Nanofibers and Their Characterizations

Nanofibers are structures whose length is greater than their diameter and offer diverse internal morphology possibilities. Various nanofibers composed of synthetic Synthetic or natural polymers materials, or their mixtures, are defined as fibrous networks with either random or ordered fiber arrangements depending on the application area containing active materials.

Characterization of Nanofibers

Nanofibers have numerous application areas, which are determined based on their fundamental properties. These properties are the morphology and molecular structure of the nanofibers. Morphological characteristics include the average diameter of the fiber, pores formed on the fiber surface, and porosity like. Surface wettability can also be considered within the concept of morphology. Molecular structure determines the fiber’s permeability, thermal behavior, and mechanical properties.

Morphology

The morphology of nanofibers is examined using SEM (Scanning Electron Microscopy), TEM (Transmission Electron Microscopy), AFM (Atomic Force Microscopy), and contact angle analysis.

SEM (Scanning Electron Microscopy)

SEM operates on the principle of detecting and converting into an image the interactions between an electron beam directed at the sample and the sample’s surface via detectors. Analysis is performed after the surface of insulating samples is coated with a conductive material to facilitate electron interactions. The solid sample surface is scanned by a high-energy electron beam. In this method, various signals are generated on the surface, including Auger electrons, secondary electrons, X-ray fluorescence photons, and other photons of varying energies. The use of scanning electron microscopy for solid samples arises from the variety of signals produced by the interaction of the electron beam with the solid. Nanofibers with diameters ranging from 50 to 100 nm may exhibit damage due to energy bombardment at high magnifications damage. Therefore, Field Emission Scanning Electron Microscopy (FESEM) is employed. The greatest advantage of FESEM is obtaining high-resolution images at low voltages.

TEM (Transmission Electron Microscopy)

TEM analysis is conducted for detailed examination of morphological structure. This method is based on the principle of using high-voltage accelerated electrons to investigate the internal structure of the sample. TEM consists of an electron source, condenser lenses, objective lenses, and projector lenses. In transmission electron microscopy, the electron source is typically a tungsten filament heated by a low-voltage power supply. The filament is held at a high negative potential, and electrons are accelerated toward the sample, which must be thinner than 100 nm. The electron beam is directed using electromagnetic lenses. The electrons passing through the sample produce an image on a fluorescent observation screen.

AFM (Atomic Force Microscopy)

AFM enables the individual resolution of atoms on both insulating and conductive surfaces. In this system, a sharp-tipped cantilever, sensitive to forces, scans the entire sample surface in a raster pattern. Forces generated between the sample and the cantilever cause deflections in the cantilever small. These deflections are detected using optical instruments. Movement of the tip or sample is achieved using a piezoelectric tube. During scanning, the force acting on the tip is maintained constant by adjusting its vertical motion to produce a topographic information. The greatest advantage of atomic force microscopy is its applicability to non-conductive samples.

Contact Angle

Contact angle measurement is performed to analyze the wettability of materials. This measurement is determined by observing the spreading behavior of a liquid droplet deposited on the material, based on the interaction between the solid, liquid, and gas phases. Spreading depends on the magnitude of the adhesive force between the material and the droplet and the cohesive force between molecules within the droplet recording. If the tangent angle θ to the droplet is less than 90⁰, the surface exhibits hydrophilic properties; if θ is greater than 90⁰, the surface exhibits hydrophobic properties. Furthermore, materials with a contact angle between 0 and 5⁰ are classified as superhydrophilic, while those with θ greater than 150⁰ are classified as superhydrophobic.

Molecular Structure

The molecular structure of nanofibers is analyzed using XRD (X-ray Diffraction), DSC (Differential Scanning Calorimetry), TGA (Thermogravimetric Analysis), and Fourier Transform Infrared Spectroscopy (FTIR).

XRD (X-ray Diffraction)

XRD is based on the principle of analyzing the crystal structure from the diffraction pattern of X-rays. Each crystal type produces a unique diffraction pattern. XRD allows determination of the unit cell dimensions of a crystal plane and the distance between atomic planes in the crystal. Scattering occurs due to interactions between X-rays and electrons in the material. When X-rays are scattered by a regular arrangement in a crystal, the spacing between the scattering centers is on the same order as the X-ray wavelength, resulting in constructive or destructive interference. This produces diffraction.

DSC (Differential Scanning Calorimetry)

Differential scanning calorimetry is a method that examines the difference in heat flow between a sample and a reference as a function of temperature under a controlled thermal program.

TGA (Thermogravimetric Analysis)

In thermogravimetric analysis, the mass of a sample in a controlled atmosphere is recorded as a function of increasing temperature (linearly with time). A plot of mass or mass percentage versus time is called a thermal degradation curve or thermogram. Thermograms provide information about the degradation mechanisms of various polymers. Additionally, since degradation patterns are unique for each polymer, they are also used for their identification.

Fourier Transform Infrared Spectroscopy (FTIR)

The measurement is based on analyzing the absorption spectrum of chemical bonds or atoms in the sample exposed to infrared (IR) radiation red.