Rheology

Linear rheology measures the frequency dependency of complex elasticity modules G(w) and provides the necessary connection between the mechanical properties and the molecular structure, as defined by scattering experiments. The dynamic modules scan the relaxation time scale in polymer melts in a sensitive way. Properties such as individual relaxation processes or plateau modules can be successfully discussed within a tube model for topological interplay between chains. This appears as a more or less distinctive maxima in G”-loss modulus at frequency proportional to the inverse relaxation time.

Dynamic studies complete the typical relaxation experiments in the macroscopic time window. They allow us to distinguish non-networked polymer melts from rubber and demonstrate, for example, within the framework of elastic materials, an analogy with temporary coupling, i.e. the topology and permanent network points of a vulcanized ensemble of chains. If one assumes that using changes in temperature, the characteristic times could be to the same extent speeded up or slowed down, then it is possible by means of the time-temperature overlay principle to establish a so-called master curve. Behaviour predictions can thus be made in areas where experimental work is either not possible, or very difficult to perform.

This simple-thermo-rheological behaviour is present in most homogenous systems. Deviations from this are observed in more complex systems such as mixtures with a different monomer chemistry or additional processes which can be activated such as all types of intermolecular interactions (complex formations, hydrogen bridges and so on).

Our understanding of polymers on the coupling scale is supplemented by non-linear rheology experiments. A major disruption of the system leads to a microscopic length scale-dependent deformation and rheological properties which are favourable to polymer processing. Due to the fact that this area has still not been investigated microscopically and dominated by over-simplistic theories, techniques such as rheology and coupling with neutron experiments are very important. The available scattering vector ranges as well as the time window open up interesting and relevant length scale and relaxation times.

Our rheometric equipment includes instruments capable of performing various measurement methods and modes:

• Deflection-controlled dynamic-mechanical rheometer (ARES, ARES-G2)(TA Instruments)
• Q800: Deflection-controlled dynamic-mechanical analysis (TA Instruments)
• AR-G2 Voltage-controlled rheometer for samples with low viscosity (liquids) with an added device for in-situ small angle light scattering for characterizing structures