To investigate the behaviors of polymers on the local scale, atomistic simulations are required, and it would be highly desirable to have a multiscale model which can include all relevant interactions. Polymer dynamics of longer chains or higher concentrations cases can only be captured by a mesoscale model. We investigate the polystyrene dynamical behaviors on multiple length scales and different environments.

Solvents significantly alter polymer properties and can lead to favorable or unfavorable changes. We investigated the liquid structure and dynamics of a mixed solvent of cyclohexane and N, N-dimethylformamide molecules in the neighborhood of atactic polystyrene oligomers¹. We found consecutive side rings to be perpendicular at local scales and to parallel each other due to the quadrupolar interaction at slightly larger scales. Polystyrene side rings are the primary targets for the solvents with a strong preference to cyclohexane over N, N-dimethylformamide. Increasing temperatures speeds up the movement of all the small molecules and increasing concentrations slows down the reorientation process.

Some phenomena, for example, entanglement or phase separation behaviors, occur on length scales not accessible by a fully atomistic simulation. We build a mesoscale polystyrene model based on the atomistic simulation of pure 48 chains of polystyrene at chain length of 15 monomers². We group the 16 atoms of a monomer together as a superatom and its center has been placed on the carbons connecting the backbone with the side ring³. “Iterative Boltzmann Inversion is used to reproduce the structure by means of radial distribution functions. Simulation results provide an extensive description of the transition from Rouse motion to reptation behaviors. Our analysis provides persistent evidence for the conclusion that the entanglement length of this coarse-grained PS model is about 85 monomers, which is not too far below the experimental value of about 130 monomers.

We elucidate the possibility to optimize a coarse-grained model for a blend of cis-polyisoprene and polystyrene as a test case⁴. Experimentally, this system is known to be miscible at short chain lengths, and demixing is observed at longer chain lengths. The generalization of Iterative Boltzmann Inversion to binary blends requires optimizing three potentials from the corresponding distribution functions: polyisoprene-polyisoprene, polystyren-polystyrene, and polyisoprene-polystyrene. The iteration of blends is technically more demanding in that mapping takes into account the effects of three sets of highly correlated potentials. It is effective to start with those potentials that are lease affected by changes in the others. We started the iteration of polyisoprene-polyisoprene and polystyrene-polystyrene pairs first as they are relatively independent of each other compared to the degree to which they interdepend on the polyisoprene-polystyrene potential. Using our mesoscale model, we can embark now on a study of the phase behavior of the blend. The phase separation sets in at around seven monomers. Various morphologies of lamella, cylinder, and sphere are observed, which are associated with equi-mass, medium unbalanced, and extreme concentration ratios⁵.  

Reference:

1. Qi Sun and Roland Faller J Phys Chem B 109(33), 15714, 2005.
2. Qi Sun and Roland Faller Macromolecules 39, 812, 2006.
3. Qi Sun and Roland Faller Comp & Chem Eng. 29, 2380, 2005.
4. Qi Sun and Roland Faller J Chem Theor Comp, 2006.
5. Qi Sun and R. Faller J Am Chem Soc in preparation.