Micellar Sphere-to-Rod Transition in an Aqueous Triblock Copolymer System. A Dynamic Light Scattering Study of Translational and Rotational Diffusion

Abstract

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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