Calorimetric and rheological characterization of polymers – Advances in RheoDSC

In polymers, intimate structure-processing-property relationships exist due to the specific macromolecular architecture. Studying the relationships between the physicochemical and rheological changes during phase transitions, such as crystallisation of semi-crystalline polymers and polymerisation reactions in thermosetting resins, is of both scientific and industrial relevance. A proof-of-principle RheoDSC concept for performing a rheological experiment inside a DSC cell was further developed into a fully functional instrument. In the new RheoDSC concept a commercial rheometer was combined with a commercial DSC, without making irreversible changes to either instrument. With the aid of heat transfer modelling, new insights and design features were gained. The main improvement was achieved by reducing the thermal mass of the rotor close to the sample, which reduced the thermal transient behaviour by achieving more quickly a steady state thermal gradient over the rotor, and resulted in a flatter baseline, overall offering a markedly better calorimetric performance. New calibration concepts as well as new measuring procedures ultimately gave rise to a heat flow rate signal approaching the quality of a standalone DSC, implying that the state-of-the-art RheoDSC enables one to measure quantitatively, reproducibly, and in a combined fashion, calorimetric and rheological information on a single sample. Due to the strides made in calorimetric performance, the RheoDSC proved to be an increasingly interesting tool for investigating the hardening curves of crystallising polymers, where the solidification observed in the rheological signals can be confronted with the extent of crystallinity, and for studying the chemorheology of reactive polymer systems. In the course of this work, the rheology of nanocomposites obtained by melt-mixing nanoclays or multi-walled nanotubes in poly(-caprolactone) was investigated in more detail. A phenomenological model from literature was further explored and extended through the use of time-temperature superposition to improve the reliability of the zero shear modulus parameter. The latter represents the stiffness of the nanofiller network, which proved to be correlated with the dispersion quality for a wide range of nanofillers and processing conditions. The extent of the influence of the nanofiller on the matrix behaviour indicates a clear correlation between the strength of the interactions between polymer and nanofiller and the dispersion quality. In future work, RheoDSC can be used to extend these rheological studies to the effect of nanofillers on the crystallo- and chemorheology in polymer nanocomposites. In general, the direct coupling of the rheological and calorimetric experiments on the same sample undergoing a given thermal treatment provides for direct viscosity-conversion relations and time-temperature-transformation and temperature-conversion-transformation diagrams, both relevant and desirable for industries.