Faramarz Afshar Taromi

Preparation of aminated-polyacrylonitrile nanofiber membranes for the adsorption of metal ions: Comparison with microfibers

Polyacrylonitrile nanofibers (PAN-nFs) were produced using the electrospinning method. Subsequently, the electrospun fibers were modified by diethylenetriamine to produce aminated polyacrylonitrile (APAN) nanofibers. Finally, the adsorbability of copper ions on the surface of the nanofibers was examined in an aqueous solution. Attenuated total internal reflection (ATIR) analysis confirmed the surface amination of the produced PAN-nFs. The grafting yield was calculated by the gravimetric method. The optimum condition was determined to yield the maximum grafting of amine groups to PAN with no losses in sample flexibility. Atomic absorption spectroscopy (AAS) was used to measure the copper ion concentration in the solution. Results indicate that the adsorption process in nanofibers is three times faster in comparison with microfibers. Moreover, the pH effect was studied based on the adsorption behavior of copper ions on the APAN nanofibers. In addition, thermodynamic parameters were calculated, revealing that the process was endothermic and demonstrating that randomness increased at the solid-solution interface during the process. The obtained enthalpy value indicates that the chelation of copper ions among the aminated polyacrylonitrile can be regarded as a chemical adsorption process. The adsorption data fit well with the Langmuir isotherm. The saturation adsorption capacity obtained from the Langmuir model for Cu(II) ions was 116.522 mg/g which is five times more than the reported value for APAN microfibers [S. Deng, R. Bai, J.P. Chen, Aminated polyacrylonitrile fibers for lead and copper removal, Langmuir,19 (2003)5058-5064]. Analysis using atomic force microscopy (AFM) showed that the surface roughness increased upon adsorption of the metal ion. Scanning electron microscopy (SEM) examination demonstrated that there were no cracks or sign of degradation on the surface after amination.

A novel ambipolar polymer: from organic thin-film transistors to enhanced air-stable blue light emitting diodes

The search for developing a deep blue long-lasting polymer light-emitting diode (PLED) has focused attention on polyfluorene (PFO) keto defect suppression with physical and chemical modifications. This study presents the synthesis and characterization of a new donor–acceptor (DA) polymer, based on naphthalene diimide (NDI) as a strong acceptor and n-phenyl-dithieno[3,2-b:2′,3′-d]pyrrole (DTP) as a strong donor. This polymer exhibits ambipolar behavior with stronger n-type properties, 0.07 cm2 V−1 s−1, versus p-type 0.006 cm2 V−1 s−1. By blending PNDIT-alt-DTP into PFO with hole and electron trapping sites, significantly enhanced electroluminescence (EL) efficiency in a simplified polymer light-emitting diode (PLED) was achieved. Further improvement is also achieved by introducing graphene oxide (GO) into the hole injection layer of PEDOT:PSS. A pure and strong long-lasting blue-color in a wide range of bias voltages can be acquired upon introducing 5.0 wt% PNDIT-alt-DTP and 0.01 wt% GO. New blend devices have lower turn-on voltages of 3.5 V, compared to 7.0–8.0 V for pure PFO PLEDs, high current efficiency up to 3.8 cd A−1 and maximum luminance exceeding 5500 cd m−2 which are at least 20 and 60 times greater than a pure PFO/0.01GO device, respectively. The enhanced efficiency can be related to improved hole injection and electron blocking nature of the GO doped PEDOT:PSS layer as well as reducing singlet exciton quenching at the interface. Additionally, the spatial confinement effect of the ambipolar polymer efficiently enhances the thermal stability of the binary blend and facilitates charge carrier balance and more efficient radiative recombination in the devices. These values are among the highest ever reported for PFO devices doped with an ambipolar polymer working in air.

Effect of bending deformation on photovoltaic performance of flexible graphene/Ag electrode-based polymer solar cells

The current work describes a simple low cost solution-based method to synthesis graphene silver (Gr/Ag) nanocomposite as electrode material in fabrication of flexible polymer solar cells (PSCs). Flexible and transparent Gr/Ag – based electrodes were easily prepared on a desired scale by spin coating of an aqueous solution of Gr/Ag nanocomposite on polyethylene terephthalate (PET) substrate at ambient conditions. With the optimization of the weight ratio of Gr/Ag and the electrode thickness, flexible electrodes with sheet resistance of as low as 83 kΩ sq−1 and transmittance of 47% were achieved. These electrodes were then utilized as transparent anodes in the fabrication of flexible PSC devices, replacing ITO. Photovoltaic performance of the fabricated devices was evaluated as a function of the anode electrode thickness under three conditions; at rest, inward bending and outward bending. The introduction of Ag into Gr resulted in a significant improvement in open circuit voltage (VOC), short circuit current density (JSC) and power conversion efficiency (PCE) compared to that considered for Gr-based devices. The Gr/Ag-based devices showed JSC of 1.46 mA cm−2 and a VOC of 0.71 V. Under bending, PSC devices showed considerably larger VOC and JSC compared to the rest position. Furthermore, PCE of the devices decreased about 5.6% when they were bent inward and increased about 34% as they were bent outward. This was in good agreement with the observed changes in the sheet resistance of the fabricated flexible electrodes under similar conditions. The advantages of the described method for fabrication of flexible transparent electrodes include simple solution-based process, production of Gr-based electrodes with high mechanical stability via a low cost and environmentally friendly method and finally, the compatibility and adaptability of the method with roll-to-roll manufacturing technique suitable for industrial production.

