Compatibility of epoxidized natural rubber and polyaniline blend

Cite this paper: Vietnam J. Chem., 2021, 59(1), 32-36 Article DOI: 10.1002/vjch.202000089 32 Wiley Online Library © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH Compatibility of epoxidized natural rubber and polyaniline blend Nguyen Thu Ha 1* , Nguyen Thi Hoa 1 , Vu Van Tung 1 , Nguyen Lan Anh 1 , Tran Thi Thuy 1 , Toshiaki Ougizawa 2 1 School of Chemical Engineering, Hanoi University of Science and Technology, No.1 Dai Co Viet, Hai Ba Trung District, Hanoi 10000, Viet Nam 2 Department of Material Science and Engineering, Tokyo Institute of Technology, 152-8550, Meguro City, Tokyo, Japan Submitted May 29, 2020; Accepted July 9, 2020 Abstract The compatibility of binary blend of epoxidized natural rubber and polyaniline was investigated. Epoxidized natural rubber with 25 mol% of epoxy group content was prepared through epoxidation of deproteinized natural rubber in latex stage. Blends of epoxidized natural rubber and polyaniline with various ratios were prepared. The compatibility of the blends was examined through differential scanning calorimetry measurement and scanning electron microscopy observation. Obtained result showed that the presence of epoxy group significantly improved the compatibility of binary blends of epoxidized natural rubber and polyaniline blends. The compatibility significantly enhanced the electrical conductivity of epoxidized natural rubber/polyaniline blends. Keywords. Epoxidized natural rubber, polyaniline, compatibility. 1. INTRODUCTION Blending natural rubber (NR) and polyaniline (PAni) was recognized to be efficient method to apply PAni in daily life. [1-3] The blend is expected to combine excellent elasticity of NR and outstanding electrical conductivity of PAni. This material may be useful for antistatic coating, corrosion protection, electromagnetic shielding application, and so forth. [4-6] However, due to the difference in polarity of NR and PAni, the interfacial adhesion between these two polymers are poor, resulting in the incompatibility of NR and PAni phase in blend. [7,8] Compatibility of components is a key factor to determine the properties of NR/PAni blend. For instance, polymer blends generally exhibit inferior electrical conductivity due to incompatibility and phase separation. Several trials were carried out to minimize phase separation of PAni and NR, and increase interfacial adhesion between them. [9,10] In the previous work, we used expanded graphite as a third component to increase the compatibility of binary blends of PAni and NR. In this work, we proposed a new strategy to improve the interaction between the constituent polymers. Through introducing a functional group into NR, it is anticipated to obtain compatible blend of PAni and NR. Depending upon the degree of molecular mixing, the blends were categorized as miscible, compatible (semi-miscible) and incompatible (immiscible) blends. In blends, the interaction such as hydrogen bonding, dipol - dipol plays a dominant to provide adhesion between phases. [11,12] In the previous work, epoxy group was used to increase adhesion between phases. The compatibility between epoxy natural rubber and polar polymers such as poly(lactic acid), chitosan, starch was investigated. [13,14] It was found that when epoxy group was introduced to natural rubber, the interaction between natural rubber phase and a polar polymer phase was improved. Therefore, we may apply the results in previous works which was introducing epoxy group to natural rubber to enhance the interaction between PAni phase and rubber phase. The aim of this work is to enhance the compatibility of PAni and natural rubber by introducing epoxy group. The role of epoxy group on compatibility of PAni and NR was examined. In this work, epoxidized natural rubber was prepared and blended with PAni. Because proteins naturally present in NR may cause side reations during the epoxidation, the removal of protein from NR was carried out [15,16] The structure of resulting epoxidized natural rubber was characterized through gel permeation chromatography (GPC) and nuclear magnetic resonance (NMR) spectroscopy. The Vietnam Journal of Chemistry Nguyen Thu Ha et al. © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 33 compatibility of PAni and rubber phases were studied through differential scanning calorimetry (DSC) analysis and scanning electron microscopy (SEM) observation. The effect of compatibility on electrical conductivity of epoxidized natural rubber/PAni blend was investigated. 