Polydiphenylamine/Zeolite Y Composites and Electrical Conductivity Responses toward Halogenated Hydrocarbons

Composites of polydiphenylamine (D-PDPA) and zeolite Y with H as the cation (Y_H) have been fabricated to be used as a sensing material towards non-halogenated and halogenated solvents (hexane, dichloromethane, 1, 2-dichloroethane, chloroform). These composites are toxic towards human and environment and are widely used as solvents in various industries. Polydiphenylamine, zeolite Y, and their composites are characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, particle size analysis, surface area, and pore size analysis. The effects of the Si/Al ratio, zeolite content, and vapor concentrations are investigated. The electrical conductivity sensitivity of the composites towards the solvents is higher than the pristine D-PDPA by ~1 order of magnitude. The composites can discriminate a non-halogenated solvent from halogenated solvents. They possess maximum electrical conductivity sensitivity values towards dichloromethane, but the composites do not respond to hexane. Generally, the sensitivity of the composites increases with increasing zeolite content and vapor concentration. The interactions between the composites and the vapors are investigated by FT-IR spectroscopy and UV-Vis spectroscopy. A mechanism for the interaction between the composites and the solvents is proposed.


Introduction
Volatile organic compounds (VOCs) are organic compounds that easily evaporate at normal pressure and room temperature.The VOCs are widely used in daily products and used as solvents in various industries 1 .Nevertheless, the toxicity of these chemical vapors causes serious environmental and human health concerns.The VOCs are divided into two groups; non-halogenated hydrocarbons, and halogenated hydrocarbons.Nonhalogenated hydrocarbons are volatile hydrocarbons that do not contain a chlorine atom within the molecule.Normally, this group is found in daily life products such as plastics, cleaning solvents, and paints, and it can affect human health through the respiratory system.Examples of non-halogenated hydrocarbons are aliphatic hydrocarbons, aromatic hydrocarbons, alcohol, aldehyde, and ketones.The other type of VOCs is halogenated hydrocarbons which consist of a chlorine atom within the molecule.They are used in dry cleaning, metal cleaning, furniture making, thermoplastics production, degreasing, printing, paper and textile production, and paint removal 1 .They are clear liquids, and vaporize at room temperature [2][3][4] .Examples of halogenated solvents are trichloroethylene, perchloroethylene, methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, and methyl chloroform.The toxicity of these chemical vapors is more severe to human health than non-halogenated hydrocarbons.The solvents pose a major source of environmental problems such as stratospheric ozone depletion, smog formation, acid rain production, and global warming.Health problems caused by exposing humans to these solvents are systemic, immunological, neurological, reproductive, developmental, and genotoxic and carcinogenic in nature; which can lead to death 5 .The severity of the chemicals on human health depends on the concentration and exposure levels.The National Institute of Occupational Safety and Health (NIOSH) defined the limit for personal exposure to the chemical that cause to death or immediate or delayed permanent adverse health effects as the immediately dangerous to life or health concentrations (IDLHs) 6 .For instance, the exposure level of chloroform that leads to death in humans is ~40,000 ppm 7 .The level of chloroform that leads to human loss of responsiveness, loss of skeleton muscle reflex, and decreased stress response is less than 22,500 ppm 5,8 .The deadly inhalation concentration for methylene chloride, or dichloromethane (DCM), in a study of animals, was ~3,000 ppm 9 .Hence, effective sensory systems are required to identify the presence of these solvents.One development of sensing materials has focused on conductive polymers due to their unique properties.They are: light weight, easy to synthesize, possess high sensitivity and short response time, and have good mechanical properties over metal oxides [10][11][12] .Polydiphenylamine (PDPA) is an N-aryl substituted derivative of polyaniline and it shows better mechanical strength, electrochemical, conductivity, and electro chromic properties than polyaniline [13][14][15][16] .Additionally, PDPA has been used as a sensing material for many sensory systems: pH sensors, CO sensors, and glucose biosensors [17][18][19] .Another group of materials that can also be used as sensing materials are zeolites.Zeolites are aluminosilicate minerals that have micro porous cages within their crystal structures.Due to their unique structures, zeolites can act as molecular sieves to separate different types of molecules.Thus, zeolites can be used as a catalyst support, an absorbent for moisture and toxic gases, and for ion exchange.Recently, there have been several studies on conductive polymer/zeolite composites as gas or vapor sensing materials [20][21][22] .
This work is aimed at fabricating the composites between D-PDPA and zeolite Y with H + as a cation (Y_H + ) to discriminate between the non-halogenated and halogenated hydrocarbon solvents, based on the electrical conductivity response when exposed to halogenated vapors.The influences of the Si/Al ratio, zeolite Y content, and vapor concentration of the halogenated solvents-dichloromethane (DCM), 1,2-dichloroethane (DCE), and chloroform-on the electrical conductivity response of D-PDPA/zeolite Y composites (D-PDPA/Y_H + ) are investigated.The physical adsorption of the halogenated solvents on the composites is examined by FT-IR and UV-Vis spectroscopy.

