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.     

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.     

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.     

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.     

反相悬浮法合成高分子量聚丙烯酸钠

探讨了体系乳化剂、中和度对聚合稳定性的影响 ,研究引发体系、链转移剂、交联剂等与分子量、凝胶含量的关系。结果表明 ,当选用 Span- 6 0作为乳化剂、单体中和度为 93%时体系稳定性较好。用一定配比的过硫酸铵氧化还原体系为引发剂 ,以异丙醇作链转移剂 ,在适量的交联剂 N ,N'-亚甲基双丙烯酰胺和盐类的存在下 ,聚合得到了粒径为 5 0～ 2 0 0μm、分子量高达 10 0 0× 10 4 左右的聚丙烯酸钠透明粒子 ,并且解决水溶液聚合后期体系粘度大、搅拌和传热困难等问题。     

Solution properties of polyamic acids and their amine salts

Solution properties such as viscosity, critical concentration, radius of gyration, and activation energy in aqueous solutions are described for [BPDA/PDA]polyamic acid (PAA) and their salts with various amines (PAS). Although PAA and their salts with Bu₃N, Hex₃N, Oct₃N, and pyridine, were insoluble in H₂O, only the salt with Et₃N (PAS(Et₃N)) was soluble in H₂O. The different solubility of PAS suggests that higher base strength as well as shorter alkyl length of amines enhance water solubility of PAS. PAS(Et₃N) showed critical concentrations at 18, 14, and 7.8 wt% in NMP, NMP/H₂O (1/1), and H₂O, respectively. The radius of gyration, which is calculated from the critical concentration, in H₂O is 1.3 times larger than that in NMP; that is, the polymer chain expands in H₂O rather than in NMP. PAS(Et₃N) in H₂O showed larger activation energy of viscosity than that in NMP. However, PAA and PAS(Et₃N) showed similar activation energies in the NMP solution. Therefore, it is concluded that the amine salts of the polyamic acids are hydrated by several water molecules in H₂O, resulting in the larger radius of gyration and lager activation energies.     

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.

     

新型高效冷冻脱水器的设计研究

传统冷冻脱水器通常为折流板换热器,由于要满足结冰要求,换热管管径一般比较大,传热效果差,不能有效脱水。冷冻脱水器的热阻主要在管程气体侧,新型高效冷冻脱水器使用变截面强化传热管代替光滑圆管,换热管扭曲段可以使流体沿螺旋线流动,形成强烈扰流,破坏边界层,提高传热效率;管内气体中的凝结水由于密度大,在气流离心力作用下,会迅速甩向管壁,凝固成冰,提高冷冻脱水效果。壳程省去折流板结构,流体流动由错流变为纵流,降低系统功耗,且节省材料,降低成本。     

Porous anodic alumina on galvanically grown PtSi layer for application in template-assisted Si nanowire growth

We report on the fabrication and morphology/structural characterization of a porous anodic alumina (PAA)/PtSi nano-template for use as matrix in template-assisted Si nanowire growth on a Si substrate. The PtSi layer was formed by electroless deposition from an aqueous solution containing the metal salt and HF, while the PAA membrane by anodizing an Al film deposited on the PtSi layer. The morphology and structure of the PtSi layer and of the alumina membrane on top were studied by Scanning and High Resolution Transmission Electron Microscopies (SEM, HRTEM). Cross sectional HRTEM images combined with electron diffraction (ED) were used to characterize the different interfaces between Si, PtSi and porous anodic alumina.     

Effect of Polyacrylic Acid as Antiscaling Agent on Heat Exchangers

Polyacrylic acid (PAA) is synthesized from acrylic acid. Different concentrations ranging from 20 to 100 ppm of PAA in water are used as an antiscaling agent on copper and mild steel (m. s.) for 4 to 24 h. The deposition of salt decreases with increasing concentration of PAA as well as with increasing time. The zero acid value of PAA is observed after refluxing for 24 h. The concentration of PAA had no effect on the heat transfer coefficient and the heat transfer rate of the shell and tube heat exchanger. When varying the amount of PAA from 20 to 100 ppm in water, a negligible change in viscosity is observed which does not effect the flow behaviour of water in heat exchanger.     

