Electrospun polymer nanofibers: Mechanical and thermodynamic perspectives

This article reviews and discusses some open problems concerning polymer materials of reduced sizes and dimensions. Such objects exhibit exceptional physical properties when compared with their macroscopic counterparts. More specifically, abrupt increases in polymer nanofiber elastic modulus have been observed when diameters drop below a certain value. In addition, temperature dependence of elastic modulus is highly influenced by fiber diameter. Mechanical (macroscopic) analyses have failed to provide satisfactory explanations for the mechanisms ruling such features, calling for detailed microscopic examination of the systems in question. A hypothesis bridging the current knowledge gaps is presented. The key element of this hypothesis is based on confinement of the supermolecular microstructure of polymer nanofibers and its dominant role in the deformation process. This suggestion challenges the commonly held view suggesting that surface effects are the most significant parameters impacting mechanical and thermodynamic nanofiber behaviors. The review will focus on the mechanical and thermodynamic properties of electrospun polymer nanofibers, selected as representatives of nanoscale polymer objects. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

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     

Preparation of three‐dimensional structure controllable nanofibers by electrospinning

The size of “bowl‐like” structures woven by nanofibers could be controlled by adjusting the distance from the nozzle to a modified collector and the voltage applied to the electrospinning device. More interestingly, the nanofibers in the side wall of the “bowl” could vibrate up and down with the changing of the voltage. This voltage‐induced vibration might have potential applications for bio‐mimic process and micro‐motor devices. Copyright © 2008 John Wiley & Sons, Ltd.     

Fluorescent PMMA/MEH‐PPV electrospun nanofibers: Investigation of morphology,solvent, and surfactant effect

Electrospinning is a powerful technique to produce nanofibers of tunable diameter and morphology for medicine and biotechnological applications. By doping electrospun nanofibers with inorganic and organic compounds, new functionalities can be provided for technological applications. Herein, we report a study on the morphology and optical properties of electrospun nanofibers based on the conjugated polymer poly[2‐methoxy‐5‐(2‐ethylhexyloxy)‐1,4‐phenylenevinylene] (MEH‐PPV) and poly(methylmethacrylate) (PMMA). Initially, we investigate the influence of the solvent, surfactant, and the polymer concentration on electrospinning of PMMA. After determining the best conditions, 0.1% MEH‐PPV was added to obtain fluorescent nanofibers. The optical characterizations display the successful impregnation of MEH‐PPV into the PMMA fibers without phase separation and the preservation of fluorescent property after fiber electrospinning. The obtained results show the ability of the electrospinning approach to obtain fluorescent PMMA/MEH‐PPV nanofibers with potential for optical devices applications. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1388–1394     

Fundamental study of crystallization,orientation, and electrical conductivity of electrospun PET/carbon nanotube nanofibers

The morphology, structure, and properties of polyethylene terephthalate (PET)/Carbon Nanotubes (CNT) conductive nanoweb were studied in this article. Nanocomposite nanofibers were obtained through electrospinning of PET solutions in trifluoroacetic acid (TFA)/dichloromethane (DCM) containing different concentrations and types of CNTs. Electrical conductivity measurements on nanofiber mats showed an electrical percolation threshold around 2 wt % multi‐wall carbon nanotubes (MWCNT). The morphological analysis results showed smoother nanofibers with less bead structures development when using a rotating drum collector especially at high concentrations of CNTs. From crystallographic measurements, a higher degree of crystallinity was observed with increasing CNT concentrations above electrical percolation. Spectroscopy results showed that both PET and CNT orientation increased with the level of alignment of the nanofibers when the nanotube concentration was below the electrical percolation threshold; while the orientation factor was reduced for aligned nanofibers with higher content in CNT. Considerable enhancement in mechanical properties, especially tensile modulus, was found in aligned nanofibers; at least six times higher than the modulus of random nanofibers at concentrations below percolation. The effect of alignment on the mechanical properties was less important at higher concentrations of CNTs, above the percolation threshold. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2052–2064, 2010     

Mass preparation of nanofibers by high pressure air‐jet split electrospinning: Effect of electric field

A novel electrospinning method using airflow, namely high pressure air‐jet split electrospinning, was proposed to fabricate polymer nanofibers with ultrahigh production rate. 7 wt % polyacrylonitrile spinning solution with a 0.157 Pa s viscosity was divided into micron size droplets by the filter screen in the front of the nozzle, and then these droplets were divided and split through high pressure airflow, which were drafted into nanofibers directly in the electric field and airflow field. In this study, the electric field distributions with different positive electrodes were simulated and their effect on fiber formation was investigated. The results show that electric field distribution and its intensity depended on electrodes area, a broader electric field distribution with a stronger intensity would bring about a larger cone angle of spraying jet region, at the same time, the contrast in the spray region enhanced. When the whole nozzle was charged, thinner fibers with about 170 nm could be prepared and the fiber production was 75.6 g/h. Compared with the conventional needle electrospinning, the throughput of nanofibers could be improved by thousands of times based on this novel electrospinning method. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 993–1001     