Synthesis of PDPA and doping process
Polydiphenylamine (PDPA) was synthesized via chemical oxidative polymerization 23,24 .A DPA solution in toluene was mixed with (NH 4 ) 2 S 2 O 8 solution in distilled water.The solution mixture was stirred by using an overhead stirrer, cooled to 0 °C for 15 minutes, then a HCl solution in distilled water was added to the mixture.The reaction mixture was continuously stirred at 0 °C for 4 hr; next the green slurry of PDPA was precipitated.The precipitate was washed with isopropyl alcohol at least 3 times.Then the HCl-doped PDPA was dedoped in a mixture of ethanol and ammonium hydroxide solution (1:4 v/v) for 24 hours.The neutralized PDPA was doped with 5M HCl solution at a mole ratio of 100:1 (N HCl /N monomer ) and stirred at room temperature for 24 hours.The resultant doped PDPA, D-PDPA, was filtered and dried in a vacuum oven at room temperature for 24 hr to remove excess HCl.

Preparation of D-PDPA pellets and D-PDPA/Y_H + composites
The D-PDPA powder was ground with the Y_H + powder at various Y_H + concentrations (0, 5, 10, 20, and 30 %v/v).The D-PDPA powder and D-PDPA/Y_H + composites were compressed into pellets for the electrical conductivity and sensitivity measurements with a hydraulic press machine (GRASEBY SPECAC) under a 4-5 ton load.Pellets were produced in disc form using a 1 cm stainless steel die.The thickness of each pellet was between 0.03-0.04cm as measured by a digital thickness gauge (PEAACOCK, dial stand type model PDN-20).

Characterization
Fourier transform infrared (FT-IR) spectra of D-PDPA were recorded with a Thermo Nicolet, Nexus 670 FT-IR spectrometer using a resolution of 2 cm -1 , a scan number of 32, and a wavenumber range of 400-4000 cm -1 .Scanning electron micrographs of D-PDPA, Y_H + , and D-PDPA/Y_H + were taken using a HITACHI, S-4800 scanning electron microscope (SEM) with a magnification of 10 000x, operating at 10 kV.The particle sizes of D-PDPA and Y_H + were measured by using a Mastersizer X particle size analyzer.The surface area and pore size of Y_H + were measured with a Physisorption, Quantachrome, Autosorb-1MP surface area analyzer.Zeolite powder was weighed and out gassed at 300 °C over night before adsorption and desorption with He and N 2 gases.During the operation, the zeolite powder was cooled by liquid N 2 .The specific density of the D-PDPA and Y_H + were measured with a pycnometer.The optical properties of the D-PDPA and Y_H + when exposed to DCM were taken by a UV-Vis spectrophotometer (SHIMADZU, UV 1800) in the wavelength number of 200 nm to 500 nm at room temperature.

