Dynamic Responses in Polymer-Clay Gels
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Dynamic Responses in Polymer-Clay Gels ELENA LOIZOU1 , LIONEL PORCAR2 , PAUL BUTLER2 , MATTHEW MALWITZ1 , GUDRUN SCHMIDT1* 1 2
Department of Chemistry, Louisiana State University, Baton Rouge, LA, 70803 National Institute of Standards and Technology, Gaithersburg, MD 20899
ABSTRACT The shear orientation of three polymer-clay gels has been investigated by means of small angle neutron scattering (SANS). The gels have the same polymer and clay concentrations but different polymer molecular weight. The polymer is adsorbed to the clay platelets. While long polymer chains can interconnect several platelets shorter polymer chains cannot. Although the polymer concentration is above c* the polymer chain length and cross linking between clay platelets strongly influence their shear orientation which leads to anisotropy in SANS. Our data suggest that the flow is strong enough to enhance and maintain a continuous increase in the shear orientation of the polymer clay gels only when the polymer chains are long enough to interconnect or strongly entangle between platelets. INTRODUCTION Polymer-clay gels, solutions and bulk nanocomposites are generating tremendous interest as novel materials exhibiting unique mechanical, electrical, optical, and thermal properties. The ability to tailor properties to the desired application by controlling nanoscopic structure can optimize the materials performance for a variety of applications. Nanocomposite applications range from pharmaceuticals, shampoos, optical switches to thin film barrier membranes and even ceramic precursor materials. Several methods have been applied to examine the polymer-clay interactions and shear induced structural changes. Among them, microscopy and light, x-ray and neutron scattering are very powerful techniques for unveiling their structure and providing a measure of size, shape and interfacial polymer conformation. 1-4 Solutions, gels as well as solution precursors experience considerable flow during processing and, sometimes, as part of their end use. Furthermore, many properties depend on the shear dependent structural changes such as the orientation of the clay platelets within the material, a property which can be greatly affected or controlled by shear flow. Thus it is imperative to understand the polymer clay interactions in solutions and their responses to shear. The chemical simplicity of poly(ethylene-oxide) (PEO) makes it a useful model system for studying the charging effect induced by ion-dipole association in water.5-7 PEO strongly absorbs onto charged Laponite clay platelets. Low molar mass polymers thus inhibit aggregation of clay particles by classic steric hindrance, while higher molar mass polymers, particularly at higher concentrations of clay, bridge between particles and lead to the formation of large clusters8 or smart gels with novel properties.3,9-11 Recent small angle neutron scattering (SANS) studies describe the adsorption of PEO polymer chains onto Laponite clay platelets at low polymer and clay concentrations using contrast va
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