Chapter II
CHARACTERIZATION AND EXPERIMENTAL TECHNIQUES
In this chapter, details on the experimental techniques involved in the preparation of pure polymer and doping, their characterization, measurement of electrical, optical and thermal properties have been explained.
2.1. MATERIALS SELECTED:The polymer used for the present study is poly(4-vinylpyridine). The dispersants & dopants used are p-toluene sulfonic acid, Iodine, Nickel Phthalocyanine & Copper Phthalocyanine.
2.1.1 Poly (4-vinylpyridine)(P4VP) : The polymer can be synthesized by free radical polymerization or ionic polymerization of 4-vinylpyridine.
The polymer is weakly polar having basic
pyridine as a branched functional group and provides source for various applications like solid state electrochemical cells, solar cells and sensors[l-5]. The structure is as follows:
Molecular formula: C7H7N. The molecular weight of monomer =105. The N atom having lone pair of electrons can form charge transfer complex with iodine and enhance the conducting properties [6-8].
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2.1.2. P-Toluene sulfonic acid (4-methyl benzene sulfonic acid)(PTSA): -
Molecular weight: 172 P-toulene sulfonic acid(PTSA) is precipitated by sulfonation of toluene with 96100% H2S04, carried out at 75°C. It is a monoclinic leaflet or prism type structure, crystallizes from H20 forming tetrahydro PTSA. But when anhydrous, its M.P. is 106-107°C. It is freely soluble in water & used in dye chemicals, oral antidiabetic drugs [9]. It is used as stabilizer or plasticizer for many polymers. It also enhances the conductivity of the polymer.
2.1.3 Iodine (I2): Iodine exists as diatomic covalent molecule and volatile in nature. The M.P. is 386K. Its oxidation states are -1, +1, +3, +5 & +7. It is steel grey solid, which sublimes on heating.
It is sparingly soluble in water but dissolves in sodium or
potassium iodide solution. It diffuses easily into polymer matrix and serves as most common dopant for polymers.
It enhances the conductivity of conjugated as well as saturated
polymers due to the formation of charge transfer complex.
2.1.4 Cu-Phthalocyanine(CuPc) & Ni-Phthalocyanine(NiPc): Phthalocyanines constitute an important class of synthetic pigments and dyes. They are colored blue to green and are stable to light, heat, acids and alkalies. Phthalocyanines contain tetra benzotetra azuporphyrin structure with or without the presence of metals like copper, cobalt, nickel, iron etc. Cu-Phthalocyanine & Ni-Phthalocyanine (CuPc & NiPc) are chosen for the present study. CuPc has a four fold axis of symmetry
in common with porphyn system, and has two
absorption bands in or close to visible region designated as Q and B-bands.
32
CuPc
NiPc
The Q band is the most important & most intense occurring at 657.6nm and Bband at 325nm makes no contribution to the blue color of the pigment. NiPc absorbs at 651nm. The Q band arises because of the loss of four-fold axis of symmetry. X-ray diffraction studies show that NiPc & CuPc are planar. CuPc sublimes at 580°C without any change and dissolves in cone. H2S04 [10,11].
2.2 PREPARATION OF POLY(4-VINYLPYRIDINE): Poly (4vinylpyridine) used for the present study is synthesized in the laboratory by the free radical polymerization of 4-vinylpyridine using bulk polymerization technique^ 2]. Monomer 4vinylpyridine is obtained from Lanchester Chemicals (UK) and purified by double distillation under low pressure. The inhibitor free monomer is then polymerized by bulk polymerization technique using azobisisobutaronitrile(AIBN) as initiator. The reaction is carried out at 30IK for 24 hours.
The solid mass obtained is dissolved in distilled methanol and precipitated by double distilled water. The precipitate is dried in oven at 333K for 48 hours to get 33
granules. The granules are kept in vacuum desiccator at 10'3 torr to remove traces of solvent and moisture. It is then stored in vacuum desiccator.
2.3 CHARACTERIZATION OF POLY(4-VINYLPYRIDINE):The polymer is characterized by viscosity, DSC, NMR, FTIR and XRD methods.
2.3.1. Viscosity Measurement : The viscosity average molecular weight of P4VP is found out from Mark-Houwink Sakurada equation [13].
