Construction Of Crosslinked Microporous Polyimide Networks And Their Carbon Dioxide Adsorption Properties

In recent years,with the rapid development of human society the excessive emission of CO₂ in the atmosphere largely contributes to environmental issues such as the rising sea levels,alteration in ecosystems and ocean acidification.For the sake of these intractable environmental issues,it is highly desirable to develop capable adsorbents for effective CO2 capture and storage?CCS?.As a newly developed functional adsorbent materials,microporous organic polymers?MOPs?have been intensively investigated because of their large specific surface area,low skeletal density,tunable porosity parameters,excellent thermal/chemical stability and synthetic flexibility.Microporous aromatic polyimides?PI?is an important class of MOPs and they featuring with abundant heteroatoms and superior thermal/chemical stability have become the feasible choice of porous adsorbent for CCS.However,the rigid anhydride or amine monomers with twisted structure or three-dimensional structure are critical needed for the construction of microporous polyimide materials,therefore,the shortage of these monomers has been limited the large-scale applications of microporous polyimides.Most of polyimides do not possess microporous structure due to their flexible polymer chains that cannot prevent the polymer chains from collapsing into a dense and nonporous state.Recently,the crosslinked polyimides have attracted a lot of attention because the crosslinked structure can endow the polyimides with excellent thermal and chemical stability,outstanding dimensional stability and strong mechanical properties.Hence,we propose to use a"two-step process"to prepare the microporous polyimide materials,the first step is the construction of cross-linkable polyimides based on the polymerization reaction,the second step is the formation of microporous structure in the polymer through the thermal cross-linking reaction.The microporous structure of the microporous polyimide can be tuned by changing the distance between the crosslinking point and the polymer skeleton which can effectively change the distance between the polymer molecular chains.Besides,the specific structure of the monomer is not necessary in the"two-step method".Therefore,more microporous polyimide materials with different structure and property can be prepared based on this method and the application of microporous polyimides can be further extended.In this thesis,choosing crosslinked polyimides as the research object,we have synthesized a series of microporous polyimides for CO₂ adsorption and separation,and investigated the influence of the distance between the crosslinking point and the polymer skeleton,polar substituent and high crosslinking density on the microporous structure and CO₂ adsorption performance of crosslinked polyimides in detail.In chapter 3,we studied the effect of the distance between the polymer skeleton and crosslinking points on the BET surface area,pore size and distribution,the absorptions of CO₂ gas and the selectivities of CO₂/N₂ and CO₂/CH₄ for the crosslinked microporous polyimides.We design and synthesized two dianhydride monomers with cross-linkable phenylacetylene side groups and cross-linkable phenylacetylenyl benzene side group?PEQDA and PEPHQDA?,respectively,which were used in polycondensation reaction with TAPM to fabricate the crosslinked polyimide networks?HBPI-CLs?with the different distance between the polymer skeleton and crosslinking points.The HBPI-CLs exhibit typical adsorption characteristics of microporous materials,their BET surface areas and micropore areas significantly increased to 339–593 m² g^-1 and 260–369 m² g^-1,and micropore volume are in the range of 0.096-0.130cm³ g^-1.This can be attributed to the cross-linked structure can effectively restrict the stretching and movement of the polymer molecular chain,hinder the collapse of the polymer skeleton and the microporous structure;at the same time,the cross-linked structure can promote the formation of new microporous channels.We also focused on the effect of the distance between the cross-linking point and the polymer backbone on the pore structure of the cross-linked microporous polyimide.The pore size distributions of HBPI-CLs are consistent with the trend of the distance between the crosslinking point and the polymer skeleton,because the shorted distance between the crosslinking point and the polymer skeleton make the polymer chains get closer to each other,which is beneficial for promoting the polymer chains segments to divide the pore structure into smaller pore structure.The crosslinked microporous polyimide with smaller pore size exhibits excellent CO₂ adsorption