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See detailA new variant of the Burgers model describing the flow of uncured styrene-butadiene rubber
Řehoř, Martin; Gansen, Alex UL; Sill, Clemens et al

E-print/Working paper (2020)

Uncured styrene-butadiene rubber (SBR) can be modelled as a viscoelastic material with at least two different relaxation mechanisms. In this paper we compare the classical Burgers model with additional ... [more ▼]

Uncured styrene-butadiene rubber (SBR) can be modelled as a viscoelastic material with at least two different relaxation mechanisms. In this paper we compare the classical Burgers model with additional dissipation, a generalised Burgers model originally developed to describe the response of asphalt binders, and a newly derived Burgers model combining the strengths of the two existing models. We select the model that best fits the experimental data obtained from a modified stress relaxation experiment in the torsional configuration of the plate-plate rheometer. The optimisation of the five model parameters for each model is achieved by minimising the weighted least-squares distance between experimental observations and the computer model output using a tree-structured Parzen estimator algorithm to find an initial guess, followed by further optimisation using the Nelder-Mead simplex algorithm. The results show that our new model is the most suitable variant to describe the observed behavior of SBR in the given regime. The predictive capabilities of the three models are further examined in changed experimental and numerical configurations. Full data and code to produce the figures in this article are included as supplementary material. [less ▲]

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See detailInvestigation of the Sharkskin melt instability using optical Fourier analysis
Gansen, Alex UL; Rehor, Martin UL; Sill, Clemens et al

in Journal of Applied Polymer Science (2019), 137(24), 48806

An optical method allowing the characterization of melt flow instabilities typically occurring during an extrusion process of polymers and polymer compounds is presented. It is based on a camera‐acquired ... [more ▼]

An optical method allowing the characterization of melt flow instabilities typically occurring during an extrusion process of polymers and polymer compounds is presented. It is based on a camera‐acquired image of the extruded compound with a reference length scale. Application of image processing and transformation of the calibrated image to the frequency domain yields the magnitude spectrum of the instability. The effectiveness of the before mentioned approach is shown on Styrene‐butadiene rubber (SBR) compounds, covering a wide range of silica filler content, extruded through a Göttfert capillary rheometer. The results of the image‐based analysis are compared with the results from the sharkskin option, a series of highly sensitive pressure transducers installed inside the rheometer. A simplified version of the code used to produce the optical analysis results is included as supplementary material. [less ▲]

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See detailIsotropic–isotropic phase separation and spinodal decomposition in liquid crystal–solvent mixtures
Reyes, Catherine UL; Baller, Jörg UL; Araki, Takeaki et al

in Soft Matter (2019), 15

Phase separation in mixtures forming liquid crystal (LC) phases is an important yet under- appreciated phenomenon that can drastically influence the behaviour of a multi-component LC. Here we demonstrate ... [more ▼]

Phase separation in mixtures forming liquid crystal (LC) phases is an important yet under- appreciated phenomenon that can drastically influence the behaviour of a multi-component LC. Here we demonstrate, using polarising microscopy with active cooling as well as differential scanning calorimetry, that the phase diagram for mixtures of the LC-forming compound 4’-n- pentylbiphenyl-4-carbonitrile (5CB) with ethanol is surprisingly complex. Binary mixtures reveal a broad miscibility gap that leads to phase separation between two distinct isotropic phases via spinodal decomposition or nucleation and growth. On further cooling the nematic phase enters on the 5CB-rich side, adding to the complexity. Significantly, water contamination dramatically raises the temperature range of the miscibility gap, bringing up the critical temperature for spinodal de- composition from ∼ 2◦C for the anhydrous case to > 50◦C if just 3 vol.% water is added to the ethanol. We support the experiments with a theoretical treatment that qualitatively reproduces the phase diagrams as well as the transition dynamics, with and without water. Our study highlights the impact of phase separation in LC-forming mixtures, spanning from equilibrium coexistence of multiple liquid phases to non-equilibrium effects due to persistent spatial concentration gradients. [less ▲]

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See detailFractionation of cellulose nanocrystals: enhancing liquid crystal ordering without promoting gelation
Honorato Rios, Camila UL; Lehr, Claudius Moritz UL; Schütz, Christina UL et al

in NPG asia materials (2018)

Colloids of electrically charged nanorods can spontaneously develop a fluid yet ordered liquid crystal phase, but this ordering competes with a tendency to form a gel of percolating rods. The threshold ... [more ▼]

Colloids of electrically charged nanorods can spontaneously develop a fluid yet ordered liquid crystal phase, but this ordering competes with a tendency to form a gel of percolating rods. The threshold for ordering is reduced by increasing the rod aspect ratio, but the percolation threshold is also reduced with this change; hence, prediction of the outcome is nontrivial. Here, we show that by establishing the phase behavior of suspensions of cellulose nanocrystals(CNCs) fractionated according to length, an increased aspect ratio can strongly favor liquid crystallinity without necessarily influencing gelation. Gelation is instead triggered by increasing the counterion concentration until theCNCs lose colloidal stability, triggering linear aggregation, which promotes percolation regardless of the original rod aspect ratio. Our results shine new light on the competition between liquid crystal formation and gelation in nanoparticle suspensions and provide a path for enhanced control of CNC self-organization for applications in photonic crystal paper or advanced composites. [less ▲]

