Hongjie Xu

Polyimide/montmorillonite nanocomposites based on thermally stable, rigid-rod aromatic amine modifiers

The preparation and processing of most of polymer/clay nanocomposites need high temperature. This limited the application of commonly used organic modifiers of long carbon-chain alkyl ammonium salts because of their low thermal stability. In this study, we synthesized two novel thermally stable, rigid-rod aromatic amines. Montmorillonite (MMT) treated by these amines exhibited larger layer-to-layer spacing, higher thermal stability than that treated by commonly used 1-hexadecylamine and also high ion-exchange ratio (>95%). They were applied to prepare nanocomposites with polyimide (PI) by in situ polymerization. XRD, TEM were used to obtain the information on morphological structure of PI/MMT nanocomposites. DMA, TGA, DSC, universal tester were applied to characterize the mechanical and thermal properties of the nanocomposites. When the MMT content was below 3 wt%, the PI/MMT nanocomposites were strengthened and toughened at the same time. The introduction of a small amount of MMT also led to improvement in thermal stability, slight increase in glass transition temperature, marked decrease in coefficient of thermal expansion and decrease in solvent uptake. MMT treated by these aromatic amines exhibited better dispersibility and (probably) interfacial interaction with PI matrix than that treated by 1-hexadecylamine. The nanocomposites based on these MMT resultantly exhibited better mechanical, thermal and solvent resistance properties than those based on 1-hexadecylamine treated MMT.

A facile approach for the preparation of cross-linked sulfonated polyimide membranes for fuel cell application

A facile approach has been successfully developed for the preparation of a series of cross-linked sulfonated polyimide (SPI) membranes via the condensation reaction between the sulfonic acid groups and the activated hydrogen atoms of SPIs in the presence of phosphorous pentoxide : methanesulfonic acid in the ratio of 1 : 10 by weight (PPMA, method 1) or phosphorous pentoxide only (method 2). The resulting sulfonyl linkages are very stable and the cross-linked SPI membranes showed greatly improved water stability in comparison with the uncross-linked ones while high proton conductivity was maintained.

Direct exfoliation of graphene in methanesulfonic acid and facile synthesis of graphene/polybenzimidazole nanocomposites

The emerging field of graphene(GP)-based polymer nanocomposites has continued to be the focus of considerable interest in recent years because of the unparalleled improvement shown in mechanical, thermal, and electrical properties compared to the neat polymer. However, these improvements largely depend on the synthesis of well-exfoliated and high-quality GP. In this paper, we report a facile method for the production of GP sheets through the liquid-phase exfoliation of graphite in methanesulfonic acid (MSA). Raman, X-ray photoelectron and infrared spectroscopies reveal that the obtained GP has a low-defect density with a low degree of oxidation. Transmission electron microscopy and atomic force microscopy further confirm that the resulting GP is in the well-exfoliated state. Using this GP/MSA solution as a reaction solvent medium, polymer nanocomposites are prepared by in situpolymerization of poly [2,2′-(p-oxydiphenylene)-5,5′-bibenzimidazole] (OPBI). Compared to pure OPBI, the resulting OPBI/GP nanocomposites show simultaneously improved Youngs modulus, tensile stress, toughness, storage modulus and thermal stability with the addition of extremely small amounts of GP. The high levels of reinforcement are attributed to the good dispersion and effective stress transfer between polymer and GP as evidenced by SEM images of the fracture surfaces, and the excellent intrinsic properties of the high-quality GP. All these features make this simple procedure a potential route for the fabrication of low-cost and high-performance polymer nanocomposites.

