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CHAPTER 1

INTRODUCTION

1.1 Organic-Inorganic composites

Differing from the other bulk materials, the nanostructured materials

have evoked tremendous interest in regard to their attractive properties. This has

been mainly attributed to their size dependent effects which are related to the large

fraction of atoms at the surface and quantum size effects [1-4]. Recently, much

speculation has been shown on the fabrication and control of one-dimensional

materials such as nanowires, nanofibers, and nanorods owing to their abeyant

appositeness in electronics, optoelectronics, energy and other related fields.

According to the National Science Foundation (NSF), nanofibers have been

described as structures having a one dimension of 100 nanometers (nm) or less,

but generally considered having a diameter of less than one micron.

Actively, Organic-Inorganic composites system has reached great

heights of interest satisfying a broad range of applicability in several fields. In

many of the current applications, the unique properties of nanoparticle have been

delivered as fillers of composites or as coating materials. This polymer-

nanoparticle composite has stipulated the flexibility, stability and the

conformational ability for complicated structures while maintaining the

nanoparticle traits. The organic-inorganic composites not only possess the

characteristics of stability, excellent luminescence, electrical or magnetic

properties of inorganic particles but also the convenience to tailor-make the

structure, the facility to process it and also deplete the cost of the organic

molecules.

  

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Predicted to possess unique electronic and optical properties, these

composite nanofibers have been tuned by ways of optimizing the doping level of

inorganic materials. These fibers carry a whole package of applications in chemical

and biological sensors, light emitting diodes, rechargeable batteries nanoelectronic

devices, electromagnetic shielding and wearable electronics. Similarly, nanofibers

derived from ceramic materials such as zinc oxide (ZnO), titanium oxide and silicon

carbide possess optical characteristics (luminescence) that has been made use of in

light and field emitters.

Applications which make use of sub-micrometer diameter fibers have

many appealing benefits namely,

High surface area: Sensors, Protective clothing, drug delivery

Reduced pore size: Filters, Scaffolds for tissue engineering,

Adsorbents

Mechanical Strength: Reinforced composites

Higher packing densities: Energy storage systems

Small dimension: Micro and nano fabrications systems

Novel magnetic, electric and optical properties

ZnO has been considered a versatile material due to its direct band gap

(3.37eV), a large exciton binding-energy (60 MeV) at room temperature and a high

melting temperature (2248k). Due to its high exciton binding-energy, the excitons

have turned thermally stable at room temperature and thus ZnO has availed

numerous appliances in optoelectronic devices such as ultra-violet photo detectors,

photovoltaic devices etc. Apart from this, ZnO has been considered an eco-friendly

subsistence which makes it suitable for antimicrobial applications. Importantly, it

has also been listed as an eminent entity under GRAS (Generally Regarded As Safe)

by the US Food and Drug Administration and hence has been extensively used in

the formulation of personal and health care products. Most modern preparation

techniques have achieved the growth of morphology-controlled one-dimensional

ZnO structures. Unlike many of the materials with which it competes, ZnO has its

  

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own attributes of being inexpensive, relatively abundant, chemically stable, easy to

prepare and non-toxic [5].

Many methods have been developed to prepare the organic-inorganic

composites. One of the earliest and easiest methods has been the direct mixing of

the nanoparticles onto the polymer matrix. In this method, the nanoparticles would

be prepared before their incorporation into the polymer matrix. The expediencies of

this method have been facile technique, morphology as well as easy control over the

size of the nanoparticles. However, the nanoparticles have shown the liability to

aggregate due to their large specific surface energy. This has caused unavoidable

circumstances where the nanoparticles inhomogeneously distribute in the polymer

matrix, resulting in a loss of their function.

The second method to be discussed has been the layer-by-layer assembly

technique developed by Decher [6]. It has been proven to be one of the most

promising au courant methods of thin film deposition. Recently, this method has

also been successfully applied to thin films of nanoparticles and other inorganic

materials. Its simplicity and universality open a wide range of possible usances for

this technique, both in fundamental research and in advanced industrial

applications. The composites prepared using this method have found innumerable

applications in electroluminescence. However, difficulty has been seen when

applied to bulk materials.

Additionally, methods of electrochemistry, sol-gel and ultrasonic irradiation

have been used to prepare organic-inorganic composites. Among them, an

important and generally adopted technique has been the in-situ approach. The size

and distribution of the nanoparticles in the polymer matrix have been tuned by

designing and tailoring the structure of polymer matrix. In terms of their

composition, architecture, and arrangement of the organic and inorganic species, 1D

organic-inorganic hybrid nanomaterials exist mainly in the following groups (Figure

1).

