Dynamics of water in polyelectrolyte multilayers: restricted diffusion and cross-relaxation

Understanding water diffusion in polyelectrolyte multilayers remains a challenge in polymer science. Prior research has shown that water diffusion in porous materials often follows Gaussian behavior. However, the structure of polyelectrolyte multilayers introduces complexity. Established methods like pulsed field gradient NMR have been used to study diffusion in such systems. Yet, the interpretation of diffusion data in these multilayers is not straightforward. No prior work had resolved the role of cross-relaxation effects in diffusion measurements. That uncertainty drove the need for more precise models. This gap motivated the investigation of non-Gaussian diffusion in PEMs. The study aimed to clarify how cross-relaxation affects pore size estimation.

The aim of this work was to assess the diffusion behavior of water in polyelectrolyte multilayers. The specific problem addressed was the discrepancy between observed diffusion patterns and Gaussian models. The motivation stemmed from the need to better understand the structure of these multilayers. The researchers sought to determine if cross-relaxation influences diffusion measurements. They also aimed to quantify pore sizes accurately. This study focused on correcting pore size estimates affected by cross-relaxation. The goal was to detect structural heterogeneities in PEMs. The findings could improve the interpretation of NMR data in similar systems.

The study used pulsed field gradient diffusion NMR to measure water diffusion in PEMs. The mean-square displacement was calculated to assess diffusion behavior. Goldman-Shen experiments were employed to investigate cross-relaxation effects. These experiments involved measuring diffusion echo decays under varying conditions. The data was analyzed to determine the impact of cross-relaxation on pore size estimates. A model of restricted diffusion was applied to extract pore sizes. Surface morphology was examined to correlate with diffusion findings. The results were compared to theoretical predictions for Gaussian diffusion.

The mean-square displacement showed non-Gaussian behavior, indicating restricted diffusion. Initial pore size estimates were several micrometers in size. However, these estimates were found to be overestimated due to cross-relaxation effects. After correction, pore sizes were approximately 4 micrometers. This suggests the presence of low-density polymer domains. These domains allow for faster water diffusion compared to the surrounding matrix. Surface morphology confirmed the existence of such heterogeneous regions. The study demonstrated that cross-relaxation significantly affects diffusion measurements.

The authors concluded that water diffusion in PEMs is restricted and non-Gaussian. They proposed that cross-relaxation must be considered when interpreting diffusion data. Corrected pore sizes of about 4 micrometers reflect structural heterogeneities. These heterogeneities arise from PEM preparation under high salt conditions. The study suggests that cross-relaxation rates influence diffusion echo decays. The corrected pore sizes align with surface morphology observations. The findings imply that structural domains affect diffusion behavior. The authors emphasized the importance of accounting for cross-relaxation in future studies.