Boron nitride nanosheets reinforced waterborne polyurethane coatings for improving corrosion resistance and antifriction properties

European Polymer Journal 104 (2018) 57–63

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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Macromolecular Nanotechnology

Boron nitride nanosheets reinforced waterborne polyurethane coatings for improving corrosion resistance and antifriction properties ⁎

Jing Lia, Lingzhu Gana, Yuchen Liub, Srikanth Matetib, Weiwei Leib, Ying Chenb, , Junhe Yanga, a b

T ⁎

School of Materials Science and Engineering, University of Shanghai for Science and Technology, No. 516 Jungong Road, Shanghai 200093, China Institute for Frontier Materials, Deakin University, Waurn Ponds 3216, Australia

A R T I C LE I N FO

A B S T R A C T

Keywords: Composite coatings Boron nitride nanosheet Antifriction Corrosion protection

Effective corrosion resistance and antifriction properties can prolong the durability of coatings. We demonstrate that hexagonal boron nitride (BN) nanosheets can be employed as the reinforcement in waterborne polyurethane (PU)-matrix composite coatings and lead to a significant improvement in their corrosion protection properties. The impedance modulus at 0.1 Hz dropped from 109 Ω·cm2 to 108 Ω·cm2 for 2% BN/PU coatings after 114 h immersion in 3.5 wt% NaCl electrolyte, while it dropped from 108 Ω·cm2 to 5 × 106 Ω·cm2 for neat PU coatings. The dynamic friction coefficient was decreased for 22.6% with the addition of 2 wt% BN in the PU composite coatings at 100 °C. The morphologies of the coating surfaces showed less abrasion than neat PU after the test. The major challenge of preparing BN/PU composite coatings was the dispersion of BN nanosheets in the waterborne polymer, which was solved by functionalization of BN nanosheets with hydroxyl groups.

1. Introduction The durability of metal materials is often shortened by their corrosion process. Since ancient times, animal fat and beeswax were applied onto the metal surface to suppress corrosion process of metallic tools. Nowadays, organic coatings were formulated to provide more efficient protection. The wildly used solvent based coatings contain a large amount of volatile organic compounds, which have caused severe environmental pollution. As an environmental-friendly substitution, waterborne polyurethane (PU) is one of the most widely used resins in organic coating formulation due to its high flexibility, excellent abrasion resistance and chemical resistance, as well as the strong adhesion to substrates. In order to improve its corrosion resistance properties, researchers have explored nano-fillers such as nanoclays [1], carbon nanotubes [2], graphene and its derivatives [3–5] to prepare polymer matrix composite coatings. Hexagonal boron nitride (BN) nanosheets own the similar structure and properties as graphene, e.g. its layered structure, high aspect ratio, excellent mechanical and barrier properties [6–10]. In addition, BN nanosheets have a higher oxidation temperature (800 °C) than graphene [6,11,12], and barely react with almost any acid and alkaline at ambient condition due to its chemical stability [13]. BN is also an electric insulator with a wide bandgap of 5–6 eV [6,11,14]. Besides, BN nanosheets have been used as a lubricating agent owning to its strong inplane bonding and the weak interlayer van der Waals forces [15,16].

⁎

It has been reported that BN films fabricated by chemical vapor deposition can protect different metals [16,17], including copper [7,18], and nickel [19], from being corroded. While graphene film grown by chemical vapor deposition method can only provide the metal surfaces a short-term protection [20], the BN monolayer can be a longterm protective barrier on Cu surfaces, as proven by experiments and theoretical calculations [18]. Moreover, researchers have found that BN is an effective reinforcement in polymer matrix, aiming for better mechanical properties and thermo-stability [21–24]. Recently, BN has been applied in composite coatings with different polymer matrix for corrosion resistance purpose, including polyvinyl butyral [7], epoxy [25], polyimide [26], PANI [27]. However, the application of BN nanosheets in waterborne polymer-matrix composite coatings remains largely unexplored. Unlike graphene oxide, BN nanosheet surfaces are usually lack of functional groups, which makes it extremely challenging to disperse them in waterborne polymer matrix. There is one report on the BN reinforced waterborne epoxy composite coatings [28]. Water-soluble carboxylated aniline trimer derivative (CAT−) was employed as the dispersant, which dyed the BN dispersion and the composite coatings black color. The composite coatings had improved corrosion resistance properties because of the BN incorporation [28]. It’s critical to disperse BN in waterborne polymer coatings without changing the appearance of the coatings. In this work, BN nanosheets were treated with sodium hydroxide solution to increase their hydrophilicity. The waterborne PU matrix

