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Recently, Friedrich proposed an empirical model for linear viscoelastic fluids corresponding to a constitutive equation with fractional derivatives [Phil. Mag. Lett., 66, 287 (1992)]. For this model, the relaxation modulus, the dynamic moduli, the relaxation time spectrum, and other material functions have been explicitly calculated as a function of the few parameters that characterize a viscoelastic fluid within this model. By fitting this model to experimental data, the model parameters can be determined and other material functions, in particular the relaxation time spectrum, can be calculated immediately. This paper reports to what extent this method, which may be called analytical inversion, is appropriate for the determination of relaxation time spectra. For that purpose, the spectra of a number of very different polymeric materials are determined with this method. The spectra calculated in this way are compared with the spectra obtained by nonlinear regularization. It turns out that the empirical model describes the linear viscoelastic properties of a variety of different materials with high accuracy. Keeping in mind that the determination of the relaxation time spectrum requires the solution of an ill-posed problem, the agreement between the relaxation time spectra obtained by analytical inversion and by regularization is satisfactory for these materials., INTRODUCTION The determination of the relaxation time spectrum from experimental data for a material function is one of the oldest tasks in rheology. It is, however, not an easy task, [...]

The present paper aims at studying the behaviour of the poly(vinyl alcohol) [PVA] cryogel in the presence of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]+[BF4]-) aqueous solutions with various concentrations. The gravimetric method showed that the swollen PVA cryogels exhibit mechanically active behaviour. PVA cryogels showed shrinking in the presence of ionic liquid, (IL), and re-swelling in the presence of distilled water. The re-swelling is not completely reversible, due to the influence of the IL ions on the gel morphology. The Fourier transform infrared (FTIR) spectra have indicated no chemical interaction between the PVA and the studied IL, but highlighted the gel crystallinity change as a function of IL concentration, as well as changes in the bound water amount. Rheological analyses showed dominating plastifying effect of the cation at a lower IL concentration and dominating kosmotropic effect of the anion at a higher IL concentration. A phenomenological kinetic equation that takes into account both fluxes of matters, in and out of the gel, is proposed, explaining the alteration of the gel properties when it comes in contact with BMIMBF4 solutions. [ABSTRACT FROM AUTHOR]

A model for the structure of filled polymeric composites has been developed. To simulate filler-filler and filler-matrix interaction on a meso level, the Voronoi polyhedron representation of smallest structural elements (filler particles) is used while fractal concepts are applied for the description of more coarse-grained structures. An iterative method based on the ideas of renormalization group transformations is presented to calculate viscoelastic properties such as the storage and loss modulus of the composite. The influence of frequency as well as the properties of filler and matrix on the effective viscoelastic properties of polystyrene melt filled with glass spheres has been elucidated in a wide concentration range. The calculations and the experiments are in good agreement. Moreover, model calculations coincide with results of percolation theory.