Stiffness Improvements and Molecular Mobility in Epoxy-Clay Nanocomposites
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Stiffness Improvements and Molecular Mobility in Epoxy-Clay Nanocomposites X. Kornmann1, L. A. Berglund1, and H. Lindberg2 1 Division of Polymer Engineering, Luleå University of Technology, S-97187 Luleå, Sweden 2 Division of Wood Material Science, Luleå University of Technology, S-93187 Skellefteå, Sweden
ABSTRACT Conventional composites filled with clay as well as intercalated nanocomposites, and exfoliated nanocomposites based on a glassy epoxy matrix have been synthesised. Flexural moduli of these materials were measured in three-point bending at various clay contents. For a given clay content, stiffness improvements depended not only on the dispersion of the clay on the microscale, but also on the exfoliation of the clay layers at the nanolevel. Dynamic mechanical measurements indicated a decrease of intensity in the glass transition peak with the extent of exfoliation of the clay and the clay content, suggesting a restriction of the molecular mobility of the polymer in the vicinity of the clay layers. A shift in Tg of 20°C towards lower temperature for the epoxy resin cured at 160°C was possibly caused by thermal degradation of compatibilizing agents at high temperature. INTRODUCTION Polymer-clay nanocomposites are attracting a great deal of attention. Because of the molecular size of their reinforcement, they exhibit substantially improved thermal and mechanical properties [1, 2] as well as exceptional barrier properties [3]. Such improvements may extend the field of applications of polymers. The nanoscale reinforcements are 1 nmthick silicate layers of high aspect ratio (typically 200). Because of its hydrophilic nature, the clay does not disperse well in organic media. For this reason, treatment with compatibilizing agents is required prior to mixing with a polymer. Polymer-clay composites are usually divided into three general types : conventional composite where the clay acts as a normal filler, intercalated nanocomposite consisting of the regular insertion of the polymer in between the clay layers and exfoliated nanocomposite where individual 1 nm-thick clay layers are dispersed in the matrix on a molecular level, forming a monolithic structure on a microscale. Toyota researchers were the first to synthesise an exfoliated nanocomposite based on montmorillonite clay treated with amino acids reacting with polyamide 6 [4]. Their material showed dramatic improvement of thermal and mechanical properties and was used to produce timing belt covers for automotive engines. The weight saving was about 25% as compared with a conventional glass fibre reinforced polyamide 6. They explained the increase in properties of their material by constrained polymer regions due to the reinforcement phase [5]. Exfoliated nanocomposites based on rubbery [6] and glassy [7, 8] epoxies have also been studied. Major improvements in modulus were reported at low clay content. The 10-fold stiffness improvement observed in the rubbery epoxy with only 7.5 vol% of exfoliated clay may be explained by the restricted mobility experienced by
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