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Surface Modification of Aramid Fibers by Catechol/ Polyamine Co-deposition Followed Silane Grafting for the Enhanced Interfacial Adhesion to Rubber Matrix Lei Wang, Yongxiang Shi, Rina Sa, Nanying Ning, Wencai Wang, Ming Tian, and Liqun Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03177 • Publication Date (Web): 16 Nov 2016 Downloaded from http://pubs.acs.org on November 21, 2016

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Industrial & Engineering Chemistry Research

Surface Modification of Aramid Fibers by Catechol/Polyamine Co-deposition Followed Silane Grafting for the Enhanced Interfacial Adhesion to Rubber Matrix Lei Wang†, §, Yongxiang Shi†, §, Rina Sa†, ‡, Nanying Ning†, ‡, Wencai Wang*, †, ‡, Ming Tian*, ‡, §, Liqun Zhang†, § † Key Laboratory of Beijing City for Preparation and Processing of Novel Polymer Materials, Beijing 100029, P. R. China ‡ State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, P. R. China § Engineering Research Center of Elastomer Materials on Energy Conservation and Resources Ministry of Education, Beijing 100029, P. R. China Corresponding Author *E-mail: [email protected]; [email protected] Tel.: +86-10-64434860; Fax: +86-10-64433964. Notes The authors declare no competing financial interest.

ACS Paragon Plus Environment

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ABSTRACT: In this work, we developed a modified mussel-inspired method to enhance interfacial adhesion of aramid fiber to rubber matrix. Through a simple dip-coating procedure, catechol and polyamine could initially co-deposit as a poly(catechol-polyamine) (PCPA) coating on the surface of aramid fiber. Then, the PCPA layer could be further grafted with silane coupling agent γ-(glycidyloxypropyltrimethoxysilane) (GPTMS). Results indicated that GPTMS was successfully grafted onto aramid fibers surface via the bridging of PCPA layer. The interfacial adhesion between the aramid fibers and the rubber matrix was improved compared to that by polydopamine in our previous study. In addition, this method is more applicable to rubber industry than polydopamine coating due to its cost-effectiveness and short reaction time.

1. Introduction Owing to their good integrated performances including high strength to weight ratio and excellent chemical and thermal resistance,1-3 aramid fibers have been widely used in aerospace, bullet proof, and engineering applications.4-8 The performances of aramid fiber reinforced composites largely depend on the fiber-matrix interface. However, due to the high crystallinity and chemical inertness of aramid fiber, a weak interfacial interaction between fiber and composite matrix was perceived.9, 10 Over the past decades, a great number of methods have been developed to improve the surface activity of aramid fibers, such as chemical etching,11, 12 plasma treatment,13-15 ultrasound,16 UV rays17 and Co60 γ-rays irradiation,18 and polymer coating.19 Physical methods such as plasma treatment and gamma irradiation have been used to improve the wettability and roughness of fibers surface.13-15 However, they are expensive and require specific reaction conditions. With respect to traditional chemical treatments, such as surface etching and grafting, is aiming to


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introduce surface active groups to form chemical bonds between fiber and matrix.11,

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Nevertheless, chemical etching seriously destroys the structure integrities of aramid fibers. For these reasons, to develop a simple, low-cost and effective strategy for surface modification of aramid fibers is highly desirable. Marine mussel adhesive proteins (MAPs) have attracted great attention owing to their amazing adhesion ability. It has been confirmed that the extraordinarily powerful adhesion properties of MAPs are mainly attributed to the 3, 4-dihydroxy-L-phenylalanie (DOPA), which are abundant in the MAPs.20 Dopamine, with a similar chemical structure with DOPA, was reported as a novel surface modification material in 2007.21 Through self-oxidation and polymerization in wet environments,22-23 the formed polydopamine (PDA) can form strong and durable bonds on the substrates. Owing to its striking properties, PDA has opened up a new route for surface modification and stimulated extensive researches.24-28 In addition, due to the presence of abundant catechol and amine groups, the PDA coating can act as a versatile platform for the further specific functionalization.29-35 Several researchers have adopted PDA for the modification of fibers surface to improve the interface interaction between fibers and polymer matrix.36-40 For example, in Chen’s study,37 catecholamine and dopamine were respectively introduced as the surface modifiers to enhance the interfacial adhesion of both carbon fiber and glass fiber reinforced composites. In another report, Chen et al. used PDA coating and the following octadecylamine grafting to modify the carbon fibers.39 The interfacial adhesion between carbon fibers and polymer matrix was highly improved by 35%. In our previous work40, the combination of PDA deposition and silane grafting on aramid fibers surface was introduced. The result showed that the interfacial adhesion force of the fiber/rubber composites was improved by 67.5%.


