We investigate the influence of filler particles on the phase stability of
a model blend of polystyrene (PS) and polybutadiene (PB). The upper critica
l solution temperature cloud point curve of a PS:PB blend is "destabilized"
(upward shift of cloud point temperature) by the addition of untreated fum
ed silica filler particles. Preliminary cloud point measurements on surface
-functionalized fumed silica particles were also performed. A small downwar
d shift of the cloud point temperature was observed by functionalizing the
filler particles with grafted polystyrene chains, while grafting with a sil
ane coupling agent resulted in a large apparent "stabilization" effect or a
decrease of the cloud point temperature. (Further measurements on surface-
functionalized filler particles over a range of polymer compositions are re
quired to determine the filler concentration dependence of the shift of the
blend critical temperature.) Surface treatments were selected to demonstra
te that modifying the filler particle surface chemistry can alter the blend
phase boundary. These phase boundary shifts have significance in commercia
l blends containing dispersions of filler particles. (C) 2000 Elsevier Scie
nce Ltd. All rights reserved.