Wetting of Macromolecules: From Linear Chain to Soft Colloid-Like Behavior

The ability to control the wetting properties of a polymeric liquid on a given surface is important for several emerging technological applications including protective coatings, lubricants and sensors. Here we show that star-shaped polystyrene (PS) molecules exhibit notably different wetting properties than their linear analogues of the same chemical structure and their wetting properties can be controlled through changes of their functionality f (number of arms per molecule). Unlike linear chains, the wetting of star-shaped macromolecules is determined by the competition between entropic forces. Wetting is enhanced due to reductions in the loss of entropy upon adsorption of the stars with increasing f; soft colloidal-like entropic repulsion effects suppress the ability of the stars to exhibit a high degree of surface adsorption and to efficiently pack, for sufficiently large values of f. This phenomenon is manifested in the existence of a minimum in the macroscopic contact angle and other related microscopic parameters embodied in the effective interface potential as a function of f.