Lisheng Zhou

Synthesis and properties of polyimide foams containing benzimidazole units

In this research paper, a series of novel polyimide (PI) foams containing benzimidazole units were prepared derived from polyester ammonium salt (PEAS) precursor powders, which were synthesized by co-polymerization of benzophenone-3,3′,4,4′-tetracarboxylic dianhydride (BTDA) with two diamines of 2-(4-aminophenyl)-5-aminobenzimidazole (BIA) and 4,4′-diaminodiphenyl ether (ODA) with various molar ratios. The effects of incorporation of BIA on the morphology, thermal and mechanical properties of co-polyimide (co-PI) foams were explored. The results show that the BIA has a significant influence on foaming degree of PEAS precursor powders. The density of co-polyimide foams increases with increasing the BIA content in the polymer chains. Moreover, the thermal stability of the resultant co-polyimide foams presents a remarkable upward trend with incorporating more BIA units into the polymer chains. As the BIA loading up to 30 mol%, the glass transition temperature of co-polyimide foams increases around 50 °C in comparison with the pristine polyimide foam. Furthermore, the compressive strength of the co-polyimide foams is in the range of 0.30–0.75 MPa, which is superior to their of commercial polyimide foams with the same density. The co-polyimide foams with higher thermal and mechanical properties expand their potential application in many high-tech fields such as aerospace and aviation industries.

Synthesis and characterization of porous polyimide films containing benzimidazole moieties

In this study, a series of porous polyimide films containing benzimidazole units were prepared through a phase separation process. The dibutyl phthalate was selected as porogen. The copolyimides were prepared by the reaction of 3,3′,4,4′-benzophenonetetracarboxylic dianhydride with two diamines of 2-(4-aminophenyl)-5-aminobenzimidazole (BIA) and 4,4′-diaminodiphenyl ether with various molar ratios. The resultant porous polyimide films exhibit optimum cell-size distributions. The effects of BIA on morphology, mechanical, and thermal properties of the porous films were explored. It was found that as the BIA content reached up to 30 mol%, the porous copolyimide film demonstrates remarkable thermal stability and admirable mechanical properties with the glass transition temperature of 294°C, 5% weight loss temperature in argon flow up to 545°C, and a tensile strength of 48 MPa. The incorporation of BIA into the polyimide chains brought the highly rigid structures and strong intermolecular interactions, resulting in the enhancement in the thermal stability and the mechanical properties.

Dynamic/quasi-static stab-resistance and mechanical properties of soft body armour composites constructed from Kevlar fabrics and shear thickening fluids

Soft body armour composites were constructed by combining Kevlar fabrics with different quantities of shear thickening fluid (STF). In particular, the quantity of the added STF and the mechanical properties of these composites were systemically explored. The dynamic and quasi-static knife-stabbing resistance and the quasi-static mechanical properties of these composites were both significantly enhanced in comparison to those of neat Kevlar fabric. The composites were much lighter, thinner and more flexible than the neat Kevlar fabric and, with an optimal quantity of added STF, could not even be penetrated. However, the dynamic knife-stabbing resistance of the fabric became poor when the amount of added STF was higher than 100 wt%. The tensile strength of the composites could be increased from 40% to 80% and the tearing strength could be increased by nearly eight times that of the neat Kevlar fabric by increasing the concentration of added STF. Yarn pull-out testing suggested that the triggered shear thickening contributed to the increase of the friction between yarns or filaments, resulting in improvements in the dynamic and quasi-static properties of these composites. Moreover, these composites are suitable for use as soft body armour materials due to their good adaptability to high and low speeds conditions.

Design of a self-healing and flame-retardant cyclotriphosphazene-based epoxy vitrimer

Abstract Reprocessability and reparability are possible with thermoplastics but are rarely encountered with thermosets, which are used much more frequently in many applications other than thermoplastics. A novel cyclolinear cyclotriphosphazene-based epoxy resin (CTP-EP) was successfully synthesized, characterized, and cured by the addition of the disulfide-containing aromatic diamine hardener DTDA to form a new epoxy vitrimer, CTP-EP/DTDA. This epoxy vitrimer behaves as a typical thermoset at ambient conditions but can be quickly reprocessed at elevated temperatures by hot-pressing, similar to a thermoplastic. The outstanding multi-self-healing performance of this dynamic epoxy system is attributed to the radical-mediated aromatic disulfide exchange mechanism. After repairing itself three times, this epoxy system still had a high healing efficiency of 89.8%. In addition, this newly obtained epoxy system also possesses good thermal stability and excellent flame retardancy due to the cyclotriphosphazene structures in the epoxy resin. The multifunctional properties of this material make it a promising candidate for a variety of applications, such as fiber-reinforced polymer composites in the aerospace and automotive industries.

