Abstract
We report on the hydrothermal polymerization (HTP) of polyimide (PI) networks using the medium H2O and the comonomers 1,3,5-tris(4-aminophenyl)benzene (TAPB) and pyromellitic acid (PMA). Full condensation is obtained at minimal reaction times of only 2 h at 200 °C. The PI networks are obtained as monoliths and feature thermal stabilities of >500 °C, and in several cases even up to 595 °C. The monoliths are built up by networks of densely packed, near-monodisperse spherical particles and annealed microfibers, and show three types of porosity: (i) intrinsic inter-segment ultramicroporosity (<0.8 nm) of the PI networks composing the particles (∼3–5 μm), (ii) interstitial voids between the particles (0.1–2 μm), and (iii) monolith cell porosity (∽10–100 μm), as studied via low pressure gas physisorption and Hg intrusion porosimetry analyses. This unique hierarchical porosity generates an outstandingly high specific pore volume of 7250 mm3 g−1. A large-scale micromorphological study screening the reaction parameters time, temperature, and the absence/presence of the additive acetic acid was performed. Through expert interpretation of hundreds of scanning electron microscopy (SEM) images of the products of these experiments, we devise a hypothesis for morphology formation and evolution: a monomer salt is initially formed and subsequently transformed to overall eight different fiber, pearl chain, and spherical morphologies, composed of PI and, at long reaction times (>48 h), also PI/SiO2 hybrids that form through reaction with the reaction vessel. Moreover, we have developed a computational image analysis pipeline that deciphers the complex morphologies of these SEM images automatically and also allows for formulating a hypothesis of morphology development in HTP that is in good agreement with the manual morphology analysis. Finally, we upscaled the HTP of PI(TAPB–PMA) and processed the resulting powder into dense cylindrical specimen by green solvent-free warm-pressing, showing that one can follow the full route from the synthesis of these PI networks to a final material without employing harmful solvents.
Highlights
Networks are three-dimensional molecular structures built up by covalently-linked building blocks, where at least one building block has a functionality f > 2
Through expert interpretation of hundreds of scanning electron microscopy (SEM) images of the products of these experiments, we devise a hypothesis for morphology formation and evolution: a monomer salt is initially formed and subsequently transformed to overall eight different fiber, pearl chain, and spherical morphologies, composed of PI and, at long reaction times (>48 h), PI/SiO2 hybrids that form through reaction with the reaction vessel
The general procedure for all hydrothermal polymerization (HTP) experiments performed in this study employs the commercially available monomers TAPB and pyromellitic acid (PMA) (Fig. 1B, Electronic supplementary information (ESI) for further details†)
Summary
Networks are three-dimensional molecular structures built up by covalently-linked building blocks, where at least one building block has a functionality f > 2. A er tr, the autoclave is cooled back to r.t. and the medium water becomes polar again, leading to the phase separation of the rather apolar reaction products, which can be collected by simple ltration (if solid) or decantation (if liquid).[28] HT conditions have been shown to promote crystallinity in small molecules,[27,29] linear polyimides,[16,28,30,31] polyamide networks,[22] or at least semi-crystallinity in PI/SiO2 hybrid materials and linear polybenzimidazoles and BBB.[24,31] To date, monomers used in HTP were mainly difunctional aromatic monomers, i.e., aromatic dianhydrides,[16,28,31] aromatic diamines,[16,23,28] and aromatic dialdehydes.[23] Some reports employ tetrafunctional aromatic monomers, such as tetraamines,[24] or in the case of PIs the free tetracarboxylic acids instead of the corresponding dianhydrides.[31] In these cases the 1,2-difunctionality is required to generate the heterocyclic linkages and does not lead to networks but linear polyheterocyclics. We believe that this approach is of great interest for the materials science/chemistry community, in light of the large analytical datasets generated nowadays, for instance by accelerated materials discovery through automation.[37,38] we show that the HTP of PI(TAPB–PMA) can be upscaled, and that bulk materials can be generated through solvent-free warm-pressing
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