Coaxial Electrospinning with Triton X‐100 Solutions as Sheath Fluids for Preparing PAN Nanofibers

Coaxial electrospinning using surfactants as sheath fluid for preparing high‐quality polymer nanofibers is studied. PAN nanofibers are fabricated using this process with Triton X‐100 solutions in DMF. FESEM demonstrates that the Triton X‐100 solution has a significant influence on the quality of the nanofibers. The nanofiber diameters can be controlled by adjusting the concentration of Triton X‐100 in the sheath fluids with a scaling law D = 640 C^?0.32. The mechanism of the influence of Triton X‐100 solutions on the formation of PAN fibers is discussed and it is demonstrated that coaxial electrospinning with surfactant solution is a facile method for achieving high‐quality polymer nanofibers.

     

A Novel Approach to Prepare Uniaxially Aligned Nanofibers and Longitudinally Aligned Seamless Tubes Through Electrospinning

A novel method to produce uniaxially aligned nanofibers is described, in which a pair of parallel auxiliary electrodes at a positive potential is placed between the needle and the collector electrodes. Charged nanofibers ejecting from the polymer solution are pre‐aligned by the electrostatic repulsion originating from the auxiliary electrodes and deposited on the collector electrodes, forming a narrow mat with the fiber segments strongly curved. By adjusting the conductivity and shape profile of the collector, the curved segments can be straightened longitudinally. A seamless tube composed of longitudinally aligned nanofibers can be obtained. Such seamless tubes may be useful as biomaterials in tissue engineering.

     

Polyacrylonitrile and Carbon Nanofibers with Controllable Nanoporous Structures by Electrospinning

In this work, polyacrylonitrile (PAN) and carbon nanofibers with controllable nanoporous structures were successfully prepared via electrospinning technique. For the preparation of porous PAN nanofibers, two kinds of polymers of PAN and polyvinylpyrrolidone (PVP) were used as electrospun precursor materials, and then the bicomponent nanofibers of PAN and PVP were extracted with water to remove the PVP in the composite polymer nanofibers. By altering the ratio of PAN/PVP in the precursor, the pore size and pore distribution of porous PAN nanofibers could be easily controlled. By using the porous PAN nanofibers as structures directing template and through heat treatment, carbon nanofibers with nanoporous structures were obtained. The porous nanofibers were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT‐IR), differential thermal analyses (DTA), Brunauer–Emmett–Teller (BET) nitrogen adsorption, X‐ray diffraction (XRD), and Raman spectra.

     

Molecular Orientation and Crystalline Structure of Aligned Electrospun Nylon‐6 Nanofibers: Effect of Gap Size

We study the effect of the gap size on the molecular orientation and crystalline structure of uniaxially well‐aligned nylon‐6 nanofibers produced in the gap between negatively charged metal plates. The relative intensities of several absorbance bands are found to be different in the parallel‐ and perpendicularly polarized FTIR spectra. X‐ray analysis indicates that the metastable γ‐form is predominant in as‐spun nylon‐6 nanofibers, and is transformed into the thermodynamically stable α‐form by increasing the gap size. The polymer chains are thought to be oriented perpendicular to the fiber direction, and the molecular orientation to the fiber axis is enhanced on increasing the gap size.

     

Thermo‐Physical and Impact Properties of Epoxy Containing Epoxidized Linseed Oil, 1

Summary: Biobased neat epoxy materials containing epoxidized linseed oil (ELO) were processed with an anhydride curing agent. A defined amount of the diglycidyl ether of bisphenol F (DGEBF) was replaced by ELO. The selection of the DGEBF, ELO, and an anhydride curing agent resulted in an excellent combination, to provide a new biobased epoxy material showing high elastic modulus, high glass transition temperature, and high heat distortion temperature (HDT) with larger amounts of ELO. The Izod impact strength was almost constant while changing the amount of ELO. This is a promising result for future industrial applications in different engineering industries.

The effect of changing ELO concentration of the anhydride‐cured neat epoxy on the storage modulus.     