[77] = K.Mva ........................ (2.1) The polymer solution is prepared in solvent ethanol [92:8]. The concentration is varied from 0.643 gms/dl to 0.2143 gms/dl. The intrinsic viscosity [p] is found to be pint= 3.85. The M.H.S constants for P4VP in ethanol [92:8] are [14], K=12.0xl0'5 dl/g, and a = 0.73 The Molecular weight of P4VP is found to be Mv « 14.8x105.
2.3.2. Nuclear Magnetic Resonance Spectrum (NMR) : The NMR spectrum of pure P4VP synthesized is given fig.2.1. Aromatic protons of the pyridine group appear at 8.32ppm and 6.41 ppm. The aliphatic CH-CH2 protons appear near 1.613ppm and 1.752ppm along with solvent. The NMR of P4VP fairly matches with the NMR spectrum found in literature.
2.3.3. Differential Scanning calorimetry (DSC): The differential scanning calorimetry is done by using Shimadzu DSC 50 analyzer with heating rate of lOK/min and for the temperature range 300-473K. It is well known that the heat flow rate (Cp) against temperature (T) of the sample undergoes a change during glass transition. The DSC of pure poly(4vinylpyridine) is given in fig.2.2. From the figure, the Tg of P4VP is determined by the change in 34
endothermic
Fig 2.1 NMR spectrum of Poly(4-vinylpyridine).
specific heat resulting in an endothermic peak found at 43 IK. The reported Tg of P4VP is 425K[13]. The higher Tg indicates higher molecular weight.
2.3.4. Infra Red-spectrum : The IR spectrum of P4VP is found to be similar to the IR spectrum reported in literature [15]. Th spectrum is given in fig.2.3. Different peaks and their characteristics are given below. 3405.67
- OH- stretching
3026.73
- CH-stretch of Pyridine ring
2932.23 - CH- stretch of backbone chain of polymer 2856.06 - CH-stretch of backbone chain of polymer 1601.59 - C=N strech of pyridine ring 1557.26 - C=C stretch of pyridine ring 1451.17 - CH deformation bending (back bone) 998.95 and 823.46 - CH- deformation
2.3.5. XRD: XRD is done by Scintag 2000 X -ray difractometer with scan rate of 4 degree/min and multiple
step of 0.03
for the
range
20
=10-80°
(fig.2.4).
The
crystalline/amorphous nature of the polymer has been tested by diffraction pattern. A broad peak is observed between 20 ~ 10-30°, related to the amorphicity of the polymer [16].
2.4 DOPING TECHNIQUES: Different doping techniques are described in section 1.2, out of which solution doping is the most versatile method for saturated polymers and is employed for the present study. The technique enables uniformity & homogeneity in distribution of dopant molecules in polymer matrix. The formation of charge transfer complex is easier in solution medium. The doping of polymers and the preparation of doped films are as follows:
35
4000
3000 2000 wave number(cm'')
1000
Fig2.3 FTIR ofP4VP.
o
-p *
oM
(N
n
O
O
to
CD
o
o
00
o
to
o
O
intensity
o^
opr
n
©
00
Fig. 2.4 XRD of P4VP.
A known quantity of polymer is dissolved in double distilled dimethyl formamide(DMF). Dopant and stabilizer are added in different proportions in order to get different doping percentage compositions. The solution is stirred well by magnetic stirrer for 6 hours and poured in to flat bottomed petridish. The films are dried in oven at 35°C for 48 hours and then by vacuum at 10'3 torr. The thick films are removed from petridish and conductive grade silver paste is used for making electrical contact. Thin films of thickness within the range 20 ~ 40pm are used for optical and electrical studies. These films are prepared by isothermal immersion technique [17] and by film casting method for dispersion systems. The details of the Isothermal immersion technique is as follows: Pre-aluminum electrode coated glass substrate is dipped in a well-stirred polymer solution, for a fixed time, which is kept in the thermostat and then removed. Film is then dried in oven for 48 hours at 35°C. A layer of Al-electrode is coated over this film by vacuum evaporation method. The sandwiched M-P-M structure is used for resistivity, dielectric and I-V- measurements.
