ABSTRACT
Abstract-The total surface free energy of poly(methyl methacrylate) (PMMA) was determined from the measured advancing and receding contact angles of water for at least 20 droplets, at varying relative humidity (RH) of the atmosphere present in the measuring chamber. At three selected humidities contact angles of diiodomethane and formamide were also determined. For determination of total surface free energy and its components two approaches were used: the contact angle hysteresis (CAH) approach and the Lifshitz-van der Waals acid-base (LWAB) approach. In the former approach the advancing and receding contact angles are employed, and in latter only the advancing angle. It was found that the apparent total surface free energy γ totS of PMMA in the RH range 35-100% decreases roughly linearly and in the range RH 1.5-10% it is practically constant. However, at RH between 10% and 35% a sharp minimum appears at RH = 15%. Possible reasons for such behaviour are discussed and a mechanism based on the coalescence of pre-adsorbed water nano-and/or micro-droplets during their contact with the water droplet deposited on the surface is proposed.
