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Investigation of optical and dielectric properties of polyvinyl chloride and polystyrene blends in terahertz regime
Optical Materials xxx (xxxx) xxx
Contents lists available at ScienceDirect
Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Investigation of optical and dielectric properties of polyvinyl chloride and polystyrene blends in terahertz regime Naima Farman a, 1, Muhammad Mumtaz a, b, 1, M. Ahsan Mahmood a, b, Sabih D. Khan a, M. Aslam Zia a, Muhammad Raffi b, Mushtaq Ahmed b, Izhar Ahmad a, * a b
National Institute of Lasers and Optronics (NILOP), Islamabad, Pakistan Pakistan Institute of Engineering and Applied Sciences (PIEAS), Islamabad, Pakistan
A R T I C L E I N F O
A B S T R A C T
Keywords: Terahertz Polymer blends Refractive index Absorption coefficient Polymers
Terahertz time domain spectroscopy has been utilized to investigate the optical, dielectric, and electrical properties of polyvinyl chloride (PVC) and polystyrene (PS) blend to highlight their utilization in terahertz technology. These blends prepared by solution casting technique, show a prominent change in these properties as a function of blending ratios. Moreover, the universal dielectric response model has been fitted to real THz conductivity which shows the super-linear behavior indicating the restricted motion of carriers. These results provide a database for the efficient utilization of such blends in THz technology.
1. Introduction Terahertz time-domain spectroscopy (THz-TDS) is a salient tech nique for characterizing the response of materials and devices in the farinfrared regime owing to its non-destructive, non-ionizing, contact free, and phase sensitive nature [1]. It is extensively being used in material, medical and agricultural sciences, as well as in security applications [2–6]. In the recent years, the research has been focused towards im provements in generation and detection of THz radiation, and devel opment of different compatible THz optical components, e.g., lenses, THz attenuator, band-pass filters, THz polarizers, windows, beam splitters, and waveguides, for effective utilization of these radiations [7–14]. Moreover, tuning of optical and dielectric properties of different optical components is also of great interest. Different polymers are being used in the generation of THz radiation [15–20] and fabrication of THz optical components [7,11–14], as they are low cost, easy to grow, and transparent to THz radiations [21–27]. Moreover, their optical and dielectric properties can be tuned or tailored in THz spectral regime using different techniques, such as change of operating temperature [28] and/or by blending the polymers, for extending their scope. Blending different polymers with variable weight ratios is a well-established technique for tailoring such properties in a specific range [29]. Polystyrene (PS)/Polyvinyl chloride (PVC) blends are important for
such applications because they individually have very different optical and dielectric properties in the THz range [30] and are beneficial for tailoring these properties in the form of blends. As compared to indi vidual polymers, the Polystyrene (PS)/Polyvinyl chloride (PVC) blends are more advantageous. Their optical and dielectric properties in THz regime can be tuned by changing the blending ratios of PS and PVC and can be deployed in THz technology to fabricate the density filters, waveguide etc. In previous studies, the mechanical and thermal prop erties of PS/PVC polymer blends have been investigated [31–35]. The optical and electrical characteristics of these blends have been studied by FTIR, UV-VIS spectroscopy, and XRD etc. [29,36–38]. In THz spectral regime, the optical, dielectric, and electrical properties of individual polymers have been explored [22,25,26,30,39], but no such work has been carried out so far for the PVC/PS blend in THz-spectral regime. However, it is necessary for efficient use of these blends in the THz-technology. In this work, PS/PVC blends with different weight ra tios have been prepared by solution casting process for tailoring the optical and dielectric properties of polymers. THz-TDS has been employed to investigate the optical, electrical and dielectric properties of PS, PVC and their blend series in the range of 0.2–1.8 THz. This article is organized as: firstly, the experimental details along with the sample preparation have been presented. Secondly, the results have been pre sented and discussed followed by the conclusions.
* Corresponding author. E-mail address: [email protected] (I. Ahmad). 1 These authors contribute equally to this work. https://doi.org/10.1016/j.optmat.2019.109534 Received 25 September 2019; Received in revised form 30 October 2019; Accepted 12 November 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Naima Farman, Optical Materials, https://doi.org/10.1016/j.optmat.2019.109534