A chemically amplified positive-working photosensitive polyimide based on a blend of poly(amic acid ethoxymethyl ester) and poly(amic acid)

We prepared a novel chemically amplified photosensitive polyimide based on a blend of poly(amic acid ethoxymethyl ester) (PAAE) and poly(amic acid); this blend produces polyimide (PI) films with improved mechanical properties after imidization with photoacid generator (PAG). PAAE and poly(amic acid) were end-capped with 5-norbornene-2,3-dicarboxylic dianhydride and 2,3-dimethyl maleic anhydride, respectively, to lower their molecular weights without compromising the properties of the resulting PI films. As a result of the blending of these PI precursors, the mechanical properties of the PI films were found to be less affected by the strong acid generated from the PAG than PI films fabricated by imidization of PAAE alone. The relatively high solubility of the blended PI precursor film in basic aqueous solutions was found to be effectively controlled by the use of a high-temperature post-exposure bake process to partially imidize the end-capped PAA. It was found that a 10-μm-thick film of the PSPI precursor system containing 13 wt% PAGs exhibits a sensitivity (D⁰) of 700 mJ/cm² when developed with 2.38 wt% aqueous tetramethyl ammonium hydroxide solution at room temperature. A fine positive pattern was fabricated in a 12 μm thick film with 1000 mJ/cm² of i-line exposure. The resultant PI film was also found to exhibit excellent mechanical and thermal properties, which are critical to its practical use as a stress buffer layer in semiconductor packaging.     

Adsorption and adhesion of poly(vinyl alcohol) and poly(ammonium acrylate) as organic additives for wet mold processing of Al2O3

Adsorption and adhesion of polyvinyl alcohol (PVA) molecules on Al₂O₃ surfaces in pH 3–10 or 0–0.1 mass% poly(ammonium acrylate) (PAA) aqueous solution was examined using the AFM colloidal probe method. The PVA behavior on the solid surface was estimated using force curve measurements obtained using colloidal probe AFM. Extensions originating from the bridging of PVA between the solid surfaces were observed primarily at less than approximately 200 nm in the pH 3 aqueous solution. The extensions, which were observed at more than approximately 600 nm for pH 6 and 10 aqueous solutions, resulted from different conformations of the PVA molecules. In the PVA–PAA system, the number of extensions decreased by increasing the PAA content. This was not observed in a PAA aqueous solution of greater than 0.1 mass%, which indicates that PAA was adsorbed selectively onto the solid surface. The force curve showed that PAA was more effective than PVA.     

声场作用下含盐液滴冻结过程及组分迁移特性的研究

为了揭示超声波辅助冻结的内部作用机理，明确声场作用下相变冻结过程中的热质传递规律及组分迁移特性，根据声场理论分析了超声波的空化作用和热效应，并在冻结过程能质守恒的基础上建立了超声波作用下液滴相变冻结及盐分迁移数学模型，研究了超声波对液滴冻结过程中气泡状态和液滴温度的影响，分析了不同盐浓度下液滴冻结过程中固液界面、溶液比例、盐度及盐水残余率的变化规律。结果表明，超声波空化作用强化了界面处的热质传递，液滴温度下降较快，有利于液滴的冻结；液滴盐浓度越高，凝固界面的移动越慢，液滴直径为2 mm时，盐浓度5wt%时达到冻结点的时间为15 s，盐浓度8wt%时达到冻结点的时间为20 s；超声波作用下盐浓度越低，冻结过程中盐分迁移变化越剧烈。     

Dispersion Behavior of Barium Titanate Nanoparticles Prepared by Using Various Polycarboxylic Dispersants

The effects of the polycarboxylic dispersant structures on the crystallinity and sedimentation behavior of prepared BaTiO₃ nanoparticles were analyzed using four types of dispersants—ethylenediaminetetraacetic acid dipotassium salt (EDTA), trans -aconitic acid (TAA), ammonium acrylate–methyl acrylate co-polymer (PAA50), and sodium polyacrylate (PAA100). In the case of EDTA and TAA, the adsorbed ratio of the dispersants on BaTiO₃ nanoparticles was relatively low, and only slight improvement of sedimentation behavior was observed. On the other hand, in the case of PAA50 and PAA100, the adsorbed ratio was high, and the sedimentation behavior was gratefully improved. Next, in order to analyze the relationships among the additive amount of polycarboxylic dispersants, crystallinity, and sedimentation behavior, various amounts of PAA100 or PAA50 were treated in the synthesis solution. The sedimentation behavior of BaTiO₃ nanoparticles improved with increasing amounts of PAA100 and PAA50 while their crystal phase became amorphous. Adding PAA50 at a molar ratio of COO⁻/Ba²⁺=0.266 resulted in BaTiO₃ nanoparticles with the best dispersion stability in an aqueous media.     