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However, dopamine is too expensive for extensive industry production. Fortunately, Lee et al. has confirmed that catechol and amine groups were crucial for achieving the superior adhesion as PDA.21 In addition, recent studies demonstrated that the contribution of primary amine group to the unique adhesion property of poly(catecholamine) coating is vital, which is attributed to the strong cation–π interaction between positively charged amine groups and the indolic crosslinks.41 Thus, molecules with catechol and amine groups may exhibit the similar properties, which could be used for the replacement of dopamine. Recent research showed that catechol and polyamine could polymerize and deposit on the surface of polypropylene separator, which exhibited similar adhesive property with PDA.42 Besides, the −NH2 or −OH group on the catechol/polyamine (CPA) modified surfaces could further react with secondary functional monomers to introduce more reactive sites for further interfacial design. However, to the best of our knowledge, the combination of CPA and silane coupling agent co-grafting to improve the interfacial adhesion of aramid fibers/rubber composites has not been reported yet. In this study, we revealed the possibility of low-cost CPA to replace dopamine to improve the interfacial adhesion of aramid fibers. After polymerization and deposition of CPA on the surfaces, the aramid fibers were further grafted by GPTMS, which can supply chemical interaction between fiber and rubber matrix. Compared with the method based on dopamine chemistry, this modification method is efficient (which takes 1 h less than PDA method), lowcost (less than 1% of the price of dopamine) and moderate without sacrificing the mechanical property of aramid fibers, which has promising practical application in rubber industry.

2. Experimental Section 2.1. Materials.


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Poly(p-phenylenediamine) (PPTA) (1000D) was purchased from Yantai Tayho Advanced Materials Co., Ltd., China. The fibers were cleaned by acetone for 3 h and dried in vacuum oven at 60 oC for 12 h. Dopamine hydrochloride and tris(hydroxymethyl aminomethane) (Tris) were bought from Alfa Aesra Company, USA. Catechol, three different kinds of polyamine including diethylenetriamine (DETA), triethylenetetramine (TETA), and tetraethylenepentamine (TEPA), anhydrous sodium carbonate, hydrochloric acid and γ-(glycidyloxypropyltrimethoxysilane) (GPTMS) were obtained from Beijing Chemical Co., China. All chemicals reagents and solvents were of analytical reagent grade and used without further purification. Natural rubber was provided by Malaysia Lee rubber industry, and styrene butadiene rubber was supplied by Sinopec Yangzi Petrochemical Co., Ltd, China. Carbon black trade marked as N330 was provided by Tianjin Cabot Chemical Products Co., Ltd. (China). The other major ingredients, such as zinc oxide, stearic acid, and sulfur, were commercially available. The cost comparison of dopamine and catechol/polyamine is shown in Table S2. Scheme 1. Schematic description of surface modification procedure of aramid fibers.

2.2. Surface Modification of Aramid Fibers by CPA Deposition and Silane Grafting.


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The detailed surface modification procedure is illustrated in Scheme 1. Catechol and polyamine (DETA, TETA, and TEPA) were dissolved in the tris-buffer solution with a predetermined molar ratio of 1:1 (5mM). Then, the molar ratios of catechol to TEPA were varied from 1:2 to 3:1. After that, the pH of the solutions (15 mM catechol and 5mM TEPA) was adjusted by tris-buffer solution to 8.5, 9.5, 10 and 10.5, respectively. Then, the aramid fibers were immersed into the catechol/polyamine solution under magnetic stirring at 20 oC for 3 h. The poly(catechol/polyamine) (PCPA) coated aramid fibers are denoted as PPTA-PCPA. After that, GPTMS with various concentration

