Electrical , Mechanical , and Thermal Analysis of Natural Rubber / Polyaniline-Dbsa Composite

A composite of natural rubber (NR) with polyaniline (PANI) was obtained by mixing an aqueous dispersion of dodecylbenzenesulfonic acid (DBSA)-doped PANI with NR latex in different concentrations. Films were obtained by the casting method and characterized by ultraviolet visible near-infrared (UV-Vis-NIR) spectroscopy, thermogravimetry/differential thermogravimetry (TG/ DTG), stress-strain testing, differential scanning calorimetry (DSC), and DC electrical conductivity measurements. The UV-vis-NIR spectrum showed that PANI remained doped in the composite, and this improved the mechanical and electrical proprieties of NR films and afforded them good thermal stability up to ~200°C. The percolation threshold did not follow the universal critical exponent, and in this case, conduction preferentially occurs by hopping and tunneling.


Introduction
Among currently available intrinsically conducting polymers (ICP), polyaniline (PANI) has emerged as one of the most promising ones for technological applications owing to its good electrical properties, environmental stability, low production cost, and ease of synthesis 1,2 .As a result, PANI is considered a strong candidate for applications such as sensors, electromagnetic shielding (EMI), corrosion protection, and actuators [1][2][3] .However, PANI's applications are limited by its poor infusibility, low solubility in organic solvents, and poor mechanical properties 2 .PANI's solubility can be improved by using organic acids such as dodecylbenzenesulfonic acid (DBSA) and p-toluenesulfonic acid (PTSA) that not only improve the compatibility of PANI with the host matrix but also acts as dopants, thus increasing the electrical conductivity of the material 4 .PANI's mechanical properties can be improved by mixing it with a polymeric host matrix to form composites or blends  . Manypolymeric materials can be used as supports for PANI, such as cellulose nanofiber 5 , epoxy resin 7 , polyvinyl chloride (PVC) 4 , polyurethane 8 , poly(methyl methacrylate) (PMMA) 9 , and rubbers [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] .Among polymeric matrixes, natural rubber (NR) has been increasingly used for forming composites owing to its unique mechanical properties.NR is extensively used in various products that require superior properties such as elasticity, flexibility, and resilience.
NR/PANI blends and composites have already been obtained using different methods such as mill mixing 14 , solution/dispersion mixing 11,12,16,22 , and electrochemical 17 and chemical polymerization 10 of aniline in the presence of a host matrix.However, few studies have focused on NR/ PANI composites obtained by a mixture of PANI dissolved in an organic solvent with NR latex [10][11][12] .
In this study, NR/PANI-DBSA composites were obtained by mixing NR latex and an aqueous dispersion of DBSAdoped PANI in different concentrations.These composites were characterized by ultraviolet visible near-infrared (UV-Vis-NIR) spectroscopy, thermogravimetric/differential thermogravimetric (TG/DTG) analysis, differential scanning calorimetry (DSC), stress-strain measurements, and DC electrical conductivity measurements.It was found that PANI-DBSA improved the mechanical proprieties of NR and that the composite showed low electrical percolation threshold.

Material
Analytical grade aniline was purchased from Sigma-Aldrich, distilled under vacuum, and stored in a refrigerator before being polymerized.Ammonium peroxydisulfate (APS) and DBSA (70 wt% in 2-propanol) were purchased from Sigma-Aldrich and used as received.NR latex was collected from Hevea brasiliensis trees (Clone RRIM 600) planted in the Experimental Farm of the University of São Paulo State (UNESP), campus of Ilha Solteira, Brazil, and stabilized in a commercial solution of ammonium hydroxide to avoid coagulation.The dry rubber content was determined by standard methods 13 .

Synthesis of PANI-DBSA
An aqueous dispersion of PANI-DBSA complex was prepared by the oxidative polymerization of aniline in the presence of DBSA in aqueous media.In a typical procedure, 1.0 mL of aniline and 7.6 mL of DBSA were mixed in 500 mL of deionized water under constant stirring.After *e-mail: mal@dfq.feis.unesp.br 1 h, 10 mL of an aqueous solution containing 0.61 g of APS was added to the mixture.The medium was kept at 5°C under magnetic stirring and after 12 h of reaction, the PANI-DBSA complex was separated from the medium by centrifugation.PANI-DBSA was re-dispersed in water and centrifuged again.This procedure was repeated three times, and the final content was either re-dispersed in water in the desirable concentration (e.g., for characterization) or kept at high concentration.

Preparation of NR/PANI-DBSA composite
The NR/PANI-DBSA composite was obtained by the mixture of PANI-DBSA aqueous solution (5.2 w/v) in NR latex (pH = 7.0, 41.0 w/v of NR) at concentrations of 3-10 wt%.The mixture was kept under constant stirring for 2 h at ambient temperature, following which it was cast on a glass substrate and dried in a conventional oven at 60°C for 12 h to obtain ~200-µm-thick films.

Methods
UV-Vis-NIR absorption spectra of NR/PANI-DBSA films were obtained using a Cary 50 spectrophotometer (Varian).Spectra were recorded from 300-1000 nm.Thermogravimetric analysis was carried out in the temperature range of 25-600°C at a heating rate of 10°C/min in nitrogen atmosphere with a flow rate of 60 mL/min using a Q500 (TA Instruments).Approximately 10 mg were used for each sample.The glass transition temperature (T g ) of the samples (10.0 mg) was measured using a MDSC 292 (TA Instruments) with a scan rate of 10°C/min within the temperature range of -100 to 150°C under nitrogen atmosphere.
Mechanical tests were conducted in accordance with ASTM D882 using an Instron tensometer at a crosshead speed of 500 mm/min and a 100-N load cell.Electrical conductivity measurements of the samples were carried out using a two-probe method.Gold electrodes were evaporated onto both faces of the film for electrical contact.A power source that provides a constant voltage and measures the current (Model 247, Keithley Instruments) was used to measure the current through the sample.The electrical conductivity σ (S/cm) was calculated according to Equation 1: . .
where d (cm) is the thickness of the film; I (A), the current driven through the sample; A e (cm 2 ), the electrode area; and V (V), the applied voltage.

