Unidirectional Freeze-drying Method Research Articles

Amine-impregnated elastic carbon nanofiber aerogel templated by bacterial cellulose for CO2 adsorption and separation

Carbon aerogel has become a promising electrode material for CO2 capture and capacitor due to its large specific surface area, well-developed pore structure, adjustable porosity and ultra-high specific power. However, most of the carbon-based materials suffer from poor mechanical properties, low recycling efficiency, and low adsorption capacity, making it difficult to be put into industrial applications on a large scale. Bacterial cellulose (BC) has an ultra-fine reticular structure, a modulus of elasticity that is several to more than ten times that of typical plant fibers, and high tensile strength. In this paper, BC was treated by unidirectional freeze-drying method, and the aerogels with ordered honeycomb pore structure were constructed by unidirectional growth of ice crystals, and the pyrolytic chemical properties of BC were adjusted by using (NH4)2SO4, which effectively prevented the shrinkage and deformation of materials during carbonization, and successfully prepared elastic nanocarbon fiber aerogels. At the same time, the introduction of (NH4)2SO4 made the material realize N doping in the carbonization process, providing more adsorption sites, which was conducive to the adsorption of CO2. Finally, the carbon fiber nanoaerogel was modified by TEPA to further improve its adsorption selectivity for CO2. The prepared elastic carbon nanofiber aerogel (CBCNT) has high CO2 adsorption performance and high selectivity, and its CO2 capture capacity is up to 4.88 mmol/g. At the same time, its capacitance also reached 246F/g.

Composite aerogel with oriented structure of polyimide@ polypyrrole /MXene for effective shielding against electromagnetic waves and insulation

Although the rapid evolution of electromagnetic wave technology make much progress to society, but has also caused electromagnetic pollution, which poses risks to health, information security, and disrupts communication and device operation. The development of new Electromagnetic Interference (EMI) shielding materials, especially those that are lightweight, easy to process and high shielding efficiency, has become the key to overcoming this challenge. In this study, MXene nanosheets with few layers were fabricated by chemical etching with lithium fluoride (LiF) and hydrochloric acid (HCl). Subsequently, the unidirectional freeze-drying method and vacuum-assisted impregnation method were applied to fabricate polyimide@ polypyrrole/MXene (PI@PPy/MXene) composite aerogel with oriented structure. The electromagnetic shielding properties of the oriented PI@PPy composite aerogel were modulated by changing the polymerization time of PPy to control its conductivity. When the polymerization time was set as 7 h, the oriented PI@PPy composite aerogel showed the highest EMI efficiency, the value of EMI SE was 19.36 dB. With the introduction of MXene, the EMI SE of the PI@PPy/MXene composite aerogel was increased compared to PI@PPy and reached to 32.64 dB. The improvement in electromagnetic shielding performance was attributed to the co-enhancement among the PI aerogel with oriented structure, PPy and MXene, which is the result of the synergisic effect of Multiple reflection losses and polarization losses within the aerogel. The infrared thermography at 200°C showed that with increasing PPy polymerization time, the external temperatures of the samples are 40.3°C, 44.8°C, 51.4°C, 55.4°C and 48.6°C, demonstrating the excellent thermal insulation properties.

Soft and flexible polyvinyl alcohol/pullulan aerogels with fast and high water absorption capacity for facial mask substrates

Facial mask substrates commonly used in skincare are often considered unhealthy and environmentally unfriendly due to their composition of premoistened nonwovens containing various preservatives. This study aims to address this issue by developing a preservative-free degradable aerogel made from polyvinyl alcohol (PVA)/pullulan (PUL) using a unidirectional freeze-drying method. The aerogels had ordered three-dimensional porous structures and exhibited desirable mechanical properties. They were soft and flexible in both dry and wet states, and their Young's moduli were comparable to that of human skin. The aerogels had high porosity, ranging from 93.0 % to 95.1 %, and exhibited a high water absorption rate and water absorption capacity (ranging from 7.5 g/g to 10.1 g/g). After 30 min of water evaporation, the aerogels showed excellent moisture retention, ranging from 88 % to 93 %. Additionally, the PVA/PUL aerogel efficiently loaded and released active ingredients, such as rapidly releasing ascorbic acid (> 90 % within 30 min). These findings suggest that the PVA/PUL aerogel has potential as a material for facial mask substrates.

