Vincent Janssens

Suspension-like hardening behavior of HDPE and time-hardening superposition

The rheology of solidifying high-density polyethylene (HDPE) is investigated. Experiments on an HDPE were performed with a novel RheoDSC device. Results agree quantitatively with simulations for a suspension of elastic spheres in a viscoelastic matrix except for very low values of space filling (<5%), indicating that the rheological behavior of the crystallizing melt in the frequency range investigated is purely suspension like. The hardening behavior of the material is characterized in two different ways; a normalized rheological function and a time-hardening superposition (THS) master curve of rheological properties. An improvement is proposed to the procedure for performing THS that was previously used in the literature. Based on this procedure, a novel method for predicting the rheological properties of crystallizing melts is presented.

RheoDSC Analysis of Hardening of Semi-Crystalline Polymers during Quiescent Isothermal Crystallization

Abstract The crystallization of semi-crystalline polymers is often analyzed by rheometry and calorimetry. By rheometry the viscosity evolution during crystallization can be followed, whereas from a calorimetric measurement, the evolution of the degree of crystallinity can be calculated. The time evolution of these material properties is valuable input for polymer processing simulation software and in order to combine the data in a reliable manner, hardening curves are used as a characterization tool. Such a hardening curve correlates the relative increase of the viscosity resulting from crystallization, to the advancing degree of crystallinity. In this study, these are extracted from simultaneous measurements on one sample using a RheoDSC device. The RheoDSC technique allows for the direct combination of the rheological and calorimetric signal without the need of combining separate stand-alone measurement results. In this study, isothermal crystallization experiments are used to discuss the benefits of this approach. This will lead to the recommendation that measuring the hardening effect in steady shear measurements at very low shear rates in a direct combined RheoDSC setup is the most reliable method to compile unambiguously a material specific hardening curve for semi-crystalline polymers.

RheoDSC: A hyphenated technique for the simultaneous measurement of calorimetric and rheological evolutions

A newly developed hyphenated technique is presented combining an existing rheometer and differential scanning calorimeter into a single experimental setup. Through the development of a fixation accessory for differential scanning calorimeter (DSC) crucibles and a novel rotor, the simultaneous measurement is performed inside the well-controlled thermal environment of a Tzero DSC cell. Hence, the evolution of thermal and flow properties of a material can be simultaneously measured using steady or oscillatory shear measurements and regular or modulated temperature DSC measurements. Along with the construction of a prototype, a validation of the design was performed. The technique offers interesting opportunities for the investigation of flow-induced transitions, for instance, crystallization or phase separation, and provides an asset for high-throughput screening of materials. The potential of the novel technique is demonstrated by two case studies: the chemorheology during the cure of a thermosetting epoxy-amine system and the flow-induced crystallization of syndiotactic polypropylene.