Quick Polymerization from Electrospun Macroinitiators for Making Thermoresponsive Nanofibers

Thermoresponsive nanofibers by very fast grafting of N,N‐isopropylacrylamide (NIPAAm) from electrospun atom transfer radical polymerization (ATRP) macroinitiator are presented in this work. The heterogenous grafting of NIPAAm onto macroinitiator fibers could be done in few minutes, i.e., in less than 5 min. The procedure involved electrospinning of an ATRP macroinitiator and subsequent PNIPAAm grafting using “grafting from” technique. The ATRP Macroinitiator was based on a copolymer of methyl methacrylate (MMA) and 2‐hydroxyethyl methacrylate (HEMA). The growth of the PNIPAAm layer on electrospun fibers was followed by IR‐spectroscopy and SEM analysis. The temperature‐dependent‐phase transition was proven by contact angle measurements and could be shown on the same surface for many cycles.

     

Photocatalytic Deposition of Silver Nanoparticles onto Organic/Inorganic Composite Nanofibers

Summary: In this work, silver nanoparticles were embedded in electrospun organic/inorganic composite nanofibers consisting of PAN and TiO₂ through photocatalytic reduction of the silver ions in silver nitrate solutions under UV irradiation. The morphology and diameter of PAN/TiO₂ composite nanofibers could be controlled by varying the initial contents of TiO₂ in the spinning solution. From TEM images and UV‐Vis spectra, it has been confirmed that monodisperse silver nanoparticles with a diameter of ≈2 nm were deposited selectively upon the titania of the as prepared composite nanofibers. The amount of Ag nanoparticles embedded on composite nanofibers was greatly influenced by the amount of TiO₂ in composite nanofibers, reflecting the role of titania as the inorganic stabilizer and photocatalyst.

Morphology of silver nanoparticles embedded on PAN/TiO₂ composite nanofibers.     

Anisotropic Electrical Transport Properties of Aligned Carbon Nanotube/PMMA Films Obtained by Electric‐Field‐Assisted Thermal Annealing

This paper demonstrates how the electric‐field‐assisted thermal annealing of octadecylamine‐functionalized SWNT/PMMA films induces an increase in the composite transversal conductivity of several orders of magnitude and a decrease in the lateral conductivity. This difference has been rationalized in terms of the nanotube alignment into the polymer matrix along the electric field direction. This result provides an initial understanding of how electric fields can be used to control the bulk physical properties of such nanocomposites.

     

One‐Step Immobilization of Protein‐Encapsulated Core/Shell Particles onto Nanofibers

A novel method is described to functionalize nanofibers to form a nanocomposite with core/shell particles in order to control protein release. The nanocomposite is produced by electrically neutralizing negatively charged poly(lactic acid) nanofibers with positively charged poly[(lactic acid)‐co‐(glycolic acid)] particles via a one‐step electrohydrodynamic jetting process. The protein‐encapsulated core/shell particles exhibited no significant initial burst release or denaturation. The protein release profile was controlled by porosity and protein/polymer interactions. The method may be promising to engineer intelligent scaffolds that can fulfill the needs of biomimetic materials.

     

Core‐Sheath Structure in Electrospun Nanofibers from Polymer Blends

Summary: Electrospinning of polymer blends offers the potential to prepare functional nanofibers for use in a variety of applications. This work focused on control of the internal morphology of nanofibers prepared by electrospinning polymer blends to obtain core‐sheath structures. Polybutadiene/polystyrene, poly(methylmethacrylate)/polystyrene, polybutadiene/poly(methylmethacrylate), polybutadiene/polycarbonate, polyaniline/polycarbonate, and poly(methylmethacrylate)/polycarbonate blends were electrospun from polymer solutions. It was found that the formation of core‐sheath structures depends on both thermodynamic and kinetic factors. Incompatibility and large solubility parameter difference of the two polymers is helpful for good phase separation, but not sufficient for the formation of core‐sheath structures. Kinetic factors, however, play a much more important role in the development of the nanofiber morphology. During the electrospinning process, the rapid solvent evaporation requires systems with high molecular mobility for the formation of core‐sheath structures. It was found that polymer blends with lower molecular weight tend to form core‐sheath structures rather than co‐continuous structures, as a result of their higher molecular mobility. Rheological factors also affect the internal phase morphology of nanofibers. It was observed the composition with higher viscosity was always located at the center and the composition with lower viscosity located outside.

