An environment‐friendly thermal insulation material from cellulose and plasma modification

Cellulose aerogels were prepared by combining the NaOH/thiourea/H₂O solvent system and the freeze‐drying technology. Hydrophobic aerogels were obtained with the cold plasma modification technology. The results showed that cellulose aerogel had good heat insulation performance, while the main factors affecting thermal conductivity were density and porosity. Thermal conductivity decreased with the decrease of density and the increase of porosity. It could be as low as 0.029 W/(m K). Cellulose aerogel adsorbed moisture easily. The moisture adsorption had a significant influence on the heat insulation performance of aerogel. After conducting hydrophobic modification using CCl₄ as plasma, cellulose aerogel was changed from hydrophilic to hydrophobic and water contact angle was as high as 102°. Hydrophobic modification did not affect the heat insulation performance of aerogel. This work provided a foundation for the possibility of applying cellulose aerogels in the insulating material field. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3652–3658, 2013     

Preparation of novel bilayer hydrogels by combination of irradiation and freeze–thawing and their physical and biological properties

BACKGROUND: Hydrogels made by irradiation or freeze–thawing often exhibit poor mechanical strength; therefore we investigated a novel synthetic method to circumvent this detrimental effect. We report a series of novel bilayer poly(vinyl alcohol) (PVA)/water‐soluble chitosan (ws‐chitosan)/glycerol hydrogels prepared by a combination of irradiation and freeze–thawing. Scanning electron microscopy morphology, swelling behavior, mechanical strength, elongation at break, PVA dissolution behavior and bovine serum albumin (BSA) release profile of the bilayer hydrogels were compared with those of hydrogels made by irradiation and freeze–thawing followed by irradiation. The cytotoxicity of the bilayer hydrogels was studied using a tetrazolium salt (MTT) assay. RESULTS: The novel bilayer hydrogels contain one layer made by freeze–thawing followed by irradiation and the other layer made by irradiation. The preparation method provides the two layers with good combination force in the wet state. However, the two layers are not combined very well in the freeze‐dried state due to the difference in microstructure. The bilayer hydrogels have large swelling capacity and good mechanical strength, and these properties can be varied by changing freeze–thawing cycles, irradiation doses and the relative thickness of the two layers. The PVA and BSA release behaviors show that the bilayer hydrogels have a small amount of dissolved PVA and can prolong the BSA release time. The MTT assay shows that extracts of the bilayer hydrogels are non‐toxic towards L929 mouse fibroblasts. CONCLUSION: The novel bilayer hydrogels prepared in this study show good physical properties with no cytotoxicity, indicating that they are suitable for biomedical applications, such as in wound dressings and drug delivery devices. Copyright © 2009 Society of Chemical Industry     

Preparation of multi-membrane alginate aerogels used for drug delivery

A significant amount of investigation has already taken place into alginate spherical hydrogels’ attractiveness for application as drug delivery carriers. One of their weaknesses is, however, their short lives within dry air conditions. In this study we used a multi-step sol–gel process for generating dry and stable complex alginate aerogels with multi-membranes. Firstly, the formation was performed of spherical hydrogels within a CaCl₂ solution. These cores were further immersed in alginate solution, filtered through a sieve, and dropped into the salt solution again. Different multi-membrane hydrogels were obtained by repeating the above process. They were further converted into aerogels by supercritical drying. The effect of the membranes’ numbers was investigated regarding the model drug nicotinic acid's loading and release. The drug loading increased and the drug release was prolonged by adding more membranes around the core. By increasing the amount of drug inside the multi-membrane spheres, the burst drug release was even further inhibited compared to the lower drug-loaded samples.     

High performances of dual network PVA hydrogel modified by PVP using borax as the structure-forming accelerator

A dual network hydrogel made up of polyvinylalcohol (PVA) crosslinked by borax and polyvinylpyrrolidone (PVP) was prepared by means of freezing-thawing circles. Here PVP was incorporated by linking with PVA to form a network structure, while the introduction of borax played the role of crosslinking PVA chains to accelerate the formation of a dual network structure in PVA/PVP composite hydrogel, thus endowing the hydrogel with high mechanical properties. The effects of both PVP and borax on the hydrogels were evaluated by comparing the two systems of PVA/PVP/borax and PVA/borax hydrogels. In the former system, adding 4.0% PVP not only increased the water content and the storage modulus but also enhanced the mechanical strength of the final hydrogel. But an overdose of PVP just as more than 4.0% tended to undermine the structure of hydrogels, and thus deteriorated hydrogels’ properties because of the weakened secondary interaction between PVP and PVA. Likewise, increasing borax could promote the gel crosslinking degree, thus making gels show a decrease in water content and swelling ratio, meanwhile shrinking the pores inside the hydrogels and finally enhancing the mechanical strength of hydrogels prominently. The developed hydrogel with high performances holds great potential for applications in biomedical and industrial fields.     