(1.0, 1.5, 2.0, and 2.5 vol%) was added into the

aforementioned solution, and the reaction was conducted under stirring for 5 h at temperature of 60 °C. Then, the temperature of the solutions (GPTMS concentration was fixed at 2.0 vol%) was changed from 20 °C to 80 °C. After washed three times with alcohol and distilled water, GPTMS functionalized-PPTA fibers (PPTA-PCPA-GPTMS) were dried under reduced pressure for 24 h. 2.3. Preparation of Aramid Fibers/Rubber Composites. At first, the rubber compounds were prepared by mixing the ingredients according to the rubber formula (See Table S1) in an internal mixer. Then, the prepared rubber compounds were compressed into rubber sheets with a thickness of 5 mm on a two-roll mill. After that, the rubber sheets with a width of 10 mm were prepared and placed in the mold, and the stretched PPTA samples were embedded into rubber compound. Finally, the as-prepared fiber/rubber composites were vulcanized at 150 °C for 30 min under a pressure of 15 MPa. The completed structure and the cross-section of the composites are shown in Scheme 2. Scheme 2. The schematic of (a) the completed structure and (b) the cross-section of the aramid fibers/rubber composites.


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2.4. Characterization. The chemical composition of the PPTA fiber surfaces was determined by X-ray photoelectron spectroscopy (XPS) measurement, which was performed on an ESCALAB 250 (Thermo Electron Corporation, USA) with an Al Kα X-ray source (1486.6 eV photons). The X-ray source was run at a reduced power of 150 W, and the core-level signals were obtained at a photoelectron takeoff angle of 45° with respect to the sample surface. All binding energies (BEs) were referenced to the C 1s hydrocarbon peak at 284.6 eV to compensate for surface charging effects. Scanning electron microscopy (SEM, Hitachi S-4800; Japan) was applied to observe the surface morphology of the PPTA samples, and it was performed at an accelerating voltage of 20 kV. Fourier transform infrared spectroscopy (FTIR) was recorded by Tensor 27 spectrophotometer (Thermo Fisher Scientific, USA) in attenuated total reflectance mode. Thermogravimetric analysis (TGA) was performed by using a METTLER-TOLEDO thermo gravimetric analyzer (Switzerland) in the temperature range of 30 to 750 °C with a heating rate of 10 °C/min. All the experiments were carried out under nitrogen atmosphere. The tensile strength of aramid fibers was measured by using an electronic tensile tester of single filament (YM-06B, Shaoxing Yuanmao Electrical and Mechanical Co. Ltd., China) at speed of 10 mm/min. The interfacial adhesion force of aramid fibers/rubber composites was characterized by pull-out test with a crosshead speed of 100 mm/min, and it was taken by the average of the maximum pull-out force


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of the composites from at least 10 times tests. The interfacial shear strength (IFSS), τ, which was calculated using the following relationship:

τ=

F π dl

where F is the pull-out force, d is the diameter of the fibers, and l is the embedded length of the fiber.

3. Result and Discussion Scheme 3. (a) Possible reaction mechanism for the preparation of PPTA-PCPA-GPTMS fibers; (b) physical interactions in the PCPA deposition layer.

The possible reaction mechanism for the preparation of PPTA-PCPA-GPTMS fibers and the physical interactions in the PCPA deposition layer are schematically presented in Scheme 3a and


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3b, respectively. Similar with the oxidation and polymerization of dopamine, catechol is oxidized in alkaline aqueous solution, and the generated quinone structures can react with TEPA through Michale addition and Schiff base reaction to form a crosslinked network, which has been proved by Burzio et al.

43

In addition, physical interactions such as π–π stacking, ionic bonds,

hydrogen bonding and cation-π interactions also contribute to the strong attraction between the PCPA network, as shown in Scheme 3b.41,

44–46

Then, silane coupling agent GPTMS is

hydrolyzed in aqueous solution and the generated silanol can react with the PCPA oligomers through hydroxyl condensation. The PCPA-GPTMS oligomers are further crosslinked and deposit on surface of PCPA coated aramid fibers. Therefore, epoxy groups are introduced on surface of aramid fibers, which can take part in the vulcanization of rubber composites.


