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Effective anticorrosion coatings prepared from sulfonated electroactive polyurea
Polymer 166 (2019) 98–107
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Effective anticorrosion coatings prepared from sulfonated electroactive polyurea
T
Kuan-Ying Chena, Yen-Shi Laia, Jun-Kai Youa, Karen S. Santiagob, Jui-Ming Yeha,∗ a Department of Chemistry, Center for Nanotechnology and Center for Membrane Technology at Chung Yuan Christian University, Chung Li District‚ 32023‚ Tao-Yuan City, Taiwan, ROC b Department of Chemistry, Research Center for the Natural and Applied Sciences, University of Santo Tomas, Espana, Manila, 1015, Philippines
H I GH L IG H T S
amino-capped aniline trimer was synthesized successful. • Sulfonated with sulfonated groups show better redox ability than those without. • Compounds • The S-EPU has good corrosion protection ability, redox ability and thermal stability.
A R T I C LE I N FO
A B S T R A C T
Keywords: Corrosion Coating Electroactive Polyurea Sulfonated
In the present study, the sulfonated electroactive polyurea (S-EPU) was first prepared and characterized, applied as an anticorrosion coating. The electroactive oligomer sulfonated amine-capped aniline trimer (S-ACAT) was synthesized by oxidative coupling reaction, followed by characterization using Fourier-transform infrared (FTIR) and mass spectroscopy (MS). For the preparation of sulfonated electroactive polyurea (S-EPU), isophorone diisocyanate (IPDI) was reacted with poly(tetramethylene ether) glycol (PTMEG) in the presence of a specific amount of diamine S-ACAT. The success of the S-EPU was confirmed through FTIR and GPC. After that the redox capability of as-prepared S-EPU was confirmed by CV studies. Chemical oxidization of S-EPU, carried out through the introduction of trace amount of ammonium persulfate, was monitored by UV-VIS spectroscopy. The electrochemical corrosion of the electroactive polymer-coated cold-rolled steel (CRS) electrode, in neutral, acidic and alkaline conditions, were measured and compared. As compared to non-electroactive polyurea (N-EPU), results showed enhanced corrosion protection of the S-EPU coating and electroactive polyurea (EPU) coating, owing to their redox capability that induces the formation of densely passive metal oxide layer (e.g., Fe2O3 and Fe3O4). The densely metal oxide layers induced by S-EPU/EPU coatings were further confirmed by Raman spectroscopy, SEM and ESCA. Moreover, the S-EPU coating exhibited better anticorrosion performance than that of EPU coating which could be attributed to the higher electro-catalytic property of S-EPU coating upon CRS electrode, causing the formation of more densely metal oxide layers to shelter the underlying metal substrate.
1. Introduction Corrosion phenomena are regarded as a common degradation behavior of distinctive metallic substrates caused by their interactions with surrounding environments and are generally considered inevitable [1]. Currently, one strategy to impart anticorrosion property to distinct metallic substrates is to treat them with versatile organic or polymeric coatings. Among many currently used polymeric coating materials, intrinsically electroactive polymers with conjugated chemical structures, showing reversible redox capability, have attracted great
∗
attention in the development of anticorrosion coatings. Generally, the mechanism by which electroactive polymeric coatings improve on distinctive metallic substrates is attributed to their electro-catalytic (i.e., redox) property, which may induce the formation of densely passive metal oxide layer to avoid underlying metallic substrates damage [2–4]. Electroactive polymers mainly include polyaniline (PANI), polythiophene (PTH), polypyrrole (PPY), and their derivatives. They possess many potential applications including rechargeable battery, electromagnetic interference shielding, photothermal therapy, chemical
Corresponding author. E-mail address: [email protected] (J.-M. Yeh).
https://doi.org/10.1016/j.polymer.2019.01.037 Received 30 October 2018; Received in revised form 9 January 2019; Accepted 16 January 2019 Available online 22 January 2019 0032-3861/ © 2019 Elsevier Ltd. All rights reserved.
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monitored by UV–visible absorption spectroscopy. Corrosion protection performance of CRS electrode coated with S-EPU/EPU/N-EPU coatings was investigated by performing a series of electrochemical corrosion measurements in NaCl(aq), HCl(aq) and NaOH(aq), to investigate the abilities of the corrosion protection in neutral, acidic and alkaline conditions [20,21]. The formation of densely passive metal oxide layers induced by the electroactive coating of S-EPU/EPU was confirmed by Raman spectroscopy, SEM and ESCA. Finally, the thermal properties were confirmed by TGA and DSC.
sensor, gas separation membrane, anticorrosion coating, microwave absorption, etc [5]. However, one drawback of electroactive polymers, such as polyaniline (PANI), is their poor solubility in common organic solvents, which limit their processing and practical application in various fields. Therefore, studies dealing on the synthesis and characterization of electroactive oligomers with good solubility have received extensive research interests. Electroactive oligomers (e.g., aniline trimer, tetramer, pentamer, etc.) are known to exhibit good solubility in many common organic solvents and exhibit comparable electroactive capability as that of polymers. For instance, Wei et al. explored the synthesis and characterization of amine-capped aniline trimer of reversibly redox and dope/de-doped properties [6]. Moreover, Zhang et al. reported the synthesis and electrochemical redox properties of aniline tetramer/pentamer [7–11]. On the other hand, many scientific literatures deal with electroactive polymers containing amine-capped aniline oligomers revealing primary amine at both ends (e.g., trimer, tetramer, pentamer, etc.) [12]. For example, electrochemical behavior of aniline oligomer-based polyimides had been published by Wang et al. [8,13,14]. The results showed that the polyimide chains with electroactive aniline oligomer segments, could lead that the polyimides have higher electroactive ability. Moreover, polyamides containing amine-capped aniline pentamer have been explored by Zhang et al. [15]. Recently, Yeh et al. reported the anticorrosion effects of CRS electrode coated with electroactive epoxy/polyimide containing ACAT by performing a series of electrochemical corrosion measurements in aqueous NaCl conditions [16,17]. Based from the literatures mention above, corrosion protection effect of electroactive polymer coatings was found to be ascribed from their electro-catalytic (i.e., redox) capability, which may induce the formation of densely passive metal oxide layers between the interface of coating and metal to protect the underlying substrate from damage. Moreover, electroactive polymer coatings with higher electro-catalytic (redox) capability were found to exhibit better anticorrosion performance. For example, Mozafari et al. [18] reported that the aniline oligomers with higher conjugated chain length were found to reveal higher redox capability. In addition, Yeh et al. explored [19] that the anticorrosion of electroactive polymer coatings by incorporating small amount of graphene nano-sheets to promote the redox capability and gas barrier property of coating. Therefore, notable strategy is to develope electroactive polymers with higher electro-catalytic capability. In this work, sulfonic acid group was designed to be functionalized the middle benzene ring of amine-capped aniline trimer (ACAT) at the ortho-position, and was denoted as S-ACAT. We therefore envision that the presence of sulfonic acid group in ACAT will facilitate its redox capability, which can be further confirmed by the electrochemical cyclic voltammetry studies. Electroactive polyureas resulting from SACAT and ACAT (denoted by S-EPU and EPU) were also prepared and characterized by FTIR spectroscopy and GPC. N-EPU was also prepared for comparative studies. Redox capability of as-prepared S-EPU/EPU/ N-EPU was identified by electrochemical CV studies. Chemical oxidization of S-EPU and EPU in the presence of trace amount of oxidant was
2. Experimental section 2.1. Materials and instrumentation Aniline (99%, Fluka) was doubly distilled prior use. poly(tetramethylene ether) glycol (PTMEG, Mn∼1000, Aldrich), isophorone diisocyanate (IPDI, 98%, Sigma-Aldrich), 5-Amino-1,3,3-trimethyl- cyclohexanemethylamine (IPDA, > 98%, Sigma) ρ-Phenylenediamine (98%, Sigma), 2,5-diaminobenzenesulfonic acid (> 97%, Sigma), ammonium persulfate (APS, > 98%, J.T. Baker), hydrochloric acid (HCl, 37.0%, Sigma-Aldrich), ammonium hydroxide solution (NH4OH, Sigma-Aldrich), sodium hydroxide (NaOH, 97%, SHOWA), ethanol (> 95%, J.T. Baker), isopropanol (IPA, > 99.5%, J.T. Baker), hydrazine hydrate (55%, Sigma-Aldrich), sodium chloride (J.T. Baker), N,N-dimethylformamide (DMF, > 99%, J.T. Baker) were used as received without further treatment. Fourier transform infrared (FTIR) analysis was carried out with JASCO FT/IR-4100 at room temperature. Ion-trap mass spectrometry (MS) was run on a Bruker Daltonics IT mass spectrometer model Esquire 2000 (Leipzig, German) with an Agilent ESI source (model G1607-6001). Cyclic voltammetry experiments were performed on VoltaLab 40 potentiostat/galvanostat in a standard corrosion cell equipped with two graphite rod counter electrode and a saturated calomel electrode (SCE) as well as the working electrode. Electrochemical impedance spectroscopy (EIS) measurements were recorded on an AutoLab (PGSTAT302 N) potentiostat/galvanostat electrochemical analyzer. UV–Visible absorption spectroscopy was obtained using a Hitachi U-2000 UV–Visible spectrometer. Raman spectroscopy (Horbi Jobin Yvon iHR320), Scanning electron microscopy (SEM, Hitachi S2300) and electron spectroscopy for chemical analysis. (ESCA, VG Scientific ESCALAB 250) were used to characterize the densely passive metal oxide layers on the surface of CRS electrode induced by electroactive polymer coatings. Thermal gravimetric analysis (TGA) was performed on a DuPont TA Q50 thermal analysis system in air. Differential scanning calorimetry (DSC) was performed on a DuPont TA Q10 differential scanning calorimeter at a heating or cooling rate of 5 °C/min in air atmosphere. 2.2. Synthesis of S-ACAT and ACAT The representative procedure to synthesize sulfonated amino-
Scheme 1. Preparation of (a) S-ACAT and (b) ACAT. 99
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3. Results and discussion
capped aniline trimer (denoted by S-ACAT) is shown in Scheme 1 (a). The detailed step to synthesis S-ACAT are given as follows: 1.5 g (i.e., 8.0 mmol) of 2,5-diamino-benzenesulfonic acid and 1.5 g (i.e., 16 mmol) of aniline were dissolved in 100 mL of 1.0 M HCl aqueous solution. Afterwards, 3.6 g (i.e., 15.7 mmol) of APS was introduced into the previous solution under magnetic stirring at 0°C for 1 h. The resulting precipitate was then collected by filtration and washed with 100 mL of 1.0 M HCl aqueous solution. Subsequently, the collected solid product was dispersed into 60 mL of 1.0 M NH4OH aqueous solution under magnetic stirring for 1 h. Moreover, the entire solution was then poured into 400 mL of isopropanol under magnetic stirring for 12 h. Eventually, the solid product of S-ACAT in dark blue color was filtered and washed with 50 mL of isopropanol, followed by drying through dynamic vacuum at 50 °C for 3 h. On the other hand, synthesis of amino-capped aniline trimer (ACAT), which served as control experiment was carried out by performing the oxidative coupling reaction between two equivalent amounts of aniline and one equivalent amounts 1,4-phenylenediamine with ammonium persulfate as oxidant, which was followed our previous report [16], as shown in Scheme 1 (b).
