Assessment of the Dispersion Quality in Polymer Nanocomposites by Rheological Methods

The properties of filled polymers and nanocomposites are strongly linked to the adequate dispersion of the solid phase into a polymeric matrix. However, obtaining the degree of dispersion within a polymer composite system is far from trivial. Typical methods for microstructural analysis such as electron and optical microscopy or scattering methods only investigate the local microstructure. In addition they are either labor intensive or may yield data that are difficult to analyze. Methods that focus more strongly on the end-use properties, such as conductivity or bulk moduli, offer a global view of the material performance, but are of course post factum. The rheological properties of (nano) particle filled matrices in the molten state offer a cost-effective alternative to evaluate the dispersion quality and can even be used during a production process. Moreover, it does not necessitate optical transparency, adequate scattering contrast, or conductivity. Typically, rheological data have been mainly used in qualitative terms and it is as yet unclear if the quality of dispersion can be quantified. The present work contains a systematic evaluation of methods to characterize the dispersion quality from rheology. Using a well-defined model sample it is shown how the dispersion quality can be assessed quantitatively by studying the viscoelastic properties as a function of volume fraction. Moreover, a novel method is proposed, where the time evolution of the linear and nonlinear rheological properties at a fixed volume fraction is studied and analyzed.