Synthesis and characterization of amorphous and impermeable poly(ethylene-co-1,4-cyclohexylenedimethylene terephthalate)/organoclay nanocomposite via in situ polymerization

Random poly(ethylene-co-1,4-cyclohexylenedimethylene terephthalate) and poly(ethylene-co-1,4-cyclohexylenedimethylene terephthalate)/organoclay nanocomposites were synthesized via in situ polymerization of terephthalic acid, ethylene glycol, 1,4-cyclohexane dimethanol, and aminosilane-modified Cloisite30B. An amorphous copolyester was produced with incorporating 30 mol% of 1,4-cyclohexane dimethanol in glycol part. Samples were characterized using Fourier transform spectroscopy, nuclear magnetic resonance, 13C-NMR and 1H-NMR, X-ray diffraction, transmission electron microscopy, differential scanning calorimetry, thermal gravimetric analysis, and dynamic mechanical thermal analysis. Intrinsic viscosities of samples were in the 0.51–0.55 dL/g range. A commercial organoclay was modified prior to add into the reactor. 1H-NMR and 13C-NMR spectra were applied to determine cis:trans isomer ratio of 1,4-cyclohexane dimethanol in polymer chains, Molar ratio of 1,4-cyclohexane dimethanol: ethylene glycol in polymer chains, degree of randomness, mean length of sequences, Mn, and Mark–Houwinks equation parameters. X-ray diffraction results of nanocomposites, including 0.5 and 1 wt% of organoclay, showed no peak in small angles, hence, modified organoclay presented exfoliated structure in polymer matrix. Differential scanning calorimetry analyses revealed that all samples were amorphous. Oxygen, nitrogen, and carbon dioxide diffusion rates in samples were investigated. Nanocomposite, including 3 wt% of organoclay, has about 60% less permeability in comparison to neat copolyester.

Visible light-assisted photoreduction of graphene oxide using CdS nanoparticles and gas sensing properties

Graphene oxide sheets suspended in ethanol interact with excited CdS nanoparticles and contributed to photocatalytic reduction by accepting electron from nanoparticle. The UV-Vis measurement showed that electrical absorbance of the CdS/graphene oxide sheets increased by decreasing the irradiation time and after 2 h it remained constant which indicates the optimum reduction time. Furthermore, the direct interaction between CdS nanoparticles and graphene sheets hinders the collapse of exfoliated sheets of graphene. The 4-point probe measurement of nanocomposite with different ratios of graphene oxide in CdS solution after irradiation shows that the conductivity of them increased by increasing the amount of GO, but further increasing causes incomplete photo reduction process due to exorbitance increasing GO sheets which contribute to decreasing the conductivity. The CdS/RGO composite material can be used as a gas sensor for CO2 based on its electrocatalytic behavior. The low-cost and easy fabrication sensor shows rapid response and high sensitivity. By varying the amount of GO the optimum concentration which shows high sensitivity is found and its good performance compared with other is attributed to its higher conductivity due to complete reduction. Moreover, the effects of thermal annealing on the conductivity of CdS/RGO film and the performance of devices are researched.

Thermoresponsive biopolymer hydrogels with tunable gel characteristics

Here we describe the design of thermosensitive biopolymer-based hydrogels with adjustable gel properties. It is demonstrated that formulations comprising predetermined contents of hydrophobically modified chitosan biopolymer with n-dodecyl groups (HC) undergo a quick fine-tunable gelation in the presence of β-glycerophosphate disodium salt (GP) triggered by increasing temperature. In-depth rheological characterizations revealed that with increasing HC content in solutions, the gel point shifts to lower temperatures as more compact networks with considerably improved gel strength are formed. The evolution of hydrogels was also investigated by the small angle light scattering (SALS) technique. It was realized that the incorporation of HC chains induce a gel to be accomplished through formation of smaller but more homogenous microdomains with the aid of pendant hydrophobic moieties in the lower temperature range. Moreover, time sweep rheological characterization of the thermogelling systems disclosed that the gelation process proceeds faster in the presence of HC chains. The Fredrickson–Larson (F–L) theory was implemented as an alternative approach to determine the gel point. In view of the obtained results which were consistent with the classical Winter–Chambon theory, the F–L theory can be proposed as a robust and less tedious method to precisely determine the gel point in such systems. The developed hydrogels can be proposed as promising injectable matrices with predetermined gel features for various biomedical applications.