Corresponding authors. E-mail addresses: [email protected] (Y. Chen), [email protected] (J. Yang).

https://doi.org/10.1016/j.eurpolymj.2018.04.042 Received 17 November 2017; Received in revised form 23 March 2018; Accepted 29 April 2018 Available online 30 April 2018 0014-3057/ © 2018 Elsevier Ltd. All rights reserved.

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Pristine BN BN-OH

Transmittance (a.u.)

(a)

(b)

-OH

4000 3500 3000 2500 2000 1500 1000 500 -1

Wavenumber (cm ) Fig. 1. (a) FTIR spectra of pristine BN and BN-OH; and (b) the reaction schematic of hydroxylation of BN.

electrolyte was a 3.5 wt% NaCl solution (reagent grade). Electrochemistry workstation (PARSTAT 4000, Princeton Applied Research) was employed with a low current interface attachment. The signal amplitude was 20 mV relative to the open circuit potential and the frequency was ranged from 0.1 to 10,000 Hz. All the EIS measurements were performed in a Faraday cage at room temperature. The wear test was carried out by a surface performance analyzer (14FW, SHINTO) with reciprocating friction of stainless steel ball according to ASTM D1894, applying a 100 g load, a moving rate of 150 mm/min, and the moving distance of 10 mm. The tests were performed at room temperature and 100 °C, respectively.

composite coatings were prepared with both pristine BN and treated BN, and then applied on the galvanized steel surfaces. The corrosion protection properties and antifriction properties of the PU-matrix coatings were improved significantly by the addition of 2 wt% functionalized BN nanosheets. 2. Experiments 2.1. Functionalization of BN with OH groups BN nanosheets were prepared following the procedure published previously [29]. The as-prepared BN nanosheets were mixed with the 5 M NaOH solution at 120 °C for 24 h to attach hydroxide groups onto the BN surfaces. The hydroxylated BN (BN-OH) nanosheets were centrifuged at 11,000 rpm for 10 min and washed with DI water repeatedly to eliminate the unreacted NaOH.

3. Results and discussion 3.1. Hydroxylated BN and its dispersibility in water Fig. S1 shows the dispersion stability of BN and BN-OH in water with a concentration of 1 mg/ml. Small amount of BN precipitation was observed after 3 days in the BN dispersion while the BN-OH dispersion did not show any precipitations. The zeta potential value of BN and BNOH dispersion were −15.7 mV and -26.0 mV at a concentration of 1 mg/ml. Although no formal charges existed in BN, B and N atoms possessed different electronegativities. B atoms display a slight positive charge while N atoms display a slight negative charge. When dispersed into H2O, whose pH was above the isoelectric point of BN (pH = 4.3), BN would adsorb more OH− than H+, showing negatively charged in general [30]. Therefore, both BN and BN-OH dispersion showed negative zeta potential values. The BN-OH dispersion had a higher absolute value of zeta potential, which led to better dispersion stability due to the electro-static repulsion between the neighboring particles. FTIR spectra in Fig. 1(a) shows that both pristine BN and BN-OH have peaks at 1380 cm−1 and 780 cm−1, corresponding to the stretching vibrations of hexagonal BN [9,24,25,30]. For BN-OH, the peak at 1380 cm−1 widens. It was reported the peak at 1437 cm−1 can be assigned to BeO− stretching vibration [31], so the widened peak is the superposition of hexagonal BN and BeO− stretch vibrations. The peak at 3400 cm−1 is assigned to the OeH stretching vibration [9,23,24]. Therefore, the chemical reaction between pristine BN and NaOH is proposed in Fig. 1(b). After the surface modification, the hydroxyl groups were introduced on the BN edges, which was responsible for the decrease of zeta potential for BN-OH.

