Graphene oxide improved thermal and mechanical properties of electrospun methyl stearate/polyacrylonitrile form-stable phase change composite nanofibers

In the present work, a novel PAN-based form-stable composite phase change materials with the methyl stearate (MES) encapsulated in the supporting matrices of polyacrylonitrile (PAN) nanofibers were fabricated through electrospunning for the storage and retrieval of thermal energy. Influences of graphene oxide (GO) addition on the chemical properties, structural morphologies, mechanical properties, thermal energy storage properties, thermal stability, and thermal energy storage/retrieval rates of electrospun MES/PAN/GO phase change composite nanofibers were systematically investigated by FT-IR, FE-SEM, tensile testing, DSC, TG, and measurement of melting/freezing times, respectively. The results revealed that the incorporation of GO effectively enhanced the mechanical properties, thermal stability, as well as heat storage and release rates of the phase change composite nanofibers. The averaged tensile strength of electrospun MES/PAN/GO phase change composite nanofibers increased significantly by 573 % with 10 mass% loading of GO, while elongation at break had a maximum 107 % increment when adding 3 mass% of GO. The DSC results indicated that the electrospun PAN-based phase change composite nanofibers with various GO loadings had suitable phase transition temperatures with the latent heat ranging from about 92 to 109 kJ kg^?1 and exhibited good thermal reliability in terms of DSC measurements during 50 melting-freezing cycles. Moreover, the melting and freezing time were significantly decreased about 44 and 43 % for the MES/PAN/GO5, as well as 59 and 64 % for the MES/PAN/GO10 after introducing the GO into the composite nanofibers systems.     

Effect of needle diameter on nanofiber diameter and thermal properties of electrospun poly(methyl methacrylate)

The effect of needle diameter on the resulting electrospun poly(methyl methacrylate) (PMMA) average nanofiber diameter has been evaluated for three different needle gauges. The resulting nanofibers were observed and analyzed by scanning electron microscopy (SEM), suggesting a lack of correlation between the needle diameter used and the resulting average nanofiber diameter. Thermogravimetric analysis (TGA) indicated an increase in the thermal stability of PMMA nanofibers when compared to powdered PMMA, while differential scanning calorimetry (DSC) studies evidenced lower glass transition temperatures (Tg) for PMMA nanofibers in the first heating cycle. Copyright © 2007 John Wiley & Sons, Ltd.     

Microstructural Study of Nylon-6/Gelatin Composite Nanofibers

Electrospinning is a simple and effective technology for fabricating nanofibers and polymer blending provides strength and minimal defects of electrospun ones. Therefore, in the present study, fabrication, and characterization of nylon-6/gelatin electrospun nanofibers using low-toxic solvents was investigated as means to improve the morphological deficiencies of gelatin nanofibers and facilitate its electrospinnability. The morphology of electrospun nylon-6/gelatin nanofibers were characterized using scanning electron microscope (SEM). SEM results showed that electrospun blend nanofibers had smooth surface with average diameter of from 40 to 100 nm; while, the miscibility of the blend and thermal behavior of nanofibers were determined using Fourier transform-infrared spectroscopy (FTIR) and differential scanning calorimeter (DSC). Water contact-angle measurement (WCA) was employed to investigating the wettability of nanofibers.     

Characterisation and comparison of biomass ashes with different thermal histories using TG-DSC

The present research is focused on the characterisation and comparison of biomass ashes from wood pellet with different thermal histories. One of the ashes is obtained in a muffle furnace until its mass stabilization reaching a temperature of 550 °C, low temperature ash (LTA); the other one came from an experimental fixed bed combustor after 4 h of stable combustion in which the temperature reached is above 1,000 °C, high temperature ash (HTA). The samples were studied using Thermogravimetry and Differential Scanning Calorimetry (TG-DSC) techniques, and they were subjected to a heating up to 900 °C under an inert atmosphere with the objective of perceiving the differences in their thermal behaviour. At these temperatures, complex phase transformations occur, related to decomposition of carbonates and formation of silicates. TG and DSC curves are compared and some differences in mass loss, temperature peaks and enthalpy associated to endothermic effects are detected and they are explained based on the different compositions of both samples obtained at different temperatures. Other techniques were applied for the determination of the chemical composition of the ashes; X-ray fluorescence and Scanning electron microscopy with energy dispersive X-ray spectroscopy showed the elements present in the ashes, and X-ray diffraction revealed the crystalline phases and confirms that LTA is mainly composed of carbonates, while HTA mostly consists of silicates.     

