Polymer Microlayer Composites
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POLYMER MICROLAYER COMPOSITES A. Hiltner, K. Sung, E. Shin, S. Bazhenov, J. Im* and E. Baer Department of Macromolecular Science and Center for Applied Research, Case Western Reserve University, Cleveland, OH 44106 *The Dow Chemical Company, Midland, MI 48674
Polymer
ABSTRACT Continuous microlayer composites of polycarbonate (PC), a ductile glassy polymer, and styrene-acrylonitrile copolymer (SAN), a glassy relatively brittle polymer, were discussed. Microlayered systems composed of 49 to 776 continuous layers, in which the primary variable was the variation in thickness of the continuous layers between 30 and 2gm, were emphasized. The bulk properties of these microlayered composites showed a dramatic improvement in the toughness or ductility and in the fatigue properties as the layer thickness decreased. Investigation of the irreversible deformation processes revealed that when the layers were thicker (301gm, 49 layers) the SAN layers crazed and the PC shear banded in the usual manner. Subsequently the composite system fractured in a relatively brittle manner due to the development of large voids in the SAN layers. When the layer thickness was reduced to 2gm (776 layers) the entire system behaved in a ductile manner and both the PC and SAN shear banded due to a new cooperative process. This was analyzed by considering the micromechanics of these irreversible processes at the PC-SAN interface. As a result of this work, it is expected that ultra-thin layered structures of other alternating composite systems will reveal synergistic properties if the interfacial properties are designed so that the ductile component will dominate the yield and failure characteristics of the entire system. INTRODUCTION Microlayer coextrusion is an advanced coextrusion process that produces sheet or film with many alternating layers of two polymers [1,2]. It differs from the conventional multilayer coextrusion in that many more layers, usually more than 50 but sometimes as many as several thousand, are produced. By utilizing the flexibility of the process, the scale of the layer thicknesses can be the macroscale, the micro-scale, ranging from tens of microns to microns, or the nano-scale
[3].
Recent studies have been made of thick microlayer sheet in which polycarbonate (PC), a tough polymer, is combined with styrene-acrylonitrile copolymer (SAN), a brittle material that exhibits excellent interfacial adhesion to polycarbonate. Not surprisingly, the tensile and impact properties show a brittleto-ductile transition when the PC/SAN volume ratio is increased but the number of layers is maintained constant. However, when the layer thickness is decreased by increasing the number of layers from 49 to 776 while maintaining the sheet thickness constant, mechanical properties such as ductility and toughness are improved for any given PC/SAN ratio. It appears that brittle fracture of the SAN layers is suppressed as the layer thickness is decreased. Good adhesion between PC and SAN creates the possibility for extensive interaction at the micro-
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