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Enhancing the interfacial strength of carbon fiber reinforced epoxy composites by green grafting of poly(oxypropylene) diamines
Accepted Manuscript Enhancing the interfacial strength of carbon fiber reinforced epoxy composites by green grafting of Poly(oxypropylene) Diamines Caifeng Wang, Lei Chen, Jun Li, Shaofan Sun, Lichun Ma, Guangshun Wu, Feng Zhao, Bo Jiang, Yudong Huang PII: DOI: Reference:
S1359-835X(17)30152-5 http://dx.doi.org/10.1016/j.compositesa.2017.04.003 JCOMA 4630
To appear in:
Composites: Part A
Received Date: Revised Date: Accepted Date:
4 November 2016 1 April 2017 5 April 2017
Please cite this article as: Wang, C., Chen, L., Li, J., Sun, S., Ma, L., Wu, G., Zhao, F., Jiang, B., Huang, Y., Enhancing the interfacial strength of carbon fiber reinforced epoxy composites by green grafting of Poly(oxypropylene) Diamines, Composites: Part A (2017), doi: http://dx.doi.org/10.1016/j.compositesa. 2017.04.003
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Enhancing the interfacial strength of carbon fiber reinforced epoxy composites by green grafting of Poly(oxypropylene) Diamines Caifeng Wang a, Lei Chen a, Jun Li, a Shaofan Sun a, Lichun Ma a b, Guangshun Wu a c, Feng Zhao a, Bo Jiang a and Yudong Huang* a a
MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion
and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150040, China. b
Institute of Material Science and Engineering, Qingdao University, Qingdao 266071,
China. c
School of Chemistry and Materials Science, Ludong University, Yantai 264025,
China. Abstract: We report on a green method of using poly(oxypropylene) diamines (D400) as coupling and curing agent to functionalize carbon fiber in water. We propose to enhance the interfacial properties of carbon fiber composites, together with the tensile strength of carbon fibers. The microstructure and mechanical properties of carbon fibers before and after modification are investigated. The results show that D400 do not change the surface morphology, but significantly increase the polarity, wettability and roughness of the carbon fiber surface. The interfacial shear strength (IFSS) of modified carbon fiber/epoxy composite and the tensile strength of carbon fibers
increase by 71.7% and 12.8%, respectively. It is believed that D400 can effectively improve the interfacial adhesion of the composites by improving resin wettability, increasing chemical bonding and mechanical interlocking. This green and simple method can have applications in continuous production of high-performance carbon fiber composites. Keywords: Carbon fiber, Grafting, Interfacial strength, poly(oxypropylene) diamines 1. Introduction Excellent tensile strength, high stiffness, light weight and great thermal resistance make carbon fiber reinforced composite an ideal structural material [1-3]. As we all know, interphase between fibers and matrix is critical in determining the properties of carbon fiber composites [4,5]. That is to say the level of interfacial adhesion determines to a great extent the transfer of stresses from the matrix to the reinforcing fiber. And an optimum interface is the necessary prerequisite to the good mechanical properties of resulting composite [6~8]. However, the interfacial adhesion of carbon fiber composites tends to be weak because carbon fibers naturally show a weak wettability and adsorption with most polymers matrix. Recently, the introduction of various nanomaterials such as carbon nanotubes [9,10], nanofibers [11] and graphene [12]/graphene oxide [13] over carbon fiber composites to improve the interfacial properties has become a subject craze. This is because the nanocomposites will have a much greater interfacial area than microcomposites at the same particle loading [14,15]. On the other hand, nanomaterials act as the reinforcing phase in polymer
composites because of its stress transfer behavior between the polymer matrix and carbon fiber [16]. In the viewpoint of carbon fiber surface configuration, grafting of micro-molecules or polymers onto the carbon fiber surface can yield a controlled, ordered and active structure, which has been proved to be an effective modification method [17~20]. D400, as a traditional cross-linker of epoxy [21], contains two active primary amine groups located on secondary carbon atoms at each end of an aliphatic polyether chain (Fig 1). The addition of coupling agent along with the materials creates the covalently bonded oxygen-carbonated functional groups at the carbon fiber surface, which is further responsible for chemical interactions with the matrix polymer. In addition, the increased crosslink density of the matrix in the interphase region could react with the organic groups of D400. Thus flexible polyether chain contributes to stress transfer between carbon fiber and epoxy [22]. In the past decades, in order to improve the interfacial properties of carbon fiber composites, researchers used different amine polymers to modify carbon fiber [17,23]. The addition of polymers phase as coupling agent which formed polymer interphase could react with carboxylic groups on acid-treated CF surface to improve interactions between carbon fiber and amine polymers. On the other hand, the amino groups are used as curing and toughening agent for matrix to enhance compatibility and interfacial strength between amine polymers and matrix [24]. However, the modification process always need long reaction time, harmful organic solvents and tedious steps [23,25].