Devisable pore structures and tunable thermal management properties of aerogels composed of carbon nanotubes and cellulose nanofibers with various aspect ratios

The anisotropic cellulose nanofiber (CNF)/carbon nanotube (CNT) aerogels hold a great promise in directional applications due to their distinct xylem-like aligned penetrating pore structures. The aspect ratio of CNF plays a crucial role in the pore structures of aerogels, directly dominating the final macroscopic properties of materials. Herein, three types of CNF with different aspect ratios were extracted through the 2,2,6,6-tetrmethylpiperidine-1-oxyl radical (TEMPO) oxidation process by changing the doses of oxidant. The corresponding anisotropic CNF/CNT aerogels were prepared by the unidirectional freeze-drying method and then their pore morphologies and properties were investigated in detail. The resulting aerogel with the shortest aspect ratio of CNF exhibited the densest porous structure, thereby obtaining the highest compressive strength of 110 kPa and elastic modulus of 383 kPa, while that containing the longest CNF possessed the highest thermal conductivity coefficient of 0.17 W m−1 K−1 and the worst thermal insulation. This research explored the relationship between the properties of the CNF/CNT aerogels and devisable pore structures caused by various aspect ratios of CNF, thus providing a new insight into the development of CNF/CNT aerogels with tunable performances.

Anisotropic MWCNT/polyimide aerogels with multifunctional EMI shielding and strain sensing capabilities

The demand for electromagnetic interference (EMI) shielding materials has become increasingly urgent due to the widespread application of electronic communication and devices. Herein, anisotropic composite polyimide aerogels (CPIAs) were prepared via unidirectional freeze-drying method using PI as the polymer matrix and MWCNTs as the filler. These CPIAs demonstrated excellent shielding behavior with an average shielding effectiveness value of 84.2 dB (CPIA-4) in the X-band. Owing to its anisotropic microstructure, an idealized model for determining the attenuation constant was proposed. The CPIA also works effectively as a lightweight sensitive strain sensor (gauge factor of 1.23 and stability cyclic testing over 1000 cycles). Remarkably, the CPIAs maintained stable response performance across a wide temperature range (25, 177, and 316 °C) and good performance when assembled into a strain alarm system, indicating promising potential for applications in harsh environments such as arc-shaped structure radar patch shielding materials or landing protection devices for aerobats.

Unidirectional thermal conduction in electrically insulating phase change composites for superior power output of thermoelectric generators

Owing to their ability to absorb excess heat within narrow temperature ranges, phase change materials (PCMs) have attracted significant interest in the thermal management of power equipment and energy devices. However, the low thermal conductivity of PCMs severely limit their application in high-power devices. Current methods for increasing the thermal conductivity are generally at the cost of lowering the phase change enthalpy (i.e., latent heat) of PCMs. In this study, we developed phase change composites (PCCs) based on highly oriented aramid nanofiber/boron nitride nanosheets (ANF/BNNS) aerogels impregnated with paraffin; the aerogels were prepared by unidirectional freeze-drying method and were impregnated with the PCM, namely, paraffin, by a vacuum assisted method. The ANF/BNNS PCCs exhibited high thermal conductivity enhancement efficiency (TCEE), ultra-high latent heat retention, and superior electrical insulation. For a loading of 2.21 wt% BNNS, the TCEE and the latent heat retention of the PCCs reached impressive values of 89.2%, and 93.64%, respectively. In addition, the PCCs are highly electrically insulating with an electrical conductivity of <1.17 × 10−14 S cm−1. The PCCs also maintain good shape stability even at temperatures higher than the melting point of the PCM (i.e., paraffin). The ANF/BNNS PCCs were shown to reduce the cold-side temperature of thermoelectric generators by more than 10 °C in comparison to that achieved by those operated by air-cooling, resulting in an approximately 300% increase in power output. These PCCs, with both high latent heat retention and TCEE, offer new opportunities for use as advanced thermal management materials in future high-power devices.