  

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Figure 1.1: Illustration of 1D organic–inorganic hybrid nanostructures.

The most elemental and primitive structure has been the homogeneous

organic-inorganic hybrid nanomaterial (Figure 1A), in which the inorganic phase

has been evenly dissolved in the organic part. A nanosized agglomeration of the

inorganic phase has been undetectable till now. Other types of 1D hybrid

nanomaterials exhibit the phase separation of the inorganic or organic part of the

nanoscale in their structures. Depending on the distribution of the nanosized

inorganic or organic domains, core-shell-type (Figure 1B), scattered type (Figure

1C, scattered phase in the entirety, core, or shell), and di-/ tri- multi block structured

1D hybrids (Figure 1D) have been reported. The spatial relationship of the inorganic

and organic phases not only determine the intrinsic properties and functions of the

corresponding hybrid, but also brings up a direct relation different synthetic

strategies.

1.2 Methods to prepare nanofibers

Many methods have so far been taken up for the preparation of fibers

namely:

i) Mechanical drawing [7]

ii) Self Assembly [8-9]

iii) Template synthesis [10]

iv) Phase separation [11]

  

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v) Electrospinning (ES)

In this research work, we have summarized the preparation of

ZnO/Polymer composites by combining in situ sol-gel and the method of

electrospinning to obtain different functional one-dimension composites for

application in the UV sensor, superhydrophobic and antibacterial field. Here, the

polymer matrix has allowed the nanometer scaled ZnO to homogeneously disperse

in the composite thus improving its stability, dispersion and mechanical strength. In

addition, the surface of the ZnO nanoparticles could be modified by the polymeric

matrix via the interactions between the two components. The most striking

character of our method has been the excellence in the compatibility between the

nanoparticles and the polymers obtained via modification of the surface of the

nanoparticles ensuring the homogeneous distribution of nanoparticles in the

polymer matrix. The existence of the polymer network not only serves as a template

medium but also stabilizes the nanoparticles. The structure of such an organic-

inorganic composite network has stabilized the nanoparticles in terms of a long

term, which has proved a landmark in the protection of their function.

1.2.1 Self assembly

Self-assembly is a process in which individual, pre-existing components

organize themselves through weak, non-covalent interactions (H-bonding,

electrostatic interactions) forces into desired patterns and structures. It is known as a

‘bottom-up’ method and it yields fibers with small diameters (less than 100 nm

thick and up to few micrometer lengths) and it offers novel properties and

functionalities, which cannot be achieved by conventional organic synthesis.

Figure 1.2 shows how small molecules are arranged in a concentric manner,

bonds form among the concentrically arranged molecules, and then nanofibers are

formed upon extension of these molecules normal to the plane. The main

disadvantage of the method is that it is a complex, long, and extremely elaborate

technique with low productivity.

  

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Figure 1.2: Schematic presentation of self-assembled nanofibers

1.2.2 Dry Spinning

Dry spinning is a method used to form polymeric fibers from solution.

However, instead of precipitating the polymer by dilution or chemical reaction as in

wet spinning, solidification is achieved by evaporating the solvent in a stream of air

or inert gas. In this method, the polymer is dissolved in a volatile solvent and the

solution is pumped through a spinneret composed of numerous holes. As the fibers

exit the spinneret, air is used to evaporate the solvent so that the fibers solidify and

can be collected on a take up wheel. Stretching of the fibers provides for orientation

of the polymer chains along the fiber axis. Dry spun fibers typically have lower void

content than wet spun fibers.

1.2.3 Drawing

Drawing is a process similar to dry spinning in fiber industry, which can

make one-by-one very long single nanofibers. However, only a viscoelastic material

  

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that can undergo strong deformations while being cohesive enough to support the

stresses developed during pulling, can be made into nanofibers through drawing. It

requires a minimum amount of equipments, and is a discontinuous process. As

shown in the Figure 1.3, a micropipette is dipped into a droplet near the solution-

solid surface contact line via a micromanipulator.

