Hybrid and Hierarchical Composite Materials

Hybrid materials generated via the combination of functional polymers with inorganic nanostructured compounds, with the latter exhibiting size-dependent physical and chemical properties, have become a major area of research and technological development owing to the remarkable properties and multifunctionalities deriving from their nanocomposite/nanohybrid structure. In this chapter, the different fabrication routes for generating organic–inorganic polymer hybrid materials are discussed. Those include blending processes, sol–gel methods, emulsion polymerization and photopolymerization, metallosupramolecular and coordination approaches, intercalation, microwave-assisted and electrochemical synthesis, synthetic routes based on surface grafting, and finally self-assembly and block copolymer-mediated synthetic strategies. The existing versatility in materials’ design in organic–inorganic polymer hybrids, in respect to the structural, compositional, and architectural characteristics, creates new prospects for many applications in very diverse areas. In the second section of this chapter, the applicability of organic–inorganic polymer hybrids in various fields including biomedicine, sensing, environmental remediation, energy, construction, automotive and coating technologies, catalysis, and optoelectronics is reviewed.

The grafting of polymeric chains to inorganic (as well as organic) particle interfaces has become an indispensable tool to engineer the physicochemical and/or biochemical properties of material interfaces. For example, polymer grafting is ubiquitously being used to compatibilize particles to polymer matrices to augment the properties of polymers in applications such as biomedical devices, lightweight aircraft wings, energy generation and storage, and for separation and environmental remediation to name a few. The recent emergence of surface-initiated controlled radical polymerization has further expanded the scope of polymer-grafted particulate materials as the precise control of the structure of the polymer grafts, offers new opportunities to tailor the properties of polymer-grafted particle systems. This chapter summarizes recent developments in synthesis of polymer-tethered nanoparticle interfaces that have afforded this fine control in the structure and properties of the resultant composite. Particular emphasis is given to the concept of “one-component hybrid materials”—that is the ability to synthesize multifunctional nanocomposite materials by the self-assembly of polymer-tethered particle systems. The role of polymer-graft modification on the interaction, dynamics, and assembly of particle brush materials is discussed to provide the context to showcase studies that have demonstrated the opportunity to harness the precision-engineered polymer-grafted particle systems for the fabrication of innovative nanocomposite material technologies.

The multiferroic magnetoelectric (ME) effect describes the coupling between the electric and magnetic fields, and is defined as a generated electric polarization P in response to an externally applied magnetic field H (direct ME effect), or an induced magnetization M with an applied electric field E (converse ME effect). Unfortunately, the ME coupling of all the known single-phase materials is usually small at room temperature to be practically applicable. Alternatively, multiferroic composites (ferroelectric and ferri/ferromagnetic phases) typically yield a giant ME coupling response above room temperature, which makes them attractive for technological applications. In the composites, the ME effect is generated as a product property of the magnetostrictive effect (magnetic/mechanical effect) and piezoelectric effect (mechanical/electric effect). To achieve a large ME response, piezoelectric constituent with a high piezoelectric coefficient, magnetostrictive constituent with a high piezomagnetic coefficient, and good coupling between the piezoelectric and magnetostrictive constituent are required. In this chapter, we begin with a brief overview of the development of each material’s constituent (piezoelectrics and magnetostriction) providing a list of state-of-the-art piezoelectric and magnetostrictive materials in multiferroic ME hybrid. Next, a discussion is provided on the composite structure and interface elastic coupling between the piezoelectric and magnetostrictive phases. After that we describe the fabrication process of several important ME hybrids with different phase connectivity, interface, and configuration. Considering the importance of nanostructure and 2–2-type ME composite, the scaling effect and theoretical modeling for these architectures are presented in some detail. Following these sections, some of the potential applications for ME hybrids are reviewed and illustrated by examples. Lastly, the chapter is concluded with a brief summary and future perspective.

