Consistent and transferable coarse-grained model for semidilute polymer solutions in good solvent

Thermodynamics of star polymer solutions: A coarse-grained study

We consider a coarse-grained (CG) model with pairwise interactions, suitable to describe low-density solutions of star-branched polymers of functionality f. Each macromolecule is represented by a CG molecule with (f + 1) interaction sites, which captures the star topology. Potentials are obtained by requiring the CG model to reproduce a set of distribution functions computed in the microscopic model in the zero-density limit. Explicit results are given for f = 6, 12, and 40. We use the CG model to compute the osmotic equation of state of the solution for concentrations c such that Φ_p=c∕c^*≲1, where c^* is the overlap concentration. We also investigate in detail the phase diagram for f = 40, identifying the boundaries of the solid intermediate phase. Finally, we investigate how the polymer size changes with c. For Φ_p≲0.3, polymers become harder as f increases at fixed reduced concentration c∕c^*. On the other hand, for Φ_p≳0.3, polymers show the opposite behavior: At fixed Φ_p, the larger the value of f, the larger their size reduction is.

Polymer models with optimal good-solvent behavior

We consider three different continuum polymer models, which all depend on a tunable parameter r that determines the strength of the excluded-volume interactions. In the first model, chains are obtained by concatenating hard spherocylinders of height b and diameter rb (we call them thick self-avoiding chains). The other two models are generalizations of the tangent hard-sphere and of the Kremer-Grest models. We show that for a specific value [Formula: see text], all models show optimal behavior: asymptotic long-chain behavior is observed for relatively short chains. For [Formula: see text], instead, the behavior can be parametrized by using the two-parameter model, which also describes the thermal crossover close to the θ point. The bonds of the thick self-avoiding chains cannot cross each other, and therefore the model is suited for the investigation of topological properties and for dynamical studies. Such a model also provides a coarse-grained description of double-stranded DNA, so that we can use our results to discuss under which conditions DNA can be considered as a model good-solvent polymer.