Here, we proposed to introduce D400 into the interface between carbon fiber and epoxy matrix via a green and simple method. Different from the previous studies, water, instead of organic solvents, was used as the reaction solvent to avoid environmental pollution. To shorten reaction time and steps, a condensing agent N-[(dimethylamino)-1H-1,2,3-triazolo[4,5,6]-pyridin-1-ylmethylene]-N-methylmetha naminium hexafluoro-phosphate N-oxide (HATU) was adopted. The influence of the concentration of D400 was studied in this paper. At an optimal concentration, the IFSS of carbon fiber/epoxy composites was significantly enhanced from 48.8 MPa to 83.8 MPa, while the tensile strength of carbon fiber showed a slight increase, from 4.73 GPa to 5.12 GPa. 2. Materials and methods 2.1. Materials Carbon fibers (T700SC-12000-50C, 12 K, tensile strength 4.9 GPa, diameter 7 μm, density 1.8 g·cm-3) [26] were purchased from Toray Industries Inc. D400 (molecular mass Mn 400) was purchased from Huntsman Corporation Australia Pty Ltd. Concentrated nitric acid (68%) was supplied by Sinopharm Chemical Reagent Co. Ltd. The HATU used was supplied by GL Biochem Ltd. Deionized water (DI water, resistivity >18 MΩ·cm) was used throughout the experiments. 2.2. Deposition of D400 onto carbon fiber via physical adsorption and chemical grafting
As received carbon fibers were cleaned with refluxed acetone for 48 h to remove the polymer sizing and pollutants before deposition, denoted as untreated CF. The untreated CF were oxidized in HNO3 solution at 80 oC for 4 h. Then the carbon fibers were taken out and washed several times with DI water until the pH of the washed water was neutral, and then dried under vacuum, denoted as ACF. Different concentrations of D400 water solutions (4×10-5, 2×10-4, 10-3 and 5×10-3mol/L) were prepared to study the effect of deposition amount of D400 on IFSS. Subsequently, 20 mg HATU was ultrasonically dissolved in D400 solution. Then, ACF (0.5 g) was immersed into the solution at 90 oC for 4 h, in order to complete the D400 chemical grafting, on condition that the HATU was used as condensing agent [17], as illustrated in Fig. 1. After that, the D400 grafted carbon fiber was washed by excess DI water and dried in a vacuum furnace, denoted as ACF/D400. Fig. 1 insert here For comparison, untreated CF was immersed in D400 water solution without HATU, following the same procedure, denoted as CF/D400. In this approach, D400 physically absorbed onto the carbon fiber surface, based on the Van der Waals interaction. 2.4. Characterization techniques The morphologies of the modified carbon fiber were observed by the scanning electron microscope (SEM) (200FEG, Quanta FEI Inc. the USA) at an accelerating voltage of 20 kV. Gold sputtering was used to improve the conductivity. The surface roughness (Ra) of carbon fiber was examined by atomic force microscopy (AFM,
Solver-P47H, NT-MDT, Russia) using tapping mode. X-ray photoelectron spectroscopy (XPS) (ESCALAB 220i-XL, VG, UK) was carried out to study the chemical state of carbon fibers using a monochromatic Al Kα source (1486.6 eV) at a base pressure of 2×10-9 mbar. The XPS Peak version 4.1 program was used for data analysis. Dynamic contact angle was measured using a dynamic contact angle meter. Tensiometer (DCAT21, Data-Physics Instruments, Germany). DI water (γd =21.8 mN m-1, γ = 72.8 mN m-1) and diiodomethane (γd =50.8 mN m-1, γ= 50.8 mN m-1, 99% purity, Alfa Aesar, USA) were used as test liquids [27]. The dispersive and polar components can be determined by solving the following equation: l cos lp fp )1 2 dl df )1 2
(1)
Where γl, γld and γlp are the surface tension of the test liquid, its dispersive and polar components, respectively. Monofilament tensile tests were conducted on a universal testing machine (Instron 5566, USA) according to the ASTM D3379-75 [28] with a load cell of 50 N. Firstly, filaments were glued on a paper frame and the paper frame was held by the clamps of testing machine. The paper frame was cut before the mechanical testing. And then, mechanical testing started and the load-extension curves were collected. The testing speed was 10 mm·min-1 and the gauge length is 20 mm. Lastly, the data from load-extension curves were normalized by the linear density of carbon fiber bundle to give the tensile strength. A total of 100 data points were collected and the results were analyzed by a Weibull statistical method [29].
IFSS was adopted to quantify the interfacial property between carbon fiber and epoxy by the interfacial evaluation equipment (FA620, Japan). Epoxy resin (WSR618) and methyl tetrahyelrophthalic anhydride hardener were mixed at the weight ratio of 100:32 to prepare microdroplets. The microdroplets were cured following a curing process: 90 °C for 2 h, 120 °C for 2 h and 150 °C for 4 h. The IFSS was calculated according to equation (2).
