Mechanism of Exfoliation and Prediction of Materials Properties of Clay-Polymer Nanocomposites from Multiscale Modeling

2D Materials in Flexible Electronics: Recent Advances and Future Prospectives

Flexible electronics have recently gained considerable attention due to their potential to provide new and innovative solutions to a wide range of challenges in various electronic fields. These electronics require specific material properties and performance because they need to be integrated into a variety of surfaces or folded and rolled for newly formatted electronics. Two-dimensional (2D) materials have emerged as promising candidates for flexible electronics due to their unique mechanical, electrical, and optical properties, as well as their compatibility with other materials, enabling the creation of various flexible electronic devices. This article provides a comprehensive review of the progress made in developing flexible electronic devices using 2D materials. In addition, it highlights the key aspects of materials, scalable material production, and device fabrication processes for flexible applications, along with important examples of demonstrations that achieved breakthroughs in various flexible and wearable electronic applications. Finally, we discuss the opportunities, current challenges, potential solutions, and future investigative directions about this field.

Influence of PEGylation on WS2 Nanosheets and Its Application in Photothermal Therapy

Photothermal therapy (PTT) is an alternative cancer therapy with minimal side effects and higher efficiency and selectivity. In this study, WS2 nanosheets were developed by ultrasonic exfoliation with different ratios of polyethylene glycol (PEG), and their effects on physicochemical properties were studied. The utilization of PEG during sonication significantly influenced the size and thickness of the resulting WS2 nanosheet layers, which was confirmed through scanning electron microscopy, atomic force microscopy, and dynamic light scattering analyses. PEG functionalization also improved the dispersibility of WS2 in aqueous solution by making its surface hydrophilic, which resulted in better biocompatibility. The intrinsic near-infrared absorbance of the nanosheets positions them as valuable agents for PTT. The study further explores the efficacy of these nanosheets as photothermal agents in the ablation of MDAMB-231 breast cancer cells. Although the use of PEG to demonstrate exfoliation and biocompatibility for WS2 has been reported previously, the effect of PEGylation on various physicochemical properties has not been studied in-depth until now. This study paves the way for the use of highly versatile PEG across a range of 2D material systems.

Systematic Study of the Nanostructures of Exfoliated Polymer Nanocomposites

High-performance bioinspired materials have shown rapid development over the last decade. Examples are brick-and-mortar hierarchical structures, which are often achieved via solvent evaporation. Although good properties are claimed, most systems are composed of stacked or intercalated platelets. Exfoliation is a crucial step to give ultimate anisotropic properties, e.g., thermal, mechanical, and barrier properties. We propose a general framework for all the various types of micro-scale structures that should be distinguished for 2D filler nanocomposites. In particular, the exfoliated state is systematically explored by the immobilization of montmorillonite platelets via (gelatin) hydrogelation. Scattering techniques were used to evaluate this strategy at the level of the particle dispersion and the regularity of spatial arrangement. The gelatin/montmorillonite exfoliated nanostructures are fully controlled by the filler volume fraction since the observed gallery d-spacings perfectly fall onto the predicted values. Surprisingly, X-ray analysis also revealed short- and quasi long-range arrangement of the montmorillonite clay at high loading.

Large-Scale Molecular Dynamics Elucidates the Mechanics of Reinforcement in Graphene-Based Composites

Using very large-scale classical molecular dynamics, the mechanics of nano-reinforcement of graphene-based nanocomposites are  examined. Simulations show that significant quantities of large, defect-free, and predominantly flat graphene flakes are required for successful enhancement of materials properties in excellent agreement with experimental and proposed continuum shear-lag theories. The critical lengths for enhancement are approximately 500 nm for graphene and 300 nm and for graphene oxide (GO). The reduction of Young's modulus in GO results in a much smaller enhancement of the composite's Young's modulus. The simulations reveal that the flakes should be aligned and planar for optimal reinforcement. Undulations substantially degrade the enhancement of materials properties.

