Ayaka Maeno

A Superamphiphobic Macroporous Silicone Monolith with Marshmallow‐like Flexibility

A number of research groups have been studying the preparation of hydrophobic and oleophobic surfaces, both for pure scientific interest and industrial applications. These studies are drawing increasing attention because of the growing demands for applications such as anti-fingerprint touch panels on electronic devices and solar panels that can prevent output fall from dust and smears on the surface by the self-cleaning effect. In nature, many examples of superhydrophobic surface exist with a water contact angle of more than 1508, such as eyes of mosquitos and lotus leaves, 2] and these are important for their survival. Their non-wetting surfaces possess a combination of nanoor microscaled roughness and low surface energy, which are known for the key of creating artificial superhydrophobic surfaces. However, most of the superhydrophobic materials can easily be wetted by organic liquids because of the lower surface tension of the liquids. In recent years, techniques for creating oleophobic surfaces have been vigorously investigated. A promising way to obtain a surface with a contact angle of more than 1508 for organic liquids is to make rough microstructures covered with perfluoroalkyl groups, which are bound on some kinds of polyhedral oligomeric silsesquioxanes (POSS), monomeric silanes, and polymers. However, the reported technologies to achieve superamphiphobicity are limited in the forms of films and fibers. As far as we know, there have been no reports on monolithic superamphiphobic materials that can be prepared in a wide range of thickness and in any shapes. We have been recently investigating marshmallow-like gels derived from triand difunctional alkoxysilanes as coprecursors through a facile one-pot sol–gel reaction. These silicone-based macroporous materials have high porosity (> 90%), flexibility both for compression and bending, and built-in superhydrophobicity. The marshmallow-like gels can be used like a sponge for quick removal of organic liquids/oils from oil–water mixtures for environmental purposes and for new solid-phase extraction media in analytical chemistry. By changing the combination of the alkoxysilanes, various kinds of marshmallow-like gels with different functional groups can be obtained. For example, in the case of methyltrimethoxysilane-dimethyldimethoxysilane copolymer system, the obtained gels are composed of the cross-linked polydimethylsiloxane (PDMS)-like molecular structure. They retain the flexible mechanical properties over a wide temperature range from 130 8C to 320 8C, as evidenced from thermal and mechanical analyses. Moreover, owing to their elasticity and bendability even at temperature of under 196 8C, we can successfully absorb and squeeze-out liquid nitrogen. In the case of (3-mercaptopropyl)trimethoxysilane-(3-mercaptopropyl)methyldimethoxysilane copolymer system, gold ions can be adsorbed on the pore surface by the mercapto groups. We employed a vinyltrimethoxysilane (VTMS)-vinylmethyldimethoxysilane (VMDMS) co-precursor system to prepare the first superamphiphobic monolith. The VTMSVMDMS marshmallow-like gel can be obtained by four simple, routine steps within half a day: 1) mixing VTMS, VMDMS, urea, and surfactant n-hexadecyltrimethylammonium chloride (CTAC) in a dilute aqueous acetic acid solution, and stirring for 60 min at room temperature for acid-catalyzed hydrolysis of alkoxysilanes; 2) transferring the resulting transparent sol to an oven for gelation and aging at 80 8C over several hours to promote the siloxane network formation under basic conditions, which is brought up by the hydrolysis of urea into ammonia; 3) washing with alcohol by hand; and 4) evaporative drying under ambient conditions (Figure 1a). The obtained gel (MG1) shows enough marshmallow-like flexibility to recover their original shape from 80% uniaxial compression and 3-point bending (Figure 2; Supporting Information, Figure S1). This material has a superhydrophobic surface with a water contact angle of 1538, which is due to the negligible amount of residual hydrophilic silanol groups, as characterized by Si solid-state cross polarization/ magic angle spinning (CP/MAS) NMR spectroscopy (Supporting Information, Figure S2). However, MG1 does not show oleophobicity, but absorbs organic liquids quickly like a sponge (Figure 3a) as mentioned before. [*] G. Hayase, Dr. K. Kanamori, Prof. K. Nakanishi Department of Chemistry, Graduate School of Science, Kyoto University Kitashirakawa, Sakyo-ku, Kyoto 606-8502 (Japan) E-mail: kanamori@kuchem.kyoto-u.ac.jp

Polymethylsilsesquioxane-cellulose nanofiber biocomposite aerogels with high thermal insulation, bendability, and superhydrophobicity.