Bioconjugation of alkaline phosphatase to mechanically processed, aqueous suspendible electrospun polymer nanofibers for use in chemiluminescent detection assays

Aqueous suspendible polymer nanostructures were prepared by simple microtome processing of electrospun nylon 6 nanofibers and were used to immobilize calf intestinal alkaline phosphatase (ALP) by either covalent or noncovalent bioconjugation chemistries. It was found that noncovalent immobilization of ALP to the mechanically cut nanofibers (mean length approximately 4 microm; mean diameter approximately 80 nm) using a multi-stacked, layer-by-layer (LBL) approach with the cationic polymer Sapphire II resulted in the highest enzyme loading (48.1 +/- 0.4 microg . mg(-1) nanofiber) when compared to other covalent immobilization methods based on glutaraldehyde crosslinking. The biofunctionalized nanofibers were also characterized for their chemiluminescent activity with the dioxetane substrate, CSPD. The results indicate that the kinetic parameters, K(m) and V(max), for the catalytic activity of the nanostructure-bound ALP enzyme were influenced by the particular types of immobilization methods employed. In terms of the overall catalytic performance of the various immobilized ALP systems, a single-stacked LBL assembly approach resulted in the highest level of enzymatic activity per unit mass of nanofiber support. To the best of our knowledge, this study represents the first report examining the preparation of mechanically shortened, aqueous dispersed electrospun polymer nanofibers for potential application as enzyme scaffolds in chemiluminescent-based assay systems.     

Dye,fluorophores and pigment coloration of nanofibers produced by electrospinning

Polymeric nanofibers produced by the electrospinning technique are widely used in industrial scale production. Nanofibers chiefly find applications in filtration media and active–barrier surfaces for medical, biological, and military applications. In such applications, the quantity and the uniformity of the nanofibers distribution play a leading role in the product characteristics. For this reason, there is considerable interest regarding the nanofibers recovering quantification and simplification of the qualitative analysis. With the aim to improve and simplify the nanofibers relevability, a coloration approach for nanofibers has been designed and tested. The coloration has been carried out by organic dyes, pigments, and organic fluorophores and the consequent nanofibers' color has been analyzed by optical analysis, colorimetry, and spectroscopy. The coloration obtained by different dyes has been compared and their effect on the nanofibers relevability has been investigated. Moreover, the leading role of the light scattering phenomenon on the nanofiber coloration efficacy has been investigated by comparing the coloration response of nanofibers and film samples on equal terms of dye content. The study has been carried out using polyamide‐6 (PA6) as the testing polymer but the recovering quantification, the coloration approach, and the interaction between light and nanofibers can be extended to all the electrospinnable polymers. Copyright © 2008 John Wiley & Sons, Ltd.     

Electrospun fibers from poly(methyl methacrylate)/vapor grown carbon nanofibers

Electrospinning has been used to obtain poly(methyl methacrylate) (PMMA) microfibers and nanofibers and PMMA/vapor grown carbon nanofibers (VGCNFs or CNFs) composite fibers with micrometer and nanometer size diameters. Thermogravimetric analysis (TGA) indicated that addition of CNFs caused a decrease in the thermal stability of the composite fibers. Scanning electron microscopy (SEM) was used to confirm the micro‐ and nano‐ nature of the fibers and transmission electron microscopy (TEM) was utilized to confirm the presence of CNFs embedded within the polymer matrix and along the surface. Copyright © 2006 John Wiley & Sons, Ltd.     

Electrospun aligned nanofibers: A review

Electrospinning (e-spinning) is famous for the construction and production of ultrafine and continuous micro-/nanofibers. Then, the alignment of electrospun (e-spun) nanofibers becomes one of the most valuable research topics. Because aligned fibers have more advantages over random fibers, such as better mechanical properties, faster charge transport, more regular spatial structure, etc. This review summarizes various electrospinning techniques of fabricating aligned e-spun nanofibers, such as early conventional methods, near-field e-spinning, and three-dimensional (3D) printing e-spinning. Among them, four auxiliary preparation methods (e.g., auxiliary solid template, auxiliary liquid, auxiliary electromagnetic field and auxiliary airflow), two collection modes (static and dynamic collection), and the controllability of near-field e-spinning and 3D printing e-spinning are highlighted. The representative applications depending on aligned nanofibers are classified and briefly introduced, emphasizing in the fields of 1D applications (e.g., field-effect transistor, nanochannel and guidance carrier), 2D applications (e.g., platform for gas detection, filter, and electrode materials storage), and 3D applications (e.g., bioengineering, supercapacitor, and nanogenerator). At last, the challenges and prospects are addressed.     

