Mechanical Behavior of Intercalated Polycarbonate Layered-Silicate Nanocomposites
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Mechanical Behavior of Intercalated Polycarbonate Layered-Silicate Nanocomposites Alex J. Hsieh, Donovan Harris, Paul Moy, and John W. Song1 U.S. Amy Research Laboratory, Polymers Research Branch, AMSRL-WM-MA, Aberdeen Proving Ground, MD 21005-5069. 1 U.S. Army Natick Soldier Center, AMSSB-RIP-B(N), Natick, MA 01760-5019. ABSTRACT The effect of layered-silicates on the mechanical response of intercalated polycarbonate (PC) nanocomposites subjected to quasi-static tensile, compressive and ballistic impact testing conditions has been investigated. These nanocomposites were melt-processed, in which good dispersion of nanoclays and adequate adhesive bonding between the nanoclay and PC matrix are achieved. However, their ductility upon tensile loading is significantly affected; a transition from ductile to brittle deformation occurs at clay loading of about 3 wt.%. Stress whitening is evident in the tensile- and ballistic-tested 1.5, 2.5, and 3.5 wt.% clay nanocomposites, and is attributed to the light scattering by microvoids, which are presumably formed from either crazing of PC or debonding of the nanoclay tactoids upon mechanical deformation. The effect of clay loading on the ballistic impact strength of the monolithic PC nanocomposites and layered PC/PC-nano/PC composites is determined. Compressive yield strength measurements are obtained at strain rate of 0.001/s for the monolithic PC nanocomposites and are utilized to correlate with the ballistic impact strength of the layered PC/PC-nano/PC composites. Thermal degradation is noted in these PC nanocomposites, and its effect on the mechanical deformation is briefly discussed. INTRODUCTION Property enhancement including decreased gas/liquid permeability and moisture uptake and increased fire/flame resistance, heat distortion temperature, strength and stiffness has been reported in the layered-silicate based polymer nanocomposites [1-24]. This is attributed to the nanometer length scale and high aspect-ratio characteristics of the individual clay platelets. Complete exfoliation of layered-silicates is the desired morphology to achieve better barrier properties; however, intercalated or partially exfoliated structures are prevailing in most engineering polymer nanocomposites. The surface chemistry of nanoclays, processing conditions, particle distribution, and the extent of platelet dispersion have a profound effect on the physical and mechanical properties of polymer layered-silicate nanocomposites. Incorporation of organo-modified nanoclays to improve the environmental durability of polycarbonate (PC) dramatically affects its ductility. For example, our previous test results show that PC nanocomposites with 5 wt.% nanoclay exhibit a brittle mode of failure upon tensile deformation [25]. On the other hand, PC nanocomposites with 2.5 wt.% nanoclay display similar but slightly decreased ductility compared to the PC-control. The effect of nanoclay loading on the mechanical response is consistent regardless of testing under quasi-static tensile, flexural or dynamic impact
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