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Electret studies of polyvinyl chloride
Solid State Communications, Vol. 17, pp. 1343—1346, 1975.
Pergainon Press.
Printed in Great Britain
ELECTRET STUDIES OF POLYVINYL CHLORIDE I.M. Taiwar Department of Physics, Punjab Agril. University, Ludhiana-141004 (Pb), India (Received 14 March 1975 by R. Fieschi)
Both types of charges hetero and homo are present in the polyvinyl. chloride thermoelectrets. The results have been explained on the basis of a qualitative model. A new phenomenon of triple charge reversal from hetero to homo, homo to hetero and finally from homo to hetero charge, in a few cases, is reported. —
—
—
THE ELECTRET phenomenon has come to stay permanently as an improtant branch of solid state physics, but so far this has not been understood completely. In an earlier communication’ the polarizing field on polyvinyichloride was varied from 2.25 to 22.5 kV/cm. In the present studies three field values are selected two much higher than those already used (35 and 50kV/cm) and the third one at 10 kV/cm intermediate in the range already used; five series of electrets at these field values were prepared at temperatures 65, 85, 105, 115 and 125°C both below and above the softening of the material (80—90°C).’
The time of change of polarity depends on the time elapsed from the end of polarization to the removal of electrodes.3 Thus the electrodes were removed, from the electret surfaces, immediately after polarization, and their charge measured immediately with the improved charge measuring assembly4 using electrostatic induction method. The polarized specimens (electrets) were then stored in a shortcircuited state in a vacuum dessicator filled with calcium chloride to keep the electrets free from humidity. Subsequently the charge of each electret was measured after 24 hr for 30 days.
—
—
—
1. RESULTS AND DISCUSSION The material used was commercially available polyvinylchloride (PVC) in the form of sheet 0.4mm thick, from MIS Bhor Industries, Bombay. All the samples had an area of 1 cm2. Full details of preparation of specimens have been described elsewhere.2 To attain thermal equilibrium, the specimen was kept in a thermostat, maintammg temperature to ±1 C for ~ hr before the field was applied. Heating was continued for 4hr and then the specimen was cooled for 6 hr more, thus the field was on for 10 hr. The temperature was decreased by equal amounts in equal intervals of time, until the room temperature was attained in 5~hr. For the next ~ hr the thermostat was kept off. This controlled rate of cooling ensured uniform polarization of the sample. Tin foil was used as polarizing electrodes.
It is observed that the final charge is homocharge on both the faces of electrets; though the nature of the initial charge depends upon the polarizing parameters. With 10 kV/cm, the initial charge, in conformity with earlier findings,’ is homocharge, whereas at other two field values this is heterocharge, the value increasing with polarizing field at all the polarizing temperatures. An altogether new phenomenon, at the field value of 35kV/cm at all the temperatures except at 125°C,of triple reversal from hetero to homo, homo to hetero and finally to homocharge takes place. This was confirmed by repeating the observations many times. Only a representative graph is shown in Fig. 1. It is well known that most of the dipolar amorphous polymers (like PVC) exhibit multiple 1343
1344
ELECTRET STUDIES OF POLYVINYL CHLORIDE
Vol. 17, No. 11
8-
2C TPME (OA~5)
FIG I. Variation of surface charge vs time for PVC electret formed at 85°Cwith polarizing field of 35 kV/cm (triple reversal is obvious). dielectric relaxation process,5 which may be the casue of this anamolous phenomenon, but no plausible explanation can yet be presented. The shape and position of peaks in thermally stimulated current of the electrets may give activation parameters of the charges responsible for this triple reversal. These studies, which have already been carried out on different systems61°will be taken up shortly on PVC sheets. Thus both hetero- and homo-charges are present in PVC. As a pedagogic explanation, heterocharge in a polar dielectric, like PVC. can be attributed to orientation of the dipolar groups, which give rise to co-operatively polarized aggregates superficially analgous to domains in ferromagnetism. But an X-ray study of polarized specimens failed to detect the expected structural change in them,1’ ruling out this possibility. Furthermore, at high fields, Maxwell— Wagner effect12 may also arise at the interface of the textural elements of the polymer giving rise to heterocharge. The presence of Maxwell—Wagner polarization in PVC has been confirmed even above Tg by Hedvig.13 Polarized regions then attract the ions of opposite polarity from either the bulk of the material or those sprayed from the breakdown of the dielectric —electrode interface. These ions settle on the surface. when the matrix gets solidified, giving rise to hornocharge. The mechanism of ionic conduction in PVC. confirmed by several workers.14 justifies the existence
