281756 Morphology Development in PC/SAN Blends: Roles of Extensional Flow and Nanoparticle Stabilization
Immiscible blends of polycarbonate (PC) and styrene-co-acrylonitrile (SAN) are commercially important engineering polymers, finding applications in automobiles, home appliances and electronic products. The mechanical properties of such blends are known to depend on the volume fraction, size, and size distribution of the dispersed phase. As a consequence, it is of interest to be able to vary the average size of the dispersed phase. Polymer blends are typically formulated using extruders or internal mixers, and the observed drop size is the result of the competition between the fluid stress that tends to deform a drop and the interfacial stress that opposes the deformation; the ratio of these two stresses is known as the capillary number, Ca. In general, large drops are more easily deformed than small ones, and droplet breakup occurs when the Ca exceeds a critical value. For Newtonian liquid pairs, the critical value of Ca depends on p, the ratio of the dispersed phase viscosity to the suspending liquid viscosity. In shear flow, drop breakup is relatively easy when the value of p ranges between 0.1 and 1. Indeed, a given drop breaks up into two daughter droplets which each undergo further break up until Cacrit is reached. A consequence of this process is a progressive reduction in the average dispersed phase size. As opposed to this, when p exceeds about 3.8, drop breakup is not possible in devices employing shear flow.
The goal of the current work was to examine ways by which one may formulate fine polymer blends, especially when the viscosity ratio exceeds four. Candidate polymers used were PC and SAN, with the former polymer being dispersed in a matrix of the latter polymer. 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 was found to decrease the dispersed phase drop size by as much as a factor of 2, and the effects of process variables such as temperature, composition, stretch rate and total strain were investigated. To further reduce the dispersed phase size, hydrophobically-treated fumed nanosilica was incorporated into the blend, and the effectiveness of this additive to prevent coalescence was examined. Electron microscopy was utilized to determine if the nanoparticles resided at the boundary between the two phases or if they preferred to remain in one of the two polymer phases.
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