Fractal Multiscale Nanoporous Polyurethanes: Flexible to Extremely Rigid Aerogels from Multifunctional Small Molecules

A large array of easily available small-molecule (as opposed to industrial oligomeric) triisocyanates and aromatic polyols render polyurethanes a suitable model system for a trend-based systematic study of structure–property relationships in nanoporous matter as a function of the monomer structure. Molecular parameters of interest include rigidity, number of functional groups per monomer (<i>n</i>), and functional group density (number of functional groups per phenyl ring, <i>r</i>). All systems were characterized from gelation to the bulk properties of the final aerogels. Molecular and nanoscopic features of interest, including skeletal composition, porous structure, nanoparticle size, and assembly, were probed with a combination of liquid- and solid-state ¹³C and ¹⁵N NMR, rheometry, N₂- and Hg-porosimetry, SEM, and small-angle X-ray scattering (SAXS). Macroscopic properties such as styrofoam-like thermal conductivities (∼0.030 W m^–1K^–1), foam-like flexibility, or armor-grade energy absorption under compression (up to 100 J g^–1) were correlated with one another and serve as a top-down probe of the interparticle connectivity, which was again related to the monomer structure. Overall, both molecular rigidity and multifunctionality control phase-separation, hence, particle size and by association porosity (e.g., meso versus macro) and internal surface area. With sufficiently rigid monomers, skeletal frameworks include intrinsic microporosity, rendering the resulting materials hierarchically nanoporous over the entire porosity regime (micro to meso to macro). Most importantly, however, clear roles have been identified not only for the absolute number of functional groups per monomer, but also for parameter <i>r</i>. The latter is expressed onto the surface of the skeletal nanoparticles (controls the surface functional group density per unit mass) and becomes the dominant structure-directing as well as property-determining parameter. By relating the molecular functional group density with the functional group density on the nanoparticle surfaces, these results establish that for three-dimensional (3D) assemblies of nanoparticles to form rigid nanoporous frameworks, they have first and foremost to be able to develop strong <i>covalent bonding</i> with one another. These findings are relevant to the rational design of 3D nanostructured matter, not limited to organic aerogels