The bulk dynamics of a compositionally asymmetric diblock copolymer studied using dynamic light scattering

We have studied the bulk dynamics of a compositionally asymmetric poly(ethy lene propylene)-poly(dimtethylsiloxane) (PEP-PDMS) diblock copolymer in a l arge temperature range both in the ordered and in the disordered state. The volume fraction of the PEP block is 0.22. Apart from the disordered state, the sample shows three ordered morphologies. Using dynamic light scatterin g, rye have investigated the dynamics in all four phases and combined these results with those obtained using pulsed field gradient NMR. In the disord ered state, me find apart from the slow cluster mode-the heterogeneity mode related to the self-diffusion of single chains. The relaxation time tau of this mode, reduced by temperature and the zero-shear viscosity eta, log(ta uT/eta), increases with temperature. In the cubic phase right below the ODT temperature, we observed two diffusive processes, and we attribute the fas ter one to the mutual diffusion of micelles and block copolymers not bound to micelles ("free chains") through the PDMS matrix. The slower mode may ei ther be due to the mutual diffusion of free chains and chains bound to PEP micelles or to the cooperative diffusion of micellar aggregates. In the non -cubic ordered state at intermediate temperatures, an additional weak diffu sive mode is observed. The low-temperature ordered state is body-centered c ubic, and here, only the mutual diffusion of micelles and free chains lies in our experimental time window.