(217dr) Effects of Extensional Flow and Nanoparticle Stabilization On the Morphology of PS/LLDPE Blends

Thompson, M. S., West Virginia University
Agarwal, S., West Virginia University
Gupta, R. K., West Virginia University

Most commercially-available polymer blends are immiscible, and there are several methods available for blending the constituents.  Blend performance depends on the properties of the ingredients, their relative amounts, and the blend morphology.  Blend morphology, in turn, depends on the methods of compatibilization and compounding.  The goal is to manufacture materials with a broad range of properties that are suitable for many different applications.  The key to effectively and efficiently developing blended products is to identify and use processing-structure-property relationships.  In this work, polystyrene (PS) and linear low density polyethylene (LLDPE) were employed as candidate blend components.  Indeed, they represent a model immiscible system with high interfacial tension and large dispersed phase domains when mixed using a predominantly shear flow field. 

            Grades of PS and LLDPE were selected such that PS viscosity was at least four times greater than that of LLDPE.  PS was loaded as the dispersed phase, and its concentration was kept less than 15 wt%.  For such blends, where the ratio of dispersed-phase viscosity to continuous-phase viscosity, p, is high, an extensional flow field is required to promote drop deformation and breakup of the dispersed phase.  Here, coarse blends were prepared in an internal mixer and then subjected to extensional flow by forcing them through various converging flow dies attached to the bottom of a capillary rheometer.  Extensional flow mixing resulted in a decrease in the number-average diameter of the droplets, but large drops were still present, possibly due to flow-induced coalescence.  To inhibit coalescence and to stabilize the small droplets produced by extensional flow, fumed nanosilica having a variety of surface treatments was incorporated into the blend.  This was found to decrease the initial coarse blend dispersed phase size and also to diminish the number of large drops present after extensional flow mixing.