Athermal Fluctuations Research Articles

Equation of state for macromolecules of variable flexibility in good solvents: A comparison of techniques for Monte Carlo simulations of lattice models

The osmotic equation of state for the athermal bond fluctuation model on the simple cubic lattice is obtained from extensive Monte Carlo simulations. For short macromolecules (chain length N=20 ) we study the influence of various choices for the chain stiffness on the equation of state. Three techniques are applied and compared in order to critically assess their efficiency and accuracy: the "repulsive wall" method, the thermodynamic integration method (which rests on the feasibility of simulations in the grand canonical ensemble), and the recently advocated sedimentation equilibrium method, which records the density profile in an external (e.g., gravitationlike) field and infers, via a local density approximation, the equation of state from the hydrostatic equilibrium condition. We confirm the conclusion that the latter technique is far more efficient than the repulsive wall method, but we find that the thermodynamic integration method is similarly efficient as the sedimentation equilibrium method. For very stiff chains the onset of nematic order enforces the formation of an isotropic-nematic interface in the sedimentation equilibrium method leading to strong rounding effects and deviations from the true equation of state in the transition regime.

Correlation effects and entropy-driven phase separation in athermal polymer blends

Polymer reference interaction site model (PRISM) theory with the Percus–Yevick closure approximation has been applied to investigate the intermolecular correlations, effective chi-parameters, and spinodal phase separation of athermal binary polymer blends. These model mixtures are composed of structurally asymmetric semiflexible chains interacting via purely hard core potentials. In strong contrast to PRISM predictions for the idealized Gaussian thread model, nonlocal entropy-driven phase separation is predicted under certain conditions. By examining the intermolecular pair correlation functions we identify the physical driving force as local packing frustration associated with the different backbone stiffnesses of the blend components, which is propagated to macromolecular scales by chain connectivity and persistence. These entropic packing effects display many nonuniversal features including a sensitive dependence on chain length, blend composition, monomer volume difference, and both the mean and relative aspect ratios of the polymers. The sensitivity of the athermal blend fluctuation phenomena to local chain rigidity and nonzero liquid compressibility is emphasized. For model parameters characteristic of most flexible polymers of experimental interest the athermal packing frustration effect is found to generate only a small amount of thermodynamic incompatibility. Perturbative estimates of the enthalpic chi-parameters associated with (local) structural asymmetries suggest they are much more important than the purely entropic contribution for hydrocarbon alloys such as the polyolefins. Recent incompressible field theories for athermal conformationally asymmetric blends are derived within the liquid state integral equation framework by identifying an alternative, mean-field-like closure approximation coupled with the imposition of a zero compressibility constraint.

Ising mean field model for antiferromagnets in the presence of spin fluctuations. Scaling approach for the non-mean field case and role of defects

Starting from the case of a single impurity we built a very simple phenomenological model for Ising antiferromagnets in the presence of spin fluctuations at zero kelvin and at finite temperatures. A qualitative scaling of critical athermal (quantum) fluctuations is enough to understand many features of the low temperature behaviour of systems such as heavy fermions, 2d antiferromagnets, unstable transition metal magnetism. In particular, although several energy scales are considered in these systems (fluctuation temperature ( T f), exchange energy between different sublattices, etc.) we show that only a single scale is relevant which is given by T f− T c f (critical energy scale).