Dynamic scattering of semirigid macromolecules.

The polarized (VV) and depolarized (VH) dynamic light scattering functions of dilute solutions of semirigid macromolecules are calculated assuming that the scattering wave vector [under q] is high compared to the chain Kuhn segment l : ql>>1. The terminal relaxation rate is Gamma proportional to q8/3. Dynamics of both uniaxial and biaxial wormlike macromolecules (filaments) are considered. Biaxial macromolecules are characterized by two persistence lengths proportional to elastic constants for bending in two perpendicular directions (easy and hard bending). We showed that biaxiality may result in a significant broadening of the relaxation spectrum. A nonmonotonous q dependence of the depolarized scattering intensity is predicted. Analyzing the short-time behavior of the dynamic structure factor for t<<1/Gamma, we show that it is characterized by two additional characteristic times: t perpendicular proportional to q(-4) and t parallel proportional to q(-8) reflecting the transverse and the longitudinal dynamics of polymer chains. The longitudinal motions (along the chain contour) increase the initial relaxation rate of the structure factor by a factor of 2. The longitudinal contribution to the dynamic structure factor is significant even for t>>t parallel.