Hybrid hydrogels of hyperbranched poly(ether amine)s (hPEAs) for selective adsorption of guest molecules and separation of dyes

We herein report a novel hybrid hydrogel (SiO1.5-hPEAs-Gels), which was fabricated by the direct hydrolysis of trimethoxysilyl groups containing hyperbranched poly(ether amine)s (TMS-hPEAs) in water. The adsorption behaviors of guest molecule dyes by the obtained SiO1.5-hPEAs-Gels were investigated systematically. The adsorption process was found to follow pseudo-first-order kinetics at the initial stage, but followed pseudo-second-order kinetics at later stages. SiO1.5-hPEAs-Gels exhibited a quick adsorption of guest dyes such as Ponceau S (PS), Rose Bengal (RB), Neutral Red (NR) and Methyl Orange (MO) with a high adsorption capacity (Qeq), and very slow adsorption of Methylene Blue trihydrate (MB) and Rhodamine 6G (R6G) with a low adsorption capacity. The big difference in the adsorption behaviors of dyes indicated that the SiO1.5-hPEAs-Gels can adsorb guest molecules selectively. Based on these results, we demonstrated a method for dynamic separation of a mixture of dyes, which can be controlled by the contact time. For example, SiO1.5-hPEA211-Gel can adsorb and remove only PS selectively from a mixture of PS/MB. These characteristics will give SiO1.5-hPEAs-Gels potential for controlled separation.

Hyperbranched poly(ether amine)@poly(vinylidene fluoride) (hPEA@PVDF) porous membranes for selective adsorption and molecular filtration of hydrophilic dyes

Porous membranes with selective adsorption are of great interest because of their wide application in molecular filtration, industrial separation and water treatment. To adsorb dyes with selectivity and high flux, the unique selective adsorption behavior of amphiphilic hyperbranched poly(ether amine) (hPEA) materials toward guest molecules and the facile preparation of a stable porous structure of poly(vinylidene fluoride) (PVDF) were combined to fabricate novel hPEA@PVDF porous membranes through non-solvent induced phase separation (NIPS). The resulting hPEA@PVDF membranes were further cross-linked through the photo-dimerization of coumarin groups in hPEA, and their morphologies were characterized using a scanning electron microscope (SEM), wide angle X-ray diffractometer (WAXD) and differential scanning calorimeter (DSC). The adsorption behavior of hPEA@PVDF porous membranes toward twelve hydrophilic dyes was investigated in detail. Regardless of their charge states, hPEA@PVDF porous membranes exhibited quick adsorption behavior toward Erythrosin B (ETB), Rose Bengal (RB) and Eosin B (EB) with a high adsorption capacity (Qeq) around 600 μmol g−1 but very slow adsorption behavior toward Calcein (Cal) and Methylene Blue Trihydrate (MB) with a low adsorption capacity. Based on their unique selective adsorption behavior toward hydrophilic dyes, hPEA@PVDF porous membranes could separate mixtures of dyes in aqueous solution through molecular filtration with a high flux rate. In addition, the hPEA@PVDF porous membranes were easily regenerated and maintained high separation efficiency over five adsorption–washing cycles. hPEA@PVDF membranes showed great advantages of large adsorption capacity, fast separation of dyes, easy regeneration and low cost due to their porous structure and unique selective adsorption behavior toward hydrophilic dyes, and might find great potential in separation and water treatment.

One-pot approach to synthesize hyperbranched poly(thiol–ether amine) (hPtEA) through sequential “thiol–ene” and “epoxy–amine” click reactions

We here demonstrated a novel strategy of a one-pot approach to synthesize hyperbranched poly(thiol–ether amine) (hPtEA) through sequential “thiol–ene” and “epoxy–amine” click reactions, both of which were well traced using in situ1H-NMR spectra. The obtained hPtEA, with a high branching degree (DB) of 0.83 and a molecular weight (Mn) of 8.6 × 103, was composed of a large amount of hydroxyl groups in the backbone and amino groups in the periphery. Moreover, hPtEA could be further functionalized orthogonally due to the difference in the chemistry between the hydroxyl and amino groups. So fluorescent anthracene (AN) and carboxyl (SA) groups were introduced to the periphery and backbone of hPtEA, respectively, and the amphiphilic and zwitterionic hPtEA (SA-hPtEA-AN) was obtained. The resulting SA-hPtEA-AN could self-assemble into microspheres in aqueous solution with uniform sizes of 750 nm in diameter, and was further cross-linked through photo-dimerization of the AN groups. The microgel of SA-hPtEA-AN is fluorescent and responsive to environmental stimuli such as temperature and pH, and can be used in the encapsulation and controlled release of guest molecules. In addition, the controlled release of guest molecules from the microgel of SA-hPtEA-AN can be monitored by its fluorescence change.