Formation and analysis of a polyimide layer in composite membranes

Composite membranes containing a thin‐film layer of aromatic polyimides (PI) ensure an advantageous combination of selectivity and permeability in gas separation. A series of rigid‐chain PI with different chemical structures were studied as a thin active layer. Composite membranes were prepared by coating a solution of poly(amic acid) (PAA) and an imidization catalyst on a poly(phenylene oxide) (PPO) support with pores filled by decane. The subsequent stage of solid‐state catalytic transformation of the PAA/PPO membrane into the PI/PPO membrane determines the specific structure of the PI layer and the transport properties of the PI/PPO composite membranes. The structure of composite membranes was determined by scanning electron microscopy and analyzed in the terms of the resistance model of gas transport in composite membranes. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1026–1032, 2000     

变截面扭曲椭圆管在冷冻脱水器中的应用

应用于四氟乙烯（TFE）生产工艺的传统单弓形折流板式冷冻脱水器存在换热管径大、裂解气冷量分布不均、脱除的冰晶不能有效在脱水器中停留而随裂解气进入下级以及因折流板原因导致的壳程流动阻力大等缺点。为此本文设计了一种新型冷冻脱水器,与传统冷冻脱水器相比,采用变截面扭曲强化传热管代替光滑圆管,使冷冻脱水器管程流体沿扭曲螺旋线流动,低雷诺数下形成非对称扰流,有效破坏边界层,提高了传热效率;管内TFE裂解气中的水分更快冷冻并在气流离心力作用下迅速甩向管壁,凝结成冰,提高脱水效果;并且壳程改为变空间纵向流动,流动阻力低,节省材料。结合案例监测数据,与传统单弓形折流板式冷冻脱水器进行数据对比,冷冻脱水效果较原换热器提高30%。     

pH-controlled uphill transport of ammonium ions through polymer membranes with sulfonic acid groups

Uphill transport of ammonium ions through a membrane with sulfonic acid groups were investigated with pH-controllers which keep the solution at a constant pH. A membrane with sulfonic acid groups was prepared by casting an aqueous solution containing poly(styrenesulfonic acid) and poly(vinyl alcohol) on a glass plate. When a membrane with one side alkaline and the other acidic was fixed as a diaphragm in a cell, ammonium ions were transported from the alkaline side to the acidic side through the membrane against the concentration gradient of the adjacent solutions. Uphill transport of ammonium ions with pH-controllers was more efficient than without, thus keeping the pH difference between both sides of the membrane constant, which is a driving force for the uphill transport. Furthermore, the effect of pH of the acidic side on the uphill transport was investigated and the mechanism of the pH-controlled uphill transport is discussed.     

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     

Heat Transfer Studies in an Agitated Vessel During Aqueous Alkaline Depolymerization of Poly(Ethylene Terephthalate) (PET) Waste

Depolymerization reactions of poly(ethylene terephthalate) (PET) waste in aqueous sodium hydroxide solution were carried out in a batch reactor at 150°C at atmospheric pressure. Disodium terephthalate (terephthalic acid salt) and ethylene glycol (EG) remain in the liquid phase. Terephthalic acid (TPA) salt was converted into TPA. The produced monomeric products (TPA and EG) were recovered. Various design parameters were estimated. Design of a batch reactor was undertaken for depolymerization of PET waste in aqueous sodium hydroxide solution. As expected, the Reynolds numbers, Prandtl numbers, Nusselt numbers, coil-side heat transfer coefficients, and overall heat transfer coefficients were consistent with the fluid velocities. It shows excellent potential for commercialization of the depolymerization of PET waste.     

The effect of pH and ionic strength on the dispersion of carbon nanotubes in poly(acrylic acid) solutions

The dispersion of three kinds of acid‐treated carbon nanotubes (CNTs) in poly(acrylic acid) (PAA) aqueous solution of different pH and ionic strengths (varied by NaCl, KCl and ZnCl₂) was investigated by visual observation, zeta potential, particle size analysis, transmission electron microscopy and scanning electron microscopy. Visual observation revealed that the dispersion of CNTs acid treated at 60 °C for 3 h and at 80 °C for 2 h was poor in aqueous solutions with pH < 2 or pH > 12. The poor dispersion of acid‐treated CNTs may be improved by adding PAA. In particular, PAA improved the dispersion of CNTs with greater COOH content. In a low pH solution (pH 1.5), a higher PAA content resulted in poorer CNT dispersion while in a high pH solution (pH 12.5), a higher PAA content led to better CNT dispersion. For superior dispersion in a basic aqueous solution (pH 12.5), experimental data showed that a greater atomic radius or a higher cationic charge of the added salt may result in faster aggregation and thus precipitation of CNTs. Copyright © 2011 Society of Chemical Industry