Properties of gradient polyimide aerogels prepared through layer-by-layer assembly

A series of density gradient polyimide (PI) aerogels were prepared using layer-by-layer assembly and radial freezing method. Briefly, a layer of poly(amic acid) (PAA) ammonium salt aqueous solution was first radially frozen. Then, the second layer of PAA ammonium salt (PAS) aqueous solution of different concentration was added on the top of the first PAS layer and radially frozen. A multilayer gradient PAS solid sample could therefore be fabricated by repeating this similar procedure. The density gradient PI aerogels were obtained after freezing drying and final thermal imidization treatment. Each layer of gradient PI aerogels had anisotropic pore structure, which consisted of tube-like pores along the radial direction and toward the center axis of the cylindrical samples. The compressive strength of five-layer gradient PI aerogel was higher than that of three-layer gradient and single-layer PI aerogels with the same density. The gradient PI aerogels exhibited anisotropic heat transfer behavior in the direction of density gradient, and heat transfer from the higher density side to the lower density side was faster.     

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.     

Preparation of polyimide/multi-walled carbon nanotubes composite aerogels with anisotropic properties

In this study, a series of polyimide/multi-walled carbon nanotubes (PI/MWCNTs) composite aerogels with anisotropic properties were fabricated. First, the poly(amic acid) ammonium salt (PAS)/MWCNTs suspension was prepared by blending poly(amic acid), deionized water, triethylamine, MWCNTs, and CNT dispersant with the aid of ultrasonication treatment. Afterwards, the aqueous PAS/MWCNTs suspension was unidirectionally frozen at −65 ± 5°C, then followed by freeze-drying. Subsequently, the PI/MWCNTs composite aerogels were obtained after thermal imidization treatment. Morphology observations revealed that PI/MWCNTs composite aerogels exhibited a “hive-like” structure while viewed along the freezing direction, whereas a typical channel-like pore structure was observed perpendicular to the freezing direction. This typical structure rendered PI/MWCNTs composite aerogels with anisotropic properties such as heat conduction, electrical conductivity as well as electromagnetic interference shielding effectiveness when the aerogels were characterized at different directions.     

Gradient structure polyimide/graphene composite aerogels fabricated by layer-by-layer assembly and unidirectional freezing

In this work, gradient polyimide (PI)/graphene composite aerogels were prepared with poly(amic acid) ammonium salt/graphene aqueous solution through layer-by-layer assembly, unidirectional freezing, freezing drying, and thermal imidization process. Each layer of gradient PI aerogels was consisted of oriented channel-like pores along the freezing direction. The gradient PI/graphene composite aerogels exhibited anisotropic conductivity and heat transfer property. The conductivity of composite aerogels in the perpendicular direction of oriented channel-like pores was higher than that along the direction of oriented pores. The heat transfer from the high-density end to the low-density end of gradient density composite aerogels was faster. Compared with those of homogeneous composite aerogel with same density, the compression yield stress of gradient density composite aerogels obviously decreased, and their compression platform region also obviously shortened. Moreover, when the compressive strain exceeded 35%, the compressive strength of gradient composite aerogel with more layers was much higher.     

The effect of solidification direction with respect to gravity on ice-templated TiO2 microstructures

Unidirectional ice-templating produces materials with aligned, elongated pores via: (i) directional solidification of particle suspensions wherein suspended particles are rejected and incorporated between aligned dendrites, (ii) sublimation of the solidified fluid, and (iii) sintering of the particles into elongated walls which are templated by the ice dendrites. Most ice-templating studies utilize upward solidification techniques, where solid ice is located at the bottom of the solidification mold (closest to the cold source), the liquid suspension is above the ice, and the solidification front advances upward, against gravity. Liquid water reaches its maximum density at 4 °C; thus, liquid nearest the solid/liquid interface, at 0ºC, is less dense than warmer liquid above (up to 4 °C, above which, a density inversion occurs, and liquid density decreases with increasing temperature). The lower density liquid nearest the solidification front is thus expected to rise due to buoyancy, promoting convective fluid motion in the liquid during solidification. Here, we investigate the effect of solidification direction with respect to the direction of gravity on ice-templated microstructures to study the role of buoyancy-driven fluid motion during solidification. We hypothesize that, for upward solidification, the convective fluid motion that results from the liquid density gradient occurs near the solidification front. For downward solidification, we expect that this fluid motion occurs farther away from the solidification front. Aqueous suspensions of TiO₂ nanoparticles (10–30 nm in size, 10, 15, and 21 vol.%) are solidified upward (against gravity, with ice on bottom and water on top), downward (water on bottom, ice on top), and horizontally (perpendicular to gravity). Microstructural investigation of sintered samples shows evidence of buoyancy-driven, convective fluid flow during solidification for samples solidified upwards (against gravity), including (i) tilting of the wall (and pore) orientation with respect to the induced temperature gradient, (ii) ice lens defects (cracks oriented perpendicular to the freezing direction), and (iii) radial macrosegregation. These features are not observed for downward nor horizontal solidification configurations, consistent with the hypothesis that convective fluid motion does not interact directly with the solidification front for downward solidification.     

