Dietmar Auhl

Experimental evaluation of the pure configurational stress assumption in the flow dynamics of entangled polymer melts

A filament stretching rheometer was used for measuring the startup of uni-axial elongational flow followed by reversed bi-axial flow, both with a constant elongational strain rate. A narrow molecular mass distribution linear polyisoprene with a molecular weight of 483 kg/mole was subjected to the flow in the non-linear flow regime. This has allowed highly elastic measurements within the limit of pure orientational stress, as the time of the flow was considerably smaller than the Rouse time. A Doi–Edwards [J. Chem. Soc., Faraday Trans. 2 74, 1818–1832 (1978)] type of constitutive model with the assumption of pure configurational stress was accurately able to predict the startup as well as the reversed flow behavior. This confirms that this commonly used theoretical picture for the flow of polymeric liquids is a correct physical principle to apply.

Determination of low amounts of long-chain branches in polypropylene using a combination of chromatographic and rheological methods.

The focus of our investigations lies on the detection of low amounts of long-chain branching (LCB) in isotactic polypropylene (iPP) created by electron beam irradiation. The modified samples were investigated by rheological experiments in shear and elongational flow and by size-exclusion chromatography. The comparison of the results demonstrates the efficiency and the detection limits of these methods, particularly at very low degrees of long-chain branching. The combination of chromatographic and rheological methods enables a sensitive and comprehensive way for the characterisation of the long-chain branching phenomena.

Cross-slot extensional rheometry and the steady-state extensional response of long chain branched polymer melts

Stress-optical measurements at a flow stagnation point in confined geometries such as the cross-slot provide an elegant way to perform extensional testing for polymer melts. This technique is especially useful for samples which have a steady-state that cannot be reached (easily) in standard elongational rheometry, for example, highly branched polymers which show a non-homogeneous deformation that occurs in stretching experiments for Hencky strains above 4. In contrast to filament stretching, the cross-slot provides one point at which steady-state extensional flow may be sustained indefinitely. In this study, a Cambridge multi-pass rheometer [Coventry, K. D., and M. R. Mackley, J. Rheol. 52, 401–415 (2008)] is used to generate planar elongational flow in a cross-slot geometry for different polyethylene melts. The experimental results are compared to finite element flow simulations using the multi-mode Pompom constitutive equations. The steady-state elongational viscosity at the stagnation point is computed...

Entanglement relaxation time of polyethylene melts from high-frequency rheometry in the mega-hertz rangea)

The determination of relevant rheological properties and parameters in a very broad frequency range can be achieved for a number of thermoplastic polymers, for example, polystyrene, by applying the time-temperature-superposition principle. In contrast, polyethylene can only be explored rheologically in a limited frequency range, due to its fast crystallization below the crystallization temperature and its weak viscosity temperature-dependence. In this paper, various commercially available polydisperse and narrowly distributed linear and branched polyethylenes and ethylene-vinylacetate-copolymers were characterized. A piezoelectric- and a new quartz (crystal resonator) rheometer (QR) with an extended frequency range were utilized for the characterization. Introduction of high frequency rheological techniques and implementation of these new measurement methods are shown. For the first time, the entanglement relaxation time in the higher MHz frequency range was determined by applying the QR-technique and com...

A study on the stability of the stress response of nonequilibrium ultrahigh molecular weight polyethylene melts during oscillatory shear flow

Dynamic shear flow and especially large-amplitude oscillatory shear have been a subject of interest to investigate limitations of theoretical approximations working under the assumption of simple shear. These studies have helped in understanding the kinematics involved in phenomena such as stick-slip and shear banding, among others. The nonequilibrium polymer melt, which transforms with time into the equilibrium state, provides a unique opportunity to investigate the influence of the thermodynamic melt state on the validity of simple shear approximations. Here, we show that during oscillatory deformation, the nonlinear rheological response of ultrahigh molecular weight polyethylene melts, having number-average molecular weight greater than one million g/mol, is strongly dependent on the entangled state of the same polymer. At sufficiently large strain amplitude, the stress response of the material departs from a periodic sinusoidal signal, with maximum stress decaying with consecutive cycles of deformatio...

