A large array of anhydrous metal ions were tested as catalysts in the preparation of shape memory poly(isocyanurate-urethane) (PIR-PUR) aerogels from the reaction of 1,3,5-tris(6-isocyanatohexyl)-1,3,5-triazinane-2,4,6-trione (Desmodur N3300A: a well-known isocyanurate-based aliphatic triisocyanate) and triethylene glycol (TEG) in anhydrous acetonitrile. The reaction yielded wet gels that were dried into aerogels in an autoclave with supercritical fluid CO2. The catalytic activity was mostly identified among CH3CN-soluble salts (mainly chlorides) of third-row d-block elements from iron to zinc, group 13 elements from aluminum to thallium, as well as cadmium, bismuth, and tin. Tin (119Sn) NMR indicated that the metal ion complexes with TEG, followed by reaction with the isocyanate. By using a fixed monomer concentration (20% w/w) and varying only the chemical identity and concentration of the catalysts, it was possible to demonstrate that the micromorphology of the resulting aerogels depended only on the gelation time. That is, for equal gelation times, the morphology was approximately the same, irrespective of the catalyst. For short gelation times (around 5 min or less), the aerogel frameworks were bicontinuous, changing to small spheroids at around 20 min and to large microspheres for gelation times around 75 min or more. Having obtained control over micromorphology, leaving other material properties such as density and porosity practically unaffected, it was possible to demonstrate that the bicontinuous structures of PIR-PUR aerogels can be up to 4 times stiffer and up to 2 times better thermal conductors than structures consisting of microspheres. This finding was attributed to the different widths of the neck zones between particles, noting that in bicontinuous morphologies, the neck diameters were almost equal to the particle diameters.




National Science Foundation, Grant CMMI-1530603

Keywords and Phrases

aerogels; bicontinuous; morphology; polyurethane; shape memory; spheroidal

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Article - Journal

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Publication Date

01 Jan 2023

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