chapter- ii :: experimental methods 2.1 polymer...
TRANSCRIPT
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CHAPTER- II :: EXPERIMENTAL METHODS
2.1 Polymer Electrolyte preparation methods:
Now a days, preparation and use of solid polymer electrolytes is an
interesting technology because of their wide applications in many fields. Recent
investigations are focusing on the different techniques to prepare the polymer
electrolytes. To prepare good quality polymers there are different methods
employed by many researchers [1-4]. Polymer preparation methods can be
classified as chemical and physical methods. Some of the commonly used
methods are given below.
1. Solution – casting method
2. Thermal evaporation method
3. Laser evaporation method
4. Photolytic Processes
5. Polymerization of monomer
6. Sputtering
7. Spin Coating
8. Spray printing
9. Photolithography
10. Screen printing
11. Gaseous discharge, etc.
The solid polymer electrolytes are easy to fabricate as soft films of a few
microns thickness and their flexibility permits interfaces with solid electrodes,
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which remain intact when the device is in use. This makes the application and
development of many electrochemical devices possible. In the present study, our
interest is in the preparation of polymer electrolytes. From the literature
available, it is found that there are various methods for the preparation of good
quality polymer membranes. Some excellent reviews are available in the
literature on increasing and sustained interest in emerging technology resulting
from the complexation of low-lattice energy salts with solvating polymers over the
past decade [5-7]. In the present study, Solution Casting method is used for the
preparations of polymer electrolytes.
Solution – Casting Method:
Polymer electrolytes are generally obtained by well known solution casting
procedure. Polymer films of thickness between 50 to 300 μm are prepared by
this method as reported by many researchers [8-11]. The polymer is the host
polymer which is dissolved in a suitable solvent and a dopant like an inorganic
salt or an organic acid is also dissolved separately in the same solvent. The
solutions of host and the dopant are then mixed together in adequate proportion
and made into a homogeneous saturated mixture. Then this mixture is allowed to
cast on to glass dishes or poly propylene dishes and evaporated slowly by
keeping in a temperature controlled oven.
2.2 Preparation of polymer electrolytes in the present work:
In the present work, Poly Vinyl Alcohol is taken as host polymer and one of
the dicarboxylic acids (Oxalic, Malonic and Succinic acid) in appropriate weight
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proportion is taken as dopant. Host and dopant are separately dissolved in
double distilled water. These solutions are then added together to form a
saturated mixture. This mixture is slowly and thoroughly stirred for about 12
hours, using magnetic stirring facility, to form a homogeneous mixture. The
stirred solution is cast on to a glass dish to evaporate slowly at 50 0C for 48 hours
to obtain thin membranes of the polymer composite. These membranes are
vacuum dried at 10-3 Torr to eliminate the residual traces of water. Now the
membranes are carefully separated from the dishes and stored in evacuated
desiccators. It is seen that the thickness of the cast membrane is in the range of
100 to 200µm. The schematic illustration of the preparation procedure of polymer
electrolyte membrane is shown in figure. 2.1.
Glass dish
Figure 2.1 :: Schematic illustration of the preparation of polymer films using solution casting method.
Polymer film
Solution of Poly
Vinyl Alcohol
Solution of the
dicarboxylic acids
Homogeneous solution
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2.3 Thickness measurement:
Quick and easy measurements of small parts and thicknesses are made
with thickness gauge. A choice of resolutions and physical dimensions is
available from several manufacturers. The dial thickness gauge (Mitutoyo Model
7301) is typically used to measure the thickness (range 10 µm to 10mm) of sheet
metal, plastic, paper, polymer film and cardboard. The pocket thickness gauge is
a miniature version, easily tucked away. These are available with an accuracy of
±0.001" or ±0.01mm making them suitable for many applications. This has a
lever which can slide in order to raise the anvil.
For a flat polymer electrolyte, a thickness gauge which has flat anvils top
and bottom such as model 7301 shown in figure 2.2 is used. Before taking a
measurement, the anvils are cleaned with alcohol (acetone) then a piece of clean
paper is placed between the anvils. They are allowed to rest together under their
own force and then the paper is pulled out slowly. This helps to polish and
further clean the surface. Initially the gauge is closed. Then the hand (the
pointer) points to the twelve o'clock position that is, to zero. If there appears a
zero error, then the bezel is rotated until the hand lines up with the zero, exactly.
Now, the anvil is lifted and repeated several times to make sure it always points
to zero. This is called repeatability and should always be less than half a
graduation. Now the anvil is lifted and the film is inserted. Let the anvil be closed
down on the film and the anvils are slid a little bit so that the lowest reading can
be found on the dial. If the hand points to 31 then the thickness is of 31 mils, or
.031". Since the gauge has an accuracy of ±.001" the reading is correctly stated
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Range : 0-10 mm
Resolution : 0.01 mm
Anvil Flatness : 5 µm
Throat Deep : 30 mm
Figure 2.2 :: Mitutoyo thickness gauge which has flat anvils top and bottom Model 7301
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as "10 mils plus or minus one mil". This, of course means that the actual
thickness might be as little as 9 mils or as much as 11 mils. There is no way to
ascertain this fact. Now the process is repeated 3 times with the same sample to
get the same reading. It should be noted that a "mil" is not the same as a
millimeter (mm). They are entirely different dimensions. If the gauge is metric,
and the graduations are in 0.01 mm and every ten graduations are equal to 0.10
mm.