IFSS
F dl
(2)
Where F is the maximum load recorded, d is the carbon fiber diameter, and l is the embedded length. The final IFSS of each sample was averaged from the data of 50 successful measurements. 3. Results and discussion 3.1. Surface Morphologies of Carbon Fibers SEM and AFM images in Fig. 2 depict the surface morphologies and roughness of the carbon fibers, respectively. It can be seen that the surface of untreated CF is smooth. After acid treatment, a few narrow shallow parallel grooves appear along the longitudinal direction of the fiber. Fig. 2 c shows that after the physical adsorption of D400, the smooth surface of carbon fiber changes to be coarse due to the absorption of D400 onto carbon fiber surface by Vander Waals interactions. Fig. 2 d-g display the surface morphologies of D400 grafted carbon fiber at different concentration. After chemical grafting of D400, the fiber surface becomes rougher. When the D400 concentration is low, like 4×10-5 mol/L, D400 molecules could not cover the whole
carbon fiber surface (Fig. 2 d). When the D400 concentration is high enough, D400 molecules would fully cover carbon fiber (Fig. 2 e), restack together (Fig. 2 f) or agglomerates clumps (Fig. 2 g). Obviously, the coverage of D400 on carbon fiber depends on D400 concentration that was used. Fig. 2 insert here Fig. 2 also reveals the surface roughness of different samples. It can be clearly seen that the surface roughness increases linearly with the concentration of D400. The surface roughness increase of CF/D400 is attributed to physical absorption of D400 by Van der Waals' force, which is strong enough to prevent D400 from undesired falling off upon washing. Compared with physical absorption, the deposition amount of D400 on carbon fiber in chemical grafting is larger, resulting higher surface roughness. When the D400 concentration is 2×10-4 mol/L, D400 deposits on carbon fibers uniformly (Fig. 2 e) and fully covers the carbon fiber surface (Ra=95.9 nm). When the D400 concentration is 10-3 mol/L, the surplus D400 results in a dramatic increase in the surface roughness (Ra=95.9 nm). This surplus D400 is more noticeable and the surface roughness is the maximum with 255 nm at D400 concentration of 5×10-3 mol/L. The untreated CF fiber profile is subtracted as a background from the Z displacements presented in Fig. S1 (the picture is given in the Supporting Information). We can see that the height difference and amplitude of variation between the modified carbon fibers curves and untreated CF curve increase from top to bottom. It also suggests that surface roughness of ACF/D400 increases with increase of D400 concentration. We
believe the increased roughness is beneficial to improve the mechanical interlocking between carbon fiber and epoxy. 3.2. Surface characteristics of Carbon Fiber Fig. 3 insert here XPS is performed to determine the surface element composition of carbon fiber, and a quantitative analysis is carried out. The wide-scan and high resolution spectra of samples are shown in Fig. 3. The untreated CF surface consists mainly of carbon and oxygen (Fig. 3 a). After acid treatment, a significant increase of oxygen content can be found (Fig. 3 b). Fig. 3 c is the XPS spectrum of CF/D400. The existence of N 1s confirms the absorption of D400 on carbon fiber. The high resolution spectrum of C 1s (Fig. 3 d) shows the relative content of C-O (286.2) and C-N (285.6 eV), owing to ether and amino groups in D400. Moreover, from the high resolution spectrum of N 1s, only one binding energy peak is found at 399.1 eV (Fig. 3 e), which is typical for C-N bonds in amino groups, indicating that D400 was physical absorbed on the carbon fiber via Van der Waals interactions. Compared with CF/D400 (Fig. 3 c), the wide-scan spectrum of ACF/D400 (Fig. 3 f) shows a significant increase of nitrogen content from 11.9% to 24.5%. A new binding energy peak at 287.9 eV is also found from the high resolution spectrum of C 1s (Fig. 3 g), which is attributed to new generated bond (-N-C=O), corresponding to chemical bonds formed between carboxyl groups on the surface of ACF and the amino groups of D400. The reaction is also confirmed by tracing N1s spectrum, as shown in Fig. 3 h,
where a new peak (400.1 eV) assigned as amide (-N-C=O) [10] appears. These results indicate amino groups of D400 react with carboxyl groups of ACF. 3.3. Wettability of Carbon Fiber Fig. 4 insert here The functionalization of D400 changes the surface free energy of carbon fiber. The wettability of carbon fiber is investigated using advancing dynamic contact angle test. The advancing contact angle (θ), the surface energy (γ), dispersion component (γ d) and polar component (γp) of untreated CF, ACF, CF/D400 and ACF/D400 are shown in Fig. 4. After D400 grafting treatment, the contact angles of both water and diiodomethane decrease, while the surface energy of carbon fiber increases. The increased polar component of surface energy can be interpreted from the increase on polar
amine
groups
in
D400.
Therefore,
the
wettability
between
the
amino-functionalized carbon fiber and the polar polymer matrix should be significantly improved [4]. In addition, the increased dispersion component, caused by the increased roughness, also contributes to the increase of surface energy [27]. Meanwhile, the surface free energy of carbon fiber becomes higher as the D400 concentration increases. In summary, the change of the fiber surface free energy confirms the successful deposition of D400 on carbon fiber surface. 3.4. Interfacial Property Testing of Carbon Fiber/Epoxy Composite Fig. 5 a and b demonstrate the images of micro-droplet before and after interfacial debonding. Fig. 5 c shows IFSS results of untreated CF, ACF, CF/D400 and ACF/D400.
After acid-treatment, the IFSS of the composite has a negligible increase in comparison with untreated CF (48.8 MPa). After adsorption of D400, the IFSS of CF/D400 composite increases to 55.8 MPa which is attributed to improvement of surface
energy.