Clay/polymer nanocomposites have been extensively studied in recent years. The present state of the art for these materials is summarized in this chapter. The development of fabrication methods for these composites is very challenging because the platelets of nanoclay exist in the form of clusters, which need to be dispersed in the matrix resin in order to obtain any benefit from the high surface area of nanoclay. Incorporation of only a small weight fraction (1–5 %) of nanoclay in polymers provides significant benefits in the properties of composites. Several tensile, flexural, and thermal properties are found to increase by 30–40 % due to the presence of nanoclay in the composite compared to the properties of the neat resin. Entrapment of air porosity at higher nanoclay content can lead to reversal of the trends and can actually reduce the mechanical properties of nanocomposites. Theoretical models have been developed to estimate the properties of nanocomposites by accounting for the microstructure that may include clustered, intercalated, or exfoliated nanoclay. The benefit in mechanical properties obtained from incorporating nanoclay is much greater than that can be achieved with microscale reinforcement at the same loading levels. The current applications of clay/epoxy nanocomposites are in the area of automotive moldings and fire retardant coatings.

Composites with hierarchical structures are considered to be one of the most prevailing materials for various electrochemical applications as they constitute a complex architecture that provides a large internal surface area and enables synergistic effects for electrochemical reactions at the interface. Considering novel nanoscale properties with large-scale processability and affordable cost, hierarchically structured composites could potentially provide the new and transformative approaches to meet the challenges for the modern society: enabling powerful electrochemical devices for renewable energy conversion and storage and environmental monitoring. This chapter reviews the applications of hierarchically structured composites for energy storage, energy conversion, and environmental monitoring. In the section of energy storage, supercapacitors and batteries are intensively reviewed with emphasis on the design, synthesis, and performance evaluation of the hierarchically structured composites; in the section of energy conversion, photoelectrochemical cells and fuel cells are introduced; and in the section of environmental monitoring, we introduce some sensing devices based on the hierarchically structured composites. The fundamental understanding of the structure–property relationship between these hierarchically structured materials and their performances in electrochemical devices will further promote the design of new electrochemical materials with unprecedented properties.

The structural design of composite materials at multiple length scales is a widespread strategy found in biological materials to optimize opposing properties or to combine multiple functional properties in a unique material system. The combination of this hierarchical structuring approach with the vast chemical repertoire available in synthetic systems is expected to lead to man-made composites with unprecedented functionalities. Alternatively, hierarchical materials can potentially achieve sufficient strength and toughness even if made out of weaker environmentally-friendly or bioresorbable building blocks. Replicating the hierarchical design principle of biological systems in synthetic materials is an exciting challenge that has been tackled by researchers across different scientific communities. In this chapter, we present state-of-the-art examples on attempts to identify fundamental design principles of hierarchical natural materials and to then mimic these bioinspired concepts in man-made materials. Three selected structural features that can be independently designed at multiple length scales in biological materials are described as examples: (i) mechanical reinforcement, (ii) porosity, and (iii) topography. By comparing biological and man-made materials exhibiting these hierarchical features, we provide an overview on the limitations of currently exploited top-down and bottom-up manufacturing technologies and on the opportunities for the future development of hierarchical composites inspired by the unique multiscale structure of biological materials.

Hierarchical composites containing carbon nanotubes (CNTs) have the potential to possess improved multifunctional properties as well as unique sensing/active capabilities due to the inherent properties of CNTs (i.e., mechanical, electrical, and thermal). The purpose of this chapter is to review the current state of the art in this research area and highlight opportunities for future research. Specifically, three construction schemes used to produce CNT hierarchical composites are reviewed: dispersed systems, fiber coatings, and CNT structures. In these construction schemes, CNTs are used as a performance additive to reduce matrix mobility, as a sizing to improve adhesion between the matrix and the microscale fiber, and as the building blocks of structures such as fibers to more fully exploit the mechanical properties of CNTs, respectively. To date, research results have indicated that these strategies produce composites with improved properties, and most frequently those improved properties are mechanical properties such as strength, modulus, and fracture toughness. Based on these results, further activities aimed at understanding these property increases in terms of modeling as well as more research activity aimed at producing CNT fibers and exploiting other CNT properties will lead to improved approaches to composite design which merit the routine use of CNTs in structural composites.