals by assigning their roles to other multifunctional components. Graphene and, by extension, polymer/graphene nanocomposites have seen huge interest in academia and industry. Among different methods to produce graphene, liquid-phase exfoliation (LPE) is one of the most commonly used. LPE is a low cost, industrially-friendly, high-yield process in which graphite flakes are exfoliated into their 2D constituents, i.e., graphene, and stabilized in a liquid medium. Water is the most desirable medium for LPE but graphene needs stabilization with surfactants or stabilizers because of the huge mismatch between the surface energies of water and graphene. There have been a myriad of synthetic surfactants and stabilizers studied for this purpose, but natural surfactants have seen less attention. Here, lignin, an abundant, undervalued, byproduct of paper industry, will be used as a naturally-occurring stabilizer for graphene. Lignin-modified graphene (LMG) is non-covalently modified meaning its sp2 structure, which is the reason for its amazing properties, is unperturbed. Different polymer/graphene systems based on this sustainable design philosophy are produced. In the first work, LMG is added to waterborne polyurethane (WPU) to produce conductive films for antistatic coating applications. Since durability against environmental damage is an important factor for these coatings, functional properties such as self-healing and UV-resistance are required. However, instead of using self-healing agents and synthetic UV blockers, LMG is tasked with enabling self-healability and improving UV stability. Thanks to its multifunctional properties, LMG can act as a photothermal converter creating heat after being illuminated by near-infrared (IR). This increases the rate of polymer diffusion at the site of damage which results in fast self-healing of the damage. Due to the anti-UV properties of LMG and lignin, these nanocomposite coatings show remarkable UV resistance as well. The combination of self-healing and UV-stability renders these films highly durable. In the second work, graphene was used to produce multi-functional sensors. Commercially-available polyurethane (PU) sponges can become conductive by dip-coating in aqueous LMG suspensions. These sponges show electrical sensitivity to compression, and thanks to their high mechanical stability, they are able to accurately sense pressure with high accuracy over 5000 compression-release cycles. Since LMG acts as a negative temperature coefficient thermistor, these sensors can measure temperature with high sensitivity as well without the need for any temperature-sensitive component. Finally, this design philosophy is taken to the next level by completely removing lignin in the third work. Here, WPU can be used as a stabilizer to produce WPU-modified graphene (PMG). In contrast to the WPU/LMG system, WPU/PMG nanocomposites have no external stabilizer. These highly conductive nanocomposites are electromechanically sensitive and show huge electromechanical responses to deformation. In addition to being the polymer matrix and stabilizer, WPU endowed the sensors with shape conformality too, which significantly increased the real-life sensitivity of the sensors in monitoring biomechanical deformations and reproducing blood-pressure waveforms. This innate shape-conformality obviated the need for an additional skin-conformal layer. By adopting these strategies, organic liquid media and synthetic surfactants/stabilizers can be substituted by water and naturally-occurring lignin, or in case of the last study, lignin can be eliminated. Other functional additives such as self-healing agents, synthetic UV-stabilizers, or active materials are not required thanks to the multifunctional and multi-stimuli responsive nature of graphene.