A facile method to fabricate monolithic alumina–silica aerogels with high surface areas and good mechanical properties

In this study, monolithic alumina–silica aerogels with high surface areas and good mechanical properties were synthesized via a facile sol–gel method without solvent exchange. Furthermore, surface areas, microstructures (up to 1300 °C), and mechanical properties of the prepared alumina–silica aerogels were investigated. The sintering and phase transformations of metastable alumina nanoparticles are suppressed owing to the uniformly distributed Si in the alumina–silica aerogels; therefore, the alumina–silica aerogels can maintain much higher specific surface areas after being calcined at 800 °C (575.5 m²/g), 1000 °C (443.2 m²/g), and 1200 °C (120.6 m²/g) compared to pristine alumina aerogels. In addition, the prepared high surface area alumina–silica aerogels show considerably higher strengths than those obtained in previous works. The compressive stress (3 % strain) and elastic modulus of the alumina–silica aerogels reached 1.78 and 65.6 MPa, respectively. The reported alumina–silica aerogels in this study can be good candidates as high-temperature thermal insulators and catalysts.     

Effect of the thermal treatment on microstructure and physical properties of low-density and high transparency silica aerogels via acetonitrile supercritical drying

In this paper, we reported the experimental results about the effect of the thermal treatment on microstructure and physical properties of low-density and high transparent silica aerogels. From our results, with tetramethyl orthosilicate as precursor and via acetonitrile supercritical drying process, silica aerogel monolith was obtained possessing the properties as low-density (0.018 g/cm³), high surface area (923 m²/g), high optical transparency (87.9 %, 800 nm). It should be noted that high transparency of silica aerogel can be maintained up to 600 °C (91.5 %, 800 nm). The mechanical properties of silica aerogel decreased with increasing heat treated temperature to 600 °C, and silica aerogels still maintained crack-free monoliths completely and possessed high homogeneous density even after 600 °C thermal treatment. Furthermore, thermal conductivity of the monoliths at desired temperatures was analyzed by the transient plane heat source method. When the temperature flowed from 25 to 600 °C, thermal conductivity coefficients of silica aerogels changed from 0.021 to 0.065 W (m K)^?1, revealed an excellent heat insulation effect in high-temperature area. Currently, the specific process developed for low-density aerogels affected by thermal treatment has not been reported in previous literature.     

Gellan/poly(vinyl alcohol) hydrogels: characterization and evaluation as delivery systems

Bioartificial polymeric materials in the form of hydrogels were prepared starting from blends of poly(vinyl alcohol) (PVOH) with gellan, using a procedure based on freeze–thawing cycles. The effect exerted by gellan on the properties of these materials was investigated. The materials were loaded with human growth hormone (GH) and the release of the drug was evaluated. The results obtained indicated that gellan favours the crystallization process of PVOH allowing the formation of a material with a more homogeneous and stable structure than that of pure PVOH hydrogels. Both the PVOH melting enthalpy and the elastic modulus increased with increasing gellan content in the hydrogels; in addition, the higher the gellan content in the samples, the lower was the amount of PVOH released. Gellan/PVOH hydrogels were able to release GH and the release was affected by the content of the biological component. The amount of GH released was within a physiological range. © 2001 Society of Chemical Industry     

Preparation of cellulose–graphene oxide aerogels with N‐methyl morpholine‐N‐oxide as a solvent

Cellulose–graphene oxide (GO) aerogel composites were successfully prepared from cellulose and GO dispersed in N‐methyl morpholine‐N‐oxide monohydrate, a nontoxic and environmentally friendly solvent, after a freeze‐drying process. Because of the strong interactions between the numerous oxygen‐containing groups located on the surface of GO and the functional groups of the cellulose molecules, the GO monolayers were well dispersed in the three‐dimensional porous structure of the cellulose aerogels. With the addition of 10 wt % GO, the swelling ratios and water contents of the composite cellulose–GO aerogels increased from 468 to 706% and from 82.4% to 87.6%, respectively. The corresponding maximum decomposition temperatures also increased from 335 to 353 °C with increasing GO content from 0 to 10%; this indicated that the thermal stability of the cellulose–GO aerogels was enhanced. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46152.     

