Stability and behaviour in aqueous solutions of the anionic cubic silsesquioxane substituted with tetramethylammonium

Unusual flexibility of transparent poly(methylsilsesquioxane) aerogels by surfactant-induced mesoscopic fiber-like assembly

High-performance thermal insulators represented by aerogels are regarded as one of the most promising materials for energy savings. However, significantly low mechanical strength has been a barrier for aerogels to be utilized in various social domains such as houses, buildings, and industrial plants. Here, we report a synthetic strategy to realize highly transparent aerogels with unusually high bending flexibility based on poly(methylsilsesquioxane) (PMSQ) network. We have constructed mesoscopic fine fiber-like structures of various sizes in PMSQ gels by the combination of phase separation suppression by tetramethylammonium hydroxide (TMAOH) and mesoscopic fiber-like assembly by nonionic poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-b-PPO-b-PEO) type surfactant. The optimized mesoscale structures of PMSQ gels have realized highly transparent and resilient monolithic aerogels with much high bendability compared to those reported in previous works. This work will provide a way to highly insulating materials with glasslike transparency and high mechanical flexibility.

Dynamic Nuclear Polarization of Selectively 29Si-Enriched Core@shell Silica Nanoparticles

²⁹Si silica nanoparticles (SiO₂ NPs) are promising magnetic resonance imaging (MRI) probes that possess advantageous properties for in vivo applications, including suitable biocompatibility, tailorable properties, and high water dispersibility. Dynamic nuclear polarization (DNP) is used to enhance ²⁹Si MR signals via enhanced nuclear spin alignment; to date, there has been limited success employing DNP for SiO₂ NPs due to the lack of endogenous electronic defects that are required for the process. To create opportunities for SiO₂-based ²⁹Si MRI probes, we synthesized variously featured SiO₂ NPs with selective ²⁹Si isotope enrichment on homogeneous and core@shell structures (shell thickness: 10 nm, core size: 40 nm), and identified the critical factors for optimal DNP signal enhancement as well as the effective hyperpolarization depth when using an exogenous radical. Based on the synthetic design, this critical factor is the proportion of ²⁹Si in the shell layer regardless of core enrichment. Furthermore, the effective depth of hyperpolarization is less than 10 nm between the surface and core, which demonstrates an approximately 40% elongated diffusion length for the shell-enriched NPs compared to the natural abundance NPs. This improved regulation of surface properties facilitates the development of isotopically enriched SiO₂ NPs as hyperpolarized contrast agents for in vivo MRI.

Impact of Molecular Architecture on Surface Properties and Aqueous Stabilities of Silicone-Based Carboxylate Surfactants

Silicone surfactants consist of siloxane or carbosilane hydrophobic groups that possess better surface activity compared with alkane surfactants. The surfactants, containing Si atoms which bring excellent bond flexibility and low cohesive energy properties are a promising class of materials for unique surface working, but there are few studies to elaborate their surface activity mechanism with regard to the molecular architecture. Herein, two novel carboxylate surfactants with different silicone hydrophobic groups (Si-O-Si and Si-C-Si) were synthesized and their surface activities, aggregate behaviors, and solution stabilities were systematically investigated. Results showed that both surfactants had excellent surface activities which are attributed to the hydrophobic structure of silicone. The hydrolysis resistance of the carbosilane-based carboxylate surfactant was better than that of the siloxane-based carboxylate surfactant. The differences in hydrolysis processes for the surfactants were confirmed by the mass spectrum and kinetic analysis. Meanwhile, the aggregation number of Si-C-Si surfactants was also determined by the fluorescence quenching method for the first time.