3.1. Characterization of S-ACAT, ACAT The Mass and FTIR spectra of S-ACAT and ACAT are shown in Fig. 1. The detailed characterizations of S-ACAT are as follows: Ion trap-MS: m/z calculated value of C18H16N4O3S = 368.0. Found (M-H)- value of C18H16N4O3S = 366.9. In the FTIR spectrum of S-ACAT, the characteristic peak found at the position of 1221 cm−1 and 1061 cm−1 corresponds to symmetric stretching of O=S=O and SO3−, respectively [22]. Moreover, the characteristic bands located at the position of 3169 cm−1, which may be ascribed to the terminal –NH2 and –OH of SACAT. The characteristic bands appearing at the position of 1601 cm−1 and 1508 cm−1 were assigned to be the vibrational band of quinoid ring and benzenoid ring of S-ACAT. The characterizations for the ACAT are summarized as follows: Ion trap-MS: m/z calculated value of C18H16N4 = 288.1. Found (M-H)+ value of C18H16N4 = 289.1. In the FTIR spectrum of ACAT, the characteristic bands located at the position of 3380 cm−1 and 3309 cm−1, which may be attributed to the terminal -NH2 of ACAT. The characteristic bands appearing at the position of 1597 cm−1 and 1508 cm−1 were assigned to the vibrational band of quinoid ring and benzenoid ring of ACAT, respectively [16].
2.3. Synthesis of S-EPU, EPU and N-EPU 3.2. Characterization of S-EPU, EPU and N-EPU
A representative procedure to prepare the S-EPU, EPU and N-EPU is given as in Scheme 2: 3 g of PTMEG-1000 was added into a 250 mL of three-neck round-bottom flask connected with a condenser, a thermometer and nitrogen gas inlet/outlet. Nitrogen gas was bubbled into the flask throughout the reaction. Another beaker containing 1.338 g of isophorone diisocyanate (IPDI) was dissolved in 2 mL of DMF and treated by ultrasonic for 10 min, followed by injected into the previous flask with reflux treatment at 85 °C under magnetic stirring for 4 h. Another beaker containing 1.104 g of S-ACAT or 0.864 g of ACAT or 0.511 g of IPDA was dissolved into 12 mL of DMF under magnetic stirring for 2 h, followed by injected into the flask and maintained the reaction condition for additional 2 h. The as-prepared S-EPU, EPU and N-EPU were then casting onto the taflon substrate with specific program heating treatment.
In this work, three polyurea chains were prepared by reacting, the IPDI was with PTMEG in the presence of specific amount of diamine (i.e., S-ACAT, ACAT and IPDA) to prepare the sulfonated electroative polyurea (S-EPU), electroactive polyuria (EPU) and non-electroactive polyurea (N-EPU). Characterization of S-EPU, EPU and N-EPU was performed by FTIR spectroscopy, as shown in Fig. 2. The FTIR spectrum of S-EPU, as shown in Fig. 2 (a), was found to show the characteristic peaks located at the position of 1644 and 1536 cm−1, corresponding C=O vibration of amide I and the CO–N–H stretching of amide II, respectively, indicating the formation of urea linkage [23]. Moreover, the stretching of C=O and N–H of S-EPU was observed at positions of 1644 and 3323 cm−1. Finally, the characteristic peaks of S-EPU appear at 1042 cm−1 and 1230 cm−1, corresponding to the symmetric stretching of O=S=O and SO3−, respectively. In Fig. 2 (b), for the studies of FTIR spectrum of EPU, the
Scheme 2. Preparation of S-EPU, EPU and N-EPU. 100
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Fig. 1. Characterization of S-ACAT and ACAT by (a) FT-IR (b) Mass spectroscopy.
characteristic peaks appeared positions 1648 and 1536 cm−1 corresponding to the C=O vibration of amide I and the CO–N–H stretching of amide II. Moreover, the characteristic band of N–H vibration of EPU was found at 3323 cm−1. The characteristic peaks of EPU appeared at 1587 cm−1 and 1506 cm−1 correspond the vibrational band of quinoid ring and benzenoid ring. For comparison, N-EPU was also prepared as shown in Fig. 2 (c). The characteristic peaks of N-EPU found at position 1633 and 1557 cm−1, correspond to C=O vibration of amide I and the CO–N–H stretching of amide II. The characteristic peak for N–H of IPDA was found at the position of 3348 cm−1. Moreover, from the GPC data, the weight-average molecular weight (Mn ), number-average molecular weight (Mw ) and polydispersity index (PDI) of N-EPU, EPU and S-EPU were also shown in Table 1. For example, Mn , Mw and PDI of N-EPU were 151 kDa, 446 kDa and 1.40, respectively, EPU were 300 kDa, 327 kDa and 1.09, respectively. Moreover, Mn , Mw and PDI of S-EPU were 889 kDa, 951 kDa and 1.07, respectively.
3.3. Electroactivity of S-ACAT/ACAT and S-EPU/EPU Fig. 2. Characterization of (a) S-EPU, (b) EPU and (c) N-EPU by FT-IR spectroscopy.
To investigate the redox properties of electroactive diamine of SACAT/ACAT and electroactive polymer of S-EPU/EPU, electrochemical cyclic voltammetry (CV) was employed. In Fig. 3 (a), the ITO electrode without organic coating was found to exhibit zero redox current. After casting with organic coating of S-ACAT/ACAT, the ITO electrode was found to reveal obvious redox current. For example, the oxidation current (Iox) of S-ACAT was found to have a three-fold increase than that of ACAT was ca. at 207 μA. However, the Iox of ACAT was found to be 83 μA. It clearly of ACAT. It indicated that the introduction of sulfonic acid group into ACAT significantly boost the electroactivity of
Table 1 GPC Test and thermal properties and of N-EPU, EPU and S-EPU. Sample code
Mn (kDa)
Mw (kDa)
PDI
Td,5% (°C)
Tg (°C)
N-EPU EPU S-EPU
151 300 889
446 327 951
1.40 1.09 1.07
267 289 291
8.1 15.2 11.7
Fig. 3. Electrochemical CV studies for ITO electrode casting by(a) S-ACAT/ACAT and (b) S-EPU, EPU and N-EPU. 101
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ACAT. The stability of the reversible redox reaction of S-ACAT and ACAT are shown in Fig. S1 and Fig. S2. After three cycles of redox current, the Iox just only decreased slightly but was still in the range of the current area. Furthermore, the electroactivity of ITO-casted by S-EPU,EPU and NEPU was also investigated by CV studies. The N-EPU was found to show zero redox current. S-EPU and EPU was found to reveal obvious redox current. As shown in Fig. 3 (b), the S-EPU was found to exhibit Iox = 88 μA, which was significantly higher than that of EPU (Iox = 20 μA). It indicates that the sulfonic acid appeared at the S-EPU may effective facilitate the redox capability of EPU. The stability of the reversible redox reaction of S-EPU and EPU are shown in Fig. S3 and Fig. S4. After three cycles of redox current, the Iox also just decrease slightly yet. Remained within the range of the current area, similar to the trend seen with S-ACAT and ACAT. The chemical reactions during the reversible oxidation-redox process of S-EPU and EPU are shown in Fig. S5.
