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

Synopsis A block copolymer may be added as a compatibilizer during polymer processing in order to promote intimate mixing of thermodynamically immiscible homopolymers. The action of this compatibilizer can only partially be attributed to its effect on the interfacial tension between the immiscible homopolymers. Here the additional contributions of the compatibilizer are directly probed by measuring the capillary number during coalescence experiments. Model blends consisting of polyisobutylene ~PIB! and polydimethylsiloxane ~PDMS!, compatibilized with various amounts of a PIB-PDMS diblock copolymer, are used for this purpose. The mean capillary number of the droplets is determined 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 droplet breakup of uncompatibilized systems. This indicates that a simple decrease in interfacial tension is not the only effect of adding the compatibilizer 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. Direct evidence of the presence of such interfacial tension gradients along the surface of compatibilized drops was obtained by optical microscopy. © 2001 The Society of Rheology. @DOI: 10.1122/1.1380424# I. INTRODUCTION Mechanical blending of immiscible polymers is a common means of achieving the desired material properties in polymer processing. The immiscible components are usu- ally blended in the melt state and then cooled, thereby freezing the nonequilibrium, multiphase microstructure. The properties of the blend are strongly influenced by the size of heterogeneity, making the relationships between processing flow conditions and the resulting microstructure a topic of intense research. Restricting ourselves to two-phase droplet-matrix morphologies, the droplet size R is governed primarily by a comparison between breakup and coalescence of droplets. For the case of uncompatibilized droplets, breakup occurs if the hydrodynamic stress deforming the droplet is significantly larger than the restoring stress a/R due to interfa- cial tension a. For a Newtonian droplet suspended in a Newtonian matrix of viscosity h, and being sheared at shear rate of g ˙, the hydrodynamic stress is characterized by gh and, hence, breakup occurs when @Grace ~1982!; Rallison ~1984!#