Polymer nanocomposites with aligned two-dimensional materials

Abstract

The use of nanomaterials with exotic mechanical and functional properties to reinforce otherwise softer and inert polymers or resins promises the development of high-performance and highly functional composite materials for advanced applications. Polymeric materials serving as matrices or substrates can translate the superior properties of different nanomaterials from nanoscale to macroscale, holding the potential to move nanomaterials from the research laboratory to tangible technological applications that revolutionize the broad plastics industry. However, even after nearly 30 years of innovative and groundbreaking research, various technical challenges still exist such as controlling the dispersion of nanomaterials in the polymer matrix during processing and the need for new synthetic concepts and processing strategies [1]. Emerging trends in 2D nanotechnology and the recent development of certain 2D materials (e.g., large-area 2D sheets and 2D heterostructures) may provide for major advancements in the field of polymer nanocomposites. The use of 2D sheets as fillers provides a means of precisely positioning nanomaterials into/onto polymer matrices/substrates with precise structural control at an atomic level, a precise control of the surface/interface layer and the distances in between the nanoinclusions, all of which are not feasible with traditional polymer nanocomposite materials. The 2D filler components have extremely high aspect ratios, which provides for maximum reinforcement at exceedingly small volume fractions. They also provide ‘multifunctional property enhancement’ and, for example, can contribute specific photoelectric or electronic functionality and mechanical reinforcement. This contribution reviews recent advances in 2D materials-based polymer nanocomposites. First, important synthetic techniques for 2D nanomaterials such as chemical vapor deposition growth and wet-chemistry are presented. Subsequently, strategies for the surface modification of 2D sheets with small molecules and polymers as well as processes for the manufacturing of polymer nanocomposites with aligned 2D inclusions are reviewed. The mechanisms of physical property enhancement are discussed and applications across a range of technology areas are presented. Finally, the challenges as well as opportunities for research in this emerging area of nanocomposite science and engineering are commented on. It is hoped that the unified perspective presented in this review will enable readers to make contributions to this exciting and rapidly advancing field.

Introduction

High performance structural and multifunctional polymer nanocomposites are obtained by the dispersion of small amounts (< 5 vol%) of nano-scale particles or fillers in polymers matrices [1], [2], [3], [4]. Nano-scale particles are those possessing at least one dimension of less than 100 nm. As a result, they tend to provide greater reinforcement compared with larger particles due to their increased specific surface area, which provides for greater interaction with the surrounding matrix material [3], [4], [5]. A wide variety of nanoparticles containing different elements [6], chemical compositions [6], dimensionalities [7], [8], [9], shapes and morphologies [10], and sizes [11] have been used to generate polymer nanocomposites. Combining these various nanoparticles with the diversity of polymeric matrices available should produce a wide variety of new composites materials possessing unusual and beneficial properties. For example, lightweight and high-strength structural composites [12], [13], [14], [15] and multifunctional materials with enhanced properties including thermal, electrical, magnetic, optical, barrier and others [4,[16], [17], [18], [19]]. Such composites can be applied in various fields such as energy [20], [21], [22], environmental [23,24], biomedical [25], [26], [27], and others [4,28,29].

Among the reinforcements utilized in polymer nanocomposites, 2D nanofillers have attracted much of the interest recently due to their excellent comprehensive properties for structural and multifunctional reinforcement [30,31]. They are sheet-like structural nanomaterials of only a single to a few atoms of thickness, i.e., they are typically less than 5 nm thick [32,33]. This characteristic provides extremely high aspect ratios and surface-to-volume ratios. Since the exfoliation of monolayer graphene from graphite by the micromechanical cleavage technique in 2004 [34], the 2D family of reinforcements has expanded quickly to now include all elements of the periodic table [35]. They can be divided into 2D allotropes such as well-known graphene [34], phosphorene [36], and 2D metals [37] amongst others; and 2D compounds like transition metal dichalcogenides [38] (TMDs), hexagonal boron nitride [39], metal/covalent organic frameworks [40,41], MXenes [42,43], and others (Fig. 1) [44]. The 2D components have been widely used in fields such as electronics/optoelectronics [45], catalysis [46], energy storage and conversion [47], sensors [48], nanocomposites [30,49], and others [50,51].

