Molecular dynamics of a polymer in mixed solvent: atactic polystyrene in a mixture of cyclohexane and N,N-dimethylformamide.

Local structural and dynamic properties of atactic polystyrene in a mixed solvent of cyclohexane (CH) and N,N-dimethylformamide (DMF) have been investigated using molecular dynamics simulations. We measure local conformations in the polymer and classify them by distance and angle distribution histograms. End-to-end distances and structure factors are employed to describe the static structure of polystyrene chains. Polystyrene concentration, including 1.6%, 4.8%, and 14% (by weight), and solution temperatures of 300, 330, or 360 K are used to elucidate the concentration and temperature dependencies of the solvation by the two solvents. Both solvent molecules preferentially approach the phenyl rings. At lower temperatures, polystyrene dissolves more favorably in cyclohexane. With rising temperature DMF molecules approach more closely with the oxygen oriented toward the phenyl rings. Additionally, the global and segmental relaxation times of the chains decrease and the system becomes more homogeneous. The segmental and global dynamics of polystyrene show different concentration behaviors: the reorientation times of solvent molecules and segments of polystyrene increase with concentration while the global dynamics of polystyrene decelerates as the concentration is changed from 1.6% to 4.8% but accelerates when the concentration rises to 14%. We conclude that the change of concentration from 4.8% to 14% qualitatively marks the change from a dilute to a semidilute solution. The diffusion constants of the small molecules and corresponding activation energies have also been measured. Our simulation data are compared with available experimental results and we find a satisfactory agreement.