Anisotropic all-aromatic polyimide aerogels with robust and high-temperature stable properties for flexible thermal protection

• Unidirectional anisotropic PIAs have been prepared by freeze-drying. • The PIA shows excellent thermal stability and stable mechanical behavior. • The u -PIA-6 exhibits great strength, which is over twice as high as that of the TEEK-HH at 177 °C with a similar density. • The u -PIA shows great thermal insulation properties in anisotropic. Polyimide aerogels (PIAs) are the new type of high-performance lightweight material that combines the excellent thermal and mechanical behavior of polyimides with the highly porous and lightweight properties of aerogels. PIAs can meet the increasing material requirements of high-tech fields such as the aerospace and transportation industries. This study reports a series of all-aromatic anisotropic polyimide aerogels (p-phenylenediamine (PDA) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA)) with unprecedented behavior. These PIAs were synthesized using a random or unidirectional freeze-drying method followed by thermal imidization of the precursor solution. Their mechanical and thermal conductivity performance were anisotropic ( u -PIA-8 strength: 1.86 and 0.14 MPa, thermal conductivity: 62.9 and 50.1 mW·m −1 ·K −1 along the axial and radial direction, respectively). Furthermore, the PIAs possessed excellent compression behavior in a wide temperature range from 25 °C to 316 °C, with a compressive strength over twice that of the commercial polyimide foam TEEK-HH ( u -PIA-6 strength: 0.83 and 0.32 MPa at 177 and 316 °C, TEEK-HH strength: 0.31 MPa at 177 °C). These aerogels also demonstrated good thermal insulation performance after being held at 300 °C for 20 min ( u -PIA-8: 76.3 °C at 25 mm). These highly anisotropic aerogels exhibit flexible thermal protection properties that extend the potential use of PIAs to applications such as high-temperature-resistant rocket fairings, sandwich structures, and arc-shaped structures for aerobats or the automobile industry.

Effects of interactions with metal ions on the thermal reduction of graphene oxide

The thermal reduction of graphene oxide (GO) is an efficient and green method for preparing graphene. Herein, the effect of metal ions interacting with oxygen functional groups on GO on its thermal reduction properties are reported for the first time. Graphene oxide aerogels (GOAs) modified with three different metal ions (Li+/Mg2+/Ce3+) have been prepared by a unidirectional freeze-drying method (UFDM). Scanning electron microscopy (SEM) images have revealed that metal ions obviously improve the 3D porous structure of GOA. Based on an analysis of Fourier-transform infrared (FTIR) spectra and X-ray photoelectron spectroscopy (XPS), different interactions, such as coordination, chelation, and hydrogen bonding, as well as ring-opening reaction, between metal ions and functional groups on GO lead to dramatic decreases in the bond energies of functional groups, transforming high-energy chemical bonds into low-energy chemical bonds. Consequently, the thermal reaction temperature of the modified GOA drops by 10–50 °C compared with the pure GOA. Moreover, interactions with metal ions promote the thermal reduction of GOA. Indeed, the exothermic energy densities of the GOA in the three metal-ion-modified GOAs are 20–580 J/g larger than that of pure GOA. Hence, the reduction products of GOAs modified by metal ions are more stable.

Cyclotriphosphazene-bridged periodic mesoporous organosilica-integrated cellulose nanofiber anisotropic foam with highly flame-retardant and thermally insulating properties

