Positron Annihilation Study Of The Pore Structure Of Hyper-cross-linked Polymers And Its Effect On Gas Adsorption Property

Since the discovery of hyper-cross-linked microporous organic polymer（HCPs）materials in the early 1970s,researchers have been paying attention to them.It is developed from the original styrene-divinylbenzene cross-linked gel,and now has developed into a universal platform for chemists to synthesized nanoporous materials with ideal properties.HCPs have received extensive attention in recent years due to the low cost of the monomers,diverse synthetic methods,mild reaction conditions and easy functionalization.At the same time,the synthesized HCPs have high specific surface area,extremely high microporosity and adjustable pore structure.So far,a variety of synthetic pathways have been developed to adjust the pore structure.HCPs will have important application prospects in gas adsorption and storage.Therefore,exploring how to accurately adjust the pore size and morphology of hyper-cross-linked materials and study how to improve the relationship between the pore structure and the gas adsorption performance are still to be further studied important topic.Positron annihilation spectroscopy is a very sensitive method for analyzing pore structure.In this thesis,we prepared several typical hyper-cross-linked organic materials,and studied the pore structures by means of positron annihilation analysis,the field emission scanning electron microscopy（FE-SEM）and N₂adsorption/desorption methods.The effect of pore structure on the gas adsorption performance was also studied.Combined with different characterization methods,we further discussed the way to characterize the pore structure by positron annihilation technique.Hypercrosslinked polystyrene（HCLPS）networks were synthesized by radical bulk polymerization and Friedel-Crafts alkylation reaction using divinylbenzene-co-vinylbenzyl chloride（DVB-VBC）as the precursors.A series of HCLPS were prepared with varying content of DVB from 0 to 10%in the precursor.Both N₂adsorption and positron annihilation measurements reveal micropores in the HCLPS.Especially,the existence of ultramicropores with size in the range of 0.63-0.7 nm is confirmed by positron lifetime measurements.With increasing DVB content from 0 to10%,the number of ultramicropores shows gradual increase.Both H₂and CO₂adsorption capacity increase monotonously with the increase of DVB content.With10%DVB in the HCPs,the H₂storage increases to 10.3 mmol g^-1（2.05 wt%）at 77 K and 1 bar and the CO₂capture reaches 2.81 mmol g^-1（12.4 wt%）at 273 K and 1 bar.The remarkable gas storage ability is ascribed to the existence of the ultramicropores,which result in a stronger affinity to the gas molecules.By using positron as a new probe for the pores,our results provide convincing evidence of the role of ultramicropores on the gas adsorption performance in microporous organic polymers.Porous knitting aryl network polymer（KAP）was synthesized using an external crosslinker.The N₂adsorption/desorption isotherm indicates that the synthesized sample contains micropores and mesoporous structures.The N₂adsorption/desorption isotherm still exhibits hysteresis in the low pressure region,and this hysteresis extends to the lowest achievable pressure,indicating that the pore diameter of KAP is similar to the diameter of the nitrogen molecule,and irreversible adsorption occurs with gas molecules at the entrance of the hole.Positron annihilation technology can accurately measure the pore size of ultramicropores and micropores in the synthesized samples.The results show that positrons are very sensitive to ultramicropores and are good probes for studying the correlation between pore structure and properties.