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.     

Phase transition of syndiotactic polypropylene in electrospun nanofibers during progressive heating

By means of high-temperature electrospinning process, syndiotactic polypropylene (sPP) nanofibers with an average diameter of 127 nm were obtained using a rotating disc as a collector. The aligned fibers were subjected to progressive heating for fiber melting. During heating, structural evolution of the sPP nanofibers was investigated in situ by means of two-dimensional wide-angle and small-angle X-ray scattering with synchrotron radiation sources. It was found that the as-spun fibers consist of the antichiral form I (9 %), mesophase (31 %), and amorphous phase (60 %), in the absence of isochiral form II. Upon heating, the mesophase started to melt and completely disappeared at 90 °C. The melting of the mesophase directly produced amorphous chains at 35–60 °C, and brought up the isochiral form II at low temperatures (60–70 °C), as well as the antichiral form I at high temperatures (70–110 °C). These events were in accordance with the DSC heating curve, which exhibited a small endotherm centered at 52 °C for the mesophase melting, followed by a shallow and broad exotherm associated with two phase-transition events, i.e., the crystal reorganization and the crystallization of supercooled liquid. The former is likely due to the solid–solid transition of meso→II phase as suggested by Lotz et al. (Macromolecules 31:9253, 1998), and the latter is relevant with crystallization of amorphous chains to develop the thermodynamic stable form I phase at high temperatures.     

Polyaniline-nylon-6 electrospun nanofibers for headspace adsorptive microextraction

A headspace adsorptive microextraction technique was developed using a novel polyaniline-nylon-6 (PANI-N6) nanofiber sheet, fabricated by electrospinning. The homogeneity and the porosity of the prepared PANI-N6 sheet were studied using the scanning electron microscopy (SEM) and nanofibers diameters were found to be around 200 nm. The novel nanofiber sheet was examined as an extracting medium to isolate some selected chlorobenzenes (CBs), as model compounds, from aquatic media. The extracted analytes were desorbed using μL-amounts of solvent and eventually an aliquot of extractant was injected into gas chromatography–mass spectrometry (GC–MS). Various parameters affecting the extraction and desorption processes were optimized. The developed method proved to be convenient and offers sufficient sensitivity and a good reproducibility. Limits of detection achieved for CBs with the developed analytical procedure ranged from 19 to 33 ng L^?1, while limits of quantification were from 50 to 60 ng L^?1. The relative standard deviations (RSD) at a concentration level of 0.1 ng mL^?1 and 1 ng mL^?1 were in the range of 8–14% and 5–11% (n = 3), respectively. The calibration curves of analytes were investigated in the range of 50–1000 ng L^?1 and R² between 0.9739 and 0.9932 were obtained. The developed method was successfully applied to the extraction of selected CBs from tap and river water samples. The relative recovery (RR) percentage obtained for the spiked real water samples at 0.1 ng mL^?1 and 1 ng mL^?1 level were 93–103% and 95–104%, respectively. The whole procedure showed to be conveniently applicable and quite easy to handle.     

The tensile properties of electrospun nylon 6 single nanofibers

In this article, we have aimed to mechanically characterize the nylon 6 single nanofiber and nanofiber mats. We have started by providing a critical review of the developed mechanical characterization testing methods of single nanofiber. It has been found that the tensile test method provides information about the mechanical properties of the nanofiber such as tensile strength, elastic modulus and strain at break. We have carried out a tensile test for nanofiber/composite MWCNTs nanofiber mats to further characterize the effect of the MWCNTs filling fiber architecture. In addition, we have designed and implemented a novel simple laboratory set‐up for performing tensile test of single nanofibers. As a result, we have established the stress–strain curve for single nylon 6 nanofibers allowing us to define the tensile strength, axial tensile modulus and ultimate strain of this nanofiber. The compared values of the tensile strength, axial modulus and ultimate strain for nylon 6 nanofiber with those of conventional nylon 6 microfiber have indicated that some of the nylon 6 nanofiber molecule chains have not been oriented well along the nanofiber axis during electrospinning and through the alignment mechanism. Finally, we have explained how we can improve the mechanical properties of nylon 6 nanofibers and discussed how to overcome the tensile testing challenges of single nanofibers. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1719–1731, 2010     

Solvent degradation of nylon-6 and its effect on fiber morphology of electrospun mats

In this work, we studied solvent-induced polymer degradation and its effect on the morphology of electrospun fibers. Nylon-6 in formic acid solvent was allowed to degrade by simply allowing it to stand for a long time, and nanofibrous mats were fabricated by taking a fraction of this solution at different time intervals via electrospinning under the same electrospinning conditions. FE-SEM images of the mats indicate that the nanofiber diameter gradually decreased with the standing time of solution, and large numbers of true nano fibers (<50 nm in diameter) were obtained. MALDI-TOF analysis revealed that the formation of low-molecular weight ions was caused by solvent degradation. FT-IR, DSC, XRD, and TGA analyses of electrospun mats showed that some physical properties, such as bond strength, crystallinity, and thermal stability also depended on solvent degradation. The obtained sub-nanofibrous mat has potential applications in different bioengineering fields.     

