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See detailDynamics of viscoelastic colloidal suspensions
Dannert, Rick UL

Doctoral thesis (2016)

The influence of different types of nanoparticles on the dynamics of glass forming matrices has been studied by small oscillatory shear rheology. Experimental measurements reveal that besides the glass ... [more ▼]

The influence of different types of nanoparticles on the dynamics of glass forming matrices has been studied by small oscillatory shear rheology. Experimental measurements reveal that besides the glass transition process of the matrix an additional relaxation process occurs in presence of nanoparticles. The latter is identified as the macroscopic signature of the microscopic temporal fluctuations of the intrinsic stress and is called Brownian relaxation. Besides the fact that Brownian relaxation has so far not been observed in colloidal suspensions with a matrix exhibiting viscoelastic behaviour in the frequency range of the experimental probe, the study reveals another important feature to be highlighted: the evolution of the Brownian relaxation times depends non-monotonously on the filler concentration. This finding challenges the use of the classical Peclet-time as a characteristic timescale for Brownian relaxation. Literature defines the Peclet-time as the specific time needed by a particle to cover –via self-diffusion- a distance comparable to its own size. As a main result it will be shown that after replacing the particle size which is relevant for the Peclet time by the mean interparticle distance depending on the filler content the non-monotonic evolution of the relaxation times can be fully described. Moreover, the introduction of the new characteristic length scale allows to include data from literature into the phenomenological description. [less ▲]

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See detailEquilibrium Liquid Crystal Phase Diagrams and Detection of Kinetic Arrest in Cellulose Nanocrystal Suspensions
Honorato Rios, Camila UL; Kuhnhold, Anja UL; Bruckner, Johanna et al

in Frontiers in Materials (2016), 3

The cholesteric liquid crystal self-assembly of water-suspended cellulose nanocrystal (CNC) into a helical arrangement was observed already more than 20 years ago, and the phenomenon was used to produce ... [more ▼]

The cholesteric liquid crystal self-assembly of water-suspended cellulose nanocrystal (CNC) into a helical arrangement was observed already more than 20 years ago, and the phenomenon was used to produce iridescent solid films by evaporating the solvent or via sol–gel processing. Yet, it remains challenging to produce optically uniform films and to control the pitch reproducibly, reflecting the complexity of the three-stage drying process that is followed in preparing the films. An equilibrium liquid crystal phase formation stage is followed by a non-equilibrium kinetic arrest, which in turn is followed by structural collapse as the remaining solvent is evaporated. Here, we focus on the first of these stages, combining a set of systematic rheology and polarizing optics experiments with computer simulations to establish a detailed phase diagram of aqueous CNC suspensions with two different values of the surface charge, up to the concentration where kinetic arrest sets in. We also study the effect of varying ionic strength of the solvent. Within the cholesteric phase regime, we measure the equilibrium helical pitch as a function of the same parameters. We report a hitherto unnoticed change in character of the isotropic–cholesteric transition at increasing ionic strength, with a continuous weakening of the first-order character up to the point where phase coexistence is difficult to detect macroscopically due to substantial critical fluctuations. [less ▲]

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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 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 detailBrownian stress decay in colloidal suspensions
Dannert, Rick UL

Presentation (2014)

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See detailStrain-induced low-frequency relaxation in colloidal DGEBA/SiO2 suspensions
Dannert, Rick UL; Sanctuary, Roland UL; Thomassey, M. et al

in Rheologica Acta (2014), 53(9), 715-723

Diglycidyl ether of bisphenol A (DGEBA) is widely exploited as an epoxy resin in adhesives and coatings. In this paper, it is used as an oligomer matrix for silica-filled nanocomposites. Rheological ... [more ▼]

Diglycidyl ether of bisphenol A (DGEBA) is widely exploited as an epoxy resin in adhesives and coatings. In this paper, it is used as an oligomer matrix for silica-filled nanocomposites. Rheological measurements show that the pure matrix obeys power-law relaxation dynamics in the vicinity of the dynamic glass transition of this low-molecular-weight glass former. In the filled systems, a low-frequency relaxation appears additionally to the structural alpha-process of the matrix. Considering the nanocomposites as Newtonian hard-sphere suspensions at low angular frequencies (or high temperatures), the modified terminal regime behavior of the matrix can be linked to strain-induced perturbations of the isotropic filler distributions. While in the low-frequency regime hydrodynamic stresses relax instantaneously, the Brownian stress relaxation is viscoelastic and can be evidenced by dynamic rheological measurements. At higher angular frequencies, the alpha-process of the matrix superimposes on the Brownian stress relaxation. In particular, we were able to depict the low-frequency anomaly for concentrated, semi-dilute, and even for dilute suspensions. [less ▲]

Detailed reference viewed: 93 (8 UL)