An Introduction to Electrospinning and Nanofibers
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Various techniques for surface modification such physical coating, blending, co-polymerization, chemical vapor deposition, chemical treatment, etc are furnished. Thereafter, the functionalized nanofibers are discussed with regard to affinity membrane, tissue engineering, sensor, protective clothing and other applications. The last chapter expounded on the many applications of nanofibers. A list of some useful websites that deal with nanofibers is given in the appendix which is correct at the point of writing.
Throughout this book, the term nanofiber membrane refers a semi-transparent membrane obtained by electrospinning. Chapter 1 Introduction 21 Whilst it is a membrane macroscopically, the membrane is a network of nanofibrous structure. Hence the terms nanofiber membrane, nanofiber mesh and nanofiber web are used interchangeably in order to reflect the diversity of viewpoints.
An Introduction to Electrospinning and Nanofibers - Seeram Ramakrishna - Google книги
These terms, as well as other technical terms, are defined and explained in the glossary of terms for the benefit of the lay readers. Chapter 2 Basics Relevant to Electrospinning To understand electrospinning, one can look at the mechanism behind the production of polymer fibers. Conventional fibers of large diameter involve the drawing of molten polymer out through a die. The resultant stretched polymer melt will dry to form individual strand of fiber. Similarly, electrospinning also involve the drawing of fluid, either in the form of molten polymer or polymer solution.
However, unlike conventional drawing method where there is an external mechanical force that pushes the molten polymer through a die, electrospinning make use of charges that are applied to the fluid to provide a stretching force to a collector where there is a potential gradient. When a sufficient high voltage is applied, a jet of polymer solution will erupt from a polymer solution droplet. The polymer chain entanglements within the solution will prevent the electrospinning jet from breaking up. While molten polymer used in both conventional fiber production method and electrospinning method cools and solidifies to yield fiber in the atmosphere, the electrospinning of polymer solution relies on the evaporation of the solvent for the polymer to solidify to form polymer fiber.
Since electrospinning is basically the drawing of a polymer fluid, there are many different types of polymers and precursors that can be electrospun to form fibers. The materials to be electrospun will depend on the applications.
Materials such as polymers and polymer nanofiber composites can be directly produced by electrospinning. Other materials such as ceramics and carbon nanotubes require post processing of the electrospun fibers. As each of the properties involved in electrospinning is a huge science of its own, this chapter aims to give some basic information relevant to electrospinning.
However, with growing interest and research into electrospinning, new fundamental properties may be discovered. Material Classes The materials and application of electrospun fibers are numerous, individual material properties must be considered depending on its applications. The electrospinning process may be modified so as to yield electrospun fiber with the desired morphology and properties. When used as composite, the nanofibers can be made as a composite on its own or it can be used as reinforcement in a matrix. In the production of ceramic fibers, post processes are required after the fibers are electrospun.
Thus it is important to have a basic understanding of the different group of materials before selecting the most appropriate electrospun fibers for specific applications.
Polymers Polymers consist of long chain of molecule with repeating units called monomers that are mostly covalently bonded to one another. An example of a polymer would be polyethylene which consist of repeating units of [CH2CH2-]n. Such single unit is also known as monomer. The monomer must either have reactive functional groups such as amino groups -NH2 or the have double bonds which may react under suitable conditions to provide the covalent linkage between the repeating units.
Such strong linkages form the backbone of the polymer chain. It is common to find weak secondary bonds between the molecule chains which allow the chain to slide over one another. Polymers exhibit several properties that are attractive for many applications. Most polymers are inexpensive as they contain simple elements and they are relatively easy to synthesize. They have found applications in many areas such as clothing, food packaging, medical devices and aircraft.
Natural polymers such as silks, collagen and agarose have found usage in many tissue engineering applications. Fundamental Classification of Polymer A widely accepted classification of polymer is their responds to heat. There are basically two types of polymer under this classification, thermoplastic and thermoset. In thermoplastics, the linear polymers melt when heat is applied but solidifies when cooled. This heating and cooling can be repeated many times without affecting the properties.
Examples of thermoplastic include polyethylene, polystyrene and vinyls. However, this would impose a limiting temperature for the material in use as a structural element above which the polymer may distort over time.