2.2. Preparation of the BN-OH/PU and BN/PU composite coatings The aqueous dispersion of BN-OH was mixed with waterborne PU supplied by Huayi Fine Chemical Ltd, Shanghai. The mixture was stirred magnetically for 10 min, followed by sonication for 10 min to obtain uniform liquid paintings. The filler content of BN-OH in PU matrix was kept at 0.5 wt%, 1 wt% and 2 wt%. Hot-dip galvanized steel sheets (80 mm × 20 mm × 0.5 mm), supplied by Baosteel Ltd. (Shanghai), were thoroughly cleaned by sonication in alkaline degreaser (Henckle, Shanghai) for 10 min, flushed with DI water and sonication in ethanol for another 10 min. The liquid paintings were applied on the steel surfaces with a bar coater, and then the specimens were baked in a vacuum oven at 110 °C for 30 min to obtain the final composite coatings. The thickness of the coatings was about 25 ± 2 μm. For comparison, the pristine BN/PU composite coatings were prepared in the same way with the same filler contents. 2.3. Characterization The surface chemistry of BN and BN-OH was characterized using Fourier transfer infrared spectrometry (FTIR, Spectrum 100 PerkinElmer) and zeta-potential analyzer (Nano-zs90, Malvern). FTIR spectra were collected in reflection mode for the wave number range 4000–500 cm−1 at a resolution of 4 cm−1. A transmission electron microscope (TEM, FEI Tecnai G2 F30) was used to evaluate the nanoscopic dispersion state of BN-OH in PU matrix at an acceleration voltage of 300 kV. For TEM sample preparation, a free-standing BN-OH reinforced PU composite thin film was embedded in an epoxy resin and microtomed (Leica UC6) into 75 nm-thick slices with a diamond knife. For the Electrochemical Impedance Spectroscopy (EIS), a three electrode cell was used: the coated steel surface as the working electrode with an exposed area of 1 cm2, the saturated Ag/AgCl (0.205 V vs. SHE) as the reference electrode and a platinum counter electrode. The

3.2. Dispersion of BN-OH in PU matrix The SEM image in Fig. 2(a) shows the morphology of the starting BN nanosheets of 100–200 nm in lateral size. Large clusters are formed to reduce the total surface area. High resolution TEM morphology of BN nanosheets (Fig. 2(b)) showed the layered structure with several layers stacking together. The TEM images in Fig. 2(c, d) shows the morphologies of PU-matrix composites reinforced by 2 wt% BN-OH. 58

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(a)

(b)

(d)

(c)

Single layer Multi-layer

Fig. 2. (a) SEM and (b) TEM morphology of BN nanosheets; (c, d) TEM images of 2 wt% BN-OH/PU composites with different magnifications.

The corrosion resistance of 0.5 wt% BN-OH reinforced PU composite coatings are presented in Fig. 4(a) and (a′). The peak value of the phase angle was 80° at the beginning of the immersion, and remained higher than 45° until 199 h of immersion. The impedance modulus at 0.1 Hz was about 8 × 108 Ω·cm2 at beginning and decreased to 4 × 106 Ω·cm2 after 199 h of immersion duration. These results indicated that 0.5 wt% BN-OH/PU composite coatings had better corrosion resistance than neat PU coatings. Fig. 4(b) and (b′) presents the EIS spectra for 1 wt% BN-OH/PU composite coatings. The peak value of phase angle remained over 70° for 153 h of immersion, suggesting the penetration of the electrolyte was restrained efficiently. After 214 h of immersion, the peak of phase angle dropped to around 45°, indicating that the corrosion protection properties of the coatings was deteriorated gradually. The impedance modulus at 0.1 Hz decreased from 3.6 × 108 Ω·cm2 to 1.6 × 107 Ω·cm2 after the 254 h of immersion. It seemed that higher BN-OH content was required to form the barrier network in the composite coatings. The barrier network formation was considered as a percolation phenomenon [34]. Critical filler content was needed to obtain the network, which in turn resulted in better corrosion protection properties. EIS results of composite coatings with 2 wt% BN-OH are shown in Fig. 4 (c) and (c′). The peak phase angle of near 90° demonstrated the capacitive nature of the composite coatings, which remained for over 100 h of immersion. After 216 h of immersion, the peak value of the phase angle was about 75°. The impedance modulus at 0.1 Hz was about 109 Ω·cm2 at beginning, and decreased to 2.6 × 106 Ω·cm2 after

