MASS ADSORBED AND SURFACE FORCES IN TERNARY SOLUTION AS NONSOLVENT WAS ADDED TO THE POINT OF PRECIPITATION - POLYSTYRENE IN MIXTURES OF CYCLOPENTANE AND LINEAR PENTANE

Abstract

The influence of ternary solvent composition on mass adsorbed and on surface forces between adsorbed polystyrene layers was investigated. Polystyrene (M(w) = 500 000 and 3.84 x 10(6)) was adsorbed onto silicon oxide and mica from dilute solution (0.15 mg mL-1) in mixtures of cyclopentane and n-pentane at 23-degrees-C. Simplicity was afforded by the fact that the two solvents were expected to mix nearly athermally. FTIR-ATR (Fourier transform infrare spectroscopy in attenuated total reflection) was used to measure mass adsorbed at the silicon oxide interface, and surface forces were measured between overlapping adsorbed layers at the muscovite mica interface. Three principal findings emerged. (1) The levels of adhesion and the thicknesses of the adsorbed polymer layers, measured in the surface force apparatus, increased significantly with the addition of nonsolvent, but there was a continuous transition between behavior in homogeneous solution and in phase-separated solution. (2) The mass of polystyrene adsorbed at the silicon oxide interface increased systematically with higher volume fraction, phi-3, of n-pentane. The relation DELTA-GAMMA = phi-3(1.13) (DELTA-GAMMA is the mass adsorbed in excess of that adsorbed from undiluted cyclopentane) described this increase empirically, amounting to a factor of 2 before the point of precipitation. (3) Possibly contrary to intuition, there occurred no detectable (< 0.1 mg m-2) depletion of the nonsolvent, n-pentane, from the adsorbed polymer layers (surface coverages 4 and 5.5 mg m-2). Two hypotheses are suggested; they pertain not only to polymers in ternary solution but also to polymer chains adsorbed from a single solvent. First, the adhesion in two-phase solution sets a resonable upper bound on the interfacial tension between a polymer-rich surface and a polymer-poor coexisting solution, gamma almost-equal-to 0.05 mJ/m2. Second, the large but still microscopic amount adsorbed just before phase separation is consistent with the expectation that adsorbed monolayers should broaden to form multilayers containing chains, none of whose segments are located directly at the substrate surface.