Steady-shear rheological properties of model compatibilized blends

Synopsis Block copolymers may be added as surface-active compatibilizers in order to control the morphology of blends of immiscible polymers. The effects of such added compatibilizers on the rheological properties of droplet-matrix blends are investigated experimentally. Model blends composed of polyisobutylene ~PIB! droplets in a polydimethylsiloxane ~PDMS! matrix, compatibilized with a diblock copolymer of PIB and PDMS, are studied here. The viscosity ratio of the blends, i.e., the ratio of the viscosity of the droplets to that of the matrix, is varied from 0.1 to 2.7. The viscosity and the first normal stress difference under steady shear conditions, and complex moduli after cessation of shear are measured. It is found that addition of the compatibilizer slightly raises the magnitude of the terminal complex viscosity of blends at all ratios of viscosity. Furthermore, with addition of the compatibilizer, the terminal relaxation time is found to increase sharply at high viscosity ratios, whereas the steady shear N1 is found to increase at low viscosity ratios. These experimental observations are consistent with Marangoni stress caused by flow-induced gradients in the compatibilizer concentration on the surface of compatibilized drops. It is shown that, qualitatively, the effects of the Marangoni stress are somewhat analogous to an increase in drop viscosity. © 2004 The Society of Rheology. @DOI: 10.1122/1.1765662# I. INTRODUCTION Blending of immiscible polymers is often the most economical means of achieving desired product properties. The processing history strongly affects the morphology and properties of a blend, hence, the relationship between flow and the structure of immis- cible blends is a topic of intense current interest. Past research has elucidated the key role played by interfacial tension, and the volume fraction and relative viscosity of the com- ponents, and has yielded insight into the various phenomena that determine the flow- induced structural evolution of immiscible blends. Key features of the dynamics of im- miscible blends as reviewed by Tucker and Moldenaers ~2002! are as follows: For droplet-matrix morphologies, structural evolution during shear flow occurs primarily by the deformation, breakup, and coalescence of droplets. Droplet deformation is governed by two dimensionless numbers: the viscosity ratio, p 5 h d /h m , and the capillary num- ber,