Polymer/Halloysite Nanotubes Composites: Mechanical Robustness and Optical Transmittance
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Polymer/Halloysite Nanotubes Composites: Mechanical Robustness and Optical Transmittance Kenan Song 1, Michael F. Rubner 1, Robert E. Cohen 2, Khalid A. Askar 3 1 Department of Materials Science and Engineering, MIT, Massachusetts 02139, United States 2 Department of Chemical Engineering, MIT, Massachusetts 02139, United States 3 Department of Materials Science and Engineering, Masdar Institute, United Arab Emirates ABSTRACT Halloysite nanotubes (HNTs) have attracted attention for their potential use in a variety of applications owing to their mechanical robustness, thermal stability, natural abundance and low cost. The inclusion of HNTs into epoxy matrix at low concentrations was found to be effective in stiffening and hardening. At 1 vol% loading, composites showed improvements up to 50% in modulus and 100% in hardness compared to pure epoxy, based on nanoindentation measurements. In addition, tribology studies using TriboIndenter and AFM showed an increase of wear resistance; depending on their orientation in the composite, HNTs can decrease the scratch volume by 50% at fixed loading levels. Adding HNTs into epoxy had almost no effect on the transmittance over the range of wavelength from 400 to 700 nm. Transmittance values of 91% were observed for HNT concentrations as high as 10 vol%. INTRODUCTION Nowadays, many types of plastics are used as optical materials due to their lightness in weight, ease of mass production, and inexpensiveness1. Among these, thermoplastics including poly (methyl methacrylate) (PMMA), polystyrene (PS), polycarbonate (PC) and cyclic olefin copolymer (COC), and thermoset resin allyl diglycol carbonate (ADC) are well-known materials for making sheets or articles of excellent optical and thermal properties2. One of the major drawbacks limiting their uses is the poor scratch resistance3-5. To overcome such a problem, organic–inorganic hybrid composites have been studied for effective hard-coating materials, which aim to mediate the mechanical robustness and optical transparency in organic and inorganic precursors6, 7. SiO2, TiO2, ZnO, ZrO2, AlN nano-particles mixed with organic compound based on sol-gel reactions have been reported8-12. Nonetheless, the addition of nonorganic particles very frequently affects the clarity and optical transmittance, therefore limiting their usage in applications requiring anti-reflection properties (i.e., solar cells require 98% transmittance)13-15. Alignment of hollow tubular fillers in polymer matrix can allow light go through the low refractive-index hollow structure and reflective light. In addition, taking advantage of these oriented particles more opportunities will be open, such as to combine functional properties including hydrophobicity and anti-static behaviors as well as mechanical durability16. However, the mechanical reinforcement and scratch resistance of aligned particles in thin-film coatings have not been studied mainly due to processing difficulty. Halloysite nanotubes (HNT) are aluminosilicate clay mineral with the empirical formula of Al2S
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