Self-diffusion investigations on a series of PEP-PDMS diblock copolymers with different morphologies by pulsed field gradient NMR

Abstract

We report on temperature-dependent self-diffusion measurements of compositionally different and non-entangled poly(ethylene-co-propylene)-b-poly(dimethylsiloxane) PEP-PDMS diblock copolymers in the melt above and below the order-to-disorder transition temperature. Depending on the dimensionality of the structures (lamellar: two-dimensional, hexagonal: one-dimensional, spherical: zero-dimensional) characteristic diffusion mechanisms are observed: in the two- and one-dimensional cases anisotropic single-chain diffusion proceeds along the geometries, i.e. parallel and perpendicular to the interfaces. The shape of the echo attenuations provides information on the anisotropy of motion. Motion perpendicular to the interfaces is markedly suppressed. In the spherical morphology composed of micelle-like structures, however, the diblock copolymers are locally confined to the micelles, and only the collective diffusion of clusters of micelles is observed. In none of the cases is a dramatic change of the self-diffusivity observed at the order-to-disorder transition temperature. The self-diffusion in the disordered state is influenced by concentration fluctuations over a broad temperature range.

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