Approaches and Perspective of Coarse-Grained Modeling and Simulation for Polymer-Nanoparticle Hybrid Systems

Molecular modeling and simulations have emerged as effective and indispensable tools to characterize polymeric systems. They provide fundamental and essential insights to design a product of the required properties and to improve the understanding of a phenomenon at the molecular level for a particular system. The polymer-nanoparticle hybrids are materials with outstanding properties and correspondingly large applications whose study has benefited from this new paradigm. However, despite the significant expansion of modern day computational powers, investigation of the long time and large length scale phenomenon in polymeric and polymer-nanoparticle systems is still a challenging task to complete through all-atom molecular dynamics (AA-MD) simulations. To circumvent this problem, a variety of coarse-grained (CG) models have been proposed, ranging from the generic CG models for qualitative properties predictions to more realistic chemically specific CG models for quantitative properties predictions. These CG models have already delivered some success stories in the study of several spatial and temporal evolutions of many processes. Some of these studies were beyond the feasibility of traditional atomistic resolution models due to either the size or the time constraints. This review captures the different types of popular CG approaches that are utilized in the investigation of the microscopic behavior of polymer-nanoparticle hybrid systems. The rationale of this article is to furnish an overview of the popular CG approaches and their applications, to review several important and most recent developments, and to delineate the perspectives on future directions in the field.

Fabrication of polyamide-12/cement nanocomposite and its testing for different dyes removal from aqueous solution: characterization, adsorption, and regeneration studies

Polyamide-12/Portland cement nanocomposite was prepared by using the exfoliated adsorption method. The fabricated nanocomposite was applied first time to remove Congo red (CR), brilliant green (BG), methylene blue (MB), and methyl red (MR) from the synthetic wastewater. The polymer nanocomposite was characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, elemental mapping, Brunauer-Emmett-Teller surface area analysis, and X-ray diffraction. The adsorption was rapid and all the studied dyes were absorbed on the surface of the polymer nanocomposite in 90 min. The point of zero charge was found at pH 5 and the factors such as pH, time, and temperature were found to affect the adsorption efficiency. Freundlich isotherm and pseudo-second-order models well-fitted the adsorption isotherm and kinetics data, respectively. The calculated maximum adsorption capacity was 161.63, 148.54, 200.40, and 146.41 mg/g for CR, BG, MB, and MR, respectively. The mode of the adsorption process was endothermic, spontaneous, and physical involving electrostatic attraction. On an industrial scale, the high percentage of desorption and slow decrease in the percentage of adsorption after every five regeneration cycles confirm the potential, practicality, and durability of the nanocomposite as a promising and advanced adsorbent for decolorization of colored wastewater.

A Coarse-Grained Model for the Mechanical Behavior of Na-Montmorillonite Clay

Sodium montmorillonite (Na-MMT) is one of the most commonly found swelling clay minerals with diverse engineering and technological applications. The nanomechanical properties of this mineral have been extensively investigated computationally utilizing molecular dynamics (MD) simulations to portray the molecular-level changes at different environmental conditions. As the environmentally found Na-MMT clays are generally sized within hundreds of nanometers, all-atomistic (AA) MD simulations of clays within such size range are particularly challenging due to computational inefficiency. Informed from atomistic modeling, a coarse-grained (CG) modeling technique can be employed to overcome the spatiotemporal limitation. The current study presents a modeling strategy to develop a computationally efficient model of Na-MMT clay with a typical size over ≃100 nm by shrinking the atomistic platelet thickness and reducing the number of center-layer atoms. Using the "strain-energy conservation" approach, the force field parameters for the CG model are obtained and the developed CG model can well preserve in-plane tension, shear, and bending behaviors of atomistic counterparts. Remarkably, the CG tactoid model of Na-MMT, a hierarchical multilayer structure, can reproduce the interlayer shear and adhesion as well as d-spacing among the clay sheets as of atomistic one to a good approximation while gaining significantly improved computational speed. Our study demonstrates the efficacy of the CG modeling framework, paving the way for the bottom-up multiscale prediction of mechanical behaviors of clay and related minerals.

Principles governing control of aggregation and dispersion of aqueous graphene oxide

