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.     

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.     

Electrospinning of nylon-6,66,1010 terpolymer

Ultrafine nylon fibers were prepared by electrospinning of nylon-6,66,1010 terpolymer solution in 2,2,2-trifluoroethanol (TFE). The morphology, crystallinity and mechanical properties of the electrospun nylon-6,66,1010 fibers were investigated by scanning electron microscope (SEM), differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD) and tensile test. The effects of electrospun process parameters such as solution concentration, voltage and tip-to-collector distance on the morphology and the average size of the electrospun fibers were also studied. The results show that the spinnable concentration of nylon-6,66,1010/TFE solution is in the range of 6-14 wt%, and higher solution concentration favors the formation of uniform fibers without beads. The diameters of the electrospun fibers increase with increasing the solution concentration and decrease slightly with increasing the voltage and needle tip-to-collector distance. But no obvious morphology changes were found with the increase of the voltage and collection distance. DSC and WAXD results suggest that the electrospun nylon-6,66,1010 membranes have lower crystallinity than those of the corresponding casting films. The electrospun nylon-6,66,1010 membrane obtained from the 14 wt% concentration exhibits the largest tensile strength and elongation at break.     

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.     

Concentration dependence of critical exponents for gelation in gellan gum aqueous solutions upon cooling

The sol-gel transition in aqueous gellan gum solutions induced upon cooling was investigated by rheology measurements. The gelation temperature was determined from the crossover point of storage and loss moduli, i.e., G′ = G′′ (T_c) and from the Winter’s criterion (T_gel), respectively, which increased with gellan concentration. T_gel was higher than T_c and the difference became larger as the gellan concentration got higher. The relaxation critical exponent n was estimated with the Winter’s method and the self-similarity was observed from the critical gel. The scaling for the zero-shear viscosity η₀ before the gel point and the equilibrium modulus G_e after the gel point was established against the relative distance ε from the gel point over the gellan concentration C_g of 1.0-2.5 wt%, giving the critical exponents k and z. The critical exponent n calculated from k and z agrees well with n from the Winter’s criterion. However, no universal n was found for the gelation in aqueous gellan gum solutions, indicating that this gelation should be classified into the cross-linking category for the physical gelation. The critical exponent n decreased with increasing C_g for the gellan gum solution. The fractal dimension d_f calculated from n with the screened hydrodynamic interaction and the excluded volume effect suggested a denser structure in the critical gel with higher C_g.     

Liquid fragility of the unsaturated polyester resin

Curing behaviors of the unsaturated polyester resin (UPR) containing 1–1.8 wt% methyl ethyl ketone peroxide (MEKP) initiator are investigated. The viscosity, gelation and vitrification transition of the UPR-MEKP systems are examined using the rotating viscometer and differential scanning calorimetry (DSC). A liquid fragility parameter, M _c, defined as the viscosity variation rate of the liquids towards the curing temperature is presented. It is found that M _c has a good negative relation with the glass transition temperature (T _g) in the systems. M _c can be used for predicting the stability of the cured amorphous systems. The relationship between the liquid and cured thermoset polymer systems is studied from both the thermodynamic and kinetic point of view.     

Fabrication of nanocomposite PAN nanofibers containing MgO and Al2O3 nanoparticles

This paper describes the effect of embedding MgO and Al₂O₃ nanoparticles on the diameter of electrospun composite polyacrylonitrile (PAN) nanofibers. Diameter of nanofibers determines the important properties of the nanofibrous mats used in a variety of developed applications such as tissue engineering scaffolds, drug delivery, catalysis, ultra filtration, sensors, and nanoelectronics. The results showed that the type and amount of nanoparticles dispersed in PAN solutions affect the conductivity as well as the viscosity of the electrospinning solutions. Increasing the amount of MgO and Al₂O₃ leads to higher conductivity and higher viscosity of the electrospinning solution and ultimately to a smaller nanofiber diameter. Moreover, the results showed that higher conductivity of the electrospinning solution overcomes the effect of higher viscosity. Finally, no interaction was detected between metal oxide nanoparticles and PAN macromolecules.     

Solution behavior of progressively quaternized poly(dimethylaminoethyl methacrylate) as a function of charge density

Hydrodynamic behavior of progressively quaternized poly(dimethylaminoethyl methacrylate) (PDMAEM) in dilute and semi-dilute salt–free aqueous solutions as a function of molar mass and charge density, which varied from 5 up to 100 mol%, was studied. The role of long range electrostatic interactions on the viscosity and on the position and value of the reduced viscosity corresponding to the peak, η_red,p, as a function of the charge density of polycation has been evaluated. The overlap concentration of polyelectrolytes, c^∗, has been evaluated as the inverse of the intrinsic viscosity determined by fitting the viscometric results on Rao equation. The values of c^∗ have been discussed as a function of polyion molar mass and charge density. The c^∗ values calculated in the frame of Odijk model for polyelectrolytes having quaternization degree ?50 were lower but comparable with those determined by viscometry, for the same molar mass.     