Conductivity and sensitivity measurements
The conductivity and the sensitivity of D-PDPA, Y_H + , and D-PDPA/Y_H + towards halogenated solvents were measured by a custom built two-point probe connected with a conductivity meter (KEITHLEY 6517A) which applied voltage; the resultant current was measured.The electrical conductivity was calculated by the equation s = (I/KVt), where I is the measured current (A), V is the applied voltage (V), t is the thickness (cm), and K is the geometric correction factor of the two-point probe as calibrated by standard silicon wafer sheets with known specific resistivity values.All measurements were taken at 27 ± 1 °C and at atmospheric pressure.The electrical conductivity responses and sensitivities under exposure to air, N 2 , and halogenated solvents were determined by the equation Ds = s halogenated solvents -s N2, initial and Ds/s N2, initial , respectively, where ∆s is the difference in the specific electrical conductivity (S cm -1 ), s N2, initial is the specific electrical conductivity in N 2 before exposure (S cm -1 ), and s halogenated solvent is the specific electrical conductivity (S cm -1 ) under halogenated solvent exposure.The effect of the halogenated solvent concentration was determined at concentrations of 5, 10, 15, 20, 25, 30, 35, and 50 %v/v in N 2 .

Characterization of PDPA and Y_H +
The FT-IR spectra of D-PDPA showed absorption peaks at 3388, 3053, 1594, 1505, 1318, 1173, 821, 748, and 748-694 cm -1 .These peaks can be assigned to: N-H stretching, C-H in the aromatic ring, quinoid ring stretching, phenyl hydrogen, benzenoid ring stretching, vibration band of N 2 in quinone, C-H out of plane aromatic, 1, 4 substituted on aromatic rings, and C-H out of plane bending vibration, respectively 15,[24][25][26] .The FT-IR spectra of Y_H + show major absorption peaks at 1213, 1080, 836, and 459 cm -1 .These peaks can be assigned to the asymmetric stretch of the internal tetrahedral, the asymmetric stretch of the external linkage, the symmetric stretch of the external linkage, and the T-O bend of the internal tetrahedral, respectively 27,28 .The average particle size of D-PDPA and Y_H + was 125.96 ± 5.51 µm and 20.66 ± 0.58 µm, respectively.The surface area, pore size, and pore volume of Y_H + are comparable at various Si/Al ratios (Table 1).The specific density of D-PDPA and Y_H + is 0.85 ± 0.01 g cm -3 and 1.61 ± 0.35 g cm -3 , respectively.The micrographs shown in Figure 1 reveal that the shapes of Y_H + are square shaped (Figure 1a) and the shape of the D-PDPA particles (Figure 1b) is irregular.Nevertheless, the Y_H + particles are dispersed uniformly within the D-PDPA matrix (Figure 1c).

