論文の公開元へDesign and Development of Soft Fiber-Reinforced Polymer Composites with Extraordinarily High Crack Resistance
概要
1.1. Overview
Engineering materials, for instance, metals, ceramics, are widely applied in industry owing to their desirable mechanical performance (high strength, stiffness, and good corrosion resistance, etc.).[1-3] However, they also show obvious shortcomings, such as heaviness, brittleness, and notch sensitivity. For decades, scientists and engineers have been attempting to overcome the limitations by developing novel structural materials that can offer prominent incorporations of stiffness, strength, toughness, and lightness at low cost. As science and technology advanced, synthetic compounds from multiple components, usually termed as composites, are invented and gradually replacing traditional industrial products due to the efficient combination of various mechanical properties that surpass those of their components by orders of magnitude.[4-6] One type of the most common composites at present, fiber-reinforced polymers (FRPs), are frequently used to substitute neat metals or ceramics in industry because of their improved comprehensive performance.[7, 8] These composites are generally composed of rigid elastic matrices (polymers, metals, ceramics, etc.) and rigid fibers, demonstrating high strength and stiffness.[9-11] Nevertheless, due to poor interfacial bonding and force transmission between matrix and fibers (Figure 1.1), the rigid/rigid combinations usually exhibit unsatisfactory fatigue crack resistance.[12-17] Namely, conventional fiber or fabric reinforced composites, with an isotropic high modulus (~several or tens of GPa), are intrinsically rigid materials with limited and resulting low toughness. More unfortunately, incorporating multiple and mutually exclusive properties into one system, as exemplified by high toughness, strength, and stiffness, results in materials that compromise one essential property at the expense of another.[18-20] Therefore, a pivotal challenge for current composite field is that how to improve material toughness without sacrificing strength, lightness, etc.
In fact, nature has managed to circumvent the dilemma and developed materials showing superior comprehensive mechanical performance (high fracture resistance, good load-bearing capacity and low density). The creations include, but not limited to, ligament and skin,[21, 22] bone and nacre.[23, 24] The strategy is to combine rigid, brittle components and soft, organic matrices into composite materials. Most of these natural materials have highly sophisticated structures with complex hierarchical designs existing over multiple length scales, which result in composite properties that far exceed what could be expected from a simple combination of the individual components (Figure 1.2). Extensive researches have been carried out in an endeavor to mimic the unique natural structure with synthetic approaches, aiming to obtain artificial composites exhibiting prominent mechanical performance which is comparable to natural materials. For example, a synthetic nacre was fabricated by predesigned matrix-directed mineralization, showing a specific toughness and strength close to natural nacre.[25] Another soft biomimetic composite was designed by incorporating stiff aramid nanofibers into poly (vinyl alcohol) system, whose mechanical properties matched or exceeded those of prototype tissues.[26] Recently, based on the delicate concept learning from nature, our group has developed a new class of tough soft composites from the soft/hard combination of polyampholyte (PA) hydrogel matrix and woven glass fiber fabric (GF).[27-29] The de-swelling tough PA gel, with multiple ionic bonds in the gel network,[30] demonstrates a self-adjustable adhesion to either positively or negatively charged surfaces.[31] Therefore, it is supposed to form good adhesion with negatively charged GF. As expected, the biomimetic composites, having a desirable interface between matrix and fiber, exhibit very high fracture toughness (~250 kJ m-2 ), strength (~65 N mm-1 ), and tensile modulus (~600 MPa), which are far superior to those of either the hydrogel or GF (Figure 1.3). As the composites contain water and are likely biocompatible, they exhibit some structural similarities with load-bearing natural tissues and hold great potential in biological applications. However, for this kind of hydrogel/fabric composites, a problem is that water evaporation during use occurs inevitably, which significantly influences their mechanical performance under industrial condition. Hence developing a universal composite system that is tough yet stable for industrial application awaits further exploration.