After
D400
grafting
treatment,
IFSS
of
composites
is
significantly higher than the untreated one. D400, which can not only react with the reactive groups on the carbon fiber surface, but also increase resin compatibility and react with the matrix system. While the roughness enhances mechanical interlocking between the fibers and epoxy by the introduction of D400 into the interfacial region. The changing trend of IFSS of composites shows ‘‘roof’’ shape as D400 concentration goes up, peaking (83.8 MPa) at 2×10-4 mol/L concentration of D400. After that, IFSS of decrease to 77.5 MPa and 71.4 MPa at 10-3 and 5×10-3 mol/L concentration of D400, respectively. In our understanding, these D400 molecules have exactly and completely covered the whole surface of carbon fibers at 2×10-4 mol/L concentration of D400. Gao et al. [30] reported that grafting polymer was highly effective in stress transfer in epoxy matrix. Fig. 5 insert here Fig. 6 depicts the surface morphology of carbon fibers after debonding from the epoxy matrix. For the untreated CF and ACF composites (Fig. 6 a and b), the surfaces are similarly neat, which indicates that the debonding between carbon fiber and epoxy is interfacial failure because of the weak van der Waals force between them. After adsorbed D400, a little of epoxy resin remains on the carbon fiber surface after debonding (Fig. 6 c), implying interface between the CF/D400 and epoxy become
stronger. On the other hand, for ACF/D400 composite, more epoxy debris adhere on the
carbon
fiber
after
debonding
due
to
the
strong
covalent
bonds formed between amino and carboxyl groups, which implies that the interface between the ACF/D400 and epoxy is stronger than that of CF/D400 composites. Fig. 6 d, e and f show debonding SEM images of carbon fiber in different concentrations composites. As D400 concentration goes up, there is more remaining epoxy resin. When the concentration of D400 is 2×10-4 mol/L, more resin fragments of resin are displayed than the others, which indicate that the interfacial strength between the carbon fiber and epoxy is the strongest. As D400 concentration continues to go up, for 10-3 mol/L, the large image clearly displays the obvious crevices (the green arrow pointed in Fig. 6 f) between bare carbon fiber and epoxy resin, which indicates that the interfacial strength between the carbon fiber and epoxy is weak [17]. Fig. 6 insert here The IFSS change is related to interfacial failure mode of the composite. Normally, the interfacial failure mode of composite includes adhesive and cohesive failure. We propose the failure models depending on the interfacial strength, as shown in Fig. 6 g~k. In the case of untreated CF and ACF, the failure mode is adhesive failure (Fig. 6 g), due to that the untreated CF and ACF have poor wettability and there is no chemical interaction between carbon fiber and epoxy (only Van der Waal interaction). This is consistent with the results from Fig. 6 a and b. For CF/D400 composite (Fig. 6 h), the failure mode is still adhesive failure, though there is a little epoxy resin remaining on the carbon fiber. This is because the interfacial failure may take place on
the surface of carbon fiber owing to the weak interactions (Van der Waals force) between CF and D400, although the epoxy groups could react with D400 via a nucleophilic ring opening mechanism [4]. When D400 grafted on carbon fiber surface, the greater amounts of D400 improve the wettability of carbon fiber (as refereed in Fig. 4 b). At the same time, covalent bond between CFs and D400 is stronger than that of Van der Waals force. This additional interphase area worked as an additional reinforcement can relieve the stress concentration effectively, transfer the loads from matrix to carbon fibers uniformly and change the failure mode from adhesive failure involving fiber-matrix interface debonding to cohesive failure within the interphase [31]. When D400 concentration is low (10-5 mol/L), D400 molecules partially cover carbon fiber surface (Fig. 6 i). As D400 concentration goes up, like 2×10-4 mol/L, D400 fully covers carbon fiber surface (Fig. 2 e). The wettability of carbon fiber becomes better and the number of the chemical bonds increases, resulting stronger interfacial strength (Fig. 6 j), which is confirmed by the fact that a layer of epoxy remains on carbon fiber surface (Fig. 6 e). Moreover, the increased fiber/ matrix compatibility is a decisive factor for final quantification of fiber/matrix interfacial adhesion in full coverage of D400. This brings an increase in the volume fraction and crosslink density of interphase in the composites and a decrease in the effective polymer chain mobility in the interphase region [8,31,32]. However, when D400 concentration is further increased (10-3 mol/L and 5×10-3 mol/L), IFSS start falling due to the D400 over covered on carbon fiber (Fig. 6f). In this way the composite failure may take place within the D400 layer, because of the weak Van der Waals interaction among D400
