ВЫСОКОМОЛЕКУЛЯРНЫЕ СОЕДИНЕНИЯ, Серия А, 2013, том 55, № 8, с. 1048-1054
СТРУКТУРА И СВОЙСТВА
УДК 541.64:535.5:537.3
ELECTRICALLY CONDUCTING PLASTIC FILMS FROM POLYETHYLENE TEREPHTHALATE FOR OPTOELECTRONIC APPLICATIONS1
© 2013 г. K. Sreelatha4 and P. Predeep"
a Laboratory for Unconventional Electronics and Photonics, Department of Physics, National Institute Technology,
Calicut, Kerala, 673601 India b Centre for Advance Scientific Research and Rural Technology, Kollam, Kerala, India
e-mail: ppredeep@gmail.com Received July 23, 2012 Revised Manuscript Received September 20, 2012
Abstract — Iodine doping of polyethylene terephthalate (PET) has been made to develop intrinsically conducting plastic films. The doped material is investigated in terms of structure and morphology, electrical and optical characteristics. Considerable decrease in the value of the degree of crystallinity is observed on doping the samples. Further there are discernible shifts found in the energy gap and band edge towards lower energies on doping with iodine. The refractive index of the complex films is also found to increase.
DOI: 10.7868/S0507547513070155
INTRODUCTION
Conducting plastics are the best choice to be employed in the fabrication of electronic devices as they possess the electronic properties of a semiconductor and the specific properties of plastics such as light weight, high mechanical strength and flexibility. Another important motivation for the interest in organic semiconductors is its easy processability and expected low cost of the end product. The conventional non-conjugated polymers such as polyesters, polyimides, and polyamides, have n-electrons, but they are all insulating as they lack extended conjugation. The oxidants like halogens and Lewis acids have been used extensively used to [1—4] tailor the physical and chemical properties of these conventional polymers. Polyethylene terephthalate (PET) used in this study belongs to the family of thermoplastics with important industrial applications, in the textile industry, for engineering and technological purposes in the pure and doped form because of its good mechanical properties, transparency, nontoxic nature, chemical resistance, with reasonable thermal stability and high thermal resistance. Doped conducting and semiconducting PET with its proven advantages of plastic flexibility and ease of processabilty has all the potential to find applications in microelectronics, as electrical fuses, switches, sensors etc. The ability to control the chemical and structural properties of polymers is very important and the related research helps for developments in the field of coatings, electronics, chemical sensing, and many other applications.
PET is formed mainly from terephthalic acid (HOOC-C6H4-COOH) and ethylene glycol (HO-
1 The article is published in the original.
C2H4-OH) and has a semi-crystalline structure composed of crystalline and amorphous regions. The reactions with OH and COOH end groups and with ester bond included into polyethylene terephthalate chains allow modification of their structure and are favourable for the [5] preparation of new materials. The doping of polymers causes changes in their structure, physical and chemical properties [6, 7] depending on the chemical nature of the doping substances and their interaction with the host matrix. A suitable dopant improves [8] the charge carrier mobility of the polymer. It is known that iodine molecule possess acceptor properties and high electron affinity and it captures a weakly localized n electron of polymer macromolecule, forming a charge-transfer complex [9] which improves the conducting behaviour of polymers. Charge-transfer complexes (CTC) are extensively studied in recent years because of their unusual electrical, mechanical and optical properties. The electrical conductivity of the iodine adducts of thermoplastics using aqueous KI/I2 solution has been studied by [10-12] various research groups. Another method of oxidant impregnation is by [10] the sublimation of iodine. This study reports an effort to develop intrinsic electric conductivity in PET films with appreciable transparency using iodine under non-aqueous solution.
PET is considered to be an excellent material [6] for making good quality transparent conducting films. The main objective of this paper is to investigate the doping of iodine oxidant on PET based on the interactions between iodine and the polymer under non-aqueous solution. The effects of iodine concentration and period of doping on the electrical conductivity of the doped films are investigated. The structure, morphology and optical properties of the doped films are
discussed. The measurements for the refractive indices of the pristine and doped films are made, to observe how the addition of dopants modifies the polymer's physical properties. Refractive index is a fundamental optical property [13] of polymers that is directly related to other optical, electrical, and magnetic properties. Information about this property is significant in the design of new optical polymeric materials.