Table 2 Electrochemical corrosion measurements of raw CRS electrode and CRS electrode coated with N-EPU, EPU and S-EPU in 3.5 wt% NaCl(aq). Sample code
E
CRS N-EPU EPU S-EPU
−706.0 −517.4 −451.3 −420.8
corr
(mV)
I
corr
(μA/cm2)
30.64 16.97 1.53 0.38
Thickness (μm) – 36 ± 1 36 ± 1 36 ± 1
three-electrode system funder three different conditions, 3.5 wt% NaCl(aq), 1.0 M HCl(aq) and 1.0 M NaOH(aq). In order to discuss the corrosion protection ability of a series PU coatings in neutral, acidic and alkaline environments. The coating thickness of as-prepared three PU samples for electrochemical measurements was 36 ± 1 μm. 3.5.1.1. Potentiodynamic polarization curves in 3.5 wt% NaCl(aq). All the values of the anticorrosion including corrosion current density (Icorr) and corrosion potential (Ecorr) are given in Table 2. It should be noted that CRS electrode coated with S-EPU/EPU coatings containing SACAT/ACAT was found to exhibit better corrosion protection performance than that of N-EPU coating. For example, Tafel plots for (a) raw CRS electrode and CRS electrode coated with (b) N-EPU, (c) EPU and (d) S-EPU were shown in Fig. 5 and Table 2. The Ecorr and Icorr of raw CRS electrode were found to be - 706 mV and 30.64 μA/cm2, respectively, as shown in Fig. 5 (a). After coated with N-EPU, the Ecorr and Icorr of CRS electrode was shifted to −517.4 mV and 16.97 μA/cm2, respectively, as shown in Fig. 5 (b). The decreasing of Ecorr and Icorr value corresponds to the better anticorrosion performance of CRS electrode. It indicated that the N-EPU provides the basic physical barrier effect of polymer coating on CRS electrode. Moreover, the Ecorr and Icorr of CRS electrode coated with EPU was further shifted to 451.3 mV and 1.53 μA/cm2, respectively, as shown in Fig. 5 (c). The further decrease of Ecorr and Icorr value of EPU corresponds to better anticorrosion performance of CRS electrode as compared to that of NEPU coating. The enhancement in corrosion protection of CRS electrode coated with EPU as compared to that of N-EPU may be attributed to the formation of densely passive metal oxide layer (e.g., Fe2O3 and Fe3O4) induced from the electro-catalytic (i.e., redox) capability of EPU. The densely passive metal oxide layers found at the surface of CRS electrode was further identified by Raman spectroscopy, which will discussed in the following section. Eventually, the Ecorr and Icorr of CRS electrode coated with S-EPU was found to be - 420.8 mV and 0.38 μA/cm2, respectively, as shown in Fig. 5 (d). It should be noted that the CRS electrode coated with S-EPU coating was found to exhibit superior anticorrosion performance as compared to that of EPU coating based on the Ecorr and Icorr of Tafel plots. The slightly decrease of Ecorr and Icorr
3.4. Chemical oxidation of S-ACAT and ACAT For the chemical oxidation study of S-ACAT and ACAT, the reduced form of S-ACAT and ACAT was separately dissolved in NMP solution. Subsequently, trace amounts of the oxidant, (NH4)2S2O8, were introduced into the S-ACAT and ACAT solutions gradually for in-situ monitoring the sequential oxidation process of S-ACAT and ACAT by UV–visible absorption spectroscopy, as shown in Fig. 4 (a) and (b). It should be noted that upon oxidation, the color of S-ACAT and ACAT solution was all found to vary from brown to green. Initially, only one absorption band was observed at positions 330 nm for S-ACAT and 315 nm for ACAT, which were associated with π –π * transition of conjugated ring system [24]. Upon the addition of trace amounts of oxidant, slow oxidation of S-ACAT/ACAT was observed. A new absorption peak appeared at locations 582 nm for S-ACAT and 581 nm for ACAT, which were designated to the exciton-type transition between the HOMO orbital of benzoid ring and the LUMO orbital of quinoid ring [25]. This phenomenon of absorption peak of S-ACAT at 582 nm is atrtbuted to the presence of the quinoid ring, having a red-shift to 778 nm. This could be explained by the incorporation of sulfonated acid group into ACAT to promote the conjugated structure of ACAT, which cause the characteristic adsorption peak shift to a longer wavelength. 3.5. Electrochemical corrosion measurements 3.5.1. Potentiodynamic measurements In this study, the corrosion studies of a series of PU coatings were investigated by electrochemical corrosion measurements. The CRS electrodes coated with S-EPU, EPU and N-EPU were equipped in a
Fig. 4. Monitoring the chemical oxidation behavior for (a) S-ACAT (b) ACAT in the presence of trace amount of ammonium persulfate by UV–visible absorption spectroscopy. 102
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Fig. 5. Tafel plots for (a) raw CRS electrode and CRS electrode coated with(b) N-EPU, (c) EPU and (d) S-EPU in 3.5 wt% NaCl(aq).
Fig. 6. Tafel plots for (a) raw CRS electrode and CRS electrode coated with(b) N-EPU, (c) EPU and (d) S-EPU in 1.0 M HCl(aq).
value of CRS electrode coated with S-EPU coating as compared to that of EPU coating may be resulted from the higher electro-catalytic (redox) capability of S-EPU inducing the more densely metal oxide layers to shelter the underlying metal substrate, which can be further confirmed by studies of the previous electrochemical CV (higher redox capability) and following Raman spectroscopy (formation of densely passive metal oxide layer). The anticorrosion performance for the raw CRS electrode and CRS electrode coated with N-EPU, SEPU, S-EPU can be further evidenced by the studies of electrochemical impedance spectroscopy (EIS) section.
Table 4 Electrochemical corrosion measurements of raw CRS electrode and CRS electrode coated with N-EPU, EPU and S-EPU in 1.0 M NaOH(aq). Sample code
E
CRS N-EPU EPU S-EPU
−738.3 −578.9 −521.9 −447.7
corr
(mV)
I
corr
(μA/cm2)
46.51 17.86 13.78 1.36
Thickness (μm) – 36 ± 1 36 ± 1 36 ± 1
3.5.1.2. Potentiodynamic polarization curves in 1.0 M HCl(aq). The values of the anticorrosion including corrosion current density (Icorr) and corrosion potential (Ecorr) are given in Table 3. It should be noted that CRS electrode coated with S-EPU/EPU was also found to exhibit better corrosion protection performance than that of N-EPU coating. And the potentiodynamic polarization curves in 1.0 M HCl(aq) (Fig. 6) also were found to show similar trend to that of the condition in neutral environment. 3.5.1.3. Potentiodynamic polarization curves in 1.0 M NaOH(aq). The values of the anticorrosion including corrosion current density (Icorr) and corrosion potential (Ecorr) are given in Table 4. It should be noted that CRS electrode coated with S-EPU was also exhibit the best corrosion protection performance than that of N-EPU or EPU coatings. And the potentiodynamic polarization curves in 1.0 M NaOH(aq) (Fig. 7) also were found to show similar trend to that of the condition in previous environments. Fig. 7. Tafel plots for (a) raw CRS electrode and CRS electrode coated with(b) N-EPU, (c) EPU and (d) S-EPU in 1.0 M NaOH(aq).
3.5.2. Electrochemical impedance spectroscopy (Nyquist plots and Bode plots) In this study, EIS was an alternative approach to evaluate the activity difference between surface of CRS electrode after treating with NEPU, EPU, and S-EPU coatings. Impedance is a complex resistance when
an electric current flow through a circuit made of capacitors, resistors, or insulators, or any possible combination of these [26]. In EIS study, the impedance (Z) usually depends on the charge transfer resistance (Rct), the solution resistance (Rs), the capacitance of electrical double layer (Cdl) and the frequency of AC signal (ω). It can be derived by using the following equation [27]:
Table 3 Electrochemical corrosion measurements of raw CRS electrode and CRS electrode coated with N-EPU, EPU and S-EPU in 1.0 M HCl(aq). Sample code
E
CRS N-EPU EPU S-EPU
−679.4 −492.5 −470.7 −443.6
corr
(mV)
I
corr
(μA/cm2)
52.31 23.85 1.28 0.74
Z= Z' + jZ'' =
Thickness (μm) – 36 ± 1 36 ± 1 36 ± 1
j (Rct2 Cdi ω) Rs + R ct + 2 1 + (R ct Cdl ω) 1 + (R ct Cdl ω)2
The high-frequency intercept is equal to the solution resistance, and the low-frequency intercept is equal to the sum of the solution and charge transfer resistances [28,29]. The higher the semicircle diameter 103
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Fig. 8. Nyquist plots for (a) raw CRS electrode and CRS electrode coated with (b) N-EPU (c) EPU and (d) S-EPU in 3.5 wt% NaCl(aq).
Fig. 9. Bode plots for (a) raw CRS electrode and CRS electrode coated with(b) N-EPU (c) EPU and (d) S-EPU in 3.5 wt% NaCl(aq).
(charge transfer resistance) indicated the lower the corrosion rate [30,31]. The electrochemical equivalent circuit adopted to derive electrochemical parameters are attached as inset of Fig. 8. The Rs represent solution resistance; RCt represent the charge transfer resistance and CPE is the constant phase element of the double layer formed at the metal-solution interface. In the following sections are the EIS studies of N-EPU, EPU and S-EPU in neutral, acidic and alkaline conditions. 3.5.2.1. EIS in 3.5 wt-% NaCl(aq). The EIS studies of raw CRS electrode and CRS electrode coated with N-EPU, EPU and S-EPU was studied in 5 wt% aqueous NaCl electrolyte after 30 min equilibrium and compared in Fig. 8.For example, we found that the charge transfer resistance of raw CRS electrode and CRS electrode coated with N-EPU, EPU and SEPU as determined by the intersection of the low frequency end of the semicircle arc with the real axis was 1.70, 17.78, 82.59, and 98.28 kcm2, respectively, as shown in Fig. 8 (a), (b), (c) and (d). This experimental result of Nyquist plot obtained from EIS studies clearly demonstrated that the anticorrosion performance showing the trend: SEPU > EPU > N-EPU > raw CRS, which is consistent with the previous studies of Tafel plot. Furthermore, EIS Bode plots of raw CRS electrode and corresponding CRS electrode coated with N-EPU, EPU and S-EPU were shown in Fig. 9. It should be noted that the impedance value of CRS electrode coated with S-EPU in the monitoring frequency region from low to high-frequency was highest, as shown in Fig. 9 (d). The EIS Bode plots were found to show similar trend to that of previous Tafel and Nyquist plots. The corrosion protection mechanism for the S-EPU/EPU was similar to that of conventional PANI coatings reported in the previous literatures [32–34].
Fig. 10. Nyquist plots for (a) raw CRS electrode and CRS electrode coated with (b) N-EPU (c) EPU and (d) S-EPU in 1.0 M HCl(aq).
3.5.2.2. EIS in 1.0 M HCl(aq). The EIS studies of raw CRS electrode and CRS electrode coated with N-EPU, EPU and S-EPU was studied in 1.0 M aqueous HCl electrolyte after 30 min equilibrium and compared in Fig. 10. For example, we found that the charge transfer resistance of raw CRS electrode and CRS electrode coated with N-EPU, EPU and SEPU as determined by the intersection of the low frequency end of the semicircle arc with the real axis was 1.34, 18.13, 60.12, and 88.26 kcm2, respectively, as shown in Fig. 10 (a), (b), (c) and (d). This experimental result of Nyquist plot obtained from EIS studies clearly demonstrated that the anticorrosion performance showing the trend: SEPU > EPU > N-EPU > raw CRS, which is consistent with the previous studies of Tafel plot. Also the EIS Bode plots of raw CRS electrode and corresponding CRS electrode coated with N-EPU, EPU and S-EPU were shown in Fig. 11.
Fig. 11. Bode plots for (a) raw CRS electrode and CRS electrode coated with (b) N-EPU (c) EPU and (d) S-EPU in 1.0 M HCl(aq).
