It has been observed that the degree to which graphene is dispersed in the polymer matrix can influence the flow or rheological properties of polymer composites. As a result, the key characteristics of the materials at the nanoscale are sensitive to the dispersion quality. This article aims to shed light on the significance of rheological analysis in graphene polymer research.
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Role of Graphene in Polymer Composite
Polymer-based nanocomposites have sparked a significant amount of research attention over the past few decades. This is because incorporating very minimal nanofiller allows for improved properties compared to their corresponding unfilled counterparts, establishing new perspectives for the ongoing demand for advanced polymer composites. In this context, a wide range of nanofillers such as ceramics, metals, and carbon-based fillers have been embedded in a polymer matrix to produce high-performance materials capable of continuously expanding polymer markets.
Graphene nanofillers have shown great potential for the framework of new polymer-based nanocomposites due to their low density, exceptional mechanical, electronic properties, and high thermal conductivity.
However, the potential for graphene to provide improved properties when loaded in polymers is strongly reliant on its state of dispersion within the host matrix; in fact, graphene tends to aggregate when immersed in a viscous medium, and this situation restricts the full realization of their theoretically inherent advantages.
Rheological Analysis and its Importance in Graphene/Polymer Research
A very effective tool for monitoring the state of dispersion of the graphene in a polymeric matrix is the evaluation of its rheological properties, such as viscosity and viscoelastic properties. The graphene dispersion state and the degree of polymer-graphene interaction significantly impact the viscoelastic properties of polymer nanocomposites. As a result, the concentration of graphene significantly impacts the continuous graphene network throughout the host polymer.
The resulting polymer network becomes increasingly interconnected as the amount of graphene increases. It eventually reaches a critical concentration, known as the rheological percolation threshold, at which a mechanically effective network forms between the graphene and the polymer. The concentration and dispersion of the graphene filler determine this point.
The rheological behavior of graphene dispersed in a polymer matrix can be divided into three states in general. At low nanofiller loading, the incorporation of graphene results in only short-range interactions; this is known as the dilute regime. The appearance of a percolation network occurs as the nanofiller content increases, resulting in a shift from the dilute to the semi-dilute state; in this state, the rheological behavior of the nanocomposite is dependent on the interactions between filler and polymer.
When the graphene content exceeds the percolation threshold, the concentrated state is met, and the rheological functions approach asymptotic values, with extremely high viscosity and dynamic modulus.
In fact, the rheological behavior of graphene/polymer-based systems reveals fundamental information about the graphene/polymer interactions established at the interface, as well as additional insight into the possible arrangement of graphene-based nanofillers within the polymer host matrix.
Research Examples of Rheological Characteristics of Graphene Polymer Materials
Using nanofillers with improved dispersion or high aspect ratios enables the percolation transition to be reached at diluted concentrations. In the case of carbon black (CB)-containing polymer composites, for example, the amount of CB required to form a percolative pathway through the matrix is typically around 10 wt. percent, but this amount is dramatically reduced to 0.2 wt. percent by using graphene-like fillers.
Researchers discovered that adding graphene to biodegradable polymers such as polylactic acid can significantly increase viscosity and dynamic modulus, resulting in increased strength and durability of biodegradable plastics. As a result, the rheological analysis provides a fundamental understanding of the processability characteristics of the nanocomposites.
Equipment Used in Rheological Analysis
Rheometers are used to evaluate the rheological properties of molten polymers as shear rates and temperatures are varied. Rheology tests for viscosity are carried out while the polymer is in the melt phase or after it has been dissolved in a solvent.
Thermo fisher ScientificTM is one example supplier of a rheometer. Specifically, their HAAKE rheometers are widely recognized for accuracy and ease of use. The instruments are designed to reliably measure the mechanical and viscosity properties of polymers in different states.
A comprehensive rheological characterization of polymeric materials can be achieved through the application of a variety of test methods. The frequency sweep data provide a direct measure of the viscous and elastic properties of a polymer. These are represented by the storage and the loss moduli (G’ & G’’) measured at different frequencies / time scales. Rotational rheometers can also be utilized to perform Dynamic Mechanical Thermal Analysis (DMTA), where the obtained data are used to identify characteristic phase transitions from a liquid-like to solid.
Summary
Evaluating rheological behavior is a very powerful tool for determining the dispersion of graphene nanofillers and its interactions between polymer chains, since it strongly influences the material's viscoelastic properties. Furthermore, rheological properties are critical when analyzing the melt flow properties of graphene/polymers nanocomposites. The understanding and design of flow behavior is critical for its processing and commercial applications.
References and Further Reading
Das, M., and Dey, A. (2022). Rheological properties of polymer-graphene composites. Polymer Nanocomposites Containing Graphene, 183-210 https://www.sciencedirect.com/science/article/pii/B9780128216392000215
C. Küchenmeister-Lehrheuser, K. Oldörp, F. Meyer, Solids clamping tool for Dynamic Mechanical Analysis (DMTA) with HAAKE MARS rheometers, Thermo Fisher Scientific Product Information P004 (2016)
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