Microscopically, BN-OH nanosheets were randomly dispersed in the PU matrix, as shown in Fig. 2(c). Higher-magnification TEM image in Fig. 2(d) shows the layered structure of BN nanosheets. The layers of BN range from single layer to about 10 layers. Agglomeration of BN-OH was not observed, owning to the good compatibility between the waterborne PU and BN-OH nanosheets. 3.3. Corrosion protection properties of the BN-OH/PU composite coatings Fig. 3 shows the Bode phase and modulus plots of neat PU coatings with different exposure durations. At the beginning, the peak value of phase angle was around 70° at the high frequency range. The impedance modulus at 0.1 Hz, represents the ability of the coating to impede the flow of current between anodic and cathodic areas, was about 108 Ω·cm2. After 130 h of immersion, the peak value of phase angle was lower than 45° and the impedance modulus at 0.1 Hz dropped to 2 × 106 Ω·cm2. A typical resistor component gives a phase angle of 0°, while a typical capacitance component gives the phase angle of 90°. The peak value of the phase angle gave corrosion characteristics of the organic coatings. The decrease of the peak value represented that the electrolyte was gradually penetrated into the organic coating and the coating characteristics were changed from capacitance to resistor [4,32,33]. The corrosion resistance of pristine BN/PU coatings was inferior to the neat PU coatings probably because the BN nanosheets were not dispersed homogeneously and thus did not function as effective barriers. The EIS results are displayed in Figs. S2–S4.

Fig. 3. EIS spectra of neat PU coatings (a) Bode phase plots and (b) Bode modulus plots. (The solid lines are the fitting curves for each sample). 59

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Fig. 4. EIS spectra of (a) (a′) 0.5 wt%, (b) (b′) 1 wt% and (c) (c′) 2 wt% BN-OH/ PU coatings. (The solid lines are the fitting curves for each sample).

ideal capacitor during real electrochemical process [28,38,39]. For the plots with one time constant, the equivalent circuit in Fig. 5(a) was used, where Rs is the resistance of solution, Rc is the resistance of coating, CPEc is the calibration of the capacitance of coating. For the EIS plots with two time constants, CPEdl is used to represent the double layer capacitance at the metal/coating interface. Rct is the charge transfer resistance of corrosion electrochemical reaction at the interface. The corresponding equivalent circuit was shown in Fig. 5(b). The fitting parameters (Rc, CPEc, CPEdl and n) were listed in Table S1, where n was defined as a CPE power. For n = 1 the CPE describes an ideal capacitor, and for 0.5 < n < 1 the CPE describes a frequency dispersion of time constants due to local heterogeneities in the dielectric material [28,38,40]. All of the values of the exponent n were lower than 1 in Table S1, justified the introduction of CPE in the equivalent circuit rather than pure capacitors. The fitting curves were shown in Figs. 3 and 4, which agreed well with the experiment data. The electrochemical parameters Rc and CPEc were plotted in Fig. 6(a) and (b) as a function of immersion duration for BN-OH/PU coatings and neat PU coatings. As the electrolyte solution penetrated through the coatings, the coating resistance Rc decreased. With increasing the content of BN-OH, the Rc was significantly increased, and

