Characterization of hydrophilic—hydrophobic polymeric surfaces by contact angle measurements

Polymeric solid surfaces were prepared by a radiation-induced graft copolymerization technique using various mixtures of 2-hydroxyethyl methacrylate (HEMA) and ethyl methacrylate (EMA). Low-density polyethylene (PE) films were used as graft substrates. Contact angles on these polymeric surfaces were determined in air and under water. The critical surface tension (γc) of each polymeric surface in air was estimated by the Zisman method. Geometric mean and harmonic mean approximation methods were utilized to estimate the dispersion force contribution (γsd) and the polar contribution (γsp) to the total surface free energy (γs) from contact angle data in air. The geometric mean approximation was also used to estimate γsd′ and γsp′ from contact angles under water. The calculated values of γsd, γsp are strongly dependent on the pair of liquids chosen for the calculation regardless of the approximation adopted. The values of γs, calculated as the sum of γsd and γsp, are close to the γc values and are less dependent on the pair of liquids used. A comparison of the ratio γsd/γsp for the same surface in air and under water suggests that major polymer chain conformational changes occur, particularly with respect to the hydroxyl side chain, when such surfaces are immersed in water.