Viscoelasticity and rheology in the regimes from dilute to concentrated in cellulose 1-ethyl-3-methylimidazolium acetate solutions

Dynamic rheological behaviors of α-cellulose 1-ethyl-3-methylimidazolium acetate ([Emim]Ac) solutions were investigated in a large range of concentrations (0.1–10 wt %) at 25 °C. On the basis of data from the dynamic viscoelastic test, the exponents of the specific viscosity η _sp versus concentration c were determined as 1.0, 2.0 and 4.7 for dilute, semidilute unentangled and entangled regimes respectively, which were in accordance with the scaling prediction for neutral polymer in θ solvent. The intrinsic viscosity [η] of the solution was determined to be 253 mL/g at 25 °C. The linear viscoelastic response of the dilute and semidilute unentangled solutions could be described successfully by the Zimm and Rouse model (ν = 0.5 for θ solution) respectively, suggesting that the motion of cellulose chain in [Emim]Ac changed from Zimm to Rouse model with increasing concentration. At low concentrations, failure of the Cox–Merz rule with steady shear viscosity larger than complex viscosity was observed. While as the concentration increased, the deviation from the Cox–Merz rule disappeared due to the formation of homogeneous entanglement structure in cellulose solution.     

Preparation of porous ultrafine polyacrylonitrile (PAN) fibers by electrospinning

A facile method for the preparation of porous ultrafine nanofibers was demonstrated. The PAN/NaHCO₃ composite nanofibers were electrospun, and then NaHCO₃ was removed by a selective dissolution and reaction with the solution of hydrochloric acid (10 wt%). The obtained PAN fibers showed highly porous surfaces after the extraction of NaHCO₃. The structure and properties of ultrafine PAN nanofibers were characterized by Fourier transform infrared (FT‐IR), X‐ray diffraction (XRD), and thermogravimetry (TG). The results indicated that NaHCO₃ could be introduced into the PAN solution and successfully electrospun. CO₂ is released and pores are formed on the fibers. The morphology image of the fibers was detected by scanning electron microscope (SEM) and showed that many pores aligned the nanofibers. Copyright © 2008 John Wiley & Sons, Ltd.     

Electrospun nanofibers of polyferrocenylsilanes with different substituents at silicon

The strained silicon-bridged [1]ferrocenophane Fe(η-C₅H₄)₂SiBuMe was prepared via a facile chloride substitution reaction at the bridging atom of a readily available SiMeCl-bridged [1]ferrocenophane precursor. Thermal ring-opening polymerization of Fe(η-C₅H₄)₂SiBuMe and Fe(η-C₅H₄)₂SiMe₂ afforded polyferrocenyldimethylsilane (PFDMS) and polyferrocenylbutylmethylsilane (PFBMS), respectively. Polyferrocenylsilane nanofibers were fabricated by electrospinning polymer solutions in 90 wt% tetrahydrofuran and 10 wt% N,N-dimethylformamide at room temperature. The effect of processing parameters such as concentration of polyferrocenylsilanes solution, applied voltage, and working distance on the diameter and morphology of resulting nanofibers were investigated. Electron diffraction patterns from polymer nanofibers revealed that PFS fibers exhibit different orientation owing to variance of the side groups at silicon.     

Optimization and predictive modelling for the diameter of nylon-6,6 nanofibers via electrospinning for coronavirus face masks

Currently, the only widely available tool for controlling the SARS-CoV-2 pandemic is nonpharmacological interventions (NPIs). Coronavirus aerosols are around 0.3–2 µm in diameter (0.9 m in mass). The present study used artificial intelligence such as gene expression programming (GEP) and genetic algorithms (GA) were used to predict and optimize the diameter of Nylon-6,6 nanofibers via electrospinning for protection against coronavirus. It is suggested that using the controlled experimental conditions such as concentration of nylon-6,6 (16% wt/v), applied voltage (26 kV), working distance (18 cm) and injection rate (0.2 mL/h) have resulted the diameter of nylon-6,6 nanofibers about 55.8 nm. Coronavirus face masks could use the obtained diameter and electrostatic interaction between viral particles and naofibers as active layers.     

