Interlamellar Incorporation of Charged Polymer Nanobeads into Sodium Montmorillonite
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Interlamellar Incorporation of Charged Polymer Nanobeads into Sodium Montmorillonite Svetlana Khvan, Sang-Soo Lee, Junkyung Kim Materials Science and Technology Division, Polymer Hybrid Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Republic of Korea ABSTRACT Complete delamination of clay in polymer matrix has been strongly prohibited due to strong adhesion of guest polymer chains between hydrophilic clay as well as degradation and desorption of organic materials in the gallery at high temperature. Incorporation of charged nanosized polystyrene beads directly into the gallery of pristine clay through exfoliation-exchange mechanism has been proposed to overcome the drawbacks. Synthesis of polymer nanobeads via emulsifier-free emulsion polymerization allowed to achieving formation of particles of appropriate particle size and surface charge density. Surface characterization, performed with XPS and ToF SIMS, has provided the results on the existence and the nature of the functional groups on the polymer particle surface, which have been found to be in a good compliance. Morphology of polymer-incorporated clay was observed from TEM, FE-SEM images. Study on mechanism of incorporation via XRD, XPS, ToF-SIMS suggested that adsorption of polymer nanobeads through cationic exchange of intergallery cation of clay for onium ion at the surface of polymer nanobead not only improves compatibility of clay with polymer matrix, but, what is essential, dramatically promotes expansion of clay gallery .
INTRODUCTION Many properties of polymer-clay nanocomposites are a function of the extent of dispersion of the individual clay platelets into a continuous polymer matrix as well as of the compatibility of the two intrinsically non-miscible materials [1-6]. In in-situ polymerization technique, the organoclay is swollen within a monomer solution so as in favorable cases the polymer formation can occur in between the intercalated sheets. The impregnation technique of the clay by different vinyl monomers followed by their polymerization has been applied by using a free-radical initiator [7-12]. However, due to impeded adsorption of monomer molecules between the lamellae the yields of the physically inserted polymer were found to be low. Only special converting the pristine layered silicate into a coreactive organophilic clay [6-9] or into initiator for controlled radical polymerization [10-12] was able to promote interfacial grafting of polystyrene onto clay and substantially improve exfoliation of the latter. For its part, fabrication of nanocomposite by direct melt intercalation of polymer into organophilic clay has problems due to thermal instability of organic materials modifying inner layer surface of clay. The interaction between onium ion and silicate layer of clay is not thermally stable enough to resist high melt processing temperature of polymer [2-4, 12]. The decrease in interlayer spacing that is attributed to the degradation and desorption of organic materials in
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