Programming Homogeneous Hydrogels Using Directional Ion Transport toward Rapid 3D Reconfiguration.

Graphene oxide (GO) sheets can be aligned at low concentration to form a liquid crystal phase. Intriguingly, the poly(amic acid) salt (PAAS) can be used as an intercalated polymer in GO liquid crystal film. The PAAS is intercalated into the aligned GO sheets, increasing water absorbing capacity and reformatting hydrogen bonds in the GO. The suspensions for each content, observed by polarizing microscope, revealed a lyotropic liquid crystal nematic dispersion, even after PAAS was added. The mechanical strength of the GO films was improved by hydrogen bonding between the PAAS and GO sheets. The ultimate tensile strength (181 MPa) and modulus (222 MPa) of the reinforced film are tougher than pristine GO film, respectively. As such, interactions through the plane between the GO sheets and PAAS also improved the films' mechanical stability at high humidity. Functional groups of PAAS also improved the water absorbing capacity and flexibility of the water channels, improving ionic conductivity (0.67 S/cm). This is extraordinary to achieve high ionic conductivity by maintaining tensile strength. Therefore, GO/PAAS films are readily applicable for battery separator and fuel cell membranes. High tensile strength can prevent internal short circuit effectively only with 5 to 10μm of thickness. Highlights We introduce the poly(amic acid) salt as an intercalated polymer in graphene oxide liquid crystal film. poly(amic acid) salt can increase water absorbing capacity and reformat hydrogen bonds in the graphene oxide. The mechanical strength of the graphene oxide films was improved by hydrogen bonding between the PAAS and graphene oxide sheets. Increased water absorbing capacity and flexibility of the water channels improved ionic conductivity (0.67 S/cm).

In situ FTIR analysis for the determination of imidization degree of polyimide precursors

Development of highly active and recyclable catalysts for selective hydrogenation of nitroarenes to amines in water at room temperature is always a challenge in chemical industry. This study reports a facile in situ method for synthesis of ultrafine palladium and platinum nanoparticles (NPs) stabilized by poly (amic acid) salt (PAAS) and their potential as catalysts for hydrogenation of nitroarenes with sodium borohydride or molecular hydrogen as reductant in water at room temperature. In the reduction of 4‐nitrophenol to 4‐aminophenol by sodium borohydride, the activity parameters of PdNPs–PAAS and PtNPs–PAAS catalyst is 6.66 × 103 and 5.58 × 103 s−1 M−1 respectively. In the hydrogenation of diverse nitroarenes under atmospheric hydrogen pressure, PdNPs–PAAS shows high activity but poor selectivity toward desired amines in some cases, while PtNPs–PAAS shows both high activity and high selectivity for selective hydrogenation of nitroarenes to corresponding anilines. The high efficiency of nanocatalyst is due to the quasi‐homogeneous dispersion of metal NPs and synergistic effects between metal NPs and PAAS. In addition, nanocatalyst can be easily recovered with pH‐sensibility of PAAS and reused at least six times without significant loss of catalytic activities.

Lightweight structures are often used for applications requiring higher strength-to-weight ratios and lower densities, such as in aircraft, vehicles, and various engine components. Three-dimensional (3D) printing technology has been widely used for lightweight polymer structures because of the superior flexibility, personalized design, and ease of operation offered by it. However, synthesis of lightweight polymeric structures that possess both high specific strength and glass transfer temperature (Tg) remains an elusive goal, because 3D printed polymers with these properties are still very few in the market. For example, 3,3',4,4'-biphenyl tetracarboxylic dianhydride (BPDA) and p-phenylenediamine (PDA)-type (UPILEX-S type) polyimides show exceptional thermal stability (Tg up to ≈400 °C) and mechanical properties (tensile strength exceeding 500 MPa) and are the first choice if extremely high temperatures of 400 °C or even higher (depending on the duration) are required, which hampers their processing using existing 3D printing techniques. However, their processing using existing 3D printing techniques is hampered due to their thermal resistance. Herein, a 3D printing approach was demonstrated for generating complex lightweight BPDA-PDA polyimide geometries with unprecedented specific strength and thermal resistance. The simple aqueous polymerization reaction of BPDA with water-soluble PDA and triethylamine (TEA) afforded the poly(amic acid) ammonium salt (PAAS) hydrogels. These PAAS solutions showed clear shear thinning and thermo-reversibility, along with high G' gel-state moduli, which ensured self-supporting features and shape fidelity in the gel state. Postprinting thermal treatment transformed the PAAS precursor to BPDA-PDA polyimide (UPILEX-S type). The resulting layer-by-layer deposition onto lightweight polyimide honeycombs in the form of triangular, square, and hexagonal structures showed tailorable mechanical strength, exceptional compressive strength-to-weight ratio (highest up to 0.127 MPa (kg m-3)-1), and remarkable thermoresistance (Tg approximately 380 °C). These high-performance 3D printed polyimide honeycombs and unique synthetic techniques with general structures are potentially useful in fields ranging from automotive to aerospace technologies.

