Effects of the degree of dispersion and the morphology of zinc powder on the thermodynamics of its interaction with polystyrene in solution and in composite films

The adsorption of block PS on the surface of spherical, round, and flaky particles of highly dispersed Zn powders with dimensions of 3.5–23 μm and the interaction of the polymer with the surface of Zn powders in the composite films are studied by refractometry and isothermal calorimetry. During establishment of adsorption equilibrium, the concentration of PS in the bulk of solution increases, while, near the surface of Zn particles, the concentration of PS decreases, thus indicating the predominance of absorption from solution of the solvent o-xylene. With the use of the thermodynamic cycle, including processes of solution of the polymer and Zn-PS film composite and wetting of the Zn powder, the enthalpies of formation of PS-Zn composites are estimated. The enthalpies of mixing of filled compositions comprise two contributions of opposite signs: In the low-filling region, the negative contribution of adhesion interactions prevails, while in the region of high-filled compositions, a positive structural contribution, which is associated with additional loosening of the glassy packing of PS near the filler surface, plays the decisive role. The contribution of adhesion interaction is largely determined by the dimensions of filler particles, while the structural contribution is controlled by their shape. The experimental evidence suggests that spherical particles possess the highest thermodynamic compatibility with the polymer matrix of the composite material.