Negative Hamaker coefficients I. Particle engulfment or rejection at solidification fronts

Abstract

Calculated Hamaker coefficients were obtained from free energies of adhesion, derived from contact angle data, for several polymers, as a function of the separation. Comparing these values with published Hamaker coefficients, an equilibrium separationd0=1.82 A for various systems was established. It was demonstrated that the Hamaker coefficients A12 calculated from the free energy of adhesion agreed very well with those calculated from the free energy of cohesion, using Berthelot's rule. Visser's procedure of modifying the combining rule to calculateA132 fromAij values was tested and confirmed. Hamaker coefficientsA132 for the interaction of various polymer particles, embedded in naphthalene melt, with the naphthalene solidification front turned out positive in some cases and negative in others, thus predicting attraction of some particles by the solidification front and repulsion of others. These predictions were compared with microscopic observations of the behaviour of particles at the solidification front by which they become either engulfed (attraction) or rejected, i. e. pushed along (repulsion). There was good agreement between prediction and experimental observation.