Effect of CNT incorporation on PAN/PPy nanofibers synthesized by electrospinning method

In this study, carbon nanotubes (CNTs) added polyacrylonitrile/polypyrrole (PAN/PPy) electrospun nanofibers were produced. Average diameters of the nanofibers were measured as 268 and 153 nm for 10 and 25 wt% of PPy contents, respectively. A relatively higher strain to failure values (23.3%) were observed for the low PPy content. When as-grown CNTs (1 and 4 wt%) were added into the PAN/PPy blends, disordered nanofibers were observed to form within the microstructure. To improve the interfacial properties of CNTs/PAN/PPy composites, CNTs were functionalized with H₂SO₄/HNO₃/HCl solution. The functionalized CNTs were well dispersed within the nanofibers and aligned along the direction of nanofibers. Therefore, beads formation on nanofibers decreased. The impedance of the nanofibers was found to decrease with the PPy content and CNT addition. These nanofibers had a great potential to be used as an electrochemical actuator or a tissue engineering scaffold.     

Charge density effects in salt‐free polyelectrolyte solution rheology

Solution rheology of 2‐vinyl pyridine and N‐methyl‐2‐vinyl pyridinium chloride random copolymers in ethylene glycol was studied over wide ranges of concentration and effective charge. The fraction of quaternized monomers α and the fraction of monomers bearing an effective charge f of these copolymers were measured using counterion titration and dielectric spectroscopy, respectively. Ethylene glycol is a good solvent for neutral poly(2‐vinyl pyridine), with very few ionic impurities. The viscosity η and relaxation time τ of dilute and semidilute unentangled solutions exhibit the scaling with concentration and effective charge expected by the Dobrynin model. Reduced viscosity data are independent of concentration in dilute solution, giving an intrinsic viscosity that depends on effective charge, and the experimental data obey the Fuoss law in the semidilute unentangled regime. Scaling concentration with the overlap concentration (c/c*) reduces these data to common curves, and c* ～ f ^?12/7 as predicted by the Dobrynin model, where f is the fraction of monomers bearing an effective charge. While the overlap concentration depends strongly on effective charge until counterion condensation occurs, the entanglement concentration c_e is surprisingly insensitive to effective charge, indicating that entanglement effects are not understood using the Dobrynin model. The terminal modulus G = η/τ depends only on the number density of chains G = ckT/N for c* < c < c_e, and G ～ c^3/2 for c > c_e independent of the effective charge. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2001–2013, 2006     

Preparation,characterization, and properties of nanofibers based on poly(vinylidene fluoride) and polyhedral oligomeric silsesquioxane

Nanostructered nanofibers based on poly(vinylidene fluoride) (PVDF) and polyhedral oligomeric silsesquioxane (POSS) have been prepared by electrospinning process. The starting solutions were prepared by dissolving both the system components in the mixture N,N‐dimethylacetamide/acetone. The characteristics of the fiber prepared, studied by scanning electron microscopy, atomic force microscopy, and wide angle X‐ray diffraction, have been compared with those of PVDF fibers. Morphological characterization has demonstrated the possibility to obtain defect‐free PVDF/POSS nanofibers by properly choosing the electrospinning conditions, such as voltage, polymer concentration, humidity, etc. Conversely, in the case of fibers based on the neat polymer, it was not possible to attain the complete elimination of beads in the electrospun nanofibers. The different behavior of the two types of solutions has been ascribed to silsesquioxane molecules, which, without influencing the solution viscosity or conductivity, favor the formation of uniform structures by decreasing the system surface tension. Concerning POSS distribution in the fibers, the morphological characterization of the electrospun films has shown a submicrometric dispersion of the silsesquioxane. It is relevant to underline that cast films, prepared by the same solutions, have been found to be characterized by POSS aggregation, thus demonstrating a scarce affinity between the two‐system components. Indeed, the peculiar solvent evaporation of the electrospun solution, which is much faster than that occurring during the cast process, prevents POSS aggregation, thus leading to the formation of nanofibers characterized by a silsesquioxane dispersion similar to that present in solution. Finally, the presence of POSS improves the electrospun film mechanical properties. Copyright © 2011 John Wiley & Sons, Ltd.     

The Effect of a Background Electrolyte on the Viscosity of Aqueous Dodecyltrimethylammonium Bromide Solutions

Conductometry and viscometry have been employed to study the effect of a background electrolyte (KBr) taken in concentrations of 0.03 and 0.1 M on the critical micelle concentration of dodecyltrimethylammonium bromide (С₁₂ТАB) and the dependence of relative viscosity η/η₀ of С₁₂ТАB micellar solutions on the overall surfactant concentration. It has been found that, as a first approximation, each of these dependences may be represented as the sum of two linear portions. Concentrations c* of С₁₂ТАB micellar solutions, which correspond to the inflections between the two linear portions in the concentration curves of relative viscosity, have been determined. The Einstein equation η/η₀ = 1 + 2.5p (p is the volume fraction of the dispersed phase and 2.5 is a theoretical parameter that takes into account the spherical shape of the particles) has been transformed into a form corresponding to the concentration dependence of the relative viscosity on the overall surfactant concentration to make it applicable to the consideration of low-concentration systems, which are uncomplicated by intermicellar interaction. In particular, the applicability of the above equation for estimating micelle radii has been studied. It has been shown that the (η/η₀–1) = f(c/с₀₁–1) concentration dependences represented in bilogarithmic coordinates (c is the overall С₁₂ТАB concentration and c₀₁ is the critical micelle concentration) are linear in the absence of a significant intermicellar interaction and have slopes equal to unity. This fact may be considered as a criterion for the applicability of the Einstein equation to micellar solutions.     