104
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Fig. 12. Nyquist plots for (a) raw CRS electrode and CRS electrode coated with (b) N-EPU (c) EPU and (d) S-EPU in 1.0 M NaOH(aq).
Fig. 14. Raman spectroscopy for the observation of (a) raw CRS electrode and the densely passive metal oxide layer formed on the CRS electrode induced by (b)N-EPU (c) EPU and (d) S-EPU.
The EIS Bode plots were found to show similar trend to that of previous Tafel and Nyquist plots.
electrochemical measurements such as Tafel, Nyquist and Bode plots. But the metal corrosion protection ability of S-EPU in neutral condition is better than in acidic or alkaline conditions slightly.
3.5.2.3. EIS in 1.0 M NaOH(aq). The EIS studies of raw CRS electrode and CRS electrode coated with N-EPU, EPU and S-EPU was studied in 1.0 M aqueous NaOH electrolyte after 30 min equilibrium and compared in Fig. 12. For example, we found that the charge transfer resistance of raw CRS electrode and CRS electrode coated with N-EPU, EPU and S-EPU as determined by the intersection of the low frequency end of the semicircle arc with the real axis was 1.81, 14.55, 55.86, and 79.57 kcm2, respectively, as shown in Fig. 12 (a), (b), (c) and (d). This experimental result of Nyquist plot obtained from EIS studies clearly demonstrated that the anticorrosion performance showing the trend: SEPU > EPU > N-EPU > raw CRS, which is consistent with the previous studies of Tafel plot. Meanwhile, the EIS Bode plots of raw CRS electrode and corresponding CRS electrode coated with N-EPU, EPU and S-EPU were shown in Fig. 13. The EIS Bode plots were found to show similar trend to that of previous Tafel and Nyquist plots. In summary, in this study, the anticorrosion performance studies of CRS electrode coated with distinctive PU samples were found whatever in neutral, acidic and alkaline environments exhibit the following tend: S-EPU > EPU > N-EPU > raw CRS electrode based on a series of
3.5.3. Identification of densely passive metal oxide layer In this study, the Raman spectroscopy, SEM and ESCA were used to confirm the formation of densely passive metal oxide layers (Fe2O3) upon the surface of CRS electrode, as shown in Fig. 14. For example, Raman spectrum for the surface of raw CRS electrode was not found any characteristic peaks of metal oxide, as shown in Fig. 14 (a). After coating with N-EPU, the Raman spectrum for the surface of CRS electrode was found to exhibit similar condition to that of raw CRS electrode, as shown in Fig. 14 (b). However, after coating with EPU for one month, Raman spectrum for the surface of CRS electrode was found to show some characteristic metal oxide peaks after detaching the EPU film. For example, in Fig. 14 (c), the Raman spectrum for surface of CRS electrode induced by EPU coating was found to exhibit seven phonon lines, corresponding to two A1g modes (221 and 497 cm−1) and three Eg mode (292, 407 and 609 cm−1) of Fe2O3 [35]. Moreover, the Raman spectrum for the surface of CRS electrode induced from S-EPU coating was found to reveal more strong/obvious characteristic peaks of Fe2O3, as shown in Fig. 14 (d). It should be noted that the stronger characteristic peaks of densely passive metal oxide layers appeared on the surface of CRS electrode induced by S-EPU coating as compared to that of EPU coating may be attributed to the higher electroactivity of S-EPU, leading to the better anticorrosion performance of S-EPU coating as compared to that of EPU coating on CRS electrode based on a series of electrochemical corrosion measurements. As shown in Fig. 15, are the surface images of the CRS. In Fig. 15 (a), pure CRS surface was shown for the comparative studies. Fig. 15 (b) and (c) were induced by EPU/S-EPU, separately. According to the SEM images observation of the passivation oxidelayers exhibited the deposition of grayish oxide layer form over the CRS surface under the EPU/S-EPU coatings on CRS electrode, are same with the published by Wei et al. [36]. The chemical nature of the passivation oxide layers was measured by ESCA. The binding energy plots vs. intensity for the iron oxide layers are shown in Fig. 16. In Fig. 16 (a), pure CRS surface was shown for the comparative studies. The passivation oxide layers shown in Fig. 16 (b) and (c) were all exhibited the Fe 2p3/2 peak binding energy. In Fig. 16 (b) is the CRS surface coated with EPU. The peak in 710.7 eV is binding energy of Fe3O4. The Fe 2p spectra of FeO and Fe3O4 are similar and it is hard to distinguish from each other. The 2p3/ 2 binding energy was about 724.1 eV. In Fig. 16 (c) is the CRS surface
Fig. 13. Bode plots for (a) raw CRS electrode and CRS electrode coated with (b) N-EPU (c) EPU and (d) S-EPU in 1.0 M NaOH(aq). 105
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Fig. 15. SEM images of (a) raw CRS metal and (b) the surface of the coated with EPU and (c) S-EPU on the CRS metal.
Fig. 18. The DSC curve of (a) N-EPU (b) EPU and (c) S-EPU.
Fig. 16. ESCA Fe 2p core level spectra of (a) raw CRS metal and (b) the surface of the coated with EPU and (c) S-EPU on the CRS metal.
highest 5% weight loss decomposition temperature (Td,5%) (291 °C), the next is EPU (289 °C), then N-EPU has the lowest Td,5% (267 °C). The reason why the S-EPU and EPU have more better thermal stability than N-EPU may due to the ACAT and S-ACAT are the structure with benzene ring. The structure of polymer with benzene ring will improve the temperature of decomposition. In the DSC investigations, the glass transition temperature (Tg) of polyurea increased as the content of S-ACAT/ACAT segments in the main chains of polyureas. For example, Tg of samples increased form 8.1 °C of N-EPU, to 11.7 °C of S-EPU, and to 15.2 °C of EPU, as shown in Fig. 18 (Table 1). The increasing of Tg for as-prepared polyureas might be attributed to the incorporation of S-ACAT and ACAT rigid aromatic chemical structures, leading to an obvious increase of Tg.
coated with S-EPU. The peak in 710.8 eV is binding energy of Fe3O4. The Fe 2p spectra of FeO and Fe3O4 are similar and it is hard to distinguish from each other. The 2p3/2 binding energy was about 724.6 eV. In summary, according to the Raman spectrum, SEM and ESCA, in the present study conform the passive metal oxide layer were induced by SEPU and EPU. 3.6. Thermal properties Thermal performance is an important indicator among the polymer in applications [37–40]. TGA analysis of N-EPU, EPU and S-EPU has been shown in Fig. 17 (Table 1). For these samples, S-EPU shows the
4. Conclusions The S-EPU was successfully synthesized and applied as anticorrosion coating. The as-prepared S-EPU was identified by FTIR spectroscopy and GPC. Redox capability of as-prepared diamine of S-ACAT/ ACAT and electroactive polymers of S-EPU/EPU/N-EPU was confirmed by electrochemical CV studies. Compounds with sulfonated groups show better redox ability than those corresponding counterpart. Chemical oxidization of S-EPU/EPU, through the introduction of trace amount of APS was monitored by UV–visible absorption spectroscopy. Electrochemical corrosion measurements of CRS electrode coated with S-EPU/EPU/N-EPU based on a series of electrochemical corrosion measurements such as Tafel plots, Nyquist plots and Bode plots were investigated in neutral, acidic and alkaline conditions. Regardless in which environments, the CRS electrode coated with different PU coatings was found to reveal following tendency: S-EPU > EPU > NEPU > raw CRS electrode based on a series of electrochemical corrosion measurements. The better corrosion protection of electroactive SEPU/EPU coatings as compared to that of non-electroactive N-EPU may
Fig. 17. The TGA curve of (a) N-EPU (b) EPU and (c) S-EPU. 106
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be attributed to their redox capability inducing the formation of densely metal oxide layer (e.g., Fe2O3 and Fe3O4), as demonstrated by the studies of Raman spectroscopy, SEM and ESCA. The S-EPU coating exhibited better anticorrosion performance than that of EPU coating which could be attributed to the higher electro-catalytic property of SEPU coating upon CRS electrode, causing the formation of more densely metal oxide layers to shelter the underlying metal substrate. Moreover, the S-EPU coating shows higher thermal properties in TGA and DSC as compared to that of EPU and N-EPU. The S-EPU has good corrosion protection ability, redox ability and thermal stability, which might be useful applied in the metal anticorrosion field.