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Figure 1. XPS wide-scan and C 1s core-level spectra of (a, f) pristine aramid fibers, (b, g) PPTA-PDA fibers, and (c, h) the aramid fibers modified by catechol/DETA, (d, i) catechol/TETA, and (e, j) catechol/TEPA. 3.1. Surface Characterization of PPTA-PCPA Fibers. The changes in the surface chemical compositions of the aramid fibers were investigated by recording the wide scan and the C 1s XPS spectra, as shown in Figure 1. Determined by XPS, the carbon, oxygen, and nitrogen concentration on the surface of pristine aramid fibers is 78.04%, 14.42% and 7.54%, respectively. After the deposition of PDA, the O/C ratio of PPTAPDA fibers increases to 0.21 and the O 1s peak is strengthened (Figure 1b). From the wide-scan spectrum of aramid fibers modified by catechol/DETA, it can be noted that the intensity of O 1s peaks is also strengthened compared with that of the pristine aramid fibers, indicating that a deposition layer is also formed on fiber surface via the polymerization of catechol and DETA. In addition, the increase in area of C−O peak in Figure 1h is attributed to the catechol groups in PCPA layer. Similar results are obtained when aramid fibers are modified by catechol/TETA and catechol/TEPA, suggesting that catechol can react with polyamine and form an adhesive coating on fiber surface, which exhibit the similar adhesive ability with PDA.


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Figure 2. SEM images of (a) pristine aramid fiber, and aramid fibers modified by different combination of CPA: (b) catechol/DETA, (c) catechol/TETA and (d) catechol/TEPA. Figure 2 shows the SEM images of pristine aramid fiber and the aramid fibers modified by catechol with DETA, TETA, and TEPA, respectively. As shown in Figure 2a, pristine aramid fiber displays a smooth surface, while the aramid fiber is covered by discontinuous granule structures when using catechol/DETA (Figure 2b). In contrast, catechol/TETA and catechol/TEPA modified fibers surface are covered with a flat and continuous deposition layer, which are shown in Figure 2c and 2d, respectively. This phenomenon is attributed to the fact that primary amine is more active than secondary amine.42 By using DETA, the distribution density of benzene rings in the resultant of catechol/DETA oligomers is high, which may facilitate the aggregation and stacking of oligomers through π-π conjugation. However, with the increase of molecular chain length, the resultant deposition is preferred to form a continuous layer on fiber surface. Thus, TEPA was selected for the subsequent experiments.


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Figure 3. N/C ratio of the aramid fibers modified by catechol/TEPA (a) with different molar ratios, and (b) in different pH condition. XPS was used to further investigate the chemical composition of the surfaces of aramid fibers modified by catechol/TEPA with different molar ratios and in different pH conditions. Here, N/C ratio of the catechol/TEPA modified PPTA fiber is chosen to determine the PCPA grafting degree. Figure 3a shows that the N/C ratio of the aramid fibers increased with the increase of catechol/TEPA molar ratio, indicating the increased content of PCPA deposition on fiber surfaces. Under pH of 9.5 (Figure 3b), N/C ratio of the fibers reaches the maximum, revealing that pH of 9.5 is the optimal reaction condition for the co-deposition of catechol/TEPA.


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Figure 4. SEM images of aramid fibers modified by catechol/TEPA with different molar ratios: (a) 1:2, (b) 1:1, (c) 2:1 and (d) 3:1; SEM images of aramid fibers modified by catechol/TEPA in different solution pH: (e) 8.5, (f) 9.5, (g) 10 and (h) 10.5. Figure 4a-d shows that the roughness of the PPTA fibers surface increases with the increase of catechol/TEPA molar ratio and when the molar ratio of catechol to TEPA is up to 3:1, the fiber surface shows a complete and rough PCPA deposition layer (the raised deposition area is highlighted with red circles). Thus, we hypothesize that the more catechol, the more accumulations of PCPA can be deposited on surface of aramid fiber. In addition, when pH of the catechol/TEPA solution reaches 9.5, the fiber is thoroughly covered by a dense and rough deposition PCPA layer, and some prominent accumulations appear


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(highlighted with red arrows), as shown in Figure 4f. To interpret this, we aware that pKa for the first hydroxyl dissociation of catechol is approximately 9.5.47 When pH is lower than 9.5, the adsorption of catechol is primary, so catechol and polyamine are more inclined to polymerize and deposit on fiber surfaces. However, the dissociation of catechol becomes the dominant effect when pH is above 9.5, so a thin and incomplete deposition layer is formed on fiber surface,48, 49 as shown in Figure 4g and Figure 4h. The above conclusions are consistent with the XPS results. TGA is used to further investigate the grafting degree of PCPA, and the results are shown in Figure S1-3, which are also in accordance with the XPS analysis. Considering the following silane grafting, the molar ratio of catechol/TEPA is determined to be 3:1 and pH value 9.5 for the further experiments.

