Volume 25, Issue 3

Organic nanoparticles of poly(styrene-co-maleimide) or SMI were synthesized in aqueous dispersion with a maximum concentration of 35 wt.% and are favorably applied in industrial coating processes. In order to evaluate the further processability and flow behavior of these nanoparticle dispersions, general rheological characterization under creep, oscillatory and rotational testing was done by applying various shear stresses, shear rates and frequencies on an air-bearing cylindrical rheometer. Creep tests at different stresses show that the nanoparticle dispersions behave like a viscous material. The crossover of G’ and G” according to oscillatory experiments also demonstrates a transition to viscoelastic behavior at high frequency. The sensitivity of shear-viscosity behavior to concentration and temperature of the dispersions has been evaluated. In parallel, the influences of gap size, repeatability and water evaporation have been statistically evaluated and could be successfully controlled. By comparing oscillatory and rotational rheometry data, flow curves under low shear rates were reconstructed.

This paper reports the rheological behavior of swelled and mineralized hydrogel prepared using polyvinylpyrrolidone (PVP) and carboxymethylcellulose (CMC) hydrogel as base polymer. Herein, the bio-mineral calcium carbonate (CaCO3) was incorporated into the hydrogel using simple liquid diffusion method. The morphology of the swelled and mineralized hydrogel was analyzed through scanning electron microscopy. Further, the normalized time of absorptivity was identified from the time dependent absorptivity behavior of calcite and water filled PVP-CMC hydrogel. The effect of the biomineral (CaCO3) and water on the dynamic viscoelastic properties, after penetrating inside the hydrogel matrix has been evaluated. The frequency sweep at 1 and 10 % strain and also strain sweep measurement were performed to determine the frequency and strain dependent viscoelastic moduli G’ and G” of both swelled and mineralized hydrogel. At higher strain the both moduli showed significant change over wide range of angular frequency region and the nature of mineralized polymer composites (MPC) turned from elastic to viscous. Based on the observed basic properties, MPC (calcite based polymer composites) can be recommended for the treatment of adyanamic bone disorder and water swelled hydrogel can be acclaimed as a scaffold for burned wound dressing.

Build-up of cement-based suspensions is a complex phenomenon affected by the mixture concentration and testing parameters as well as the shear history. Accurate measurements of build-up rely on the efficiency of the applied pre-shear regime to achieve an initial defined and dispersed structure to eliminate the shear history. This can therefore enable understanding mechanisms of build-up and quantifying the structuration of cement suspension from a reproducible dispersed state. Dispersing efficiency of various disruptive shear techniques, including rotational, oscillatory, and combination of both was evaluated. The initial and final states of suspension’s structure were determined by applying small-amplitude oscillatory shear (SAOS). Test results showed that oscillatory shear has a greater effect on dispersing concentrated cement suspension than the rotational shear. Furthermore, the increase in shear strain in oscillatory technique enhanced the breakdown of suspension’s structure until a critical point, after which thickening effects dominate. An effective dispersing method is then proposed. This consists in applying a rotational shear around the transitional value between shear-thinning and shear-thickening followed by an oscillatory shear at the crossover shear strain and high angular frequency of 100 rad/s.

The creep and recovery of asphalt modified with Elvaloy 4170 and polyphosphoric acid were studied at low temperatures, by inductive phenomenological methods. Two models of the tensile compliance function were investigated. Both models were derived from the linear viscoelastic retardation spectra and successfully used for the description of the creep and recovery tests in the studied asphalt binders. Large effects due to oxidative aging in a rolling thin film oven were found from the recovered compliance function recorded in a bending beam rheometer at a temperature of - 20 °C. The studied compliance function models worked well at higher and lower temperatures in creep and recovery experiments on conventional and modified asphalt binders for both shear and tensile creep.

Measuring the rheological properties of multi-component (and multi-phase) systems meets with many special problems which are absent in flows of homogeneous materials. Such complex fluids have inherent structure and all the peculiarities of their behavior are determined by stress-induced temporal-spatial structure rearrangements. This paper is a review devoted to the physical origin and classification of problems encountered in the flow of multi-component materials. Stress-driven phenomena can be related to phase transformations (the formation of a new phase in polymerization, crystallization, amorphous phase separation), molecular and structure orientation, and various forms of self-organization. Some of these time effects are considered to be thixotropic phenomena. Thixotropy of multi-component matters leads to absence of an upper Newtonian plateau, time (rate)-dependence of yield stress and the layered flow in the range of high shear rates. The flow of such matters can lead to the formation of spatially divided structures with different properties and displacement of structures at the macroscopic level that excludes traditional measures of their rheological properties. In addition, the flow of multi-component systems is accompanied by the appearance of anisotropy of their properties. It is emphasized that the stressdriven evolution of rheological properties are not taken into account in the existing widely used constitutive equations.