TEM image of electrospun polybutadiene/polycarbonate nanofibers at 25/75 wt.‐% ratio after staining by osmium tetroxide. The dark regions are polybutadiene and the light region is polycarbonate.     

Structural Properties and Enzymatic Degradation Behavior of PLLA and Stereocomplexed PLA Nanofibers

PLLA and stereocomplexed polylactide (sc‐PLA) nanofibers were formed by electrospinning solutions of the polymers in HFIP. A highly semi‐crystalline sc‐PLA nanofiber having only sc crystallites was confirmed by WAXD analysis. The diameters of the nanofibers of both polymers decreased slightly when they were annealed at 60 °C, which was near T_g. Enzyme degradation of both as‐spun PLLA and sc‐PLA nanofibers by proteinase K from Tritirachium album was carried out. The rate of degradation of the nanofibers can be controlled by varying annealing conditions, hence the extent of crystallinity.

     

Preparation of Ultrafine Ethylene/Propylene/Diene Terpolymer Rubber Fibers by Coaxial Electrospinning

Elastomeric EPDM fibers with diameters of 200–400 nm are prepared by coaxial electrospinning of PVP/EPDM fibers, subsequent vulcanization of the polymers and finally removal of the outer PVP layer using ethanol. The initially applied PVP layer restricts the elastic recovery of the EPDM fibers. The crosslinking density of the EPDM fibers reaches 8.44 × 10^?5 mol · cm^?3. The original morphology of EPDM is preserved after removing the PVP layer. The ultrafine EPDM fibers are expected to be useful in many fields, such as brittle plastics toughening, as well as applications in extremely high or low temperatures.

     

Formaldehyde Resins Bearing Diaminodiphenylmethane Groups: Synthesis,Characterization and Properties

Summary: Novel formaldehyde resins bearing diaminodiphenylmethane groups were synthesized by the polymerization of a mixture of diaminodiphenylmethane (DDM), cyclohexanone (CHx) and o‐cresol (o‐Cz) with formaldehyde (FA) in the presence of an acid catalyst (HCl). The resins obtained were characterized by spectral, elemental and thermal analysis and used as a hardener for epoxy resins. The curing and temperature behavior of these epoxy resin/formaldehyde systems were investigated using differential scanning calorimetry and thermogravimetry techniques. The resins had good thermal stability and the activation energies of degradation reactions had values between 70–98 kJ · mol^?1.

The curing reaction of epoxy resins with the DDM/CHx/o‐Cz/formaldehyde resins.     

Effect of Different Bicomponent Electrospinning Techniques on the Formation of Polymeric Nanosprings

The effect of different bicomponent electrospinning techniques i.e., off‐centered coaxial electrospinning and side‐by‐side electrospinning, on the formation of tight nanocoils (nanosprings) was studied. Since right balance between the longitudinal compressive forces arising from the shrinking thermoelastic components and the rigidity emerging from the stiff component and conductivity is required for nanospring formation, conductive solutions of flexible and rigid components were used for the electrospinning. Under optimum conditions, nanofiber mats with almost 100% nanospring morphology were generated using off‐centered and side‐by‐side electrospinning techniques. Mechanical properties of aligned nanomats with and without nanosprings, produced at different collecting speeds are provided.

     

Fabrication of Uniaxially Aligned Poly(propylene) Nanofibers via Handspinning

A straightforward method, which is termed novel handspinning, is reported for producing uniaxially aligned sPP nanofibers. As demonstrated by SEM analysis, the morphologies of handspun sPP nanofibers are strongly dependent upon the processing conditions such as spinning method and solvent system. Compared to the normal electrospun sPP nanofibers, the handspun sPP nanofibers show smoother morphologies. FT‐IR analysis demonstrates a significant difference in polymer chain conformation between the handspun and electrospun sPP nanofibers. Moreover, interestingly, the handspun sPP single nanofibers show higher Young's modulus and tensile strength than electrospun sPP single nanofibers.