Thermally insulating materials derived from renewable cellulose nanofibers (CNF) have attracted an increasing attention from researchers. However, CNF insulating materials exhibit high combustion characteristic, restricting the potential for constructing the energy-efficient buildings. In this study, we synthesized cyclotriphosphazene-bridged periodic mesoporous organosilica (PMOPZ) for the first time. Afterwards, PMOPZ was integrated with CNF to design an anisotropic composite foam via the unidirectional freeze-drying method. The anisotropic composite foam exhibits superior flame-retardant and thermally insulating performance with low density (16.6 kg/m3) and high porosity (99.2%). PMOPZ/CNF foam has an ultra-low thermal conductivity of 27 mW m−1 K−1 in the radial direction, approaching the value of air (25 mW m−1 K−1). Compared to CNF foam, the integration of PMOPZ results in the lowest peak heat release rate (18.2 kW/m2) and total heat release (0.53 MJ/m2), corresponding to a 72.4% and 68.5% reduction respectively. The gaseous and condensed analysis reveals that silica wrapped with intumescent char residues assembles together to restrain the permeation of exterior heat and oxygen and inhibit the effusion of pyrolytic volatiles. Moreover, the composite foam also has high mechanical strength with a specific Young’s modulus of 96 kN m kg−1. Therefore, this work provides a novel strategy to achieve cellulose-based insulating materials with high performance and fire safety.

A Directional Strain Sensor Based on Anisotropic Microhoneycomb Cellulose Nanofiber-Carbon Nanotube Hybrid Aerogels Prepared by Unidirectional Freeze Drying.

Aerogels are one of the most popular composite reinforcement materials because of their high porosity and their continuous and homogeneous network. Most aerogels are isotropic, thus leading to isotropic composites when they are used as fillers. This fundamentally limits their applications in areas where anisotropy is needed. Here, an anisotropic microhoneycomb cellulose nanofiber- (CellF)-carbon nanotube (CNT) aerogel (denoted MCCA) is reported that contains unidirectionally aligned penetrating microchannels, which is prepared by a unidirectional freeze-drying method, using the structure-directing function of the CellFs. Due to its anisotropic nature, MCCA-reinforced polydimethylsilexane (denoted MCCA/PDMS) shows distinct anisotropic behavior, with the electrical conductivity and Young's modulus along the direction of penetrating microchannels being approximately twice those in the orthogonal direction. MCCA/PDMS is used to make "directional" strain sensors with electrical resistance as the output signal. They demonstrate a 92% sensitivity difference between the microchannel direction and its orthogonal direction. This approach can be used to prepare anisotropic MCCA-based composites with other polymers for different applications.

Scaffold with Orientated Microtubule Structure Containing Polylysine-Heparin Sodium Nanoparticles for the Controlled Release of TGF-β1 in Cartilage Tissue Engineering.

Articular cartilage defects cannot adequately self-repair because of avascular and complex tissues. Although various scaffold materials have been reported to promote cartilage repair, challenges remain in this field. In the present study, an orientated microtubules scaffold that consisted of collagen, chitosan, silk fibroin, and polylysine-heparin sodium nanoparticles containing transforming growth factor β1 (TGF-β1), was constructed using a unidirectional freeze-drying method. The collagen/chitosan/0.5silk fibroin scaffolds showed optimal properties via the evaluation of physicochemical characterization, mechanical properties and biocompatibility. The collagen/chitosan/0.5silk fibroin scaffolds containing polylysine-heparin sodium nanoparticles loaded with TGF-β1 (COL/CS/0.5SF-TPHNs) could control the slow release of TGF-β1. Simultaneously, the in vitro COL/CS/0.5SF-TPHNs scaffolds exhibited not only an excellent biocompatibility to support mouse mesenchymal stem cells (mBMSCs) adhesion and proliferation, but also exhibited excellent repair effect in cartilage defects of rabbit models in vivo. Our results showed that the COL/CS/0.5SF-TPHNs scaffolds have the potential to induce articular cartilage formation, mainly because TGF-β1 could induce the mBMSCs to differentiate into chondrocytes and osteoblasts. Consequently, the current study can achieve cartilage repair, and COL/CS/0.5SF-TPHNs scaffolds are expected to be potential composites for cartilage tissue engineering.