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See detailFractionation of cellulose nanocrystals enhances liquid crystal ordering without promoting gelation
Honorato, C.; Lehr, Claudius Moritz UL; Schutz, C. et al

in Abstracts of Papers of the American Chemical Society (2018), 256

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See detailDetermination of the rheological properties of Matrigel for optimum seeding conditions in microfluidic cell cultures
Kane, K. I. W.; Moreno, E. Lucumi; Lehr, Claudius Moritz UL et al

in AIP ADVANCES (2018), 8(12),

Hydrogels are increasingly used as a surrogate extracellular matrix in three-dimensional cell culture systems, including microfluidic cell culture. Matrigel is a hydrogel of natural origin widely used in ... [more ▼]

Hydrogels are increasingly used as a surrogate extracellular matrix in three-dimensional cell culture systems, including microfluidic cell culture. Matrigel is a hydrogel of natural origin widely used in cell culture, particularly in the culture of stem cell-derived cell lines. The use of Matrigel as a surrogate extracellular matrix in microfluidic systems is challenging due to its biochemical, biophysical, and biomechanical properties. Therefore, understanding and characterising these properties is a prerequisite for optimal use of Matrigel in microfluidic systems. We used rheological measurements and particle image velocimetry to characterise the fluid flow dynamics of liquefied Matrigel during loading into a three-dimensional microfluidic cell culture device. Using fluorescence microscopy and fluorescent beads for particle image velocimetry measurements (velocity profiles) in combination with classical rheological measurements of Matrigel (viscosity versus shear rate), we characterised the shear rates experienced by cells in a microfluidic device for three-dimensional cell culture. This study provides a better understanding of the mechanical stress experienced by cells, during seeding of a mixture of hydrogel and cells, into three-dimensional microfluidic cell culture devices. (C) 2018 Author(s). [less ▲]

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See detailInfluence of suspension viscosity on Brownian relaxation of filler particles
Dannert, R.; Winter, H. H.; Sanctuary, Roland UL et al

in Rheologica Acta (2017), 56(7-8), 615-622

Brownian relaxation caused by Brownian movement of particles in suspensions can macroscopically be probed by small-amplitude oscillatory shear experiments. Phenomenological considerations suggest a direct ... [more ▼]

Brownian relaxation caused by Brownian movement of particles in suspensions can macroscopically be probed by small-amplitude oscillatory shear experiments. Phenomenological considerations suggest a direct proportionality between suspension viscosity and Brownian relaxation times. To verify this relation experimentally, a set of nanocomposite suspensions with viscosities varying over five decades is presented. The suspensions are chosen in a way to ensure that particle-particle interactions and average particle-particle distances are identical so that they can be used as a model system to study the mere influence of suspension viscosity on Brownian relaxation. The suggested linear relationship between suspension viscosity and Brownian relaxation time can be confirmed. Moreover, a verification of a recently introduced characteristic timescale for Brownian relaxation is presented. [less ▲]

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See detailRelaxations in the metastable rotator phase of n-eicosane
Di Giambattista, Carlo UL; Sanctuary, Roland UL; Perigo, Elio Alberto UL et al

in The Journal of Chemical Physics (2015), 143

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See detailSlow dynamics in sheared DGEBA/SiO2 suspensions
Dannert, Rick UL; Sanctuary, Roland UL; Baller, Jörg UL

Presentation (2015, March)

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See detailUnexpected flow and relaxation behaviour of a low molecular weight glass former
Baller, Jörg UL

Presentation (2015, January 22)

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See detailMobility restrictions and glass transition behaviour of an epoxy resin under confinement
Djemour, A.; Sanctuary, Roland UL; Baller, Jörg UL

in Soft Matter (2015), 11(13), 2683-2690

Confinement can have a big influence on the dynamics of glass formers in the vicinity of the glass transition. Already 40 to 50 K above the glass transition temperature, thermal equilibration of glass ... [more ▼]

Confinement can have a big influence on the dynamics of glass formers in the vicinity of the glass transition. Already 40 to 50 K above the glass transition temperature, thermal equilibration of glass formers can be strongly influenced by the confining substrate. We investigate the linear thermal expansion and the specific heat capacity c(p) of an epoxy resin (diglycidyl ether of bisphenol A, DGEBA) in a temperature interval of 120 K around the glass transition temperature. The epoxy resin is filled into controlled pore glasses with pore diameters between 4 and 111 nm. Since DGEBA can form H-bonds with silica surfaces, we also investigate the influence of surface silanization of the porous substrates. In untreated substrates a core/shell structure of the epoxy resin can be identified. The glass transition behaviours of the bulk phase and that of the shell phase are different. In silanized substrates, the shell phase disappears. At a temperature well above the glass transition, a second transition is found for the bulk phase - both in the linear expansion data as well as in the specific heat capacity. The c(p) data do not allow excluding the glass transition of a third phase as being the cause for this transition, whereas the linear expansion data do so. The additional transition temperature is interpreted as a separation between two regimes: above this temperature, macroscopic flow of the bulk phase inside the porous structure is possible to balance the mismatch of thermal expansion coefficients between DGEBA and the substrate. Below the transition temperature, this degree of freedom is hindered by geometrical constraints of the porous substrates. Moreover, this second transition could also be found in the linear expansion data of the shell phase. [less ▲]

