A confinement-induced mode was discovered in thin cis-1,4-polyisoprene (PI) layers if the film thickness becomes comparable with the size of the PI coil (A. Serghei, F. Kremer, to be published in Phys. Rev. Lett.). It was assigned to the fluctuation of the terminal subchains which are formed by the immobilization of chain segments at the contact with a confining interface. In the present paper we discuss the results of simulations done in order to gain an additional insight into the nature of this novel relaxation process. It turns out that the simulations of the chains as pinned random walks reproduce most of the essential features observed in the experiment.

We use molecular-dynamics computer simulations to study the relaxation dynamics of a confined simple liquid. Two types of confining walls are considered: A rough wall and a smooth wall. The simulation is set up in such a way that the static properties of the confined system are identical to the ones of the bulk. Nevertheless, we find that upon cooling the relaxation dynamics of the confined systems differ strongly from the one of the bulk. In particular, we find that close to the rough/smooth wall this dynamics is slowed down/accelerated by orders of magnitude. Using these results we are able to extract a dynamical length scale of the system and we show that this length shows an Arrhenius dependence.

We present the results of large-scale computer simulations in order to discuss the structural and dynamic properties of sodium silicate melts with the compositions (Na2O)2(SiO2) (NS2) and (Na2O)20(SiO2) (NS20). We show that, compared to silica (SiO2), these systems exhibit additional intermediate range order on intermediate length scales that stem from the tetrahedral network structure. By means of intermediate-scattering functions, we characterize the dynamics of sodium in the system under consideration. Whereas in NS2 the incoherent scattering functions for Na decay much faster to zero than the coherent ones for Na-Na, in NS20 this different behaviour of the incoherent and coherent functions is not very pronounced. On the other hand, the incoherent functions of the two systems share a very peculiar feature: their long-time decay can be described by a Kohlrausch law with a constant exponent beta for q > q(th) where q(th) is significantly below the location of the main peak in the static structure factor for the Na-Na correlations.

We present the results of Monte Carlo simulations of two different three-dimensional-Potts glass models with short-range random interactions. In the first model, a +/-J-distribution of the bonds is chosen, in the second model a Gaussian distribution. In both cases, the first two moments of the distribution are chosen to be J(0) = -1, DeltaJ = +1, so that no ferromagnetic ordering of the Potts spins can occur. We find that for all temperatures investigated the spin glass susceptibility remains finite, that the spin glass order parameter remains zero, and that the specific heat has only a smooth Schottky-like peak. These results can be understood quantitatively by considering small but independent clusters of spins. Hence, we have evidence that there is no static phase transition at any nonzero temperature. Consistent with these findings, only very minor size effects are observed, which implies that all correlation lengths of the models remain very short. We also compute for both models the time autocorrelation function C(t) of the Potts spins. While in the Gaussian model C(t) shows a smooth uniform decay, the correlator for the +/-J model has several distinct steps. These steps correspond to the breaking of bonds in small clusters of ferromagnetically coupled spins (dimers, trimers, etc). The relaxation times follow simple Arrhenius laws, with activation energies that are readily interpreted within the cluster picture, giving evidence that the system does not have a dynamic transition at a finite temperature. Hence we find that for the present models, all the transitions known for the mean-field version of the model are completely wiped out. Finally, we also determine the time auto-correlation functions of individual spins, and show that the system is dynamically very heterogeneous.