N. Farman et al.
Optical Materials xxx (xxxx) xxx
2. Experimental details
3. Results and discussion
The polymers blends of PVC and PS with weight ratios of 100:0, 75:25, 50:50, 25:75, and 0:100 were prepared in tetrahydrofuran (THF) using solution casting process. First of all, the required amounts of in dividual polymers, for one specific weight ratio, were dissolved in the solvent separately with the help of magnetic stirrer up to the formation of clear solution. This was followed by the magnetic stirring for 2 h to mix both solutions in the form of a clear solution. The solution was then spread uniformly in a petri-dish and placed in an oven at 60 � C for 2 h, followed by slow evaporation of the THF and drying of the sample inside the oven at a maintained temperature of 40 � C for approximately 48 h. In this way dried films of all the weight ratios were prepared separately. The dried films were crushed and subsequently compressed in a hot press mount (Metapress A. Metkon Instrument Inc.) at 180 � C and 300 bar to fabricate thin sample pellets of thicknesses range 0.65–0.86 (� 0:01) mm. Fig. 1b depicts the dimensions of the samples. The optical and dielectric properties of all prepared samples have been investigated in transmission mode by employing the THz-TDS system shown in Fig. 1. The output from the femtosecond pulsed laser source (Toptica 100 fs, 80 MHz, central wavelength 780 nm) was focused on a photoconductive antenna (LT GaAs) for generating the THz pulses. The generated THz pulses were collimated and passed through the sample placed at the Fourier plane of the 1:1 lens telescope. The electric-field of transmitted pulses is thereby retrieved, in time-domain, using an antenna similar to the emitter [30]. In this way, the time-domain electric-field of the THz pulses is measured for the sample as well as for the reference (without sample) alternatively. The system was purged with dry air to reduce the moisture down to approximately 5% to avoid the artifacts from THz absorption in moisture.
The optical (refractive index and absorption coefficients), dielectric, and conductive properties of all samples have been measured using method described in Ref. [30]. Fig. 2a illustrates the refractive indices (nðω)) of PVC, PS and their blend series having different weight ratios in the frequency range of 0.2–1.8 THz. It has been observed that the refractive indices of individual PVC and PS polymers have good agree ment with the previous published data [30]. Moreover, these parameters for the blends depict decreasing trend with frequency but overall value increase by increasing concentration of PVC in the blends. The values of n(ω) at 1.0 THz plotted against weight ratios of blends are shown in Fig. 2c. They provide a tunable range of the refractive index from 1.577 to 1.689, which can be effective for the design of THz devices depending upon the refractive index variations. The frequency dependence of refractive index can be determined with Sellmeier coefficients which have been extracted by fitting the Sellmeier equation given as [40]. n2 ¼ ao þ
c2
a1 v2 a2 v2
(1)
where ao , a1 , and a2 are Sellmeier coefficients, c is speed of light in
μm=ps, and ν is frequency in THz. The calculated Sellmeier coefficients
are presented in Table 1. Similarly, the absorption coefficients of the PVC, PS and their blends in the frequency range 0.2–1.8 THz are shown in Figure (2b). Its values are increasing from 10 cm 1 to 43 cm 1 with frequency for pure PVC and is almost constant at 10 cm 1 for pure PS in THz spectral range of 0.2–1.8 THz, whereas, for their blend, they lie in between of them. It represents that the absorption of THz in this range can be tuned ac cording to the device requirement by blending these two polymers at different weight ratios. Figure (2c) clarifies the variation of refractive
Fig. 1. a) Experimental setup of THz-TDS in transmission mode, fs Laser: femtosecond laser, λc : central wavelength, THz: Terahertz, d: sample thickness, Photo conductive Emitter and Detector: Low temperature grown GaAs photo-conductive antenna for emission and detection of THz waves. Teflon Lens: f ¼ 50 mm. The setup was purged using a close-loop clean-and-dry air circulation unit to automatically maintain the humidity level down to 5 � 1 % to avoid unwanted THz ab sorptions in the moisture during measurements. b) Depicts the dimensions of sample where the sample thickness for different blends ranges from 0.65 to 0.86 (� 0:01) mm and diameter is 25 mm for each sample. c) Represents time domain THz pulse of reference and sample whereas d) is the frequency spectrum obtained by Fourier transformation of THz time domain pulse of reference and sample. 2
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Fig. 2. a) The variation of refractive indices of polymers with frequency and weight ratios of PS/PVC in blends. The symbols are the measured values and solid line is the Sellmeier equation fitting. b) Absorption coefficients of PS/PVC blends, c) The effect of PS/PVC weight ratios on refractive index and absorption coefficient measured at 1.0 THz. In each figure, the error bars are the standard deviation of four measurements.