Porosity control in nylon-6/nylon-6, 6 membranes

The possibility of porosity control in N6/N66 membranes prepared from N6/N66 blend solutions of calcium chloride – methanol mixture was investigated. For N6/N66 blend solutions at 10 °C and 20 °C, the four-phase structure of solid, gel 1, gel 2 and solution was observed clearly in the pore formation process after adding water on a solution surface. In the boundary part between gel 1 and gel 2, the phase separation of N6 and N66 was predicted. The macroscopic pores of blend membranes prepared at 10°C and 20 °C were almost spherical.     

Thermal shrinkage of electrospun PVP nanofibers

This paper outlines the shrinkage of electrospun polyvinylpyrrolidone (PVP) fiber mats during thermal treatment. The thermal behavior and phase changes within the fibers were investigated by DSC and TGA/DTA. Five precursors with different PVP loading in ethanol were electrospun. The mats shrinkage as function of temperature was measured in the RT–200 °C range. Shrinkage rate drastically increased above the polymer glass transition point, T_g (150–180 °C), due to increase in polymer chain mobility. Mats shrinkage at 200 °C as function of PVP concentration showed a minimum at ∼10%wt. Below 10% PVP the mats morphology is non‐uniform, consisting of beads and fibers. Above 10% PVP, only flat and uniform fibers were observed. This paper outlines the dominant mechanism governing the mats shrinkage during heating. In addition, the effect of PVP concentration on the expansion of fibers diameter was investigated and found to be consistent with the linear shrinkage observing a minimum at ∼10% PVP. The effect of applied voltage on mat shrinkage was investigated, and showed a minimum at 12 kV. Understanding the interplay between fibers morphology and thermal shrinkage allows precursor composition and system optimization needed for minimizing shrinkage negative effects on the structure and properties of electrospun fiber mats. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 248–254     

Polymorphism of nylon-6 in nylon-6/nanoclay/functionalized polyolefin blends

The crystallization behavior of Nylon-6 and the interaction in Nylon-6/nanoclay/functionalized polyolefin blends were investigated by X-ray diffraction and Fourier transform infrared spectroscopy. For samples without any thermal history, the interaction between Nylon-6 and nanoclay or the interaction between Nylon-6 and functionalized polyolefin favors the formation of γ form crystal. In contrast, the presence of both nanoclay and functionalized polyolefin together in Nylon-6 was found to have an antagonistic effect on each other's ability to promote the formation of γ form crystal. This was attributed to the complex interactions between the constituents. The crystallization behavior of Nylon-6 in Nylon-6/nanoclay/functionalized polyolefin blends is clearly affected by the cointeraction of these effects. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1494–1502, 2007     

Perceived color of undoped electrospun polyacrylonitrile nanofibers

This work reveals influence of electrospinning of polyacrylonitrile–N ,N‐dimethylformamide solution of different concentrations on nanofiber web color parameters, molecular structure, and heat stability. It is found that fiber diameters depend on concentration through the power law relationship; however, the medium concentration‐based web is characterized by a green–yellow hue, representative of the chromophore color; while, the solvent‐rich and solvent‐poor solution‐based webs give rise to Stokes shifts and ultraviolet‐blue emission bands, attributed to fluorescence. The chromophore structure, present in the neat powder, undergoes changes as a result of electrospinning reflected by the enamine‐to‐ketonitrile conversion and the fraction of C?N conjugation. Blue‐shifting of the C?N conjugation is indicative of a reduction of the π‐electron system, which is coincident with the decreased color saturation value but observed only in the nanofibers prepared from the medium concentration solution. A decrease in the glass transition and an increase in cyclization temperatures also support these findings. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55 , 1278–1285     