Robust carbon nanotube foam for efficient electromagnetic interference shielding and microwave absorption

Lightweight and robust carbon nanotube (CNT)/chitosan (CS) foams were assembled by a facile unidirectional freeze-drying method in this work. The CNT/CS foam exhibited an excellent electromagnetic interference (EMI) shielding effectiveness (SE) of 37.6 dB while the density was only 17.6 mg·cm−3, and thus the corresponding specific SE was up to 8556 dB·cm2·g−1. The superior EMI shielding performance was mainly attributed to the perfect conductive networks. Additionally, the absorption coefficient of CNT/CS foam was up to 81.73% under high EMI SE of 37.6 dB, which was remarkable among the reported EMI shielding materials with comparable EMI shielding level. More importantly, the addition of CS significantly increased the compressive strength and modulus of CNT/CS foam to 34.1 KPa and 177.1 KPa, which were 84% and 149% higher than those for the pure CNT foam, respectively. These results indicate that the CNT/CS foam is an ideal high-efficient EMI shielding material, which has high potential applications in the fields of aerospace, automotive, and electronic devices.

Microhoneycomb Monoliths Prepared by the Unidirectional Freeze-drying of Cellulose Nanofiber Based Sols: Method and Extensions.

Monolithic honeycomb structures have been attractive to multidisciplinary fields due to their high strength-to-weight ratio. Particularly, microhoneycomb monoliths (MHMs) with micrometer-scale channels are expected as efficient platforms for reactions and separations because of their large surface areas. Up to now, MHMs have been prepared by a unidirectional freeze-drying (UDF) method only from very limited precursors. Herein, we report a protocol from which a series of MHMs consisting of different components can be obtained. Recently, we found that cellulose nanofibers function as a distinct structure-directing agent towards the formation of MHMs through the UDF process. By mixing the cellulose nanofibers with water soluble substances which do not yield MHMs, a variety of composite MHMs can be prepared. This significantly enriches the chemical constitution of MHMs towards versatile applications.

A tracheal scaffold of gelatin-chondroitin sulfate-hyaluronan-polyvinyl alcohol with orientated porous structure

Scaffold of gelatin-chondroitin sulfate-hyaluronan-polyvinyl alcohol (GCH-PVA) with orientated microtubule structure and good hydrophilicity was fabricated by unidirectional freeze-drying method mimicking the composition and structure of tracheal cartilage extracellular matrix. PVA was incorporated to improve flexibility and viscoelasticity of GCH scaffold. All wet scaffolds showed similar compressive elastic modulus with native cartilage. GCH-PVA scaffolds showed high relative remaining stress during relaxation indicating good mechanical stability. The hysteresis ratio during cyclic compression increased gradually with PVA content and close to native cartilage. During multiple frequency compression, all scaffolds showed a low loss tangent close to native cartilage, and PVA incorporation enhanced the elasticity of scaffolds when they were stressed under high frequency. The incorporation of PVA promoted gene expression of adhesion related integrin α5β1 and actin by mouse bone marrow mesenchymal stem cells (mBMSCs). With the orientated microtubule structure, cells ingrowth into scaffolds was facilitated by dynamic culture method.

Cluster-associated filling of water molecules in graphene-based mesopores

Graphene monoliths were prepared through unidirectional freeze-drying method of graphene oxide colloids-KOH mixed solution and successive reduction by heating at 573 K in Ar. The porosity- and crystallinity-controlled graphene monoliths were prepared by the KOH activation at different temperature and the post-heating in Ar. These activated graphene monoliths were characterized by N2 adsorption at 77 K, X-ray diffraction and Raman spectroscopy. Water adsorption isotherms show a typical hydrophobicity below P/P 0 = 0.5 and a marked hydrophilicity above P/P 0 = 0.6, which depends on the pore width. In the water adsorption isotherms of porous graphene monoliths activated at different temperature, the higher the activation temperature, the larger the rising P/P 0. No essential change in the shape of the water adsorption isotherm for the post-heated nanoporous graphene monoliths is observed except for the decrease in water adsorption amount with higher post-heating temperature. The linear relationship between the saturated water adsorption and pore volume whose width is smaller than 4 nm indicates clearly that water molecules are adsorbed in small mesopores by the cluster-associated filling mechanism.