Controlling anisotropy of porous B4C structures through magnetic field-assisted freeze-casting

Anisotropic porous boron carbide (B₄C) structures were successfully produced, for the first time, using the magnetic field-assisted freeze casting method. The effect of the magnetic field on the structure and mechanical strength of the formed porous B₄C was compared for two different magnetic field directions that were either aligned with ice growth (vertical), or perpendicular to the ice growth direction (horizontal). It was shown that applying even a weak horizontal magnetic field of 0.1–0.3 T noticeably affected the alignment of mineral bridges between lamellar walls. Both the porosity and the channel widths decreased with increasing horizontal magnetic field strength. In the case of a vertical magnetic field, a larger strength of 0.4 T was required for highly aligned lamellar walls and larger channel widths. Compression strength tests indicated that the application of magnetic fields led to more homogeneously aligned channels, which resulted in increased compression strength in the longitudinal (parallel to the ice growth) direction. Applying a vertical magnetic field of 0.4 T with a cooling rate of 2 °C/min during the freezing step of the magnetic field-assisted freeze-casting method was found to result in the best conditions for producing highly anisotropic structures with large channel widths and fewer mineral bridges, which led to an increase in the mechanical strength.     

Ice templating of ZrB2 porous architectures

For the first time, freeze casting of aqueous ZrB₂ based suspension was performed in order to produce porous architectures with main unidirectional anisotropic pores interconnected by diffuse globular-isotropic pores. The porosity characterizing the green bodies after the sublimation process is the replica of the ice crystals. The effects of solid content, suspension stabilization and cold transmission depending on mold type were studied since micro and macrostructure and, hence, the properties of the sintered body are determined by controlling the growth direction of the ice crystals during freezing. Improved mechanical performances are obtained by changing the above mentioned parameters, in virtue of the formation of thinner and more homogenously distributed ceramic lamellae. Possible applications of these materials include high temperature porous volumetric absorbers for concentrating solar power systems.     

Ultralight and Flexible MWNTs/Polyimide Hybrid Aerogels for Elastic Conductors

Aerogels have showed tremendous potential applications because of its unique and outstanding properties. Herein, a novel two‐step approach to form self‐assembly nanocomposite aerogels driven by the strong interactions between water‐soluble polyimide (PI) precursor polyamic acid salt (PAAs) and hydroxyl multiwalled carbon nanotubes (MWNTs‐OH) is reported. The PI therein constitutes the framework of the nanocomposite and raises the strength of the cell walls, which endows aerogels with superelasticity and robustness. The MWNTs‐OH is distributed uniformly into water via physical ultrasonic method followed by blending with PAA molecular. During the imidization process, electrically insulating polyamic acid (PAA)/MWNTs‐OH aerogels are converted to conductive PI/MWNTs‐OH nanocomposite aerogels owning to the removal of their oxygenic functional groups of  OH functionalized MWNTs. Moreover, adding multi‐walled carbon nanotube (MWNTs) contributes to the reduction of shrinkage notably, which can be evidenced by scanning electron microscopy measurement and density data. The nanocomposite aerogels display a high elastic modulus, high compressive stress, superior robustness, and high stress‐sensitive electrical conductivity. Interestingly, the variation trend of the electric resistance with compressive strain (R /R ₀–ε) plots is consistent with the compressive stress–strain (σ–ε) curves, which can be explained by the “interface contact spots” theory. And this finding could facilitate the development of polymer‐based nanocomposite aerogels as elastic conductors for various applications.