Thermal and rheological properties of poly(vinyl alcohol) and water‐soluble chitosan hydrogels prepared by a combination of γ‐ray irradiation and freeze thawing

Poly(vinyl alcohol) (PVA)/water‐soluble chitosan (ws‐chitosan) hydrogels were prepared by a combination of γ‐irradiation and freeze thawing. The thermal and rheological properties of these hydrogels were compared with those of hydrogels prepared by pure irradiation and pure freeze thawing. Irradiation reduced the crystallinity of PVA, whereas freeze thawing increased it. Hydrogels made by freeze thawing followed by irradiation had higher degrees of crystallinity and higher melting temperatures than those made by irradiation followed by freeze thawing. ws‐Chitosan disrupted the ordered association of PVA molecules and decreased the thermal stability of both physical blends and hydrogels. All the hydrogels showed shear‐thinning behavior in the frequency range of 0.2–100 rad/s. Hydrogels made by freeze thawing dissolved into sol solutions at about 80°C, whereas those made by irradiation showed no temperature dependence up to 100°C. The chemical crosslinking density of the hydrogels made by irradiation followed by freeze thawing was much greater than that of hydrogels made by freeze thawing followed by irradiation. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Effects of Solvent Exchange Period and Heat Treatment on Physical and Chemical Properties of Rice Husk Derived Silica Aerogels

In this study, hydrophobic silica aerogels were synthesized from rice husk ash-derived sodium silicate through sol-gel processing, solvent exchange, surface modification and ambient pressure drying. By volume, 10% of trimethylchlorosilane (TMCS) in 90% of n-hexane was used as a hydrophobic solution in the surface modification process. The physical and chemical properties of silica aerogels were characterized by density and porosity measurements, scanning electron microscopy (SEM), Fourier transforms infrared (FTIR) spectroscopy, Brunauer–Emmett–Teller theory (BET) and dynamic scanning calorimetry (DSC). The hydrogels prepared were in the form of 2.5 ± 0.5 mm beads and then converted into alcogels through solvent exchange with ethanol for repetition of 3, 6 and 9 days. It is found that the optimal quality of silica aerogels with the BET surface area as high as 668.82 m²/g was obtained from the alcogels of the solvent exchange period of 9 days. Depending on the size of the gel’s block, a longer solvent exchange period will ensure adequate removal of pore water. Post heat treatment on silica aerogels obtained from the 9 days of solvent exchange at 200, 300 and 400 °C for 2 h results in slight decreased of aerogel’s density from 0.048 g/cm³ to 0.039 g/cm³ and the hydrophobicity of the aerogels is decreased above 380 °C as confirmed by DSC analysis.

     

Potential applications of poly(vinyl alcohol)‐congo red aqueous solutions and hydrogels as liquids for hydraulic fracturing

The authors report on the viscoelastic characterization of guar hydrogels obtained through complexation reactions with borax ions. These gels are compared with hydrogels obtained from poly(vinyl alcohol) of different degree of hydrolysis through complexation reactions with congo red. The effect of the degree of hydrolysis and both, the concentration of PVA and the concentration of congo red, on the viscoelastic properties of the hydrogels is analyzed. The potential use of the PVA‐based hydrogels as hydraulic fracturing liquids is discussed in relation to the commonly used fracturing liquid based on the guar–borax system. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Review od ADVANCES IN DRYING,Vol. 3,

RF hydrogels were synthesized by the sol-gel polycondensation of resorcinol with formaldehyde and RF cryogels were prepared by freeze drying of the hydrogels with t-butanol. The cryogels were characterized by nitrogen adsorption, density measurements, and scanning electron microscope. Their porous properties were compared with those of the aerogels prepared by supercritical drying with carbon dioxide. RF cryogels were mesoporous materials with large mesopore volumes >5.8× 10^?4m³/kg. Although surface areas and mesopore volumes of the cryogels were smaller than those of the aerogels, the cryogels were useful precursors of mesoporous carbons. Aerogel-like carbons (carbon cryogels) were obtained by pyrolyzing RF cryogels in an inert atmosphere. The carbon cryogels were mesoporous materials with high surface areas >8.0× 10⁵m²/kg and large mesopore volumes >5.5× 10^?4m³/kg. When pyrolyzed, micropores were formed inside the cryogels more easily than inside the aerogels.     