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Acknowledgement Thanks for the financial support from the Ministry of Science and Technology in R.O·C., under the project 107-2113-M-033-007. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.polymer.2019.01.037. References [1] B.K. Shaw, G. Robert, What is corrosion? Electrochemical Society Interface 15 (2006) 24–26. [2] N. Aravindan, M.V. Sangaranarayanan, Influence of solvent composition on the anti-corrosion performance of copper–polypyrrole (Cu–PPy) coated 304 stainless steel, Prog. Org. Coating 95 (2016) 38–45. [3] S.K. Phanrapee Varakirkkulchaia, Pattarapan Prasassarakichc, Polyaniline/polyacrylate core–shell composites: preparation, morphology and anticorrosive properties, Prog. Org. Coating 85 (2015) 84–91. [4] B.Y.T. T¨uken ∗, M. Erbil, Polypyrrole/polythiophene coating for copper protection, Prog. Org. Coating 53 (2005) 38–45. [5] G. Liao, Q. Li, Z. Xu, The chemical modification of polyaniline with enhanced properties: a review, Prog. Org. Coating 126 (2019) 35–43. [6] Y. Wei, C. Yang, T. Ding, A one-step method to synthesize N,N′-bis(4′-aminophenyl)-1,4-quinonenediimine and its derivatives, Tetrahedron Lett. 37 (1996) 731–734. [7] D. Chao, X. Ma, X. Lu, L. Cui, H. Mao, W. Zhang, Y. Wei, Design, synthesis and characterization of novel electroactive polyamide with amine-capped aniline pentamer in the main chain via oxidative coupling polymerization, J. Appl. Polym. Sci. 104 (2007) 1603–1608. [8] D. Chao, L. Cui, X. Lu, H. Mao, W. Zhang, Y. Wei, Electroactive polyimide with oligoaniline in the main chain via oxidative coupling polymerization, European Polym. J. 43 (2007) 2641–2647. [9] D. Chao, X. Ma, Q. Liu, X. Lu, J. Chen, L. Wang, W. Zhang, Y. Wei, Synthesis and characterization of electroactive copolymer with phenyl-capped aniline tetramer in the main chain by oxidative coupling polymerization, European Polym. J. 42 (2006) 3078–3084. [10] D. Chao, X. Lu, J. Chen, X. Zhao, L. Wang, W. Zhang, Y. Wei, New method of synthesis of electroactive polyamide with amine-capped aniline pentamer in the main chain, J. Polym. Sci. Polym. Chem. 44 (2006) 477–482. [11] D. Chao, X. Lu, J. Chen, X. Liu, W. Zhang, Y. Wei, Synthesis and characterization of electroactive polyamide with amine-capped aniline pentamer and ferrocene in the main chain by oxidative coupling polymerization, Polymer 47 (2006) 2643–2648. [12] L. Chen, Y. Yu, H. Mao, X. Lu, W. Zhang, Y. Wei, Synthesis of parent aniline tetramer and pentamer and redox properties, Mater. Lett. 59 (2005) 2446–2450. [13] W. Lu, X. Sheng Meng, Z. Yuan Wang, Electrochemical behavior of a new electroactive polyimide derived from aniline trimer, J. Polym. Sci. Polym. Chem. 37 (1999) 4295–4301. [14] Z.Y. Wang, C. Yang, J.P. Gao, J. Lin, X.S. Meng, Y. Wei, S. Li, Electroactive polyimides derived from amino-terminated aniline trimer, Macromolecules 31 (1998) 2702–2704. [15] L. Chen, Y. Yu, H. Mao, X. Lu, L. Yao, W. Zhang, Y. Wei, Synthesis of a new electroactive poly(aryl ether ketone), Polymer 46 (2005) 2825–2829.
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Effective anticorrosion coatings prepared from sulfonated electroactive polyurea
T
Kuan-Ying Chena, Yen-Shi Laia, Jun-Kai Youa, Karen S. Santiagob, Jui-Ming Yeha,∗ a Department of Chemistry, Center for Nanotechnology and Center for Membrane Technology at Chung Yuan Christian University, Chung Li District‚ 32023‚ Tao-Yuan City, Taiwan, ROC b Department of Chemistry, Research Center for the Natural and Applied Sciences, University of Santo Tomas, Espana, Manila, 1015, Philippines
H I GH L IG H T S
amino-capped aniline trimer was synthesized successful. • Sulfonated with sulfonated groups show better redox ability than those without. • Compounds • The S-EPU has good corrosion protection ability, redox ability and thermal stability.
A R T I C LE I N FO
A B S T R A C T
Keywords: Corrosion Coating Electroactive Polyurea Sulfonated
In the present study, the sulfonated electroactive polyurea (S-EPU) was first prepared and characterized, applied as an anticorrosion coating. The electroactive oligomer sulfonated amine-capped aniline trimer (S-ACAT) was synthesized by oxidative coupling reaction, followed by characterization using Fourier-transform infrared (FTIR) and mass spectroscopy (MS). For the preparation of sulfonated electroactive polyurea (S-EPU), isophorone diisocyanate (IPDI) was reacted with poly(tetramethylene ether) glycol (PTMEG) in the presence of a specific amount of diamine S-ACAT. The success of the S-EPU was confirmed through FTIR and GPC. After that the redox capability of as-prepared S-EPU was confirmed by CV studies. Chemical oxidization of S-EPU, carried out through the introduction of trace amount of ammonium persulfate, was monitored by UV-VIS spectroscopy. The electrochemical corrosion of the electroactive polymer-coated cold-rolled steel (CRS) electrode, in neutral, acidic and alkaline conditions, were measured and compared. As compared to non-electroactive polyurea (N-EPU), results showed enhanced corrosion protection of the S-EPU coating and electroactive polyurea (EPU) coating, owing to their redox capability that induces the formation of densely passive metal oxide layer (e.g., Fe2O3 and Fe3O4). The densely metal oxide layers induced by S-EPU/EPU coatings were further confirmed by Raman spectroscopy, SEM and ESCA. Moreover, the S-EPU coating exhibited better anticorrosion performance than that of EPU coating which could be attributed to the higher electro-catalytic property of S-EPU coating upon CRS electrode, causing the formation of more densely metal oxide layers to shelter the underlying metal substrate.
1. Introduction Corrosion phenomena are regarded as a common degradation behavior of distinctive metallic substrates caused by their interactions with surrounding environments and are generally considered inevitable [1]. Currently, one strategy to impart anticorrosion property to distinct metallic substrates is to treat them with versatile organic or polymeric coatings. Among many currently used polymeric coating materials, intrinsically electroactive polymers with conjugated chemical structures, showing reversible redox capability, have attracted great
∗
attention in the development of anticorrosion coatings. Generally, the mechanism by which electroactive polymeric coatings improve on distinctive metallic substrates is attributed to their electro-catalytic (i.e., redox) property, which may induce the formation of densely passive metal oxide layer to avoid underlying metallic substrates damage [2–4]. Electroactive polymers mainly include polyaniline (PANI), polythiophene (PTH), polypyrrole (PPY), and their derivatives. They possess many potential applications including rechargeable battery, electromagnetic interference shielding, photothermal therapy, chemical
Corresponding author. E-mail address: [email protected] (J.-M. Yeh).
https://doi.org/10.1016/j.polymer.2019.01.037 Received 30 October 2018; Received in revised form 9 January 2019; Accepted 16 January 2019 Available online 22 January 2019 0032-3861/ © 2019 Elsevier Ltd. All rights reserved.
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monitored by UV–visible absorption spectroscopy. Corrosion protection performance of CRS electrode coated with S-EPU/EPU/N-EPU coatings was investigated by performing a series of electrochemical corrosion measurements in NaCl(aq), HCl(aq) and NaOH(aq), to investigate the abilities of the corrosion protection in neutral, acidic and alkaline conditions [20,21]. The formation of densely passive metal oxide layers induced by the electroactive coating of S-EPU/EPU was confirmed by Raman spectroscopy, SEM and ESCA. Finally, the thermal properties were confirmed by TGA and DSC.
sensor, gas separation membrane, anticorrosion coating, microwave absorption, etc [5]. However, one drawback of electroactive polymers, such as polyaniline (PANI), is their poor solubility in common organic solvents, which limit their processing and practical application in various fields. Therefore, studies dealing on the synthesis and characterization of electroactive oligomers with good solubility have received extensive research interests. Electroactive oligomers (e.g., aniline trimer, tetramer, pentamer, etc.) are known to exhibit good solubility in many common organic solvents and exhibit comparable electroactive capability as that of polymers. For instance, Wei et al. explored the synthesis and characterization of amine-capped aniline trimer of reversibly redox and dope/de-doped properties [6]. Moreover, Zhang et al. reported the synthesis and electrochemical redox properties of aniline tetramer/pentamer [7–11]. On the other hand, many scientific literatures deal with electroactive polymers containing amine-capped aniline oligomers revealing primary amine at both ends (e.g., trimer, tetramer, pentamer, etc.) [12]. For example, electrochemical behavior of aniline oligomer-based polyimides had been published by Wang et al. [8,13,14]. The results showed that the polyimide chains with electroactive aniline oligomer segments, could lead that the polyimides have higher electroactive ability. Moreover, polyamides containing amine-capped aniline pentamer have been explored by Zhang et al. [15]. Recently, Yeh et al. reported the anticorrosion effects of CRS electrode coated with electroactive epoxy/polyimide containing ACAT by performing a series of electrochemical corrosion measurements in aqueous NaCl conditions [16,17]. Based from the literatures mention above, corrosion protection effect of electroactive polymer coatings was found to be ascribed from their electro-catalytic (i.e., redox) capability, which may induce the formation of densely passive metal oxide layers between the interface of coating and metal to protect the underlying substrate from damage. Moreover, electroactive polymer coatings with higher electro-catalytic (redox) capability were found to exhibit better anticorrosion performance. For example, Mozafari et al. [18] reported that the aniline oligomers with higher conjugated chain length were found to reveal higher redox capability. In addition, Yeh et al. explored [19] that the anticorrosion of electroactive polymer coatings by incorporating small amount of graphene nano-sheets to promote the redox capability and gas barrier property of coating. Therefore, notable strategy is to develope electroactive polymers with higher electro-catalytic capability. In this work, sulfonic acid group was designed to be functionalized the middle benzene ring of amine-capped aniline trimer (ACAT) at the ortho-position, and was denoted as S-ACAT. We therefore envision that the presence of sulfonic acid group in ACAT will facilitate its redox capability, which can be further confirmed by the electrochemical cyclic voltammetry studies. Electroactive polyureas resulting from SACAT and ACAT (denoted by S-EPU and EPU) were also prepared and characterized by FTIR spectroscopy and GPC. N-EPU was also prepared for comparative studies. Redox capability of as-prepared S-EPU/EPU/ N-EPU was identified by electrochemical CV studies. Chemical oxidization of S-EPU and EPU in the presence of trace amount of oxidant was
2. Experimental section 2.1. Materials and instrumentation Aniline (99%, Fluka) was doubly distilled prior use. poly(tetramethylene ether) glycol (PTMEG, Mn∼1000, Aldrich), isophorone diisocyanate (IPDI, 98%, Sigma-Aldrich), 5-Amino-1,3,3-trimethyl- cyclohexanemethylamine (IPDA, > 98%, Sigma) ρ-Phenylenediamine (98%, Sigma), 2,5-diaminobenzenesulfonic acid (> 97%, Sigma), ammonium persulfate (APS, > 98%, J.T. Baker), hydrochloric acid (HCl, 37.0%, Sigma-Aldrich), ammonium hydroxide solution (NH4OH, Sigma-Aldrich), sodium hydroxide (NaOH, 97%, SHOWA), ethanol (> 95%, J.T. Baker), isopropanol (IPA, > 99.5%, J.T. Baker), hydrazine hydrate (55%, Sigma-Aldrich), sodium chloride (J.T. Baker), N,N-dimethylformamide (DMF, > 99%, J.T. Baker) were used as received without further treatment. Fourier transform infrared (FTIR) analysis was carried out with JASCO FT/IR-4100 at room temperature. Ion-trap mass spectrometry (MS) was run on a Bruker Daltonics IT mass spectrometer model Esquire 2000 (Leipzig, German) with an Agilent ESI source (model G1607-6001). Cyclic voltammetry experiments were performed on VoltaLab 40 potentiostat/galvanostat in a standard corrosion cell equipped with two graphite rod counter electrode and a saturated calomel electrode (SCE) as well as the working electrode. Electrochemical impedance spectroscopy (EIS) measurements were recorded on an AutoLab (PGSTAT302 N) potentiostat/galvanostat electrochemical analyzer. UV–Visible absorption spectroscopy was obtained using a Hitachi U-2000 UV–Visible spectrometer. Raman spectroscopy (Horbi Jobin Yvon iHR320), Scanning electron microscopy (SEM, Hitachi S2300) and electron spectroscopy for chemical analysis. (ESCA, VG Scientific ESCALAB 250) were used to characterize the densely passive metal oxide layers on the surface of CRS electrode induced by electroactive polymer coatings. Thermal gravimetric analysis (TGA) was performed on a DuPont TA Q50 thermal analysis system in air. Differential scanning calorimetry (DSC) was performed on a DuPont TA Q10 differential scanning calorimeter at a heating or cooling rate of 5 °C/min in air atmosphere. 2.2. Synthesis of S-ACAT and ACAT The representative procedure to synthesize sulfonated amino-
Scheme 1. Preparation of (a) S-ACAT and (b) ACAT. 99
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3. Results and discussion
capped aniline trimer (denoted by S-ACAT) is shown in Scheme 1 (a). The detailed step to synthesis S-ACAT are given as follows: 1.5 g (i.e., 8.0 mmol) of 2,5-diamino-benzenesulfonic acid and 1.5 g (i.e., 16 mmol) of aniline were dissolved in 100 mL of 1.0 M HCl aqueous solution. Afterwards, 3.6 g (i.e., 15.7 mmol) of APS was introduced into the previous solution under magnetic stirring at 0°C for 1 h. The resulting precipitate was then collected by filtration and washed with 100 mL of 1.0 M HCl aqueous solution. Subsequently, the collected solid product was dispersed into 60 mL of 1.0 M NH4OH aqueous solution under magnetic stirring for 1 h. Moreover, the entire solution was then poured into 400 mL of isopropanol under magnetic stirring for 12 h. Eventually, the solid product of S-ACAT in dark blue color was filtered and washed with 50 mL of isopropanol, followed by drying through dynamic vacuum at 50 °C for 3 h. On the other hand, synthesis of amino-capped aniline trimer (ACAT), which served as control experiment was carried out by performing the oxidative coupling reaction between two equivalent amounts of aniline and one equivalent amounts 1,4-phenylenediamine with ammonium persulfate as oxidant, which was followed our previous report [16], as shown in Scheme 1 (b).