Polyimide (PI)-based nanocomposites were prepared by the in-situ generation of organic silica nano-particles through a sol-gel process, combined with spin coating and multi-step curing process. In this study, silica nano-particles were introduced into the PI matrix to promote its thermal properties. Nanometer-scale composites were successfully obtained from poly(amic acid) (PAA) mixture loaded with silica. 3,3’,4,4’- Biphenyl tetracarboxylic dianhydride (BPDA) and p-phenylenediamine (p-PDA) were employed to synthesize the PAA precursor. The hybrid thin film was prepared from aminoalkoxysilane-capped PAA and tetraethoxysilane (TEOS). In this work, we investigated the effect of coupling agent, 3-aminopropyltriethoxysilane (APrTEOS). The TGA results indicate that the thermal stability of the nanocomposites can be enhanced as silica content increases. Without the incorporation of APrTEOS, less improvement can be achieved.

Polyimides were prepared in the classical two-step method via poly(amic acids). Poly(amic acids) were obtained from 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), 4,4'- (hexafluoroisopropylidene)diphthalic anhydride (6FDA), pyromellitic dianhydride (PMDA), 3,3',4,4'- diphenylsulfonetetracarboxylic dianhydride (DSDA), 4,4'- oxydiphthalic anhydride (ODPA) and amines 4,4'-oxydianiline (ODA), 1,3-phenylenediamine (MPD), 1,4-phenylenediamine (PPD), 4,4'-diaminodiphenylmethane (MDA), 4,4'- ethylenedianiline (DAB), 2,4,6-trimethyl-1,3- phenylenediamine (TMPD), 4-methyl-1,3-phenylenediamine (MMPD) and 2,3,5,6-tetramethyl-1,4-phenylenediamine (DAD) in dimethylformamide. The indium tin oxide (ITO)-glass plates were spin-coated with the poly(amic acids) solutions and dried. A thermal imidization process was then carried out at 250 degree(s)C for 4 h. In this study the anchoring energies of 6CHBT molecules were evaluated on rubbing aligning layers of PI films. The polar anchoring energy coefficient was determined by wedge cell method. The surface free energy and its components of polyimide layers were determined by measuring the contact angles of water, ethylene glycol, formamide and diiodomethane drops on the rubbing polymer surfaces. The Lifshitz-van der Waals and acidic-basic components of surface free energies were found from van Oss equation.

A diamine monomer containing isopropylidene and methyl substituted arylene ether, 2,2-bis[3,5-dimethyl-4-(4-aminophenoxy)phenyl]propane (TBAPP), was prepared in two steps. The monomer was reacted with six different aromatic tetracarboxylic dianhydrides in N,N-dimethylacetamide (DMAc) to obtain the corresponding new polyimides via the poly(amic acid) precursors and thermal or chemical imidization. The poly(amic acid)s obtained had inherent viscosities ranging from 1.38–2.10 dL · g−1. All the poly(amic acid)s could be cast from DMAc solutions and thermally converted into transparent, flexible, and tough polyimide films. The polyimide films had a tensile strength in the range from 52–95 MPa, an elongation at break within a range of 4–8%, and a tensile modulus in the range from 1.60–2.09 GPa. All polyimides were amorphous. The polyimides derived from diamine TBAPP had excellent solubility in various solvents except for those derived from rigid dianhydrides such as pyromellitic dianhydride and 3,3′,4,4′-biphenyltetracarboxylic dianhydride. These polyimides showed glass transition temperatures between 249–293°C and decomposition temperature at 10% mass loss temperatures ranging from 460–485°C in nitrogen.