and PS. They show increasing trend with increasing concentration of PVC in PS. Moreover, it has been demonstrated that real part of dielectric constant decreases with frequency for PVC and for all blends whereas PS almost shows a constant behavior throughout the frequency range from 0.2 to 1.8 THz. The dielectric loss (tanδ ¼ εi =εr ) quantifies the dissipation of electromagnetic energy in a material. Its value for PVC/PS blends at 1.0 THz have been shown in Fig. 3b which shows that PVC has high dielectric loss as compared to PS and their blends. As discussed in Refs. [38,41], the PS/PVC blends are immiscible due to weak interactions and the chemical structure of the individual polymers remain preserved. However, the density and hence the free volume vary which may be the cause of tuning these optical and dielectric properties. Similarly, the real conductivity of all samples extracted is shown in Fig. 4a. It is clear from Fig. 4a that its value for PS is lowest (4 S/m) and has small variation with frequency but its values for PVC is higher than PS and is increasing (3–17 S/m) in frequency range of 0.2–1.8 THz. However, its values for PVC/PS blends vary between these two extreme values for different blending ratios. The polymer blends studied form a disordered system and the conductivity of such systems can be under stand using universal dielectric response (UDR) model described by following equation [42,43]:
Table 1 Calculated coefficients of Sellmeier Equation for different polymers blends. Sample
ao
a1
a2
100PS/0PVC
2.548
888:964
0.001
75PS/25PVC
2.631
541:764
131.247
50PS/50PVC
2.683
686:881
120.887
25PS/75PVC
2.762
1682:063
92.236
0PS/100PVC
2.809
3111:850
0.0009
Table 2 The values of σ o , n, and A extracted by fitting of equation (2). Parameter
100PS/ 0PVC
75PS/ 25PVC
50PS/ 50PVC
25PS/ 75PVC
0PS/ 100PVC
n A
1.12 6:02 � 10 15
1.47 3:04 � 10 19
1.62 6:07 � 10 21
1.65 2:42 � 10 22
1.82 2:71 � 10 23
σo
3.49
3.39
2.94
2.63
3.82
index and absorption coefficient of blends at 1.0 THz. These results will be beneficial in THz technology such as density filters, band pas filters, lenses, and step index or graded index waveguides. The real (εr ) and imaginary part (εi ) of dielectric constant calculated using the above mentioned method are shown in Fig. 3a. Prominent tuning of εr & εi has been observed with different blending ratios of PVC
σ r ðωÞ ¼ σ o þ Aωn
(2)
where (σo ) is DC plateau at low frequency regime up to critical frequency (ωc ) defined as σ ðω ¼ ωc Þ ¼ 1:1σ o [42,43], A is the pre-exponent parameter which depends upon the temperature and n is frequency exponent depending upon the temperature and the conductive
Fig. 3. The variation of real(left y-axis) and imaginary (right y-axis) dielectric constant with frequency and weight ratio of PS and PVC in blends. The error bars are the standard deviation of four measurements. b) The variation of dielectric loss as a function of PS/PVC concentration measured at 1.0 THz. 3
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Fig. 4. a) The real THz conductivity of polymer blends, symbols are the experimental values whereas solid line is UDR model fit. The error bars are the standard deviation of four measurements. b) The variation of real conductivity vs PS/PVC weight ratios measured at 1.0 THz.
properties of sample and its value lie in the limit 0 < n < 1. Beyond ωc , sub-linear frequency dependence (n < 1) is maintained by the conductivity. The real conductivity of all sample have been fitted with this model leaving A, n and σ o as adjustable parameters. The extracted values of these parameters are given in Table 2 and indicate super-linear behavior (1 < n < 2), a deviation from UDR model. The reason of this deviation is the coupling between the hooping and vibrational motion of charge carriers. In such systems the motion of charge carriers is confined in the vicinity of Coulomb traps. Such deviations from UDR have also been observed in different studies [42]. The sub-linear frequency dependence (0 < n < 1) describe the conduction of carriers as a result of tunneling or hooping between chains whereas, in the case of super-linear frequency dependence (1 < n < 2), the Coulombic trap disturb the motion of car riers and their movement is limited in local environment. The real THz conductivities as a function of blending ratios at 1.0 THz have been plotted in Fig. 4. It is found that the THz conductivities increase with increasing the concentration of PVC in blends, indicating the tuning of conductivity. The tuning of conductivity of sample has potential appli cations in THz technology.
interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors are thankful to material division of Pakistan Institute of Nuclear Science and Technology (PINSTECH), Islamabad, Pakistan for helping in preparation of samples. References [1] P. Jepsen, D. Cooke, M. Koch, Thz Spectroscopy and Imaging–Modern Techniques and Applications, laser & photonics reviews, 2011. [2] H.J. Joyce, J.L. Boland, C.L. Davies, S.A. Baig, M.B. Johnston, A review of the electrical properties of semiconductor nanowires: insights gained from terahertz conductivity spectroscopy, Semicond. Sci. Technol. 31 (10) (2016) 103003. [3] T.-I. Jeon, K.-J. Kim, C. Kang, I.H. Maeng, J.-H. Son, K.H. An, J.Y. Lee, Y.H. Lee, Optical and electrical properties of preferentially anisotropic single-walled carbonnanotube films in terahertz region, J. Appl. Phys. 95 (10) (2004) 5736–5740. [4] T. Aoki, M.W. Takeda, J.W. Haus, Z. Yuan, M. Tani, K. Sakai, N. Kawai, K. Inoue, Terahertz time-domain study of a pseudo-simple-cubic photonic lattice, Phys. Rev. B 64 (4) (2001), 045106. [5] M. Tonouchi, Cutting-edge terahertz technology, Nat. Photonics 1 (2) (2007) 97. [6] J.F. Federici, B. Schulkin, F. Huang, D. Gary, R. Barat, F. Oliveira, D. Zimdars, Thz imaging and sensing for security applications—explosives, weapons and drugs, Semicond. Sci. Technol. 20 (7) (2005) S266. [7] A. Argyros, M.A. van Eijkelenborg, M.C. Large, I.M. Bassett, Hollow-core microstructured polymer optical fiber, Opt. Lett. 31 (2) (2006) 172–174. [8] T. Bartel, P. Gaal, K. Reimann, M. Woerner, T. Elsaesser, Generation of single-cycle thz transients with high electric-field amplitudes, Opt. Lett. 30 (20) (2005) 2805–2807. [9] F. Blanchard, G. Sharma, L. Razzari, X. Ropagnol, H.-C. Bandulet, F. Vidal, R. Morandotti, J.-C. Kieffer, T. Ozaki, H. Tiedje, et al., Generation of intense terahertz radiation via optical methods, IEEE J. Sel. Top. Quantum Electron. 17 (1) (2011) 5–16. [10] J. Dai, J. Liu, X.-C. Zhang, Terahertz wave air photonics: terahertz wave generation and detection with laser-induced gas plasma, IEEE J. Sel. Top. Quantum Electron. 17 (1) (2011) 183–190. [11] D. Chen, H. Chen, A novel low-loss terahertz waveguide: polymer tube, Opt. Express 18 (4) (2010) 3762–3767. [12] C. Jansen, S. Wietzke, V. Astley, D.M. Mittleman, M. Koch, Mechanically flexible polymeric compound one-dimensional photonic crystals for terahertz frequencies, Appl. Phys. Lett. 96 (11) (2010) 111108. [13] B. Scherger, M. Scheller, C. Jansen, M. Koch, K. Wiesauer, Terahertz lenses made by compression molding of micropowders, Appl. Opt. 50 (15) (2011) 2256–2262. [14] C. J€ ordens, K.L. Chee, I.A. Al-Naib, I. Pupeza, S. Peik, G. Wenke, M. Koch, Dielectric fibres for low-loss transmission of millimetre waves and its application in couplers and splitters, J. Infrared, Millim. Terahertz Waves 31 (2) (2010) 214. [15] A. Nahata, D.H. Auston, C. Wu, J.T. Yardley, Generation of terahertz radiation from a poled polymer, Appl. Phys. Lett. 67 (10) (1995) 1358–1360. [16] A.M. Sinyukov, L.M. Hayden, Generation and detection of terahertz radiation with multilayered electro-optic polymer films, Opt. Lett. 27 (1) (2002) 55–57.