To overcome the dilemma for hydrogel/fabric composites, replacing water-contained matrices with more suitable ones is necessary. Similar to hydrogels, elastomers are also soft and energy dissipative, and they are usually tougher and water-free, enabling them to be applied in industry. Conventional elastomers, such as polydimethylsiloxane (PDMS), polyurethane (PU), are highly elastic and can hardly form good interfacial bonding with diverse surfaces. Composites from them usually show a limited mechanical performance. Lately, our group has successfully designed and developed a series of novel viscoelastic elastomers via a simple one-step radical polymerization of two kinds of acrylate monomers.[32] The resulting elastomers are not only soft and tough, but also adhesive to diverse surfaces (Figure 1.4), which allows the fabrication of composites with superior mechanical properties feasibly. 1.2. Outline of this thesis The aim of this study is to design extraordinarily crack-resistant, yet strong and lightweight fiber-reinforced polymers (FRPs), and generate a universal criterion by understanding the fracture mechanism. To address these issues, we mainly focus on the following three parts: 1) Selecting suitable matrices to construct crack-resistant FRPs; 2) Investigating the mechanical behaviors of resulting FRPs; 3) Understanding the fracture mechanism of tough FRPs by virtue of mechanics models.
In Chapter 2, a brief view on the concept of crack resistance is introduced. Meanwhile, common experimental methods to characterize the crack resistance of a material are explained. Based on the lessons learned from nature, basic strategy to enhance the crack resistance of materials is put forward. This chapter is helpful to initially understand why composite materials always show fantastic crack resistance.
In Chapter 3, the strategy to design an extraordinarily tough fiberreinforced polymers is introduced in detail. Viscoelastic matrices that are adhesive, soft, and tough are selected to combine with commercial fiber fabrics. The three key properties result in composites showing unique features that are totally different from traditional composites with thermosetting plastics as matrices. The good adhesion between fibers and matrix enables a strong interface, which ensures both components to fully dissipate stored energy; The softness of matrices gives extremely high fiber/matrix modulus ratio, leading to energy dissipation zones several orders of magnitude larger than common composites from rigid matrices; The tough matrices show strain energy density comparable to fibers, highly enhancing the energy dissipation density of composites in the dissipation zone. Therefore, we reasonably expect the composites are able to have a satisfying crack resistance.
In Chapter 4, the prepared fiber-reinforced polymer composites are tested to investigate their mechanical properties. The highly anisotropic composites demonstrate multiple fantastic properties such as high strength, high toughness, and low density, which can be rarely achieved by other material systems. The strong interface between matrix and fibers is the premise of high mechanical performance. Moreover, the soft composites can also be polymerized from thermal initiation besides photo initiation, extending the possible application in industry. The design strategy is also universal, strong and tough composites can be obtained by combining various fabrics and matrices that are adhesive, soft, and tough. High temperature influences the performance of soft composites. However, preparing composites from matrices with high glass transition temperature is a way to solve this problem.
In Chapter 5, the crack-resistant mechanism of soft composites is analyzed. Consistent with the tearing behaviors at different width, the composites show a size-dependent fracture energy. Two characteristic widths are defined to divide the fracture behaviors of soft composites. According to the characteristic widths, the fracture energy of the materials is determined by the matrix toughness, fiber geometry, and width, when the fracture behavior of composites is fiber pullout and matrix failure. When the fracture behavior of composites is mainly fiber fracture and matrix failure, then the fracture energy is decided by the force transfer length as well as the energy dissipation density, and the fracture energy become size-independent above this width, reflecting the intrinsic crack resistance of the composites. We show that force transfer length is related to the component modulus ratio while the energy dissipation density results from the volume weighed average work of extension of components. The results point out the way to fabricate tough composite materials. That is, maximizing force transfer length by increasing modulus ratio and enhancing energy dissipation density by using energy dissipative components. Based on this principle, we successfully fabricate composites that show fracture energy of as high as 2500 kJ m-2 , 100 times that of current toughest composites and are even tougher than metals. In Chapter 6, we apply the design strategy to hydrogel system, aiming to develop crack resistant composite hydrogels. Alginate hydrogels dried in confined condition are employed as the rigid skeleton. Polyacrylamide hydrogels are used as the soft matrix. The modulus ratio of rigid to soft can be as high as 105 , which is conducive for a large force transfer length. Meanwhile, the alginate skeleton has an energy dissipation density comparable to commercial fibers, which facilitates high energy dissipation density. The resulting composite hydrogels show improved tensile and tearing properties compared with components and are higher than current toughest hydrogels.
In Chapter 7, conclusions of the whole dissertation are summarized.
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