Figure 5. FTIR spectra of (a) the pristine aramid fibers, (b) PPTA-PCPA fibers, (c) pure GPTMS and (d) PPTA-PCPA-GPTMS fibers. 3.2. Chemical Structure of PPTA-PCPA-GPTMS Fibers.


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Direct evidence for the successful surface functionalization of aramid fibers was provided by FTIR spectra. Figure 5 illustrates the FTIR spectra of (a) pristine aramid fibers, (b) PPTA-PCPA fibers, (c) pure GPTMS, and (d) PPTA-PCPA-GPTMS fibers. The pristine aramid fibers shows some characteristic peaks including the N−H bending peak (1543 cm−1), C=O stretching peak (1642 cm−1), and the synergetic C−N and O−H stretching vibration peak (3325 cm−1). After coated by PCPA, the band around 3325 cm−1 becomes broader and stronger due to the O−H stretching vibration of catechol and N−H stretching vibration from PCPA layer. In the spectrum of PPTA-PCPA-GPTMS fibers, the absorption peaks at 2940 and 2845 cm−1 attributed to the C−H stretching vibration appeared. Furthermore, the absorption peaks of 1199 cm−1, 1100 cm−1, and 1022 cm−1 are assigned to stretching vibrations of Si−CH2−R, Si−O−Si, and Si−O−C, respectively. These newly appeared peaks suggest the successful grafting of GPTMS on the surface of aramid fibers. PPTA-PCPA-GPTMS fibers also exhibit the newly absorption peaks at 910 cm−1 and 845 cm−1, which correspond to the nonsymmetric stretch vibration of the epoxy group, providing further evidence for the grafting of GPTMS. The FTIR results confirmed the occurrence of the deposition of PCPA and grafting reaction on the surfaces of aramid fibers.


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Figure 6. XPS wide-scan and C 1s core-level spectra of (a, d) pristine aramid fibers, (b, e) PPTA-PCPA fibers and (c, f) PPTA-PCPA-GPTMS fibers treated with 2.0 wt % of GPTMS at 60 °C. To confirm the successful deposition of PCPA and grafting of GPTMS, XPS was used to analyze the chemical composition of the fibers surface. Figure 6 shows the wide scan and C 1s core-level spectra of pristine aramid fibers (Figure 6a and 6d), PPTA-PCPA fibers (Figure 6b and 6e), and PPTA-PCPA-GPTMS fibers (Figure 6c and 6f), respectively. Compared with that of pristine aramid fibers, the wide scan spectra of PPTA-PCPA shows enhanced O 1s and N 1s peaks because the coated PCPA layer has higher content of O and N element than those of PPTA. This statement is further confirmed by the appearance of the new peaks C−O (Figure 6e) besides C−Si, C−C, C−N, C=O, and O−C=O (Figure 6d). For PPTA-PCPA-GPTMS, the


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increased intensity of Si peak is attributed to the introduction of GPTMS. In addition, because GPTMS has high content of carbon oxygen bonds, the increase in the C−O peak area can further verify the successful grafting of GPTMS, as shown in Figure 6f.

Figure 7. Thermogravimetric curves of (a) pristine aramid fibers, (b) PCPA, (c) PPTA-PCPA fibers, and (d) PPTA-PCPA-GPTMS fibers. Table 1. Thermodegradation data of pristine aramid fibers, PCPA, and modified aramid fibers.