2.5 EXPERIMENTAL TECHNIQUES:-
2.5.1 Film thickness measurement: The thickness of the polymer film can be measured by gravimetric method, capacitance method and mechanical stylus method. The thickness in the micron range can be directly measured by Mitutoyo Digit outside Micro meter(Japan) to the accuracy of ± 2pm. In the present study, the above instrument is used for the measurement of thickness and further confirmation is done by mechanical stylus method.
The thickness of the film used for different studies is mentioned in
respective chapters.
36
2.5.2 Differential scanning calorimetry(DSC), Thermo gravimetric analysis (TGA) and Differential thermo gravimetry (DTG): DSC experiments are done as described in the section 2.3.3. Thermo gravimetric analysis of the pure and complex samples is done by Mettler TA 4000 thermal analyzer. About lOmg of the dried sample in the fine powdered form is taken in a crucible and heated at a heating rate of 10°C /minute in the range 25-600°C for P4VP, P4VP/PTSA samples and 25-800° C for complexes. The sample is held at 50°C for one minute in order to remove the traces of solvent and moisture. The DTG curves are obtained from the plots of dw/dt as a function of temperature .
2.5.3. Optical properties (UV-VIS and FTIR): The optical absorption spectrum of different films is measured by Shimadzu UVVISIBLE recording Spectrophotometer, Model 240, in the range 190-700nm and SECOMAM UV-VIS spectrophotometer in the range 200-1 OOOnm (The spectrum is obtained by Dathelie software). The thickness of the complex films used was ~20pm. For pure polymers and dopants dilute solution in DMF was used to obtain the spectra. FTIR studies are done by JASCO FTIR analyzer. The powder sample is well mixed with the spectroscopy grade KBr and the mixture is taken in the pellet form to obtain FTIR spectrum in the range 400 - 4000cm'1.
2.5.4. D.C. conductivity: D.C. conductivity is measured by two-probe method by measuring resistance of the specimen in a vacuum jig, which is connected to a rotary pump of 10'3 torr. The schematic diagram of the jig is given in fig 2.5. The jig is connected by a CuConstantan
thermocouple
to
measure
the
change
in
temperature.
The
d.c.conductivity data is obtained by Keithley 617 programmable electrometer in the temperature range 300K to 440K at a heating rate of lOK/min.
37
TEFLON AMPHENOLS
Fig. 2.5 : Schematic sketch of Experimental Jig used for electrical measurements.
The resistance of the conducting complex is directly obtained and d.c. conductivity is calculated by ohms law. All connections are perfectly insulated by teflon tapes and amphenols and uniform pressure contact is given to the measuring area of the films.
2.5.5.1-V Characteristics: Current-Voltage characteristics is determined by Keithley 236 I-V source measure unit. The software used to obtain spectrum was METRICES-ICS (Interactive characterization software). The current is measured (by keeping the sample in resister mode) for a voltage sweep of 0-100volts in multiple steps of 2 volts. The dependence of I-V property on temperature is determined by keeping the sample in the jig and varying the temperature from 300-440K. The temperature is measured by Cu-Constantan thermocouple, which is connected to digital multimeter. All connections are perfectly insulated by Teflon tape.
2.5.6. Dielectric properties: Dielectric datas are obtained using GenRad Digibridge (model 1689) in the frequency range 50 Hz - 100kHz and in the temperature range 300-450K. The above mentioned jig is used to create vacuum of 10'3 torr and to make external connections. The specimen is kept under uniform pressure contact to avoid stray capacitance. The temperature is monitored by Cu- Constantan thermocouple kept in close vicinity of the sample.