2.4 X-Ray Diffraction:
After the discovery of X-ray diffraction by Von Laue in 1912, a treasure of
important information is obtained by using X-ray diffraction in science and
industry. The diffraction studies help to determine the arrangement and the
spacing of atoms in crystalline materials. These studies have contributed
immensely in understanding the physical properties of metals, polymeric
materials and other solids. The X-ray diffraction method primarily aid to chemical
analysis in the identification of compounds present from their diffraction patterns
and to determine the relative concentrations by the intensities of pattern lines.
W.L. Bragg showed that the x-rays reflected from a lattice plane and the effect
associated with it could be analyzed by the equation.2.1
2 d Sin θ = n λ (Bragg’s law) --------- (2.1)
In which ‘n’ is an integer (the “order”), “λ” the wavelength of the X-rays‘d’ the
inter-planar spacing and “θ” is the angle of incidence of the X-ray beam on the
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lattice plane. The schematic diagram of a general representation of Bragg’s X-
Ray condition is shown in figure.2.3.
In powdered crystal, the millions of tiny crystals are oriented at random in
all directions; all possible diffraction planes will be available for Bragg reflection to
take place. If an X-ray beam strikes such a powdered sample and an X-ray
diffraction pattern is obtained then Bragg’s law is simultaneously satisfied. It is
ascertained that the X-ray beam passes directly through the sample and all
possible planes are exposed to the beam. Also, care is taken that all diffracted
beams are collected by a detector (scintillation counter), which is rotated by 2θ.
Its output is processed before feeding into an automatic recorder. The result is a
graph which gives a record of counts per second (proportional to diffracted beam
intensity) as a function of diffraction angle 2θ. In the present study X-ray
diffraction analysis of all polymer samples are carried out by using SEIFERT X-
ray diffractometer (Cu Kα radiation). The block diagram of SEIFERT X-ray
diffractometer used in the present study is shown in figure 2.4.
2.5. Infrared spectroscopy:
The identification of the functional groups in organic compounds is
extensively done using Infrared spectroscopy [12-14]. As their spectra are
generally complex and comprises of numerous maxima and minima which are
useful for comparison of organic compounds. In fact, some of the physical
properties of any organic compound can be truly inferred from its infrared
absorption spectrum. This method functions on the fact that atoms in the
molecule vibrate relative to each other with very well defined frequencies for a
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Figure 2.3 :: General representation of Bragg’s law
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Figure 2.4 :: SEIFERT X-ray diffractometer (Dept of Physics, OU)
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given set of atoms or functional groups. The molecule is irradiated with infrared
radiation of different photon energies as a function of wavelengths.
When the energy needed to increase a molecular vibration one level up
matches with the photon energy of irradiation, it is absorbed by the molecule
while photons of mismatch energies pass through the molecule. An absorption
spectrum is obtained by measuring the transmitted light intensity for a broad
range of wavelengths. The intensity of the transmitted light (the transmittance) is
depicted as a function of the wave-number in cm-1. To assist the chemical
identification of a sample the observed absorption peaks (depicted as a decrease
in transmittance) between 4000– 400 cm-1 may be assigned to different functional
groups in the sample.
In the present study FTIR spectra of all polymer electrolytes were recorded
on Bruker Optics spectrophotometer, Germany Model: Tensor 27 in the wave
number range 400 – 4000 cm-1. The schematic diagram of spectrophotometer
used in the present study is shown in figure 2.5. The figure 2.6 shows the
Bruker Optics spectrophotometer, Germany Model: Tensor 27.
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Figure 2.5 :: The schematic diagram of a general spectrophotometer
Figure 2.6 :: FTIR spectrometer Bruker Optics, Germany Model: Tensor 27: CFRD, OU.
Sampling
Area Monochromator
Detector
Recorder Amplifier
Radiation
Source
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2.6. Scanning electron microscope (SEM):
Scanning Electron Microscopy producing higher resolution images is used
to analyze the surface morphology of solid objects. SEM produces three-
dimensional representations of samples for a diverse range of materials. Back-
scatter and cathode luminescence (SEM-CL) are the basic principles used in
Micro- structural characterization to evaluate a wide range of samples such as
polymers, films, coatings, geological and mineral core samples, pharmaceuticals,
raw materials, metals, plastics, ceramics, glass, food, dust and contaminants.
SEM studies provide elemental analysis of samples, impurities and the
identification of physical and chemical defects. SEM studies also support surface
analysis, study of the morphological features of materials, failure analysis,
troubleshooting, quality control and other requirements. The equipment used to
obtain the SEM micrographs is shown in figure.2.7.