A negative‐type photosensitive polyimide (PSPI) based on semialicyclic poly(amic acid) (PAA), poly(trans‐1,4‐cyclohexylenediphenylene amic acid), and {[(4,5‐dimethoxy‐2‐nitrobenzyl)oxy]carbonyl} 2,6‐dimethylpiperidine (DNCDP) as a photobase generator has been developed as a next‐generation buffer coat material. The semialicyclic PAA was synthesized from 3,3′,4,4′‐biphenyltetracarboxylic dianhydride and trans‐1,4‐cyclohexyldiamine in the presence of acetic acid, and the PAA polymerization solution was directly used for PSPI formulation. This PSPI, consisting of PAA (80 wt %) and DNCDP (20 wt %), showed high sensitivity of 70 mJ/cm2 and high contrast of 10.3, when it was exposed to a 365‐nm line (i‐line), postexposure baked at 190 °C for 5 min, and developed with 2.38 wt % tetramethylammonium hydroxide aqueous solution containing 20 wt % isopropanol at 25 °C. A clear negative image of 6‐μm line and space pattern was printed on a film, which was exposed to 500 mJ/cm2 of i‐line by a contact printing mode and fully converted to poly(trans‐1,4‐cyclohexylenebiphenylene imide) pattern upon heating at 250 °C for 1 h. The PSPI film had a low coefficient of thermal expansion of 16 ppm/K compared to typical PIs, such as prepared from 3,3′,4,4′‐biphenyltetracarboxylic dianhydride and 4,4′‐oxydianiline. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1317–1323, 2010

Environmentally adaptive hydrogels that are capable of reconfiguration in response to external stimuli have shown great potential toward bioinspired actuation and soft robotics. Previous efforts have focused mainly on either the sophisticated design of heterogeneously structured hydrogels or the complex manipulation of external stimuli, and achieving self-regulated reversal shape deformation in homogenous hydrogels under a constant stimulus has been challenging. Here, we report the molecular design of structurally homogenous hydrogels containing simultaneously two spiropyrans that exhibit self-regulated transient deformation reversal when subjected to constant illumination. The deformation reversal mechanism originates from the molecular sequential descending-ascending charge variation of two coexisting spiropyrans upon irradiation, resulting in a macroscale volumetric contraction-expansion of the hydrogels. Hydrogel film actuators were developed to display complex temporary bidirectional shape transformations and self-regulated reversal rolling under constant illumination. Our work represents an innovative strategy for programming complex shape transformations of homogeneous hydrogels using a single constant stimulus.

A fluidic relaxation oscillator for reprogrammable sequential actuation in soft robots

Novel aromatic polyimides containing bis(phenoxy)naphthalene units were synthesized from 1,5‐bis(4‐aminophenoxy)naphthalene (APN) and various aromatic tetracarboxylic dianhydrides by the usual two‐step procedure that included ring‐opening polyaddition in a polar solvent such as N,N‐dimethylacetamide (DMAc) to give poly(amic acid)s, followed by cyclodehydration to polyimides. The poly(amic acid)s had inherent viscosities between 0.72 and 1.94 dL/g, depending on the tetracarboxylic dianhydrides used. Excepting the polyimide IVb obtained from 3,3′,4,4′‐biphenyltetracarboxylic dianhydride (BPDA), all other polyimides formed brown, flexible, and tough films by casting from the poly(amic acid) solutions. The polyimide synthesized from BPDA was characterized as semicrystalline, whereas the other polyimides showed amorphous patterns as shown by the x‐ray diffraction studies. Tensile strength, initial moduli, and elongation at break of the APN‐based polyimide films ranged from 105–135 MPa, 1.92–2.50 GPa, and 6–7%, respectively. These polyimides had glass transition temperatures between 228 and 317°C. Thermal analyses indicated that these polymers were fairly stable, and the 10% weight loss temperatures by TGA were recorded in the range of 543–574°C in nitrogen and 540–566°C in air atmosphere, respectively. © 1993 John Wiley & Sons, Inc.