Thermal,morphological, and mechanical properties of ethyl vanillin immobilized in polyvinyl alcohol by electrospinning process

In this study, polyvinyl alcohol (PVA) nanofibers with ethyl vanillin as an active compound were prepared using electrospinning technique. The final products of electrospinning process were in the form of nanofibers films. PVA/ethyl vanillin nanofibers, having fibers diameters in the range 100–1700 nm, were successfully electrospun from ethanol/water mixture of PVA and ethyl vanillin. The effects of immobilization process on ethyl vanillin thermal properties were investigated by differential scanning calorimetry (DSC). The results of DSC showed significant influence of immobilization process on thermal properties of ethyl vanillin. It was noticed that melting point of immobilized ethyl vanillin was lower (~55 °C) compared to free flavor (~77 °C). Our results showed that films based on PVA/ethyl vanillin nanofibers are mechanically stable.     

Electrospinning Process of Thermo-sensitive Poly(N-isopropylacrylamide) /poly (2-acrylamido-2-methylpropanesulfonic acid) Nanofibers

Poly (N-isopropylacrylamide)/poly (2-acrylamido-2-methylpropanesulfonic acid) (PNIPAAm/PAMPS) nanofibers was prepared using the electrospinning technique. The electrospinning process parameters such as solution concentration, voltage, receiver distance and flow rate were determined by the orthogonal experiments. The appropriate electrospinning parameters were 7.0% of solution concentration, 10.0 kV of voltage, 20 cm of distance and 3.1 μL·min^?1 of flow rate, respectively. The major factor affecting the nanofibers diameter was the solution concentration and the diameter increased with the solution concentration. The Fourier-transform infrared spectroscopy (FTIR) was conducted to characterize the structure of the components for electrospinning. Scanning electron microscopy (SEM) was taken to observe the morphology, and the contact angle (CA) measuring was carried out to determine the wettability of the nanofibers with temperatures. The results of SEM observation showed that the surfaces of nanofibers were smooth with uniform fibrous diameters and without the formation of beads. The CA detections showed that the electrospun PNIPAAm/PAMPS nanofibers exhibited thermo-sensitivity of hydrophilicity at 20°C and hydrophobicity at 40°C.     

Thermal and mechanical properties of electrospun PMMA,PVC, Nylon 6, and Nylon 6,6

Poly(methyl methacrylate) (PMMA), poly(vinyl chloride) (PVC), Nylon 6, and Nylon 6,6 have been electrospun successfully. The nanofibers have been characterized by scanning electron microscopy (SEM), confirming the presence of bead free and fiber‐bead free morphologies. Thermogravimetric analysis (TGA) indicated differences between the thermal stability of PMMA nanofibers and PMMA powder. However, no significant differences were observed between the starting physical form (powder or pellet) of PVC, Nylon 6 and Nylon 6,6, and their corresponding electrospun nanofibers. Differential scanning calorimetry (DSC) demonstrated a lower glass transition temperature (T_g) and water absorption for PMMA electrospun nanofibers. Furthermore, electrospun Nylon 6 and Nylon 6,6 had a slight decrease in crystallinity. Tensile testing was performed on the electrospun nanofibers to obtain the Young modulus, peak stress, strain at break, and energy to break, revealing that the non‐woven mats obtained had modest mechanical properties that need to be enhanced. Copyright © 2007 John Wiley & Sons, Ltd.     

