MELT RHEOLOGY AND MORPHOLOGY OF PHYSICALLY COMPATIBILIZED NATURAL RUBBER-POLYSTYRENE BLENDS BY THE ADDITION OF NATURAL RUBBER-G-POLYSTYRENE

Blends of natural rubber (NR) and polystyrene (PS) were prepared by me lt mixing in a Brabender plasticorder and by solution casting using ch loroform as the casting solvent. Earlier studies have indicated that t hese blends are incompatible and immiscible, and their compatibility c an be improved by the addition of a graft copolymer of NR and PS (NR-g -PS). The rheological behavior of these blends has been carried out in the presence and absence of the compatibilizer using a capillary rheo meter and a melt flow indexer. The effects of blend ratio, processing techniques (melt mixing versus solution casting), shear stress, and te mperature on the rheological behavior have been studied in detail. Bot h in the presence and absence of the copolymer, the blends showed a de crease in viscosity with an increase of shear stress, indicating pseud oplastic nature. Solution-cast blends showed a higher viscosity as com pared to melt-mixed blends. The viscosity versus composition curve of both melt-mixed and solution-cast blends showed negative deviation fro m the additivity at a higher shear rate region. This is associated wit h the interlayer slip between the highly incompatible NR and PS phases . The effects of graft copolymer loading and temperature on solution-c ast blends were studied, and it was found that as the copolymer loadin g increases, the shear viscosity increases. This is due to the high in terfacial interaction between the two components in the presence of th e copolymer. The copolymer, in fact, locates at the interface and make s the interface more broad. However, at higher loading of the copolyme r, the viscosity of the blends decreases. This may be associated with the formation of micelles, which have a plasticizing action on the vis cosity of the blends. Melt elasticity parameters like principal normal stress difference, recoverable elastic shear strain, and die swell we re evaluated. Master curves have been generated using modified viscosi ty and shear rate functions that contain the melt flow index as a para meter. The extrudate morphology of the blends was studied using a scan ning electron microscope. Addition of the copolymer reduces the domain size of the dispersed phase, followed by a leveling off at a higher c oncentration. The leveling off is an indication of interfacial saturat ion. The interparticle distance also decreased followed by a leveling off at a higher loading of the copolymer. (C) 1998 John Wiley & Sons, Inc.