Controlled Placement of Microcapsules in Polymeric Materials
A wide variety of functional materials are based on microcapsules, including self-healing and self-sensing composites. In this paper, we demonstrate the ability to guide microcapsules to a desired location in an epoxy specimen using magnetic fields. Guida
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Controlled Placement of Microcapsules in Polymeric Materials Matthew D. Crall and Michael W. Keller
Abstract A wide variety of functional materials are based on microcapsules, including self-healing and self-sensing composites. In this paper, we demonstrate the ability to guide microcapsules to a desired location in an epoxy specimen using magnetic fields. Guidable microcapsules are synthesized by the inclusion of magnetic nanoparticles in the microcapsule core and shell. Nanoparticles are surface modified to enhance compatibility with emulsion-based encapsulation processes. Transmission Electron Microscopy (TEM) is used to determine nanoparticle crystal structure and overall size, found to be 7 nm in diameter. Microcapsules are characterized using optical microscopy and Scanning Electron Microscopy (SEM). The influence of microcapsule diameter and nanoparticle concentration is studied to optimize the placement efficiency of the microcapsules. Microcapsule dispersion and location are analyzed using optical and electron microscopy. The impact of the placed microcapsules on fracture toughness is evaluated using a Tapered Double Cantilevered Beam (TDCB) specimen geometry. Keywords Microcapsules • Magnetic nanoparticles • Self-healing • Controlled placement • Fracture toughness
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Introduction
Within the past 15 years, materials science has seen significant advances in the area of multi-functional materials. A subset of this field studies materials with the capability to self-repair, called self-healing materials. There are several mechanisms that can be used to achieve self-healing functionality. One particularly successful approach is to sequester a liquid “healing agent” that can be delivered to autonomically repair damaged regions [1–3]. Healing agents can be delivered using vascular networks embedded in the material or through the use of liquids sequestered in microcapsules. Microcapsule-based materials are synthesized by simply mixing in microcapsules into a polymer before it cures. An approaching crack ruptures the embedded microcapsules and releases the healing agent. The healing agent then reacts and bonds the crack faces, restoring the fracture toughness of the material [2, 3]. Various types of microcapsules have been synthesized for these types of applications, including microcapsules with diverse core and shell materials, different shell thicknesses, and varying numbers of shell layers. These improvements serve to make the microcapsules more robust or to expand the range of healing chemistries that can be used [4–6]. Such developments are geared towards creating intelligently designed materials with unique properties desirable for specific applications. The inclusion of microcapsules in polymeric materials can alter material properties in several ways. Typically, fracture toughness increases, as should be expected for the inclusion of particles in the matrix [7]. In addition, overall material modulus and ultimate strength can decrease as microcapsules are added [7]. Therefore, optimization of the micr
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