Structure and Dynamics at the solid-liquid interface

Max Wolff (Chair for solid-state physics/EP IV, Ruhr-University Bochum, Germany)

Understanding the self organization of molecules on the atomistic scale becomes more and more important as the miniaturization down to nano-devices proceeds. Neutron scattering can provide this particular information as the wave-length and energy of the probe allows addressing atomic or molecular distances and relaxation times from micro- to picoseconds. Moreover the high penetration power for engineering materials makes the investigation of samples in complex geometries, such as buried interfaces or shear cells, possible. Shear or an interface may introduce self organization to complex polymer systems.

The presentation will focus on the near surface crystallization of polymer micelles. Our samples are three block copolymers called Pluronics and tending to agglomerate into micelles when solved in water. Solid surfaces with different degrees of hydrophobicity were employed as solid substrates used as wall materials in the experiments.

The crystallization process of the polymer micelles close to a solid interface was investigated in great detail by neutron reflectivity, grazing incident small angle scattering and spectroscopy. The most important results can be summarized as follows:

1. We find a preferred crystallization of the micelles close to an attractive interface (with respect to the micelle shell), whereas crystallization is suppressed close to a repulsive interface.

2. The epitaxial growth of the crystallites starts at well separated germs at the solid interface resulting first in crystallite sizes of several microns. Deeper in the crystalline phase different crystallites start to interpenetrate resulting a. in a reorientation (texture) of the crystallites but on the other hand b. in a reduction of the correlation length parallel to the interface due to strain induced changes of the form factor of the micelles and of the rotating crystallites.

3. For low micelle stabilities and high shear rates the correlations can become enhanced for the attractive interface while they are further disappearing for the repulsive one as defects formed at the interface become dissolved. This results in a reduced diffusivity of polymer monomers along the shear gradient for the attractive interface whereas for the repulsive interface the diffusivity remains isotropic. For high micelle stabilities and low shear rates the influence of the interface on the crystallization is reduced.

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