Effect of compatibilization on the breakup of polymeric drops in shear flow

A block copolymer may be added as a compatibilizer during polymer processin g in order to promote intimate mixing of thermodynamically immiscible homop olymers. The action of this compatibilizer can only partially be attributed to its effect on the interfacial tension between the immiscible homopolyme rs. Here the additional contributions of the compatibilizer are directly pr obed by measuring the capillary number during coalescence experiments. Mode l blends consisting of polyisobutylene (PIB) and polydimethylsiloxane (PDMS ), compatibilized with various amounts of a PLB-PDMS diblock copolymer, are used for this purpose. The mean capillary number of the droplets is determ ined from the mechanical frequency response of the blends. With increasing amounts of compatibilizer, a systematic increase in steady shear capillary number is seen, to values well above the critical capillary number for drop let breakup of uncompatibilized systems. This indicates that a simple decre ase in interfacial tension is not the only effect of adding the compatibili zer to these immiscible blends. Past simulations suggest that these results are associated with gradients in interfacial tension (Marangoni stresses) induced by the gradients of compatibilizer concentration due to flow. Direc t evidence of the presence of such interfacial tension gradients along the surface of compatibilized drops was obtained by optical microscopy. (C) 200 1 The Society of Rheology.