Droplet (millimetric size)

Micropipette is brougThe contact 

Micropipedr

ht towardline

tte is touched the oplet surface

Nanofibeprod

Micropipette is withdrawn to Produce nanofibers

rbeing uced

Figure 1.3: Illustration of drawing method for preparing the fibers

Then the micropipette is withdrawn from the liquid at a certain speed,

yielding nanofibers. These steps are repeated many times on each droplet. The

solution viscosity, however, increases with solvent evaporation and some fiber

breaking occurs due to instabilities that occur during the process. Drawing process

is disadvantageous since the fiber size is dependent on the orifice size of the

extrusion mould; it is difficult to obtain fibers diameters less than 100 nm.

1.2.4 Template Synthesis

The template synthesis, as the name suggests, uses a nanoporous membrane

as a template to make nanofibers of solid (fibril) or hollow (tube) shape. The most

  

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important feature of this method lies in the fact that, nanometer tubes and fibrils of

different materials such as electronically conducting polymers, metals,

semiconductors, and carbons can be fabricated. On the other hand, the method

cannot make long continuous nanofibers. Extrusion of the polymer solution through

the porous membrane is achieved by water pressure. As soon as the polymer comes

into contact with the solidifying solution, fibers with diameter dependent on the

template pore size are produced as shown in Figure 1.4. The resultant fiber

diameters vary from a few to hundreds nanometers. On the other hand, this method

is limited in that nanofibers only a few micrometers long are obtained.

Water

Polymer Solu

Al2O3 Memb

Solidification Solution

tion

rane

Pressurized Water

Extruded fibers

STAGE 2

STAGE 1

Figure 1.4: Schematic representation of the template based method for preparation of fibers

  

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1.2.5 Phase Separation

The phase separation method of preparing fibers consists of dissolution,

gelation, and extraction using a different solvent, freezing, and drying, resulting in

nanoscale porous foam. The process takes relatively long periods of time to transfer

the solid polymer into the nano-porous foam. In this process, the polymer is

dissolved in an appropriate solvent at the desired concentration. The solution is then

stirred at a certain temperature for a period of time until a homogeneous solution is

obtained. This is followed by transferring the solution into a refrigerator set to the

gelation temperature of the polymer. The resultant gel is immersed in water several

times to allow solvent exchange. Finally, the gel is removed from water, transferred

to a freezer (-70oC), and then the frozen gel is lyophilized. A simple representation

of this process is given in figure 1.5, which shows how nanoporous polymer foam is

produced.

Figure 1.5: Schematic representation of the phase separation process

In this process, phase separation occurs due to physical incompatibility and

yields nanofibers; however, a long period is needed to transfer a solid polymer into

nano-porous foam. Fiber dimensions vary from 50 to 500 nm with a length of a few

  

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micrometers. Therefore, the limitation of this method is that no long continuous

fibers are produced and only the polymers that have gelation capability can be used

to produce the nanofibrous structure.

1.2.6 Electrospinning

Electrospinning is a process that creates nanofibers through an electrically

charged jet of polymer solution which is based on the uniaxial stretching of a

viscoelastic solution. The diameter of the fibers obtained through this method as

low in the range of 10µm to 10nm, which are typically 1 to 3 orders less than that

obtained by the conventional spinning process. To understand and appreciate the

process that enables the formation of various nanofiber assemblies, the principles of

electrospinning and the different parameters that affect the process have to be

considered. Unlike conventional fiber spinning methods like dry-spinning and melt-

spinning, electrospinning makes use of electrostatic forces to stretch the solution as

it solidifies. Similar to conventional fiber spinning methods, the drawing of the

solution to form the fiber will continue as long as there is enough solution to feed

the electrospinning jet. Thus without any disruption to the electrospinning jet the

formation of the fiber will be continuous.

Electrospun fibers show very high surface area to volume ratio, which makes

these fibers suitable for variety of applications such as sensors, adsorbents, filters

and energy storage materials. The main advantage of electrospinning method is,

relatively low cost and simplicity compared to that of most bottom-up methods. The

resulting nanofiber samples are often uniform in diameter and do not require

expensive further purification. Unlike submicron-diameter structures such as

whiskers, nanorods, carbon nanotubes, and nanowires, the electrospun nanofibers

are continuous. However, the use of conducting or semiconducting nanofibers for

electronic, opto-electronic, photonic or sensor devices is relatively new.

  

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Figure 1.6: Schematic diagram of the various electrospinning apparatus to obtain the different morphology of fibers.