Results and Discussion
UV-Vis-NIR absorption spectra of DBSA-doped PANI and the NR/PANI-DBSA composite are shown in Figure 1.Three bands are observed in the PANI-DBSA spectrum, indicating that the polymer is in its emeraldine salt form.The band at 340 nm is assigned to the π-π* transition of benzene rings and those at ~800 nm and ~420 nm, to polaron bands related to the doping process and conductivity of PANI 5,10,19 .The same bands are observed in the UV-VIS-NIR spectra of NR/PANI-DBSA composites, indicating that the high pH of natural latex did not lead to the dedoping of PANI.
Figure 2 shows typical TG/DTG curves obtained for the neat NR, PANI-DBSA, and NR/PANI-DBSA composite with 10 wt% of PANI-DBSA.The TG curve of DBSA-PANI shows three main weight loss stages.The first weight loss occurred before 100°C owing to the loss of water and other volatiles; the second, in a temperature range of 200-350°C owing to the evaporation and degradation of DBSA and the oxidation of the PANI structure 20,21 ; and the third, in a relatively wide temperature range of 400-500°C owing to the degradation of the bound PANI-DBSA and the decomposition of PANI 20,21 .
The TG/DTG curves of the neat NR and composite basically show the same decomposition mechanism.The NR TG profile shows remarkable weight loss in the temperature range of 300-450°C corresponding to the structural decomposition of rubber in nitrogen atmosphere 26 .The NR/PANI-DBSA composite also showed a peak at ~370°C (major peak) that was mainly attributed to rubber decomposition and two discrete weight loss steps in the temperature ranges of 60-100°C and 400-500°C, corresponding to the loss of water and the degradation of PANI and DBSA bounded in the composite, respectively.
The effect of the addition of PANI-DBSA on the T g of NR was investigated by DSC, and the results did not show a significant change in the T g of NR with addition of up to 10 wt% of PANI-DBSA, as shown in Figure 3.In both samples, the T g was around -63°C.
Stress-strain tests were performed under uniaxial extension; Table 1 shows the analytical results of the mechanical properties and Figure 4, the tensile curves.The addition of PANI-DBSA improved the mechanical properties of the composites.Both Young's modulus (determined from the initial slope of the tensile curves) and tensile strength significantly increased with the addition of PANI-DBSA to rubber.
This effect is attributed to the rigidity of PANI.Increasing the PANI-DBSA content in the composite to 10 wt% led to no significant change in the tensile profile compared to a proportion of 5 wt%.This behavior can be related to the amount of water uptake in the composite owing to the hygroscopic characteristics of DBSA-doped PANI 27 .
Figure 5 shows the electrical conductivity of the composite films as a function of the PANI-DBSA content.The conductivity increased with an increase in the PANI-DBSA content in the NR matrix, reaching a value of 10 -6 S/cm for 10 wt% of PANI-DBSA.The percolation threshold was found to be ~3.1 wt%.
By percolation theory, when a conducting continuous network is formed in the composite through connections    between adjacent conducting particles, the electrical conductivity behavior can be calculated using a typical power-law 28,29 : where p c is the critical concentration or percolation threshold; p, the concentration of the conductive phase; k, a constant; and t, the conductivity critical exponent.The data from Figure 5 were fitted to a plot of log (σ) versus log (p-p c ) according to Equation 2, as illustrated in Figure 6, to estimate the values of the critical exponent (t) and constant (k).The values t and k were estimated by fitting the data shown in Figure 6, in which the values were t = 3.3 and k = 3.7 × 10 -9 .
In polymeric composites filled with a low proportion of conducting particles, the mean distance between particles or clusters is sufficiently large and the conductivity is restricted by the presence of the polymeric matrix.However, by increasing the conducting phase content at the percolation threshold, a physical path is formed.The t value obtained is larger than that obtained by the universal percolation theory.The behavior of the nonuniversal critical exponent of conductivity in polymer composites has been reported in literature 30,31 ; it is attributed to the formation of an electrical percolation network in which the particles are not in direct physical contact 32 .In this case, the conduction process in the composite preferentially occurs via hopping and tunneling of charge carriers between neighboring particles or particles clusters 32 .
Above the percolation threshold, the electrical conductivity of the composite increased by seven orders of magnitude compared to that of neat NR (10 -14 S/cm).

Conclusion
An NR/PANI-DBSA composite was obtained by incorporating an aqueous dispersion of DBSA-doped PANI into NR latex, and films of the composite were obtained by casting methods.The mechanical and electrical proprieties of the NR films were improved by the incorporation of PANI-DBSA with good thermal stability up to ~200°C.The percolation threshold (p c ) and critical exponent values were p c = 3.1 and t = 3.3, respectively.The behavior of the nonuniversal critical exponent was attributed to electrical percolation.For a composite with 10% of PANI-DBSA content, an electrical conductivity of ~10 -6 S/cm was attained, which is seven orders of magnitude higher than that of neat NR.

Figure 3 .
Figure 3. DSC curves of the neat NR sample and NR/PANI-DBSA composite.

Figure 4 .
Figure 4. Stress-strain curves of the samples of the NR/PANI-DBSA composite films with proportions of 95/05 and 90/10 and neat NR.

Figure 5 .
Figure 5. Electrical conductivity of NR/PANI-DBSA composites as a function of PANI-DBSA content.