4. Conclusions In summary, the optical, dielectric, and conductive properties of PS/ PVC blends prepared by solution casting method have been examined in the frequency range of 0.2–1.8 THz using THz-TDS in transmission mode. A prominent change in these properties has been observed with the weight ratios of 0–100% of PVC in PS. The refractive index at 1.0 THz has been changed in the range of 1.59–1.67 with standard deviation of 0.0019 and absorption coefficient in the range of 10–21 cm 1 with standard deviation of 0.6 cm 1 . Similarly the 10% change in dielectric properties has been observed. The dielectric loss of blend series observed at 1.0 THz frequency varies in the range of 0.05–0.1. THz conductivity has also been observed increasing with weight ratio in blends as well as with frequency and it is analyzed using UDR model. According to this model, conductivity of all samples have shown super-linear behavior (n > 1), indicating the restricted motion of carriers due to Coulombic traps. This controlled way of tuning the optical, dielectric, and electrical properties of polymers will help the future applications of these bends in THz technology. Declaration of competing interest The authors declare that they have no known competing financial 4
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Contents lists available at ScienceDirect
Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Investigation of optical and dielectric properties of polyvinyl chloride and polystyrene blends in terahertz regime Naima Farman a, 1, Muhammad Mumtaz a, b, 1, M. Ahsan Mahmood a, b, Sabih D. Khan a, M. Aslam Zia a, Muhammad Raffi b, Mushtaq Ahmed b, Izhar Ahmad a, * a b
National Institute of Lasers and Optronics (NILOP), Islamabad, Pakistan Pakistan Institute of Engineering and Applied Sciences (PIEAS), Islamabad, Pakistan
A R T I C L E I N F O
A B S T R A C T
Keywords: Terahertz Polymer blends Refractive index Absorption coefficient Polymers
Terahertz time domain spectroscopy has been utilized to investigate the optical, dielectric, and electrical properties of polyvinyl chloride (PVC) and polystyrene (PS) blend to highlight their utilization in terahertz technology. These blends prepared by solution casting technique, show a prominent change in these properties as a function of blending ratios. Moreover, the universal dielectric response model has been fitted to real THz conductivity which shows the super-linear behavior indicating the restricted motion of carriers. These results provide a database for the efficient utilization of such blends in THz technology.
1. Introduction Terahertz time-domain spectroscopy (THz-TDS) is a salient tech nique for characterizing the response of materials and devices in the farinfrared regime owing to its non-destructive, non-ionizing, contact free, and phase sensitive nature [1]. It is extensively being used in material, medical and agricultural sciences, as well as in security applications [2–6]. In the recent years, the research has been focused towards im provements in generation and detection of THz radiation, and devel opment of different compatible THz optical components, e.g., lenses, THz attenuator, band-pass filters, THz polarizers, windows, beam splitters, and waveguides, for effective utilization of these radiations [7–14]. Moreover, tuning of optical and dielectric properties of different optical components is also of great interest. Different polymers are being used in the generation of THz radiation [15–20] and fabrication of THz optical components [7,11–14], as they are low cost, easy to grow, and transparent to THz radiations [21–27]. Moreover, their optical and dielectric properties can be tuned or tailored in THz spectral regime using different techniques, such as change of operating temperature [28] and/or by blending the polymers, for extending their scope. Blending different polymers with variable weight ratios is a well-established technique for tailoring such properties in a specific range [29]. Polystyrene (PS)/Polyvinyl chloride (PVC) blends are important for
such applications because they individually have very different optical and dielectric properties in the THz range [30] and are beneficial for tailoring these properties in the form of blends. As compared to indi vidual polymers, the Polystyrene (PS)/Polyvinyl chloride (PVC) blends are more advantageous. Their optical and dielectric properties in THz regime can be tuned by changing the blending ratios of PS and PVC and can be deployed in THz technology to fabricate the density filters, waveguide etc. In previous studies, the mechanical and thermal prop erties of PS/PVC polymer blends have been investigated [31–35]. The optical and electrical characteristics of these blends have been studied by FTIR, UV-VIS spectroscopy, and XRD etc. [29,36–38]. In THz spectral regime, the optical, dielectric, and electrical properties of individual polymers have been explored [22,25,26,30,39], but no such work has been carried out so far for the PVC/PS blend in THz-spectral regime. However, it is necessary for efficient use of these blends in the THz-technology. In this work, PS/PVC blends with different weight ra tios have been prepared by solution casting process for tailoring the optical and dielectric properties of polymers. THz-TDS has been employed to investigate the optical, electrical and dielectric properties of PS, PVC and their blend series in the range of 0.2–1.8 THz. This article is organized as: firstly, the experimental details along with the sample preparation have been presented. Secondly, the results have been pre sented and discussed followed by the conclusions.
* Corresponding author. E-mail address: [email protected] (I. Ahmad). 1 These authors contribute equally to this work. https://doi.org/10.1016/j.optmat.2019.109534 Received 25 September 2019; Received in revised form 30 October 2019; Accepted 12 November 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Naima Farman, Optical Materials, https://doi.org/10.1016/j.optmat.2019.109534
