DYNAMIC LIGHT-SCATTERING FROM BLOCK-COPOLYMER MELTS NEAR THE ORDER-DISORDER TRANSITION

Dynamic light scattering measurements have been performed on four symm etric diblock copolymer melts, as a function of temperature, both abov e and below the order-disorder transition (ODT). The materials were po ly(ethylenepropylene)-poly(ethylethylene) (PEP-PEE) with M(w) = 5.0 x 10(4), poly(vinylcyclohexane) (PVCH)-PEE with M(w) = 5.3 x 10(4), PVCH -polyethylene (PE) with M(w) = 1.6 x 10(4), and PE-PEE with M(w) = 2.7 x 10(4). Up to four relaxation modes were resolved. In all cases, the correlation functions exhibited a very strong, very slow diffusive mo de, similar to that previously observed in polymeric and small-molecul e glass formers and attributed to long-range density fluctuations. Via the Kawasaki-Stokes-Einstein relation, a correlation length or cluste r size, xi was associated with this process. Above the ODT, xi was on the order of 100 nm and independent of temperature. However, below the ODT, xi apparently increased by at least 2 orders of magnitude for PV CH-PE and PE-PEE, while remaining nearly independent of temperature fo r the other two copolymers. For PEP-PEE, the angle dependence of the s cattered intensity also reflected a correlation length of 100 nm. The cluster mode was subtracted from the correlation functions, and the re sidual decays were reanalyzed by Laplace inversion. Two other modes, t he diffusive heterogeneity mode and the structural internal mode, were then resolved, in accordance with theory. For the PEP-PEE sample, the heterogeneity diffusion coefficient was in quantitative agreement wit h the self-diffusion coefficient as measured by forced Rayleigh scatte ring, and at least in the disordered state, the relaxation time of the internal mode was in quantitative agreement with the longest relaxati on time determined by rheological measurements. For this sample, a fou rth, diffusive mode was also apparent, with a time scale intermediate between the longest relaxation time and the translational diffusion of the chains. The temperature dependence of this mode was weaker than t hat of the viscosity or the chain diffusion, and its specific origin i s unclear; however, it appears to be correlated with the frequency at which time-temperature superposition breaks down in the rheological pr operties.