     

Microstructural Control of Colloidal‐Based Ceramics by Directional Solidification Under Weak Magnetic Fields

The use of weak magnetic fields to control the microstructural evolution of colloidal‐based systems in conjunction with directional solidification is demonstrated as a convenient processing route to fabricate anisotropic ceramic scaffolds with complex microarchitectures. A variety of graded and aligned microstructures were formed by applying external static magnetic fields oriented radially, axially, and transversely with respect to the solidification direction of freezing slurries containing micro/nanoparticles of ZrO₂ and Fe₃O₄. The graded structures, formed by the radial and axial fields, resemble core–shell architectures composed of dense outer perimeters surrounding porous inner cores. The aligned structures, formed by transverse fields, exhibit two modes of microstructural alignment: lamellar walls aligned by the growing ice crystals and mineral bridges aligned by the magnetic fields. The alignment of mineral bridges that connect adjacent lamellae, provide these scaffolds enhanced strength and stiffness when compressed parallel to their orientation (parallel to the direction of the magnetic field).     

Novel freeze-casting fabrication of aligned lamellar porous alumina with a centrosymmetric structure

A novel freeze-casting method is used to fabricate aligned lamellar porous alumina with a centrosymmetric structure from aqueous alumina slurries. Two cold fronts oriented perpendicularly to each other, originating from the bottom and side of the cylindrical copper mold, induce the growth of ice crystals in specific directions along the radius of the cylindrical mold. Lamellar channels of porous alumina are arranged centrosymmetrically along the radial axis. The pore distribution of the currently prepared porous ceramics is more regular when compared with that of porous ceramics prepared by conventional freeze casting. This affords porous ceramics with improved mechanical properties and stability. The current method addresses the issue of partial failure as induced by the randomly distributed channels in lamellar porous ceramics.     

Tough organogels based on polyisobutylene with aligned porous structures

Macroporous gels with aligned porous structures were prepared by solution crosslinking of butyl rubber (PIB) in cyclohexane at subzero temperatures. Sulfur monochloride was used as a crosslinker in the organogel preparation. The reactions were carried out at various temperatures between 20 and −22 °C as well as at various freezing rates. The structure of the gel networks formed at −2 °C consists of pores of about 100 μm in length and 50 μm in width, separated by polymer domains of 10-20 μm in thickness. The aligned porous structure of PIB gels indicates directional freezing of the solvent crystals in the direction of the temperature gradient. The size of the pores in the organogels could be regulated by changing the freezing rate of the reaction solution. The results suggest that frozen cyclohexane templates are responsible for the porosity formation in cyclohexane. In contrast to the regular morphology of the gels formed in cyclohexane, benzene as a crosslinking solvent produces irregular pores with a broad size distribution from micrometer to millimeter sizes due to the phase separation of PIB chains at low temperatures. Macroporous organogels prepared at subzero temperatures are very tough and can be compressed up to about 100% strain without any crack development. The gels also exhibit superfast swelling and deswelling properties as well as reversible swelling-deswelling cycles in toluene and methanol, respectively.     

Fabrication of lamellar porous alumina with regular structure via directional solidification with multiple cold sources

Porous ceramics obtained with aqueous slurries by directional solidification have randomly distributed lamellar pore channels and thus have unstable mechanical properties. According to the anisotropic principle of ice crystal growth, the growth of lamellar ice crystals is regular when multiple cold sources are used, and porous ceramics with regular pore channels are then obtained after drying. Multiple cold sources are formed with a bottom cold plate and copper sides in rectangular molds. The copper sides are in contact with the bottom cold plate, thus forming the side cold source with temperature gradient distribution by heat transfer. The interaction between the side cold source and the bottom cold plate facilitates the regular distribution and continuous growth in parallel of ice crystals. The use of parallel copper sides of the mold results in porous ceramics with an axisymmetric pore structure and high aspect ratios of pore channel in porous alumina. The positive compressive strength of fabricated porous ceramics with an axisymmetric structure is similar with those of conventional directional solidification, but the lateral side direction compressive strength of fabricated porous ceramics with an axisymmetric structure is significantly increased.     

