| Aerogels are porous lightweight materials, unique among all solids in terms of low density, low thermal conductivity, low dielectric constants and high acoustic attenuation. The penalty for those exceptional properties is fragility. That issue has been addressed by bridging covalently (cross linking) the elementary nanoscopic building blocks of silica aerogels with polymers. To streamline the crosslinking process we synthesized a bidentate free radical initiator (Si-AIBN) that gets attached on the skeletal nanoparticles and induces crosslinking by surface initiated polymerization (SIP). Here, we extend the scope of Si-AIBN in two directions: (a) we demonstrate extremely adhering, covalently attached polymer coatings on oxidized surfaces (glass, metal) and eventually we adapt the method into highly durable conducting polymer films; and, (b) we engage a carbonizable crosslinker (acrylonitrile), whereas the resulting polymer coating, polyacrylonitrile (PAN), can react carbothermally with the underlying silica backbone to yield monolithic, highly porous (>70% v/v) silicon carbide (SiC) suitable as a high temperature catalyst support. Meanwhile, PAN is the major starting material of carbon fiber used industrially in high-strength composites for automotive and aerospace applications. It was thus deemed desirable to explore the synthesis of PAN-derived porous carbons. Although free-radical solution polymerization of acrylonitrile may afford gels, those linear-polymer based gels collapse upon drying and they cannot be converted to aerogels. Use of 1.6-hexanediol diacrylate (HDDA) or ethyleneglycol dimethacrylate (EGDMA) as molecular crosslinkers induces phase-separation of "live" nanoparticles that react with one another forming a robust, covalently bonded 30 network than can be dried into carbonizable and eventually graphitizable aerogels. Phase-separation can be also induced prior to polymerization by surfactants, and thus eventually we demonstrate an environmentally friendly aqueous route to PAN aerogels and carbons. |
