Fourier Transform Rheology: A New Tool to Characterize Material Properties

Liquid multiphase systems such as polymer blends or emulsions are ubiquitous in many applications, including plastic production, food processing, pharmaceutical and cosmetic production. When the constituents of the multiphase system are incompatible the phases are immiscible, and, depending on their relative amount, the microstructure can consist of droplets in a matrix, elongated fibrils or a co-continuous structure (Utracki, 2003) as schematically shown in Figure 1. The morphology of the liquid multiphase system is important in the applications as it strongly affects processing properties, and the properties of the final products. With the term “morphology” we here indicate not only the overall form or shape of the physical structure of the system, but also the distribution and orientation of the phases, the interfacial area, and the volume of the interphase. Hence, a profound knowledge of the relation between processing parameters, material properties and morphology is essential to optimize the performances of the liquid multiphase systems.Substantial efforts were done in the last decades to set up experimental protocols aimed at evaluating the morphological properties of polymer blends and emulsions via rheological measurements. So far, the most reliable strategy for morphological characterization through rheological measurements is based on the dynamic small amplitude oscillatory shear (SAOS) experiment: the samples are subjected to small amplitude shearing oscillations, and the measured shear stress response is used to gain information on the blend properties (e.g. Palierne, 1990). Here, we present an alternative technique we have recently proposed to characterize the liquid two-phase system morphology. This methodology is based on Large Amplitude Oscillatory Shear (LAOS) flows. This kind of analysis is often referred in the literature as Fourier Transform Rheology (FTR) (Wilhelm et al., 1998), since the stress response is usually analyzed in the Fourier domain. It will be shown that Fourier Transform Rheology possesses a high sensitivity in the characterization of the morphology, thus allowing evaluation of properties that might otherwise be hardly appreciated with traditional linear methodologies.