Co‐electrospinning of core–shell polymer nanofibers (see Figure) is introduced. This process can be used for manufacturing of coaxial nanofibers made of pairs of different materials. Non‐spinnable materials can be forced into 1D arrangements by co‐electrospinning using a spinnable shell polymer. The method results in a novel two‐stage approach for fabrication of nanotubes instead of the previously used three‐stage process.
This paper describes an electrostatic field-assisted assembly technique combined with an electrospinning process used to position and align individual nanofibres (NFs) on a tapered and grounded wheel-like bobbin. The bobbin is able to wind a continuously as-spun nanofibre at its tip-like edge. The alignment approach has resulted in polyethylene oxide-based NFs with diameters ranging from 100-300 nm and lengths of up to hundreds of microns. The results demonstrate the effectiveness of this new approach for assembling NFs in parallel arrays while being able to control the average separation between the fibres.
The electrospinning process was used successfully to fabricate nanofibers of poly(ethylene oxide) (PEO)
in which multiwalled carbon nanotubes (MWCNT) are embedded. Initial dispersion of MWCNTs in water
was achieved using amphiphiles, either as small molecules (sodium dodecyl sulfate, SDS) or as a high
molecular weight, highly branched polymer (Gum Arabic). These dispersions provided separation of the
MWCNTs and their individual incorporation into the PEO nanofibers by subsequent electrospinning. The
focus of this work is on the development of axial orientations in these multicomponent nanofibers. A
theoretical model is presented for the behavior of rodlike particles representing CNTs in electrospinning.
Initially the rods are randomly oriented, but due to the sinklike flow in a wedge they are gradually oriented
mainly along the stream lines, so that straight CNTs are almost oriented upon entering the electrospun
jet. The degrees of orientation of polymer, surfactant, and MWCNT were studied using X-ray diffraction
and transmission electron microscopy. Oriented ropes of the nanofibers were fabricated in a converging
electric field by a rotating disk with a tapered edge. A high degree of alignment of PEO crystals was found
in electrospun nanofibers containing only PEO, as well as PEO/SDS. The latter also exhibited a high
degree of alignment of the SDS layers. The axial orientation of PEO and SDS is significantly reduced in
MWCNT-containing nanofibers. Transmission electron microscopy (TEM) images indicated that the
MWCNTs were embedded in the nanofibers as individual elements, mostly aligned along the fiber axis.
Nevertheless, there are also many cases in which the nanotubes appear twisted, bent, or with other
irregularities. Comparison of cryo-TEM images of vitrified MWCNT dispersions with TEM images of the
raw nanotubes indicated that sonication during the dispersion process may be responsible for the
irregularities observed in some of the nanotubes.
Polymer materials of reduced size and dimensionality, such as thin films, polymer nanofibres and nanotubes, exhibit exceptional mechanical properties compared with those of their macroscopic counterparts. We discuss here the abrupt increase in Young's modulus in polymer nanofibres. Using scaling estimation we show that this effect occurs when, in the amorphous (non-crystalline) part of the nanofibres, the transversal size of regions consisting of orientation-correlated macromolecules is comparable to the nanofibre diameter, thereby resulting in confinement of the supramolecular structure. We suggest that in polymer nanofibres the resulting supramolecular microstructure plays a more dominant role in the deformation process than previously thought, challenging the commonly held view that surface effects are most significant. The concept we develop also provides a way to interpret the observed--but not yet understood--temperature dependence of Young's modulus in nanofibres of different diameters.
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