Haitao Wang

Hierarchical polyimides (PIs) not only show outstanding thermal stability and high mechanical strength but also have great advantages in terms of microstructure and surface area, which makes them highly valuable in various fields such as aerospace, microelectronics, adsorption, catalysis, and energy storage. However, great challenges still remain in the synthesis of hierarchical PIs with well‐defined microstructure. Herein, polyamide acid salts (PAAS) with tunable ionization degree are synthesized first via the polymerization of dianhydride and diamine monomers in deionized water with 1,2‐dimethylimidazole (DMIZ). Then cationic cetyltrimethylammonium chloride (CTAC) is added to the PAAS aqueous solution to induce the formation of polyelectrolyte‐surfactant complexes based on electrostatic interaction. After a typical hydrothermal reaction (HTR) procedure, hierarchical PIs with different microstructures such as urchin‐like PI microparticles, flower‐like PI microparticles, and lamellar PI petals can be fabricated simply by changing the additive amount of DMIZ and CTAC. The nanostructure self‐assemblies of PAAS are dominated by the charges on macromolecular chains and the formation of hierarchical structures of polymers is ascribed to a geometrical selection process during crystal growth. This work provides valuable insights into the self‐assembly behaviors of polyelectrolyte systems for synthesizing well‐defined hierarchical polymers.

Water‐in‐water (w/w) emulsions have been recognized for their broad applications in foods, cosmetics, and biomedical engineering. In this work, silica Janus nanosheets (JNs) with polyacrylic acid (PAA) chains grafted on one surface via crushing functional silica foams, and used silica JNs as Pickering stabilizer to produce stable water‐in‐water (w/w) emulsions from the aqueous two‐phase system (ATPS) containing methacrylic acid (MAA) and NaCl are prepared. The interfacial area of w/w emulsions increases linearly with the concentration of silica JNs, and the interfacial coverage of nanosheets is calculated to be about 98%. After polymerizing w/w emulsions prepared from MAA/NaCl ATPS, it is found that silica JNs are entrapped at the interface of w/w emulsions with the smooth PAA‐grafted surface located toward MAA‐rich phase due to their specific interaction. These results show that functional silica JNs can be used as a promising amphiphilic Pickering stabilizer to produce well‐defined w/w emulsions for numerous application fields.

Multifunctional silica-based aerogels are an emerging material due to their unique properties and wide applications. However, their large-scale production and application are limited due to the high cost and cumbersome preparation process. Herein, we prepare graphene oxide (GO)/silica-based composite aerogels via a simple in situ sol–gel reaction. GO nanosheets (GOs) are functionalized with polyethylenimine (PEI) and 3-glycidyloxypropyltrimethoxysilane (GPTMS) successively. After a cohydrolysis and condensation of trimethoxymethylsilane (MTMS) and dimethoxydimethylsilane (DMDMS) in the presence of GOs and a convenient ambient-pressure drying process, the composite aerogels are obtained. In addition to the normal cross-linking of MTMS and DMDMS, the GOs also behave as cross-linking points to significantly enhance the mechanical properties and thermal stability of the network of the composite aerogels. The pore structure of the aerogels is tailored by varying the GO loads as well as its surface modification. The Young’s modulus of a composite aerogel with a GO load of 0.5 wt % is about 5 times that for a neat polysiloxane aerogel, and the maximum degradation rate temperature is increased to over 90 °C. Compared with pure polysiloxane aerogel, the thermal insulation and flame resistance are also improved by a small addition of GOs. Moreover, GO/silica-based composite aerogels show stable piezo-resistive behavior. With the excellent mechanical properties, thermal stability, and multifunctionality, GO/silica-based composite aerogels show promising applications under some harsh and extreme conditions.

Functional porous materials show extensive applications in environment, biology, aerospace and so on. In this work, generation of silica foams and functionalization of pore surface were simultaneously realized through an interfacial sol-gel reaction within high internal phase emulsion (HIPE) microreactors, where a hyperbranched polyethoxysiloxane (PEOS) was used as the sole stabilizer for the HIPEs. With various functional substances containing amino, epoxy and carboxyl groups initially dissolved in the aqueous phase of HIPEs, these functional groups could be grafted onto the pore surface in the process of forming silica foams. Amino-functionalized silica foam showed fast adsorption of sunset yellow, and the adsorption capacity could reach as high as 1213.13 mg/g. Sodium polyacrylate-modified silica foam exhibited good adsorption capacity of cationic dyes and metal ions, e.g. 280.11 mg/g to methylene and 226.24 mg/g to Cu(II). Epoxy-functionalized silica foam particles were confirmed with a pronounced activity at oil/water interface due to their Janus-like surface, which could be used as Pickering stabilizer. This HIPEs-based synthesis strategy for silica foams shows promising future in adsorption, emulsion stabilizer and compatilizer.