Stress, birefringence, and conformational relaxation of an initially straight stiff bead-rod polymer.

The stress and optical relaxation of an initially straight stiff polymer chain are studied through Brownian dynamics simulations (based on a semiflexible bead-rod model) covering a broad range of time scales and polymer lengths. The strong stress component sigma11 (where "1" is the direction of the original alignment) is shown to be associated with the chain's longitudinal relaxation while the weak stress component sigma22 = sigma33 is shown to depend on the chain's transverse relaxation. The two independent stress components follow a different relaxation; this anisotropy is shown to result from the participation of the different relaxation modes in the transverse direction. The chain's optical relaxation is shown to be affected by the longitudinal dynamics only. The early relaxation of the strong stress component sigma11 and that of the chain's optical properties constitute a universal behavior--i.e., valid for any stiffness of the bead-rod chain, since at the early times the bending forces do not affect the longitudinal dynamics. Based on the knowledge of the physical mechanism and the chain's conformational behavior, we predict and explain the polymer stress and optical relaxation. A nonlinear stress-optic law (valid for any time and chain stiffness) is derived based on the identified relation of the chain configuration with the optical properties and the polymer stress. A coarse-grain model describing extended semiflexible bead road chains is also derived.