Poly­(ethylene oxide), PEO, based solid polymer electrolyte with different loadings of a lithium salt, lithium bis­(trifluoromethanesulfonyl)­imide (LiTFSI), has been investigated to study the role of PEO–Li cross-linking on segmental dynamics and free volume structure of PEO which consequently determine thermal, mechanical, and ion conduction properties of the electrolyte. In order to investigate the interrelation between segmental dynamics, free volume structure, and ion conduction mechanism, broadband dielectric spectroscopy and positron annihilation spectroscopy have been employed. The ion conduction process in the polymer electrolyte has been explained according to Almond–West formalism considering two different universalities dominating at different temperature or frequency regimes. Ionic conductivity was observed to increase in a nonlinear trend with salt loading, confirming the additional role of the ion diffusion process. The present study has shown that segmental dynamics and free volume structure of PEO-electrolyte which primarily govern the ion diffusion process are interrelated. These have been invoked to explain the observed variations in ionic conductivity, crystallinity, ductility, and thermal stability of PEO–LiTFSI electrolytes.

Crystal downsizing of zeolitic imidazolate framework-8 (ZIF-8) has been shown to affect the flexibility of its framework, leading to the occurrence of “gate-opening” phenomenon at higher applied gas pressures compared to larger size crystals. In the present study, we have investigated the crystal size-dependent pore architecture of crystalline desolvated ZIF-8 samples which is expected to play the most deterministic role in its gas adsorption behavior. In order to avoid the pressurization or gas molecule–ZIF interaction, positron annihilation spectroscopy has been used to investigate the pores’ sizes, pores’ interconnectivity, and pores’ elemental surface characteristics of desolvated ZIF-8 samples. The studied ZIF-8 samples (size ∼14 nm −1.4 μm) have been synthesized at room temperature and systematically characterized for their phase purity, structural integrity, morphology, and thermal stability using complementary characterization techniques. Positronium annihilation from inter-crystal voids as a result of its diffusion through the crystals indicates that the pore network is highly interconnected in smaller size crystal samples. However, some interconnections from deep inside of larger crystals to the outer surface are observed to be blocked due to the imperfections produced in pore networking during crystal growth. The present study confirms that crystal downsizing results in modifications of average pore size corresponding to the aperture and central cavity. The average aperture size in nanometer size ZIF-8 crystals (size ∼14 and 45 nm) was measured to be smaller than the perfect lattice aperture (0.34 nm). We attribute this smaller aperture size in ZIF-8 nanocrystals to the occurrence of “gate-opening” phenomenon at higher applied gas pressure. The surface of nanocrystals is observed to be enriched by Zn compared to imidazole.

Limited ionic conductivity of polymer electrolytes is a major issue in their industrial application. Enhancement in ionic conductivity in Poly (ethylene oxide), PEO, based electrolyte have been achieved by loading...

Fine tuning of the pore architecture and flexibility of zeolitic imidazolate frameworks (ZIFs) is highly crucial for realizing their applications in molecular gas separation. Mixed ligand frameworks (ZIF-7−8) synthesized by mixing 2-methylimidazole (2meIm) and benzimidazole (bIm) ligands show enhanced gas separation performance, attributable to pore and flexibility tuning. In the present study, positron annihilation lifetime spectroscopy (PALS) measurements under CO 2 pressure have been used to experimentally investigate the tuning of the pore architecture and flexibility of mixed ligand frameworks ZIF-7−8 having a ZIF-8 structure and similar morphology with varying bIm content up to 18.2%. The aperture and cavity of frameworks begin to open up with an increasing bIm ligand content followed by a decrease at a higher content. On the contrary, flexibility of the frameworks indexed from PALS measurements carried out under CO 2 pressure shows a decreasing trend followed by an increase. The present study shows that mixed ligand frameworks having a larger aperture size are less flexible as a result of inherent open configurations of ligands in the framework lattice. On the other hand, frameworks having a comparatively smaller aperture size show higher flexibility as a result of a possibility of twisting of the ligands under CO 2 pressure, resulting in aperture opening. The pore-opening phenomenon as a result of lattice flexibility under CO 2 pressure is observed to be fully reversible for ZIF-7−8.

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Although Zeolitic Imidazolate Frameworks (ZIFs) show a great promise in the field of molecular separation owing to their crystalline morphology of pores, the sieving performance of ZIFs remains limited due to their flexible frameworks leading to “gate opening” phenomenon under gas pressure. However, the pore size expansion due to gate opening may ensure the use of flexible ZIFs as an efficient gas storage material at high pressure. In the present study, we have investigated the pore volume expansion in three flexible ZIFs (ZIF-8, ZIF-67, and ZIF-7) during CO2 adsorption up to high pressure ∼60 kg cm–2 using positron annihilation lifetime spectroscopy. Our result provides first direct in situ observation of pore volume expansion due to the gate opening in ZIF frameworks under high CO2 gas pressure. The enhancement in pore volume at all the available gas adsorption sites in these frameworks is caused due to the linker rotation and phase transformations. ZIF-8 framework flexibility was observed to be much higher as compared to its cobalt-based analogue, i.e., ZIF-67. ZIF-7 is observed to show two step pore volume expansions first in low pressure range and second in high pressure range. The low pressure transition corresponds to well-known ZIF-7(np) to ZIF-7(lp) phase transformation. The second step is observed to be associated with much higher expansion of pore volume and must be associated with a crystal structural transformation in ZIF-7. All the phase transformations observed under CO2 pressure up to 60 kg cm–2 at 298 K in ZIF-8, ZIF-67, and ZIF-7 are observed to be fully reversible. The present study suggests that positronium is a powerful probe for the in situ investigation of the changes in pore architecture of ZIFs during gas adsorption, which plays an important role for the application of these frameworks in high level gas storage.