| In this work, we have studied the equilibrium dynamics and the ordering kinetics of an asymmetric poly(styrene-block-isoprene) diblock copolymer melt using a combination of rheology, Small Angle X-ray Scattering (SAXS) and X-ray Photon Correlation Spectroscopy (XPCS). The starting point of any dynamic study has to be the determination of phase behavior. Using a combination of optical birefringence under both quiescent and shear-flow conditions as well as high-resolution SAXS measurements, we determined that the system undergoes an order-to-disorder transition at 70°C from a hexagonally packed cylinder phase to a disordered phase containing micelles.; We then proceed to explore the relationship between structural and stress relaxation dynamics in the disordered micellar state using XPCS and rheology. The experimentally determined characteristic times for structural relaxation are 1-2 orders of magnitude larger than those for stress relaxation. In order to shed light on the interplay between microscopic relaxation processes and macroscopic stress relaxation, we employed the Fredrickson-Larson theory. We were able to attribute the rheologically measured terminal relaxation time to the relaxation of spontaneous concentration fluctuations in the disordered bulk and the structural relaxation time measured by XPCS to the diffusion of intact micelles.; With the help of the understanding gleaned from studying the equilibrium dynamics in the disordered state, we proceeded to study the ordering kinetics using a combination of time-resolved SAXS, XPCS and rheology. When the polymer is quenched close to the order-to-disorder transition temperature, nucleation barriers preclude the formation of an ordered phase, and no changes in the structure factor or the shear moduli are observed. However, the microscopic relaxation time, measured by XPCS, increases with time and levels off at a time scale that is consistent with that expected for diffusing micelles in the metastable disordered fluid. Time-resolved rheology was used to determine the various time scales involved in the nucleation of the ordered phase when the polymer was quenched deep into the two-phase region. |
