Coalescence in polymer blends during shearing

Control of coalescence is a critical factor in preparing polymer blends with specific morphologies. Coalescence was studied as a function of shear rate, volume fraction, viscosity ratio, and amount of block copolymer in a model system comprising polystyrene (PS) and high density polyethylene (HDPE). Small drops of HDPE (approximately 2 μm) were initially developed by shearing at 10 s^-1 in a cup-cone shear cell for 20 min. The shear rate γ was then decreased to between 0.1 and 2.5 s^-1, and the state of coalescence was determined after quenching samples following specified periods of steady shearing. The PS portions of quenched samples were dissolved in chloroform, and the HDPE particles filtered from the suspension. These particles were imaged by scanning electron microscopy, which showed a sharp increase in the volume-average particle diameter at specific times that depend on shear rate volume fraction, and viscosity ratio. Little change was observed in the number average diameter (D_n) at low shear rates up to a total strain of 360. At higher shear rates, however, D_n significantly increased. Interestingly, adding small amounts of block copolymer to the blends significantly suppressed coalescence. These results are discussed in terms of particle trajectory and deformation during flow.