Temperature dependence of the surface behaviour of a side-chain liquid crystalline polymer probed by contact angle measurements

In this study the temperature dependence of the surface properties of a side-chain liquid crystalline polyacrylate is investigated through contact angle measurements. In particular, the effect of the glass transition (T-g = 46.7 degrees C) on the surface tension of the liquid crystalline polymer is analysed. The total surface tension as well as its dispersive and polar components are determined as a function of temperature, between 24 degrees C and 73 degrees C, from contact angle values of water, diiodomethane and glycerol. For water and diiodomethane the proximity of the glass transition temperature induces an abrupt change in the temperature dependence of the contact angle; the same is not observed for glycerol. The calculations were performed using the most common techniques: the Neumann equation of state for interfacial tensions, the Owens-Wendt approach (also known as the geometric mean approach) and Lifshitz-van der Waals donor-acceptor approach (also known as the acid-base approach). The results obtained for the total surface tension of the polymer surface were compared and discussed. The temperature dependence of the surface tension exibits the behaviour characteristic of a second-order transition: the surface tension is continuous in the vicinity of the glass transition temperature but its first derivative is discontinuous. Furthermore, the decomposition of the surface tension into its components shows that the polar component, gamma(s)(p), has a small contribution throughout the entire temperature range, while the dispersive component, gamma(s)(d), largely predominates. The temperature dependence of both components gamma(s)(d) and gamma(s)(p) is clearly different in the two phases (glassy and liquid crystalline), despite the fact that the total surface tension does not present any appreciable discontinuity across the glass transition.