1.3 History of the electrospinning

The term “Electrospinning” derived from “electrostatic spinning”, was used

relatively recently (around 1994), but its fundamental idea dates back to more than

60 years. During 1934 to 1944, Formhals obtained a series of patents, describing an

experimental setup for the production of polymer filaments using an electrostatic

force [12-14]. A polymer solution, such as cellulose acetate was introduced into the

electric field. The polymer filaments were formed from the solution between two

electrodes bearing electrical charges of opposite polarity. One of the electrodes was

  

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placed into the solution and the other onto a collector. One ejected out of a metal

spinneret with a small hole, the charged solution jets evaporated to become fibers

which were collected on the collector. The potential differences in the fiber

characteristics depended on the properties of the spinning solution such as polymer

molecular weight and viscosity. When the distance between the spinneret and the

collecting device was short, spun fibers tended to stick to the collecting device as

well as to each other, due to incomplete solvent evaporation.

In 1952, Vonnegut and Neunauer were able to produce streams of highly

electrified uniform droplets of about 0.1mm in diameter. They invented a simple

apparatus for electrical atomization. A glass tube was drawn down to a capillary

having a diameter in the order of few tenths of millimeter. The tube was filled with

water or some other liquid and an electric wire connected with the source of

variable high voltage was introduced into the liquid [16].

In 1955, Drozin investigated the dispersion of a series of liquids into

aerosols under high electric potentials. He used a glass tube ending in a fine

capillary similar to the one employed by Vonnegut and Neubauer. He found that for

certain liquids and under proper condition, the liquid emerged from capillary as

highly dispersed aerosol consisting of droplets with relatively uniform in size. He

also captured different stages of dispersion [17].

In 1966, Simons patented an apparatus for the production of non-woven

fabrics of ultrathin and very light weight fibers forming different patterns using

electrical spinning [18]. The positive electrode was immersed into the polymer

solution and negative one was connected to a belt where the non-woven fabric was

collected. He found that fibers from low viscosity solutions tended to shorter and

finer whereas those from viscous solutions were relatively continuous.

In 1971, Baumgarten made an apparatus to electrospin acrylic fibers with

diameters in the range of 0.05-1.1µm. The spinning drop was suspended from

  

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stainless steel capillary tube and maintained constant in size by adjusting the feed

rate of an infusion pump [19]. A high voltage DC current was connected to the

capillary tube while the fibers were collected on a grounded metal screen. Since

1980’s and especially in the recent years, the Electrospinning process essentially

similar to that described by Baumgarten has regained more attention probably due

in part to surging interest in nanotechnology, as ultrafine fibers or fibrous structures

of various polymers with diameters down to submicron or nanometers can be easily

fabricated with this process [20-25].

Figure 1.7: The increasing number of publications on Electrospinning over the years is a clear indication of the importance of the area of research

Until 1993, this technique has been known as electrostatic spinning, are there

were only a few publications dealing with its use in the fabrication of thin fibers. In

early 1990s several research groups revived interest in this technique by

demonstrating the fabrication of thin fibers from broad range of organic polymers.

At this time the term Electrospinning is coined and is now widely used in literature.

This timely demonstration triggered a lot of experimental and theoretical studies

  

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related to Electrospinning. Number of publications in this field has been increasing

exponentially in the past few years, on account of remarkable simplicity, versatility

and potential uses of this technique. Many researchers began to explore the field.

Since then the number of publication in each year is increasing exponentially and

the figure 1.7 shows the exponential growth of the number of publications in this

area over the years.

1.4 Preparation of defect free fibers by controlling the process parameters

The morphology and diameter of the electrospun fibers are dependent on a

number of processing parameters that include: 1) the intrinsic properties of the

solution such as type of polymer, viscosity, etc., and 2) process parameters such as

applied electric field potential, flow rate, distance between the electrodes and

ambient parameters (temperature, humidity and air velocity in the chamber). The

presence of beads in electrospun fibers is a common problem [25-26]. Other shapes,

in particular ribbon like shapes with rectangular cross-section have also been

reported. A number of research articles discuss about the factors affecting the fiber

diameter. The major factors that affect the diameter of the electrospun fibers include

the concentration of the polymer, electrical conductivity of the polymer solution,

electric field strength and flow rate of the polymer solution etc.

Fridrikh et al. presented a simple analytical model for the forces that

determine jet diameter during electrospinning as a function of surface tension, flow

rate and electric current in the jet. Voltage can be considered the most essential

parameter in electrospinning, since it initiates the jetting and causes instabilities,

which stretch the jet [28].

