The hydrothermal route to imide-linked network- and frameworks materials

Abstract

Highly stable and functional organic polymer networks (OPNs) are of interest for green applications such as gas sorption and separation, catalysis, and electronics.Especially, network polyimides (PIs) are promising due to their outstanding chemical,thermal and mechanical stabilities as well as their anity for CO2 gas molecules.Unfortunately, PIs' preparation typically requires hazardous solvents and catalysts,which gravely diminish the materials' sustainability. In recent years, hydrothermalpolymerization (HTP) which employs solely water as solvent was developed and thoroughly investigated as a green synthetic alternative to generate linear PIs. Until2021, HTP had been limited to linear PI materials.Within in this thesis it was shown that it is possible to extend HTP of PIs to three dimensions: Amorphous PI networks were obtained, which exhibited thermalstabilities up to 595 °C and a trimodal porosity which ranged from ultra-microporous(< 0.8 nm), to meso- as well as macroporous. By varying the reaction conditions,the networks' behavior in HT conditions was elucidated. I.e., the formation of anintermediate multitopic monomer salt could be identied as well as a hypothesis forthe micromorphological evolution was devised. Finally, it was shown that up-scalingand processing via warm-pressing of the PI network powder is feasible.Furthermore, it was proven that HTP provides imide reversibility, which is necessary to produce crystalline PI networks (aka covalent organic frameworks (COFs)),using a small molecule model system. However, it was shown that the addition ofaniline was necessary to generate extended 2D PI-COF sheets, acting similarly tomineralizers in inorganic HTP. Unfortunately, aniline also led to a mix of inseparableside-products, which prevented the sheets' three dimensional (3D) arrangementand hence no permanent porosity could be achieved.Finally, it was shown that HTP is indeed capable of generating crystalline frameworkmaterials without the necessity of aniline as additive. On the basis of several solidstate techniques it is proposed that H-bonded frameworks (HOFs) were obtained,which are based on linear PI-backbones that are laterally H-bonded via side-groups.The PI-HOFs' formation is believed to be T-dependent, where elevated reaction temperatures (TRs) lead to a transformation to linear PIs accompanied by sidegroup removal. HTP was assumed crucial for both products' crystallinity, as solidstate heat treatment led to largely amorphous products.Overall, this thesis showed that hot water is able to not only produce a morphousnetwork PIs but also provides reversibility conditions for inducing long range orderin PI framework materials.iii