2.5.7. TSDC: Thermally stimulated depolarization current was measured in case of pure P4VP and P4VP/PTSA complexes. Thin films of about 18-20pm thickness of the polymer/PTSA complex were grown on vacuum coated aluminum electrodes on a glass substrate by isothermal immersion technique. The films were dried in oven at
38
40°C and vacuum dried for 24 hours at 10'3 Torr. Aluminum electrodes were vacuum coated on to these films. The area of the polymer covered by aluminum electrode was 9mm2. The M-P-M sandwich structure was subjected to a d.c electric field of 5kV/cm for a constant poling time of one hour and at different polarizing temperatures. The poled sample was then subjected to heating at a rate of 10-12K/min and the corresponding depolarizing current was noted on an Electrometer Amplifier (EA815 made by ECIL). The change in temperature was measured using a Cu-Constantan thermocouple kept in close contact with the specimen. All measurements are carried out in vacuum of 10'3 Torr.
2.5.8. Electron Irradiation studies: Irradiation of pure polymer & complexes were done at Microtron center, using the electron beam (by lanthanum hexa fluorite source). The monochromatic beam was made to fall on sample kept at particular distance and the following beam parameters are maintained:
i) Beam energy = 8 meV ii) Beam current = 15mA - 20mA iii) Pulse repetition rate = 5Hz. iv) Pulse width = 15- 2p sec. v) Target to sample distance = 30 cms vi) Time of exposure = 1-4 hours.
The dose delivered to different polymer complexes is measured by keeping alanine dosimeter
with
sample
during
irradiation
spectrophotometric method.
39
and
analyzing
them
using
2.6 ACCURACY OF THE MEASUREMENTS: -
For measurement of I/V characteristics the accuracy is as follows. For current range ± 1 nA
w ± (0.3% + 100fA)2
For current range ±10nA
« ±(0.3%+lpA)
For Current range ±100nA~ ±(0.21% + 6pA) For Current range ± 1 p A
» ± (0.04% + 60pA)
The accuracy of measurement of resistance is as follows: For resistance range:
GQ
«
1.5%
200 Mn 20 MQ
0.3% »
0.25%
2MQ
0.25%
200 kQ
0.25%
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2.7. REFERENCES: [1] Y.Sakai, Y.Sadaoka & H.Fukumoto, Sens. & Actu. 13 (1988) 243. [2] B.A.Gregg & A.Heller, Anal. Chem. 62 (1990) 258. [3] Y.I.Degani, & A.Heller, J.Am.Chem.Soc.,111(1988) 2387. [4] P.V.Kamat & M.A.Fox, J.Electro. Chem.Soc ; Electrochem. Sci., & Tech. 131 (5)(1984) 1032. [5] P.V.Kamat & M.A.Fox, Chem.Phys. lettrs. 92 (1982) 595. [6] H.Sakai, T.Matsuyama, Y.Maeda & H.Yamaoka, J.Chem. Phys. 75 (10) (1981) 15. [7] H.M.Sakai, Y.Maeda, S.Ichiba & H.Nagitha, J.Chem. Phys. 72 (1980) 6192. [8] R.I.Stankovic, R.W.Lenz, & F.E.Karasz, Eur.Polym.J.,26 (1990) 359. [9] M.Nindholz, S. Budavari, R.F. Blumetti & E.S.Oherbein “The Merck Index” Merck & Co., Inc, Rahway,N.J. U.S.A.10 (1983) 1364. [10] O.D.Tyagi & M.Yadav “ A text book of synthetic dyes” Anmol Publications, New Delhi (1993). [11] G.R.Chatwal “Synthetic Dyes” Himalaya Publishing House, Bombay (1990). [12] M.Chanda, O.F.O’Driscoll & G.L.Rempel, J.Mol.Catal. 7 (1980) 389. [13] J.Brandrup & E.H.Immergut “Polymer Handbook” John Wiley & Sons, New York,(1975). [14] A.G.Boyes, U.P.Strauss, J.Polym. Sci. 22 (1956) 463. [15] M. Audenaert, G. Gusman, M. Mehbod, R. Deltour, B. Noirhomme & E. V. Donckt, Solid St. Commun., 30 (1979) 797. [16] P.G. Roth & F.J. Boerio. J. Polym. Sci. Part B: Polym Physics, 25 (1987) 1923. [17] A.C. Rastogi & K.L.Chopra, Thin Solid films, 18 (1973) 187.
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