Temperature range

30-325 oC 325-520 oC

520-600 oC

600-800 oC

>800 oC

PPTA

4.1%

2.6%

50.2%

6.1%

37.0%

PCPA

22.2%

23.2%

5.3%

7.9%

50.7%

PPTA-PCPA

1.8%

1.8%

41.9%

6.4%

48.1%

PPTA-PCPA-GPTMS

2.2%

37.6%

14.7%

2.1%

45.7%

The thermal decomposition curves of aramid fibers, PCPA, PPTA-PCPA fibers, and PPTAPCPA-GPTMS fibers are shown in Figure 7, and the detailed thermo degradation data of the curves are listed in Table 1. The curve of pristine aramid fibers shows a weight loss of 50.2%


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between 520 and 600 °C, which is ascribed to the partial dehydroxylation and alkoxide decomposition of aramid fibers. This stage can be used to evaluate the grafting degree of the fibers. Figure 7b shows that PCPA has a residual weight of 50.7% at 800 °C, which is similar with PDA.50 A comparison of curve of PPTA-PCPA to PPTA shows decreased weight loss at the temperature range of 520 and 600 °C to 41.9%, indicating that the weight of PCPA deposition layer is 8.8%. Moreover, PPTA-PCPA-GPTMS fibers begin to decompose at around 325 °C due to the grafting of GPTMS, and the weight loss at the temperature range of 520 °C and 600 °C further decreases to 14.7%, suggesting that about 27.2 % GPTMS is grafted on the PPTA-PCPA fibers.

Figure 8. O/C atomic ratio of PPTA-PCPA-GPTMS fibers as a function of (a) temperature and (b) GPTMS concentration. Due to the different carbon and oxygen elements composition of GPTMS and aramid fibers, the changes of the O/C atomic ratio of the PPTA-PCPA-GPTMS fibers were used to investigate the grafting efficiency of GPTMS. Figure 8a shows the O/C atomic ratio of PPTA-PCPAGPTMS fibers as a function of temperature. The O/C ratio increases with the increase of temperature, but decreases when the temperature is above 60 °C. This is mainly due to the fact


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that proper temperature is needed for the adequate hydrolysis of GPTMS and its rapid reaction with catechol. However, high temperature may cause self poly-condensation of GPTMS rather than co-grafting. The changes of O/C atomic ratio as a function of the concentration of GPTMS are presented in Figure 8b. The O/C atomic ratio increases rapidly with the increase of GPTMS concentration until the concentration is up to 2.0 vol%. It may be ascribed to the fact that excessive GPTMS monomers are unable to join the reaction due to the limited reaction sites of catechol and the hydroxyl groups on fiber surfaces. These results indicate that the optimal temperature and GPTMS concentration for treating PPTA are 60 °C and 2.0 vol%, respectively.

Figure 9. SEM images of PPTA-PCPA-GPTMS fibers with different grafting temperatures: (a) 20 °C, (b) 40 °C, (c) 60 °C, and (d) 80 °C; and PPTA-PCPA-GPTMS fibers treated with GPTMS at different concentrations: (e) 1.0 vol %, (f) 1.5 vol %, (g) 2.0 vol %, and (h) 2.5 vol %.


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The surface morphology of aramid fibers before and after treatment was observed by SEM. As shown in Figure 4f, after the deposition of the PCPA, the aramid fibers are covered with a distinct rough coating. However, PPTA-PCPA-GPTMS fibers display a relatively smooth surface, as shown in Figure 9a. With increase in the reaction temperature, the fibers exhibit denser and thicker grafting layer of GPTMS (Figure 9b-c). However, 80 oC may lead to the selfcondensation of GPTMS monomers, resulting in a thin grafting layer (Figure 9d). Furthermore, the diameters of the aramid fibers increase with the increase of GPTMS concentration (Figure 9e-h). Nevertheless, bulky grains appeared on the fiber surfaces when the concentration is above 2 vol%, which was detrimental for the subsequent vulcanization of fibers/rubber composites. This phenomenon is well consistent with the XPS analysis. Table 2. Adhesion strength of aramid fibers/rubber composites

sample PPTA/rubber PPTA-PCPA/rubber PPTA-PCPA-GMPTS/rubber

pull-out force (N)

interfacial shear strength (IFSS) (MPa)

33±3

2.1±0.2

46.7±4.5

2.9±0.3

60.5±5

3.9±0.3

3.3. Interfacial Adhesion of aramid Fibers/Rubber Composites. Pull-out test was carried out to investigate the interfacial adhesion property of aramid fibers/rubber composites, and the results are presented in Table 2. Here, the PPTA-PCPAGPTMS fibers were prepared under the optimal reaction condition: a GPTMS concentration of 2.0 vol% and temperature of 60 °C. By comparison with that of the aramid fibers/rubber composites, the interfacial shear strength (IFSS) of PPTA-PCPA fibers/rubber composites is


