Rheologica Acta最新文献

The focus of the present paper is the rheological study of poly(D,L-lactic-acid) (PDLLA) towards a modeling of their healing properties during 3D direct pellet printing extrusion (DPPE). The viscoelastic properties of PDLLA and the filament temperature during deposition are first characterized. The influence of DPPE processing conditions is investigated in terms of temperature, time, and printing speed. For this, we propose a modeling of the process-induced interphase thickness between two deposited layers considering the non-isothermal polymer relaxation and accounting for the contribution of entanglement rate through the Convective constraint release model. Hence, taking into account the induced chain orientation and mobility coming from filament deposition, this model quantifies the degree of healing between 3D-printed layers. Eventually, the proposed model is validated by comparing the theoretically calculated degree of healing with experimental tensile properties and lap shear results.

Graphical Abstract

The flow curves of polymers often reveal the onset of melt instabilities such as sharkskin, stick–slip, or gross melt fracture, in order of increasing shear rates. The focus of this work lies in the application of the Göttfert sharkskin option to the investigation of flow curves of styrene-butadiene rubber (SBR) compounds. The sharkskin option consists of highly sensitive pressure transducers located inside a slit die of a capillary rheometer. This tool allows the detection of in-situ pressure fluctuation characteristics of different melt instabilities. It is shown that the change of slope of the transition region in the flow curves is only linked to slip. Dynamic Mechanical Analysis (DMA) measurements furthermore show that the shear rate at which the change of slope can be observed shows the same temperature dependency as the viscous and elastic properties of the compounds.

In fast elongational flows, linear polymer melts exhibit a monotonic decrease of the viscosity with increasing strain rate, even beyond the contraction rate of the polymer defined by the Rouse time. We consider two possible explanations of this phenomenon: (a) the reduction of monomeric friction and (b) the reduction of the tube diameter with increasing deformation leading to an Enhanced Relaxation of Stretch (ERS) on smaller length scales. (Masubuchi et al. (2022) reported Primitive Chain Network (PCN) simulations using an empirical friction reduction model depending on segmental orientation and could reproduce the elongational viscosity data of three poly(propylene carbonate) melts and a polystyrene melt. Here, we show that the mesoscopic tube-based ESR model (Wagner and Narimissa 2021) provides quantitative agreement with the same data set based exclusively on the linear-viscoelastic characterization and the Rouse time. From the ERS model, a parameter-free universal relation of monomeric friction reduction as a function of segmental stretch can be derived. PCN simulations using this friction reduction relation are shown to reproduce quantitatively the experimental data even without any fitting parameter. The comparison with results of the earlier PCN simulation results with friction depending on segmental orientation demonstrates that the two friction relations examined work equally well which implies that the physical mechanisms of friction reduction are still open for discussion.

In-extruder measurements of shear viscosity and normal stresses are important as these measurement techniques allow determining the rheological state of the polymer melt at processing conditions up to high shear rates. However, validation of viscosity and normal stress data obtained by in-line slit rheometers at high shear rates is difficult due to a lack of overlap of the in-line data and the off-line measurements by rotational rheometers limited to lower shear rates. Here, shear viscosity and normal stress data measured in-line at large shear rates during extrusion and off-line at low shear rates are compared to predictions of the Doi-Edwards model and the Hierarchical Multi-Mode Molecular Stress Function (HMMSF) model using linear-viscoelastic off-line small amplitude oscillating shear data of two polystyrenes and a low-density polyethylene as input parameters. For polystyrene, the results of this investigation do not only validate the experimental data obtained by rotational as well as slit-die rheometry, but also demonstrate the agreement between experiments and models up to very high shear rates, which were not experimentally accessible earlier. The low-density polyethylene shows a more complex behaviour, which follows the HMMSF model at low shear rates, but approaches the Doi-Edwards model at high shear rates.

Platinum group metals (PGM) particles are generally found in nuclear borosilicate glasses resulting from the melting, at 1200 °C, of a glass precursor and fission products issued from spent fuel reprocessing. Contrary to some other elements, such as iron, nickel, and chromium, these particles are not incorporated chemically in molten glasses. During the melting step, the presence of these metals as suspended particles of a few microns has an impact on the rheological properties of the material, leading to a non-Newtonian behavior. Their impact on the process is the object of characterization and modeling of many studies that have established that the melt presents a shear-thinning and thixotropic behavior. In this work, a deeper analysis of the thixotropic behavior of a simulated nuclear glass melt containing 3.0 wt% (1.02 vol%) of PGM particles is presented. Steady and transient state rheological measurements were performed over a wide shear rate range using a stress-imposed rheometer at temperatures ranging from 1100 to 1250 °C. A mathematical modeling of the glass melt suspension thixotropic behavior is presented for the first time, using a thixotropic model akin to that proposed by Moore. This model is found to explain and predict successfully the rheological behavior of the material. In particular, it allows predicting the transient behavior of the samples from steady-state experiments, without additional adjustable parameters. The present study thus provides an important input for the modeling of the vitrification process.