One-Pot Sol-Gel Synthesis of Highly Insulative Hybrid P(AAm-CO-AAc)-Silica Aerogels with Improved Mechanical and Thermal Properties

Silica aerogels and their derivatives have outstanding thermal properties with exceptional values in the thermal insulation industry. However, their brittle nature restricts their large-scale commercialization. Thus, enhancing their mechanical strength without affecting their thermal insulating properties is essential. Therefore, for the first time, highly thermally stable poly(acrylamide-co-acrylic acid) partial sodium salt is used as a reinforcing polymer to synthesize hybrid P(AAm-CO-AAc)-silica aerogels via epoxy ring-opening polymerization in the present study. Functional groups in P(AAm-CO-AAc) partial sodium salts, such as CONH2 and COOH, acted as nucleophiles for the epoxy ring-opening reaction with (3-glycidyloxypropyl)trimethoxysilane, which resulted in a seven-fold enhancement in mechanical strength compared to that of pristine silica aerogel while maintaining thermal conductivity at less than 30.6 mW/mK and porosity of more than 93.68%. Moreover, the hybrid P(AAm-CO-AAc)-silica aerogel demonstrated improved thermal stability up to 343 °C, owing to the synergetic effect between the P(AAm-CO-AAc) and the silica aerogel, corresponding to the thermal stability and strong covalent bonding among them. These excellent results illustrate that this new synthetic approach for producing hybrid P(AAm-CO-AAc)-silica aerogels is useful for enhancing the mechanical strength of pristine silica aerogel without impairing its thermal insulating property and shows potential as an industrial heat insulation material.

Fabrication of Mechanically Strong Silica Aerogels with the Thermally Induced Phase Separation (TIPS) Method of Poly(methyl methacrylate)

Constructing and maintaining a three-dimensional network structure with high porosity is critical to the preparation of silica aerogel materials because this structure provides excellent properties. However, due to the pearl-necklace-like structure and narrow interparticle necks, aerogels have poor mechanical strength and a brittle nature. Developing and designing lightweight silica aerogels with distinct mechanical properties is significant to extend their practical applications. In this work, thermally induced phase separation (TIPS) of poly(methyl methacrylate) (PMMA) from a mixture of ethanol and water was used to strengthen the skeletal network of aerogels. Strong and lightweight PMMA-modified silica aerogels were synthesized via the TIPS method and supercritically dried with carbon dioxide. The cloud point temperature of PMMA solutions, physical characteristics, morphological properties, microstructure, thermal conductivities, and mechanical properties were investigated. The resultant composited aerogels not only exhibit a homogenous mesoporous structure but also achieve a significant improvement in mechanical properties. The addition of PMMA increased the flexural strength and compressive strength by as much as 120% and 1400%, respectively, with the greatest amount of PMMA (M_w = 35,000 g/mole), while the density just increased by 28%. Overall, this research suggests that the TIPS method has great efficiency in reinforcing silica aerogels with less sacrifice of low density and large porosity.

Hollow polymer microcapsule embedded transparent and heat-insulating film

We herein report a facile and scalable approach to manufacturing optically transparent and heat-insulating films by incorporating hollow poly(methyl methacrylate) microcapsules into a transparent polymeric matrix. The microcapsule was prepared via emulsion polymerization. The size of the microcapsules could be easily controlled from ∼1 to 3 μm by varying the polymerization time in a narrow size distribution. The microcapsules were then mixed with a UV-curable transparent liquid resin and cured by a subsequent light irradiation. The current approach could enhance the thermal barrier property of the films without a significant reduction in the optical transparency. The solid film possessing 30 wt% microcapsules, for example, exhibited a high visible light transmittance (∼80% as measured by UV-vis spectroscopy) and the thermal conductivity was reduced to 0.06 W mK⁻¹, corresponding to 46% of the capsule free film. To quantify and verify this result, theoretical models describing a heat transfer in a hollow microsphere composite were used, and the model showed a good agreement with our experimental observations.

Highly transparent, heat-insulating films were manufactured by incorporating hollow poly(methyl methacrylate) microcapsules into a transparent polymeric film.

One-Pot Synthesis and Characterization of Polyrotaxane-Silica Hybrid Aerogel

A novel kind of polyrotaxane-silica hybrid aerogel is successfully prepared via one-pot sol-gel synthesis in this work. The polyrotaxane can chemically interpenetrate with Si particles homogeneously in nanoscale, so as to shorten the gelation time and construct a flexible and mechanically strong skeleton. The supramolecular effect ascribable to the sliding motion of cyclic components in polyrotaxane is introduced into the hybrid aerogel for the first time. Compared with the brittle pure silica aerogel, the obtained polyrotaxane-silica hybrid aerogels show very low density, low thermal conductivity, and more than two orders magnitude improvement in the compression strength without compromising transparency.

Aerogel Microparticles from Oil-in-Oil Emulsion Systems

This paper reports preparation of polymer aerogel microparticles via sol-gel reactions inside micrometer size droplets created in an oil-in-oil emulsion system. The oil-in-oil emulsion system is obtained by dispersing in cyclohexane the droplets of the sols of polybenzoxazine (PBZ) or polyimide (PI) prepared in dimethylformamide. The sol droplets transform into harder gel microparticles due to sol-gel reactions. Finally, the aerogel microparticles are recovered using supercritical drying of the gel microparticles. The PBZ and PI aerogel microparticles prepared in this manner show mean diameter 32.7 and 40.0 μm, respectively, mesoporous internal structures, and surface area 55.4 and 512.0 m(2)/g, respectively. Carbonization of PBZ aerogel microparticles maintains the mesoporous internal structures but yields narrower pore size distribution.