Interfacial-interaction induced modifications in segmental dynamics and free volume of the poly(ethyleneimine) canopy in nanoparticle-organic hybrid materials: an investigation by temperature dependent broadband dielectric and positron annihilation spectroscopy.

Broadband dielectric spectroscopy, positron annihilation lifetime spectroscopy and differential scanning calorimetry are used to investigate the molecular dynamics, free volume and thermal behaviour of a poly(ethyleneimine), PEI, canopy in liquid-like nanoparticle-organic hybrid materials (NOHMs) consisting of alumina nanoparticles and nanorods as inorganic nanocores. It is confirmed that the highly branched PEI canopy in liquid-like NOHMs possesses an ordered structure having less entanglements of the side chains as compared to neat PEI. The size of the free volumes associated with the PEI canopy for nanoparticle- and nanorod-based NOHMs is larger and smaller, respectively, as compared to neat PEI. The time scales characterizing the segmental dynamics and side chain relaxations of the canopy in the nanorod-based NOHM are slowed down drastically as compared to the nanoparticle-based NOHM and neat PEI. These results confirm that the shape of the inorganic cores plays a deterministic role toward the structural arrangement (free volume) and segmental dynamics of the canopy polymer in liquid-like NOHMs.

Capture of Volatile Iodine Gas and Identification of Adsorption Sites in the Pore Network of Unsubstituted Imidazole Linker-Incorporated Zeolitic Imidazolate Framework-8.

The incorporation of unsubstituted imidazole (Im) in zeolitic imidazolate framework-8 (ZIF-8) crystallized in sodalite topology is proposed to improve gas capture and gas separation performance drastically. However, the incorporation of unsubstituted Im in ZIF-8 has remained challenging due to the thermodynamic instability of zinc and Im bonding in sodalite topology. We have incorporated up to 24.4 (mol %) Im linker in highly crystalline ZIF-8 with similar morphology and sodalite topology using a delayed linker addition strategy at room temperature. Im incorporation brings significant tuning to the pore architecture of ZIF-8, as confirmed by positron annihilation lifetime spectroscopy. The modifications in the pore architecture are primarily due to linker defects produced in the frameworks during crystallization and the elimination of steric hindrance due to the absence of a methyl group on Im. The Im-incorporated ZIF-8 shows significant enhancement in iodine capture as well as higher crystal structure stability under iodine vapor exposure as compared to pristine ZIF-8. Through ortho-positronium interaction with the adsorbed molecular iodine in the pore network of the frameworks, it is confirmed that iodine is preferentially adsorbed at cavity and intercrystalline voids, whereas aperture sites remain unoccupied by iodine molecules.

Bimetallic Zeolitic Imidazole Frameworks for Improved Stability and Performance of Intrusion-Extrusion Energy Applications.

Hydrophobic flexible zeolitic imidazole frameworks (ZIFs) represent reference microporous materials in the area of mechanical energy storage, conversion, and dissipation via non-wetting liquid intrusion-extrusion cycle. However, some of them exhibit drawbacks such as lack of stability, high intrusion pressure, or low intrusion volume that make them non-ideal materials to consider as candidates for real applications. In this work, we face these limitations by exploiting the hybrid ZIF concept. Concretely, a bimetallic SOD-like ZIF consisting of Co and Zn ions was synthesized and compared with Co-ZIF (ZIF-67) and Zn-ZIF (ZIF-8) showing for the first time that the hybrid ZIF combines the good stability of ZIF-8 with the higher water intrusion volume of ZIF-67. Moreover, it is shown that the hybrid-ZIF approach can be used to tune the intrusion/extrusion pressure, which is crucial for technological applications.

Fine-Tuning of the Pore Aperture and Framework Flexibility of Mixed-Metal (Zn/Co) Zeolitic Imidazolate Framework-8: An In Situ Positron Annihilation Lifetime Spectroscopy Study under CO2 Gas Pressure.

The mixed-metal (Zn/Co) strategy has been used to enhance the gas separation selectivity of zeolitic imidazolate framework-8 (ZIF-8)-based membranes. The enhancement in selectivity has been attributed to possible modifications in the grain boundary structure, pore architecture, and flexibility of the frameworks. In the present study, we used in situ positron annihilation lifetime spectroscopy (PALS) under varying CO2 pressure to investigate the tuning of the pore architecture and framework flexibility of mixed-metal (Zn/Co) ZIF-8 frameworks with varying Co contents. The random distribution of Zn and Co metal nodes within the highly crystalline frameworks having an SOD topology was established using electron microscopy, Fourier transform infrared spectroscopy, and Raman spectroscopy. The inherent aperture as well as cavity size of the frameworks, and the pore interconnectivity to the outer surface, were observed to vary with the Co content in ZIF-8 due to the random distribution of Zn and Co metal nodes in the frameworks. The aperture size is reduced with the incorporation of an additional metal (Zn or Co) in ZIF-67 or ZIF-8, respectively. The aperture size remains the smallest for a lower Co content (∼0.20) in ZIF-8. The framework flexibility determined by in situ PALS measurements under CO2 pressure continuously reduces with increasing Co content in ZIF-8. A smaller aperture size as well as low flexibility of ZIF-8 with a low Co content is seen to be directly correlated to a higher separation selectivity of membranes prepared with this mixed-metal composition.

Flexibility of Mixed Ligand Zeolitic Imidazolate Frameworks (ZIF-7-8) under CO2 Pressure: An Investigation Using Positron Annihilation Lifetime Spectroscopy.

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 CO2 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 CO2 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 CO2 pressure, resulting in aperture opening. The pore-opening phenomenon as a result of lattice flexibility under CO2 pressure is observed to be fully reversible for ZIF-7-8.