318 h. The 2 wt% BN-OH/PU composite coatings showed much better corrosion protection properties than the composite coatings with lower BN-OH content. Two time constants were observed after 300 h of immersion which were showed in the Bode phase plot. The appearance of two time constants suggested that the electrolyte was penetrated through the organic coating and reached the metal substrates. The corrosion reaction of the metal substrate was initiated. The time constant at high frequency range represented the corrosion resistance characteristics of organic coating layer, and the time constant at lower frequency range was corresponding to the properties at coating-metal interface layer. Compared to the corrosion protection properties of pristine BN/PU composite coatings, the BN-OH showed much better efficiency on improving the barrier properties of the coatings. It was well-known that strong interface benefited the resistance of environmental attack for composite materials [35,36]. The hydroxyl groups on the BN-OH surfaces favored the interface interactions between BN-OH and the waterborne PU. The EIS results were fitted by the equivalent circuits [4,37], as shown in Fig. 5. Constant phase element (CPE) was used to replace the pure capacitance in the equivalent circuits, representing a shift from the

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Fig. 5. The equivalent circuits to fit the EIS results with (a) one time constant and (b) two time constants. 10

10

9

10 Rc (Ω cm )

8

2

with the absorption of the electrolyte solution in the coating. The water absorption in the organic coating can be calculated from the coating capacitance according to the Brasher-Kingsbury’s equation CPEct Xv = log CPEc0 /log80 , where Xv is the volume fraction of water in the organic coatings, CPEct is the coating capacitance at a certain immersion time, CPEc0 is the initial capacitance of dry coating [41]. According to the equation, the higher value of CPEc meant the higher amount of water absorption. The addition of BN-OH decreased the value of CPEc, suggesting the BN-OH functioned as the physical barriers to the electrolyte. The 2 wt% BN-OH/PU coatings showed the lowest CPEc values. The frequency at 45° phase angle was defined as the Breakpoint frequency (fb), obtained from the Bode phase plot. The porosity of the coatings and the reaction at the metal/coating interface could be analyzed by the variation of fb as a function of immersion duration. Their relations are presented by fb = K(At/A0), where At is the delaminated area, A0 is the total area of the sample, K = (1/2)ρεε0, ρ is the resistivity of the coating, ε is the dielectric constant of electrolyte in the coating, and ε0 is the vacuum permittivity [33]. With the electrolyte solution penetrating through the organic coating, the value of ρ declines, while the value of ε increases accordingly. The compensation effect makes it possible that K can be approximately seen as a constant, given ε0 is a constant. Therefore, the delaminated area of the coating is approximately proportional to fb. The fb was regarded as a boundary where a capacitive region first transited to a resistive region when the frequency shifts from high to low values [3,33]. The fb shifted to higher value with increasing of immersion duration, as shown in Fig. 6(c), suggesting the increase of the defected area. The BN-OH addition delayed the increase of fb. The fb value of 2 wt% BN-OH/PU coatings showed a platform without obvious increase at the first 220 h of immersion, which was much lower than that of neat PU. Based on the above results, the superior corrosion protection properties were achieved by adding 2 wt% BN-OH in PU coatings, which can be attributed to three reasons: (1) the two-dimensional geometry of BN nanosheets provided a physical barrier in the coatings; (2) the nanoscopic dispersion of BN nanosheets (Fig. 2) promoted the formation of the barrier network; and (3) the excellent insulating property of BN retarded the corrosion reaction. It was reported that the electron transport in the horizontal direction is prohibited because BN is electrically insulating, in which the close loop of an electrochemical cell was disconnected and the corrosion was slowed down [18,33].

0% 0.5% 1% 2%

(a)

10

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6

10

0

100 150 200 250 300 350

t(h)

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CPEc(Fcm )

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10

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100 150 200 250 300 350 t(h)

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fb (Hz)

600 400

0% 0.5% 1% 2%

200 0 0

50

100 150 200 250 300 350 t (h)

3.4. Antifriction properties of the BN-OH/PU composite coatings

Fig. 6. Evolution of (a) Rc, (b) CPE-T and (c) fb of the coatings with immersion duration.

