Rheol Acta (2009) 48:527–541 DOI 10.1007/s00397-009-0349-9 ORIGINAL CONTRIBUTION Stress relaxation behavior of PMMA/PS polymer blends Márcio Yee · Adriana M. C. Souza · Ticiane S. Valera · Nicole R. Demarquette Received: 28 August 2008 / Accepted: 3 February 2009 / Published online: 18 March 2009 © Springer-Verlag 2009 Abstract In this work, the stress relaxation behavior of PMMA/PS blends, with or without random copolymer addition, submitted to step shear strain experiments in the linear and nonlinear regime was studied. The effect of blend composition (ranging from 10 to 30 wt.% of dispersed phase), viscosity ratio (ranging from 0.1 to 7.5), and random copolymer addition (for concen- trations up to 8 wt.% with respect to the dispersed phase) was evaluated and correlated to the evolution of the morphology of the blends. All blends presented three relaxation stages: a first fast relaxation which was attributed to the relaxation of the pure phases, a sec- ond one which was characterized by the presence of a plateau, and a third fast one. The relaxation was shown to be faster for less extended and smaller droplets and to be influenced by coalescence for blends with a dispersed phase concentration larger than 20 wt.%. The relaxation of the blend was strongly influenced by the matrix viscosity. The addition of random copolymer resulted in a slower relaxation of the droplets. Keywords Rheological behavior · Step shear · PMMA/PS blends · Random copolymer · Compatibilization · Morphology evolution M. Yee · A. M. C. Souza · T. S. Valera · N. R. Demarquette (B ) Metallurgical and Materials Engineering Department, Escola Politécnica—University of São Paulo, Av. Prof. Mello Moraes, 2463, CEP 05508-900, São Paulo, SP, Brazil e-mail: nick@usp.br Introduction Due to their useful properties, polymer blends have many applications in different industries such as auto- motive, packaging, and aerospace. Their properties de- pend strongly on their morphology that can be tailored and controlled during processing. During processing, the blend suffers high shear and elongational flows and its dispersed phase is broken into small droplets or sheets (Migler et al. 1999). Rheology can be an inter- esting tool to quantify the evolution of morphology of blends during flow. In particular, stress relaxation experiments (Yamane et al. 1998; Okamoto et al. 1999; Iza and Bousmina 2000; Hayashi et al. 2001a, b; Van Puyvelde et al. 2002; Jansseune et al. 2003; Almusallam et al. 2003, Ansari et al. 2006; Macaúbas et al. 2007; Mechbal and Bousmina 2007) can bring a substantial amount of information regarding the evolution of mor- phology after flow. If conducted in the linear viscoelas- tic regime, these experiments can be used to infer the interfacial tension between the components of the blends or characterize the morphology of the blends similarly to small-amplitude oscillatory shear exper- iments (Palierne 1990; Gramespacher and Meissner 1992). If conducted in the nonlinear viscoelastic regime, these experiments can be a nice tool to understand the relaxation of deformed droplets (Martin and Velankar 2007; Wang and Velankar 2006a, b). Therefore, several studies have already been con- ducted in order to understand the evolution of mor- phology during step strain experiments. One of the pioneer works, conducted by the group of Takahashi at Kyoto University, consisted of the observation of the evolution of a single droplet when submitted to step shear of different amplitudes (Yamane et al. 1998). The