Preparation And Microstructure Of Microporous Materials Studied By Positron Annihilation

As an advanced functional material,microporous organic material has a large num-ber of pores with size below 2nm.The high microporosity of microporous materials generally leads to a larger specific surface area.In addition,due to the chemical di-versity and the designability of building unit,microporous organic materials have been widely applied in gas adsorption,separation,energy storage and microelectronics ow-ing to their fascinating properties such as adjustable aperture,easy functionalization,and good stability.The performance of microporous organic materials is related to the pore structure.Rational regulation of pore structure and detailed insights into the pore architecture are very significant to optimize the macroscopic properties of microporous materials.In this paper,positron annihilation lifetime spectroscopy was used to study the micropore structure and explore the relationship between the microstructure and the performance of the material by measuring the annihilation life and momentum distribu-tion of the positronium.The lifetime and the chemical environment of annihilation were studied by changing the geometric configuration of the construction unit and the molec-ular structure of the repeating unit,and compared with the traditional characterization methods of the pore structure.The physical adsorption instrument,gas separation de-vice and AC impedance are measured to test the performance of the material.The research content and conclusions are as follows:(1)A series of amorphous melamine-based polymer networks synthesized by Schiff base chemistry(SNW)were successfully prepared by varying the strut length.The pore structure was analyzed by gas adsorption and positron annihilation methods.Positron lifetime measurements indicate the existence of ultramicropores in the SNW materials.The sizes of micropores is almost the same in these samples,which is about 0.7nm.The relative number of micropores increases in the order of SNW-1<SNW-2<SNW-3.N₂adsorption/desorption measurements also reveal micropores and mesopores in these materials.However it gives an underestimation of the micropore volume.Benefiting from the abundant nitrogen-content and high microporosity,the SNW materials exhibit exceptionally high CO₂capture ability,which reaches a maximum value of 18.3 wt%in SNW-3 at 273 K and 1 bar,followed by SNW-2 and SNW-1.This order is exactly the same as the order of micropore volume revealed by positron annihilation measurement,suggesting that micropores play a crucial role in the CO₂uptake.(2)Polyimides with ortho-functional groups(PIOFG-1 and PIOFG-2)were pre-pared by a traditional two-step method,and thermal rearrangement polymers(TR-1and TR-2)were obtained through thermal rearrangement reaction under vacuum con-ditions.Tests such as FT-IR,TGA,and WAXD have shown that the transition of the aggregation structure of the PIOFG-1 precursor depends on the degree of ther-mal rearrangement,and the degree of thermal rearrangement is directly related to the temperature.After the treatment temperature is higher than 350?C,the thermal re-arrangement of the molecular chain start reacting,and the rearrangement degree is greatest at 400?C.As the rearrangement reaction goes on,the flexible molecular chain gradually transforms into a rigid twisted structure.The positron annihilation measure-ment shows that when the temperature rises to 400?C,the free volume fraction of the film changes most significantly,and it increases from 1.25%to 13.82%.By analyzing the S–W parameters of Doppler broadening,the chemical environment of the annihi-lation during the transformation of the chain structure remain unnchanged,only the concentration of free volume elements has changed.When the chemical structure of the diamine monomer was changed,the chemical environment of the annihilation process began to change.The porosity inside the film effectively reduces the number of dipoles.Compared to the commercial polyimide Kapton(1MHz,3.49),the dielectric constant of TR-1 measured under the same conditions is only 1.93,moreover it has an outstanding stability.(3)The ZIF-8 crystals were prepared by the solvent method,and the structural transformation of the crystals was studied through a series of changes by annealing treatment.The WAXD and N₂adsorption/desorption shows that the ZIF-8 crystals maintain an intact crystal structure after annealing at 400?C for three hours.But when the annealing temperature is higher than 400?C,the crystal structure starts to collapse.The N₂adsorption/desorption shows that there are a large number of cavity structures with a pore size of 10?inside the crystal.The positron annihilation lifetime spectrum can get not only the exact size of the cage cavity inside the crystal,but also the annihilation information at the junction of the holes.The continuous distribution of the life can also reflect the collapse process of the pore structure.During the high temperature annealing process,the Zn-N bond in the skeleton structure is broken,which exposes the nitrogen atoms on the ligand heterocyclic ring,resulting in the improved the CO₂adsorption capacity.