Plasma modification of polyvinyltoluene and polystyrene latices
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I. INTRODUCTION
Plasma etching of polymeric materials is not a new concept. The field of lithography makes extensive use of plasma stripping of photoresist masks. Much less information is available on plasma effects on fine particles, which may cause changes in their morphology, as well as modify the specific surface areas, surface functionalities, and charge densities. Such alterations in properties could affect wetting, adhesion, stability, optics, etc. of the treated powders.1 This study describes the changes in characteristics of polyvinyltoluene and polystyrene latices etched by an oxygen plasma, with specific emphasis on the surface charges, as evaluated by electrophoresis. Electrokinetic behavior of fine particles is defined by the nature of ionizable surface groups and by the composition of the liquid medium. In principle, one should be able to evaluate the physico-chemical parameters, characterizing surface association and dissociation equilibria, from the electrokinetic data. Sprycha and Szczypa2'3 estimated surface ionization constants for SiO2, FeOOH, A12O3, and TiO2 from the corresponding electrokinetic potentials on the basis of a surface complexation model. In their approach the negative surface sites were neglected in the "positive" region (pH < pHiep) while the positive sites were not considered at pH > pHiep. The charge densities were directly calculated from the electrophoretic potentials on the basis of the Gouy-Chapman theory, assuming ^-potential to be equal to the electrostatic potential at the onset (inner boundary) of the diffuse layer. The effect of the surface potential on the interfacial ionization equilibria was taken into account by the semiempirical "double-extrapolation" procedure,4"6 and applied a)
On leave of absence from Faculty of Science, University of Zagreb, Zagreb, Yugoslavia. J. Mater. Res., Vol. 6, No. 4, Apr 1991
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to the results at different ionic strengths and pH values. In this article the electrokinetic data are interpreted on the basis of the surface complexation model,4'9 combined with the Stern-Gouy-Chapman description of the interfacial layer, taking the separation of the electrokinetic slipping plane from the inner boundary of the diffuse layer to equal ~7 A.6"9 The investigated systems consist of polyvinyltoluene and polystyrene latex particles, which undergo changes of surface properties on treatment with an O2 and CF4/O2 plasma, respectively. II. EXPERIMENTAL
All etch studies were carried out at 25 °C using a PlasmaTerm Reactor Model 730 in the RIE mode. The pressure and the total oxygen flow rate were held constant at 198-200 mTorr and 100 cm3/min (seem), respectively. The total gas flow was established at 86 cm3/min (CF4) and 16 cm3/min (O2) for CF4/O2 mixtures. Powders were evenly distributed on an aluminum foil placed on the bottom powered electrode, in the aluminum plasma chamber, with another electrode positioned at its top. The chamber becomes an asymmetric parallel plate etcher since the top elec
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