The dynamic friction coefficients of the coatings decreased with increasing BN-OH content, as shown in Fig. 7. For all the specimens, the dynamic friction coefficients increased with the increase in reciprocating cycles. The addition of BN-OH delayed the increase of the dynamic friction coefficients (Fig. 7a). The antifriction effect was more significant at 100 °C. After 500 cycles, the dynamic friction coefficients

reached the highest value when the filler content was 2 wt%. A higher Rc implied that less electrolytes were penetrated into the coatings, as well as the improved barrier properties of the coatings. CPEc increased 61

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(a)

composite coatings was increased, attributed to the good compatibility between the BN-OH and PU matrix [42]. The sample surface morphologies after dynamic friction tests were studied using optical microscopy (OM), as presented in Fig. 8. The top surface images of the coatings showed inhomogeneous color due to the microphase separation of the soft segment and the rigid segment of PU. The soft segment and the rigid segment of PU were not compatible thermodynamically. They form individual microcell during solidification, and therefore develop microphase separation construction [43]. Neat PU coatings showed a wear track with about 1251 μm in width after 500 reciprocating cycles at room temperature (Fig. 8a). The wear track width of 0.5 wt% BN-OH/PU coatings was about 641 μm, which was reduced by 48.7% (Fig. 8b). The wear track width was 548 μm for 1 wt% BN-OH/PU coatings (Fig. 8c). For the 2 wt% BN-OH/PU coatings, no wear trach can be observed under OM (Fig. 8d). The similar results were observed for the coatings tested at 100 °C, as shown in Fig. S5. It seems that the addition of BN-OH not only increased the antifriction properties of the composite coatings, but also increased their abrasion-resistance. It was reported that the incorporation of BN in polymer matrix can improve the elastic modulus, tensile strength and toughness of the composites [21,22], which were the factors that may contribute to the abrasion-resistance of the composite coatings.

0.5 0.4 0.3

0% 0.5% 1% 2%

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60 120 180 240 300 360 420 480 Cycles

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0.5 0.4 0.3

0% 0.5% 1% 2%

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4. Conclusions

60 120 180 240 300 360 420 480 Cycles

BN nanosheets were used as the reinforcement in PU-matrix composite coatings. A stable aqueous dispersion of BN nanosheets was achieved by introducing hydroxyl groups on the edges of BN nanosheets. The functionalized BN (BN-OH) had good interface compatibility with waterborne PU. The BN-OH thus achieved homogenous dispersion in the PU matrix, and act as effective barriers to inhibit the permeation of electrolyte, which in turn improved the corrosion protection properties of the composite coatings. The addition of 2 wt% BN nanosheets significantly delayed the decrease of the impedance modulus at 0.1 Hz and the peak value of phase angle, as characterized by

Fig. 7. Dynamic friction coefficients for neat PU coatings, 0.5 wt%, 1 wt% and 2 wt% BN-OH/PU composite coatings (a) at room temperature and (b) 100 °C.

of 2% BN-OH/PU composite coatings were 22.6% lower than those of all the other specimens (Fig. 7b). This can be explained by the good lubricating property of BN. At high temperature, the composite coatings with 2% BN-OH showed much lower dynamic friction coefficient than neat PU. It was plausible to explain that the thermal stability of the

(b)

(a)

1251ȝm

641ȝm

2000 μm

2000 μm

(c)

(d)

548ȝm

2000 μm

2000 μm

Fig. 8. OM images of wear tracks after the dynamic friction tests at room temperature for (a) neat PU coatings; (b) 0.5 wt% BN-OH/PU coatings; (c) 1 wt% BN-OH/PU coatings; (d) 2 wt% BN-OH/PU coatings. 62

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EIS tests. The dynamic friction coefficient of 2 wt% BN-OH/PU composite coatings was 22.6% lower than neat PU at 100 °C. The abrasion track was not observed under OM for 2 wt% BN-OH/PU composite coatings. Once the homogeneous dispersion was achieved, the BN nanosheets can be an ideal reinforcement of waterborne composite coatings, especially for application as the top coatings, due to the good durability and colorless appearance.

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Acknowledgements

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The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (U1560108, U1760119) and Science and Technology Commission of Shanghai Municipality (17511101603, 17XD1403000, 18ZR1426300), as well as the Discovery projects awarded by the Australian Research Council.

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Appendix A. Supplementary material [25]

Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.eurpolymj.2018.04.042.

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