of the excessive ions in the bulk of the material. Under the impact of heating these ions may get trapped near the surface’5 of the electret giving rise to homocharge. This is further confirmed by the large value of activation energy (~ 1 .5 eV) for PVC, determined from log conductivity (temperature) curves,16 associated with ions trapped in partially crystalline regions. —
The incompatibility of the presence of heterocharge possibly due to dipolar orientation of the polar material and the absence of explicit detection by the X-ray studies can be resolved by a qualitative model already suggested,4 after Caserta and Serra,17 based upon band theory of solids, in which some of the charges suffer microscopic displacement during polarization and are trapped. There is a distribution of localized energy levels in the bulk of the material, as well as in the forbidden gap.18 The plasticizer and fillers used in the preparation of PVC sheet may give rise to these energy levels electron traps at the conduction band and hole traps at the valence band. The polarization P, due to these energy levels may be assumed to be proportional to their number Nd i.e. —
P~Nd. Electrons and holes are injected from the electrodes.’9 With the changed history of thermal treatment of the material, in the presence of electric field, there is a change in the state of these energy levels brought
Vol. 17, No. 11
ELECTRET STUDIES OF POLYVINYL CHLORIDE
about by the mobility of the injected charge carriers, The difference in the magnitude and nature of the injected charge carriers by the different types of electrode, and consequent change in the state of the energy levels existing in the material is reflected in the difference in magnitude and sign of initial and final charge on the electrets prepared under identical polarizing conditions but with different electrodes.’ The presence of hetero- and homo-charge on both the faces supports the injections of electrons and holes, When the specimen is heated with high forming field, charge carriers are produced from the electrodes! dielectric—electrode interface breakdown. Simultaneously free charge carriers are also produced from the valence band. On cooling the sample, these free charges get trapped near the electrodes, forming a charge layer of the same sign as of the polarizing electrode, giving rise to homocharge. The electrons arising out of valence band are trapped in the deep
1345
electron traps and the holes in the deep hole traps forming a pair of opposite charges, which is oriented by the applied field. Thus the heterocharge, involving the bulk of the material takes place. Since the external field of these dipoles would tend to annul the applied field, little alignment of the dipoles would be detectable2°as in the present case. The final net homocharge persists, the injected charges being dominant. At high temperatures, the greater polarizing field ejects out more electrons and holes from the valence band to be trapped in electron and hole traps giving rise to more induced dipoles and hence more heterocharge with high field, in conformity with earlier COfllmiinications.1’2’4 Binder,2’ by dusting the surface with suitable powder, has shown that homocharge produced on PVC thermoelectrets is due to discharge between the electrode—dielectric interface.
REFERENCES 1.
TALWAR I.M. & BHAWALKAR D.R., md. J. Pure. App!. Phys. 7,685 (1969).
2.
TALWAR I.M., Ph.D. Thesis, University of Sagar, Sagar, India (1968).
3.
PIECH T. & HANDEREK 3., Phys. Status Solidi 9, 361 (1965).
4.
TALWAR I.M.,Ind. J. PureAppi. Phys. 11,334(1973).
5.
ISHIDAY.,J. Polymer Sci. A2, 1835 (1969).
6.
PILLAI P.K.C., JAIN K. & JAIN V.K., Phys. Lett. 39A, 216 (1962).
7. 8.
LATOUR M., Thesis, Montpellier, France (1972). HEDVIG P. & FOLDES E., Die Ang. Makromol. Chem. 35, 147 (1974).
9.
VANDERSCHUEREN J., Thesis, Liege, Belgium (1974).
10.
VAN TURNHOUT J., Thermally Stimulated Discharge ofPolymer Electrets, Elsevier, Amsterdam/Oxford/NY (1975).
11. 12.
TALWAR I.M. & KHANNA S.L., md. J. PureAppi. Phys. 8,851(1970). HASTED J.B., Aqueous Dielectrics, p. 126. Chapman and Hall, London (1973).
13. 14.
HEDVIG P., mt. Microsymposium on Polarization and Electrical Conduction in Polymers, Bratislava, Czechosolovakia (May, 1972). KOSAKI M., SUGIYAMA K. & IEDAM.,J. App!. Phys. 42, 3388 (1971).
15. 16.
IEDA M., KOSAKI M., OSHIMA H. & SHINOHRA U., J. Phys. Soc. Japan 25, 1742 (1968). TALWAR I.M. & BHAWALKAR D.R., md. J. Pure App!. Phys. 7,681(1969).
17.
CASERTAG. & SERRAA.,J. App!. Phys. 42, 3778 (1971).
18.
MOTT N,S. & ALLGAIER R.S., Phys. Status Solidi 21, 343 (1967).
1346
ELECTRET STUDIES OF POLYVINYL CHLORIDE
19. 20.
BOGRODITISKII N.P., TAIROVA D.A. & SOROKIN V.S., Fiz. Tverd. Tela 6,2301(1954). GERSON R. & ROHRBAUGH J.H., J. Chem. Phys. 23. 2381 (1955).
21.