2.7 Differential Scanning Calorimetry (DSC):
The differential scanning calorimeter DSC determines the temperature and
the heat flow associated with sample transitions as a function of time and
temperature [15, 16]. It also provides quantitative and qualitative data on
endothermic (heat absorption) and exothermic (heat evolution) process of
materials during physical transitions that are caused by phase changes, melting,
oxidation and other heat related changes. This information helps the scientist or
an engineer to identify the processing and end-use performance. The DSC
instrument works in conjunction with a controller and associated software to
make up a thermal analysis system. The DSC has two major parts: The DSC
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instrument, which contain its own thermocouples (temperature sensors) for
monitoring differential heat flow and temperatures.
The DSC cell is used to measure differential heat flow. The test sample and
reference materials are placed in pans that sit or raised platforms on a
constantan disc, and heat is transferred through the disc up into the sample and
reference. The differential heat flow is monitored by thermocouples beneath the
disc. In the present investigation DSC (SHIMADZU, DSC-60) instrument is used
to study the polymer complexes [17]. The photograph of DSC is shown in
figure.2.8.
2.8 Optical absorption spectroscopy:
Optical absorption spectroscopy using UV rays is a useful technique for
characterization of organic polymers. The absorption spectra of poly vinyl alcohol
and its dopants (Oxalic, Malonic and Succinic acid) are dissolved in a suitable
solvent gives qualitative interpretation regarding the level of doping, extent of
conjugation in the polymer chain. The conductor to insulator transition arising can
be clearly monitored using this technique. Further, UV spectroscopy helps to
understand the inter conversion of different oxidation states of PVA and allows
correlating the electrical conductivity with the observed change in the absorbance
values. Also from the spectral data optical constants such as optical band gap
(both direct and indirect) can be determined.
In the present study the optical absorption spectra of pure PVA and PVA
doped with Oxalic, Malonic and Succinic acid with different concentrations were
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carried out at room temperature over the wavelength range 190- 800 nm using
LABINDIA UV-3000+ model as shown in figure.2.9.
Figure 2.7 :: Scanning Electron microscope (Dept. of Physics, OU)
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Figure 2.8::DSC (SHIMADZU,DSC-60) instrument:College of Technology,OU
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Figure 2.9 ::UV-VIS Spectro Photometer (LABINDIA UV- 3000+ Model):CFRD,OU
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2.9 D.C. Conductivity Measurements:
The experimental set up used for conductivity measurements is
shown in fig 2.10 and fig 2.11. A sample holder is fabricated and used for the
conductivity measurements. It consists of two electrodes A and B (blocks of
nickel), the lower one being circular one of diameter 2 cm and the upper one
being square of 1.2 cm2 surface area. The lower circular electrode is supported
on a single centrally placed “Viterosil” cylinder of 2 cm length. This Viterosil is a
good insulator even at high temperature. The lower end of Viterosil Cylinder is
fixed to the steel frame F. The upper electrode B is also fixed to the Viterosil
cylinder of length 2 cms. The other end of the Viterosil cylinder is inserted in a
socket which is the end point of a steel rod “R” of about 8 cm length. This rod is
arranged such that it can move up and down through a groove made in a steel
frame. The steel frame consists of a circular steel plate supporting the lower
electrode and another circular plate at the top as shown in figure 2.9. The two
plates are held in their position by three steel rods. At the end of the steel rod a
spring is provided so that a well pressed contact is given to the sample under test
“c”. This spring is just outside the furnace and retains the spring action even
though it acquires certain temperature. Two flexible chromyl wires are brazed in
to two nickel electrodes and are brought out of the furnace. The two leads are
fixed to amphienel connections J1 and J2. The two wires are shielded by fine
bored alumina tubes which prevent the wires from touching the steel frame. “S” is
a thin copper foil which is earthen and acts as a shield. The furnace is a
resistance type furnace and consists of an alumina muffle of 30 cm long and 10
cm in diameter. The heating element is super kanthol carbon, wound on the
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muffle. The windings are such that the numbers of windings are maximum at the
place where the sample is situated. The temperature of the furnace has been
controlled by a temperature controller. The outer surface of the furnace is
covered with a metallic sheet which in turn is earthen in order to avoid pick up
currents.
The stainless steel frame “F” hangs in to muffle of the furnace over a steel
rod support. The points which are connected to the electrometer are fixed to a
Perspex box as shown in fig. 2.10. The battery is arranged in a separate box. The
box is thermally insulated. The temperature of the sample is measured by means
of choromel/ alumel thermocouple arranged very close to the sample and the
output of the thermocouple is fed to the digital temperature controller. The
currents are measured by means of Keithley electrometer [model No. 196]. The
polarizing effects during the measurements of dc conductivity were minimized by
applying smaller voltages for a short interval of time and short-circuiting after
every reading.
Figure 2.10 :: Block diaagram of exm
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xperimentmeasureme
al set-up uents
used for coonductivity
y
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Figure 2.11 :: Keithley electrometer [model 196]. Dept of Physics, OU
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[14] W.Brugel, An introduction to Infrared spectroscopy, London: Methuen and Co Ltd; 1962 [15] E.S.Watson, US patent No: 3,263,484
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[17] DSC 2010 operations manual; PN 925604.001 REW .D (Text and Binder), PN 925604.002 REW. D (Text only) issues June 1997