2. MATERIALS AND METHODS 2.1. Materials High ammoniated natural rubber (HANR)with dried rubber content (DRC) of 60 w/w% was the commercial product of Dau Tieng Rubber Corporation (Vietnam). Polyaniline (99.9 w/w%) with conductivity of 7.5 S/cm was supplied by Beijing Mayor Chemical Technology Co., Ltd. Urea (99.5 %), sodium dodecyl sulfate (SDS, 99.0 %), hydrogen peroxide (30 %), anhydride acetic (99 %), ammonia solution (28 %) was purchased from Sigma Aldrich. 2.2. Sample preparation DRC of initial HANR latex was adjusted DRC 30 %w/w by distilled water. The obtained dilute HANR latex was incubated with urea 0.1 w/w% and SDS 1 w/w% for 1 hour at room temperature. Thereafter, the mixture was centrifuged at 10,000 g at 15 o C in 30 minutes. The obtained cream fraction was purified by re-dispersing in SDS solution and centrifuging at 10,000 g at 15 o C in 30 minutes. This step was repeated three times. Finally, the cream fraction was dispersed in SDS 0.1 w/w% solution to obtain deproteinized natural rubber (DPNR) latex. DPNR latex adjusted DRC to 10 w/w% was epoxidized in latex stage with various amounts of fresh peracetic acid (concentration of 33 w/w%) at 10 o C for 3 hours. After the completion of reaction, pH of resulting solution was adjusted to 7 by ammonia solution, then centrifugated at 9,000 g for 30 minutes to obtain epoxidized deproteinized natural rubber (EDPNR). EDPNR was dried under reduced pressure at 50 o C for a week. EDPNR and PAni was dissolved in CHCl3 and stirred in 24 hours. The obtained solution was casted into film in a petri dish. CHCl3 was allowed to evaporate in a fume cupboard at room temperature to obtain EDPNR/PAni blend. The composition and abbreviation of blends are shown in table 1. 2.3. Characterization GPC measurements for samples were made in a TOSOH GPC consisting of a TOSOH CCPD pump, a RI-8012 differential refractometer, and a UV-8011 UV detector. The flow rate of the mobile phase, i.e. THF of 0.5 ml/min. Standard polyisoprenes purchased from PSS Polymer Standards Service GmbH (Mainz, Germany), were used for preparing a calibration curve. Table 1: Composition and abbreviation of blends Sample EDPNR (w/w%) PAni (w/w%) 2EDPNR/8PAni 20 80 4EDPNR/6PAni 40 60 5EDPNR/5PAni 50 50 DSC measurement for samples was run in a DSC 7020 (SII NanoTechnology Inc.). The sample was heated from -80 to 150 o C at a heating rate of 10 o C/min. The surface of samples was sputter coated with a gold layer before SEM observation. SEM images was taken in a Hitachi TM4000 Plus at the accelerating voltage of 15 kV. The sample with the thickness of 0.1 cm was cut into rectange shape of 1.0-2.0 cm. The electrical conductivity of sample was measured using a four- probe system coupled to a Keithley 2000 electrometer. 3. RESULTS AND DISCUSSION 3.1. Structure characterization of EDPNR The epoxy group content of EDPNR was calculated from 1 H-NMR spectrum. [17] The 1 H-NMR measurement was carried out on a JEOL FT-NMR ECA-400 at 400 MHz at 25 o C. It was estimated that the epoxy group content of EDPNR prepared in this study was 25 mol%. The molecular weight distribution of DPNR and EDPNR is shown in figure 1. The weight-average molecular weight (Mw) of DPNR and EDPNR was 8.3105 and 9.6104 Da, respectively. In addition, the molecular weight distribution of EDPNR was found to be more broaden than that of DPNR. The low molecular weight of EDPNR was expected to contribute to the compatibility of EDPNR and PAni. 3.2. DSC results Glass transistion temperature (Tg) of a sample was determined as a point of inflection in DSC Vietnam Journal of Chemistry Compatibility of epoxidized natural rubber © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 34 Figure 1: GPC curves of (a) DPNR, (b) EDPNR Figure 2: DSC thermograms of (a) EDPNR, (b) PAni, (c) 2EDPNR/8PAni, (d) 4EDPNR/6PAni, (e) 5EDPNR/5PAni thermogram. The compatibility of a multicomponent system can be evaluated by Tg determination. It is classified into the following three: one Tg for miscible system, closed two Tg’s for compatible system, and distinct two Tg’s for