Preparation and characterization of aligned porous PCL/zein scaffolds as drug delivery systems via improved unidirectional freeze-drying method

A novel type of drug-delivery scaffold based on poly(ε-caprolactone) (PCL) and zein blends was prepared by improved unidirectional freeze-drying. Scaffolds with tube-like pore structure and high porosity, up to 89%, were obtained by adjusting the concentration of the PCL and zein solutions. Characters of the prepared scaffolds, such as microstructural, porosity, and compressive strength, were evaluated. The hydrophilicity and the degradability of the composite films were investigated in contact with phosphate buffer saline (PBS). It was found that the presence of zein accelerates the degradation rate of the scaffolds in the period time of investigation (28days). The results showed an acceptable way for controlling the in vitro degradation behavior of PCL composite scaffolds by adapting the concentration of zein. In vitro protein release and degradation results revealed that the absolute weight loss of the PCL/zein scaffolds exhibited an increasing trend by increasing the amount of zein concentration in the scaffolds. The drug delivery capability of the scaffolds was tested using tetracycline hydrochloride (TCH). Sustained release of the drug was obtained, and it was found that the proportion of zein in the scaffold had a great impact on the drug release kinetics. The results demonstrated the potential of the PCL/zein biocomposite scaffolds as a suitable candidate in tissue engineering strategies for bone defect treatment.

Activation routes for high surface area graphene monoliths from graphene oxide colloids

Graphene monoliths made from graphene oxide colloids by unidirectional freeze-drying method were activated by typical activation processes of CO2 activation, chemical activation using ZnCl2 or H3PO4, and KOH activation. The porosity development of graphene monolith markedly depends on the activation method. The monoliths with highest surface area are obtained by the KOH activation method; only the KOH activation is effective for production of the graphene monolith of which surface area is in the range of 1760–2150m2g−1. The mechanism of the porosity development by KOH activation method is proposed. This work provides a promising route for the bottom-up design of pore width-tunable nanoporous carbons.

Porous graphene oxide/carboxymethyl cellulose monoliths, with high metal ion adsorption

Orderly porous graphene oxide/carboxymethyl cellulose (GO/CMC) monoliths were prepared by a unidirectional freeze-drying method. The porous monoliths were characterized by Fourier transform infrared spectra, X-ray diffraction and scanning electron microscopy. Their properties including compressive strength and moisture adsorption were measured. The incorporation of GO changed the porous structure of the GO/CMC monoliths and significantly increased their compressive strength. The porous GO/CMC monoliths exhibited a strong ability to adsorb metal ions, and the Ni2+ ions adsorbed on GO/CMC monolith were reduced by NaBH4 to obtain Ni GO/CMC monolith which could be used as catalyst in the reduction of 4-nitrophenol to 4-aminophenol. Since CMC is biodegradable and non-toxic, the porous GO/CMC monoliths are potential environmental adsorbents.

Graphene oxide foams and their excellent adsorption ability for acetone gas

Graphene oxide (GO) and reduced graphene oxide (RGO) foams were prepared using a unidirectional freeze-drying method. These porous carbon materials were characterized by thermal gravimetric analysis, differential scanning calorimetry, X-ray photoelectron spectroscopy and scanning electron microscopy. The adsorption behavior of the two kinds of foams for acetone was studied. The result showed that the saturated adsorption efficiency of the GO foams was over 100%, and was higher than that of RGO foams and other carbon materials.

Reduction of graphene oxide with l-lysine to prepare reduced graphene oxide stabilized with polysaccharide polyelectrolyte

An environmentally friendly approach to reduce graphene oxide (GO) with L-lysine (L-Lys) was developed by using carboxymethyl starch (CMS) as a stabilizing agent and a stable suspension of reduced graphene oxide (RGO) was obtained. UV visible absorption spectroscopy was used to monitor the deoxygenating process and the factors that affect the GO reduction, such as the ratio of GO/L-Lys, the temperature and pH were optimized. The reduction of the GO was verified by Fourier transform infrared spectroscopy, X-ray diffraction, thermo-gravimetric analysis, Raman spectroscopy and X-ray photoelectron spectroscopy. Ordered porous RGO/CMS foams were prepared by a unidirectional freeze-drying method (UFDM) and used as absorbents for copper ions. Since L-Lys and CMS are natural and edible chemicals, this approach provides a green method to produce stable RGO from GO on a large scale. The nontoxic biodegradable RGO/CMS foams show potential applications for metal ions removal from wastewater.