N. Farman et al.
Optical Materials xxx (xxxx) xxx
2. Experimental details
3. Results and discussion
The polymers blends of PVC and PS with weight ratios of 100:0, 75:25, 50:50, 25:75, and 0:100 were prepared in tetrahydrofuran (THF) using solution casting process. First of all, the required amounts of in dividual polymers, for one specific weight ratio, were dissolved in the solvent separately with the help of magnetic stirrer up to the formation of clear solution. This was followed by the magnetic stirring for 2 h to mix both solutions in the form of a clear solution. The solution was then spread uniformly in a petri-dish and placed in an oven at 60 � C for 2 h, followed by slow evaporation of the THF and drying of the sample inside the oven at a maintained temperature of 40 � C for approximately 48 h. In this way dried films of all the weight ratios were prepared separately. The dried films were crushed and subsequently compressed in a hot press mount (Metapress A. Metkon Instrument Inc.) at 180 � C and 300 bar to fabricate thin sample pellets of thicknesses range 0.65–0.86 (� 0:01) mm. Fig. 1b depicts the dimensions of the samples. The optical and dielectric properties of all prepared samples have been investigated in transmission mode by employing the THz-TDS system shown in Fig. 1. The output from the femtosecond pulsed laser source (Toptica 100 fs, 80 MHz, central wavelength 780 nm) was focused on a photoconductive antenna (LT GaAs) for generating the THz pulses. The generated THz pulses were collimated and passed through the sample placed at the Fourier plane of the 1:1 lens telescope. The electric-field of transmitted pulses is thereby retrieved, in time-domain, using an antenna similar to the emitter [30]. In this way, the time-domain electric-field of the THz pulses is measured for the sample as well as for the reference (without sample) alternatively. The system was purged with dry air to reduce the moisture down to approximately 5% to avoid the artifacts from THz absorption in moisture.
The optical (refractive index and absorption coefficients), dielectric, and conductive properties of all samples have been measured using method described in Ref. [30]. Fig. 2a illustrates the refractive indices (nðω)) of PVC, PS and their blend series having different weight ratios in the frequency range of 0.2–1.8 THz. It has been observed that the refractive indices of individual PVC and PS polymers have good agree ment with the previous published data [30]. Moreover, these parameters for the blends depict decreasing trend with frequency but overall value increase by increasing concentration of PVC in the blends. The values of n(ω) at 1.0 THz plotted against weight ratios of blends are shown in Fig. 2c. They provide a tunable range of the refractive index from 1.577 to 1.689, which can be effective for the design of THz devices depending upon the refractive index variations. The frequency dependence of refractive index can be determined with Sellmeier coefficients which have been extracted by fitting the Sellmeier equation given as [40]. n2 ¼ ao þ
c2
a1 v2 a2 v2
(1)
where ao , a1 , and a2 are Sellmeier coefficients, c is speed of light in
μm=ps, and ν is frequency in THz. The calculated Sellmeier coefficients
are presented in Table 1. Similarly, the absorption coefficients of the PVC, PS and their blends in the frequency range 0.2–1.8 THz are shown in Figure (2b). Its values are increasing from 10 cm 1 to 43 cm 1 with frequency for pure PVC and is almost constant at 10 cm 1 for pure PS in THz spectral range of 0.2–1.8 THz, whereas, for their blend, they lie in between of them. It represents that the absorption of THz in this range can be tuned ac cording to the device requirement by blending these two polymers at different weight ratios. Figure (2c) clarifies the variation of refractive
Fig. 1. a) Experimental setup of THz-TDS in transmission mode, fs Laser: femtosecond laser, λc : central wavelength, THz: Terahertz, d: sample thickness, Photo conductive Emitter and Detector: Low temperature grown GaAs photo-conductive antenna for emission and detection of THz waves. Teflon Lens: f ¼ 50 mm. The setup was purged using a close-loop clean-and-dry air circulation unit to automatically maintain the humidity level down to 5 � 1 % to avoid unwanted THz ab sorptions in the moisture during measurements. b) Depicts the dimensions of sample where the sample thickness for different blends ranges from 0.65 to 0.86 (� 0:01) mm and diameter is 25 mm for each sample. c) Represents time domain THz pulse of reference and sample whereas d) is the frequency spectrum obtained by Fourier transformation of THz time domain pulse of reference and sample. 2
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Fig. 2. a) The variation of refractive indices of polymers with frequency and weight ratios of PS/PVC in blends. The symbols are the measured values and solid line is the Sellmeier equation fitting. b) Absorption coefficients of PS/PVC blends, c) The effect of PS/PVC weight ratios on refractive index and absorption coefficient measured at 1.0 THz. In each figure, the error bars are the standard deviation of four measurements.