BINDER F.,Naturf 6a, 714 (1952).
Vol. 17, No. 11
Pergainon Press.
Printed in Great Britain
ELECTRET STUDIES OF POLYVINYL CHLORIDE I.M. Taiwar Department of Physics, Punjab Agril. University, Ludhiana-141004 (Pb), India (Received 14 March 1975 by R. Fieschi)
Both types of charges hetero and homo are present in the polyvinyl. chloride thermoelectrets. The results have been explained on the basis of a qualitative model. A new phenomenon of triple charge reversal from hetero to homo, homo to hetero and finally from homo to hetero charge, in a few cases, is reported. —
—
—
THE ELECTRET phenomenon has come to stay permanently as an improtant branch of solid state physics, but so far this has not been understood completely. In an earlier communication’ the polarizing field on polyvinyichloride was varied from 2.25 to 22.5 kV/cm. In the present studies three field values are selected two much higher than those already used (35 and 50kV/cm) and the third one at 10 kV/cm intermediate in the range already used; five series of electrets at these field values were prepared at temperatures 65, 85, 105, 115 and 125°C both below and above the softening of the material (80—90°C).’
The time of change of polarity depends on the time elapsed from the end of polarization to the removal of electrodes.3 Thus the electrodes were removed, from the electret surfaces, immediately after polarization, and their charge measured immediately with the improved charge measuring assembly4 using electrostatic induction method. The polarized specimens (electrets) were then stored in a shortcircuited state in a vacuum dessicator filled with calcium chloride to keep the electrets free from humidity. Subsequently the charge of each electret was measured after 24 hr for 30 days.
—
—
—
1. RESULTS AND DISCUSSION The material used was commercially available polyvinylchloride (PVC) in the form of sheet 0.4mm thick, from MIS Bhor Industries, Bombay. All the samples had an area of 1 cm2. Full details of preparation of specimens have been described elsewhere.2 To attain thermal equilibrium, the specimen was kept in a thermostat, maintammg temperature to ±1 C for ~ hr before the field was applied. Heating was continued for 4hr and then the specimen was cooled for 6 hr more, thus the field was on for 10 hr. The temperature was decreased by equal amounts in equal intervals of time, until the room temperature was attained in 5~hr. For the next ~ hr the thermostat was kept off. This controlled rate of cooling ensured uniform polarization of the sample. Tin foil was used as polarizing electrodes.
It is observed that the final charge is homocharge on both the faces of electrets; though the nature of the initial charge depends upon the polarizing parameters. With 10 kV/cm, the initial charge, in conformity with earlier findings,’ is homocharge, whereas at other two field values this is heterocharge, the value increasing with polarizing field at all the polarizing temperatures. An altogether new phenomenon, at the field value of 35kV/cm at all the temperatures except at 125°C,of triple reversal from hetero to homo, homo to hetero and finally to homocharge takes place. This was confirmed by repeating the observations many times. Only a representative graph is shown in Fig. 1. It is well known that most of the dipolar amorphous polymers (like PVC) exhibit multiple 1343
1344
ELECTRET STUDIES OF POLYVINYL CHLORIDE
Vol. 17, No. 11
8-
2C TPME (OA~5)
FIG I. Variation of surface charge vs time for PVC electret formed at 85°Cwith polarizing field of 35 kV/cm (triple reversal is obvious). dielectric relaxation process,5 which may be the casue of this anamolous phenomenon, but no plausible explanation can yet be presented. The shape and position of peaks in thermally stimulated current of the electrets may give activation parameters of the charges responsible for this triple reversal. These studies, which have already been carried out on different systems61°will be taken up shortly on PVC sheets. Thus both hetero- and homo-charges are present in PVC. As a pedagogic explanation, heterocharge in a polar dielectric, like PVC. can be attributed to orientation of the dipolar groups, which give rise to co-operatively polarized aggregates superficially analgous to domains in ferromagnetism. But an X-ray study of polarized specimens failed to detect the expected structural change in them,1’ ruling out this possibility. Furthermore, at high fields, Maxwell— Wagner effect12 may also arise at the interface of the textural elements of the polymer giving rise to heterocharge. The presence of Maxwell—Wagner polarization in PVC has been confirmed even above Tg by Hedvig.13 Polarized regions then attract the ions of opposite polarity from either the bulk of the material or those sprayed from the breakdown of the dielectric —electrode interface. These ions settle on the surface. when the matrix gets solidified, giving rise to hornocharge. The mechanism of ionic conduction in PVC. confirmed by several workers.14 justifies the existence
