We have carried out simulations of particle diffusion through polyacrylamide gel networks. The model structures were built on a diamond lattice, in a simulation box with periodic boundary conditions. The method of structure generation consists of a random distribution of knots on the lattice and interconnection between randomly chosen pairs of knots. The structures generated by this procedure approximate the topology of real polymer gels. Parameters that control the distance between knots and the degree of stretching of the chain permit us to simulate a polyacrylamide system in which the concentration of species as well as the degree of crosslinking can be compared to realistic gels as prepared by the available experimental procedures. These structures were geometrically characterized by the analysis of the pore size distribution and excluded volume. The structures thus generated are used as model networks for Monte Carlo studies of the diffusion of hard spheres in the restricted geometry. Modeling the deviations from the normal diffusion behavior as a purely geometrical phenomenon, these simulations can give us insights into the factors which lead to anomalous diffusion in gel-like systems. In these simulations a sphere of variable radius is allowed to perform an off-lattice random walk through the space between the generated structures, which are assumed to be rigid. It will be shown that the study of the influence of tracer size on diffusion is complementary to the study of the influence of obstacle concentration on the diffusion of monodispersed particles. While the latter method can give us information about the fractal nature of the internetwork space the former method provides information about the fractal nature of the network and the connectivity of the internetwork space.

We have investigated particle diffusion through different obstacle geometries by computer simulations. The model structures used in this work — randomly placed point obstacles and cage-like structures — were chosen with the aim of represent a broad range of geometrical structures similar to gels and in order to be compared with our previous simulations of particle diffusion through polyacrylamide gels. The diffusion behavior was studied as a function of tracer size and obstacle concentration. The isomorphism between the diffusion of finite-sized tracers and the diffusion of point tracers in the presence of expanded obstacles was applied. Only hard-sphere interactions of the tracer with the immobile obstacles were considered and the theoretical description was made in terms of theory of the obstruction effect. In the case of randomly placed point obstacles an analytical expression for the dependence of the diffusion coefficient on tracer radius and obstacle concentration, applying the model of spherical cells, could be deduced. The same description was applied numerically to the other model systems. Up to moderatly high fractions of excluded volume this description was found to be successful. For very high fractions of excluded volume — higher concentrations or larger tracers — the validity of Fick’s second equation for describing diffusion breaks down and anomalous diffusion was found. The anomalous diffusion exponent diverges as the tracer size becomes comparable to the size of the pores. Analysis of the trajectory of tracers in the cases where an anomalous diffusion takes place shows a Levy-flight-like characteristic.

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The interactions between polyacrylamide and SPC/E water molecules in polymer gels and solutions were analyzed using computer simulations. Several polymer structures with different concentrations, connectivity characteristics, and pore sizes were used in order to investigate different polymer environments. It was shown that the structure of water was strongly modified in the presence of the polymer. This polymer-induced modification could be characterized in a very detailed way using the spatial distribution function, which considers both the distance as well as the orientation of the water molecules relative to the amide side groups. The dynamics of water, as manifested in the translational and rotational diffusion and residence probability of water is slowed down in the presence of the polymer. The strongest modifications are found in the shell of bound water, but also long-ranged effects were detected. The hydrogen bonds in the vicinity of amide side groups become stronger and longer lived. By setting the partial charges in the amide side groups to zero in some systems, artificial apolar polymer network model systems were defined, whose influence on the structure of water is remarkably different from the influence of the polar networks, although their influence on the transport properties of the surrounding water molecules was found to be quite similar.

We report results of the photon correlation polarized light scattering study on the dynamics of bulk polymethylacrylate (PMA) in the temperature range of 7–76 °C and in the pressure range 1–2100 bar. A single relaxation process with a distribution of relaxation times represents the experimental photon correlation functions. The temperature and pressure dependence of the mean relaxation time are analyzed in terms of the VFT and WLF equations. The parameters obtained are discussed in terms of the rotational isomerization model and the free volume model. The results obtained in light scattering agree well with dielectric and mechanical relaxation data. In the case of polyethylmethacrylate (PEMA) and polybutylmethacrylate (PBMA), two overlapping relaxation modes are found. The values of the activation parameters and the relaxation time at the glass transition temperature (Tg) thus obtained characterizing the dynamics of a polymer melt above Tg are compared for PMA, PEA, PEMA, and PBMA.