In situ polymerization induced supramolecular hydrogels of chitosan and poly(acrylic acid-acrylamide) with high toughness

A novel type of supramolecular hydrogel was developed by in situ polymerization of acrylic acid (AA) and acrylamide (AM) monomers in the aqueous solution of chitosan (CS), in which nanofiber-structured CS and polyelectrolyte chains of poly(acrylic acid-acrylamide) P(AA-r-AM) form the dynamic cross-linked network through the electrostatic interaction of ions. 1H NMR, WAXD, SEM and TEM were used to trace the formation of this supramolecular hydrogel, and revealed that CS chains aggregate into a nanofiber when the copolymerization of AA and AM proceeded. The obtained hydrogels exhibited good comprehensive mechanical properties. With the excellent fatigue resistance and a high toughness of ∼500 J m−3, the tensile strength and elongation of hydrogel-5 are 120 kPa and 1600%, respectively. The obtained supramolecular hydrogel can self-heal and exhibited no residual strain after continuous deformation-resting processes and loading–unloading tests. Furthermore, the obtained hydrogels can be used to adsorb metal ions in water, interestingly their tensile modulus enhanced several times after adsorption of metal ions.

Self-Assembly of Amphiphilic Anthracene-Functionalized β-Cyclodextrin (CD-AN) through Multi-Micelle Aggregation.

Multi-micelle aggregation (MMA) mechanism is widely acknowledged to explicate large spherical micelles self-assembly, but the process of MMA during self-assembly is hard to observe. Herein, a novel kind of strong, regular microspheres fabricated from self-assembly of amphiphilic anthracene-functionalized β-cyclodextrin (CD-AN) via Cu(I)-catalyzed azide-alkyne click reactions is reported. The obtained CD-AN amphiphiles can self-assemble in water from primary core-shell micelles to secondary aggregates with the diameter changing from several tens nm to around 600-700 nm via MMA process according to the images of scanning electron microscopy, transmission electron microscopy, and atomic force microscopy as well as the dynamic light scattering measurements, followed by further crosslinking through photo-dimerization of anthracene. What merits special attention is that such photo-crosslinked self-assemblies are able to disaggregate reversibly into primary nanoparticles when changing the solution conditions, which is benefited from the designed regular structure of CD-AN and the rigid ranging of anthracene during assembly, thus confirming the process of MMA.

Multi-responsive wholly aromatic sulfonated polyamide ultra-sensitive to pH value

We demonstrated here a new family of multi-responsive polymer: wholly aromatic sulfonated polyamide (SPA). SPA exhibited the unusual response to temperature and pH with the tunable low critical solution temperature (LCST). LCST of the obtained SPA decreased sharply with the increasing pH, and the difference of LCST between pH 6.0–6.8 is about 60 °C.

Inspired by elastomers: fabrication of hydrogels with tunable properties and re-shaping ability via photo-crosslinking at a macromolecular level

Conventional hydrogels cross-linked by small molecular cross-linkers usually exhibit poor mechanical properties, limiting the application of hydrogels to a certain degree. Here, inspired by the fabrication technologies of elastomers, we firstly fabricate a macromolecular hydrogel pre-polymer (MHP) via thermal polymerization and then crosslink the MHP at a macromolecular level by UV irradiation. In this novel technology, polyetheramine (PEA) plays an important role in the fabrication of crosslinked hydrogels. In the thermal polymerization stage, N–CH3 units in PEA acted as co-initiators and grafting agents to form an MHP. Under the UV irradiation, benzophenone (BP) moieties in PEA could be excited as photo-cross-linkers. More importantly, the tensile stress of PEA5%-PAAm hydrogel could be effectively tailored from 180 kPa to 500 kPa by controlling the crosslinking degree under different irradiation times. Through this important characteristic, we could also effectively build a hydrogel with a complex three dimensional (3D) geometry, which is not easily obtained by conventional polymerization methods.