molecules [17]. Therefore, the cohesive failure happens both in epoxy and D400 layer, thus the remaining epoxy becomes less and the IFSS slightly decreases, as observed in Fig. 6 k. Based on the above analysis, we can conclude that when the concentration of D400 is 2×10-4 mol/L, the strong interface would lead to cohesive failure in matrix, where the IFSS is the highest. 3.5. Tensile Strength Table. 1 insert here The tensile tests are carried out and the results are summarized in Table. 1. It can be seen that the tensile strength of all D400 treated carbon fibers are a little higher than that of the untreated CF (4.73 GPa). We also note that the tensile strength of D400 grafted carbon fiber increased slightly with the increase of concentration, which could be attributed to the increasing surface coverage of D400. The results imply that D400 grafting can not only make up the tensile strength losses due to the etching effect during the acid treatment, but also mildly enhance tensile strength of carbon fiber. 4. Conclusion In this study, a green grafting method is proposed to chemically functionalize carbon fibers by D400 in water. It is found that the IFSS of carbon fiber composites was improved significantly. When the concentration of D400 solution is 2×10-4 mol/L, the IFSS of ACF/D400 composites is improved by 69.9%. We believe the presence of D400 can increase the polarity, wettability and surface roughness of the carbon fiber, which are responsible for the enhancement of IFSS of the ACF/D400 composites. After
the surface functionalization by D400, the tensile strength of carbon fibers have not been sacrificed, but mildly enhanced. This green and simple grafting method can be applied in versatile and scalable fabrication of high-performance carbon fibers, as well as excellent carbon fiber reinforced composites. 5. Acknowledgements The authors gratefully acknowledge financial supports from the Chang Jiang Scholars Program and National Natural Science Foundation of China (51073047) 6. Refrences [1] P.C. Varelidis, R.L. McCullough, C.D. Papaspyrides. The effect on the mechanical properties of carbon/epoxy composites of polyamide coatings on the fibers. Compos Sci Technol 1999;59:1813-23. [2] M.A. Montes-Moran, R.J. Young. Raman spectroscopy study of HM carbon fibres: effect of plasma treatment on the interfacial properties of single fibre/epoxy composites. Part I: fibre characterization. Carbon 2002;40:845-855. [3] F. Li, Y. Liu, C.B. Qu, H.M. Xiao, S.Y. Fu. Enhanced mechanical properties of short carbon fiber reinforced polyethersulfone composites by graphene oxide coating. Polymer 2015;59:155-165. [4] L.C. Ma, Y.D. Huang. Improving the interfacial properties of carbon fiber-reinforced epoxy composites by grafting of branched polyethyleneimine on carbon fiber surface in supercritical methanol. Compos Sci Technol 2015;114:64-71.
[5] H.L. Tekinalp, V. Kunc, G.M. Velez-garcia, C.E. Duty, L.J. Love, A.K. Naskar, C.A. Blue, and S. Ozcan. Highly oriented carbon fiber-polymer composites via additive manufacturing. Compos Sci Technol 2014;105:144-150. [6] X.Q. Zhang, X.Y. Fan. Interfacial Microstructure and Properties of Carbon Fiber Composites Modified with Graphene Oxide. ACS Appl Mater Inter 2012;4: 1543-1552. [7] M. Ashuri, F. Moztarzadeh, and N. Nezafati. Development of a composite based on hydroxyapatite and magnesium and zinc‐ containing sol-gel-derived bioactive glass for bone substitute applications. Mater Sci Eng C 2012;32:2330-2339. [8] M. Sharma, S. Gao, E. Mäder, H. Sharma, L. Yew, and J. Bijwe. Carbon fiber surfaces and composite interphases. Compos Sci Technol 2014;102:35-50. [9] F. Zhao, Y. Huang, L. Liu, Y. Bai, and L. Xu. Formation of a carbon fiber/polyhedral oligomeric silsesquioxane/carbon nanotube hybrid reinforcement and its effect on the interfacial properties of carbon fiber/epoxy composites. Carbon 2011;49:2624-2632. [10] A.Y. Boroujeni, M. Tehrani, A.J. Nelson, and M. Al-haik. Hybrid carbon nanotube – carbon fiber composites with improved in-plane mechanical properties. Composites : Part B 2014;66:475-483. [11] J.Q. Zhao, Q.Y. Li, X.F. Zhang, M.J. Xiao, W. Zhang, and C. Lu. Grafting of polyethylenimine onto cellulose nanofibers for interfacial enhancement in their epoxy nanocomposites. Carbohydr Polym 2017;157:1419-1425.
[12] T. Ramanathan1, A.A. Abdala, S. Stankovich, D. A. Dikin1, M. Herrera-Alonso R.K. Prud'Homme, L.C. Brinson. Functionalized graphene sheets for polymer nanocomposites. Nat Nanotechnol 2008;3:327-331. [13] Y. Mo, M. Yang, Z. Lu, and F. Huang. Preparation and tribological performance of chemically-modified reduced graphene oxide/polyacrylonitrile composites. Composites: Part A 201;54:153-158. [14] H. Abdizadeh, M. Ashuri, P. Tavakoli, A. Nouribahadory, and H. Reza. Improvement in physical and mechanical properties of aluminum/zircon composites fabricated by powder metallurgy method. Mater Des 2011;32:4417-4423. [15] M. Ashuri and L.L. Shaw. Silicon as a potential anode material for Li-ion batteries: where size, geometry and structure matter. Nanoscale 2016;8:74-103. [16] G.S. Wu, L.C. Ma, L. Liu, and Y.D. Huang. Interfacial properties and impact toughness of methylphenylsilicone resin composites by chemically grafting POSS and tetraethylenepentamine onto carbon fibers. Composites: Part A 2016;84:1-8. [17] Q. Peng, Y. Li, X. He, H. Lv, P. Hu, Y. Shang, C. Wang, R. Wang, T. Sritharan, and S. Du. Interfacial enhancement of carbon fiber composites by poly(amido amine) functionalization. Compos Sci Technol 2013;74:37-42. [18] S. Chen, Y. Cao, and J. Feng. Polydopamine as an efficient and robust platform to functionalize carbon fiber for high-performance polymer composites. ACS Appl Mater Interfaces 2014;6:349-356.
