Specific Interactions and Contact Angle Measurements on Polymer Solids

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Abstract
The present work examined the susceptibility of contact angle data to specific interactions taking place between solids and contacting liquids. The polymers involved were polystyrene, polyvinyl chloride and polyethylene, representing respectively basic, acidic and neutral substrates. Contacting fluids also were chosen to represent acid and base interaction categories. Significant time-dependent changes in contact angles were observed when acid/base pairs were involved in the experimental sequence. In specific cases it was possible to identify initial (zero contact time) contact angles, as well as equilibrium values, attained after prolongued contact times. Local solvation, or plasticization, of the polymer by the wetting fluid was postulated as the operative mechanism. The differences between initial and final values of the contact angles were correlated with parameters of specific interaction, calculated from the acceptor/donor numbers for the pertinent materials as measured by inverse gas chromatography. In contrast, when acid/acid or base/base combinations of polymer and wetting fluid were studied, equilibrium values of the contact angle were established rapidly. Since accurate information on acid/base properties of polymers and wetting fluids is not always available, it seems prudent to record contact angles as a function of contact time, and by extrapolation to determine the initial (true) value for further use in surface characterizations of polymers.

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