Polymethylsilsesquioxane-cellulose nanofiber (PMSQ-CNF) composite aerogels have been prepared through sol-gel in a solvent containing a small amount of CNFs as suspension. Since these composite aerogels do not show excessive aggregation of PMSQ and CNF, the original PMSQ networks are not disturbed. Composite aerogels with low density (0.020 g cm(-3) at lowest), low thermal conductivity (15 mW m(-1) K(-1)), visible light translucency, bending flexibility, and superhydrophobicity thus have been successfully obtained. In particular, the lowest density and bending flexibility have been achieved with the aid of the physical supporting effect of CNFs, and the lowest thermal conductivity is comparable with the original PMSQ aerogels and standard silica aerogels. The PMSQ-CNF composite aerogels would be a candidate to practical high-performance thermal insulating materials.

Grafted Polymethylhydrosiloxane on Hierarchically Porous Silica Monoliths: A New Path to Monolith-Supported Palladium Nanoparticles for Continuous Flow Catalysis Applications

Polymethylhydrosiloxane has been grafted on the surface of a hierarchically porous silica monolith using a facile catalytic reaction between Si-H and silanol to anchor the polymer. This easy methodology leads to the functionalization of the surface of a silica monolith, where a large amount of free Si-H bonds remain available for reducing metal ions in solution. Palladium nanoparticles of 15 nm have been synthesized homogeneously inside the mesopores of the monolith without any stabilizers, using a flow of a solution containing Pd2+. This monolith was used as column-type fixed bed catalyst for continuous flow hydrogenation of styrene and selective hydrogenation of 3-hexyn-1-ol, in each case without a significant decrease of the catalytic activity after several hours or days. Conversion, selectivity, and stereoselectivity of the alkyne hydrogenation can be tuned by flow rates of hydrogen and the substrate solution, leading to high productivity (1.57 mol g(Pd)-1 h-1) of the corresponding cis-alkene.

Transparent, Superflexible Doubly Cross-Linked Polyvinylpolymethylsiloxane Aerogel Superinsulators via Ambient Pressure Drying

Aerogels have many attractive properties but are usually costly and mechanically brittle, which always limit their practical applications. While many efforts have been made to reinforce the aerogels, most of the reinforcement efforts sacrifice the transparency or superinsulating properties. Here we report superflexible polyvinylpolymethylsiloxane, (CH2CH(Si(CH3)O2/2))n, aerogels that are facilely prepared from a single precursor vinylmethyldimethoxysilane or vinylmethyldiethoxysilane without organic cross-linkers. The method is based on consecutive processes involving radical polymerization and hydrolytic polycondensation, followed by ultralow-cost, highly scalable, ambient-pressure drying directly from alcohol as a drying medium without any modification or additional solvent exchange. The resulting aerogels and xerogels show a homogeneous, tunable, highly porous, doubly cross-linked nanostructure with the elastic polymethylsiloxane network cross-linked with flexible hydrocarbon chains. An outstanding combination of ultralow cost, high scalability, uniform pore size, high surface area, high transparency, high hydrophobicity, excellent machinability, superflexibility in compression, superflexibility in bending, and superinsulating properties has been achieved in a single aerogel or xerogel. This study represents a significant progress of porous materials and makes the practical applications of transparent flexible aerogel-based superinsulators realistic.