电纺聚氧化乙烯纤维的分散形态和结晶行为研究

将聚氧化乙烯(PEO)水溶液在不同的工艺条件下进行电纺,制备了PEO纤维.用SEM研究了纤维的分散形态;用DSC和XRD研究了纤维的结晶性能.电纺纤维分散形态是由浓度、电压、固化距离等因素综合作用的结果.其中,浓度是最关键的因素.降低溶液浓度,提高静电压和增加固化距离均会使纤维变细.电纺得到的纤维与原粉相比,电纺使纤维结晶度下降,理论上分析了可能的机理.     

Characterization of gelatin nanofibers electrospun using ethanol/formic acid/water as a solvent

Gelatin nanofibers were prepared via electrospinning using aqueous solutions of formic acid and ethanol as the solvent instead of cytotoxic solvents. The resulting mat was further crosslinked with glutaraldehyde (GTA). The influence of the storing time on the viscosity and gel point of the solution was investigated. The gelatin nanofibers were examined using a field emission scanning electron microscope (FESEM) for the fiber size and morphology. The lowest diameter of gelatin fiber (85 nm, without beads) was achieved when the gelatin concentration was 20 wt% and electrospinning was conducted with a voltage of 20 kV over a distance of 10 cm at ambient temperature. The results from differential scanning calorimetry (DSC) showed that the softening temperature of gelatin nanofibers crosslinked with GTA was elevated. In addition, GTA‐crosslinked gelatin nanofibers exhibited cell compatibility for mouse mesangial cells (CRL 1927). Copyright © 2008 John Wiley & Sons, Ltd.     

Crosslinking kinetics of polyethylene with small amount of peroxide and its influence on the subsequent crystallization behaviors

Crosslinking reactions of high density polyethylene with low peroxide concentrations ranging from 0.1 wt% to 1.0 wt% at temperatures of 170, 180 and 190 ° C were monitored by rheological measurements. A critical gel forms at the peroxide concentration of 0.2 wt%, where the transition from long chain branching generation to crosslinking network formation could occur. Rheokinetics of crosslinking can be fitted well by Ding-Leonov's model. The curing rate k_2 at the earlier stage exhibits about 3 times acceleration per 10 °C with increasing temperature, while the equilibrium modulus G′ at the fully cured stage is almost independent of temperature. Influences of crosslinking on the subsequent crystallization behaviors were detected by DSC measurements. Above the critical gel concentration, crystallization is largely retarded as evidenced by the lower crystallization temperature Tc and crystallinity X_c due to the network formation. The secondary crystallization valley located at the temperature near 80 °C can be observed above the critical concentration, which becomes more evident with the increasing peroxide concentration and curing temperature. This phenomenon provides another evidence of crystallization retardation by the crosslinking network.     

Functionalized carbon nanotube/polyacrylonitrile composite nanofibers: fabrication and properties

The functionalized multi‐walled carbon nanotubes (f‐MWCNTs) were obtained by Friedel–Crafts acylation, which introduced aromatic amine groups onto the sidewall. And the grafted yield was adjusted by controlling the concentration of the catalyst. The composite solutions containing f‐MWCNTs and polyacrylonitrile (PAN) were then prepared by in‐situ or ex‐situ solution polymerization. The resulting solutions were electrospun into composite nanofibers. In the in‐situ polymerization, morphological observation revealed that f‐MWCNTs was uniformly dispersed along the axes of the nanofibers and increased interfacial adhesion between f‐MWCNTs and PAN. Furthermore, two kinds of f‐MWCNTs/PAN composite nanofibers had a higher degree of crystallization and a larger crystal size than PAN nanofibers had, so the specific tensile strengths and modulus of the composite nanofibers were enhanced. And the thermal stability of f‐MWCNTs/PAN from in‐situ method was higher than that of ex‐situ system. When the f‐MWCNTs content was less than 1 wt%, the specific tensile strengths and modulus of nanofibers were enhanced with increase in the amounts of f‐MWCNTs, and f‐MWCNTs/PAN of in‐situ system provided better mechanical properties than that of ex‐situ system. Copyright © 2010 John Wiley & Sons, Ltd.