Electrospun Nanofibers Sorbents for Pre-Concentration of 1,1-dichloro-2,2 bis-(4-chlorophenyl)ethylene with Subsequent Desorption by Pressurized Hot Water Extraction

Electrospun polystyrene (PS) nanofibers (130–500 nm) incorporating a potassium salt of imidazole-1-carbodithioate were evaluated as potential sorbents for the pre-concentration of a model organochlorine pesticide; 1,1-dichloro-2,2bis-(4-chlorophenyl)ethylene (DDE). The efficiencies of DDE (0.25–1.0 μg L^?1) adsorption by the nanofiber sorbent followed by desorption employing pressurized hot water extraction (PHWE) were investigated and monitored using gas chromatography with electron capture detection (GC-ECD). Parameters such as time, temperature and pressure of extraction, sample volume, DDE concentration and sorbent mass were optimized. The maximum adsorption of DDE (0.50 μg L^?1) on electrospun PS and carbodithioate incorporated PS nanofibers was at 43.7 and 94.6%, respectively, in 20 min. Incorporation of carbodithiote doubled the adsorption efficiency of PS and achieved LOD of 0.000234 μg L^?1 for DDE. The optimal DDE desorption on the PHWE system was 93.8% in 10 min. It would seem that the use of electrospun nanofibers as sorbent material with subsequent desorption by PHWE has great potential and thus warrants further investigations. This approach as it uses water as an extraction solvent for an organochlorine pesticide provides an opportunity to eliminate organic solvents, especially for procedures aimed at monitoring organic pollutants in the environment.     

Accuracy of electrospun fiber diameters: The importance of sampling and person-to-person variation

Potential sampling errors (regional variation) on an electrospun mat were explored and person-to-person (analyst affect) variation in image analysis of the fiber diameter were investigated via detailed statistical analyses. Scanning electron microscope (SEM) samples were prepared from the vertical midline of a single non-woven mat of electrospun polyethylene oxide. Thirteen analysts with identical training and instructions measured the diameters of the nanofibers from the six SEM images and statistical analyses were performed on the resulting data. The fiber diameters were significantly different in the lower region than the upper and center regions. Furthermore, the fiber diameters in the lower region—from micrographs taken only millimeters apart—were statistically different demonstrating a statistically significant regional variation in the sample. Furthermore, statistically significant variation between the analysts also was observed, with the average fiber diameter ranging from 166 nm to 276 nm.     

Electrospinning of nylon-6,6 solutions into nanofibers: Rheology and morphology relationships

The relationship between the rheological properties of nylon-6,6 solutions and the morphology of their electrospun nanofibers was established. The viscosity of nylon-6,6 in formic acid(90%) was measured in the concentration range of 5 wt%-25 wt% using a programmable viscometer. Electrospinning of nylon-6,6 solutions was carried out under controlled parameters. The chemical structure, morphology and thermal properties of the obtained nanofibers were investigated using Fourier transform infrared spectroscopy(FTIR), scanning electron microscopy(SEM) and differential scanning calorimetry(DSC), respectively. Entanglement concentration(ce) was found to be 15 wt% and a power law relationship between specific viscosity and solution concentration was observed with exponents of 2.0 and 3.3 for semi-dilute unentangled(c ce) and semi-dilute entangled(c ce) regimes, respectively. The diameter and uniformity of the nanofibers were found to be dependent on the viscosity. Moreover, the average diameter of electrospun nanofibers was found to be dependent on zero shear rate viscosity and normalized concentration(c/ce) in a power law relationship with exponents of 0.298 and 0.816, respectively. For nylon-6,6 solutions, the entanglement concentration(ce = 15 wt%) provides the threshold viscosity required for the formation of a stable polymeric jet during electrospinning and producing uniform beadless fibers. For concentrations less than ce, beaded fibers with some irregularities are formed. DSC analysis showed an increase in crystallinity of all electrospun samples compared to original polymer. Furthermore, Based on FTIR spectroscopy, α phase is dominant in electrospun nanofibers and minor amount of β and γ phases is also available.     