Preparation of a unique,multihollow‐core honeycomb structure via the unidirectional freezing of a binary solvent system

Unidirectional freezing is a simple and environmentally friendly method for preparing polymeric porous materials from polymer solutions for use in various applications. In this study, a unique, multihollow‐core honeycomb structure was prepared from diurethandimethacrylate (DUDM) in a 1,4‐dioxane (Dx) and tertiary butanol (TBA) binary solvent system via unidirectional freezing, subsequent photopolymerization and freeze‐drying. The multihollow‐core honeycomb consists of two different hollow tubular structures: one structure is noncircular with an atypical cross‐sectional area, and the other is circular and measures approximately 5–10 μm in diameter. Both structures are aligned parallel to the freezing direction. These hollow structures were formed by using the sequential growth of Dx and TBA crystals as a template. During the unidirectional freezing process, the Dx crystals grew in the solution along the freezing direction and expelled DUDM and TBA from its crystalline phase into the solution. When the freezing temperature was further decreased, small, needle‐shaped TBA crystals grew along the freezing direction and were confined by the Dx crystals. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Studies on ionic salt of polyamic acid and related compounds

Amic acid compound namely bisphthalamic acid of 2, 2-dimethyl-4, 4-diaminobiphenyl (amic acid) and its ionic salt with 3-(dimethylamino)propyl methacrylate (amic acid salt) were synthesized and characterized by FT-IR, mass spectroscopy, NMR and DSC. Effect of temperature and water content on these compounds was studied by ¹H-NMR and potentiometric titration. In the absence of added water, both amic acid and its ionic salt had undergone imidization followed by hydrolysis, which was attributed to the water formed as a result of an initial imidization reaction. Polyamic acid (PAA) was synthesized by reacting 4, 4-oxydiphthalic anhydride and 2, 2-dimethyl 4, 4-diaminobiphenyl (m-tolidine) in N-methyl-2-pyrrolidone, which was then reacted with 3-(dimethylamino)propyl methacrylate to obtain PAS. Their storage stabilities were studied by monitoring their bulk viscosities and acid numbers as a function of time and temperature. PAS was found to be less stable than PAA. Similar observations were made for amic acid compounds. PAA and PAS showed two-step thermal degradation in air and nitrogen.     

Anisotropic films photopolymerized from aligned cross-linkable gemini ammonium liquid crystals for ion conduction

Cross-linkable gemini ionic liquid crystals are prepared by jacketing the diammonium moiety between two biphenyl benzoate mesogens. Anisotropic films are obtained by photopolymerization of the macroscopically aligned gemini ionic liquid crystals for ion conduction. Small angle X-ray scattering (SAXS) measurements indicate that the monolayer nanostructure is formed in the films and scanning electron microscope (SEM) observations reveal that the smectic layers are perpendicular to the film surface. Electrochemical impedance spectroscopy (EIS) characterization shows that the films exhibit strong anisotropy in ion conduction. The ion conduction across the film is enhanced while that within the plane of the film is impeded. The ionic conductivity in vertical direction of the film reaches up to 10⁻³ cm S⁻¹ at 180 °C and the measured anisotropy (ratio of the measured conductivities in vertical direction of the film versus in parallel to the film) is 100–350. The photopolymerization of the cross-linkable gemini ammonium liquid crystals offers excellent potential for the development of solid electrolytes for electrochemical devices. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47349.     

Facile fabrication of macroporous polyimide films with pores distributing on one side

Herein, we report on direct preparation of macroporous polyimide (PI) films with pores distributing on one side, the method of which relies on sedimentation of ceramic spheres in polyamic acid (PAA) solutions in a gravitational field and imidization of PAA/ceramic spheres mixtures to obtain PI/ceramic spheres hybrid films followed by curing in dilute hydrofluoric (HF) acid. In this strategy, the curing of the hybrid films in HF acid leads to the formation of pores. The introduction of pores makes the room‐temperature dielectric constants of the macroporous films lower than that of pure PI film. Moreover, the macroporous PI films have improved Young's moduli and higher thermal stability in nitrogen atmosphere. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 261–266, 2007     

Temperatures required for eutectic freezing of natural waters

The eutectic freezing process separates aqueous solutions of inorganic salts into solid salt and fresh water. It is useful for treating cooling tower and boiler blowdown and the brine effluent from inland desalting plants.By freezing the water out of these aqueous solutions until they are adequately concentrated to precipitate the salt simultaneously, one ends up with no brine product. The ice and salt crystals nucleate and grow independently and are easily separated since the ice floats and the salt sinks.Laboratory investigation of the eutectic temperatures of various proportions of the ions commonly found in natural waters (Na, K, Ca, Mg, Cl, SO₄, HCO₃) shows that the process will operate well at temperatures no lower than ?25°C.     