Controlling the structure of graphene oxide (GO) phases and their smaller analogues, graphene (oxide) quantum dots (GOQDs), is vitally important for any of their widespread intended applications: highly ordered arrangements of nanoparticles for thin-film or membrane applications of GO, dispersed nanoparticles for composite materials and three-dimensional porous arrangements for hydrogels. In aqueous environments, it is not only the chemical composition of the GO flakes that determines their morphologies; external factors such as pH and the coexisting cations also influence the structures formed. By using accurate models of GO that capture the heterogeneity of surface oxidation and very large-scale coarse-grained molecular dynamics that can simulate the behaviour of GO at realistic sizes of GOQDs, the driving forces that lead to the various morphologies in aqueous solution are resolved. We find the morphologies are determined by a complex interplay between electrostatic, [Formula: see text]-[Formula: see text] and hydrogen bonding interactions. Assembled morphologies can be controlled by changing the degree of oxidation and the pH. In acidic aqueous solution, the GO flakes vary from fully aggregated over graphitic domains to partial aggregation via hydrogen bonding between hydroxylated domains, leading to the formation of planar extended flakes at high oxidation ratios and stacks at low oxidation ratios. At high pH, where the edge carboxylic acid groups are deprotonated, electrostatic repulsion leads to more dispersion, but a variety of aggregation behaviour is surprisingly still observed: over graphitic regions, via hydrogen bonding and "face-edge" interactions. Calcium ions cause additional aggregation, with a greater number of "face-face" and "edge-edge" aggregation mechanisms, leading to irregular aggregated structures. "Face-face" aggregation mechanisms are enhanced by the GO flakes possessing distinct domains of hydroxylated and graphitic regions, with [Formula: see text]-[Formula: see text] and hydrogen bonding interactions prevalent between these regions on aggregated flakes respectively. These findings furnish explanations for the aggregation characteristics of GO and GOQDs, and provide computational methods to design directed synthesis routes for self-assembled and associated applications.

Selective dispersion of neutral nanoplates and the interfacial structure of copolymers based on coarse-grained molecular dynamics simulations

The selective dispersion of neutral nanoplates (NNP) and the control of the interfacial structure of copolymers are challenging. In this work, we employ coarse-grained molecular dynamics (CGMD) to investigate the dispersion of NNP and the interfacial structure. The introduction of NNP significantly changes the interfacial structure and formation mechanism of diblock copolymers (DBCP), which is related to the matrix phase, distribution, composition, and length of two different chain segments (A and B) in A_mB_n-DBCP. The phase-weak groups that have a poor interaction with NNP will stack easily, whereas the stacking degree for the phase-rich groups that have a strong interaction with NNP decreases due to the addition of NNP. The interaction between two phases will be enhanced, which is favorable for the formation of a random network structure. Due to the strong interaction of the phase-rich groups with NNP, the NNP change the accumulation types of phase-weak groups and enhances the combination of two chain segments in favor of the formation of a cylindrical micelle-like structure. The transmission electron microscopy (TEM) images show that layered double hydroxide (LDH) orientationally distributes in the acrylic acid chain segments in ethylene acrylic acid (EAA) random copolymers, which is in agreement with the theoretical simulation results. This proves that the selective dispersion of LDH in copolymers affects their interfacial structure.

From digital hype to analogue reality: Universal simulation beyond the quantum and exascale eras

Many believe that the future of innovation lies in simulation. However, as computers are becoming ever more powerful, so does the hyperbole used to discuss their potential in modelling across a vast range of domains, from subatomic physics to chemistry, climate science, epidemiology, economics and cosmology. As we are about to enter the era of quantum and exascale computing, machine learning and artificial intelligence have entered the field in a significant way. In this article we give a brief history of simulation, discuss how machine learning can be more powerful if underpinned by deeper mechanistic understanding, outline the potential of exascale and quantum computing, highlight the limits of digital computing - classical and quantum - and distinguish rhetoric from reality in assessing the future of modelling and simulation, when we believe analogue computing will play an increasingly important role.

Principles Governing Control of Aggregation and Dispersion of Graphene and Graphene Oxide in Polymer Melts

Controlling the structure of graphene and graphene oxide (GO) phases is vitally important for any of its widespread intended applications: highly ordered arrangements of nanoparticles are needed for thin-film or membrane applications of GO, dispersed nanoparticles for composite materials, and 3D porous arrangements for hydrogels. By combining coarse-grained molecular dynamics and newly developed accurate models of GO, the driving forces that lead to the various morphologies are resolved. Two hydrophilic polymers, poly(ethylene glycol) (PEG) and poly(vinyl alcohol) (PVA), are used to illustrate the thermodynamically stable morphologies of GO and relevant dispersion mechanisms. GO self-assembly can be controlled by changing the degree of oxidation, varying from fully aggregated over graphitic domains to intercalated assemblies with polymer bilayers between sheets. The long-term stability of a dispersion is extremely important for many commercial applications of GO composites. For any degree of oxidation, GO does not disperse in PVA as a thermodynamic equilibrium product, whereas in PEG dispersions are only thermodynamically stable for highly oxidized GO. These findings-validated against the extensive literature on GO systems in organic solvents-furnish quantitative explanations for the empirically unpredictable aggregation characteristics of GO and provide computational methods to design directed synthesis routes for diverse self-assemblies and applications.

