Evaluation of Deformation Mechanisms at Mineral-Protein Composite Interface Using Steered Molecular Dynamics Simulations
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Evaluation of Deformation Mechanisms at Mineral-Protein Composite Interface Using Steered Molecular Dynamics Simulations Dinesh R. Katti, Pijush Ghosh, Kalpana Katti Department of Civil Engineering, North Dakota State University, Fargo, ND 58105, USA ABSTRACT In the area of clay-polymer nanocomposites, recently montmorillonite is extensively used because of its unique characteristics of swelling. In this work, steered molecular dynamics is used to evaluate the mechanical behavior of a new class of nanocomposites, using amino acids to intercalate clay interlayers. Two positively charged amino acids, lysine and arginine, are used here. Our simulation indicates that both the amino acids have preferred orientation inside the clay interlayer. Our simulations also indicate that the clay-amino acid interlayer is about three times stiffer under tension as compared to under compression. On the other hand, dry montmorillonite shows similar stiffness under tension and compression. The fundamental mechanism of deformation during tension and compression is intrinsically different in the amino acid-clay composite. The stress-strain behavior of this clay-amino acid interlayer is predominantly linear until a stress of 1.5 GPa. This study is a first step towards the potential use of biomacromolecules as modifiers in clay nanocomposites. INTRODUCTION Montmorillonite, a commonly known swelling clay, belongs to the smectite group of minerals. It exhibits the characteristics of swelling owing to its interlayer spacing between two segments. The interlayer spacing is present between two layers with oxygen atoms on the clay surface facing the interlayer and carrying partial negative charges. Due to the swelling behavior of the mineral, researchers are using montmorillonite in designing polymer-clay nanocomposites (PCN). In PCN, montmorillonite is intercalated with an organic modifier and dispersed in a polymer matrix to form a composite material with significantly improved engineering properties than that of the parent polymer forming the matrix. Specifically, the elastic modulus [1, 2], tensile strength and elongation properties [3], thermal resistance and flammability [4, 5] are enhanced. In addition, the interlayer in montmorillonite can act as a potential site of polymerization. Polystyreneclay and polypropylene-clay nanocomposites have been prepared in-situ using this interlayer [3]. The degree and nature of polymerization controls the d-spacing of the clay crystals. Higher d-spacing causes more polymers to enter into the interlayer, thus increasing the interaction between matrix phase and dispersed phase. In nanocomposites, the interfacial properties play an important role in determining the overall properties of the composites. Besides this, another factor which influences the property of a nanocomposite is the miscibility of the components involved in it. The mechanical properties of PCNs to a large extent depend on the mechanical response of the clay interlayer. Thus the study of mechanical response of clay with different
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