Rheol Acta (2006) 45: 877–889 DOI 10.1007/s00397-005-0076-9 ORIGINAL CONTRIBUTION Khalil El Mabrouk Mosto Bousmina Received: 13 July 2005 Accepted: 7 December 2005 Published online: 16 May 2006 # Springer-Verlag 2006 Assessment of morphology in transient and steady state regimes resulting from phase separation in polystyrene/poly (vinyl methyl ether) blend Abstract Morphology development after phase separation in polystyrene (PS)/poly(vinyl methyl ether) (PVME) blend was assessed both in transient and in steady state regimes. Phase segregation was evaluated by various techniques including optical microscopy, light transmission, dy- namic scanning calorimetry and rheo- logical analyses. The steady state particle size resulted from phase sep- aration was determined experimen- tally and then compared to the pre- dictions of both the emulsion models that assume zero-thickness interfacial boundaries and to the asymptotic value of the Cahn–Hilliard theory that assumes rather a diffuse interphase. Keywords Phase diagram . Phase-separating morphology . Rheological properties Introduction While the classical thermodynamics of phase stability in polymer mixtures is well established, the transient regime and the variation of structure in time remains poorly understood. This is due to the complex coupling between chemical potential and kinetics that involves diffusion, nucleation, growth and variation in size and shape of the dispersed domains. This is the case for instance in binary mixtures in the neighbourhood of their mixing–demixing temperature or in block copolymers in the vicinity of their order–disorder transition temperature. The key analysis of such a transient process is the fluctuation of concentration near the critical point (Mani et al. 1991; Onuki 2002). In a free-stress medium and upon variation in temperature, pressure and/or composition, phase segregation may occur either by nucleation and growth mechanism or by spinodal decomposition (SD). Spinodal decomposition is a sponta- neous process that involves a continuous evolution from homogeneous to dispersed phase morphology passing through a percolating co-continuous type structure (Lauger et al. 1994). A necessary condition for spinodal decompo- sition is that the parent phase must be in the unstable region. Thus, the composition of the system must be within the two spinodal compositions at the temperature of phase separation. Thermodynamics theories of non-homogeneous solu- tions have been developed by Cahn (1965), Debye (1959) and Cahn and Hilliard (1958). Owing to the theory, SD is induced by a spontaneous amplification of concentration fluctuation that brings the homogenous mixture into a phase-separated state. The whole process is generally divided into three distinct stages: (1) the early stage, (2) the intermediate stage and (3) the late stage. Bates and Witzius (1989) reported the existence of an additional transitional stage. The time evolution of the structure during the first stage can be described by the linearized Cahn–Hilliard theory that describes the stability of the system in terms of the amplitude of concentration fluctuation. When the wavelength of the fluctuations becomes higher than a certain critical value, the mixture becomes unstable and the concentration fluctuations grow exponentially at the long wavelength leading to a phase separation, while the small wavelengths relax back to equilibrium. The long wave- length λ m is given by λ m ¼ 2π q m ¼ 2 ffiffiffiffiffi 2π p  @ 2 ΔG  @ϕ 2 2κ   12 = (1) where the second derivative of the free energy density (ΔG) has a negative value in the spinodal region. κ is an K. El Mabrouk . M. Bousmina (*) Canada Research Chair on Polymer Physics and Nanomaterials, Department of Chemical Engineering, CREPEC, Laval University, G1K 7P4, Quebec, Canada e-mail: Bousmina@gch.ulaval.ca