In dilute water solutions of a PEO-PPO-PEO triblock copolymer, polarized dynamic light scattering measurement data reveal a sphere-to-rod transition in the shape of the micelles at a temperature close to 70 OC. This transition is anticipated from theory and was also inferred from earlier small-angle neutron scattering measurements. Regularized inverse Laplace transformation of the polarized dynamic light scattering data typically showed a fast rotational diffusion component of low amplitude in addition to a major slow component from translational diffusion. Good agreement was obtained between the rotational diffusion coefficient estimated from the polarized measurements and the value measured directly using depolarized dynamic light scattering. From the rotational diffusion coefficient a length of the P-85 rodlike micelle can be estimated, via Broersma's expressions for the rigid rod, as 1050 A in the dilute limit. This corresponds to a 1engtWdiameter ratio of 7. Intensity autocorrelation functions were modeled on the measured dynamic light scattering data using Pecora's theoretical expressions for the time correlation function in terms of the dynamic form factors. Good agreement between theory and experiment was found at the lowest concentrations. Concentrations c > 0.05 7% correspond to the semidilute regime where interparticle friction effects preclude determination of the length.

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This communication describes dynamic light scattering measurements on three low molar mass symmetric polystyrene–polybutadiene diblock copolymer melts. Above but close to the order–disorder transition, four modes contribute to the time-dependent correlation function. In the depolarized geometry there is a very fast, broad relaxation typifying the segmental reorientational dynamics and a slower, single-exponential mode related to chain orientation and stretching. In the polarized experiments two additional processes are seen: the heterogeneity mode related to the self-diffusion of individual copolymer molecules and a slow mode representing the translational diffusion of large (≂100 nm) clusters. Just below the order–disorder transition (ODT), the slowest mode is shifted to very long times. The heterogeneity mode gives rise to depolarized scattering below the ODT because of the anisotropy induced by the lamellar structure. With the lowest molar mass copolymer above the ODT, it could be seen that the amplitude of the mode related to chain orientation and stretching decreases the deeper the sample is in the disordered state. A new subtraction technique was used for data analysis.