3.1. Characterization of S-ACAT, ACAT The Mass and FTIR spectra of S-ACAT and ACAT are shown in Fig. 1. The detailed characterizations of S-ACAT are as follows: Ion trap-MS: m/z calculated value of C18H16N4O3S = 368.0. Found (M-H)- value of C18H16N4O3S = 366.9. In the FTIR spectrum of S-ACAT, the characteristic peak found at the position of 1221 cm−1 and 1061 cm−1 corresponds to symmetric stretching of O=S=O and SO3−, respectively [22]. Moreover, the characteristic bands located at the position of 3169 cm−1, which may be ascribed to the terminal –NH2 and –OH of SACAT. The characteristic bands appearing at the position of 1601 cm−1 and 1508 cm−1 were assigned to be the vibrational band of quinoid ring and benzenoid ring of S-ACAT. The characterizations for the ACAT are summarized as follows: Ion trap-MS: m/z calculated value of C18H16N4 = 288.1. Found (M-H)+ value of C18H16N4 = 289.1. In the FTIR spectrum of ACAT, the characteristic bands located at the position of 3380 cm−1 and 3309 cm−1, which may be attributed to the terminal -NH2 of ACAT. The characteristic bands appearing at the position of 1597 cm−1 and 1508 cm−1 were assigned to the vibrational band of quinoid ring and benzenoid ring of ACAT, respectively [16].
2.3. Synthesis of S-EPU, EPU and N-EPU 3.2. Characterization of S-EPU, EPU and N-EPU
A representative procedure to prepare the S-EPU, EPU and N-EPU is given as in Scheme 2: 3 g of PTMEG-1000 was added into a 250 mL of three-neck round-bottom flask connected with a condenser, a thermometer and nitrogen gas inlet/outlet. Nitrogen gas was bubbled into the flask throughout the reaction. Another beaker containing 1.338 g of isophorone diisocyanate (IPDI) was dissolved in 2 mL of DMF and treated by ultrasonic for 10 min, followed by injected into the previous flask with reflux treatment at 85 °C under magnetic stirring for 4 h. Another beaker containing 1.104 g of S-ACAT or 0.864 g of ACAT or 0.511 g of IPDA was dissolved into 12 mL of DMF under magnetic stirring for 2 h, followed by injected into the flask and maintained the reaction condition for additional 2 h. The as-prepared S-EPU, EPU and N-EPU were then casting onto the taflon substrate with specific program heating treatment.
In this work, three polyurea chains were prepared by reacting, the IPDI was with PTMEG in the presence of specific amount of diamine (i.e., S-ACAT, ACAT and IPDA) to prepare the sulfonated electroative polyurea (S-EPU), electroactive polyuria (EPU) and non-electroactive polyurea (N-EPU). Characterization of S-EPU, EPU and N-EPU was performed by FTIR spectroscopy, as shown in Fig. 2. The FTIR spectrum of S-EPU, as shown in Fig. 2 (a), was found to show the characteristic peaks located at the position of 1644 and 1536 cm−1, corresponding C=O vibration of amide I and the CO–N–H stretching of amide II, respectively, indicating the formation of urea linkage [23]. Moreover, the stretching of C=O and N–H of S-EPU was observed at positions of 1644 and 3323 cm−1. Finally, the characteristic peaks of S-EPU appear at 1042 cm−1 and 1230 cm−1, corresponding to the symmetric stretching of O=S=O and SO3−, respectively. In Fig. 2 (b), for the studies of FTIR spectrum of EPU, the
Scheme 2. Preparation of S-EPU, EPU and N-EPU. 100
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Fig. 1. Characterization of S-ACAT and ACAT by (a) FT-IR (b) Mass spectroscopy.
characteristic peaks appeared positions 1648 and 1536 cm−1 corresponding to the C=O vibration of amide I and the CO–N–H stretching of amide II. Moreover, the characteristic band of N–H vibration of EPU was found at 3323 cm−1. The characteristic peaks of EPU appeared at 1587 cm−1 and 1506 cm−1 correspond the vibrational band of quinoid ring and benzenoid ring. For comparison, N-EPU was also prepared as shown in Fig. 2 (c). The characteristic peaks of N-EPU found at position 1633 and 1557 cm−1, correspond to C=O vibration of amide I and the CO–N–H stretching of amide II. The characteristic peak for N–H of IPDA was found at the position of 3348 cm−1. Moreover, from the GPC data, the weight-average molecular weight (Mn ), number-average molecular weight (Mw ) and polydispersity index (PDI) of N-EPU, EPU and S-EPU were also shown in Table 1. For example, Mn , Mw and PDI of N-EPU were 151 kDa, 446 kDa and 1.40, respectively, EPU were 300 kDa, 327 kDa and 1.09, respectively. Moreover, Mn , Mw and PDI of S-EPU were 889 kDa, 951 kDa and 1.07, respectively.
3.3. Electroactivity of S-ACAT/ACAT and S-EPU/EPU Fig. 2. Characterization of (a) S-EPU, (b) EPU and (c) N-EPU by FT-IR spectroscopy.
To investigate the redox properties of electroactive diamine of SACAT/ACAT and electroactive polymer of S-EPU/EPU, electrochemical cyclic voltammetry (CV) was employed. In Fig. 3 (a), the ITO electrode without organic coating was found to exhibit zero redox current. After casting with organic coating of S-ACAT/ACAT, the ITO electrode was found to reveal obvious redox current. For example, the oxidation current (Iox) of S-ACAT was found to have a three-fold increase than that of ACAT was ca. at 207 μA. However, the Iox of ACAT was found to be 83 μA. It clearly of ACAT. It indicated that the introduction of sulfonic acid group into ACAT significantly boost the electroactivity of
Table 1 GPC Test and thermal properties and of N-EPU, EPU and S-EPU. Sample code
Mn (kDa)
Mw (kDa)
PDI
Td,5% (°C)
Tg (°C)
N-EPU EPU S-EPU
151 300 889
446 327 951
1.40 1.09 1.07
267 289 291
8.1 15.2 11.7
Fig. 3. Electrochemical CV studies for ITO electrode casting by(a) S-ACAT/ACAT and (b) S-EPU, EPU and N-EPU. 101
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ACAT. The stability of the reversible redox reaction of S-ACAT and ACAT are shown in Fig. S1 and Fig. S2. After three cycles of redox current, the Iox just only decreased slightly but was still in the range of the current area. Furthermore, the electroactivity of ITO-casted by S-EPU,EPU and NEPU was also investigated by CV studies. The N-EPU was found to show zero redox current. S-EPU and EPU was found to reveal obvious redox current. As shown in Fig. 3 (b), the S-EPU was found to exhibit Iox = 88 μA, which was significantly higher than that of EPU (Iox = 20 μA). It indicates that the sulfonic acid appeared at the S-EPU may effective facilitate the redox capability of EPU. The stability of the reversible redox reaction of S-EPU and EPU are shown in Fig. S3 and Fig. S4. After three cycles of redox current, the Iox also just decrease slightly yet. Remained within the range of the current area, similar to the trend seen with S-ACAT and ACAT. The chemical reactions during the reversible oxidation-redox process of S-EPU and EPU are shown in Fig. S5.