of the excessive ions in the bulk of the material. Under the impact of heating these ions may get trapped near the surface’5 of the electret giving rise to homocharge. This is further confirmed by the large value of activation energy (~ 1 .5 eV) for PVC, determined from log conductivity (temperature) curves,16 associated with ions trapped in partially crystalline regions. —
The incompatibility of the presence of heterocharge possibly due to dipolar orientation of the polar material and the absence of explicit detection by the X-ray studies can be resolved by a qualitative model already suggested,4 after Caserta and Serra,17 based upon band theory of solids, in which some of the charges suffer microscopic displacement during polarization and are trapped. There is a distribution of localized energy levels in the bulk of the material, as well as in the forbidden gap.18 The plasticizer and fillers used in the preparation of PVC sheet may give rise to these energy levels electron traps at the conduction band and hole traps at the valence band. The polarization P, due to these energy levels may be assumed to be proportional to their number Nd i.e. —
P~Nd. Electrons and holes are injected from the electrodes.’9 With the changed history of thermal treatment of the material, in the presence of electric field, there is a change in the state of these energy levels brought
Vol. 17, No. 11
ELECTRET STUDIES OF POLYVINYL CHLORIDE
about by the mobility of the injected charge carriers, The difference in the magnitude and nature of the injected charge carriers by the different types of electrode, and consequent change in the state of the energy levels existing in the material is reflected in the difference in magnitude and sign of initial and final charge on the electrets prepared under identical polarizing conditions but with different electrodes.’ The presence of hetero- and homo-charge on both the faces supports the injections of electrons and holes, When the specimen is heated with high forming field, charge carriers are produced from the electrodes! dielectric—electrode interface breakdown. Simultaneously free charge carriers are also produced from the valence band. On cooling the sample, these free charges get trapped near the electrodes, forming a charge layer of the same sign as of the polarizing electrode, giving rise to homocharge. The electrons arising out of valence band are trapped in the deep
1345
electron traps and the holes in the deep hole traps forming a pair of opposite charges, which is oriented by the applied field. Thus the heterocharge, involving the bulk of the material takes place. Since the external field of these dipoles would tend to annul the applied field, little alignment of the dipoles would be detectable2°as in the present case. The final net homocharge persists, the injected charges being dominant. At high temperatures, the greater polarizing field ejects out more electrons and holes from the valence band to be trapped in electron and hole traps giving rise to more induced dipoles and hence more heterocharge with high field, in conformity with earlier COfllmiinications.1’2’4 Binder,2’ by dusting the surface with suitable powder, has shown that homocharge produced on PVC thermoelectrets is due to discharge between the electrode—dielectric interface.
REFERENCES 1.
TALWAR I.M. & BHAWALKAR D.R., md. J. Pure. App!. Phys. 7,685 (1969).
2.
TALWAR I.M., Ph.D. Thesis, University of Sagar, Sagar, India (1968).
3.
PIECH T. & HANDEREK 3., Phys. Status Solidi 9, 361 (1965).
4.
TALWAR I.M.,Ind. J. PureAppi. Phys. 11,334(1973).
5.
ISHIDAY.,J. Polymer Sci. A2, 1835 (1969).
6.
PILLAI P.K.C., JAIN K. & JAIN V.K., Phys. Lett. 39A, 216 (1962).
7. 8.
LATOUR M., Thesis, Montpellier, France (1972). HEDVIG P. & FOLDES E., Die Ang. Makromol. Chem. 35, 147 (1974).
9.
VANDERSCHUEREN J., Thesis, Liege, Belgium (1974).
10.
VAN TURNHOUT J., Thermally Stimulated Discharge ofPolymer Electrets, Elsevier, Amsterdam/Oxford/NY (1975).
11. 12.
TALWAR I.M. & KHANNA S.L., md. J. PureAppi. Phys. 8,851(1970). HASTED J.B., Aqueous Dielectrics, p. 126. Chapman and Hall, London (1973).
13. 14.
HEDVIG P., mt. Microsymposium on Polarization and Electrical Conduction in Polymers, Bratislava, Czechosolovakia (May, 1972). KOSAKI M., SUGIYAMA K. & IEDAM.,J. App!. Phys. 42, 3388 (1971).
15. 16.
IEDA M., KOSAKI M., OSHIMA H. & SHINOHRA U., J. Phys. Soc. Japan 25, 1742 (1968). TALWAR I.M. & BHAWALKAR D.R., md. J. Pure App!. Phys. 7,681(1969).
17.
CASERTAG. & SERRAA.,J. App!. Phys. 42, 3778 (1971).
18.
MOTT N,S. & ALLGAIER R.S., Phys. Status Solidi 21, 343 (1967).
1346
ELECTRET STUDIES OF POLYVINYL CHLORIDE
19. 20.
BOGRODITISKII N.P., TAIROVA D.A. & SOROKIN V.S., Fiz. Tverd. Tela 6,2301(1954). GERSON R. & ROHRBAUGH J.H., J. Chem. Phys. 23. 2381 (1955).
21.
BINDER F.,Naturf 6a, 714 (1952).
Vol. 17, No. 11