EXPERIMENTAL
Materials
Polyethylene terephthalate films of thickness 0.023 mm are purchased from Good Fellow materials; U. K. Iodine and toluene (Merck) of analytical grade were purchased from local suppliers and used as received.
Sample Preparation
Iodine sorption is carried out by immersing the PET films into the iodine solution in toluene in an air tight glass apparatus for different time periods and concentrations at room temperature. For DC conductivity measurements, the films are doped by using I2 solution of different concentrations (0.04, 0.08, 0.16, 0.32 and 0.4 M) for 16 days. The films are removed from the solution and washed with pure toluene to remove excess iodine. They are then drained and left to dry at room temperature. The experiment is repeated for various doping periods (2 to 16 days), keeping the concentration of iodine solution as 0.4 M. Sample mass is measured with an electronic balance of resolution of 0.1 mg and the relative increase in mass percentage, Am%, is calculated. Iodine content of the films is estimated using Eq. (1).
Percent iodine uptake = Increase in weight of PET film
x 100
(1)
Original weight of PET film
For other studies, heavily doped films (0.4 mol of I2/l for 16 days) are used. Measurements are made for five specimens in each case to ensure homogeneity of the doped films.
Characterization
The DC conductivity measurements of the doped films are made by the four probe technique using the Keithly 6514 Electrometer and Keithly 2400 source meter. The structure of the films is observed using infrared spectra taken with Nicolet 5700 FTIR Spectrometer using an attenuated total reflectance accessory (ATR). Uniform films of the sample are placed on the ATR crystal and suitable pressure is applied to obtain optimal signal. IR studies are used to observe the appearance of new absorption bands and the weakening or disappearance of certain bands confirming the
structural transformation of PET-iodine complex films. Surface morphologies of complex films are observed by FESEM using the JSM-6390. The UV-visi-ble absorption spectra of the samples are recorded at room temperature on Cary 5000 at a scan rate of 600 nm per minute in the wavelength range 300— 900 nm. From this data, the optical band gap is determined. Changes in the refractive index and extinction coefficient of the pure and doped films are determined from the reflectance (R) and transmission (T) spectra in the ultraviolet-visible range by obtaining the absorption coefficient a using the conventional method. Since a is related to the extinction coefficient k, which is defined as the imaginary part of the complex refractive index where n, is the real part of refractive index, an accurate determination of n and k is possible. X-ray diffraction patterns are registered with Bruker AXS D8 Advance Diffractometer using CuZ"a radiation of wavelength X = 1.5406 Â between a 29 angle of 0 and 90°.
RESULTS AND DISCUSSION
D. C. Conductivity
Iodine treatment changes the colour of the PET films from colourless to dark brown with appreciable transparency and the film becomes electrically conductive in the semiconducting range. The colour change observed here indicates the formation of Charge Transfer Complexes (CTCs) on account of the donating nature of the conjugate pair of n electrons from the carbonyl group because of the strong affinity of iodine towards these sites. The current-voltage characteristic of PET iodine complex films (0.4 mol of I2/l for 8 days) measured at room temperature (Fig. 1) also supports the observation based on colour change. The complex formation provides conducting pathways through the amorphous regions of the polymer and their hoping from chain to chain resulting in the enhancement of conductivity. The I—V characteristic curve is a linear plot (the solid line is the linear fit to the experimental value) indicating the Ohmic characteristics of the charge transfer complexes. On doping the polymer films with iodine for various concentrations ranging from 0.04 to 0.4 M for a period of 16 days at room temperature it is found that the iodine content in the film improved with the concentration of the solution and enhanced the DC conductivity of the films.
Figure 2 shows the variation of the electrical conductivity and percent iodine uptake as a function of the period of doping. The dopant concentration is 0.4M and the period of doping is varied from 2 to 16 days. The DC conductivity and iodine mass uptake increases with the period of doping. For heavily doped films the conductivity reached a maximum value of about 1.45 x 10-4 S/cm.
It can be seen from Fig. 2 that the conductivity starts saturating after a doping period of about twelve
Current x 106, A 1.2
0 3 6 9
Voltage, V
Fig. 1. I-V Characteristic of PET iodine complex films.
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£ 10-8 O
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