Effects of Polymer Branching Structure on the Hydrodynamic Adsorbed Layer Thickness Formed on Colloidal Particles

Branched flocculants are more efficient at improving flocculation rates than linear flocculants, but the reason is not well understood. The adsorption of linear polymers is known to increase the hydrodynamic radius of particles for effective collisions between particles and subsequently improve flocculation rate. To reveal such an effect with branched polymers, we measured the thickness of the adsorbed polyelectrolyte layer and their electrophoretic mobility as a function of time for polyelectrolytes with different degrees of branching. The results show that all polyelectrolytes adsorbed on the particles undergo a relaxation effect, where the thickness of the adsorbed layer continuously decreases as a function of time. The branched polymers exhibited a thick layer regardless of ionic strength due to branching points that disturbed smooth reconformation when adsorbed to the particles. However, linear polymers consistently showed a thinner layer thickness than branched polymers because of their flexible structure, which was prone to relaxation effects. The most significant layer thickness was observed at low ionic strength, and the electrophoretic mobility confirmed that adsorption was kinetically controlled. The thick adsorbed layer of branched polymers explains the commonly observed trend, where branched polymers show better flocculating abilities than linear polymers.