     

Dual‐Curing of Waterborne Urethane‐Acrylate Coatings by UV and Thermal Processing

A waterbased dual‐cure urethane‐acrylate oligomer has been synthesized by polycondensation of monomers bearing hydroxyl, isocyanate and acrylate groups. To obtain a stable aqueous dispersion, carboxylate groups were grafted on the oligomer chain and the isocyanate groups were protected by a blocking agent. After water release by a brief heating, the dry films were cured either by a short UV exposure in the presence of a photoinitiator to induce the polymerization of the acrylate double bonds, or by heating up to 150 °C to release the isocyanates and promote the polycondensation by reaction with the hydroxyl groups, but mainly by a combination of UV and thermal cure. Both processes have been followed quantitatively by infrared spectroscopy to evaluate the influence of the temperature on the reaction rate and on the cure extent. The newly developed waterbased dual‐cure coatings were found to be quite resistant to accelerated weathering because of their aliphatic structure and their high crosslink density. Their light stability was substantially improved by the addition of a hydroxyphenyltriazine UV absorber and a hindered amine radical scavenger.

Thermal curing of the dual‐cure waterborne PUA formulation at a temperature of 150 °C.     

Fabrication and Characterization of Ionic Conducting Nanofibers

The electrospinning method was used to fabricate nanostructures of Nafion‐poly(vinyl alcohol) (PVA) and Nafion‐poly(ethylene oxide) (PEO). Depending on the ratio between the two polymers, nanospheres and/or nanofibers could be obtained in a reproducible manner. The Nafion‐PVA mats were found to be more conductive than the Nafion‐PEO ones, possibly because of their better mechanical properties when swollen by water. The fiber morphology was always found to be more conductive than the sphere morphology. However, all electrospun mats presented ionic conductivities slightly lower than extruded Nafion 115 or Nafion‐PVA cast films.

     

Polyurethane Foams of Improved Thermal Stability

In reactions of polyetherols prepared from hydroxymethyl derivatives of uric acid and typical oxiranes with isocyanates and water one obtains a new group of polyurethane foams containing purine rings in their structure. These polyurethanes withstand prolonged heating at temperatures as high as 200 °C. Preliminary results on the effect of the type of polyetherol and the composition used in preparation of the polyurethane foams on their properties are presented. The results of studies on thermal stability and mechanical properties of the foams before and after thermal treatment are also reported.

     

Monodisperse Silica‐Polymer Core‐Shell Microspheres via Surface Grafting and Emulsion Polymerization

We describe a flexible method for preparing monodisperse silica‐polystyrene core‐shell microspheres. In this method, silica nanoparticles grafted with 3‐(trimethoxysilyl)propyl methacrylate (MPS) were employed as seeds in an emulsion polymerization. The thickness of the shells could be changed through varying the amount of the monomer. The monodispersity and diameters of the core‐shell microspheres were found to depend on the size of the grafted silica nanoparticles and the concentration of emulsifier. In addition, we investigated the formation mechanism of the core‐shell microspheres.

Schematic outline of the experimental procedure and TEM image of the monodisperse silica/PS core‐shell microspheres; inset: grafted silica nanoparticles (scale bar = 200 nm).     

Biological,Chemical, and Electronic Applications of Nanofibers

With their high‐surface‐to‐volume ratio, nanofibers have been postulated to increase interactions between nanofibrous materials and targeted substrates, which are helpful to overcome many obstacles and enhance the efficiency in a diverse number of applications. Over the past decade, many studies have been published on the fabrication of nanofibers and their applications in various fields. In this review, novel biological, chemical, and electrical characteristics of nanofibers as well as their recent status and achievements in medicine, chemistry, and electronics are analyzed. It is found that nanofibers can induce fast regeneration of many tissues/organs in medical applications and improve the efficiency of many chemical and electronics applications.