The incorporation of organically modified Lucentite nanoclay dramatically modifies the structure and morphology of the PVDF electrospun fibers. In a molecular level, the nanoclay preferentially stabilizes the all-trans conformation of the polymer chain, promoting an
Polyvinylidene fluoride (PVDF) is a widespread, low-cost engineering material that combines remarkable levels of chemical inertness, thermal resistance, mechanical strength, and flexibility [
Among the various approaches followed for the preparation of the nanofibrous membranes, electrospinning presents distinct advantages due to its versatility and compatibility with standard industrial processing that allows the continuous production of long nanofibers [
A large gallery of nano- and submicron particles has been incorporated into the initial polymer solution to give composite electrospun fibers with superior performance and desirable functionalities [
In this work, we demonstrate that the incorporation of organically modified Lucentite nanoclay into PVDF solutions dramatically modifies the morphology of the electrospun membranes in a multiple fashion. First, while the fibers of neat polymer suffer a dense population of bulky beads, the introduction of minor amounts of nanoclay eliminates this effect. Second, the alignment of the clay layers parallel to the fiber axis gives rise to a highly laminated superstructure. Third, the clay stabilizes the formation of the fiber-like
PVDF with
Lucentite was first dispersed in DMF by vigorous stirring for 24 hours followed by ultrasonication for an hour. The polymer powder was added gradually to the clay suspension that was kept at 50°C for the first few hours and then at room temperature for 1 day. The suspensions were electrospun onto aluminum foil at an electric field strength of 1.7 kV cm−1 and a solution flow speed of 20
Fiber images were taken by scanning Electron Microscopy (SEM), using a LEO 1550 equipped with a Schottky field emission gun (10 kV) and a Robinson backscatter detector. The specimens were subject to cold coating to minimize the charging effect. Transmission electron microscopy (TEM) images of the single fibers were collected by a JEOL JEM1200ex (the samples were prepared by electrospinning directly onto the copper-carbon TEM grids).
The electrospun membrane samples (length = 2.54 cm, width = 6 mm, and thickness = 90
Fourier transform Infra-red (FTIR) spectra of the samples were recorded using a Nicolet iZ10 spectrophotometer (Thermo Scientific, USA). Samples were scanned 128 times at a resolution 2 cm−1. X-ray diffraction (XRD) spectra were recorded at room temperature using a Scintag Inc.
An Anton-Paar Physica MCR 301 rheometer equipped with a couette geometry (diameter 27 mm) was used to measure the shear viscosity of solutions. The measurements were carried out at room temperature.
The XRD pattern of the neat PVDF fibers (Figure
XRD patterns of electrospun nanofibers: (a) PVDF and (b) PVDF/1.5 wt% Lucentite.
FTIR spectra of electrospun nanofibers: (a) PVDF and (b) PVDF/1.5 wt% Lucentite.
The polymorphism of PVDF crystals stems from the symmetry and flexibility of the chains and the size proximity between the fluorine and the hydrogen atoms. Among the five crystalline phases of PVDF (
Accordingly, substantial efforts have been devoted in order to promote the
It is worth mentioning that uniaxial deformation (application of a mechanical force) and pooling (application of an electrical field), both present in electrospinning, can effectively promote an
A comparison between the SEM images of the neat PVDF and PVDF/nanoclay electrospun fibers (Figure
SEM images of electrospun nanofibers derived from DMF solutions containing PVDF ((a), (c), and (e)) or PVDF and Lucentite ((b), (d)) at the concentrations indicated.
The superior morphological characteristics of the PVDF/Lucentite electrospun fiber can be attributed to the higher viscosity and conductivity values of the polymer/clay dispersions in DMF compared to the binary polymer/DMF solutions. In Figure
Shear viscosity of 15 wt% PVDF solution (open symbols) and 15 wt% PVDF/10 wt% Lucentite dispersion (close symbols) in DMF at room temperature.
At the same time, addition of clay nanoparticles increases the density of the charge carriers in the system. We note that the cation exchange capacity, a measure of the charge density that accounts for the electric conductivity of Lucentite clays, has been estimated to be close to 0.65 mequiv/g [
The representative TEM images of the hybrid fiber (Figure
TEM micrographs of electrospun nanofibers: (a) PVDF and (b) PVDF/1.5 wt% Lucentite.
Low angle XRD patterns of (a) Lucentite clay and (b) PVDF/1.5 wt% Lucentite.
The nanocomposite fibers exhibit simultaneous enhancements in both mechanical strength and toughness (defined by the area under the stress-strain curves) as shown in Figure
Stress-strain curves for electrospun membranes: PVDF (open circles), PVDF/1 wt% Lucentite (half-filled circles), and PVDF/10 wt% Lucentite (filled circles).
In general, incorporation of rigid particles within a polymer matrix improves the stiffness of the materials at the expense of their toughness. Substantial improvements in toughness have been reported for a limited number of clay based nanocomposite systems including PVDF [
In this report, we present a unique case of nanoclay directed structure and morphology in PVDF electrospun fibers. The introduction of organically modified Lucentite to the jet solution increases the viscosity and the density of the charged carriers, facilitating the formation of uniform and bead-free fibers. At the same time, the introduction of nanoclays promotes the crystallization of the highly desirable
Schematic representation of the structure and morphology of neat PVDF (lower scheme) and PVDF/clay (upper scheme) electrospun membranes.
The authors declare that there is no conflict of interests regarding the publication of this paper.