Aqueous-only exfoliation of pristine graphite to graphene : towards sustainable, multifunctional polymer nanocomposites
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
... ing 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, industriallyfriendly, 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. Ligninmodified graphene (LMG) is non-covalently modified meaning its sp 2 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 Abstract ii 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 dipcoating 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 UVstabilizers, or active materials are not required thanks to the multifunctional and multistimuli responsive nature of graphene. Lay Summary iii Lay Summary The conventional economical model which was a by-product of the industrial revolution is a linear route from extraction to landfill. This linear model will soon reach its dead-end since it fails to consider the impacts on ecosystem degradation, pollution, and resource depletion. As a result, a circular economy based on value retention by reducing extraction, reusing, and recycling is gaining popularity. Since products remain in service for longer in circular economy, a cleaner product design by reducing or removing hazardous chemicals is necessary. Therefore, the motivation for this study is to produce polymeric materials without synthetic chemicals by substitution with components found in nature or, if possible, elimination of those chemicals. Graphene is a two-dimensional honeycomb sheet of carbon with many interesting properties, and it can be added to plastics to achieve desirable functionalities. There are different ways to produce graphene but one of the easiest and cheapest methods is to break the three-dimensional structure of graphite into two-dimensional graphene in a liquid medium. Water is an attractive choice, but graphene is not stable in water; therefore, certain materials such as surfactants or stabilizers should be added to produce stable graphene suspension in water. There have been many synthetic surfactants and stabilizers used for this purpose, but natural surfactants are more desirable. Here, lignin, an abundant, undervalued, byproduct of paper industry, was used as a natural stabilizer for graphene in water. Lignin does not damage the structure of graphene hence many of graphene's unique properties are unchanged. Polymer/graphene nanocomposites were produced for different applications such as coating and sensing. Firstly, graphene was mixed with waterborne polyurethane to make conductive films. These films can be used as antistatic coatings to counter electrostatic discharge, which is hazardous to many applications such as electronics. Since durability against environmental damage is an important factor for coatings, they should be able to repair and heal environmental damage and resist UV irradiation. One way to enable healing is to add healing components that can be released in a controlled manner. Synthetic UV Lay Summary iv blockers can also be used to improve UV stability. However, instead of using these compounds, graphene can be used to enable self-healability by Increasing temperature through converting near infrared (IR) light into heat and increase the rate of polymer diffusion at the damage which results in fast self-healing of the damage. Graphene can also absorb the damaging UV light and improve UV resistance remarkably. Graphene can be used to produce sensors as well. Sensor is a device that can detect a change, such as pressure, strain, and temperature, in its environment. By submerging commercially-available polyurethane sponges in aqueous graphene suspensions and evaporating water they become conductive and their conductivity can change when they are pressed. The conductivity of graphene can also change with temperature which means that these sensors can measure temperature as well. Normally, this sensor would require separate pressure-sensitive and temperature-sensitive components, but graphene can serve both functions at the same time. Finally, polymer/graphene nanocomposites can be produced without lignin as well, reducing the constituents to only two. Here, the polymer itself is used to stabilize graphene in water. These polymer/graphene nanocomposites are highly conductive and show changes in conductivity under deformation, so they can be used as strain sensors. In addition, the polymer that is used is very soft which allows it to conform to the shape of any substrate. Shape-conformality can improve sensor/substrate interface, enhance signal collection, and give rise to the sensitivity of the sensor, making these sensors very accurate in real-life applications such as monitoring bodily movements and blood pressure.