Effect of Shear Rate on the Orientation and Relaxation of a Vanillic Acid Based Liquid Crystalline Polymer

In this study, we report on the visco-elastic response during start-up and cessation of shear of a novel bio-based liquid crystal polymer. The ensuing morphological changes are analyzed at different length scales by in-situ polarized optical microscopy and wide-angle X-ray diffraction. Upon inception of shear, the polydomain texture is initially stretched, at larger strain break up processes become increasingly important, and eventually a steady state texture is obtained. The shear stress response showed good coherence between optical and rheo-X-ray data. The evolution of the orientation parameter coincides with the evolution of the texture: the order parameter increases as the texture stretches, drops slightly in the break up regime, and reaches a constant value in the plateau regime. The relaxation of the shear stress and the polydomain texture showed two distinct processes with different timescales: The first is fast contraction of the stretched domain texture; the second is the slow coalescence of the polydomain texture. The timescale of the orientation parameter’s relaxation matched with that of the slow coalescence process. All processes were found to scale with shear rate in the tested regime. These observations can have far reaching implications for the processing of liquid crystal polymers as they indicate that increased shear rates during processing can correspond to an increased relaxation rate of the orientation parameter and, therefore, a decrease in anisotropy and material properties after cooling.

Rheological and Film‐Casting Properties of Well‐Characterised Polyethylenes with Different Branching Structure

We investigated the influence of branching structure on cast‐film properties, i.e. the neck‐in profile and final cast‐film dimensions, for a series of metallocene catalysed PE as well as tubular reactor PE with various branching levels. For this study a lab extruder has been employed due to the small sample quantities available. The materials and flow conditions have been chosen specifically in order to match for various polyethylenes with different molecular structures at the die exit. Non‐linear visco‐elastic properties in shear and uniaxial elongation have been determined from constant strain‐rate tests and fitted with a multi‐mode Pom‐Pom model, the parameters of which are discussed with respect to the molecular analysis of the branching structure. The experimental results for the film‐casting behaviour as a function of long‐chain branching content and draw ratio will be further compared to model predictions using recent theoretical approaches together with the Pom‐Pom constitutive equation.

A Critical Analysis of Using Step‐Strain and Extensional Rheology to Obtain the Multi‐mode “Pom‐Pom” Model Parameters for Branched High‐Density Polyethylenes

The focus of this research is to evaluate the viability of using a rheological approach that combines shear step‐strain and transient extensional rheology to determine the model parameters for the McLeish‐Larson multi‐mode pom‐pom model for a series of structurally well‐defined high density polyethylene. Presented here are two areas fundamental to this work. First is a novel encapsulation technique designed to overcome material failures (“necking”) in transient extensional flow using a LLDPE sheath. Second is a discussion of the effect of branching content on time‐strain factorability in step‐strain.

Non‐linear Step Strain of Branched Polymer Melts

The Pom‐pom model by McLeish and Larson (Journal of Rheology 42(81–110), 1998) is a highly successful molecular theory for describing the rheology of long chain branched melts. However, there is a long‐standing puzzle in step strain: how can a model that is intrinsically non‐separable recover empirical strain‐time separation? We investigate the Pom‐pom model in step‐strain, comparing the qualitatively different behaviour of the single mode integral and differential orientation. Despite this difference when both models are used in a multi‐mode form, their behaviour is shown to be comparable. Although neither integral nor differential model can predict exact time‐strain separability, both can create a region in which the approximation is a very good one before the longest stretch time has been reached.By transforming to a continuous spectrum we find under certain assumptions, a parameter sub‐space where an analytic damping function can be derived. We survey a range of materials produced by two different syn...

From Reactor to Rheology in LDPE Modeling

In recent years the association between molecular structure and linear rheology has been established and well‐understood through the tube concept and its extensions for well‐characterized materials (e.g. McLeish, Adv. Phys. 2002). However, for industrial branched polymeric material at processing conditions this piece of information is missing. A large number of phenomenological models have been developed to describe the nonlinear response of polymers. But none of these models takes into account the underlying molecular structure, leading to a fitting procedure with arbitrary fitting parameters. The goal of applied molecular rheology is a predictive scheme that runs in its entirety from the molecular structure from the reactor to the non‐linear rheology of the resin. In our approach, we use a model for the industrial reactor to explicitly generate the molecular structure ensemble of LDPEs, (Tobita, J. Polym. Sci. B 2001), which are consistent with the analytical information. We calculate the linear rheolo...