Fabrication of polyimide/graphene nanosheet composite fibers via microwave-assisted imidization strategy

Here, a rapid and efficient strategy was introduced to prepare polyimide/graphene nanosheet (PI/GN) composite fibers by microwave-assisted imidization. The mechanical properties of the PI/GNs (1 wt%) fibers treated by microwave-assisted imidization were apparently improved with the tensile strength of 1.12 GPa at 350 °C, which was approximately 1.7 times as much as those treated with traditional thermal imidization. The PI/GNs (1 wt%) fibers heated by the microwave-assisted imidization method exhibited excellent thermal stabilities of up to 570.3 °C in nitrogen for a 5% weight loss, and a glass transition temperature above 339 °C. The results of the infrared spectrum and thermal properties indicated that the microwave-assisted treatment could promote the imidization degree of the PI/GN fibers prominently. Meanwhile, as a microwave absorber, graphene nanosheets (GNs) could also promote the imidization process by converting microwave energy into thermal energy. The microwave-polyimide/graphene nanosheet (MW-PI/GN) fibers possessed an optimum tensile strength of 1.38 GPa and modulus of 56.82 GPa at the GN content of 0.25 wt%. The 5% weight loss temperature in nitrogen ranged from 520.9 °C to 570.3 °C, and the glass transition temperature was increased from 305.7 °C to 339.1 °C with increasing the GN content.

Here, a rapid and efficient strategy was introduced to prepare polyimide/graphene nanosheet (PI/GN) composite fibers by microwave-assisted imidization.

Degree of crosslinking in β-cyclodextrin-based nanosponges and their effect on piperine encapsulation

Piperine (PIP) is an alkaloid which is potent as a therapeutic agent. However, its applications are restricted by its poor water solubility. Nanosponges (NS) derived from polymers are versatile carriers for poor water-soluble substances. The aim of this work was to synthesize β-cyclodextrin NS, by microwave-assisted fusion, for the encapsulation of PIP. Different formulations of NS were synthesized by varying the molar ratio of β-cyclodextrin:diphenyl carbonate (β-CD:DPC; 1:2, 1:6 and 1:10). NS specimens derived from 1:2, 1:6 and 1:10 β-CD:DPC molar ratios exhibited degree of substitution values of 0.345, 0.629 and 0.878, respectively. The crystallinity of NS was enhanced by increasing diphenyl carbonate concentration. A high degree of crosslinking in the NS increased the loading efficiency due to increased surface area available for bioactive inclusion. This study demonstrated the feasibility of synthesizing NS derived from β-cyclodextrin of high crystallinity for the encapsulation of PIP at high loading capacity.

Lewis Acid Promoted, One-Pot Synthesis of Fluoroquinolone Clubbed 1,3,4-Thiadiazole Motifs under Microwave Irradiation: Their Biological Activities

Background:

A Lewis acid promoted efficient and facile procedure for one-pot synthesis of a novel series of fluoroquinolone clubbed with thiadiazoles motifs under microwave irradiation is described here. This technique has more advantages such as high yield, a clean procedure, low reaction time, simple work-up and use of Lewis acid catalyst.

Objective:

Our aim is to generate a biologically active 1,3,4- thiadiazole ring system by using a onepot synthesis method and microwave-assisted heating. High yield and low reaction time were the main purposes to synthesize bioactive fluoroquinolone clubbed 1,3,4- thiadiazole moiety.

Methods:

Fluoroquinolone Clubbed 1,3,4-Thiadiazole Motifs was prepared by Lewis acid promoted, one-pot synthesis, under microwave irradiation. All the synthesized molecules were determined by IR, 1H NMR, 13C NMR, and Mass spectra. The antimicrobial activity of synthesized compounds was examined against two Gram-negative bacteria (Escherichia coli, Pseudomonas aeruginosa), two Gram-positive bacteria (Staphylococcus aureus, Streptococcus pyogenes), and three fungi (Candida albicans, Aspergillus niger, Aspergillus clavatus) using the MIC (Minimal Inhibitory Concentration) method and antitubercular activity H37Rv using L. J. Slope Method.

Results:

Lewis acid promoted, one-pot synthesis of Fluoroquinolone clubbed 1,3,4-Thiadiazole motifs under microwave irradiation is an extremely beneficial method because of its low reaction time and good yield. Some of these novel derivatives showed moderate to good in vitro antibacterial, antifungal, and antitubercular activity.

Conclusion:

One-pot synthesis of 1,3,4-Thiadiazole by using Lewis acid catalyst gives a good result for saving time and also getting more production of novel heterocyclic compounds with good antimicrobial properties via microwave heating method.