Fabrication and characterization of NiTiO3 nanofibers by sol–gel assisted electrospinning

Continuous NiTiO₃ nanofibers have been successfully synthesized by a sol–gel assisted electrospinning method followed by calcination at 600 °C in air. These nanofibers were characterized for the morphological, structural and optical properties by scanning electron microscopy (SEM), energy-dispersive X-ray spectrum (EDS), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectra (XPS) and UV–visible (UV–vis) diffuse reflectance spectroscopy (DRS). SEM results reveal that the obtained NiTiO₃ nanofibers are 175 nm in diameter and several micrometers in length after annealing at 600 °C. The XRD analysis shows that the nanofibers possess highly crystalline structure with no impurity phase. In contrast, the NiTiO₃ nanoparticles synthesized at the identical conditions by a sol–gel route have impurities including TiO₂ and NiO. Moreover, the electrospun NiTiO₃ nanofibers are endowed with an obvious optical absorbance in the visible range, demonstrating they have visible light photoresponse.     

Preparation of Cross-Linked Enzyme Aggregates of Trehalose Synthase via Co-aggregation with Polyethyleneimine

Trehalose synthase (TreS) from Meiothermus ruber was co-aggregated with polyethyleneimine (PEI) and precipitated with polyethylene glycol (PEG), followed by cross-linking with glutaraldehyde to obtain TreS-polyethyleneimine cross-linked enzyme aggregates (termed as CLEAs-PEI-PEG). The TreS solution at 0.5 mg mL^?1 protein concentration, with PEI at a mass ratio of 1:0.8 (enzyme/PEI, w/w) and 25 % (w/v) PEG concentration were found to be most adequate for the co-aggregation of TreS. CLEAs-PEI-PEG was most active with glutaraldehyde at a mass ratio of 1:0.5 (enzyme/glutaraldehyde, w/w) to cross-link the co-aggregates. The CLEAs-PEI-PEG prepared in this work had an optimum pH of 6.5 and optimum temperature of 60 °C. For lower concentrations of enzyme, using PEI could enhance the cross-linking efficiency of TreS. The thermal stability and pH tolerance of CLEAs-PEI-PEG were significantly improved. Scanning electron microscopy revealed that the main structure of CLEAs-PEI-PEG showed scaffolding morphology which was constituted by structured ball-like particles with a size of 1–2.5 μm in diameter.     

Preparation and investigation on tetradecanol and myristic acid/cellulose form-stable phase change material

In this paper, a novel form-stable phase change material (FS PCM) was prepared by incorporating the eutectic mixture of tetradecanol (TD) and myristic acid (MA) into the hydroxylpropyl methyl cellulose (HPMC). HPMC is used as support material, and the eutectic mixture is used as phase change material. The Fourier-transform infrared spectroscopy (FT-IR), X-ray diffractometer (XRD) and scanning electron microscopy (SEM) were used to study the chemical structure, crystallization behavior and morphology of the FS PCM, respectively. FT-IR, XRD and SEM showed that the TD–MA was distributed uniformly in HPMC by physical interaction. Specific surface area (BET) and pore size analysis determined the pore characteristics of the composite, and the results showed the porosity of HPMC. The thermal properties, thermal stability and thermal reliability were detected by differential scanning calorimetry (DSC), thermogravimetric analysis (TG), thermal cycling test and leakage test. The TG, DSC and leakage analysis results revealed that the absorption of eutectic mixture into HPMC is nearly 50% and without seepage from the composite. The peak temperatures of melting and solidifying were 34.61 and 31.09 °C, and latent heat was 102.11/84.58 J g^?1 by DSC. TG and cycling experiment detected that the FS PCM showed good thermal stability and reliability performance.     

Thermal and mechanical properties of nanofibers-based form-stable PCMs consisting of glycerol monostearate and polyethylene terephthalate

