Calculations of the conformational preferences of isolated single chains of poly(2,6-dimethylphenylene oxide) (PPO) and bisphenol-A polycarbonate (PC) predict that the rings of both chains are nearly free rotors at room temperature. However, experimental dipolar rotational spin-echo 13C NMR shows that in the glass, the rings of PPO execute only small-amplitude motions while those of PC undergo primarily 180°r ing flips (a hindered rotation) superimposed on some wiggles. Geometrical considerations of the dense packing of chains in the glass suggest that the rings of adjacent chains block rotational freedom. We propose that the mobility of the PC main chain results in lattice distortions which allow ring flips not permitted by the stiffer PPO main chain.

We use molecular dynamics to study the nucleation of AgBr in water. After first testing our Born–Mayer–Huggins potentials for Ag+ and Br− by looking at bulk AgBr and at AgBr clusters in vacuo, we consider small numbers of Ag+ and Br− ions immersed in water. The system shows the expected qualitative features of nucleation form solution, including a critical cluster size that decreases with increasing concentration. However, we find that for cluster sizes at least as large as Ag18Br18, the most stable cluster is disordered. This is in stark contrast to clusters in vacuo where clusters as small as Ag4Br4 from ordered fragments of the lattice. These results lend some support to the conjecture that nucleation of crystals from solution is a two-stage process with the first stage consisting of the formation of disordered clusters of solute and the second stage involving the nucleation of a crystal from this solute “melt.”

Models of polymers can generally be divided into two classes: (a) flexible, wherein bond lengths and valence angles are constrained to nearly constant values by strong, harmonic potentials, and (b) rigid, wherein the constraints are geometric, i.e., fixed bond lengths and angles. In general, the statistical mechanics of such systems differ. By introduction of a compensating potential based on the metric determinant of the unconstrained coordinates (the Fixman potential), the rigid model can be made to reproduce the equilibrium flexible results. It is not clear whether the corresponding dynamics are also reproduced. The purpose of the present work is to investigate the effect of the Fixman potential on dynamics. We have performed Brownian dynamics simulations of three models of a n-butane-like molecule: flexible, rigid, and rigid-plus-Fixman potential.Results show that for certain autocorrelation functions of the dihedral angle, there is a definite, though small, difference in dynamics between the rigid and flexible models in the low-friction limit. At high friction the difference has decreased and the dynamics are essentially the same. The effect of the Fixman potential is minimal in both cases.

We present a simple minimal model which allows numerical and analytical study of a glass transition. This is a model of rigid rods with fixed centers of rotation. phenomena and that is simple enough to allow numerical and analytical study. (The best kno,vn example of such a model is the Ising model in magnetics.) An important step towards creating a minimal model for glass the transition was recently made by Edwards and E,>ans [3j, (see also Edwards and Vilgis [4]). They proposed a model of randomly placed and randomly oriented rigid rods. This model is simple enough to be treated analytically. The rotational mobility of the rods is blocked by the presence of other rods. The only ,vay rods are allowed to move is along their longest dimension (along the direction of their orientation). The mobilityof each rod depends on the mobility of the rods that can block