On Origins of Time-Dependence in Contact Angle Measurements

Time-dependent variations have been observed in the contact angles of wetting fluids on surfaces of homopolymers poly(styrene) (PS) and poly(methyl methacrylate) (PMMA), and copolymers of styrene and 4-vinyl pyridine (P(S-4VP)) and styrene and methacrylic acid (P(S-MAA)). Variations in the case of PS were unimportant, but significant variations occurred with the other polymers, allowing for the definition of both initial and equilibrium contact angle values. The total change in contact angles was strongly dependent on the acid-base contribution to the free energy of adsorption of the wetting fluids on the respective polymer surfaces, as determined from inverse gas chromatography experiments. A tentative correlation also linked the total change in contact angle with the ability of the polymer surfaces to reorganize when in contact with the polar wetting fluids of this study. Careful consideration must be given to the possibility of polymer surface reorganization when selecting fluids for use in the collection of reliable contact angle data.