Model for Assembly and Gelation of Four-Armed DNA Dendrimers

We introduce and numerically study a model designed to mimic the bulk behavior of a system composed of single-stranded DNA dendrimers. Complementarity of the base sequences of different strands results in the formation of strong cooperative intermolecular links. We find that in an extremely narrow temperature range the system forms a large-scale, low-density disordered network via a thermo-reversible gel transition. By controlling the strand length, the gel transition temperature can be made arbitrarily close to the percolation transition, in contrast with recent model systems of physical gelation. This study helps the understanding of self-assembly in this class of new biomaterials and provides an excellent bridge between physical and chemical gels

Interpenetration as a Mechanism for Liquid-Liquid Phase Transitions

We study simple lattice systems to demonstrate the influence of interpenetrating bond networks on phase behavior. We promote interpenetration by using a Hamiltonian with a weakly repulsive interaction with nearest neighbors and an attractive interaction with second-nearest neighbors. In this way, bond networks will form between second-nearest neighbors, allowing for two (locally) distinct networks to form. We obtain the phase behavior from analytic solution in the mean-field approximation and exact solution on the Bethe lattice. We compare these results with exact numerical results for the phase behavior from grand canonical Monte Carlo simulations on square, cubic, and tetrahedral lattices. All results show that these simple systems exhibit rich phase diagrams with two fluid-fluid critical points and three thermodynamically distinct phases. We also consider including third-nearest-neighbor interactions, which give rise to a phase diagram with four critical points and five thermodynamically distinct phases. Thus the interpenetration mechanism provides a simple route to generate multiple liquid phases in single-component systems, such as hypothesized in water and observed in several model and experimental systems. Additionally, interpenetration of many such networks appears plausible in a recently considered material made from nanoparticles functionalized by single strands of DNA.Comment: 12 pages, 9 figures, submitted to Phys. Rev.

Modifying Fragility and Collective Motion in Polymer Melts with Nanoparticles

We investigate the impact of nanoparticles (NP) on the fragility and cooperative string-like motion in a model glass-forming polymer melt by molecular dynamics simulation. The NP cause significant changes to both the fragility and the average length of string-like motion, where the effect depends on the NP-polymer interaction and the NP concentration. We interpret these changes via the Adam-Gibbs (AG) theory, assuming the strings can be identified with the "cooperatively rearranging regions" of AG. Our findings indicate fragility is primarily a measure of the temperature dependence of the cooperativity of molecular motion.Comment: To appear in Physical Review Letter

Dynamics of simulated water under pressure

We present molecular dynamics simulations of the SPC/E model of water to probe the dynamic properties at temperatures from 350 K down to 190 K and pressures from 2.5GPa (25kbar) down to -300MPa (-3kbar). We compare our results with those obtained experimentally, both of which show a diffusivity maximum as a function of pressure. We find that our simulation results are consistent with the predictions of the mode-coupling theory (MCT) for the dynamics of weakly supercooled liquids -- strongly supporting the hypothesis that the apparent divergences of {\it dynamic} properties observed experimentally may be independent of a possible thermodynamic singularity at low temperature. The dramatic change in water's dynamic and structural properties as a function of pressure allows us to confirm the predictions of MCT over a much broader range of the von Schweidler exponent values than has been studied for simple atomic liquids. We also show how structural changes are reflected in the wave-vector dependence of dynamic properties of the liquid along a path of nearly constant diffusivity. For temperatures below the crossover temperature of MCT (where the predictions of MCT are expected to fail), we find tentative evidence for a crossover of the temperature dependence of the diffusivity from power-law to Arrhenius behavior, with an activation energy typical of a strong liquid.Comment: 14 pages, 15 figure