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raised by 41.5% from 2.1 MPa to 2.9 MPa. This increase can be attributed to the hydrogen bond between PCPA layer and the carbon black and/or silica in rubber composites. Here, carbon black and silica can be considered as physical crosslink points in the rubber matrix.51 Therefore, the PCPA modified aramid fibers exhibits an improved interfacial adhesion force with rubber composites. As to PPTA-PCPA-GPTMS fibers, a more prominent improvement, namely 83.3% increase (3.9 MPa), is achieved. Our previous work reported that aramid fibers modified by PDA and GPTMS exhibited a 67.5% increase in pull-out force,40 compared with pristine aramid fibers. Therefore, PCPA performs a comparative modification effect with PDA. The successful introduction of epoxy groups is the major reason for the adhesion improvement between fibers and rubber matrix. During the vulcanization process of aramid fibers/rubber composites, the grafted epoxy groups on the PPTA fiber surfaces can form chemical bonds with the rubber matrix through intermediary agent of sulfur. The possible reaction mechanism is shown in Scheme 4. As shown in Table S1, sulfur was chosen as the crosslink agent for the aramid fibers/rubber composites. Under high temperature, sulfur is turned into sulfur radicals in homolysis reaction, then the sulfur radicals would react with the double bonds of rubber and produce mercapto groups, which then react with the epoxy groups to form chemical bonds between fiber and rubber matrix.52 Meanwhile, it is noticeable that tensile strength of the aramid fibers did not show obviously decrease, which means that this modification method is moderate without sacrificing the mechanical property of the fibers (in Figure S4).


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Scheme 4. Possible mechanism for epoxy groups reacting with rubber matrix.


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Figure 10. SEM images of fracture surfaces of aramid fibers/rubber composites after the pullout test: (a) aramid fibers/rubber, (b) PPTA-PCPA fibers/rubber and (c) PPTA-PCPA-GPTMS fibers/rubber. SEM images of the fracture surfaces of the aramid fibers/rubber composites are shown in Figure 10. It can be seen from Figure 10a that there is almost no rubber residual left on the surface of the pristine aramid fibers, indicating a poor interfacial adhesion between fibers and rubber. For PPTA-PCPA fibers, the increasing content of rubber residual is attributed to the physical bonds between PCPA layer and rubber composites. However, more content of rubber residual is adhered to the PPTA-PCPA-GPTMS fibers surface, as shown in Figure 10c. The increased content of rubber residual can be attributed to the fact that the chemical bonds formed between epoxy groups of GPTMS and rubber matrix enhance the interfacial adhesion. XPS spectra of the pull-out surface of modified fibers and the rubber interface can further confirm the good interfacial interaction between fibers and rubber matrix (See Figure S5).

4. CONCLUSION Surface modification of aramid fibers was successfully developed by the catechol/polyamines (CPA) co-deposition and followed silane GPTMS grafting. By selecting the polyamine, the molar ratio of catechol to TEPA, and pH value of the solution, the optimal reaction conditions were determined. Results show that CPA exhibits the similar adhesive ability with PDA. The grafting efficiency of GPTMS was controlled by adjusting the reaction temperature and monomer concentration. Through introducing the epoxy groups on fibers surface which could take part in the vulcanization of rubber, the interfacial adhesion of aramid fibers/rubber composites was significantly improved. Compared with the method by dopamine, this method


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shows less pre-deposition time on aramid fibers surface of PCPA (3 h) than that of PDA (4 h), and lead to a higher improvement in adhesion force (83.3%) than that by dopamine (67.5%). In addition, the cost of CPA is less than 1% of that of dopamine. With advantages of controllable, efficient and low cost, this modification method shows great potential application in rubber industry high performance fiber reinforced composites.

ACKNOWLEDGMENT The authors gratefully acknowledge the financial support from the Natural Science Foundation of China (Grant No. 51373010, 51525301 and 51521062).

Supporting Information Rubber formula for adhesion test. The cost comparison of dopamine and catechol/polyamine. TGA curves of PCPA modified aramid fibers. The wide scan and C 1s core-level spectra of pulled-out surface of modified fibers and the rubber interface

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Polyamine

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