The Canadian Journal of Chemical Engineering, Volume 80, December 2002 1057 T he interrelations between thermodynamics and rheology of partially miscible blends are an issue of increasing interest in the scientific and industrial communities. Considerable effort has been directed toward investigating the properties of blends exhibiting Upper Critical Solution Temperature (UCST) and Lower Critical Solution Temperature (LCST) (Utracki, 1990). The study of the effect of process- ing conditions, such as shearing, on the phase behaviour of these blends (Fernandez et al., 1995; Mani et al., 1992; Chopra et al., 1998) is of great interest as well. Although interrelations between rheology and morphology are well established for immiscible blends (Choi and Schowalter, 1975; Palierne, 1990; Graebling and Muller, 1993; Minale et al., 1997; Lacroix et al., 1998; Vinckier et al., 1996; Grizzuti et al., 2000), systematic investiga- tions involving partially miscible blends are relatively scarce (Vinckier and Laun, 1999). In recent years a growing number of studies have focused on assess- ing the effect of phase separation on the rheology of partially miscible blends, with particular focus on the regime lying in the vicinity of phase separation. Lack of time–temperature superposition has been frequently cited as manifestation of phase-separation leading to the determination of the (binodal) phase separation temperature (Mani et al., 1992; Kapnistos et al., 1996a; Vlassopoulos, 1996; Chopra et al., 1998, 2002). Concentration fluctuations in the pretransitional regime affect the linear, as well as the nonlinear, viscoelastic properties of the blends, and have been quantified, yielding the determination of the spinodal temperature (Vlassopoulos, 1996; Kapnistos et al., 1996a, b). In this work we investigate the rheological properties of a model UCST polymer blend across its phase boundary and relate them to its phase behaviour. To achieve our goal we employ shear rheometry, comple- Interrelations between Rheology and Phase Behaviour in Partially Miscible Blends: The Case of Polydimethylsiloxane/Polyethylmethylsiloxane (PDMS/PEMS) Divya Chopra 1 , Marianna Kontopoulou 1* , Dimitris Vlassopoulos 2 and Savvas G. Hatzikiriakos 3 1 Department of Chemical Engineering, Queen’s University, Kingston, ON K7L 3N6, Canada 2 Foundation for Research and Technology-Hellas (FO.R.T.H), Institute of Electronic Structure & Laser, 71110 Heraklion, Crete, Greece 3 Department of Chemical & Biological Engineering, The University of British Columbia, Vancouver, BC V6T 1Z4, Canada A rheological study of the phase behaviour and the dynamics in the immiscible region of a model partially miscible Upper Critical Solution Temperature (UCST) blend was carried out by using a series of nearly monodisperse polydimethylsiloxane (PDMS) of varying molecular weights blended with polyethyl- methylsiloxane (PEMS). The complete phase diagram was established through steady shear rheology, optical microscopy and light scattering. The UCST, interfacial tension and morphology of these blends are very sensitive to the molecular weight of the constituents. Partial miscibility was observed only when the M w of PDMS was sufficiently low. The viscoelastic behaviour in the phase separated region can be modeled success- fully using the Palierne model. Une étude rhéologique du comportement et de la dynamique de phases dans la région immiscible d’un mélange modèle à température de solution critique supérieure (UCST) partiellement miscible, a été réalisée au moyen d’une série de polydiméthylsiloxane (PMDS) quasi monodispersé de différents poids moléculaires mélangé à du polyéthylméthylsiloxane (PEMS). Le diagramme de phases complet a été établi par rhéologie en cisaillement permanent, microscopie optique et diffusion de la lumière. La UCST, la tension interfaciale et la morphologie de ces mélanges sont très sensibles au poids moléculaire de ces constituants. Une miscibilité partielle est observée uniquement lorsque le M w du PMDS est suffisamment bas. Le comportement viscoélastique dans la région où les phases sont séparées peut être modélisé avec succès à l’aide du modèle de Palierne. Keywords: partially miscible blends, UCST, rheology, phase diagram. *Author to whom correspondence may be addressed. E-mail address: kontop@ chee.queensu.ca