Rapid and high-concentration exfoliation of montmorillonite into high-quality and mono-layered nanosheets

After montmorillonite (MTM) was first exfoliated into nanosheets as a reinforcing filler in the 1980s, layered clay became a hotspot of interest. However, to date, the exfoliation of the resource-rich and inexpensive layered MTM into high-quality nanosheets still remains a significant challenge. Herein, a simple and effective strategy to exfoliate layered MTM into mono-layered sheets via the aggregation of polyethyl-phosphate glycol ester (Exolit OP 550) is proposed. A significant decrease in exfoliation time from 120 min to 3 min was observed at room temperature only via a gentle stirring process. Moreover, various factors that reduce the viscosity of the mixture could be utilized to boost the exfoliated concentration to a record high value of 100 wt%, which is an increase of 460-2400% compared with that in other works. A tentative model was also proposed to illustrate the exfoliation mechanism based on the detection of segmental confined movement, structural evolution, and polymer-clay interaction. Particularly, the as-observed critical concentration of 200 wt% MTM indicated a saturation effect for the surface-adsorbed polymer. The critical concentration for the onset of exfoliation was 150 wt%. In addition, the structure of the exfoliated nanosheets in Exolit OP 550 underwent a temperature-sensitive and irreversible transformation. Thus, our study may provide new insight for the exfoliation of clay.

Charge-Dependent Regulation in DNA Adsorption on 2D Clay Minerals

DNA purification is essential for the detection of human clinical specimens. A non-destructive, controllable, and low reagent consuming DNA extraction method is described. Negatively charged DNA is absorbed onto a negatively charged montmorillonite to achieve non-destructive DNA extraction based on cation bridge construction and electric double layer formation. Different valence cation modified montmorillonite forms were used to validate the charge-dependent nature of DNA adsorption on montmorillonite. Electric double layer thickness thinning/thickening with the high/lower valence cations exists, and the minerals tended to be sedimentation-stable due to the Van der Waals attraction/electrostatic repulsion. Li-modified montmorillonite with the lowest charge states showed the best DNA adsorption efficiency of 8–10 ng/μg. Charge-dependent regulating research provides a new perspective for controllable DNA extraction and a deep analysis of interface engineering mechanisms.

Micromechanical exfoliation of graphene on the atomistic scale

Mechanical exfoliation techniques are widely used to create high quality graphene samples for analytical use. Increasingly, mechanical methods are used to create large quantities of graphene, yet there is surprisingly little molecular insight into the mechanisms involved. We study the exfoliation of graphene with sticky tape using molecular dynamics. This is made possible by using a recently developed molecular dynamics forcefield, GraFF, to represent graphene's dispersion interactions. For nano-sized flakes we observe two different mechanisms depending on the polymer-adhesive used. A peeling mechanism which mixes shearing and normal mode exfoliation promotes synthesis of graphene rather than many-layered graphite. Armed with this new chemical insight we discuss the experimental methods that could preferentially produce graphene by mechanical exfoliation. We also introduce a mathematical model describing the repeated exfoliation of graphite.

Thermal insulation with 2D materials: liquid phase exfoliated vermiculite functional nanosheets

Phyllosilicates are layered materials possessing unique thermal properties, commonly exploited in their multilayered crystalline form as refractory insulators and heating elements. A more versatile use of such materials, however, would require their existence in the form of inks and dispersions ready to be patterned. Within this framework, the liquid-phase exfoliation of low-cost, low-purity materials such as bulk multiphasic minerals and powders represents an economically advantageous approach for the production of 2D nano-sized objects with a defined composition, size and morphology. Here, ultrasound-assisted exfoliation and shear-mixing of a multi-phasic vermiculite in mild acidic aqueous solutions were employed to successfully obtain dispersions of mono- and few-layer thick clay nanosheets. The exfoliated materials were thoroughly investigated through granulometry, X-Ray Diffraction (XRD), specific surface area measurements and AFM imaging. Despite the fact that the lateral size and the thickness distribution of exfoliated flakes obtained with the two approaches appear similar, the ultrasound-assisted exfoliation process yielded a larger amount of mono- and bi-layer thick flakes as well as materials with a higher specific surface area. XRD analysis revealed that the use of ultrasound waves in an acidic environment results in the complete exfoliation of the vermiculite layer, whereas the use of shear forces under the same conditions results in the exfoliation of hydrobiotite and mica crystalline phases. Thermal conductivity measurements provided clear evidence on how the structural changes - arising from the exfoliation process - have a direct impact on the properties of the exfoliated clay. Remarkably, compared to the raw material, the thermal conductivity of the exfoliated material decreases by 25%, reaching the ultra-low thermal conductivity regime (<0.1 W m^-1 K^-1). Our approach may enable in the future the generation of patterns of thermal insulators onto different surfaces by applying vermiculite nanosheets in the form of dispersions and printable inks.

