(180bj) 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 Ca_crit 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.