Effect of incorporating bis(2-hydroxyethyl) terephthalate on thermal and mechanical properties and degradability of poly(butylene succinate)

AbstractA series of novel random copolymers of poly(butylene succinate-co-ethylene terephthalate) were synthesized and characterized in terms of thermal and mechanical properties, crystallinity and biodegradability. The composition and microstructure of the prepared copolyesters were characterized by 1H NMR and 13C NMR, respectively. It was seen that the PBS sequence length decreases with ethylene terephthalate content. All copolymers are semi-crystalline and crystallinity and crystallite size decrease slightly with the comonomer content up to 10%, but the introduction of 20% comonomer leads to decrease the crystallinity up to 29%. The melting temperature of copolyesters decreases with the comonomer content according to the Baur’s equation that indicates only PBS blocks crystallize and crystallite size is decreased with the comonomer content. It was also investigated that the elastic modulus also decreases slightly with the comonomer content. However, the elongation at break increases by 500% due to the decrease in crystallite size and crystallinity. Incorporating non-biodegradable aromatic comonomer has a little effect on copolyester degradability because of the randomness and lower crystallite size.

Preparation of poly(butylene terephthalate)/modified organoclay nanocomposite via in-situ polymerization Characterization, thermal properties and flame retardancy

Nanocomposites of poly(butylene terephthalate) (PBT)/modified montmorillonite organoclay were prepared via in-situ polymerization of terephthalic acid and butanediol in the presence of different clay content. Cloisite 30B, a commercially available amino modified montmorillonite, was modified with 3-aminopropytriethoxysilane (APS) through a silylation reaction. Morphology and properties of all samples were investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infrared (FTIR) spectroscopy, proton nuclear magnetic resonance (1HNMR), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The extent of clay layers in samples was confirmed by XRD. Results of XRD and TEM showed that an exfoliation structure was produced in nanocomposites. Studies of crystallization showed that the presence of nanoclay leads to an increase in the crystallization rate and enhances the thermal stability of nanocomposites. Crystallization kinetics were described by the Avrami equation. Crystal growth was spherulitic. According to dynamic mechanical analysis (DMA), storage modulus of nanocomposites was remarkably improved compared with homo PBT. Moreover, a shrinkage test was carried out. Results showed a reduction in shrinkages along and across the flow direction that means a decrease in free volume. Flammability based on the UL-94 test was applied to study the flame retardancy effect of nanoclay on the samples.

Flexible polyurethane nanocomposite foam: Synthesis and properties

Flexible polyurethane (PU) nanocomposite foams were synthesized using organically modified montmorillonite clay (Cloisite 30B). The dispersion of organoclay was considered both in the isocyanate and polyol matrixes. Silicate layers of organoclay can be exfoliated in PU matrix by use of two steps mixing process. The presence of clay increased the cell density and reduced the cell size compared to the conventional PU foam. Clay dispersion was investigated by X-ray diffraction (XRD). The morphology and properties of PU nanocomposite foams were also studied. Generally, mechanical properties by addition of clay were improved. Foams in which clay was firstly dispersed in the isocyanate, showed better dispersion due to affinity of OH group on the clay surface to react with NCO groups. Better properties have been achieved with these nanofoams.

Effect of Hydroxyapatite Nanoparticles on the Degradability of Random Poly(butylene terephthalate-co-aliphatic dicarboxylate)s Having a High Content of Terephthalic Units

Copolyesters derived from 1,4-butanediol and constituted also of aliphatic and aromatic dicarboxylate units in a molar ratio of 3:7 were synthesized by a two-step polycondensation procedure. Succinic, adipic, and sebacic acids were specifically selected as the aliphatic component whereas terephthalic acid was chosen as the aromatic moiety. The second synthesis step was a thermal transesterification between the corresponding homopolymers, always attaining a random distribution as verified by NMR spectroscopy. Hybrid polymer composites containing 2.5 wt % of hydroxyapatite (HAp) were also prepared by in situ polymerization. Hydroxyl groups on the nanoparticle surface allowed the grafting of polymer chains in such a way that composites were mostly insoluble in the typical solvents of the parent copolyesters. HAp had some influence on crystallization from the melt, thermal stability, and mechanical properties. HAp also improved the biocompatibility of samples due to the presence of Ca2+ cations and the damping effect of phosphate groups. Interestingly, HAp resulted in a significant increase in the hydrophilicity of samples, which considerably affected both enzymatic and hydrolytic degradability. Slight differences were also found in the function of the dicarboxylic component, as the lowest degradation rates was found for the sample constituted of the most hydrophobic sebacic acid units.