Diameter control of electrospun polyacrylonitrile/iron acetylacetonate ultrafine nanofibers

Electrospinning is the process of producing ultrafine fibers by overcoming the surface tension of a polymer solution using high voltage. In this work, the effects of both solution properties (viscosity, conductivity, and surface tension) and operational conditions (voltage, feed rate, and spinneret‐collector distance), on the structure of electrospun polyacrylonitrile nanofibers, were systematically investigated. Iron acetylacetonate was added to the electrospinning solution to control fiber diameter by selectively adjusting solution properties. It was found that, with increased salt concentration, the fiber diameter increases and then passes through a maximum due to changes in solution viscosity, conductivity, and surface tension. In addition, the fiber diameter increases with increase in voltage, feed rate, and spinneret‐collector distance. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1611–1618, 2008     

Core–shell structure PEO/CS nanofibers based on electric field induced phase separation via electrospinning and its application

Core–shell structured PEO‐chitosan nanofibers have been produced from electric field inducing phase separation. Chitosan, a positive charged polymer, was dissolved in 50 wt % aqueous acetic acid and the amino group on polycation would protonize, which would endow chitosan electrical properties. Chitosan molecules would move along the direction of the electric field under the electrostatic force and formed the shell layer of nanofibers. Preparation process of core – shell structure is quite simple and efficient without any post‐treatment. The core–shell structure and existence of chitosan on the shell layer were confirmed before and after post‐treatment by TEM and further supported by SEM, FTIR, XRD, DSC, and XPS studies. Blending ratio of PEO and chitosan, molecular weight of chitosan for the mobility of chitosan are thought to be the key influence factors on formation of core–shell structure. Drug release studies show that the prepared core–shell structure nanofibers has a potential application in the biomedical fields involving drug delivery. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2298–2311     

Electrospun poly(aniline‐co‐ethyl 3‐aminobenzoate)/poly(lactic acid) nanofibers and their potential in biomedical applications

Poly(aniline‐co‐ethyl 3‐aminobenzoate) (3EABPANI) copolymer was blended with poly(lactic acid) (PLA) and co‐electrospun into nanofibers to investigate its potential in biomedical applications. The relationship between electrospinning parameters and fiber diameter has been investigated. The mechanical and electrical properties of electrospun 3EABPANI‐PLA nanofibers were also evaluated. To assess cell morphology and biocompatibility, nanofibrous mats of pure PLA and 3EABPANI‐PLA were deposited on glass substrates and the proliferation of COS‐1 fibroblast cells on the nanofibrous polymer surfaces determined. The nanofibrous 3EABPANI‐PLA blends were easily fabricated by electrospinning and gave enhanced mammalian cell growth, antioxidant and antimicrobial capabilities, and electrical conductivity. These results suggest that 3EABPANI‐PLA nanofibrous blends might provide a novel bioactive conductive material for biomedical applications. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011.     

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.     

Electrospun nanofibers for drug delivery applications: Methods and mechanism

Electrospinning procedures such as blend electrospinning, coaxial electrospinning, and emulsion electrospinning have been used for the fabrication of electrospun nanofibers (ENFs) for biomedical applications. These ENFs are attracted great interest especially in drug delivery applications due to their small size, high surface area-to-volume, and porosity. The aim of this review is to focus on the controlled release mechanism among the different electrospinning methods, and the selectivity of hydrophilic, water-soluble polymers as a carrier for drug. The mechanism for the drug delivery depends mainly on the method of drug loading, polymeric interactions, and the nature of polymer swelling, erosion, or degradation. This review compressed on the literature survey about the fabrication of nanofibers by different electrospinning methods, factors affecting the nanofiber morphologies, selectivity of polymeric blends for successful controlled release behavior, and the mechanism involved in the drug release steps.     

Infrared spectroscopy studies of structure and orientation in clay-reinforced polyamide-6 nanocomposites

Infrared (IR) spectroscopy was used to study the surface (by means of external reflection) and the bulk (by means of transmission measurements on microtomed slices) of specimens obtained by injection moulding of a commercial polyamide-6 clay-based nanocomposite material (NCH = nylon clay hybrid) at different mould temperatures and with different part geometries. Comparisons were made with equivalent non-reinforced polymer (PA-6). For the PA-6, the mould temperature influences the crystalline structure, with the γ phase predominating at 50°C and the α phase at 80°C. However, in the NCH material the ( phase is favoured, even at 80°C. In all cases the crystallinity increases on going from the surface to the core. The polymer chains are oriented in the flow direction, and the orientation is higher for parts with more elongated shapes. It does not vary greatly across the part thickness, except for a thin surface layer, where it is significantly higher. Both at the surface and in the bulk, the crystalline phase orientation is higher for the NCH than for the PA-6.     

