Alginate Aerogels Crosslinked with Isophorone-Based Polyurea: The Missing Link between Rigid Aromatic and Flexible...

Polyurea-cross-linked alginate (X-Ca-alg-IP) aerogels, prepared from calcium alginate (Ca-alg) gels and the isocyanurate trimer of rigid aliphatic isophorone diisocyanate (IP), provide insight into the role of the cross-linker molecular structure in the nanoscale and macroscopic properties of X-biopolymer aerogels. Keeping all other synthetic conditions the same, the polyurea (PUA) loading in X-Ca-alg-IP (40-46% w/w) was lower than what was obtained using flexible aliphatic (56% w/w) or rigid aromatic (60% w/w) triisocyanate monomers, reflecting differences in monomer reactivity and packing efficiency of the resulting PUA. Cross-linking preserved the macro/mesoporous nanoscale architecture of the native Ca-alg backbone, maintaining high porosity (up to 95% v/v) and specific surface area (up to 393 m² g^-1). Small-angle neutron scattering (SANS) confirmed that the nanostructure and mass-fractal organization are largely preserved by the cross-linking process that produced a conformal interfacial layer of PUA around the primary Ca-alg skeletal particles, with an estimated thickness on the order of 2-3 nm. Compared to other X-biopolymer aerogels, X-Ca-alg-IP aerogels showed significantly enhanced stiffness, with Young's moduli in the range of 11-16 MPa. These results highlight the critical role of the molecular structure of the cross-linker in tailoring the structure-property relationships in X-biopolymer aerogels for advanced and sustainable environmental and biomedical applications.