[19] S. Ahmad, A. Najafabadi, M. Ashuri, and M. Tahriri. Surface Modification of Castor Oil-Based Polyurethane by Polyacrylic Acid Graft using a Two-Step Plasma Treatment for Biomedical Applications. Adv Polym Technol 2014;33:1-8. [20] S.A.A. Najafabadi, H. Keshvari, Y. Ganji, M. Tahriri and M. Ashur. Chitosan/heparin surface modified polyacrylic acid grafted polyurethane film by two step plasma treatment. Surface Engineering 2012;28:710-714. [21] W. Wan, L. Li, Z. Zhao, H. Hu, X. Hao, D. A. Winkler, L. Xi, T. C. Hughes, and J. Qiu. Ultrafast fabrication of covalently cross-linked multifunctional graphene oxide monoliths. Adv Funct Mater 2014;24:4915-4921. [22]
T.Y.
Lee,
J.
Carioscia,
Thiol-allylether-methacrylate
ternary
Z.
Smith,
systems.
and
Evolution
C.N.
Bowman.
mechanism
of
polymerization-induced shrinkage stress and mechanical properties. Macromolecules 2007;40:1473-1479. [23] L.C. Ma, L.H. Meng, and Y.D. Huang. Interfacial enhancement of carbon fiber composites by generation 1–3 dendritic hexamethylenetetramine functionalization. Appl Surf Sci 2014;296:61-68. [24] A.P. Mouritzas, K.H. Leongb, and I. Herszbergc. A review of the effect of stitching on the in-plane mechanical properties of fibre-reinforced polymer composites. Composites: Part A 1997;97:979-991. [25] H.B. Xu, X.Q. Zhang, D. Hui, and Y. Zhu. Cyclomatrix-type polyphosphazene coating: Improving interfacial property of carbon fiber/epoxy composites and preserving fiber tensile strength. Composites: Part B 2016;93:244-251.
[26] T700S data sheet, Torayca. http://www.toraycfa.com/pdfs/T700SDataSheet. pdf [27] C.F. Wang, J. Li, S.F. Sun, and Y.D. Huang Electrophoretic Deposition of Graphene Oxide on Continuous Carbon Fibers for Reinforcement of both Tensile and Interfacial Strength Compos Sci Technol 2016;135:46-53. [28] ASTM D3379-75. Standard test method for tensile strength and Young's Modulus for high-modulus single-filament materials. 1989. [29] W. Weibull, J. Appl. Mech. 1951;18:293-297. [30] B. Gao, R.L. Zhang, and H.Z. Cui. Interfacial Microstructure and Enhanced Mechanical Properties of Carbon Fiber Composites Caused by Growing Generation 1-4 Dendritic Poly(amidoamine) on a Fiber Surface. Langmuir 2016;32:8339-8349. [31] F. Zhao and Y.D. Huang. Grafting of polyhedral oligomeric silsesquioxanes on a carbon fiber surface: novel coupling agents for fiber/polymer matrix composites. J Mater Chem 2011;21:3695-3703. [32] J.F. Che, W. Yuan, G.H. Jiang, J. Dai, S.Y. Lim, and M.B. Chan-park. Epoxy Composite Fibers Reinforced with Aligned Single-Walled Carbon Nanotubes Functionalized with Generation 0 - 2 Dendritic Poly ( amidoamine ). Chem Mater 2009;11:1471-1479.
7. Figure Captions
Fig. 1 Schematic illustration of D400 grafted to carbon fiber surface in water.
Fig. 2 SEM, AFM images and their Z displacement of respective AFM images of (a) untreated CF, (b) ACF, (c) CF/D400, and D400 grafted carbon fiber in (d) 4×10-5, (e) 2×10-4, (f) 10-3 mol/L and (g) 5×10-3 mol/L D400/water solution.
Fig. 3 Wide-scan XPS spectra, C 1s and N 1s high-resolution XPS element spectra of (a) untreated CF, (b) ACF, (c-e) CF/D400 and (f-h) ACF/D400
Fig. 4 (a) Contact angles of different carbon fiber samples and (b) surface free energies of different carbon fiber samples.
Fig. 5 SEM image of micro-droplet. (a) Before debonding, (b) after debonding and (c) interfacial shear strength of the carbon fiber/epoxy composites.
Fig. 6 Surface morphologies of after debonding samples and sketch of failure mode: (a,g) untreated CF, (b,g) ACF, (c,h) CF/D400, and D400 grafted carbon fiber in (d,i) 4×10-5, (e,j) 2×10-4 and (f,k) 10-3 mol/L D400/water solution (d~f insert high-magnification pictures).