Thermo-mechanical behavior of electrospun thermoplastic polyurethane nanofibers

Analysis of the thermo-mechanical behavior of electrospun thermoplastic polyurethane (TPU) block co-polymer nanofibers (glass transition temperature ∼−50 °C) is presented. Upon heating, nanofibers began to massively contract, at ∼70 °C, whereas TPU cast films started to expand. Radial wide-angle X-ray scattering (WAXS) profiles of the nanofibers and the films showed no diffraction peaks related to crystals, whereas their amorphous halo had an asymmetric shape, which can be approximated by two components, associated with hard and soft segments. During heating, noticeable changes in the contribution of these components were only observed in nanofibers. These changes, which were accompanied with an endothermic DSC peak, coinciding with the start of the nanofibers contraction, can be attributed to relaxation of an oriented stretched amorphous phase created during electrospinning. Such structure relaxation becomes possible when a portion of the hard segment clusters, forming an effective physical network, is destroyed upon heating.     

Functionalized electrospun nanofibers as bioseparators in microfluidic systems

Functionalized electrospun nanofibers were integrated into microfluidic channels to serve as on-chip bioseparators. Specifically, poly(vinyl alcohol) (PVA) nanofiber mats were shown to successfully serve as bioseparators for negatively charged nanoparticles. Nanofibers were electrospun onto gold microelectrodes, which were incorporated into poly(methyl methacrylate) (PMMA) microfluidic devices using UV-assisted thermal bonding. PVA nanofibers functionalized with poly(hexadimethrine bromide) (polybrene) were positively charged and successfully filtered negatively charged liposomes out of a buffer solution, while negatively charged nanofibers functionalized with Poly(methyl vinyl ether-alt-maleic anhydride) (POLY(MVE/MA)) were shown to repel the liposomes. The effect of fiber mat thickness was studied using confocal fluorescence microscopy, determining a quite broad optimal range of thicknesses for specific liposome retention, which simplifies fiber mat production with respect to retention reliability. Finally, it was demonstrated that liposomes bound to positively charged nanofibers could be selectively released using a 4-(2-Hydroxyethyl)piperazine-1-ethanesulfonic acid (HEPES)-sucrose-saline (HSS) solution of pH 9, which dramatically changes the nanofiber zeta potential and renders the positively charged nanofibers negatively charged. This is the first demonstration of functional electrospun nanofibers used to enable sample preparation procedures of isolation and concentration in lab-on-a-chip devices. This has far reaching impact on the ability to integrate functional surfaces and materials into microfluidic devices and to significantly expand their ability toward simple lab-on-a-chip devices.     

Inorganic reinforcement in PET/silica electrospun nanofibers

PET/silica nanocomposite fibers of high quality were fabricated from electrospinning by choosing appropriate surface modification of inorganic fillers, solution properties, and processing conditions. The existence of an immobilized layer around silane-modified silica particles in PET fibers was verified by Fourier transform infrared spectroscopy, the results of which confirm previous thermal analysis studies. The influence of silica particles on the crystal growth during isothermal crystallization as well as the phase structure of the crystallized nanocomposite fibers were examined using differential scanning calorimetry. The PET crystallization rate increases significantly with increasing silica content, which indicates that the silica nanoparticles act as an efficient nucleating agent to facilitate PET crystallization. Using Avrami analysis, for the first time, preferred 1-D crystal growth was confirmed for geometrically confined nanocomposite fibers. Addition of silica particles makes the crystal growth more likely to occur in a 1-D manner.     

Aligned electrospun nanofibers for ultra-thin layer chromatography

The fabrication and implementation of aligned electrospun polyacrylonitrile (PAN) nanofibers as a stationary phase for ultra-thin layer chromatography (UTLC) is described. The aligned electrospun UTLC plates (AE-UTLC) were characterized to give an optimized electrospun mat consisting of high nanofiber alignment and a mat thickness of ∼25 μm. The AE-UTLC devices were used to separate a mixture of β-blockers and steroidal compounds to illustrate the properties of AE-UTLC. The AE-UTLC plates provided shorter analysis time (∼2–2.5 times faster) with improved reproducibility (as high as 2 times) as well as an improvement in efficiency (up to100 times greater) relative to non-aligned electrospun-UTLC (E-UTLC) devices.     

Solutions of nylon-6 in antimony trichloride

Dilute solution viscosity measurements of nylon-6 in molten SbCl₃ reveal a polyelectrolyte effect that becomes more pronounced with increased molecular weight of the polymer sample. Intrinisic viscosities result in a relationship of [η] = 2.35 × 10^?6M^1.45_w for nylon-6 in SbCl₃ at 100°C, which indicates a high polymeric chain extension in molten SbCl₃ in the limit of zero concentration. Infrared (IR) and nuclear magnetic resonance (NMR) spectra indicate that a substantial fraction of the amide groups in each polymer chain remains unaffected, whereas the rest is interacted, probably, with SbCl₄-ions originating from the self-ionization of SbCl₃.     

