Intermediate Wavelength Composition Fluctuations in Diblock Copolymers:  Modification of the Diffusive Mode near the Ordering Transition

The relaxation of composition fluctuations of diblock copolymers in the intermediate wavevector range 0.08 < q/q* < 0.61 is investigated by photon correlation spectroscopy, thus bridging the q/q* much less than 1 and q/q* similar to 1 regimes (q* is the wavevector of the most probable composition fluctuations). Although away from the ordering transition (ODT) the modes of relaxation follow the behavior established for q/q* much less than 1, the enhanced thermodynamic interactions by approaching the ODT lead to a modification of the intensity characteristics of the polydispersity (slower) mode, which now increases with increasing q and shows a concentration dependence stronger than phi(1). The diffusive character of this process, however, is not affected, and it still reflects the self-diffusion of the chains. The characteristics of the internal mode remain unchanged. The findings can be accounted for by a theory recently developed for the dynamic structure factor of polydisperse entangled diblocks.