Hyperbranched, Functional Polyethoxysiloxanes: Tunable Molecular Building Blocks.

Functional silanes are multifaceted cross-linkers, compatibilizers, coupling agents, and surface modifiers. Herein, we present organofunctional polysiloxane building blocks that offer great versatility in terms of molecular weight, degree of condensation, and the choice and loading of organic substituent groups. The organofunctional polyethoxysilanes (funPEOS) are prepared in a one-pot, two-step process: synthesis of the PEOS carrier/substrate, followed by grafting a functional silane "shell", both based on condensation with acetic anhydride. The reaction was optimized at the lab scale and scaled up to a 7 L reactor. The acetylation, condensation, and hyperbranched structure of the carrier were confirmed by 29Si NMR, while 29Si-29Si 2D INADEQUATE NMR provides strong evidence for the grafting of functional silanes onto the carrier (Q-T coupling). IR, 1H, and 13C NMR spectroscopy demonstrate that the functional groups remain intact. The molar mass can be tailored by stoichiometric control of the acetic anhydride to silane monomer ratio (M n 3500-20,000 g/mol). The compounds are stable organic liquids with a long shelf life. Selected applications are presented: scratch-resistant coatings with water contact angles of ∼90°, stable water emulsions, and surfactant-free, mesoporous silica foams.

Strong, Machinable, and Insulating Chitosan-Urea Aerogels: Toward Ambient Pressure Drying of Biopolymer Aerogel Monoliths.

Biopolymer aerogels are an emerging class of materials with potential applications in drug delivery, thermal insulation, separation, and filtration. Chitosan is of particular interest as a sustainable, biocompatible, and abundant raw material. Here, we present urea-modified chitosan aerogels with a high surface area and excellent thermal and mechanical properties. The irreversible gelation of an acidic chitosan solution is triggered by the thermal decomposition of urea at 80 °C through an increase in pH and, more importantly, the formation of abundant ureido terminal groups. The hydrogels are dried using either supercritical CO2 drying (SCD) or ambient pressure drying (APD) methods to elucidate the influence of the drying process on the final aerogel properties. The hydrogels are exchanged into ethanol prior to SCD, and into ethanol and then heptane prior to APD. The surface chemistry and microstructure are monitored by solid-state NMR and Fourier transform infrared spectroscopy, scanning electron microscopy, and nitrogen sorption. Surprisingly, large monolithic aerogel plates (70 × 70 mm2) can be produced by APD, albeit at a somewhat higher density (0.17-0.42 g/cm3). The as prepared aerogels have thermal conductivities of ∼24 and ∼31 mW/(m·K) and surface areas of 160-170 and 85-230 m2/g, for SCD and APD, respectively. For a primarily biopolymer-based material, these aerogels are exceptionally stable at elevated temperature (TGA) and char and self-extinguish after direct flame exposure. The urea-modified chitosan aerogels display superior mechanical properties compared to traditional silica aerogels, with no brittle rupture up to at least 80% strain, and depending on the chitosan concentration, relatively high E-moduli (1.0-11.6 MPa), and stress at 80% strain values (σ80 of 3.5-17.9 MPa). Remarkably, the aerogel monoliths can be shaped and machined with standard tools, for example, drilling and sawing. This first demonstration to produce monolithic and machinable, mesoporous aerogels from bio-sourced, renewable, and nontoxic precursors, combined with the potential for reduced production cost by means of simple APD, opens up new opportunities for biopolymer aerogel applications and marks an important step toward commercialization of biopolymer aerogels.