Fundamental Skeletal Nanostructure of Nanoporous Polymer-Cross-Linked Alginate Aerogels and Its Relevance To Environmental Remediation

Nanoporous polyurea-cross-linked Ca-alginate (X-Ca-alginate) aerogels were prepared by reacting an aliphatic or aromatic triisocyanate with the preformed biopolymer network post gelation and drying in supercritical CO2. The nanomorphology of native Ca-alginate aerogels together with those of the different X-Ca-alginate aerogels were investigated using low-voltage scanning electron microscopy, N2-sorption porosimetry, and contrast variation small-angle neutron scattering. Native Ca-alginate aerogels were built from primary nanoparticles (8.3 ± 0.1 nm in radius) that attach to one another forming secondary particles. In X-Ca-alginate aerogels, the aliphatic and aromatic polyureas attach to primary nanoparticles (which increase in size up to 10.0 ± 0.1 nm) via urethane linkages, and then they extend into the empty space within secondary particles in different ways. Cross-linking with an aliphatic triisocyanate leads to the formation of a dense polyurea layer over the primary nanoparticles, following the contours of the Ca-alginate skeletal framework. The rigid aromatic triisocyanate forms a more loose and randomly oriented polymer structure that more or less fills the empty space between the primary nanoparticles within the secondary particles. Both processes leave the primary Ca-alginate structure practically undisturbed, while it does affect the structure at the most fundamental level, increasing the primary particle size and reducing the porosity. The different fundamental skeletal nanostructures of X-Ca-alginate aerogels affect not only their material properties but also their potential for application in environmental remediation.