For thermosetting polymer, once an initial heat is applied, there is crosslinking between polymer chains. Subsequent application of heat would only degrade the polymer. Examples of thermoset include phenolics, urea and epoxies. This means that such polymer has much higher upper limiting temperature.
Polymer Crystallinity In bulk polymer, there are usually both regions of crystalline and amorphous parts as shown in Fig. The ratio of the two regions would determine the properties of the polymer. A polymer is said to be amorphous when the arrangement of the linear molecules is completely random. A crystalline polymer has its linear adjacent linear chains are aligned. The more commonly accepted theory for crystalline polymer is the folded chain theory.
The polymer chains are first folded and stacked on top of one another held together by amorphous tie-molecules to from crystallites. Chapter 2 Basics Relevant to Electrospinning 25 Fig. Model of structure of partially crystalline polymer. These are then twisted and turned to form a ribbon-like supramolecules called spherulites as shown in Fig.
Electrospinning of ceramic nanofibers: Fabrication, assembly and applications
Polymers that are of higher crystallinity show higher yield strength, modulus and hardness. When crystalline polymers are stretched, the polymer chains are oriented in the direction of the stress and destroy the spherulites structure. A phenomenon called necking is then observed. They also have better wear and chemical resistance. However, crystalline polymers are more brittle. The optical properties of polymers are also affected by the crystallinity.
More crystalline polymers have a higher refractive index than amorphous matrix making them either opaque or translucent. Amorphous polymer may be completely transparent [Farag ]. Spherulite in polypropylene [Aboulfaraj et.
Polymer Molecular Weight As polymer chains are made of repeating units, the molecular weight of the polymer is the sum of the molecular weight of the individual monomers. Generally, a higher molecular weight increases the polymer's resistance to solvent dissolution. The molecular weight of the polymer also has a direct influence on its viscosity. Mn is the total weight of the individual molecular weight by the number of molecules. Mn is independent of molecular size but is highly sensitive to small molecules present in the mixture. Glass Transition Temperature Tg The glass transition temperature is a very important property of polymers.
This temperature defines the mobility state of the polymer molecules. Below its Tg, the amorphous polymer is brittle as the molecules are frozen but above Tg, the polymer is ductile and the molecular chains have sufficient thermal energy to slide. This causes the elastic modulus of amorphous polymers to decrease by several orders of magnitude at temperature above Tg.
It is important to note that the mechanical behavior of the polymer at temperature above Tg is affected by the loading rate. Tg affects the mobility of polymer molecules, however, the motion of the molecular chain is not instantaneous. For slow loading rate, the molecular chain have time to move at temperature near Tg but if the loading rate is fast, there may not be enough time for the molecular chain to move thus increasing the effective Tg.
At temperature below Tg, it can be said that the relaxation time of the molecule is too long for equilibrium to occur under the slowest of experimental duration. When the temperature is at Tg, the molecules within the polymer bulk move as a temperature-dependent "coorperatively rearranging" region. The size of this "coorperatively rearranging" region is dependent on the configuration restrictions due to amorphous packing [Adam and Gibbs ].
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Molecular dynamic simulation of a polymer melt has shown that there is an increasing clustering of mobile monomer unit as the temperature decreases to a critical temperature. Thus the measuring condition must be noted when making comparison between different experiments. In the study of thin polymer films, the glass transition temperature was found to found to be lower than the bulk [Ellison et. However, the bulk dynamics beneath the surface area can slow down the mobility at the surface leading to a higher surface T g.
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When the film thickness is reduced sufficiently about 14nm , the T g at the surface is the same as the T g of the bulk [Ellison et. From the studies polymer film, it can be seen that T g is affected by the surface mobility. For a nanofiber, with the surface area larger than film, it has been experimentally shown to exhibit a lower T g than cast film [Zong et. There are many factors that affect bulk T g. Polymers with more freevolume allows easier movement of molecular chain thus lowering the T g.
Stronger secondary bonds such as H bonds would increase the T g. With greater the chain length, there are more entanglements within the polymer structure thus increasing the T g. Table 2.