Diffusion processes in homogeneous and phase-separated binary fluid mixtures

Area of Science:

• Materials Science
• Physical Chemistry
• Polymer Physics

Background:

• Dynamically asymmetric binary fluid mixtures, such as polystyrene (PS) and 5CB liquid crystals, exhibit complex phase behavior.
• Understanding diffusion dynamics is crucial for predicting material properties and morphology evolution.

Purpose of the Study:

• To investigate diffusion processes in PS/5CB mixtures near the phase-separation temperature.
• To elucidate the relationship between phase separation, polymer chain mobility, and dynamic heterogeneity.

Main Methods:

• Pulsed-field gradient spin echo Nuclear Magnetic Resonance (NMR) spectroscopy was employed.
• Diffusion coefficients were measured across a range of temperatures, including slow cooling and deep quenches.
• Echo attenuation curves were modeled to estimate characteristic length scales.

Main Results:

• Two distinct self-diffusion coefficients (Dfast and Dslow) were observed, corresponding to molecules in the polymer matrix and phase-separated domains, respectively.
• Simultaneous phase separation and loss of polymer chain mobility at the glass transition temperature (Tg) were noted.
• The temperature dependence of Dfast followed Arrhenius behavior under slow cooling and Vogel-Fulcher-Tamman-Hesse law under deep quenches, indicating dynamic heterogeneities.

Conclusions:

• The study demonstrates significant dynamic heterogeneities in PS/5CB mixtures below the upper critical solution temperature (UCST).
• Elasticity plays a critical role in sample morphology during phase separation.
• NMR is effective in characterizing diffusion and estimating length scales in complex fluid mixtures.