An innovative type of nanofibers-based form-stable composite phase change materials for the storage and retrieval of thermal energy was successfully prepared by encapsulating glycerol monostearate (GMS) into the porous structure of polyethylene terephthalate (PET)-supporting matrices on the nanoscale through electrospinning. The field-emission scanning electron microscopy and transmission electron microscopy images revealed that the composite nanofibers possessed desired morphologies with the average fiber diameters ranging from about 290 to 1000 nm which increased with the contents of GMS. The two phase separation (e.g., GMS phase and PET phase) was clearly observed from the images. When GMS content reached 60 %, the amount of the GMS distributing on the surface of the composite nanofibers was significantly increased during the electrospinning. The Fourier transform infrared spectroscopy spectrum proved that the PET supporting matrices were physically combined with GMS molecules. The differential scanning calorimetry analysis indicated that the GMS/PET composite nanofibers had reversible phase change behaviors, and the melting enthalpies increased from 32.63 to 66.99 kJ kg^?1 with increasing GMS amount. The TG results showed that both the onset thermal degradation temperature and charred residue of the GMS/PET composite nanofibers at 700 °C were higher than those of pristine GMS powder owing to the better thermal stability of the PET molecules. The tensile testing revealed that the averaged tensile strength and elongation at break of the all GMS/PET composite nanofibers varied from 3.29 to 10.30 MPa and from 2.42 to 42.30 %, respectively.     

Simple fabrication of cellulose nanofibers via electrospinning of dissolving pulp and tunicate

This work demonstrates a simple fabrication of cellulose nanofibers by direct electrospinning of dissolved cellulose solutions. The hard- and softwood pulps and the outer mantles of tunicate were dissolved in a mixture of trifluoroacetic acid and dichloroethane by stirring and ultrasonication to give highly viscoelastic, clear solutions. These solutions were electrospun to form continuous nanofibers made of unsubstituted cellulose, which were confirmed by scanning electron microscopy (SEM) and IR spectroscopy. Statistical analysis of the SEM images of the nanofibers suggested that there are positive correlations between diameters of the nanofibers and concentration of the cellulose solution. The mean diameters of the nanofibers obtained from softwood pulp (DP of cellulose ≈ 1200) solutions were larger than those from hardwood pulp (DP of cellulose ≈ 500) at the same concentrations. This indicates that the DP of cellulose is one of the important parameters to control the diameters of the electrospun cellulose nanofibers.     

Effect of Ag Loading on the Microstructure of TiO<Subscript>2</Subscript> Electrospun Nanofibers

In this research work, crystalline structure, phase transformation, morphology and mean size of titanium dioxide (TiO₂) electrospun nanofibers have been tailored by loading with 2.5, 5.0 and 7.5 wt.% of silver (Ag) which was followed by calcination. The as prepared non woven mats of nanofibers were calcinated at 500 °C to allow the reaction moieties to leave the TiO₂ matrix and subsequently formation of Ag clusters. The effect of Ag loading and calcination on the transformation of microstructure of these electrospun nanofibers have been characterized by XRD, FESEM, FT-IR and Raman spectroscopy (RS). The mean diameter of Ag loaded nanofibers has been found to decrease upon calcination which was estimated to 70 nm whereas length was in the order of mm range. XRD and RS results have strongly supported the transformation of crystalline phase from rutile (A) to anatase (R) above 2.5 wt.% of Ag loading in TiO₂ after calcination. The roughness on the outer surfaces of these nanofibers has been observed to increase with the Ag loading consequent to calcination, which has been attributed to the formation Ag nanoparticles that were found adsorbed at the surfaces. An interesting finding of this study is the existence of 1D nanofibers’ structure even at higher (7.5 wt.%) Ag loading, as observed by the SEM micrographs.     

Stabilization of electrospun poly(vinyl alcohol) nanofibrous mats in aqueous solutions

<正>The stability ofpoly(vinyl alcohol)(PVA) nanofibrous mats in water media was improved by post-electrospinning treatments.Bifunctional glutaraldehyde(GA) in methanol was used as a crosslinking agent to stabilize PVA nanofiber,but fiber twinning was observed frequently,and the highly porous structure of PVA nanofibrous mats was destroyed when the crosslinked fiber was soaked in water.To overcome this shortcoming,chitosan(CS) was introduced into the PVA spinning solution to prepare PVA/CS composite nanofibers.Their treatment in GA/methanol solution could retain the fiber morphology of PVA/CS nanofibers and porous structure of PVA/CS nanofibrous mats even if they were soaked in aqueous solutions for 1 month.Scanning electron microscopy(SEM),X-ray diffraction(XRD),thermal gravimetric analysis(TGA) and differential scanning calorimetry(DSC) were applied to characterize the physicochemical structure and thermal properties of PVA nanofibers.It was found that the water resistance of PVA nanofibrous mats was enhanced because of the improvement of the degree of crosslinking and crystallinity in the electrospun PVA fibers after soaking in GA/methanol solution.     

