TRANSIENT ELECTRIC BIREFRINGENCE OF FLEXIBLE POLYMERS - ORIENTATION AND RELAXATION DYNAMICS

The orientation and relaxation dynamics of flexible polymers in an ele ctric field is analyzed by following the change in electric birefringe nce with time. The polymer chain is represented by two beads (dipole) connected by a Hookean spring and interacts with the electric field th rough a local induced dipole which is proportional to the end-to-end d istance between the beads. Equations of motion are written for the bea ds taking into account (a) the hydrodynamic drag force, (b) the Browni an force, (c) the spring force, and (d) the electric force acting on t he molecule. The electric field produces a strong anisotropic orientat ion of the polymer chain. Thus, equations are derived and analytically solved to yield the time dependence of the rise of birefringence in t he electric field, the relaxation of the birefringence from a nonequil ibrium state, and the change in the mean-squared end-to-end distance o f the molecule with time. The dynamics are found to be governed by the dimensionless number mu(o)E(2)/H, where mu(o) is a constant related t o the polarizability of the molecule, E is the electric field strength , and H the Hookean spring constant. A merit of this analysis lies in the fact that expressions for the time course of the birefringence ris e in the presence of an orienting field of strength E as well as relax ation in the presence of a reduced electric field, E(r) (E(r)<E) can b e readily derived; hence cases other than the field-free decay (E(r)=O ) can be interpreted. Moreover, for a biological system, a state of eq uilibrium with E(r)=O may never be reached. Finally, the field depende nce of the birefringence and the orientation and relaxation time const ants has been obtained using the model and comparison of the steady st ate and relaxation behavior of Hookean and finitely extensible nonline ar elastic (FENE) polymer chains has been carried out. (C) 1995 Americ an Institute of Physics.