Effects of the parameters of the electric field on electrospinning process and

forming fibers are, once again, diversified. An increase in voltage and, thus, in

electric field, was found mainly to decrease, but also increase the fibers diameter.

Similarly, an increase in distance (decrease in field) has found to both increase and

decrease the fibers diameter. These findings indicate that by appropriately varying

  

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one or more of the above parameters, nanofibers can be successfully electrospun

from a rich variety of materials that include polymers, biopolymers, DNA, protein,

composites, and ceramics and even relatively small macromolecules such as

phospholipids.

The fibers obtained in the Electrospinning processes are randomly oriented.

For several useful applications one need well aligned and long fibers. Several

research groups attempted to obtain well-aligned fibers as illustrated in figure 1.6

[29-44]. More recently Lin and coworkers developed near-field Electrospinning

process to deposit solid nanofibers in a direct, continuous and controllable manner.

Sundaray et al fabricated the well aligned electrospun fibers using a modified

rotating drum with sharp pin inside [40].

Intrigued by their potential application as scaffolds in cell biology and tissue

engineering, a large number of biodegradable that include poly(caprolactone),

poly(L-lactide) and poly(glycolide) have been directly electrospun into nanofibers

[45-48]. In addition to these synthetic organic polymers, natural biopolymers such

as DNA, silk fibroin, human or bovine fibrinogens, dextran, collagens and even

viruses have also been successfully used for Electrospinning. In general, the

condition controlling the Electrospinning using these biomacromolecules are

identical to those observed for the case of conventional synthetic polymers.

Lrsen et al. were the first to combine Electrospinning with sol-gel methods

to design vesicles and nanofibers made from inorganic oxides [49]. A variety of

functional components can be directly added to the solution for Electrospinning to

obtain nanofibers within diversified range of compositions and well defined

functionalities. To this end, incorporation of nanoparticles such as Zinc Oxide,

Carbon nanotube, silver and iron oxides have all been demonstrated. Various

ceramic meal oxides fibers were obtained by high temperature calcinations of the

precursor organic-inorganic composite fibers assembled by Electrospinning. It was

  

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generally observed that the calcinations temperature has a great influence on both

the crystalline phase and the surface morphology of the fibers.

These provide a wide range of properties such as strength, weight, elasticity,

porosity and charged surface areas. Moreover electrospinning also provides the

capacity to lace together a variety of nanoparticles or nanofillers types that can be

encapsulated into a nanofibers matrix. Functional micro/nano particles may be

dispersed in polymer solutions, which are then electrospun to form composites in

the form of continuous nanofibers and nanofibrous assemblies. All these endow

electrospinning with outstanding manufacturing capabilities but utilizing an easy

process and capable of excellent flexibility. Additionally, electrospinning seems to

be the only method that can be further developed for mass production of one-by-one

continuous nanofibers from various polymers. A number of processing techniques

such as drawing, template synthesis, phase separation and self-assembly have been

used to prepare polymer nanofibers in recent years. However these methods have

disadvantages such as: material limitation, they are time-consuming and they

require complicated processing systems. As far as electrospinning is concerned it is

not only a simple one-step top-down process for fabricating nanofibers, but also the

co-processing of polymer mixtures, chemical cross- linking can be carried out that

provide a variety of path-ways for controlling the chemical composition of the

nanofibers.

1.5 Characteristics of electrospun fibers

Electrospun nanofibers possess unique traits, such as: extraordinary high

surface area, coupled with remarkable porosity, excellent structural and mechanical

properties, flexibility, low basis weight, and cost effectiveness among others.

Another interesting aspect of using nanofibers has been its feasibility to modify not

only their morphology and their content but also the surface structure to carry

various functionalities. Nanofibers have been post-synthetically functionalized in

an. Furthermore, its feasibility to control secondary structures of nanofibers has

  

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been successful in preparing nanofibers with core/sheath structures, nanofibers with

hollow interiors and nanofibers with porous structures.

Economically, the electrospinning process has been relatively cheaper when

compared to that of the bottom-up nano-fiber fabricating methods. The resulting

nanofibers have been prone to be uniform, continuous and require no expensive

purification protocols. These nanofibers have been found relatively apparent to be

scaled up for productivity due to the top-down process and the design of multiple

jets for synchronous electrospinning. Moreover, the nanofibers have one dimension

on the microscopic scale but another dimension macroscopically. This unique

characteristic endows nano-fiber mats with appealing benefits possessed by

functional materials on the nanometer scale, gaining advantage over conventional

solid membrane in regard to ease in processing, ease of packaging and shipping.