Enhanced mechanical and thermal properties of polyimide/organically modified montmorillonite hybrid film based on stable poly(amic acid) ammonium salt

In this study, polyimide/organically modified montmorillonite (PI/OMMT) hybrid film was prepared by in situ polymerization from the stable poly(amic acid) ammonium salt/OMMT (PAAS/OMMT) precursor hybrid. PAAS was obtained by incorporating calculated triethylamine into terpolymer poly(amic acid) (PAA), which was synthesized by pyromellitic dianhydride (PMDA), 4,4′‐oxydianiline and p‐phenylenediamine in dimethylacetamide (DMAc). OMMT as a type of layered clays was prepared through surface treatment of montmorillonite (MMT) with 1‐hexadecylamine. Mechanical property measurements of PI/OMMT hybrid film indicated that the addition of 5 wt% of OMMT increased the Young's modulus of PI film up to 11.24 GPa, which is 58% higher than the pristine PI film from PAAS. Besides, the tensile strength increased to 168.36 MPa, which was higher than that of PI film derived from PAA (164.3 MPa) and PI film derived from PAAS (145.2 MPa). Moreover, the thermal stabilities of PI/OMMT hybrid film with appropriate OMMT content were also better than those of original PI films. POLYM. COMPOS., 34:2076–2081, 2013. © 2013 Society of Plastics Engineers     

Superhydrophobic polyimide aerogels via conformal coating strategy with excellent underwater performances

The hydrophobilization of aerogels is critical for their long-term application due to their highly open-porous structures, while the hydrophobilization strategies for polyimide (PI) aerogels are limited. In the present work, a feasible strategy to synthesize superhydrophobic PI aerogels is proposed via a conformal-coating process. The resulting PI aerogels (F-PI) exhibit high contact angles up to 156°. Impressively, the F-PI aerogels show excellent water-resistant capacity not only in the ambient condition but also in the high-pressure water up to 0.5 MPa, where the liquid cannot or only partially permeate into the aerogel's open pores. Besides, the F-PI aerogels possess high Brunauer–Emmett–Teller surfaces areas (185 m² g⁻¹), high Young's modulus (3 MPa), low thermal conductivity (0.056 W m⁻¹·k⁻¹), and variable densities. All these parameters are found to be well preserved after high pressure (0.1–0.2 MPa) water treatment, further indicating the excellent water-resistant performance of the F-PI aerogels. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48849.     

Porous YSZ ceramics with unidirectionally aligned pore channel structure: Lowering thermal conductivity by silica aerogels impregnation

The previous report of this work has demonstrated the fabrication and properties of porous yttria-stabilized zirconia (YSZ) ceramics with unidirectionally aligned pore channels. As a follow-up study, the present work aims at lowering the thermal conductivity of the porous YSZ ceramics by silica aerogels impregnation. The porous YSZ ceramics were immersed in an about-to-gel silica sol. Both the unidirectionally aligned pore channels and the inter-grain pores by grain stacking in the channel-pore wall of the porous YSZ ceramics were impregnated with the silica sol. After aging and supercritical drying, silica aerogels formed in the macroporous network of the porous YSZ ceramics with unidirectionally aligned pore channels. The influences of silica aerogel impregnation on the microstructure and properties of porous YSZ ceramics with unidirectional aligned pore channels were investigated. The porosity decreased after impregnation with silica aerogels. Both microstructure observation and pore size distribution indicated that both channel-pore size and inter-grain pore-size decreased significantly after impregnation with silica aerogels. Impregnating porous YSZ ceramics with silica aerogels remarkably lowered the room-temperature thermal conductivity and enhanced the compressive strength. The as-fabricated materials are thus suitable for applications in bulk thermal isolators.