8. Tables Table. 1 Single fiber tensile strength of the samples. Samples
Number of samples
R2
Weibull shape
Expectation (GPa)
paramenter (m) Untreated CF
100
0.96
4.97
4.73
ACF
100
0.98
4.65
4.54
CF/D400
100
0.96
5.03
4.78
ACF/D400-4×10-5
100
0.95
5.24
4.95
ACF/D400-2×10-4
100
0.96
5.28
5.01
ACF/D400-10-3
100
0.94
5.41
5.12
ACF/D400-5×10-3
100
0.94
5.43
5.15
S1359-835X(17)30152-5 http://dx.doi.org/10.1016/j.compositesa.2017.04.003 JCOMA 4630
To appear in:
Composites: Part A
Received Date: Revised Date: Accepted Date:
4 November 2016 1 April 2017 5 April 2017
Please cite this article as: Wang, C., Chen, L., Li, J., Sun, S., Ma, L., Wu, G., Zhao, F., Jiang, B., Huang, Y., Enhancing the interfacial strength of carbon fiber reinforced epoxy composites by green grafting of Poly(oxypropylene) Diamines, Composites: Part A (2017), doi: http://dx.doi.org/10.1016/j.compositesa. 2017.04.003
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Enhancing the interfacial strength of carbon fiber reinforced epoxy composites by green grafting of Poly(oxypropylene) Diamines Caifeng Wang a, Lei Chen a, Jun Li, a Shaofan Sun a, Lichun Ma a b, Guangshun Wu a c, Feng Zhao a, Bo Jiang a and Yudong Huang* a a
MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion
and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150040, China. b
Institute of Material Science and Engineering, Qingdao University, Qingdao 266071,
China. c
School of Chemistry and Materials Science, Ludong University, Yantai 264025,
China. Abstract: We report on a green method of using poly(oxypropylene) diamines (D400) as coupling and curing agent to functionalize carbon fiber in water. We propose to enhance the interfacial properties of carbon fiber composites, together with the tensile strength of carbon fibers. The microstructure and mechanical properties of carbon fibers before and after modification are investigated. The results show that D400 do not change the surface morphology, but significantly increase the polarity, wettability and roughness of the carbon fiber surface. The interfacial shear strength (IFSS) of modified carbon fiber/epoxy composite and the tensile strength of carbon fibers
increase by 71.7% and 12.8%, respectively. It is believed that D400 can effectively improve the interfacial adhesion of the composites by improving resin wettability, increasing chemical bonding and mechanical interlocking. This green and simple method can have applications in continuous production of high-performance carbon fiber composites. Keywords: Carbon fiber, Grafting, Interfacial strength, poly(oxypropylene) diamines 1. Introduction Excellent tensile strength, high stiffness, light weight and great thermal resistance make carbon fiber reinforced composite an ideal structural material [1-3]. As we all know, interphase between fibers and matrix is critical in determining the properties of carbon fiber composites [4,5]. That is to say the level of interfacial adhesion determines to a great extent the transfer of stresses from the matrix to the reinforcing fiber. And an optimum interface is the necessary prerequisite to the good mechanical properties of resulting composite [6~8]. However, the interfacial adhesion of carbon fiber composites tends to be weak because carbon fibers naturally show a weak wettability and adsorption with most polymers matrix. Recently, the introduction of various nanomaterials such as carbon nanotubes [9,10], nanofibers [11] and graphene [12]/graphene oxide [13] over carbon fiber composites to improve the interfacial properties has become a subject craze. This is because the nanocomposites will have a much greater interfacial area than microcomposites at the same particle loading [14,15]. On the other hand, nanomaterials act as the reinforcing phase in polymer
composites because of its stress transfer behavior between the polymer matrix and carbon fiber [16]. In the viewpoint of carbon fiber surface configuration, grafting of micro-molecules or polymers onto the carbon fiber surface can yield a controlled, ordered and active structure, which has been proved to be an effective modification method [17~20]. D400, as a traditional cross-linker of epoxy [21], contains two active primary amine groups located on secondary carbon atoms at each end of an aliphatic polyether chain (Fig 1). The addition of coupling agent along with the materials creates the covalently bonded oxygen-carbonated functional groups at the carbon fiber surface, which is further responsible for chemical interactions with the matrix polymer. In addition, the increased crosslink density of the matrix in the interphase region could react with the organic groups of D400. Thus flexible polyether chain contributes to stress transfer between carbon fiber and epoxy [22]. In the past decades, in order to improve the interfacial properties of carbon fiber composites, researchers used different amine polymers to modify carbon fiber [17,23]. The addition of polymers phase as coupling agent which formed polymer interphase could react with carboxylic groups on acid-treated CF surface to improve interactions between carbon fiber and amine polymers. On the other hand, the amino groups are used as curing and toughening agent for matrix to enhance compatibility and interfacial strength between amine polymers and matrix [24]. However, the modification process always need long reaction time, harmful organic solvents and tedious steps [23,25].