Hydrogels have been widely studied in the field of soft robots in recent years due to their good biocompatibility, controllable electrical properties and mechanical properties, and hydrogels are expected to be promising materials for manufacturing soft robots and sensors. Here, we focus on the simulation of the mechanical properties of hydrogels. A novel simulation model based on discrete element method (DEM) is presented to simulate the fracture properties of notched hydrogels. The method used particle model and wall model to simulate the hydrogel and the force applied to it. Elastic module, viscous module and bonding module were applied between each particle to simulate the real microstructure inside hydrogel. Here, we used regular arrangement particles to simulate the homogeneous hydrogel, and staggered arrangement particles to simulate the inhomogeneous hydrogel. A single edge notched tension specimen, as a typical specimen for testing fracture properties is used to simulate the real broken. The morphology of hydrogels during the single edge notched hydrogel was simulated in detail, and the force-displacement curve, stress-strain curve and strain energy was calculated from the method. The results of the simulation show that the fracture properties were quite different while the arrangement of the particle was changed and the irregular arrangement particles had a better fracture property which was consistent to the real experiment that the fracture property of inhomogeneous hydrogel was better than homogeneous hydrogel. The applicability, simplicity and flexibility of the new approach were demonstrated and the results show that the model has potential in study the movement and the mechanical response for the research of soft materials and robotics.

Poly(amic acid)s (PAAs) having the high solution stability and transmittance at 365 nm for photosensitive polyimides have been developed. PAAs with a twisted conformation in the main chains were prepared from 2,2′,6,6′‐biphenyltetracarboxylic dianhydride (2,2′,6,6′‐BPDA) and aromatic diamines. Imidization of PAAs was achieved by chemical treatment using trifluoroacetic anhydride. Among them, the PAA derived from 2,2′,6,6′‐BPDA and 4,4′‐(1,3‐phenylenedioxy)dianiline was converted to the polyimide by thermal treatment. The heating at 300 °C under nitrogen did not complete thermal imidization of PAAs having glass‐transition temperatures (Tg)s higher than 300 °C to the corresponding PIs. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6385–6393, 2006

A series of novel electroactive copolymers of poly(amic acid) (PAA) bearing pendant aniline tetramer groups were prepared via direct polycondensation from novel electroactive diamine (EDA), 4,4'-diaminodiphenyl ether (p-ODA) and 3,3',4,4'-biphenyltetracarboxylic dianhydride (s-BPDA). The structures of the copolymers were confirmed by FT-IR and 1H NMR measurements. Then the prepared PAA copolymers were heated at 300 °C to induce cyclization, and transformed into polyimide (PI) films. The obtained PI films exhibited excellent thermal stability and outstanding mechanical properties. The PAA copolymers films had reversible redox couples with good cycle stability. The stepwise oxidation of as-prepared PAA copolymers by ammonium persulfate is monitored by UV-vis spectroscopy. The absorption peak at 571 nm underwent a red shift when it transformed from emeraldine state to pernigraniline state. Moreover, the PAA copolymers films revealed electrochromic behaviors with a color change from yellowish at its neutral state, to green, and finally to dark blue. They also exhibited extremely high optical contrast (%ΔT) up to 63% at 700 nm, moderate coloration efficiency (CE ≈ 100 cm2 C−1) and low switching times of 3.9 s at 0.8 V and 1.3 s for fast bleaching.

Refractive indices of in-plane and out-of-plane directions were measured for a poly(amic acid) (PAA) synthesized from biphenyltetracarboxylic dianhydride and p-phenylenediamine and PAA synthesized from biphenyltetracarboxylic dianhydride and p,p′-diaminodiphenylether. Depth dependence of in-plane orientation was observed for both PAAs. The degree of in-plane orientation is larger around the air/film interface than the film/substrate interface. Factors which contribute to increase in average refractive index are solvent evaporation, thermal imidization and dense packing during heating. Solvent evaporation is dominant for curing at temperatures below 100°C, thermal imidization is dominant for curing at temperatures from 100°C to 200°C and change in packing becomes important for curing at temperatures above 200°C.