Table 2 Electrochemical corrosion measurements of raw CRS electrode and CRS electrode coated with N-EPU, EPU and S-EPU in 3.5 wt% NaCl(aq). Sample code
E
CRS N-EPU EPU S-EPU
−706.0 −517.4 −451.3 −420.8
corr
(mV)
I
corr
(μA/cm2)
30.64 16.97 1.53 0.38
Thickness (μm) – 36 ± 1 36 ± 1 36 ± 1
three-electrode system funder three different conditions, 3.5 wt% NaCl(aq), 1.0 M HCl(aq) and 1.0 M NaOH(aq). In order to discuss the corrosion protection ability of a series PU coatings in neutral, acidic and alkaline environments. The coating thickness of as-prepared three PU samples for electrochemical measurements was 36 ± 1 μm. 3.5.1.1. Potentiodynamic polarization curves in 3.5 wt% NaCl(aq). All the values of the anticorrosion including corrosion current density (Icorr) and corrosion potential (Ecorr) are given in Table 2. It should be noted that CRS electrode coated with S-EPU/EPU coatings containing SACAT/ACAT was found to exhibit better corrosion protection performance than that of N-EPU coating. For example, Tafel plots for (a) raw CRS electrode and CRS electrode coated with (b) N-EPU, (c) EPU and (d) S-EPU were shown in Fig. 5 and Table 2. The Ecorr and Icorr of raw CRS electrode were found to be - 706 mV and 30.64 μA/cm2, respectively, as shown in Fig. 5 (a). After coated with N-EPU, the Ecorr and Icorr of CRS electrode was shifted to −517.4 mV and 16.97 μA/cm2, respectively, as shown in Fig. 5 (b). The decreasing of Ecorr and Icorr value corresponds to the better anticorrosion performance of CRS electrode. It indicated that the N-EPU provides the basic physical barrier effect of polymer coating on CRS electrode. Moreover, the Ecorr and Icorr of CRS electrode coated with EPU was further shifted to 451.3 mV and 1.53 μA/cm2, respectively, as shown in Fig. 5 (c). The further decrease of Ecorr and Icorr value of EPU corresponds to better anticorrosion performance of CRS electrode as compared to that of NEPU coating. The enhancement in corrosion protection of CRS electrode coated with EPU as compared to that of N-EPU may be attributed to the formation of densely passive metal oxide layer (e.g., Fe2O3 and Fe3O4) induced from the electro-catalytic (i.e., redox) capability of EPU. The densely passive metal oxide layers found at the surface of CRS electrode was further identified by Raman spectroscopy, which will discussed in the following section. Eventually, the Ecorr and Icorr of CRS electrode coated with S-EPU was found to be - 420.8 mV and 0.38 μA/cm2, respectively, as shown in Fig. 5 (d). It should be noted that the CRS electrode coated with S-EPU coating was found to exhibit superior anticorrosion performance as compared to that of EPU coating based on the Ecorr and Icorr of Tafel plots. The slightly decrease of Ecorr and Icorr
3.4. Chemical oxidation of S-ACAT and ACAT For the chemical oxidation study of S-ACAT and ACAT, the reduced form of S-ACAT and ACAT was separately dissolved in NMP solution. Subsequently, trace amounts of the oxidant, (NH4)2S2O8, were introduced into the S-ACAT and ACAT solutions gradually for in-situ monitoring the sequential oxidation process of S-ACAT and ACAT by UV–visible absorption spectroscopy, as shown in Fig. 4 (a) and (b). It should be noted that upon oxidation, the color of S-ACAT and ACAT solution was all found to vary from brown to green. Initially, only one absorption band was observed at positions 330 nm for S-ACAT and 315 nm for ACAT, which were associated with π –π * transition of conjugated ring system [24]. Upon the addition of trace amounts of oxidant, slow oxidation of S-ACAT/ACAT was observed. A new absorption peak appeared at locations 582 nm for S-ACAT and 581 nm for ACAT, which were designated to the exciton-type transition between the HOMO orbital of benzoid ring and the LUMO orbital of quinoid ring [25]. This phenomenon of absorption peak of S-ACAT at 582 nm is atrtbuted to the presence of the quinoid ring, having a red-shift to 778 nm. This could be explained by the incorporation of sulfonated acid group into ACAT to promote the conjugated structure of ACAT, which cause the characteristic adsorption peak shift to a longer wavelength. 3.5. Electrochemical corrosion measurements 3.5.1. Potentiodynamic measurements In this study, the corrosion studies of a series of PU coatings were investigated by electrochemical corrosion measurements. The CRS electrodes coated with S-EPU, EPU and N-EPU were equipped in a
Fig. 4. Monitoring the chemical oxidation behavior for (a) S-ACAT (b) ACAT in the presence of trace amount of ammonium persulfate by UV–visible absorption spectroscopy. 102
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Fig. 5. Tafel plots for (a) raw CRS electrode and CRS electrode coated with(b) N-EPU, (c) EPU and (d) S-EPU in 3.5 wt% NaCl(aq).
Fig. 6. Tafel plots for (a) raw CRS electrode and CRS electrode coated with(b) N-EPU, (c) EPU and (d) S-EPU in 1.0 M HCl(aq).
value of CRS electrode coated with S-EPU coating as compared to that of EPU coating may be resulted from the higher electro-catalytic (redox) capability of S-EPU inducing the more densely metal oxide layers to shelter the underlying metal substrate, which can be further confirmed by studies of the previous electrochemical CV (higher redox capability) and following Raman spectroscopy (formation of densely passive metal oxide layer). The anticorrosion performance for the raw CRS electrode and CRS electrode coated with N-EPU, SEPU, S-EPU can be further evidenced by the studies of electrochemical impedance spectroscopy (EIS) section.
Table 4 Electrochemical corrosion measurements of raw CRS electrode and CRS electrode coated with N-EPU, EPU and S-EPU in 1.0 M NaOH(aq). Sample code
E
CRS N-EPU EPU S-EPU
−738.3 −578.9 −521.9 −447.7
corr
(mV)
I
corr
(μA/cm2)
46.51 17.86 13.78 1.36
Thickness (μm) – 36 ± 1 36 ± 1 36 ± 1
3.5.1.2. Potentiodynamic polarization curves in 1.0 M HCl(aq). The values of the anticorrosion including corrosion current density (Icorr) and corrosion potential (Ecorr) are given in Table 3. It should be noted that CRS electrode coated with S-EPU/EPU was also found to exhibit better corrosion protection performance than that of N-EPU coating. And the potentiodynamic polarization curves in 1.0 M HCl(aq) (Fig. 6) also were found to show similar trend to that of the condition in neutral environment. 3.5.1.3. Potentiodynamic polarization curves in 1.0 M NaOH(aq). The values of the anticorrosion including corrosion current density (Icorr) and corrosion potential (Ecorr) are given in Table 4. It should be noted that CRS electrode coated with S-EPU was also exhibit the best corrosion protection performance than that of N-EPU or EPU coatings. And the potentiodynamic polarization curves in 1.0 M NaOH(aq) (Fig. 7) also were found to show similar trend to that of the condition in previous environments. Fig. 7. Tafel plots for (a) raw CRS electrode and CRS electrode coated with(b) N-EPU, (c) EPU and (d) S-EPU in 1.0 M NaOH(aq).
3.5.2. Electrochemical impedance spectroscopy (Nyquist plots and Bode plots) In this study, EIS was an alternative approach to evaluate the activity difference between surface of CRS electrode after treating with NEPU, EPU, and S-EPU coatings. Impedance is a complex resistance when
an electric current flow through a circuit made of capacitors, resistors, or insulators, or any possible combination of these [26]. In EIS study, the impedance (Z) usually depends on the charge transfer resistance (Rct), the solution resistance (Rs), the capacitance of electrical double layer (Cdl) and the frequency of AC signal (ω). It can be derived by using the following equation [27]:
Table 3 Electrochemical corrosion measurements of raw CRS electrode and CRS electrode coated with N-EPU, EPU and S-EPU in 1.0 M HCl(aq). Sample code
E
CRS N-EPU EPU S-EPU
−679.4 −492.5 −470.7 −443.6
corr
(mV)
I
corr
(μA/cm2)
52.31 23.85 1.28 0.74
Z= Z' + jZ'' =
Thickness (μm) – 36 ± 1 36 ± 1 36 ± 1
j (Rct2 Cdi ω) Rs + R ct + 2 1 + (R ct Cdl ω) 1 + (R ct Cdl ω)2
The high-frequency intercept is equal to the solution resistance, and the low-frequency intercept is equal to the sum of the solution and charge transfer resistances [28,29]. The higher the semicircle diameter 103
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Fig. 8. Nyquist plots for (a) raw CRS electrode and CRS electrode coated with (b) N-EPU (c) EPU and (d) S-EPU in 3.5 wt% NaCl(aq).
Fig. 9. Bode plots for (a) raw CRS electrode and CRS electrode coated with(b) N-EPU (c) EPU and (d) S-EPU in 3.5 wt% NaCl(aq).
(charge transfer resistance) indicated the lower the corrosion rate [30,31]. The electrochemical equivalent circuit adopted to derive electrochemical parameters are attached as inset of Fig. 8. The Rs represent solution resistance; RCt represent the charge transfer resistance and CPE is the constant phase element of the double layer formed at the metal-solution interface. In the following sections are the EIS studies of N-EPU, EPU and S-EPU in neutral, acidic and alkaline conditions. 3.5.2.1. EIS in 3.5 wt-% NaCl(aq). The EIS studies of raw CRS electrode and CRS electrode coated with N-EPU, EPU and S-EPU was studied in 5 wt% aqueous NaCl electrolyte after 30 min equilibrium and compared in Fig. 8.For example, we found that the charge transfer resistance of raw CRS electrode and CRS electrode coated with N-EPU, EPU and SEPU as determined by the intersection of the low frequency end of the semicircle arc with the real axis was 1.70, 17.78, 82.59, and 98.28 kcm2, respectively, as shown in Fig. 8 (a), (b), (c) and (d). This experimental result of Nyquist plot obtained from EIS studies clearly demonstrated that the anticorrosion performance showing the trend: SEPU > EPU > N-EPU > raw CRS, which is consistent with the previous studies of Tafel plot. Furthermore, EIS Bode plots of raw CRS electrode and corresponding CRS electrode coated with N-EPU, EPU and S-EPU were shown in Fig. 9. It should be noted that the impedance value of CRS electrode coated with S-EPU in the monitoring frequency region from low to high-frequency was highest, as shown in Fig. 9 (d). The EIS Bode plots were found to show similar trend to that of previous Tafel and Nyquist plots. The corrosion protection mechanism for the S-EPU/EPU was similar to that of conventional PANI coatings reported in the previous literatures [32–34].
