Nanomechanical Properties of UV Degraded TiO 2 /Epoxy Nanocomposites
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Nanomechanical Properties of UV Degraded TiO2/Epoxy Nanocomposites Stephanie Scierka*, Peter L. Drzal*, Amanda L. Forster, and Stephanie Svetlik National Institute of Standards and Technology, Building and Fire Research Laboratory 100 Bureau Drive, Stop 8615, Gaithersburg, MD 20899-8615, U.S.A. ABSTRACT Model epoxy nanocomposite thin films containing one of three types of titanium dioxide (TiO2) particles were degraded using an integrating sphere-based ultraviolet weathering chamber. Instrumental Indentation Testing (IIT) was used to measure nanomechanical changes in the surface region of thin films resulting from UV exposure. Attenuated Total ReflectanceFourier Transform Infrared Spectroscopy (ATR-FTIR) and Differential Scanning Calorimetry (DSC) were used to support the mechanical results with chemical and thermal data. The unfilled epoxy was the most photosensitive sample tested, exhibiting the highest rates of chemical oxidation, the largest decrease in the glass transition (Tg), and the greatest increase in elastic modulus with increased exposure. Similar trends were observed in the nanocomposite films, but the rates of change were much lower than the unfilled epoxy and decreased with increasing volume fraction of nanoparticles. INTRODUCTION TiO2 particles and other pigments/fillers are commonly utilized in building and construction applications. For most applications, these components are added to increase the opacity, improve the appearance, or to increase the stiffness of coating systems. However, the addition of pigments has been shown to change the properties of the coating by decreasing its durability in outdoor environments, particularly when measuring gloss or chalking [1-4]. This is primarily because the titanium dioxide pigment is a semiconductor material. When a semiconductor is irradiated with light of a given wavelength, electrons in the valence band of the material are excited into the conduction band, forming holes, or positively charged species. Both electrons and holes are capable of causing the rapid destruction of organic materials that they contact, which is naturally a major area of concern for the coatings industry. Perhaps the greatest challenge in working with nanoparticles has been adequately dispersing them into the polymeric matrix. Nanoparticles have inherently strong interparticle attraction and thus, lead to the aggregates causing a decrease in the effective volume fraction of filler and leading to poorer mechanical improvement. One thrust of this work is to examine model nanocomposites by evaluating the mechanical enhancement due to the addition of nanoparticles using nanoindentation. In recent years, Instrumented Indentation Testing (IIT) has emerged as a powerful technique for measuring the hardness and elastic modulus of materials over smaller length scales than can be obtained with conventional mechanical characterization. Originally developed for indentation of metals and ceramics, IIT has many applications in the polymer community [5,6]. This is partly due to recent
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