and PS. They show increasing trend with increasing concentration of PVC in PS. Moreover, it has been demonstrated that real part of dielectric constant decreases with frequency for PVC and for all blends whereas PS almost shows a constant behavior throughout the frequency range from 0.2 to 1.8 THz. The dielectric loss (tanδ ¼ εi =εr ) quantifies the dissipation of electromagnetic energy in a material. Its value for PVC/PS blends at 1.0 THz have been shown in Fig. 3b which shows that PVC has high dielectric loss as compared to PS and their blends. As discussed in Refs. [38,41], the PS/PVC blends are immiscible due to weak interactions and the chemical structure of the individual polymers remain preserved. However, the density and hence the free volume vary which may be the cause of tuning these optical and dielectric properties. Similarly, the real conductivity of all samples extracted is shown in Fig. 4a. It is clear from Fig. 4a that its value for PS is lowest (4 S/m) and has small variation with frequency but its values for PVC is higher than PS and is increasing (3–17 S/m) in frequency range of 0.2–1.8 THz. However, its values for PVC/PS blends vary between these two extreme values for different blending ratios. The polymer blends studied form a disordered system and the conductivity of such systems can be under stand using universal dielectric response (UDR) model described by following equation [42,43]:
Table 1 Calculated coefficients of Sellmeier Equation for different polymers blends. Sample
ao
a1
a2
100PS/0PVC
2.548
888:964
0.001
75PS/25PVC
2.631
541:764
131.247
50PS/50PVC
2.683
686:881
120.887
25PS/75PVC
2.762
1682:063
92.236
0PS/100PVC
2.809
3111:850
0.0009
Table 2 The values of σ o , n, and A extracted by fitting of equation (2). Parameter
100PS/ 0PVC
75PS/ 25PVC
50PS/ 50PVC
25PS/ 75PVC
0PS/ 100PVC
n A
1.12 6:02 � 10 15
1.47 3:04 � 10 19
1.62 6:07 � 10 21
1.65 2:42 � 10 22
1.82 2:71 � 10 23
σo
3.49
3.39
2.94
2.63
3.82
index and absorption coefficient of blends at 1.0 THz. These results will be beneficial in THz technology such as density filters, band pas filters, lenses, and step index or graded index waveguides. The real (εr ) and imaginary part (εi ) of dielectric constant calculated using the above mentioned method are shown in Fig. 3a. Prominent tuning of εr & εi has been observed with different blending ratios of PVC
σ r ðωÞ ¼ σ o þ Aωn
(2)
where (σo ) is DC plateau at low frequency regime up to critical frequency (ωc ) defined as σ ðω ¼ ωc Þ ¼ 1:1σ o [42,43], A is the pre-exponent parameter which depends upon the temperature and n is frequency exponent depending upon the temperature and the conductive
Fig. 3. The variation of real(left y-axis) and imaginary (right y-axis) dielectric constant with frequency and weight ratio of PS and PVC in blends. The error bars are the standard deviation of four measurements. b) The variation of dielectric loss as a function of PS/PVC concentration measured at 1.0 THz. 3
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Fig. 4. a) The real THz conductivity of polymer blends, symbols are the experimental values whereas solid line is UDR model fit. The error bars are the standard deviation of four measurements. b) The variation of real conductivity vs PS/PVC weight ratios measured at 1.0 THz.
properties of sample and its value lie in the limit 0 < n < 1. Beyond ωc , sub-linear frequency dependence (n < 1) is maintained by the conductivity. The real conductivity of all sample have been fitted with this model leaving A, n and σ o as adjustable parameters. The extracted values of these parameters are given in Table 2 and indicate super-linear behavior (1 < n < 2), a deviation from UDR model. The reason of this deviation is the coupling between the hooping and vibrational motion of charge carriers. In such systems the motion of charge carriers is confined in the vicinity of Coulomb traps. Such deviations from UDR have also been observed in different studies [42]. The sub-linear frequency dependence (0 < n < 1) describe the conduction of carriers as a result of tunneling or hooping between chains whereas, in the case of super-linear frequency dependence (1 < n < 2), the Coulombic trap disturb the motion of car riers and their movement is limited in local environment. The real THz conductivities as a function of blending ratios at 1.0 THz have been plotted in Fig. 4. It is found that the THz conductivities increase with increasing the concentration of PVC in blends, indicating the tuning of conductivity. The tuning of conductivity of sample has potential appli cations in THz technology.
interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors are thankful to material division of Pakistan Institute of Nuclear Science and Technology (PINSTECH), Islamabad, Pakistan for helping in preparation of samples. References [1] P. Jepsen, D. Cooke, M. Koch, Thz Spectroscopy and Imaging–Modern Techniques and Applications, laser & photonics reviews, 2011. [2] H.J. Joyce, J.L. Boland, C.L. Davies, S.A. Baig, M.B. Johnston, A review of the electrical properties of semiconductor nanowires: insights gained from terahertz conductivity spectroscopy, Semicond. Sci. Technol. 31 (10) (2016) 103003. [3] T.-I. Jeon, K.-J. Kim, C. Kang, I.H. Maeng, J.-H. Son, K.H. An, J.Y. Lee, Y.H. Lee, Optical and electrical properties of preferentially anisotropic single-walled carbonnanotube films in terahertz region, J. Appl. Phys. 95 (10) (2004) 5736–5740. [4] T. Aoki, M.W. Takeda, J.W. Haus, Z. Yuan, M. Tani, K. Sakai, N. Kawai, K. Inoue, Terahertz time-domain study of a pseudo-simple-cubic photonic lattice, Phys. Rev. B 64 (4) (2001), 045106. [5] M. Tonouchi, Cutting-edge terahertz technology, Nat. Photonics 1 (2) (2007) 97. [6] J.F. Federici, B. Schulkin, F. Huang, D. Gary, R. Barat, F. Oliveira, D. Zimdars, Thz imaging and sensing for security applications—explosives, weapons and drugs, Semicond. Sci. Technol. 20 (7) (2005) S266. [7] A. Argyros, M.A. van Eijkelenborg, M.C. Large, I.M. Bassett, Hollow-core microstructured polymer optical fiber, Opt. Lett. 31 (2) (2006) 172–174. [8] T. Bartel, P. Gaal, K. Reimann, M. Woerner, T. Elsaesser, Generation of single-cycle thz transients with high electric-field amplitudes, Opt. Lett. 30 (20) (2005) 2805–2807. [9] F. Blanchard, G. Sharma, L. Razzari, X. Ropagnol, H.-C. Bandulet, F. Vidal, R. Morandotti, J.-C. Kieffer, T. Ozaki, H. Tiedje, et al., Generation of intense terahertz radiation via optical methods, IEEE J. Sel. Top. Quantum Electron. 17 (1) (2011) 5–16. [10] J. Dai, J. Liu, X.-C. Zhang, Terahertz wave air photonics: terahertz wave generation and detection with laser-induced gas plasma, IEEE J. Sel. Top. Quantum Electron. 17 (1) (2011) 183–190. [11] D. Chen, H. Chen, A novel low-loss terahertz waveguide: polymer tube, Opt. Express 18 (4) (2010) 3762–3767. [12] C. Jansen, S. Wietzke, V. Astley, D.M. Mittleman, M. Koch, Mechanically flexible polymeric compound one-dimensional photonic crystals for terahertz frequencies, Appl. Phys. Lett. 96 (11) (2010) 111108. [13] B. Scherger, M. Scheller, C. Jansen, M. Koch, K. Wiesauer, Terahertz lenses made by compression molding of micropowders, Appl. Opt. 50 (15) (2011) 2256–2262. [14] C. J€ ordens, K.L. Chee, I.A. Al-Naib, I. Pupeza, S. Peik, G. Wenke, M. Koch, Dielectric fibres for low-loss transmission of millimetre waves and its application in couplers and splitters, J. Infrared, Millim. Terahertz Waves 31 (2) (2010) 214. [15] A. Nahata, D.H. Auston, C. Wu, J.T. Yardley, Generation of terahertz radiation from a poled polymer, Appl. Phys. Lett. 67 (10) (1995) 1358–1360. [16] A.M. Sinyukov, L.M. Hayden, Generation and detection of terahertz radiation with multilayered electro-optic polymer films, Opt. Lett. 27 (1) (2002) 55–57.
4. Conclusions In summary, the optical, dielectric, and conductive properties of PS/ PVC blends prepared by solution casting method have been examined in the frequency range of 0.2–1.8 THz using THz-TDS in transmission mode. A prominent change in these properties has been observed with the weight ratios of 0–100% of PVC in PS. The refractive index at 1.0 THz has been changed in the range of 1.59–1.67 with standard deviation of 0.0019 and absorption coefficient in the range of 10–21 cm 1 with standard deviation of 0.6 cm 1 . Similarly the 10% change in dielectric properties has been observed. The dielectric loss of blend series observed at 1.0 THz frequency varies in the range of 0.05–0.1. THz conductivity has also been observed increasing with weight ratio in blends as well as with frequency and it is analyzed using UDR model. According to this model, conductivity of all samples have shown super-linear behavior (n > 1), indicating the restricted motion of carriers due to Coulombic traps. This controlled way of tuning the optical, dielectric, and electrical properties of polymers will help the future applications of these bends in THz technology. Declaration of competing interest The authors declare that they have no known competing financial 4
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