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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,

     

24  

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

     

25  

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

     

26  

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

     

28  

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

     

29  

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

     

30  

Figure 2.3 :: General representation of Bragg’s law

     

31  

Figure 2.4 :: SEIFERT X-ray diffractometer (Dept of Physics, OU)

     

32  

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.

     

33  

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 

     

34  

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

     

35  

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

     

36  

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)

     

37  

Figure 2.8::DSC (SHIMADZU,DSC-60) instrument:College of Technology,OU

     

38  

Figure 2.9 ::UV-VIS Spectro Photometer (LABINDIA UV- 3000+ Model):CFRD,OU

     

39  

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

     

40  

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

41

xperimentmeasureme

al set-up uents

used for coonductivity

y

     

42  

Figure 2.11 :: Keithley electrometer [model 196]. Dept of Physics, OU

     

43  

References

[1] Gregov.L.V., Hass.G. and Thun.R.E.,. Physics of thin films (eds.) Vol.3(1966)144.

[2] Jackson.G.H., Thin solid films, 5(1970)209.

[3] White.J.B. and Wagner.C. Thin Solid Films 18(1973)157.

[4] Mana.E.B., and Durow.L.K., Impedance spectroscopy Emphsizing Solid material and system, Wiley, New York 1987. [5] MacCallum J R and Vincent C A (ed)., Polymer Electrolytes Review-I,2 (1987/1989) (London: Elsevier).

[6] Gray F M., Solid Polymer Electrolytes,(1991) (New York:VCH).

[7] Skotheim T A (ed)., Electroresponsive Molecular and Polymeric Systems vol 1(1988) (New York: Dekker). [8] Ch.V.Subbareddy, A.K.Sharma, V.V.R.Narasimha Rao; J.Polymer 47 (2006) 1318

[9] S.Rajendran, M.Sivakumar, R.Subadevi; J. Power sources 124 (2003) 225

[10] R.Baskaran, S.Selvasekharapandian, G.Hiran Kumar, M.S.Bhuvaneswari; Ionics 10 (2004) 129 [11] M.Jaipal Reddy, T.Sreekanth, M.Chandrasekhar, U.V.Subbarao; J.Mater.Sci 35 (2000) 2841-2845

[12] H.M.Randall, R.G.Fowler, N.Fuson and J.R.Dangl, Infrared determination of organic structures; New York: D.Van Norstand, 1949 [13] R.P. Bauman, John Wiley and Sons, Absorption Spectroscopy, New York Inc., 1962

     

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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

[16] M. J. O'Neill (1964). "The Analysis of a Temperature-Controlled Scanning Calorimeter". Anal.Chem. 36 (7): 1238–1245

[17] DSC 2010 operations manual; PN 925604.001 REW .D (Text and Binder), PN 925604.002 REW. D (Text only) issues June 1997