Relationships between conductivity and local topology in heterocyclic polymer networks.

The low frequency complex dielectric relaxation above the glass transition temperature T(g) for a series of well-characterized heterocyclic polymer networks has been analyzed in terms of the electric moduli formalism. It was established that the contribution of ionic conductivity to the electric modulus can be quantitatively separated from the alpha relaxation by using a combination of two Havriliak-Negami (HN) functions. A strong correlation between the mechanisms of both conductivity and segmental mobility was inferred from the similarity of the shape of the HN function for conductivity relaxation to those for the main relaxation. This correlation is further supported by the similarity of the temperature dependencies of the relevant relaxation times corresponding to both processes. The overwhelming contribution of the preexponents D0 in the Arrhenius behavior of the apparent diffusion coefficients can be explained by considering a model implying decreased mean free paths of the diffusing elements and lower activation entropies of diffusion for polymer networks with higher apparent network densities.

Probing the subglass relaxation behavior in model heterocyclic polymer networks by dielectric spectroscopy.

The subglass relaxation (beta) in model heterocyclic polymer networks (HPNs) with a controlled ratio of trimerized mono- and diisocyanates was characterized by dielectric spectroscopy in the frequency domain. The beta relaxation in the investigated HPNs follows the Arrhenius law with unusually low values of the preexponential factor (10(-17)

Temporal fractal model for the anomalous dielectric relaxation of inhomogeneous media with chaotic structure.

The potential of the fractional derivative technique is demonstrated on the example of derivation of all three known patterns of anomalous, nonexponential dielectric relaxation of an inhomogeneous medium in the time domain. It is explicitly assumed that the fractional derivative is related to the dimensionality of a temporal fractal ensemble (in a sense that the relaxation times are distributed over a self-similar fractal system). The proposed fractal model of a microstructure of inhomogeneous media exhibiting nonexponential dielectric relaxation is built by singling out groups of hierarchically subordinated ensembles (subclusters, clusters, superclusters, etc.) from the entire statistical set available. Different relaxation functions are derived assuming that the real (physical) ensemble of relaxation times is confined between the upper and lower limits of self-similarity. It is predicted that at times, shorter than the relaxation time at the lowest (primitive) self-similarity level, the relaxation should be of a classical, Debye-like type, whatever the pattern of nonclassical relaxation at longer times.

Elastic properties of inhomogeneous media with chaotic structure.

The elastic properties of an inhomogeneous medium with chaotic structure were derived within the framework of a fractal model using the iterative averaging approach. The predicted values of a critical index for the bulk elastic modulus and of the Poisson ratio in the vicinity of a percolation threshold were in fair agreement with the available experimental data for inhomogeneous composites.