Investigation into the Thermal and Mechanical Behavior of PMMA/Alumina Nanocomposites
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Investigation into the Thermal and Mechanical Behavior of PMMA/Alumina Nanocomposites Benjamin J. Ash, Jason Stone, Diana F. Rogers*, Linda S. Schadler, Richard W. Siegel, Brian C. Benicewicz*, Thomas Apple* Department of Materials Science and Engineering,*Department of Chemistry Rensselaer Polytechnic Institute, Troy, NY 12180-3590. Abstract
Polymethylmethacrylate (PMMA) nanocomposites were synthesized by free radical polymerization in the presence of various weight percentages of alumina (Al2O3) nanoparticles. The resulting nanocomposites show an average increase of 600% in strain-to-failure and the appearance of a well-defined yield point. Concurrently, the glass transition temperature (Tg) of the composites decreased 20ºC, while the ultimate strength and the Young’s modulus decreased by 20% and 15%, respectively. Introduction
Nanocomposites make up a particular class of polymer composites that have recently garnered much attention. This growing class of materials can lead to reinforcement of a polymer without the undesirable property changes exhibited by micron particle filled systems. For example, Sumita found dramatic improvements in the yield stress (30%) and Young’s modulus (170%) in nano-filled polypropylene compared to micron-filled polypropylene that lowered the yield stress and provided only modest improvements to the modulus [1]. Similarly, Ou et al., filled Nylon 6 with 50 nm silica particles and reported increases in tensile strength (15%), strainto-failure (150%), Young’s modulus (23%), and impact strength (78%) [2]. One of the key aspects of nanoparticles as a filler is their significant surface to volume ratio. When any particle is well dispersed in a polymer matrix, the volume directly surrounding the particle has polymer chains in contact with the particle surface. If the particle has a strong affinity for the matrix, these chains can lose some of their mobility and a region of low-mobility polymer will exist around each particle [3]. This layer of affected polymer has been estimated to be somewhere between 2 and 9 nm thick [4] and is generally termed the “bound polymer” (BP) layer. In conventional micron-filled composites, the volume fraction of BP is insignificant due to the relatively large diameter of the particles and the corresponding low surface to volume ratio. In the case of nanocomposites, however, this region of affected polymer can represent a significant volume of the total matrix polymer. Therefore, the BP’s behavior will directly impact the observed mechanical and thermal behavior of the composite [5]. For example, Becker found an increase in the Tg and a 100% increase in storage modulus above Tg in a nano-size silica/ PMMA-HEMA system [6]. Iisaka and Shibayama also reported an increase in Tg directly related to the strength of interaction between glass beads and PMMA [5]. The presence of the nanoparticles changes the overall mechanical properties of the polymer matrix. In general, well-dispersed particles or voids can change the stress state of a large volume fraction of the pol
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