Coalescence in polymer blends during shearing

Control of coalescence is a critical factor in preparing polymer blends wit h specific morphologies. Coalescence was studied as a function of shear rat e, volume fraction, viscosity ratio, and amount of block copolymer in a mod el system comprising polystyrene (PS) and high density polyethylene (HDPE). Small drops of HDPE (similar to 2 mu m) were initially developed by sheari ng at 10 s(-1) in a cup-cone shear cell for 20 min. The shear rate (gamma) over dot was then decreased to between 0.1 and 2.5 s(-1), and the state of coalescence was determined after quenching samples following specified peri ods of steady shearing. The PS portions of quenched samples were dissolved in chloroform, and the HDPE particles filtered from the suspension. These p articles were imaged by scanning electron microscopy, which showed a sharp increase in the volume-average particle diameter at specific times that dep end on shear rate volume fraction, and viscosity ratio. Little change was o bserved in the number average diameter (D-n) at low shear rates up to a tot al strain of 360. At higher shear rates, however, D-n significantly increas ed. Interestingly, adding small amounts of block copolymer to the blends si gnificantly suppressed coalescence. These results are discussed in terms of particle trajectory and deformation during flow.