These outstanding properties have shown up polymer nanofibers and composite

fibers as good candidates for several applications.

1.6 Modifications of electrospinning

Polymer fibers obtained by using conventional electrospinning apparatus

have been randomly oriented. Researchers have attempted diverse approaches to

achieve alignment in electrospun fibers. During a course of electrospinning,

production of fibers in a continuous and controllable manner has been reported.

With this technique, known as near-field electrospinning, the production of

nanofibers with desired pattern has been made possible. Recently, Atomic force

microscope (AFM) based voltage-assisted electrospinning technique has been

reported to achieve aligned fibers.

Single fiber of polyethylene oxide (PEO) polymer with nanometer scale

diameters have been formed by this method. Ceramic hollow nanofibers have been

developed by Li and Xia using coaxial electrospinning. They have been able to

control the wall thickness by acclimatizing the experimental parameters. They have

  

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also proposed the use of these hollow ceramic fibers in nano fluidic devices or

optical waveguides [50-52].

Figure 1.8: (a) Flat polymer nanofibers, (b) Beaded polymer nanofibers, (c) Non-porous smooth polymer nanofibers, (d) porous submicron polymer fibers, (e) Randomly oriented ceramic nanofibers and (f) Aligned ceramic nanofibers 1.7 Applications of Electrospinning

The simplicity of the fabrication scheme, the usage of commensurable

materials, as well as the unique and profiting features associated with electrospun

nanofibers, have together made this technique attractive in a number of applications

[53-54]. Major areas of research on nanofibers include biomedical applications,

energy storage such as solar cells and fuel cells, sensors, and filtration. The

potential applications of electrospun nanofibers have been summarized in figure

1.9.

  

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Figure1.9: Application of electrospun fibers in different field Modification of electrospinning 1.8 Motivation and Statement of the problem

In particular, one-dimensional structures such as nanofibers have been

found more attractive due to their large specific surface area, uniform diameter

distribution and well-defined charge carriers transporting path. In the recent times,

many researchers among the academic and industrial communities have shown keen

  

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interest towards the preparation of polymer nanofibers. Our interest has also been

on making composite fibers through electrospinning for varied applications. In

addition to a wide variety of polymer and composite nanofibers, inorganic

nanofibers such as ZnO have also been fabricated using this simple technique,

through calcination of polymer composite nanofibers containing inorganic

precursors. However, difficulty has been observed in obtaining direct ultra fine

fibers of inorganic materials having lengths in the order of millimeter as they tend

to break during formation pertaining to their thermal and other mechanical stresses.

Importance has been stressed on the understanding of the causes behind the

formation of defects in the fibers and the breakage of fibers during thermal

treatment encouraging development methods to control their formation. Here, in our

work, we have investigated the effect of thermal treatment on fiber morphology and

the possible mechanism behind such structural changes in fibers.

Dispersing nanoparticles in a polymer matrix has been one of the best

methods to stabilize the nanoparticles and to achieve properties that combine the

excellent thermal and mechanical properties of polymer matrix with the attractive

functional properties of the stable inorganic filler materials. Diverse approaches

such as spin coating, film casting etc have been employed to achieve this. However,

major difficulties associated with these methods are random distribution and

aggregation of nanoparticles in polymer matrix. In this work, we have focused on

the dispersion of Nanocrystalline ZnO in fibrous polymer matrix through two

different approaches for comparison namely the Insitu sol-gel method and the Ex-

situ method (Direct dispersion of ZnO nanoparticles in polymer solution). Detailed

studies of the fibrous membrane prepared through different routes have been

systematically studied and the results have been compared.

For application in ultraviolet photo detectors, requirement of high

quality ZnO nanostructures have been essential since the defects in the material

introduce additional transition states leading to unwanted emissions in the visible

region. In order to achieve this high quality, the importance of understanding the

  

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mechanism behind the defect-formation and means of eliminating them during the

preparation of ZnO nanomaterials has been compulsive. Also, conspicuousness in

maintaining low cost of synthesis and easy affordability has to be kept in mind.

There have not been many investigations in the scientific literature on the

fabrication of UV photo-detectors-based on polymer/ZnO fibrous composites.