Novel silica-modified boehmite aerogels and fiber-reinforced insulation composites with ultra-high thermal stability and low thermal conductivity

Alumina–silica composite aerogels have drawn vast attention due to their enhanced thermal stability compared to pristine alumina aerogels. However, they are generally weakly-crystallized and tend to experience inevitable sintering and significant surface area loss especially above 1200 °C. In this study, we developed a hydrothermal treatment and supercritical drying strategy for synthesizing novel, well-crystallized, silica-modified boehmite aerogels and fiber-reinforced composites. For the prepared aerogel, network coarsening was significantly hindered and the α-Al₂O₃ transition was completely prevented even at 1400 °C. As a result, the aerogel exhibits extremely high surface area maintenance (87 % and 53 % after 1300 °C and 1400 °C calcination, respectively) and low linear shrinkage (14 % after 1300 °C calcination) at elevated temperatures. The composite with good toughness shows excellent heat resistance and thermal insulating performance up to 1500 °C. These findings provide a general, direct new idea to improve the thermal tolerance of alumina-based aerogels and extend their applications to higher temperatures.     

SiC junction enhanced carbon nanofiber aerogels with extremely high specific strength and ultra-low thermal conductivity

C/SiC aerogels with both ultra-low thermal conductivity and extremely high strength were fabricated by freeze casting. SiC junctions originated from pyrolysis of polycarbosilane (PCS) were formed between carbon nanofibers (C_f) to enhance the strength of aerogels. The effects of PCS content and total solid content on the phase composition, pore structure, thermal conductivity and compressive property were studied. The fabricated aerogels possess hierarchical pore structure. In the micro-scale, it contains circular pores with size of about 15 µm, while it is mesoporous and macroporous in the nano-scale. Both thermal conductivity and compressive strength increase with the increase in PCS content. Through tailoring PCS content and total solid content, C_f/SiC aerogels with porosity of 99.5%, thermal conductivity of 33 mW·m⁻¹·K⁻¹ and compressive strength of 7.14 MPa can be obtained. The specific strength of the fabricated C_f/SiC aerogels is up to 467.6 MPa/(g/cm³), which is the highest value for ceramic aerogels.     

Harmless hydrogels based on PVA/Na+‐MMT nanocomposites for biomedical applications: Fabrication and characterization

A series of nanocomposite hydrogels were prepared by a freeze‐thaw process, using polyvinyl alcohol (PVA) as polymer matrix and 0–10 wt% of hydrophilic natural Na‐montmorillonite (Na⁺‐MMT), free from any modification, as composite aggregates. The effect of nanoclay content and the sonication process on the nanocomposite microstructure and morphology as well as its properties (physical, mechanical, and thermal) were investigated. The microstructure and morphology were investigated by Fourier transform infrared spectroscopy, field emission scanning electron microscopy, transmission electron microscopy, atomic force microscopy, and X‐ray diffraction technique. The thermal stability and mechanical properties of nanocomposite hydrogels were examined using thermogravimetric analysis, differential scanning calorimetry, dynamic mechanical analysis; moreover hardness and water vapor transmission rate measurements. It was concluded that the microstructure, morphology, physical (thermal) and mechanical properties of nanocomposite hydrogels have been modified followed by addition of nanoclay aggregates. The results showed that Na⁺‐MMT may act as a co‐crosslinker. Based on the results obtained, the nanocomposite hydrogel PVA/Na⁺‐MMT synthesized by a freeze‐thaw process, appeared to be a good candidate for biomedical applications. POLYM. COMPOS., 38:1135–1143, 2017. © 2015 Society of Plastics Engineers     

Fabrication and properties of anisotropic polyimide aerogels with aligned tube-like pore structure

Polyimide (PI) aerogels with highly aligned tube-like pores were fabricated by unidirectional ice crystal-induced self-assembly method. During this process, the mold bottom contacted with the freezing medium, the aqueous solution of poly(amic acid) (PAA) ammonium salt in the mold was unidirectionally frozen, the ice crystals grew from the bottom to top of PAA ammonium salt (PAS) solution along the freezing direction, which endowed PI aerogels with aligned tube-like pores after sublimation of ice crystals and thermal imidization of PAS. The obtained aerogels had low densities (0.077–0.222 g cm⁻³) and high porosities (83.8–94.2%) and exhibited anisotropic morphology and properties. Their compression strength in vertical direction (parallel to freezing direction) was higher than that in horizontal direction (perpendicular to freezing direction). Their heat transport in horizontal direction was much slower than that in vertical direction; the aerogels had better thermal insulating property in horizontal direction. This facile approach contributed to prepare new type of PI aerogel materials. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48769.     