Sensitivity of D-PDPA and Y_H + towards halogenated solvents
The electrical conductivity sensitivity of D-PDPA, Y_H + towards the four solvents is shown in Figure 2. The D-PDPA and Y_H + exhibit negative responses towards all four solvents.Table 2 shows the initial electrical conductivity values of D-PDPA, Y_H + , D-PDPA/Y_H + [80] when exposed to halogenated solvents.The sensitivity values of D-PDPA, Y_H + towards the four solvents, shown in Table 3, show that D-PDPA and Y_H + possess maximum sensitivity responses towards DCM followed by DCE and chloroform, but they hardly respond to hexane.This is presumably due to the differences in the molecular structure, the strength of the electrostatic interaction, the dipole moment, and the dielectric constant 29 (Table 4).The interaction between D-PDPA and the vapors occurs at the N-H + site and the Cl - atom.Since, the dipole moment of DCM (1.80) is higher than that of DCE (1.60) and chloroform (1.10), the dipole-dipole interaction between the Cl -atom of the DCM molecule and the N-H + site of D-PDPA is stronger than those of DCE and chloroform 30 .Therefore, the sensitivity of D-PDPA toward DCM is higher than those of other vapors.Because the hexane structure is an aliphatic hydrocarbon chain and it has no dipole moment, the interaction between D-PDPA and     hexane vapor is suspected to be physical adsorption on the matrix surface without the dipole-dipole interaction, unlike the other chlorinated hydrocarbon molecules [31][32][33] .Therefore, the sensitivity of D-PDPA towards hexane is low.For zeolite, the interaction between Y_H + and the vapors is the hydrogen bonding at the Si-O -or Al-O -site and the -H atom of the vapor molecules.The sensitivity of Y_H + towards DCM is also higher than those of DCE (~50%) and chloroform (~60%).For DCM, there are two hydrogen atoms that form the hydrogen bond with D-PDPA and Y_H + .For DCE, a lower sensitivity than DCM occurs due to the larger chemical structure of DCE over DCM, known as the steric effect.For chloroform, since there is only one hydrogen atom in the chemical structure, and the dipole moment and dielectric constant are also very low, the strength of the hydrogen bonding between Y_H + and chloroform is lower than the strength of the hydrogen bonding between Y_H + and DCM and Y_H + and DCE 30 .Moreover, the sensitivity of Y_H + increases with increasing Si/Al ratio.This occurs because the hydrophobicity of zeolite increases with the Si/Al ratio 34 .Hence, the halogenated solvents, which are non-polar solvents, produce more favorable responses towards the zeolites Y_H + with higher Si/Al ratios.Therefore, the zeolite Y_H + [80] was chosen to form composites with D-PDPA, in order to further improve the sensitivity of D-PDPA towards the halogenated solvents.

Sensitivity of D-PDPA/Y_H + [80] composites toward halogenated solvents: Effect of zeolite Y contents
The effect of Y_H + [80] content on the sensitivity of D-PDPA/Y_H + [80] towards the halogenated solvents was investigated.The composites show negative sensitivity towards the solvents (Table 3).According to the doping theory 35 , D-PDPA is doped via the p type doping process.When it is exposed to the vapors, which are electron-donating groups, the vapors will give up an electron to fill the hole on the D-PDPA backbone.Thus, the electron mobility along the D-PDPA backbone is more difficult, resulting in a decrease in conductivity, which is why the composites show negative sensitivity towards the solvents.It appears that the sensitivities of the composites are higher than those of pure D-PDPA by about 1 order of magnitude towards DCM and DCE; the sensitivity increases with increasing Y_H + [80] content (Figure 3).With a higher zeolite content, a greater interaction results between the micro-porous structure and the target vapor.With a Y_H + [80] content of ~30 %v/v, the composite shows the highest sensitivity.The sensitivity of chloroform is quite constant with respect to the Y_H + [80] content because it has no dipole moment and very low dielectric constants 36 .The composite does not respond to hexane, even when the zeolite content increases to 30 %v/v, because the interaction between the composite and hexane is only physical absorption.The selectivity of the composite towards the three halogenated solvents is in this order: DCM>DCE>chloroform.Therefore, it can be concluded that the addition of zeolites can improve the sensitivity of D-PDPA and it can discriminate a nonhalogenated solvent from halogenated solvents.

