Dependence of the contact angle on the molecular structure of poly(propene-alt-N-maleimide) and poly(styrene-alt-N-maleimide) copolymers

Abstract

Dynamic contact angle measurements on poly(propene-alt-N-maleimide) and poly(styrene-alt-N-maleimide) copolymers are presented. The copolymers were altered by side chains of saturated hydrocarbons of increasing length. Films of these copolymers were formed on silica wafers by casting from polymer solutions. Axisymmetric Drop Shape Analysis for Profiles (ADSA-P) was used for low rate dynamic contact angle measurements. The solid surface tension γ_SV was calculated using an “Equation of State”. For poly(propene-alt-N-maleimide) copolymers increasing length of the side chains lead to an increasing contact angle. The solid surface tension γ_SV varied in the range from 22.6 to 41.5 mJ/m². For a side chain with 12 carbon atoms the surface tension γ_SV is at the value reported in the literature for surfaces consisting mainly of CH₃-groups. The wetting behavior for poly(styrene-alt-N-maleimide) copolymers, however, showed a very strange behavior for water and glycerol, respectively. No trend of the contact angle versus the length of the side chain could be found. Instead, two different ranges of the contact angle appeared. At three carbon atoms it is inconsistent. Molecular modeling suggested a conformation for poly(styrene-alt-N-maleimides) to explain the unexpected behavior. This model was supported by wide angle X-ray diffraction (WAXS). A similar conformation was also suggested by Natta. From this it can be concluded that contact angle measurements are very sensitive to structural changes and represent a simple, yet powerful technique to characterize solid surfaces.