Chemical solution approach to SrTiO3 synthesis using a new precursor

We have synthesized SrTiO3 (STO) using a newly developed precursor route by chemical solution approach. The new STO precursor solution was prepared in ambient atmosphere. Various characteristic methods, including thermal analyses, infrared spectroscopy, and transmission electron microscopic analyses techniques, were applied to study the thermal decomposition and crystallization behavior of STO precursor gel. The acquirement of single-phase STO powders at the different annealing atmospheres demonstrates that STO with perovskite structure is prone to tolerate the oxygen vacancies defect. Epitaxially grown STO film on textured Ni–W substrate by a seeded nucleation method shows a high c-axis orientation and a good out-of-plane texture. Scanning electron microscopy and atomic force microscopy investigations of STO film reveal the continuous, crack-free, and smooth surface morphology. The results suggest that STO film fabricated by the newly developed precursor route may be suitable to be used as buffer layer for subsequent growth of YBCO in coated conductors.

Preparation and mechanical properties of thermosetting epoxy foams based on epoxy/ 2-ethyl-4-methylimidazol system with different curing agent contents:

Epoxy/2-ethyl-4-methylimidazol system with different curing agent content was completely cured for foaming, and the effect of a systematic variation in 2-ethyl-4-methylimidazol content on the crosslinking density of cured epoxy resins was investigated. It was found that the crosslinking density of completed cured epoxy reduced as the 2-ethyl-4-methylimidazol content increased in certain range of contents (10–50 mol%). Then the precursors were foamed by a batch foaming process with supercritical carbon dioxide. The cellular morphologies of foamed epoxy resins were analyzed by scanning electron microscopy. The results revealed that the reduced crosslinking density would improve the foamability of cured epoxy resin. The microcellular epoxy foams could be obtained by maintaining a moderate crosslinking density, which can be controlled by varying 2-ethyl-4-methylimidazol content. For the completely cured epoxy with different curing agent content, when the crosslinking density of epoxy resin was 232.40 mol m–3 (the 2-ethyl-4-methylimidazol content was 35 mol%) or lower, microcellular structure was obtained by adjusting the foaming conditions. The effects of foaming on the mechanical properties were also discussed. The results indicated that microcellular epoxy foams had higher impact strength but lower tensile strength and tensile modulus, validating that the introduction of microcellular structure in epoxy matrix was conducive to the improvement of the ductility of epoxy foams.

Synthesis of α-hydroxyphosphonate cyclotriphosphazene under solvent-free conditions with a basic catalyst

GRAPHICAL ABSTRACT ABSTRACT An α-hydroxyphosphonate cyclotriphosphazene compound, hexa-(4-diethylphosphate-hydroxymethyl-phenoxy)-cyclotriphosphazene) (HDHPCP), was synthesized in two steps. First, the intermediate hexa-(4-aldehyde-phenoxy)-cyclotriphosphazene (HAPCP) was synthesized by the elimination reaction of hexachlorocyctriphosphazene (HCCP) with 4-hydroxybenzaldehyde. Then HDHPCP was obtained by the Pudovik reaction under solvent-free condition. In this step, the effects of the type and amount of catalyst on reaction time and yield were investigated, and it was found that triethylamine (TEA) has better activity than KF and K2CO3. When the molar ratio of HAPCP to TEA was 1:5, the reaction gave an excellent yield of 95.4% in 70 min. Under these conditions, the reaction was easy to work-up, time-saving, environmentally benign, and with high yield. Fourier transform infrared, Nuclear Magnetic Resonance, and elemental analyses results confirmed that HDHPCP was synthesized successfully, and the thermal gravimetric analysis measurement indicated that it presented good thermal stability and high char residual.