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See detailAnomalous glass transition behavior of SBR–Al2O3 nanocomposites at small filler concentrations
Sushko, Rymma UL; Filimon, Marlena UL; Dannert, Rick UL et al

in Nanotechnology (2014), 25(42), 425704

Elastomers filled with hard nanoparticles are of great technical importance for the rubber industry. In general, fillers improve mechanical properties of polymer materials, e.g. elastic moduli, tensile ... [more ▼]

Elastomers filled with hard nanoparticles are of great technical importance for the rubber industry. In general, fillers improve mechanical properties of polymer materials, e.g. elastic moduli, tensile strength etc. The smaller the size of the particles the larger is the interface where interactions between polymer molecules and fillers can generate new properties. Using temperature-modulated differential scanning calorimetry and dynamic mechanical analysis, we investigated the properties of pure styrene-butadiene rubber (SBR) and SBR/alumina nanoparticles. Beside a reinforcement effect seen in the complex elastic moduli, small amounts of nanoparticles of about 2 wt% interestingly lead to an acceleration of the relaxation modes responsible for the thermal glass transition. This leads to a minimum in the glass transition temperature as a function of nanoparticle content in the vicinity of this critical concentration. The frequency dependent elastic moduli are used to discuss the possible reduction of the entanglement of rubber molecules as one cause for this unexpected behavior. [less ▲]

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See detailComparative study of the effect of untreated, silanized and grafted alumina nanoparticles on thermal and dynamic mechanical properties of the styrene-butadiene rubber
Sushko, Rymma UL; Baller, Jörg UL; Filimon, Marlena UL et al

in AIP Conferences Proceedings (2014, May 15), 1599

Elastomers filled with hard nanoparticles are of great technical importance for the rubber industry. In general, fillers improve mechanical properties of polymer materials, e.g. elastic moduli, tensile ... [more ▼]

Elastomers filled with hard nanoparticles are of great technical importance for the rubber industry. In general, fillers improve mechanical properties of polymer materials, e.g. elastic moduli, tensile strength etc. The smaller the size of the particles the larger is the interface where interactions between polymer molecules and fillers can generate new properties. Using Temperature Modulated Differential Scanning Calorimetry (TMDSC) and Dynamic Mechanical Analysis (DMA), we investigated the properties of the pure styrene-butadiene rubber (SBR), SBR/ alumina nanoparticles, SBR/silanized alumina and SBR/alumina grafted to polymer chains. Beside a general reinforcement effect seen in the complex elastic moduli, the studies revealed that: i) small concentrations of nanoparticles (of 1.5-2 wt%) lead to a minimum in the glass transition temperature as a function of nanoparticle content; ii) for the grafted nanocomposites increasing the nanoparticle concentration beyond 4 wt% yields an increase of Tg by 4 K; iii) DMA mastercurves showed that in case of untreated and silanized alumina mechanical behaviour of the composite systems is rather near to the one of the SBR matrix, but the grafting of elastomer molecules to the silanized fillers induces a quasi-solid like response of the system in the low frequency regime. [less ▲]

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See detailAnomaly in thermal and mechanical properties of SBR/alumina nanocomposites
Sushko, Rymma UL; Baller, Jörg UL; Filimon, Marlena UL et al

Scientific Conference (2014, April 03)

Filling elastomers with nanoparticles generally leads to changes in the relaxation behavior of the matrix molecules. Using dynamic mechanical analysis (DMA) and temperature modulated calorimetry (TMDSC ... [more ▼]

Filling elastomers with nanoparticles generally leads to changes in the relaxation behavior of the matrix molecules. Using dynamic mechanical analysis (DMA) and temperature modulated calorimetry (TMDSC), we investigate the influence of different amounts of untreated, hydrophilic alumina nanoparticles on the properties of a model rubber system (SBR). Beside a reinforcement effect seen in the complex elastic moduli, small amounts of nanoparticles of about 2 wt% interestingly lead to an acceleration of the relaxation modes responsible for the thermal glass transition. This leads to a minimum in the glass transition temperature as a function of nanoparticle content in the vicinity of this critical concentration. The frequency dependent elastic moduli are used to discuss the possible reduction of the entanglement of rubber molecules as one cause for this unexpected behavior. [less ▲]

Detailed reference viewed: 58 (7 UL)