The relaxation of the flow dichroism signal of native DNB molecules after rapid (-0.004 sec) stoppage of the flow has been studied in the narrow-channel flow dichroism apparatus previously described. The flow dichroism signal decays with a spectrum of relaxation times. The slowest relasation time accounts for about 0.40 of the total signal for shear gradients in the range of 100-300 see-1 and for about 0.25 of the total signal for shear gradients in the range of 5,000-20,000 sec-l. This longest relaxation time in dilute aqueous salt solution a t 25Β°C for T4-I)XA molecules is measured as 0.15 sec; in the molecnlar weight range from 12.5 X 1 0 6 to 12 X loE, the relaxation time varies as Al(1 G * " 1 ) .
The relaxation time is proportional to qj2' for large (sixfold) changes in solvent viscosity and for temperature changes between 25 and O'C, showing that the molecular relasation is hydrodynamically limited. An accurate analysis of the data at short times to determine the shorter relaxation times was not possible. A theoretical prediction based on the Zimm-Rouse theory of polymer dynamics for the longest relaxation time is in good agreement with the observed value, but the theory predicts that 87% of the total signal should decay with this relaxation time, whereas experimentally only a smaller fraction of the total signal does so. This discrepancy occurs because the theory is applicable to low hydrodynamic shear stress flows and the experiments were made a t higher shear stresses.