Structure and Electrical Study of New Chemically Modified Poly(vinyl chloride)

The aim of this work was to study the structural and electrical properties of a new polymer obtained by functionalization of a commercial poly(vinyl chloride) (PVC) (Mw= 48000) by grafting aminoalkyl and aminoaryl groups.Modified poly(vinyl chloride) was prepared in two steps. The structural properties of the polymer were systematically investigated by varieties of techniques as differential scanning calorimetric (DSC), thermogravimetry analysis (TG), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. The electrical properties of the polymer were studied by electrochemical impedance spectroscopy (EIS).


Introduction
Taking into account its excellent miscibility, compatibility properties, good mechanical strength, low energy consumption, and its low cost [1][2][3][4][5], poly(vinyl chloride) (PVC) has been considered as a good candidate polymer for use in a great variety of electrochemical devices such as high energy density batteries, fuel cells, sensors, and electrochromic devices [6][7][8].The development of PVC polymer with high ionic conductivity is one of the main objectives in polymer research.In fact, various approaches have been made to modify the structure of PVC polymer in order to improve their electrical, electrochemical, and mechanical properties.Among the modified PVC, we found those cited in many literatures [9][10][11].In the present work, differential scanning calorimetric (DSC) studies, thermogravimetry analysis (TG), X-ray diffraction (XRD), Fourier transform infrared (FTIR), and temperature dependent conductivity were performed on a new polymer obtained by functionalization in two steps of a commercial poly(vinyl chloride) by grafting aminoalkyl and aminoaryl groups.

Structure of Amino-p-anisidine-PVC (P 2
). Synthesis is described in our precedent work [11] as shown in Scheme 1.

TG Analysis.
In order to study the thermal stability of polymer (P 2 ), TG analysis has been done from room temperature to 800 ∘ C as shown in Figure 2. It can be seen that when (P 2 ) is heated at 156-169 ∘ C, the weight loss is about 3 mg and 38 mg while heated at 572-577 ∘ C. It is due to two factors including the elimination of HCl and HI groups and complete decomposition of the polymer.

Analysis by XRD.
The X-ray diffraction pattern shows the amorphous nature of the commercial PVC and the studied polymer (P 2 ) (Figure 3).Transmission (%)

𝛿 Ct é
t-H (Me) at 1332 and  Ct é t-H (aromp-sub) at 828 cm −1 ; we also note that the band ] C-Cl at 690 cm −1 becomes very low compared to that corresponding to the PVC as a consequence to the increase of the number of chlorine atoms substituted by diethylenetriamine groups.The IR spectrum of the polymer (P 2 ) heated at 190 ∘ C shows the appearance of a band of vibration ] C=C = 1605 cm −1 and the disappearance of the band ] C-Cl at 690 cm −1 which indicates the removal of HCl and HI groups giving rise to a conjugated system having an alternate of single and double bonds.

Electrical Measurements.
The powders were ground in an agate mortar and then pressed at 4 tons into cylindrical pellets with 11.8 mm in diameter and 2.7 mm in thickness.Electrochemical Impedance Spectra (EIS) were obtained using a Hewlett-Packard HP 4192 analyzer.The impedance measurements were taken in an open circuit using two electrode configurations with signal amplitude of 50 mV and a frequency band ranging from 5 Hz to 13 MHz.Both pellet surfaces were coated with silver pastes electrodes while the platinum wires attached to the electrodes were used as current collectors.All these measurements were performed at equilibrium potential at a temperature ranging between 160 ∘ C and 220 ∘ C.

Electrochemical Characterization.
Figure 5 shows the Nyquist diagram obtained from pellet of amino-p-anisidine-PVC (P 2 ) between 160 ∘ C and 220 ∘ C which appears as a semicircle and the amplitude of this arc is thermally dependent.The impedance spectra plotted in Figure 5 were analyzed by fitting the data with the equivalent circuit shown in Figure 6.In this figure,  corresponds to an inductance which is usually   associated with the platinum current-voltage probes;   is the ohmic resistance of the amino-p-anisidine-PVC (P 2 );  is resistance and CPE is a constant phase element representing time-dependent capacitive elements.
(1) dc Conductivity Study.For a pellet with thickness  and section area , the dc conductivity was calculated using the following relation: Activation energies (  ) were obtained by fitting the conductivity data to the Arrhenius relation for thermally activated conduction which is calculated using the following equation: where ,  0 ,   , , and  are, respectively, the conductivity, preexponential factor, activation energy, Boltzmann constant, and absolute temperature.
Figure 7 shows the Arrhenius plot of the dc electrical conductivity for the sample named amino-p-anisidine-PVC (P 2 ) in the temperature range 160-220 ∘ C. As can be seen in Figure 7, the Arrhenius plot shows a significant curve which may be interpreted as a transition behavior of the polymer.This curvature is observed at around 190-200 ∘ C and arises from the differences in the conduction mechanism due to the changes in the behaviors of the polymer at 160-190 ∘ C and 200-220 ∘ C temperatures.On the other hand, most likely the transport of ions must occur via indirect motion along a convoluted path restricted to the plasticizer-phase, which is responsible for low conductivity at PVC polymer [4].
The increase in conductivity may be explained by the formation of a conjugate system with the elimination of  HCl and HI groups as shown in the TG analysis described previously.
(2) Frequency Dependence of ac Conductivity.The dependence of  ac with frequency at different temperature is shown in Figure 8.Its value is related to frequency by the following relation [14]: where  is a temperature dependent constant and  is the power exponent.The evolution of  with temperature is tributary to the conduction mechanism.The estimated power exponent  values are shown in Table 1 in terms of temperature.This power exponent fluctuates with temperature between 0.84 and 0.95 in 170 ∘ C-220 ∘ C domain.

Figure 5 :
Figure 5: Complex plane impedance plots of polymer (P 2 ) at different temperatures.

Figure 6 :
Figure 6: Equivalent circuit used for fitting the impedance data.

Figure 7 :
Figure 7: Arrhenius plots of the conductivity of polymer (P 2 ) solid solutions.

Table 1 :
Power exponent of the polymer (P 2 ) for different temperatures.These results show that the polymer (P 2 ) may be useful in a great variety of electrochemical devices such as rechargeable batteries, super-capacitors, and gas sensors.