Chemically Specific Multiscale Modeling of the Shear-Induced Exfoliation of Clay-Polymer Nanocomposites

We recently showed, using chemically specific modeling and simulation, how the process of intercalation of polymers within clay sheets occurs, transforming the large-scale materials properties by a specific set of spatial and temporal processes that can lead to exfoliation. Here, we use the same hierarchal multiscale modeling scheme to understand the processes that occur during the shear-induced processing of clay-polymer nanocomposites. For both hydrophobic polymers (polyethylene) and hydrophilic polymers (poly(ethylene glycol)), we used free-energy methods to identify the lowest-free-energy separation of the clay sheets; the polymer molecules spontaneously intercalate into the clay interlayer from the surrounding polymer melt. We apply shear forces to investigate exfoliation and find that while exfoliation is promoted by shearing, it is the surfactant molecules that play the dominant role in resisting it.

Direct Exchange Mechanism for Interlayer Ions in Non-Swelling Clays

The mobility of radiocesium in the environment is largely mediated by cation exchange in micaceous clays, in particular Illite-a non-swelling clay mineral that naturally contains interlayer K⁺ and has high affinity for Cs⁺. Although exchange of interlayer K⁺ for Cs⁺ is nearly thermodynamically nonselective, recent experiments show that direct, anhydrous Cs⁺-K⁺ exchange is kinetically viable and leads to the formation of phase-separated interlayers through a mechanism that remains unclear. Here, using classical atomistic simulations and density functional theory calculations, we identify a molecular-scale positive feedback mechanism in which exchange of the larger Cs⁺ for the smaller K⁺ significantly lowers the migration barrier of neighboring K⁺, allowing exchange to propagate rapidly once initiated at the clay edge. Barrier lowering upon slight increase in layer spacing (∼0.7 Å) during Cs⁺ exchange is an example of "chemical-mechanical coupling" that likely explains the observed sharp exchange fronts leading to interstratification. Interestingly, we find that these features are thermodynamically favored even in the absence of a heterogeneous layer charge distribution.

Interdigitated crystalline MMT–MCA in polyamide 6

A novel interdigitated crystalline MMT–MCA shows outstanding fire retardancy owing to homogeneous dispersion of MMT and MCA in PA6.

Big data need big theory too

The current interest in big data, machine learning and data analytics has generated the widespread impression that such methods are capable of solving most problems without the need for conventional scientific methods of inquiry. Interest in these methods is intensifying, accelerated by the ease with which digitized data can be acquired in virtually all fields of endeavour, from science, healthcare and cybersecurity to economics, social sciences and the humanities. In multiscale modelling, machine learning appears to provide a shortcut to reveal correlations of arbitrary complexity between processes at the atomic, molecular, meso- and macroscales. Here, we point out the weaknesses of pure big data approaches with particular focus on biology and medicine, which fail to provide conceptual accounts for the processes to which they are applied. No matter their 'depth' and the sophistication of data-driven methods, such as artificial neural nets, in the end they merely fit curves to existing data. Not only do these methods invariably require far larger quantities of data than anticipated by big data aficionados in order to produce statistically reliable results, but they can also fail in circumstances beyond the range of the data used to train them because they are not designed to model the structural characteristics of the underlying system. We argue that it is vital to use theory as a guide to experimental design for maximal efficiency of data collection and to produce reliable predictive models and conceptual knowledge. Rather than continuing to fund, pursue and promote 'blind' big data projects with massive budgets, we call for more funding to be allocated to the elucidation of the multiscale and stochastic processes controlling the behaviour of complex systems, including those of life, medicine and healthcare.This article is part of the themed issue 'Multiscale modelling at the physics-chemistry-biology interface'.