A Direct Method of Studying Polymer Adsorption on to Mica Surfaces Using a Commercial Mettler Ultramicrobalance
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ABSTRACT A simple and direct method has been developed of determining absolute values of adsorbance, i.e. mass per unit area, of polymers adsorbed from solution on to mica surfaces. A commercially available Mettler UMT2 ultramicrobalance is used to measure the weight of mica sheets before and after the immersion of the mica sheets into polymer solution for a given period of time. The increase in weight of mica sheets is divided by the total area of mica surfaces to derive the adsorbance. Practically measurable change in adsorbance was evaluated to be 0.1 mg mi2 . As an example of the practical application of the present method, an experimental result is presented of the adsorption kinetics of polystyrene (M=66,000) adsorbed from cyclohexane on to mica surfaces at the 0 -temperature. INTRODUCTION Muscovite mica is now widely used as a material to provide a molecularly smooth surface for the surface force measurement [1]. The surface of mica is modified by the adsorption of various materials to investigate the interaction between the surfaces of these materials. Especially, the interactions between polymers adsorbed on mica surface have been studied to know the mechanism of steric stabilization of colloidal dispersion [2,3]. The lubricating actions have also been studied of polymer chains adsorbed on two surfaces set facing each other and being in lateral motion. [4-6].
Although the technique of surface force measurement is useful for the study of the interactions between adsorbed polymer layers, there arises a problem how much amount of adsorbed polymers affect the interactions between them. Therefore, an independent technique to determine the amount of polymers adsorbed on mica surface has been required [7]. Any existing methods used usually for the study of polymer adsorption are useless for mica. The area of mica surface is too small to apply the usual depletion method [8] based on the measurement of the difference in solution concentration before and after the adsorption. The surface of mica is not a reflecting mirror suitable for optical methods like ellipsometry [9,10] or spectroscopy on a mode of attenuated total reflection [11]. An attempt based on FECO (Fringes of Equal Chromatic Order) has been made to determine the adsorbed amount of polymers in the course of the surface force measurement [3], but the precision was unsatisfactory. Formally, in our laboratory, a direct weighing of adsorbed polymers on mica surface has been made using a fused quartz torsion microbalance of the author's own making [12,13]. In practice, the operational difficulties and the lack of the sensitivity were the serious obstacles to develop further. Recently, these difficulties have been entirely cleared by the employment of a commercially available Mettler ultramicrobalance, which is sufficiently sensitive and stable in operation. The object of this paper is to show the usefulness of a commercial Mettler ultramicrobalance for the study of polymer adsorption on to mica surface. A preliminary 225 Mat. Res. Soc. Symp. Proc. Vol. 543
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