There are several reports on the fabrication of UV detectors using different

ZnO structures such as particles, films, wires, etc. Still, extensive research needs to

be carried out on this prominent arena. In particular, improvements have been taken

up to enhance the quality of the ZnO nanostructures in order to achieve esteemed

performance and reliability. Solution processed optoelectronic devices have some

advantages in terms of ease of fabrication, low cost, etc. However, difficulty in

realizing the feasible UV photodetectors has been observed since complications

pertaining to the uniform dispersion between the two electrodes. Further, UV

detectors based on wet chemical synthesized semiconductors have shown slow

photo response because of the high density of defects.

Basically, photodiodes have been found to be light-sensitive devices

used to detect optical signals through electronic processes, and generally work

under photoconductive mode. The operation involves three steps: (1) Carrier

generation by absorption of the incident light photon of energy higher than the band

gap of the device (2) Carrier transport and (3) Current flow in the external circuit to

provide the output signal. Based on the requirement of the device and earlier

reports, we have noticed tremendous scope for improvement namely: (1) To

eliminate intrinsic defects in ZnO materials during fabrication (2) To enhance the

pathway for carrier transport in the active material (3) To improve the interaction of

ZnO nanostructures with electrodes.

Our research on fabrication of ZnO nanoparticles in fibrous polymer

matrix has proven the high optical quality of ZnO obtained, which has further

elevated the interest regarding the UV sensor applications of ZnO. Electrospinning

  

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process allows the direct fabrication of high quality ZnO nanofibers and their

composites without introducing the intrinsic defects. Compared with the device

based on ZnO nanowires, particles and films fabricated by other techniques, the UV

sensor based on electrospun fibers has been simple to fabricate.

The main advantages of the scheme proposed here been the ease of

preparation under controlled conditions, enhanced interfacial interaction between

organic and inorganic materials and an overall stabilizing effect during preparation.

Defined material structure i.e. controlling the morphology and particle size has led

to higher efficiency of exciton generation under UV irradiation and charge

separation. Therefore, it might be possible to enhance the UV sensitivity of the

device if we replace the Nano ZnO particles, films with fibers and their composites.

Increased surface area of the materials has shown to improve the performance of the

device. In this respect, nanofibers might fulfill the requirements because they have

high surface area to volume ratio, defined structure, and high aspect ratio.

Super hydrophobic surfaces, with a water contact angle (WCA) greater than

120°, have proved very useful in applications such as self cleaning, antisnow/fog

and contamination prevention. The wettability of the solid surface has turned out to

be a characteristic property of materials and has been strongly dependent on both

the surface energy and surface roughness. Conventionally, super-hydrophobic

surfaces have been produced mainly in two ways by creating a rough surface or by

modifying the surface with materials having low surface energy, such as fluorinated

or silicon compounds. Of late, a variety of techniques have been proposed for

constructing superhydrophobic materials, such as etching (chemical etching and

plasma etching), chemical/physical vapor deposition CVD/PVD), densely packed

aligned carbon, sol–gel processing, etc. These tend to modify surface topography

and enhance hydrophobicity by coating a hydrophobic thin layer or monolayer. A

large number of materials both inorganic (such as ZnO, CNTs, Fluorinated

materials, SiO2, etc.) and hydrophobic polymers have been used to prepare

superhydrophobic surfaces.

  

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In some cases, the reported approaches employ either expensive

materials or complicated procedures, thereby limiting the applications of

superhydrophobic surfaces. In most practical cases, it has not been possible to

modify the surface roughness without simultaneously affecting the chemical nature

of the surface. In addition, common acknowledgement has been made on the

difficulty in obtaining super hydrophobic surfaces from water soluble polymers.

Primarily, our objective has been to attempt a scheme to prepare superhydrophobic

surface using initially water soluble polymers with the addition of ZnO

nanoparticles by using electrospinning method. Here, the wettability of fibrous

composite membranes has been studied in different ways namely: (1) Effect of

concentration of precursor (2) Effect of thermal treatment (3) various routes to

prepare fibrous membrane, etc.