Sensitivity of D-PDPA/Y_H + [80] composites toward halogenated solvents: Effect of vapor concentration
The composite consisting of 30 %v/v of Y_H + [80] with D-PDPA (D-PDPA/30% Y_H + [80]) was next investigated with respect to DCM and DCE at various vapor concentrations.The sensitivity of the composite, which is exposed to DCM, increases from (-1.23 ± 0.004) × 10 -2 to (-3.47 ± 1.64) × 10 -1 as the vapor concentration increases from 7,696 ppm to 153,914 ppm, respectively (Figure 4).When the composite is exposed to DCE, the sensitivity increases from (-1.09 ± 0.96) × 10 -1 to (-1.77 ± 0.04) × 10 -1 as the vapor concentration increases from 1,015 ppm to 20,299 ppm, respectively.When the composites are exposed to both of DCM and DCE, they exhibit linear relationships between the sensitivity and vapor concentration.As the vapor concentration increases, the sensitivity of D-PDPA/30% Y_H + [80] increases.A similar result has been also found by Jiang et al. 37 .They prepared a polypyrrole and polyvinylalcohol composite via in situ vapor state polymerization to detect methanol vapors.The composite showed a linear relationship between sensitivity and gas concentration in the gas range of 50-1059 ppm.The minimum concentrations of DCM and DCE that the composite can detect are 7696 ppm and 1015 ppm, respectively; these values are equivalent to the IDLHs 30,38,39 .The chemical interactions between DCM and the composites were investigated by FT-IR spectroscopy and UV-VIS spectroscopy.Figure 5 shows the FT-IR spectra of the vapors.Before exposure to DCM, the absorption peaks at 1594, 1505, 1318, 1213, 1080, 836, and 748 cm -1 can be assigned to the quinoid ring stretching of D-PDPA 25 , the phenyl hydrogen of D-PDPA 15 , the benzenoid ring stretching of D-PDPA, the asymmetric stretch of the internal tetrahedral in Y_H + [80], the asymmetric stretch of the external linkage, the symmetric stretch of external linkage 27,28 , and the 1, 4 substituted on aromatic rings 24 , respectively.During exposure to DCM, new peaks occur at 3097, 1285, and 752 cm -1 which can be assigned to the hydrogen bonding between the oxygen atom of Y_H + and the hydrogen atom   of DCM molecule 40 , CH 2 in CH 2 -Cl 41 , and the interaction of the chlorine atom of DCM and the nitrogen atom of D-PDPA backbone 42 , respectively.After exposure to DCM, the peaks at 3097, 1285, and 752 cm -1 disappear.This indicates that no interaction occurs between the composite and DCM molecules.The proposed mechanism is shown in Figure 6.From the proposed mechanism, the FT-IR absorption band of D-PDPA cannot be clearly discerned when exposed to DCM, thus the second derivative of the spectrum is used instead.During exposure to DCM, the absorption bands at 1596 and 1318 cm -1 corresponding to the quinoid ring stretching and the benzenoid ring stretching, respectively shift from the original positions before exposed to DCM by 4 cm -1 for thequinoid ring stretching and by 7 cm -1 for the benzenoid ring stretching as shown in Figure 7. Anitha and Subramanian 43 have also observed that the FT-IR absorption band of polyaniline when exposed to DCM shifted by 3 cm -1 due to the electron displacement from chlorine atom of DCM to the polyaniline backbone.
The UV-Vis spectra are used to confirm the interaction between DCM and the composite.The spectrum of the D-PDPA is shown in Figure 8a.There are two absorption bands, one at 282 nm which corresponds to the characteristic absorption bands of DCM, and the other at 342 nm which corresponds to the n-π* transition at -N + = of the D-PDPA backbone 44 .It is clear from Figure 8a that the absorption band is blue shifted from 342 nm to 334 nm and the absorbance intensity decreases with increasing DCM concentration from 5%v/v to 25 %v/v.This indicates that the shift in the absorption band is due to DCM giving up an electron to fill hole on D-PDPA back bone.Thus, the electron mobility along the D-PDPA backbone becomes more difficult, and the band gap of D-PDPA increases.It needs more energy to accelerate the electron to a higher conduction band, resulting in the shift of the electronic band of D-PDPA to a higher energy or a lower wavelength 16,45 .The absorption band observed at 271 nm corresponds to the -Si-O 4 2-site on the Y_H + framework 46 (Figure 8b).The absorption band is red shifted from 271 nm to 277 nm and the absorbance intensity decreases with increasing DCM concentration from 5%v/v to 25 %v/v because the electron