performance,the selectivities of CO₂/N₂ and CO₂/CH₄ reach to 109 and 15 at 273 K,respectively,which are among the best results for both porous organic and inorganic materials.Therefore,the properties of the obtained microporous hyperbranched polyimide such as SBET,micropore area and volume,micropore size and gas sorption performance can be finely tuned by changing the distance between the polymer skeleton and crosslinking points.In chapter 4,we studied the effects of different polar substituents on the BET surface area,pore structure parameters,CO₂ affinity and CO₂ adsorption performance of crosslinked microporous polyimides.We design and synthesized triamine monomers with different substituents and used to react with the dianhydride monomer with cross-linkable phenylacetylene side groups to prepare the crosslinked polyimides.The crosslinked polyimides exhibit typical adsorption characteristics of microporous materials with the micropore?<1 nm?BET surface areas and volume in the range of79-89 m² g^-1 and 0.03-0.05 cm³ g^-1,respectively.We also investigated the effects of polar trifluoromethyl on pore structure parameters,CO₂ affinity and CO₂ adsorption performance of crosslinked microporous polyimides.The trifluoromethyl-containing polymer has smaller pore size ultramicropore?<0.7 nm?and larger ultramicropore volume because the rotation of the imide bond can be hindered by the large volume of trifluoromethyl in the ortho-position,which can facilitate the micropores formation in the polymer structure.At the same time,the fluorine atom can increase the polarity of the microporous polyimide skeleton,hence strengthening the affinity with the CO₂molecule,increasing the corresponding adsorption heat and promoting the adsorption and selectivity of CO₂.In chapter 5,the effect of crosslinked structure and trifluoromethyl group on pore structure and CO₂ adsorption properties of crosslinked polyimides were investigated in detail.We prepared the cross-linkable linear polyimides based on the polycondensation reaction of trifluoromethyl-containing diamine monomer and the dianhydride monomer possessing cross-linkable phenylacetylene side groups.The uncrosslinked linear polyimide exhibits the non-porous polymer characteristics due to the skeleton collapse.After heat treatment,the BET surface area and micropore area of the crosslinked polyimides significantly increase from 21 m² g^-1and 2 m² g^-1 to 575 m² g^-1and 190 m²g^-1 respectively.This phenomenon can be ascribed to the crosslinked structure that can effectively restrict the conformation changes of flexible linear polyimide molecular chains,thus preventing the collapse of polymer chains and facilitating the formation of microporous structures.Meanwhile,large volume trifluoromethyl can hinder the conformational change of polymer chain and further promote the formation of micriporous structure.The abundant heteroatoms and ultramicroporous structures endow the crosslinked polyimides with relatively high CO₂ adsorption capacity?9.4 wt%at 273 K and 1 bar?and excellent CO₂ separation performance?the selectivity of CO₂/N₂ is 72?.Therefore,the introduction of cross-linkable group and trifluoromethyl group into linear polyimides structure facilitates the construction of microporous polyimides with high surface area and excellent CO₂ adsorption properties.In chapter 6,we studied the effect of high cross-linking density on the pore structure and gas adsorption behavior of crosslinked polyimides.The cross-linkable styryl and phenylacetylene were introduced into the linear polyimides through the polycondensation reaction of cross-linkable styryl-containing diamine monomer and the dianhydride monomer possessing phenylacetylenyl benzene side group.The crosslinked polyimides exhibit the typical adsorption characteristics of microporous materials,its BET surface area significantly increase to 635-727 m² g^-1,which are among the best results for linear microporous polyimides.We also studied the effect of different diamine structures on the polymers pore structure.The rigid biphenyl structure can restrict the conformational transition of the polymer segment,hinder the collapse of formed micropores and promote the formation of ultramicropores in the crosslinked polyimide.While,the large free volume hexafluoropropyl endow the polymer molecular chain with more flexibility,resulting in some collapse of ultramicroporous structures and decreased ultramicropores volume.The CO₂adsorption amounts of the fabricated crosslinked microporous polyimides are in the range of 7.28-10.03 wt%?273 K,1 bar?.Therefore,increasing the crosslink density facilitates the fabrication of microporous polyimide networks possessing high surface area and highly selective CO₂ capture capacity.