Investigation on the Brill transition of polyamide 618

The Brill transition of even-even polyamide 618 was investigated using differential scanning calorimetry (DSC), temperature-dependent wide angle X-ray diffraction (WAXD) and Fourier transform infrared (FTIR). The X-ray diffraction results indicate that the melt-crystallized sample of polyamide 618 transforms from the triclinic unit cell to the pseudo-hexagonal phase in the range of 120–180°C. In this range, the thermograph of polyamide 618 presents a broad endothermal peak. From the FTIR spectra, it was found that during the transition process of polyamide 618, the intensity of the intra-sheet hydrogen bonds becomes weak. At the same time, the CH₂-amide bonds twist, and the all-trans conformation of methylene sequences is disordered by inserting the gauche conformation. The CH₂ segments are in a mobile state because of the enhanced stretching and twisting vibrations of the C-CO and C-N bonds. Translated from Chemical Journal of Chinese Universities, 2006, 27 (2) (in Chinese)     

Thermochemical stabilization and analysis of continuously electrospun nanofibers

Carbon nanotube (CNT)-loaded and neat polyacrylonitrile nanofibers were produced by a needleless continuous electrospinning method as carbon nanofiber precursors. The details of the stabilization, which is a crucial issue during carbon fiber production, were investigated as these nanofibers are especially sensitive to degradation. In order to determine the optimal parameters, the nanofibers were stabilized at different temperatures. The stabilized samples were analyzed by Fourier-transform infrared spectroscopic and differential scanning calorimetric (DSC) measurements and by the determination of the color changes. The chemical changes during the stabilization (the formation of the so-called ladder-polymer) can be followed by infrared spectrometry, while the conversion can be monitored by DSC. The formation of the ladder-polymer occurs according to the Gaussian distribution function, where the temperature of the stabilization is the statistical parameter, which was also determined. In the case of CNT-loaded samples, the range of stabilization temperature was wider, which provides better controllability of the process. Based on the established models, an appropriate multi-step heat-treatment program could be determined, which led to completely stabilized nanofibers, suitable for carbonization.     

Application of silicate electrospun nanofibers for cell culture

Silicate produced via the sol–gel process is a biocompatible material that has high purity and high homogeneity. In this study, we evaluated the feasibility of electrospun fibers of silicate formed into silicate nonwoven fabrics (SNF) developed via the sol–gel process as substrates for substance production using Chinese hamster ovarian cells CHO-K1, and as substrates for producing drug metabolism simulators from the human cell line HepG2. We compared the adherent and proliferation profiles of the two cell types on SNF with those profiles produced on a hydroxyapatite-pulp composite fiber sheet (HAPS). During 14 days of cultivation, a greater number of CHO-K1 and HepG2 cells continued to grow on SNF compared to those on HAPS. Per unit volume, the HepG2 cells on SNF showed higher hepatic-specific functions than those on HAPS. These results demonstrate the feasibility of SNF as a cell culture substrate for substrate production, and for producing drug metabolism simulators.     

Development of electrospun metallic hybrid nanofibers via metallization

The metallized hybrid nanofiber webs were prepared by using a combined technology of electrostatic spinning and metallization. The electrospun polyurethane (PU) nanofibers were metallized with different thicknesses of copper layer via metal vapor deposition technique. The thickness of the copper layer, which ranges from 10 to 100 nm, was monitored and controlled. The resultant metallized hybrid nanofiber webs were characterized using field emission scanning electron microscopy (FE‐SEM), wide angle X‐ray diffraction (WAXD), and thermogravimetric analysis (TGA). FE‐SEM images demonstrated that the nanoscaled copper layers are well deposited on the surface of the PU nanofibers. TGA result indicated that the thermal stability of the metallized hybrid PU nanofibers was dramatically enhanced due to the barrier effects of thin metallic copper layer. WAXD data confirmed that the crystalline copper layers were well deposited onto the PU nanofibers. Moreover, the mechanical properties of the metallized hybrid PU nanofiber webs were increased with increase in the thickness of deposited copper layer. Unlike the organic PU nanofiber webs, it was observed that the metallized hybrid PU nanofiber webs showed higher conductive properties depending on the thickness of the deposited copper layer. Copyright © 2009 John Wiley & Sons, Ltd.