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Filament stretching rheometry is a prominent experimental method to determine rheological properties in extensional flow whereby the separating plates determine the extension rate. In literature, several correction factors that can compensate for the errors introduced by the shear contribution near the plates have been introduced and validated in the linear viscoelastic regime. In this work, a systematic analysis is conducted to determine if a material-independent correction factor can be found for non-linear viscoelastic polymers. To this end, a finite element model is presented to describe the flow and resulting stresses in the filament stretching rheometer. The model incorporates non-linear viscoelasticity and a radius-based controller for the plate speed is added to mimic the typical extensional flow in filament stretching rheometry. The model is validated by comparing force simulations with analytical solutions. The effects of the end-plates on the extensional flow and resulting force measurements are investigated, and a modification of the shear correction factor is proposed for the non-linear viscoelastic flow regime. This shows good agreement with simulations performed at multiple initial aspect ratios and strain rates and is shown to be valid for a range of polymers with non-linear rheological behaviour. [ABSTRACT FROM AUTHOR]

A finite element model is presented to describe the flow, resulting stresses and crystallization in a filament stretching extensional rheometer (FiSER). This model incorporates nonlinear viscoelasticity, nonisothermal processes due to heat release originating from crystallization and viscous dissipation as well as the effect of crystallization on the rheological behavior. To apply a uniaxial extension with constant extension rate, the FiSER plate speed is continuously adjusted via a radius‐based controller. The onset of crystallization during filament stretching is investigated in detail. Even before crystallization starts, the rheology of the material can change due to the effects of flow‐induced nucleation on the relaxation times. Both nucleation and structure formation are found to be strongly dependent on temperature, strain rate and sample aspect ratio. The latter dependence is caused by a clear distribution of crystallinity over the radius of the filament, which is a result of the nonhomogeneous flow history in the FiSER. Therefore, this numerical model opens the possibility to a priori determine sample geometries resulting in a homogeneous crystallinity or to account for the nonhomogeneity. [ABSTRACT FROM AUTHOR]

It is well established that polymer chain architecture and the distribution of molecular weight play a key role in the flow behavior (processing) and performance of a given polymer material. Long-chain branching (LCB) in particular is known to strongly affect the processability and the material performance of polymers. Often branching is a result of the polymerization process and therefore must be quantified in every sample. We study four commercial high-density polyethylene (HDPE) samples with unknown degrees of polydispersity and LCB. We first use size-exclusion chromatography and linear shear rheology to identify differences in molecular weight, polydispersity, and LCB. Each material is then tested in constant rate and constant stress uniaxial extension using a filament stretching rheometer to quantify extensional viscosity and strain hardening. Correlations between nonlinear extensional rheology, LCB and polydispersity are discussed. We show that the combination of the van Gurp-Palmen plot and extensional rheology allows for a full characterization of the LCB fraction and their effect on extensional rheology. [ABSTRACT FROM AUTHOR]

The extensional rheology of polymer melts and dilute polymer solutions has been extensively examined through experiments and theoretical predictions. However, a systematic study of the extensional rheology of polymer solutions in the semidilute regime, in terms of examining the effects of concentration and molecular weight, has not been carried out so far. Previous experimental studies of the shear rheology of semidilute polymer solutions have demonstrated that their behaviour is distinctively different from that observed in the dilute and concentrated regimes. This difference in behaviour is anticipated to be even more pronounced in extensional flows, which play a critical role in a number of industrial contexts such as fiber spinning and ink-jet printing. In this work, the extensional rheology of linear, double-stranded DNA molecules, spanning an order of magnitude of molecular weights (25-289 kilobasepairs) and concentrations (0.03-0.3 mg/ml), has been investigated. DNA solutions are now used routinely as model polymeric systems due to their near-perfect monodispersity. Measurements have been carried out with a filament stretching rheometer since it is the most reliable method for obtaining an estimate of the elongational stress growth of a polymer solution. Transient and steady-state uniaxial extensional viscosities of DNA dissolved in a solvent under excess salt conditions, with a high concentration of sucrose in order to achieve a sufficiently high solvent viscosity, have been determined in the semidilute regime at room temperature. The dependence of the steady state uniaxial extensional viscosity on molecular weight, concentration and extension rate is measured with a view to determining if data collapse can be observed with an appropriate choice of variables. Steady state shear viscosity measurements suggest that sucrose-DNA interactions might play a role in determining the observed rheological behaviour of semidilute DNA solutions with sucrose as a component in the solvent. [ABSTRACT FROM AUTHOR]

