Positron annihilation in cardo-based polymer membranes.

Area of Science:

• Materials Science
• Polymer Chemistry
• Condensed Matter Physics

Background:

• Positron annihilation lifetime spectroscopy (PALS) is a sensitive probe of free volume in polymers.
• Cardo polymers, featuring bulky pendant groups, exhibit unique free volume characteristics.
• Understanding free volume is crucial for predicting gas transport properties in polymers.

Purpose of the Study:

• To investigate the formation and behavior of positronium (Ps) in cardo polyimides and polysulfones using PALS.
• To correlate positronium lifetimes with polymer structure and free volume.
• To explore the relationship between free volume and gas diffusion (O2, N2) in these polymers.

Main Methods:

• Positron annihilation lifetime spectroscopy (PALS) was employed.
• A series of bis(aniline)fluorene and bis(xylidine)fluorene-based cardo polyimides and bis(phenol)fluorene-based polysulfone membranes were synthesized and analyzed.
• Gas diffusion coefficients (O2, N2) were measured.

Main Results:

• Favorable positronium (Ps) formation was observed in cardo polyimides with the 6FDA moiety and in bis(phenol)fluorene-based cardo polysulfone.
• Minimal Ps formation occurred in polyimides containing PMDA and BTDA moieties.
• An anomalous shortening of ortho-positronium (o-Ps) lifetime with increasing temperature was noted in a specific polyimide, attributed to o-Ps chemical reactions.
• A correlation between hole size (V(h)) and gas diffusion coefficients was discussed for polymers showing favorable Ps formation, based on free volume theory.

Conclusions:

• The presence of the 6FDA moiety and cardo structures significantly influences Ps formation and free volume in polymers.
• o-Ps lifetime is a valuable indicator of free volume characteristics in these complex polymer systems.
• o-Ps tends to probe larger holes in rigid chain polymers with wide hole size distributions, potentially deviating from correlations observed in simpler polymers.