2D materials of different molecular structures possess a variety of properties providing for a range of mechanical and functional reinforcement performance. For example, graphene has a Young's modulus of ∼1.0 TPa, fracture strength of 125 GPa [52], ultrahigh carrier mobility [53] (about 200,000 cm² V⁻¹ s⁻¹), and high thermal conductivities [54], [55], [56], [57] in the range of 1500 - 3000 W⋅m⁻¹⋅K⁻¹. Other 2D materials such as molybdenum disulfide (MoS₂), phosphorene, and hexagonal boron nitride (hBN) also possess intriguing properties. For example, MoS₂ and black phosphorus (BP) are semiconductors with varying bandgaps while hBN is an insulator [58]. These 2D materials also have good tolerance to a tensile strain up to 20-25% [52]. Combining and stacking different 2D sheets can form Van der Waals heterostructures vertically (Fig. 2), which could provide a variety of properties and functions that cannot be achieved with a single 2D component alone [59,60]. In particular, such 2D heterostructures could be used as nanoelectronics for a range of applications, e.g., transistors, photovoltaic and light-emitting devices, plasmonic devices amongst others. In this context, 2D materials are quite different from traditional nanofillers such as nanosilicas, nanoclays, and others. Such 2D particles could be used to generate nanodevices with unique electronic or optoelectronic functions. In this way, new nanocomposites with programmable functions could be developed, and they have the potential to be applied as new composite materials that may couple sensing, actuation, computation, and communications capabilities [61,62].

The use of 2D materials as nanofillers also provides a unique opportunity to explore the maximum enhancement of various properties at the theoretical limit. Imagine that if we can control the dispersion status of the nanofillers in the polymer matrix from a random dispersion (Fig. 3, A), gradually to a uniform and aligned state (B), and finally to an ideal case with uniform, aligned, and continuous 2D fillers (C). According to the platelet-filler model [64] and the percolation theory [65], the nanocomposite body in Fig 3C can achieve a maximum reinforcement in terms of mechanical properties such as elastic modulus and fracture strength [66], barrier performance [67], electrical and thermal conductance [66,68] among others. Note that such enhancements are highly anisotropic, being electrical/thermal conductive in plane while insulated across plane for example.

The emergent two-dimensional (2D) nanotechnology also provides the capability of manipulating the surface and interface of composite materials with atomic-level precision, which is crucial for the development of high-performance and functional nanocomposites [69,70]. Specifically, the use of large-area and high-quality 2D materials and their heterostructures, i.e., those grown by chemical vapor deposition (CVD) [33], provides a facile method to precisely engineer the surface or interface between layers of the polymer nanocomposite body. These 2D sheets with fewer defects have high aspect ratios, one-cm sized monolayer CVD graphene has an aspect ratio about 3 $\times$ 10⁷ for example. They are not strong enough for free-standing, off-substrate applications, i.e., a matrix or substrate material as physical support is usually required. Typically, polymers such as the thermoplastic poly(methyl methacrylate) (PMMA), polystyrene (PS), polycarbonate (PC), polyolefins, and others having good compatibility with 2D materials are used as a support. 2D sheets can be conveniently transferred from their grown substrates such as copper foil or silica to the said polymer support, and composite films with a 2D sheet as the surface layer can be generated, which can be used as precursors or building blocks for further processing such as the fabrication of nanocomposites or devices.

In this review, we focus primarily on polymer nanocomposites generated with large-area, high-quality 2D sheets, i.e., those grown by the CVD method or other bottom-up methods. There have been a number of review papers about polymer nanocomposites with 2D materials such as graphene oxides and their derivatives [30,[71], [72], [73], [74], [75], [76], [77], [78], [79], [80]], MoS₂ [32,81], hBN [82], [83], [84], [85], and others. But 2D nanofillers discussed in those review papers are typically small-sized (<1.5 µm) nanoflakes produced by top-down exfoliation methods with a relatively low cost [33], [86], [87]. The exfoliated 2D flakes usually have varied thickness of multiple layers and a high content of defects [88,89] that can significantly deteriorate their raw performance for reinforcement. Consequently, it is almost impossible to use them for developing polymer nanocomposites with performance at the theoretical limit mentioned above.

We begin with a review of CVD synthesis of large-area and high-quality 2D materials such as graphene, MoS₂, hBN, and others, including the recent progress in the wet-chemistry synthesis of large-area 2D sheets. This will be followed by a discussion of the transfer of these 2D materials into various polymer substrates matrices. The fabrication of polymer nanocomposites with continuous 2D layers is then described including processing techniques and the resulting nanocomposite structures, their properties and details of the reinforcement mechanisms imparted. This will include a look at the application of these lightweight and strong structural materials for energy storage and conversion, sensor, separation membrane, colloidal electronics and elsewhere. Finally, a summary and outlook of this field will be presented. We believe that this review will provide a helpful unifying perspective on some of the most important topics in the field of 2D materials and polymer nanocomposites, particularly for readers interested in the next generation of polymer composite materials.