Fig. 10. Nyquist plots for (a) raw CRS electrode and CRS electrode coated with (b) N-EPU (c) EPU and (d) S-EPU in 1.0 M HCl(aq).
3.5.2.2. EIS in 1.0 M HCl(aq). The EIS studies of raw CRS electrode and CRS electrode coated with N-EPU, EPU and S-EPU was studied in 1.0 M aqueous HCl electrolyte after 30 min equilibrium and compared in Fig. 10. For example, we found that the charge transfer resistance of raw CRS electrode and CRS electrode coated with N-EPU, EPU and SEPU as determined by the intersection of the low frequency end of the semicircle arc with the real axis was 1.34, 18.13, 60.12, and 88.26 kcm2, respectively, as shown in Fig. 10 (a), (b), (c) and (d). This experimental result of Nyquist plot obtained from EIS studies clearly demonstrated that the anticorrosion performance showing the trend: SEPU > EPU > N-EPU > raw CRS, which is consistent with the previous studies of Tafel plot. Also the EIS Bode plots of raw CRS electrode and corresponding CRS electrode coated with N-EPU, EPU and S-EPU were shown in Fig. 11.
Fig. 11. Bode plots for (a) raw CRS electrode and CRS electrode coated with (b) N-EPU (c) EPU and (d) S-EPU in 1.0 M HCl(aq).
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Fig. 12. Nyquist plots for (a) raw CRS electrode and CRS electrode coated with (b) N-EPU (c) EPU and (d) S-EPU in 1.0 M NaOH(aq).
Fig. 14. Raman spectroscopy for the observation of (a) raw CRS electrode and the densely passive metal oxide layer formed on the CRS electrode induced by (b)N-EPU (c) EPU and (d) S-EPU.
The EIS Bode plots were found to show similar trend to that of previous Tafel and Nyquist plots.
electrochemical measurements such as Tafel, Nyquist and Bode plots. But the metal corrosion protection ability of S-EPU in neutral condition is better than in acidic or alkaline conditions slightly.
3.5.2.3. EIS in 1.0 M NaOH(aq). The EIS studies of raw CRS electrode and CRS electrode coated with N-EPU, EPU and S-EPU was studied in 1.0 M aqueous NaOH electrolyte after 30 min equilibrium and compared in Fig. 12. For example, we found that the charge transfer resistance of raw CRS electrode and CRS electrode coated with N-EPU, EPU and S-EPU as determined by the intersection of the low frequency end of the semicircle arc with the real axis was 1.81, 14.55, 55.86, and 79.57 kcm2, respectively, as shown in Fig. 12 (a), (b), (c) and (d). This experimental result of Nyquist plot obtained from EIS studies clearly demonstrated that the anticorrosion performance showing the trend: SEPU > EPU > N-EPU > raw CRS, which is consistent with the previous studies of Tafel plot. Meanwhile, the EIS Bode plots of raw CRS electrode and corresponding CRS electrode coated with N-EPU, EPU and S-EPU were shown in Fig. 13. The EIS Bode plots were found to show similar trend to that of previous Tafel and Nyquist plots. In summary, in this study, the anticorrosion performance studies of CRS electrode coated with distinctive PU samples were found whatever in neutral, acidic and alkaline environments exhibit the following tend: S-EPU > EPU > N-EPU > raw CRS electrode based on a series of
3.5.3. Identification of densely passive metal oxide layer In this study, the Raman spectroscopy, SEM and ESCA were used to confirm the formation of densely passive metal oxide layers (Fe2O3) upon the surface of CRS electrode, as shown in Fig. 14. For example, Raman spectrum for the surface of raw CRS electrode was not found any characteristic peaks of metal oxide, as shown in Fig. 14 (a). After coating with N-EPU, the Raman spectrum for the surface of CRS electrode was found to exhibit similar condition to that of raw CRS electrode, as shown in Fig. 14 (b). However, after coating with EPU for one month, Raman spectrum for the surface of CRS electrode was found to show some characteristic metal oxide peaks after detaching the EPU film. For example, in Fig. 14 (c), the Raman spectrum for surface of CRS electrode induced by EPU coating was found to exhibit seven phonon lines, corresponding to two A1g modes (221 and 497 cm−1) and three Eg mode (292, 407 and 609 cm−1) of Fe2O3 [35]. Moreover, the Raman spectrum for the surface of CRS electrode induced from S-EPU coating was found to reveal more strong/obvious characteristic peaks of Fe2O3, as shown in Fig. 14 (d). It should be noted that the stronger characteristic peaks of densely passive metal oxide layers appeared on the surface of CRS electrode induced by S-EPU coating as compared to that of EPU coating may be attributed to the higher electroactivity of S-EPU, leading to the better anticorrosion performance of S-EPU coating as compared to that of EPU coating on CRS electrode based on a series of electrochemical corrosion measurements. As shown in Fig. 15, are the surface images of the CRS. In Fig. 15 (a), pure CRS surface was shown for the comparative studies. Fig. 15 (b) and (c) were induced by EPU/S-EPU, separately. According to the SEM images observation of the passivation oxidelayers exhibited the deposition of grayish oxide layer form over the CRS surface under the EPU/S-EPU coatings on CRS electrode, are same with the published by Wei et al. [36]. The chemical nature of the passivation oxide layers was measured by ESCA. The binding energy plots vs. intensity for the iron oxide layers are shown in Fig. 16. In Fig. 16 (a), pure CRS surface was shown for the comparative studies. The passivation oxide layers shown in Fig. 16 (b) and (c) were all exhibited the Fe 2p3/2 peak binding energy. In Fig. 16 (b) is the CRS surface coated with EPU. The peak in 710.7 eV is binding energy of Fe3O4. The Fe 2p spectra of FeO and Fe3O4 are similar and it is hard to distinguish from each other. The 2p3/ 2 binding energy was about 724.1 eV. In Fig. 16 (c) is the CRS surface
Fig. 13. Bode plots for (a) raw CRS electrode and CRS electrode coated with (b) N-EPU (c) EPU and (d) S-EPU in 1.0 M NaOH(aq). 105
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Fig. 15. SEM images of (a) raw CRS metal and (b) the surface of the coated with EPU and (c) S-EPU on the CRS metal.
Fig. 18. The DSC curve of (a) N-EPU (b) EPU and (c) S-EPU.
Fig. 16. ESCA Fe 2p core level spectra of (a) raw CRS metal and (b) the surface of the coated with EPU and (c) S-EPU on the CRS metal.
highest 5% weight loss decomposition temperature (Td,5%) (291 °C), the next is EPU (289 °C), then N-EPU has the lowest Td,5% (267 °C). The reason why the S-EPU and EPU have more better thermal stability than N-EPU may due to the ACAT and S-ACAT are the structure with benzene ring. The structure of polymer with benzene ring will improve the temperature of decomposition. In the DSC investigations, the glass transition temperature (Tg) of polyurea increased as the content of S-ACAT/ACAT segments in the main chains of polyureas. For example, Tg of samples increased form 8.1 °C of N-EPU, to 11.7 °C of S-EPU, and to 15.2 °C of EPU, as shown in Fig. 18 (Table 1). The increasing of Tg for as-prepared polyureas might be attributed to the incorporation of S-ACAT and ACAT rigid aromatic chemical structures, leading to an obvious increase of Tg.
coated with S-EPU. The peak in 710.8 eV is binding energy of Fe3O4. The Fe 2p spectra of FeO and Fe3O4 are similar and it is hard to distinguish from each other. The 2p3/2 binding energy was about 724.6 eV. In summary, according to the Raman spectrum, SEM and ESCA, in the present study conform the passive metal oxide layer were induced by SEPU and EPU. 3.6. Thermal properties Thermal performance is an important indicator among the polymer in applications [37–40]. TGA analysis of N-EPU, EPU and S-EPU has been shown in Fig. 17 (Table 1). For these samples, S-EPU shows the
4. Conclusions The S-EPU was successfully synthesized and applied as anticorrosion coating. The as-prepared S-EPU was identified by FTIR spectroscopy and GPC. Redox capability of as-prepared diamine of S-ACAT/ ACAT and electroactive polymers of S-EPU/EPU/N-EPU was confirmed by electrochemical CV studies. Compounds with sulfonated groups show better redox ability than those corresponding counterpart. Chemical oxidization of S-EPU/EPU, through the introduction of trace amount of APS was monitored by UV–visible absorption spectroscopy. Electrochemical corrosion measurements of CRS electrode coated with S-EPU/EPU/N-EPU based on a series of electrochemical corrosion measurements such as Tafel plots, Nyquist plots and Bode plots were investigated in neutral, acidic and alkaline conditions. Regardless in which environments, the CRS electrode coated with different PU coatings was found to reveal following tendency: S-EPU > EPU > NEPU > raw CRS electrode based on a series of electrochemical corrosion measurements. The better corrosion protection of electroactive SEPU/EPU coatings as compared to that of non-electroactive N-EPU may
Fig. 17. The TGA curve of (a) N-EPU (b) EPU and (c) S-EPU. 106
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be attributed to their redox capability inducing the formation of densely metal oxide layer (e.g., Fe2O3 and Fe3O4), as demonstrated by the studies of Raman spectroscopy, SEM and ESCA. The S-EPU coating exhibited better anticorrosion performance than that of EPU coating which could be attributed to the higher electro-catalytic property of SEPU coating upon CRS electrode, causing the formation of more densely metal oxide layers to shelter the underlying metal substrate. Moreover, the S-EPU coating shows higher thermal properties in TGA and DSC as compared to that of EPU and N-EPU. The S-EPU has good corrosion protection ability, redox ability and thermal stability, which might be useful applied in the metal anticorrosion field.
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