Rheology of compatibilized immiscible blends with droplet-matrix and cocontinuous morphologies during coarsening

It is well known that addition of block copolymers to immiscible polymer blends may result in finer and more stable microstructures by reducing the driving force for coarsening, namely, the interfacial tension. This compatibilization is also reflected in the rheological behavior of the blends. We present an experimental study on the morphology and rheology of model blends composed of fluorescently labeled polystyrene and styrene-ran-acrylonitrile copolymer compatibilized with a polystyrene-b-polymethyl methacrylate diblock copolymer (BC). Three different weight ratios (wt/ wt) are studied: 20/80, 35/65, and 50/50, with the following morphologies: Droplet/matrix, metastable cocontinuous, and abiding cocontinuous, respectively. It is found that regardless the type of initial morphology, the addition of BC reduces the characteristic size and increases the elastic modulus in the terminal regime, where the relaxation of the interface is probed. The relaxation spectra in the terminal zone are greatly affected by the presence of BC, which reflects the strong dependence of the viscoelastic behavior on the local microstructure (shape and area) of the interface. Furthermore, the addition of BC reduces the rate of coarsening, which is characterized by the time evolution of both the elastic modulus and the specific interfacial area. These effects are more pronounced when the molecular weight or concentration of the BC is higher. Finally, two regimes of coarsening in the symmetric (50/50) blends were identified and characterized by two different power laws.