CHARACTERIZATION OF POLYMER SURFACE SITES WITH CONTACT ANGLES OF TEST SOLUTIONS .1. PHENOL AND IODINE ADSORPTION FROM CH2I2 ONTO PMMA FILMS

The surface composition of solids is not predictable from bulk measurements, so surface chemical analysis methods are needed for understanding and predicting surface chemical interactions of solids with other materials. The calorimetric and spectrometric techniques used for powders and fibers are not sensitive enough for flat surfaces such as polymer films, and therefore, we are developing a contact angle technique that appears to allow direct determination of the surface concentration and strength of acidic or basic sites. Methylene iodide, a van der Waals liquid of high surface tension has finite contact angles on many organic polymers. Adsorption of test acids or bases from CH2I2 solutions onto basic or acidic surface sites of a polymer film is determined with the Gibbs adsorption isotherm by following the change in gamma-L cos theta with concentration. Then the Langmuir isotherm is determined, from which the concentration of acidic or basic sites at the polymer surface is calculated. Heats of acid-base interaction can be determined by making such measurements at two or more temperatures by use of the Langmuir equilibrium constant. Upon generation of more data, using hard and soft test acids or bases, the Drago E and C constants for acidic or basic surface sites may be determined. Since these measurements provide values of both the free energy and enthalpy changes due to interfacial acid-base interaction, the entropy change may also be determined. In this initial study, solution-cast films of poly(methyl methacrylate) (PMMA) were characterized by the adsorption of phenol and iodine at 23 and 10-degrees-C. The surface concentration of basic sites was found to be only 0.53-mu-mol/m2, indicating that most ester groups are buried and not available at the surface for acid-base interaction. The heat of acid-base interaction of phenol with PMMA determined from the temperature dependence of adsorption isotherms (-22 kJ/mol) agrees with that measured by infrared spectral shifts in solution.