Electrospun nylon-4,6 nanofibers: solution rheology and Brill transition

Polyelectrolyte solutions of nylon-4,6 in 99 vol.% formic acid were electrospun, and then the concentration effect on the solution spinnability was studied. The microstructure of the as-spun nanofibers was characterized by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). Based on the solution rheology, the concentration of the entangled regime and the concentrated regime (? _D ) were 1 and 10 wt.%, respectively. To prepare bead-free fibers, the minimum polymer concentration used was 10 wt.%, yielding a fiber diameter of 49?±?13 nm. The fiber diameter (d _f) was dependent on the solution viscosity ( $ {\eta_{\mathrm{o}}} $ ) or the polymer concentration (?_w) through the following simple scaling law relation: d _f?～? $ \eta_{\mathrm{o}}^{0.62 } $ and d _f?～? $ {{({\phi_{\mathrm{w}}}/{\phi_{\mathrm{D}}})}^{2.25 }} $ . DSC heating trace on the as-spun nanofibers exhibited double-melting behavior. However, after cooling, the second heating trace showed a single melting peak. WAXD intensity profiles showed that the as-spun nanofibers possessed lamellae with small lateral dimensions, and the lattice parameter difference between a-axis and b-axis was significantly reduced due to the rapid electrospinning process. Both structural features induce the occurrence of the Brill transition of nylon-4,6 in the nanofibers at a much lower temperature of 80 °C than that in the melt-processed film, as-revealed by the temperature-variable WAXD.     

Melting Temperature of Individual Electrospun Poly(vinylidene fluoride) Fibers Studied by AFM-based Local Thermal Analysis

Thermal properties such as melting temperature can well reflect the microstructure of the polymer material, and have practical implications in the application of nanofibers. In this work, we investigated the melting temperature of individual electrospun poly(vinylidene fluoride)(PVDF) nanofibers with diameters ranging from smaller than 200 nm to greater than 2 μm by the local thermal analysis technique. The PVDF fibers obtained under four different conditions were found to crystallize into α and β phases, and the fiber mats showed typical values in the crystallinity and T_m with no significant difference among the four. However, analyses at single fiber level revealed broad distribution in diameter and T_m for the fibers produced under identical electrospinning condition. The T_m of individual nanofibers was found to remain constant at large diameters and increase quickly when reducing the fiber diameter toward the nanoscale, and T_m values of 220-230 ℃ were observed for the thinnest nanofibers, much higher than the typical values reported for bulk PVDF. The T_m and molecular orientation at different positions along a beaded fiber were analyzed, showing a similar distribution pattern with a minimum at the bead center and higher values when moving toward both directions. The results indicate that molecular orientation is the driving mechanism for the observed correlation between the T_m and the diameter of the nanofibers.     

Development of silver-containing nanocellulosics for effective water disinfection

Electrospun cellulose nanofibers and cellulose-graft-polyacrylonitrile (Cell-g-PAN) copolymer nanofibers containing silver nanoparticles (AgNPs) were synthesized for effective water disinfection. Surface morphology, AgNPs content, physical distribution of AgNPs, levels of silver leaching from the fibers in water and antimicrobial efficacy were studied. Scanning electron microscope images revealed that AgNPs in cellulose nanofibers were more evenly dispersed than in Cell-g-PAN copolymer nanofibers, but with the certainty that Cell-g-PAN copolymer nanofibers had higher AgNPs content. This was confirmed by energy dispersive X-ray analysis and atomic absorption analysis. Both cellulose nanofibers and Cell-g-PAN copolymer nanofibers containing AgNPs had excellent antimicrobial activity against Escherichia coli, Salmonella typhi, and Staphylococcus aureus, with cellulose-nAg nanofibers killing between 91 and 99 % of bacteria in a contaminated water sample and Cell-g-PAN-nAg copolymer nanofibers killed 100 %. Neither Cell-g-PAN copolymer nanofibers nor cellulose nanofibers leached silver into water.