Ziegler V., Wolf, B. A.

"On the role of drop breakup for coalescence processes and morphology development in polymer blends under shear"

Macromolecules 2005, 38, 5826-5833.

Abstract: Drop sizes and drop-size distributions were determined as a function of time at constant shear rates,, by means of an optical shear cell in combination with an optical light microscope after preshearing the samples at high. The systems under investigation were PIB 3/PDMS 152 (PIB: polyisobutylene, PDMS: poly(dimethylsiloxane), numbers: average molar masses in kg/mol) and COP 26*/PDMS 48 (COP: poly(dimethyl-co-methylphenylsiloxane), asterisk: apparent molar mass); all measurements refer to 25 degrees C. Systems and conditions were chosen such that shear rates in the vicinity of the intersection of the coalescence and the break up curves become experimentally accessible. Under these conditions, it is important to evaluate the different processes not only in terms of average drop radii but also in terms of their distribution. Such an analysis demonstrates that there exists a kind of transition from the typical behavior at low shear rates (far below the intersection, only coalescence, approach of dead-end drop radii) to that at high shear rates (far above the intersection, simultaneous coalescence and break up, approach of stationary drop radii). Within the entire transition regime between these two ordinary types of behavior, some drops are larger and others smaller than the corresponding break up radius, and the fraction of larger drops increases from zero to unity as one raises the shear rate. In some cases, one observes bimodal distributions of drop size where the ratio of the volumes of the two kinds of drops is either four or eight.

preprint number: 268