Morphology and structure of nylon‐2,14 single crystals from dilute solution

A perfect single crystal of nylon‐2,14 was prepared from 0.02% (w/v) 1,4‐butanediol solution by a “self‐seeding” technique and isothermal crystallization at 120 and 145 °C. The morphology and structure features were examined by transmission electron microscopy with both image and diffraction modes, atomic force microscopy, and wide‐angle X‐ray diffraction (WAXD). The nylon‐2,14 single crystal grown from 1,4‐butanediol at 145 °C inhabited a lathlike shape with a lamellar thickness of about 9 nm. Electron diffraction and WAXD data indicated that nylon‐2,14 crystallized in a triclinic system with lattice dimensions a = 0.49 nm, b = 0.51 nm, c = 2.23 nm, α = 60.4°, β = 77°, and γ = 59°. The crystal structure is different from that of nylon‐6,6 but similar to that of other members of nylon‐2Y. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1913–1918, 2002     

Electrospun carbon nanofibers for lithium metal anodes: Progress and perspectives

Li metal anodes(LMAs) has attracted extensive research interest because of its extremely high theoretical capacity(3860 m Ah/g) at low redox potential(-3.04 V vs. standard hydrogen electrode). However, the extremely high chemical reactivity and the intrinsic “hostless” nature of LMAs bring about serious dendritic growth and dramatic volume change during the plating/strapping process, thus resulting in poor Coulombic efficiency, short lifespan, and severe safety concerns. Of various strategies, t...     

Structures for monodisperse oligoamides. A novel structure for unfolded three-amide nylon 6 and relationship with a three-amide nylon 6 6

Three-amide oligomers of nylon 6 and nylon 6 6 have been investigated using electron microscopy (imaging and diffraction), X-ray diffraction, and computational modeling. A new crystal structure has been discovered for the three-amide oligomer of nylon 6. This material crystallizes from chloroform/dodecane solutions into an unfolded crystal form that has progressively sheared hydrogen bonding in two directions between polar (unidirectional) chains. This structure is quite different from the usual room temperature α-phase structure of chain-folded nylon 6 crystals, in which alternatingly sheared hydrogen bonding occurs between chains of opposite polarity in only one direction. The occurrence of this new structure illustrates the extent to which progressively sheared hydrogen bonding is preferred over alternatingly sheared hydrogen bonding. Indeed, the progressive hydrogen bonding scheme occurs in the three-amide nylon 6 material even though it requires a disruption to the lowest potential energy all-trans conformation of the chain backbone, and requires all the chains in each hydrogen-bonded layer to be aligned in the same direction. We believe the presence of chain folding, which necessarily incorporates adjacent chains of opposite polarity into the crystal structure, prevents the formation of this new crystal structure in the nylon 6 polymer. In contrast, the three-amide nylon 6 6 crystal structure is analogous to the polymeric nylon 6 6 α-phase structure, found in both fibers and chain-folded crystals, and consists of progressive hydrogen-bonded sheets which stack with a progressive shear. In both structures, the molecules (≈ 3 nm in length) form smectic C-like layers with well-orchestrated stacking of 2.2 nm to form a three-dimensional crystal. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2849–2863, 1998     

Polymer entanglement loss in extensional flow: Evidence from electrospun short nanofibers

High strain rate extensional flow of a semidilute polymer solution can result in fragmentation caused by polymer entanglement loss, evidenced by appearance of short nanofibers during electrospinning. The typically desired outcome of electrospinning is long continuous fibers or beads, but, under certain material and process conditions, short nanofibers can be obtained, a morphology that has scarcely been studied. Here we study the conditions that lead to the creation of short nanofibers, and find a distinct parametric space in which they are likely to appear, requiring a combination of low entanglement of the polymer chains and high strain rate of the electrospinning jet. Measurements of the length and diameter of short nanofibers, electrospun from PMMA dissolved in a blend of CHCl₃ and DMF, confirm the theoretical prediction that the fragmentation of the jet into short fibers is brought about by elastic stretching and loss of entanglement of the polymer network. The ability to tune nanofiber length, diameter and nanostructure, by modifying variables such as the molar mass, concentration, solvent quality, electric field intensity, and flow rate, can be exploited for improving their mechanical and thermodynamic properties, leading to novel applications in engineering and life sciences. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1377–1391