We use computer simulations to study the relaxation dynamics of a model for oil-in-water microemulsion droplets linked with telechelic polymers. This system exhibits both gel and glass phases and we show that the competition between these two arrest mechanisms can result in a complex, three-step decay of the time correlation functions, controlled by two different localization lengthscales. For certain combinations of the parameters, this competition gives rise to an anomalous logarithmic decay of the correlation functions and a subdiffusive particle motion, which can be understood as a simple crossover effect between the two relaxation processes. We establish a simple criterion for this logarithmic decay to be observed. We also find a further logarithmically slow relaxation related to the relaxation of floppy clusters of particles in a crowded environment, in agreement with recent findings in other models for dense chemical gels. Finally, we characterize how the competition of gel and glass arrest mechanisms affects the dynamical heterogeneities and show that for certain combination of parameters these heterogeneities can be unusually large. By measuring the four-point dynamical susceptibility, we probe the cooperativity of the motion and find that with increasing coupling this cooperativity shows a maximum before it decreases again, indicating the change in the nature of the relaxation dynamics. Our results suggest that compressing gels to large densities produces novel arrested phases that have a new and complex dynamics.

We use ab initio simulations to investigate the properties of a sodium borosilicate glass of composition 3Na2 O-B2 O3 -6SiO2 . We find that the broadening of the first peak in the radial distribution functions gBO (r) and gBNa (r) is due to the presence of trigonal and tetrahedral boron units as well as to nonbridging oxygen atoms connected to BO3 units. In agreement with experimental results, we find that the [3] B units involve a significant number of nonbridging oxygens, whereas the vast majority of [4] B have only bridging oxygens. We determine the three-dimensional distribution of the Na atoms around the [3] B and [4] B units and use this information to explain why the sodium atoms associated with the latter share more oxygen atoms with the central boron atoms than the former units. From the distribution of the electrons we calculate the total electronic density of states, as well its decomposition into angular momentum contributions. The vibrational density of states shows at high frequencies a band that originates from the motion of the boron atoms. We find that the [3] B and [4] B units give rise to well-defined features in the spectrum, which thus can be used to estimate the concentration of these structural entities. The contribution of [3] B can be decomposed further into symmetric and asymmetric parts that can also be easily identified in the spectrum. Furthermore, it is found that certain features in the spectrum can be used to obtain information on the type of atom that is the second-nearest neighbor of a boron in the [4] B unit. We calculate the average Born charges on the bridging and nonbridging oxygen atoms and show that these depend linearly on the angle between the two bonds and the distance from the connected cation, respectively. Finally, we have determined the frequency dependence of the dielectric function, as well as the absorption spectra. The latter is in good quantitative agreement with the experimental data.

We use ab initio simulations to study the static and dynamic properties of a sodium borosilicate liquid with composition 3Na2 O–B2 O3 –6SiO2 , i.e., a system that is the basis of many glass-forming materials. In particular, we focus on the question how boron is embedded into the local structure of the silicate network liquid. From the partial structure factors we conclude that there is a weak nanoscale phase separation between silicon and boron and that the sodium atoms form channel-like structures as they have been found in previous studies of sodosilicate glass-formers. Our results for the x-ray and neutron structure factor show that this feature is basically not detectable in the former but should be visible in the latter as a small peak at small wave vectors. At high temperatures we find a high concentration of threefold coordinated boron atoms which decreases rapidly with decreasing T , whereas the number of fourfold coordinated boron atoms increases. Therefore, we conclude that at the experimental glass transition temperature most boron atoms will be fourfold coordinated. We show that the transformation of [3] B into [4] B with decreasing T is not just related to the diminution of nonbridging oxygen atoms as claimed in previous studies, but to a restructuring of the silicate matrix. The diffusion constants of the various elements show an Arrhenius behavior and we find that the one for boron has the same value as the one of oxygen and is significantly larger than the one of silicon. This shows that these two network-formers have rather different dynamical properties, a result that is also confirmed from the time dependence of the van Hove functions. Finally, we show that the coherent intermediate scattering function for the sodium atoms is very different from the incoherent one and that it tracks the one of the matrix atoms.