Anisotropic polyimide aerogels fabricated by directional freezing

In this study, we adapted a simple, low-cost, and environmentally friendly method to fabricate anisotropic polyimide (PI) aerogels. During directional freezing of an aqueous poly(amic acid) ammonium salt solution from the profile to the center axis of the cylindrical mold, the ice crystals preferred to grow along the radial direction of the cylindrical mold. After the ice crystals were sublimated by freeze drying, an anisotropic pore structure was formed in the aerogels. The prepared PI aerogels had lower densities (0.04–0.22 g/cm³) and higher porosities (84–97%) and exhibited anisotropy in both their pore structures and properties. Their compressive modulus and strength in the horizontal direction were both higher than those in the vertical direction, and they also had good compression recovery in the vertical direction. Moreover, their heat-transfer performance also exhibited anisotropy. The heat transfer in the horizontal direction of the aerogels was much faster than that in the vertical direction. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47179.     

Rapid preparation of Palygorskite/Al2O3 composite aerogels with superhydrophobicity,high-temperature thermal insulation and improved mechanical properties

Al₂O₃ aerogels are widely employed in heat insulation and flame retardancy because of their unique combination of low thermal conductivity and exceptional high-temperature stability. However, the mechanical properties of Al₂O₃ aerogel are poor, and the preparation time is considerably long. In this study, we present a simple and scalable approach to construct monolithic Pal/Al₂O₃ composite aerogels using solvothermal treatment instead of traditional solvent replacement, which remarkably shortened the preparation time. Subsequently, to obtain stable superhydrophobicity (θ > 152°), the Pal/Al₂O₃ aerogel was modified by gas-phase modification method. The obtained Pal/Al₂O₃ composite aerogels demonstrate the integrated properties of low density (0.078–0.106 g/cm³), low thermal conductivity (1000 °C, 0.143 W/(m·K)), good mechanical properties (Young's modulus, 1.6 MPa), and good heat resistance. The monolithic Pal/Al₂O₃ composite aerogels with improved mechanical performance and improved thermal stability can show great potential in the field of thermal insulation.     

A new strategy for porous carbon synthesis: Starch hydrogel as a carbon source for Co3O4@C supercapacitor electrodes

A novel Co₃O₄@C composite with a three-dimensional (3D) interconnected network morphology was successfully fabricated by anchoring cobalt oxide nanocrystals onto porous carbon originating from starch hydrogels via freeze drying, precarbonization and thermal treatment in an aqueous system. Benefiting from unique structural features, the optimized electrode delivers an excellent capacitance of 1314.0 F g^?1 (1 A g^?1) and outstanding durability in terms of capacity preservation (93.5% over 10,000 cycles). In addition, an asymmetric supercapacitor consisting of DF-2 and active carbon exhibits an energy density of 149.1 Wh?kg^?1 at 800 W kg^?1 while maintaining great stability. The observed excellent performance is attributed to the unique 3D network, good conductivity and high surface-to-volumetric ratio of the carbon skeleton derived from the starch gel, which has wide scope for applications.     

Investigation on the formation mechanisms of hydrogels made by combination of γ‐ray irradiation and freeze‐thawing

Hydrogels based on poly(vinyl alcohol) (PVA)/water‐soluble chitosan (ws‐chitosan)/glycerol were prepared by γ‐ray irradiation, freeze‐thawing, and combination of γ‐ray irradiation and freeze‐thawing, respectively. The influence of freeze‐thawing cycles, the irradiation doses, and the sequence of freeze‐thawing and irradiation processes on the rheological, swelling, and thermal properties of these hydrogels was investigated to evaluate the formation mechanisms of hydrogels made by combination of irradiation and freeze‐thawing. For hydrogels made by freeze‐thawing followed by irradiation, the physical crosslinking is destroyed partially while chemical crosslinking is formed by irradiation. However, the chemical crosslinking density reduces with the increase of freeze‐thawing cycles. Hydrogels made by irradiation followed by freeze‐thawing bear less degree of physical crosslinking with the increase of irradiation dose for the increased chemical crosslinking density. It is found that these hydrogels own larger swelling capacity and better transparent appearance than those made by freeze‐thawing followed by irradiation. Moreover, the former hydrogels have larger mechanical strength than the latter at low freeze‐thawing cycles. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008