WETTABTILITY OF TRI-BLOCK COPOLYMER COATED HYDROPHOBIC SURFACES PREDICTIONS AND MEASUREMENTS

Hydrophobic surfaces with adsorbed tri-block copolymers are wetted by oil in spite of the hydrophilic buoy groups of the block copolymer tha t are present near the surface. The effect of the buoy group length of the adsorbed molecules on the wettability of hydrophobic surfaces is studied by contact angle measurements and by computer modelling. The c omputer model predicts an increase in interfacial free energy with inc reasing buoy group length for equilibrium adsorption of block copolyme r from water. Molecules with large buoy groups occupy more lateral spa ce; therefore the ''bare'' surface gets more exposed and the anchor gr oups contribute less to the interfacial free energy which thus increas es with the buoy group length. The calculations showed that the variat ion of the interaction parameter between solvent and buoy group hardly influences the interfacial free energy. In contrast the interaction p arameter between solvent and surface influences the interfacial free e nergy to a large extent because the oil/surface interactions have a lo wer energetic value as compared to water/surface interactions and ther efore the interfacial free energy is lower than in water. The interfac ial free energy varies slightly with increasing buoy group length, dep ending on the value chosen for the solvent/surface interaction paramet er. Advancing and receding contact angles of hexadecane, sunflower oil and hydrolysate (partly hydrolysed sunflower oil) were measured on hy drophobic surfaces. All oil/water contact angles were small, indicatin g a hydrophobic apolar surface character. It was found that, for oils with a ''good'' interaction with the surface (hexadecane and sunflower oil), the contact angle has a minimum value at a certain buoy group l ength. For hydrolysate (less-strong interaction with the surface) the contact angle decreases monotonically with increasing buoy group lengt h. The results for hexadecane, sunflower oil and hydrolysate are in re asonable agreement with the model predictions. The effect of increasin g buoy group length is weak; both decreasing and increasing angles are found, depending on the type of oil used.