Role of External Field in Polymerization: Mechanism and Kinetics

The past decades have witnessed an increasing interest in developing advanced polymerization techniques subjected to external fields. Various physical modulations, such as temperature, light, electricity, magnetic field, ultrasound, and microwave irradiation, are noninvasive means, having superb but distinct abilities to regulate polymerizations in terms of process intensification and spatial and temporal controls. Gas as an emerging regulator plays a distinctive role in controlling polymerization and resembles a physical regulator in some cases. This review provides a systematic overview of seven types of external-field-regulated polymerizations, ranging from chain-growth to step-growth polymerization. A detailed account of the relevant mechanism and kinetics is provided to better understand the role of each external field in polymerization. In addition, given the crucial role of modeling and simulation in mechanisms and kinetics investigation, an overview of model construction and typical numerical methods used in this field as well as highlights of the interaction between experiment and simulation toward kinetics in the existing systems are given. At the end, limitations and future perspectives for this field are critically discussed. This state-of-the-art research progress not only provides the fundamental principles underlying external-field-regulated polymerizations but also stimulates new development of advanced polymerization methods.

Microwave-accelerated diastereoselective catalyst-free one-pot four-component synthesis of 2-(N-carbamoylacetamide)-substituted 2,3-dihydrothiophenes in glycerol

In the current study, glycerol was successfully employed as a nonvolatile, non-toxic, non-flammable, biodegradable, very cheap, easily accessible, and efficient reaction medium for the microwave-enhanced diastereoselective synthesis of 2-(N-carbamoylacetamide)-substituted 2,3-dihydrothiophenes via a catalyst-free one-pot four-component reaction. A versatile range of starting materials were used, and diverse products were obtained in good to excellent yields and very short reaction times. Moreover, the reaction medium was recovered and reused several times without any loss of the efficiency.

Enhancement of the Mechanical Properties of Polyimide Film by Microwave Irradiation

Polyimide films have conventionally been prepared by thermal imidization of poly(amic acid)s (PAAs). Here we report that the improvement of tensile strength while increasing (or maintaining) film flexibility of polyimide films was accomplished by simple microwave (MW) irradiation of the PAAs. This improvement in mechanical properties can be attributed to the increase in molecular weight of the polyimides by MW irradiation. Our results show that the mechanical properties of polyimide films can be improved by MW irradiation, which is a green approach that requires relatively low MW power, very short irradiation time, and no incorporation of any additional inorganic substance.

Preparation of polyimide films via microwave-assisted thermal imidization

A series of polyimide (PI) films based on aromatic heterocyclic monomers of 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), p-phenylenediamine (p-PDA) and 4,4′-oxydianiline (ODA) were prepared via a microwave-assisted thermal imidization and conventional thermal imidization method at different temperatures. The effects of microwave irradiation on the imidization degree, microstructures, mechanical and thermal properties of PI films were investigated. The imidization degree of the PI films treated with microwave-assisted heating reached a relatively high value at 250 °C, which was twice as much as those treated with traditional thermal imidization. The tensile strength and modulus of PI films treated with microwave-assisted imidization at 300 °C were 187.61 MPa and 2.71 GPa respectively, which were 30% higher than those of PI films treated with thermal imidization. Moreover, the order degree of polymer chains was improved by the microwave-assisted imidization method. The PI films prepared by the microwave-assisted imidization method showed excellent thermal stability with a 5% weight loss temperature of 573 °C under N₂. The microwave-assisted thermal imidization proved to be a rapid and efficient way to prepare high-performance polyimide materials.

A series of polyimide films were prepared via a microwave-assisted thermal imidization and conventional thermal imidization method at different temperatures.

Formation of epoxide-amine oligo-adducts as OH-functionalized initiators for the ring-opening polymerization of ε-caprolactone

Epoxide-amine oligo-adducts were synthesized via a one-pot microwave assisted heterogeneous catalytic transfer hydrogenation. Accordingly, 4-nitroanisole was reduced under microwave conditions to give 4-aminoanisole which reacted immediately with the diglycidyl ether of bisphenol A in an addition polymerization reaction to yield oligo(amino alcohol)s. The hydroxy groups of the new formed oligomers were used as the initiator for the ring-opening polymerization of ε-caprolactone to produce a graft copolymer.

Understanding microwave heating effects in single mode type cavities-theory and experiment

This paper explains the phenomena which occur in commercially available laboratory microwave equipment, and highlights several situations where experimental observations are often misinterpreted as a 'microwave effect'. Electromagnetic simulations and heating experiments were used to show the quantitative effects of solvent type, solvent volume, vessel material, vessel internals and stirring rate on the distribution of the electric field, the power density and the rate of heating. The simulations and experiments show how significant temperature gradients can exist within the heated materials, and that very different results can be obtained depending on the method used to measure temperature. The overall energy balance is shown for a number of different solvents, and the interpretation and implications of using the results from commercially available microwave equipment are discussed.