The temporal response D-PDPA/Y_H + [80] composites
The induction time of the D-PDPA when exposed to DCM, DCE, and chloroform was around 13.50 ± 2.12, 8.56 ± 1.25, 15.00 ± 1.41 min, respectively.For the D-PDPA/30% Y_H + [80] composite, the induction time was about 18.50 ± 0.71 min for DCM, 17.65 ± 0.43 min for DCE, 15.95 ± 1.34 min for chloroform.The results showed that the induction time of the composite was higher than D-PDPA.This suggests that the presence of zeolite, a micro porous structure, introduces more active sites for the vapors to absorb.D-PDPA showed a reduction time when exposed to DCM, DCE, and chloroform of 9.50 ± 4.86, 6.35 ± 1.57, and 13.75 ± 7.84 min, respectively.The reduction times of the composite when exposed to DCM, DCE, and chloroform were 6.00 ± 3.04, 6.10 ± 0.49, and 7.05 ± 2.87 min, respectively.The reduction time of the D-PDPA/30% Y_H + [80] composite was also higher than the D-PDPA due to the micro porous structure of the zeolite.A similar result was also found by Thuwachaowsoan and co-worker 48 .They fabricated a composites of poly(3-thiopheneacetic acid) (Pth) with zeolites L, mordenite (MOR), and beta (BETA) as H 2 gas sensors.They found that the induction time of the Pth 200:1/BETA being available for the vapor molecules.The UV-Vis spectra show that the interaction between the composite and DCM does exist; the UV absorption band of D-PDPA and Y_H + is shifted by the delocalization of the Cl -atom.The electron donor, acceptor interaction occurs between Y_H+ and the -H atom when exposed to DCM.It may be concluded that the fabricated composites possess a potential use as a sensing material for discriminating non-halogenated solvents from halogenated hydrocarbon solvents.

Conclusions
D-PDPA was fabricated into a composite with Y_H + to detect the toxic halogenated solvent vapors DCM, DCE, chloroform, and hexane.The composites show improved sensitivity responses towards DCM, DCE, and chloroform than hexane, because of the differences in the chemical structures of the dipole moment, and the dielectric constants of the target vapors.The sensitivities of the composites increase with increasing Si/Al ratio, zeolite content, and vapor concentration due to greater interactions between the zeolite and target vapors.For the temporal response, the induction and recovery times of the composite were higher than those of the pristine D-PDPA due to more active sites

Figure 2 .
Figure 2. The sensitivity of D-PDPA, Y_H + at various Si/Al ratios when exposed to halogenated solvents at 27±1°C and 1 atm.

Table 4 .
Values of dielectric constant (ε), dipole moment (µ) of the solvent tested29 .interact between the -Si-O 4 2-site of the Y_H + framework and the -H atom of the DCM molecule.For D-PDPA/30% Y_H + [80] composite (Figure8c), they are also two absorption bands at ~340 nm and ~265 nm which correspond to the n-π* transition at -N + = of the D-PDPA backbone and the -Si-O 4 2-site on the Y_H + framework, respectively.The absorption band at 342 nm is blue shifted from 340 nm to 336 nm.The absorption band at 262 nm is red shifted to 275 nm.This occurs similarly to D-PDPA and Y_H + [80].It can be concluded that the interaction between the composite of D-PDPA/30% Y_H + [80] and DCM occurs as shown in the proposed mechanism (Figure7).Choi and co-worker47 studied iodine absorption on various zeolites.The absorption band was gradually red shifted when the iodine molecules increasingly absorbed on the zeolite Y framework.They attributed the phenomenon to -Si-O 4 2donating an electron to the adsorbed iodine molecules.
a ∆s = the electrical response; b ∆s/s N2 = electrical conductivity sensitivity.