Transparent Ethenylene-Bridged Polymethylsiloxane Aerogels: Mechanical Flexibility and Strength and Availability for Addition Reaction

Transparent, low-density ethenylene-bridged polymethylsiloxane [Ethe-BPMS, O2/2(CH3)Si-CH═CH-Si(CH3)O2/2] aerogels from 1,2-bis(methyldiethoxysilyl)ethene have successfully been synthesized via a sol-gel process. A two-step sol-gel process composed of hydrolysis under acidic conditions and polycondensation under basic conditions in a liquid surfactant produces a homogeneous pore structure based on cross-linked nanosized colloidal particles. Visible-light transmittance of the aerogels varies with the concentration of the base catalyst and reaches as high as 87% (at a wavelength of 550 nm for a 10 mm thick sample). Gelation and aging temperature strongly affect the deformation behavior of the resultant aerogels against uniaxial compression, and the obtained aerogels prepared at 80 °C show high elasticity after being unloaded. This highly resilient behavior is primarily derived from the rigidity of ethenylene groups, which is confirmed by a comparison with other aerogels with similar molecular structures, ethylene-bridged polymethylsiloxane and polymethylsilsesquioxane. Applicability of the addition reaction using a Diels-Alder reaction of benzocyclobutene has also been investigated, revealing that a successful addition takes place on the ethenylene linkings, which is verified using Raman and solid-state NMR spectroscopies. Insights into the effect of molecular structure on mechanical properties and the availability of surface functionalization provided in this study are important for realizing transparent aerogels with the desired functionality.

Aerogels from Chloromethyltrimethoxysilane and Their Functionalizations

Reactions of chloromethyltrimethoxysilane (CMTMS) and its derived colloidal network polychloromethylsilsesquioxane (PCMSQ) have been investigated to extend the material design strategy toward functionalized and mechanically reinforced aerogels. In a carefully designed sol-gel system, CMTMS has afforded transparent aerogels in the presence of cationic surfactant. The surface chloromethyl groups with polarity and reactivity are shown to be useful for supporting nanostructures, with photoluminescent carbon dots (C-dots) prepared from polyethylenimine and citric acid as an example. Furthermore, since nucleophilic substitution (SN2) reactions on the surface chloromethyl groups are found to control the equilibrium of formation/dissociation of siloxane bonds, a new gelation strategy triggered by SN2 reactions in sol-gel has been developed. In the presence of nucleophilic organic species such as polyamines, a hybrid network consisting of PCMSQ cross-linked with a polyamine nucleophile can be prepared to enhance mechanical properties of aerogel.

Fabrication of hydrophobic polymethylsilsesquioxane aerogels by a surfactant-free method using alkoxysilane with ionic group

Abstract Phase separation control is an important factor to prepare a porous monolith by an aqueous sol–gel reaction. Here, we report a surfactant-free synthesis method to obtain hydrophobic polymethylsilsesquioxane aerogels by copolymerizing a cationic-functionalized alkoxysilane N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride. The resultant materials have low-density, high visible-light transmittance, and high thermal insulating equivalent to those of prepared under the presence of surfactant.

Superflexible Multifunctional Polyvinylpolydimethylsiloxane-Based Aerogels as Efficient Absorbents, Thermal Superinsulators, and Strain Sensors

Aerogels are porous materials but show poor mechanical properties and limited functionality, which significantly restrict their practical applications. Preparation of highly bendable and processable aerogels with multifunctionality remains a challenge. Herein we report unprecedented superflexible aerogels based on polyvinylpolydimethylsiloxane (PVPDMS) networks, PVPDMS/polyvinylpolymethylsiloxane (PVPMS) copolymer networks, and PVPDMS/PVPMS/graphene nanocomposites by a facile radical polymerization/hydrolytic polycondensation strategy and ambient pressure drying or freeze drying. The aerogels have a doubly cross-linked organic-inorganic network structure consisting of flexible polydimethylsiloxanes and hydrocarbon chains with tunable cross-linking density, tunable pore size and bulk density. They have a high hydrophobicity and superflexibility and combine selective absorption, efficient separation of oil and water, thermal superinsulation, and strain sensing.