The effect of molecular weight on the steady shear rheology of lyotropic solutions. A phenomenological study

Steady shear rheological measurements are carried out on lyotropic aqueous solutions of hydroxypropylcellulose. The molecular weight is the main parameter investigated. Careful sealing of the sample permits very long time experiments, thus leading to steady-state measurements at shear rates as low as 10E3 s-1. At such low deformation rates, for two of the three molecularweights investigated, a Newtonian plateau is found before region I in the viscosity curve, thus rulingout the possibility of a yield stress in this material. A phenomenological model is proposed in orderto interpret the effects of molecular weight on the rheological behavior. The polymer system is modeled as a structured fluid, in which the low shear rate response is dominated by the polydomain structure, whereas molecular dynamics dominates the high velocity gradient region. The model is able to predict the observed qualitative changes in the viscosity vs shearr ate curve as a function of molecular weight, as well as the scaling of the first normal stress difference with a molecular time scale of the system. Finally, intrinsic viscosity measurements show that the molecular weight of the samples used in this work is systematically larger than the value indicated by the supplie

Morphology evolution of immiscible polymer blends in complex flow fields

A transparent flow cell apparatus has been used to obtain quantitative information on the morphology evolution of a model polymer blend flowing in complex flow fields. A dilute emulsion of poly-dimethylsiloxane droplets immersed in a poly-isobutene matrix, both Newtonianliquids at room temperature, has been chosen as a model system. Optical microscopy coupled with image acquisition analysis allowed tomonitor the behavior of droplets flowing through a sudden contraction and through a gradual converging channel followed by a sudden expansion. The experimental results indicate that drop deformation and breakup are sensitive to both shear and extensional components. A simple criterion, based on the evaluation of a local critical capillary number, is proposed to predict the morphological evolution of the flowing blend. By using computer simulations to estimate the velocity gradients in the flow field, it is shown that the proposed criterion is able tosuccessfully predict the observed morpholog

Comparative measurements of interfacial tension in a model polymer blend

This work shows the application of several experimental methods to the measurement of the interfacial tension between two immiscible polymers. A quantitative knowledge of the interfacial tension is important in view of the crucial role that this parameter plays in polymer blend processing. Common to all methods presented here are two main points. The first is that the interfacial tension is obtained from experiments where the shape of the interface between the liquids is directly observed by means of optical microscopy techniques. The second point is that the interface geometry is controlled by a balance between the interfacial force and the viscous stresses generated by some flow applied to the system. Measurements have been carried out on a model polymer blend, whose constituents are a poly-isobutylene and a polydimethylsiloxane, both transparent and liquid at room temperature. When compared with each other, the values of interfacial tension obtained from the different methods show a good quantitative agreement. Excellent agreement is also found with results for the same system previously published in the literatur

In vitro regulation of rat Sertoli cell transferrin expression by tumor necrosis factor alpha and retinoic acid

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