immiscible system. [18] DSC thermograms of the samples is depicted in figure 2. In the case of EDPNR, a Tg was found at -44 o C. For PAni, a Tg appeared at 108 o C. In 2EDPNR/8PAni blend, the Tg of EDPNR could not be seen, wheareas a Tg was found at 105 o C. The presence of one Tg may indicate that only one phase occurred in 2EDPNR/8PAni blend. When the percentage of EDPNR went up to 40 w/w%, the DSC curve of the blend showed only one Tg as well. The Tg of 4EDPNR/6PAni slightly decreased in comparison with that of 2EDPNR/8PAni. In these blends, PAni was considered as “solvent” and EDPNR was as “solute”. The blends is expected to exhibit the uniformity in properties, since one phase was found for EDPNR - PAni system. When EDPNR percentage is higher than 40 w/w%, the two-component system of EDPNR - PAni displayed two phases. As seen in DSC thermogram of 5EDPNR/5PAni, there were two Tg - present at -47 o C and 93 o C. This may be attributed to two phases, which are EDPNR-rich phase and PAni-rich phase, respectively. In the previous work, it was recognized that NR/PAni was an immiscible system. [9] In this present work, EDPNR/PAni system was found to be miscible when EDPNR percentage was less than or equal to 40 w/w%, and compatible when EDPNR percentage was higher than 40 w/w%. The compatibility of EDPNR in PAni was attributed to the presence of epoxy group. The strong interaction between epoxy group of natural rubber and NH group of polyaniline enhanced the compatibility of these two phases. In addition, the low molecular weight of EDPNR was probable to improve the compatibility of EDPNR in PAni because EDPNR was easy to “dissolve” in PAni. 3.3. SEM observation The observation of phases in blend was carried out through SEM. Figure 3 illustrates the SEM image of different blends, i.e. 2EDPNR/8PAni, 4EDPNR/6PAni, and 5EDPNR/5PAni. In SEM image of 2EDPNR/8PAni, the uniform surface was observed. The particles were finer in diameter. This observation suggested that only one phase occurred in 2EDPNR/8PAni. In 4EDPNR/6PAni, the region of EDPNR-rich phase with the diameter of 100 µm appeared. However, in 5EDPNR/5PAni, the region of EDPNR-rich phase became larger. The observation may suggest that the blends have co- continuous structure. [19] The existence of this phase was confirmed in DSC thermogram. The observation from SEM was in good agreement with the result from DSC. 3.4. Electrical conductivity measuring results Electrical conductivity of samples was tabulated in table 2. When EDPNR was added into PAni, electrical conductivity of sample reduced. The worthy note was that the electrical conductivity of 2EDPNR/8PAni and 4EDPNR/6PAni was remarkably higher than that of 5EDPNR/5PAni. This may be due the presence of EDPNR-rich phase W (l o g M ) 107106105104 0 0.2 0.4 0.6 0.8 1.0 Molar mass (Da) (a)(b) Vietnam Journal of Chemistry Nguyen Thu Ha et al. © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 35 in the blend when EDPNR percentage was high. The region of EDPNR-rich phase restricted the current transportation, which lowered the electrical conductivity of blends. When EDPNR percentage was low, EDPNR which was miscible in PAni did not affect significantly on electrical conductivity of the blend. This result was consistent with the DSC measurement and SEM observation. Figure 3: SEM images of (a) 2EDPNR/8PAni, (b) 4EDPNR/6PAni, (c) 5EDPNR/5PAni Table 2: Electrical conductivity of samples Samples Conductivity (S/cm) EDPNR - 2EDPNR/8PAni 8.910-2 4EDPNR/6PAni 1.510-2 5EDPNR/5PAni 2.810-5 4 CONCLUSION Epoxy group in natural rubber improved the compatibility of EDPNR and PAni. When EDPNR percentage was higher than 40 w/w%, EDPNR-rich phase occurred, which was confirmed by DSC and SEM results. The compatibility of EDPNR and PAni enhanced the electrical conductivity of blends. Acknowledgment. This work was supported by National Foundation for Science and Technology Development (grant code NAFOSTED 104.02- 2017.20). REFERENCES 1. P. Sukitpaneenit, T. Thanpitcha, A. Sirivat, C. Weder, R. Rujiravanit. 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Sci., Polym. Rev., 2003, C43, 87-141. Corresponding author: Nguyen Thu Ha Hanoi University of Science and Technology 1, Dai Co Viet, Hai Ba Trung, Hanoi 10000, Viet Nam E-mail: ha.nguyenthu5@hust.edu.vn Tel.: +84- 983671674.