Even in the 21st century, infectious diseases continue to pose a dominant

public health threat in many developing countries. The World Health Organization

has reported that the developing countries contribute to 25% of the world’s death

caused by microbes. Development of resistance to antibiotics has been a major

drawback in the treatment of many infectious diseases. Therefore, the need for new

strategies has been in need to identify and develop alternative antimicrobial agents

to control bacterial infections. Since the survival of microorganisms on surfaces in

the environment could also be the result of the increased spread of diseases,

antimicrobial coatings on surfaces have been of great interest. In addition, nano

structured coatings have been expected to provide better safety and stability to the

surfaces below. In recent years, numerous antibacterial materials including metals,

semiconducting oxides and polymeric/composite materials have been extensively

investigated. Among the many inorganic antibacterial agents, ZnO has been

extensively used because of its ability to withstand harsh processing conditions, UV

blocking property and superior durability apart from being less toxic and cost

effective when compared to the organic antibacterial materials. It has been observed

from the available literatures that several mechanisms have been proposed to

  

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interpret the antibacterial behavior of ZnO, be it nanopowder or film. Though

polymer/ZnO composite has received considerable attention, its application in the

field of microbial protection has not been explored in detail. The defined

morphology and large surface to volume ratio of fibers may provide better

interaction with microorganism. Therefore it has been anticipated that these Fibrous

Composite Membranes (FCM) may exhibit much stronger antibacterial activity.

Hence bactericidal tests of Free standing-FCM (FS-FCM) have been surveyed here.

Our interest has been to prepare the “ZnO particle enriched fibers”. To

prepare the pure ZnO fibers, the component polymer inside the composite fibers has

to be removed under high temperature. Although pure inorganic oxide nanofibers

such as TiO2, ZnO could easily be synthesized in this manner, the removal of the

polymer reduces the elasticity and mechanical strength of composite nanofibers.

Improved properties might result if the inorganic precursors inside the composite

nanofibers could be converted into inorganic oxides while the component polymer

inside the composite nanofibers is being retained. Very little attention has been

directed towards the preparation of ZnO nanostructures on the fiber surface to form

functional nanostructures. Methods of assembling inorganic nanoparticles into

polymer matrixes include a mixture of preformed nanoparticles and polymers,

plasma deposition, and in situ growth. A new interest has been incited in the latter

mode of synthesis inside polymer matrixes. However, the in situ synthesis of ZnO

nanocrystals in solid polymer matrixes has still remained a highly sophisticated

challenge.

Thus the objectives of the present studies have been:

1. To Prepare and characterize the fibrous membrane (Polymer/ZnO and ZnO

nanofibers using combination of sol-gel and electrospinning methods.

2. To investigate the effect of process parameter (e.g. Electric field, viscosity,

electrode distance, flow rate, etc.) on fibers morphology

3. To obtain the ultra long ZnO fibers by controlled thermal treatment (Heat

flow rate, Different calcinations temperature etc) of composite fibers.

  

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4. To study the optical quality of the composite fibers.

5. To fabricate the UV sensor

6. To study the UV shielding property of the composite fibrous membrane.

7. To investigate the bactericidal property of fibrous membrane.

8. To study the wettability nature of composites membrane.

9. To develop a scheme for preparation of “ZnO particle enriched” fibers

10. To compare the efficiency of FCM prepared by different approaches and

to prove the role of electrospinning to enhance the properties of the

prepared material.

1.9 Organization of the thesis

The overview of the research work has been summarized into six

chapters, the contents of which have been briefly outlined:

In chapter 1, a brief outline of nanofibers and their application in various

fields, together with the current research has been presented. In addition, the

objectives and the scope of the present research work have been given.

In chapter 2, an overview of Electrospinning technique along with the

details of the experimental facilities employed for characterizing the prepared

samples has been detailed.

In chapter 3, the details of the preparation and characterization of fibrous

composite membrane and pure ZnO fibrous membrane have been explained. The

influence of various process parameters (Viscosity, strength of electric field,

distance between the electric field, flow rate, etc) on the morphology of electrospun

fibers have been discussed. This study has been extended to prepare an ultra long

and unbroken ZnO nano fibers and a thorough investigation on the effects of

thermal treatment on fibers morphology has been performed.

  

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In chapter 4, the optical properties of the composite membranes have

been investigated and the results described. The fabrication of UV sensor has been

given. UV shielding property of the fibrous composite membrane has been studied.

In chapter 5, the preparation of superhydrophobic surface without

additional chemical modification has been described. The wettability of the

composite fibrous membrane has been studied using water contact angle

measurements and the results have been presented. The bactericidal properties of

the membrane have also been studied for antibacterial applications. A method of the

preparation of “ZnO particles enriched fibers” has been developed and the results

discussed.

In chapter 6, Summary of all the results obtained and a brief scope for the

future work has also been given. A list of references and publications based on this

work has been confined.