Anomalous diffusion in anisotropic media

The structure and dynamics of three 'soft-matter' systems was investigated by pulsed field gradient diffusion NMR and complementary deuterium line shape analysis. Restricted diffusion was studied in two lyotropic lamellar systems, the first consisting of crosslinked flat lamellae in anisotropic hydrogels and the second consisting of spherically bent lamellae in multilamellar vesicles. In the third system, a swollen PDMS network, obstructed diffusion was investigated in the fractal topology. <br> <br>Anisotropic Hydrogels: The structure and ordering of macroscopically oriented anisotropic hydrogels was investigated on different length scales. Diffusion NMR proved the existence of defects in the lamellae on a mesoscopic scale and gave an estimate of the minimal extension of homogeneous domains. Temperature dependent obstruction factors for diffusion parallel to the layer normal showed an anomalous step at around 312 - 314 K. This was interpreted as the generation of further defects and microscopic cracks due to elastic forces from the network. The network tends to restore its original shape of a coil and becomes the dominating factor in the coupled system of mesophase and polymer network above this temperature range. The anisotropy of diffusion for D2O on the mesoscopic level was determined to be 10 : 1. For the diffusion of a dye the anisotropy decreased to 5.3 : 1 on the macroscopic level. The comparison of two hydrogels with different content of water showed higher order and fewer structural defects in the system with lower content of water. <br> <br>Multilamellar Vesicles: The dynamics of D2O in the shear-induced multilamellar vesicle (MLV) phase was investigated using the system C10E3/D2O as an example. An empirical relationship between vesicle size and the D2O single pulse line shape was established. Deuterium line shapes of the solid and stimulated echo showed a splitting at long evolution delays, which was identified as the 90° singularity of the motionally averaged Pake spectrum of the normal, macroscopically disordered lamellar phase. The stimulated echo line shape is dominated by rotational motions with correlation times in the order of the evolution delays. Pulsed gradient diffusion measurements showed a strong dependence of the echo decay on the diffusion time. This was attributed to the restricted diffusion along the spherical lamellae in the MLVs. A theoretical model for diffusion in an idealized MLV structure was tested for its ability to describe the measured echo decay. The observed differences in comparison to the measured data were mainly attributed to three factors: first the incomplete description of the line shape in the stimulated echo, second the neglect of diffusion during the finite gradient duration and third the diffusion through the layers. <br> <br>PDMS Model Networks: PDMS model networks were investigated by pulsed gradient diffusion NMR in order to detect anomalous diffusional behavior as a test for recently developed theories on inhomogeneous network swelling. Diffusion time dependent measurements were carried out on different samples. No evidence for inhomogeneous swelling of the networks was found. Neither the shape of the echo decay, which is Gaussian, nor diffusion time dependencies nor variation of the degree of swelling showed a significant effect from which obstructed diffusion could have been deduced. Even in a deliberately inhomogeneously synthesized bimodal networks, where heterogeneities were expected, no such evidence was found.

Author dc:creator

• Kleinschmidt, Felix

Contributors dc:contributor

• Schmidt, Claudia

• multilamellare Vesikel
• heterogenes PDMS Netzwerk
• scherinduzierte Struktur
• Scherung
• Anomale Diffusion
• multilamellar vesicle
• hydrogel
• heterogeneous network
• pulsed field gradient
• deuterium NMR

Repository record source_url

https://freidok.uni-freiburg.de/data/1808

OAI identifier oai:identifier

oai:freidok.uni-freiburg.de:1808