A linear polystyrene melt with broad molecular weight distribution (CAS 0993-53-6 from Aldrich) was characterized in extensional flow using a filament stretching rheometer. A series of measurements were performed, including (1) the startup of uniaxial extensional flow until steady state; (2) the strain controlled reversed (i.e., biaxial) flow following the startup of uniaxial flow; and (3) stress relaxation (i.e., keeping the strain unchanged) also following the startup of uniaxial flow. The measurements were compared with the model by Narimissa and Wagner [J. Rheol. 60, 625–636 (2016)]. Good agreements at low extension rates and low strain values were observed. Higher deviations were observed at high rates under steady state conditions as well as during stress relaxation and reversed flow. [ABSTRACT FROM AUTHOR]

Abstract: We propose a new control scheme to maintain a constant strain rate of the mid-filament diameter in a filament stretching rheometer for polymer melts. The scheme is cast as a velocity algorithm and consists of a feed-back and a feed-forward contribution. The performance of the controller is demonstrated on a commercial low-density polyethylene. Several strain rates and experimental conditions are tested to demonstrate the necessity of the control parameters and the limits of both the control scheme and the experimental apparatus. When the control parameters are properly tuned, the algorithm ensures that the applied strain stays within 2% of the set point and measurements can be made up to Hencky strains of 6.5. [Copyright &y& Elsevier]

Using a filament stretching rheometer surrounded by a thermostatic chamber and equipped with a micrometric laser, it is possible to measure large amplitude oscillatory elongation (LAOE) on elastomeric based networks with no base flow as in the LAOE method for polymer melts. Poly(dimethylsiloxane)-based networks, considered as a model system for studying the development of soft elasticity, are mechanically characterised through reversed LAOE deformation, applying oscillatory measurements with several amplitudes and frequencies. [ABSTRACT FROM AUTHOR]

Abstract: A filament stretching rheometer (FSR) was used for measuring the start-up of uni-axial elongational flow followed by reversed bi-axial flow, both with a constant elongational rate. A narrow molecular mass distribution linear polystyrene with a molecular weight of 145kg mole was subjected to the start-up of elongation for three Hencky strain units and subsequently the reversed flow. The integral molecular stress function formulation within the ‘interchain pressure’ concept agrees with the experiments. In the experiments the Hencky strain at which the stress becomes zero (the recovery strain) in the reversed flow has been identified. The recovery strain is found to increase with elongational rate, and has a maximum value of approximately . The Doi Edwards model using any stretch evolution equation is not able to predict the correct level of the recovery strain. [Copyright &y& Elsevier]

A filament stretching rheometer has been equipped with a bottom plate with a number of equally distributed perturbations of the circular rim. The temporal development of the resulting disturbance of the meniscus is followed with video recording and with simulations. At high strain rates the surface perturbations grow into a number of fluid columns. It is shown that the growth is associated with strain hardening of the material. The relevant Deborah number is the product of a time constant associated with the non-linear properties of the fluid and the imposed extension rate. [Copyright &y& Elsevier]

The filament stretching device which is used increasingly as an apparatus for measuring extensional properties of polymeric liquids is analysed. A force balance that includes the effects of inertia and surface tension is derived. The force balance may be used to correct for the effects of inertia and surface tension, provided online measurements of the filament surface shape are available. In addition, the question of initial asymmetry due to gravity is addressed. [ABSTRACT FROM AUTHOR]