Here, we proposed to introduce D400 into the interface between carbon fiber and epoxy matrix via a green and simple method. Different from the previous studies, water, instead of organic solvents, was used as the reaction solvent to avoid environmental pollution. To shorten reaction time and steps, a condensing agent N-[(dimethylamino)-1H-1,2,3-triazolo[4,5,6]-pyridin-1-ylmethylene]-N-methylmetha naminium hexafluoro-phosphate N-oxide (HATU) was adopted. The influence of the concentration of D400 was studied in this paper. At an optimal concentration, the IFSS of carbon fiber/epoxy composites was significantly enhanced from 48.8 MPa to 83.8 MPa, while the tensile strength of carbon fiber showed a slight increase, from 4.73 GPa to 5.12 GPa. 2. Materials and methods 2.1. Materials Carbon fibers (T700SC-12000-50C, 12 K, tensile strength 4.9 GPa, diameter 7 μm, density 1.8 g·cm-3) [26] were purchased from Toray Industries Inc. D400 (molecular mass Mn 400) was purchased from Huntsman Corporation Australia Pty Ltd. Concentrated nitric acid (68%) was supplied by Sinopharm Chemical Reagent Co. Ltd. The HATU used was supplied by GL Biochem Ltd. Deionized water (DI water, resistivity >18 MΩ·cm) was used throughout the experiments. 2.2. Deposition of D400 onto carbon fiber via physical adsorption and chemical grafting
As received carbon fibers were cleaned with refluxed acetone for 48 h to remove the polymer sizing and pollutants before deposition, denoted as untreated CF. The untreated CF were oxidized in HNO3 solution at 80 oC for 4 h. Then the carbon fibers were taken out and washed several times with DI water until the pH of the washed water was neutral, and then dried under vacuum, denoted as ACF. Different concentrations of D400 water solutions (4×10-5, 2×10-4, 10-3 and 5×10-3mol/L) were prepared to study the effect of deposition amount of D400 on IFSS. Subsequently, 20 mg HATU was ultrasonically dissolved in D400 solution. Then, ACF (0.5 g) was immersed into the solution at 90 oC for 4 h, in order to complete the D400 chemical grafting, on condition that the HATU was used as condensing agent [17], as illustrated in Fig. 1. After that, the D400 grafted carbon fiber was washed by excess DI water and dried in a vacuum furnace, denoted as ACF/D400. Fig. 1 insert here For comparison, untreated CF was immersed in D400 water solution without HATU, following the same procedure, denoted as CF/D400. In this approach, D400 physically absorbed onto the carbon fiber surface, based on the Van der Waals interaction. 2.4. Characterization techniques The morphologies of the modified carbon fiber were observed by the scanning electron microscope (SEM) (200FEG, Quanta FEI Inc. the USA) at an accelerating voltage of 20 kV. Gold sputtering was used to improve the conductivity. The surface roughness (Ra) of carbon fiber was examined by atomic force microscopy (AFM,
Solver-P47H, NT-MDT, Russia) using tapping mode. X-ray photoelectron spectroscopy (XPS) (ESCALAB 220i-XL, VG, UK) was carried out to study the chemical state of carbon fibers using a monochromatic Al Kα source (1486.6 eV) at a base pressure of 2×10-9 mbar. The XPS Peak version 4.1 program was used for data analysis. Dynamic contact angle was measured using a dynamic contact angle meter. Tensiometer (DCAT21, Data-Physics Instruments, Germany). DI water (γd =21.8 mN m-1, γ = 72.8 mN m-1) and diiodomethane (γd =50.8 mN m-1, γ= 50.8 mN m-1, 99% purity, Alfa Aesar, USA) were used as test liquids [27]. The dispersive and polar components can be determined by solving the following equation: l cos lp fp )1 2 dl df )1 2
(1)
Where γl, γld and γlp are the surface tension of the test liquid, its dispersive and polar components, respectively. Monofilament tensile tests were conducted on a universal testing machine (Instron 5566, USA) according to the ASTM D3379-75 [28] with a load cell of 50 N. Firstly, filaments were glued on a paper frame and the paper frame was held by the clamps of testing machine. The paper frame was cut before the mechanical testing. And then, mechanical testing started and the load-extension curves were collected. The testing speed was 10 mm·min-1 and the gauge length is 20 mm. Lastly, the data from load-extension curves were normalized by the linear density of carbon fiber bundle to give the tensile strength. A total of 100 data points were collected and the results were analyzed by a Weibull statistical method [29].
IFSS was adopted to quantify the interfacial property between carbon fiber and epoxy by the interfacial evaluation equipment (FA620, Japan). Epoxy resin (WSR618) and methyl tetrahyelrophthalic anhydride hardener were mixed at the weight ratio of 100:32 to prepare microdroplets. The microdroplets were cured following a curing process: 90 °C for 2 h, 120 °C for 2 h and 150 °C for 4 h. The IFSS was calculated according to equation (2).
IFSS
F dl
(2)
Where F is the maximum load recorded, d is the carbon fiber diameter, and l is the embedded length. The final IFSS of each sample was averaged from the data of 50 successful measurements. 3. Results and discussion 3.1. Surface Morphologies of Carbon Fibers SEM and AFM images in Fig. 2 depict the surface morphologies and roughness of the carbon fibers, respectively. It can be seen that the surface of untreated CF is smooth. After acid treatment, a few narrow shallow parallel grooves appear along the longitudinal direction of the fiber. Fig. 2 c shows that after the physical adsorption of D400, the smooth surface of carbon fiber changes to be coarse due to the absorption of D400 onto carbon fiber surface by Vander Waals interactions. Fig. 2 d-g display the surface morphologies of D400 grafted carbon fiber at different concentration. After chemical grafting of D400, the fiber surface becomes rougher. When the D400 concentration is low, like 4×10-5 mol/L, D400 molecules could not cover the whole
carbon fiber surface (Fig. 2 d). When the D400 concentration is high enough, D400 molecules would fully cover carbon fiber (Fig. 2 e), restack together (Fig. 2 f) or agglomerates clumps (Fig. 2 g). Obviously, the coverage of D400 on carbon fiber depends on D400 concentration that was used. Fig. 2 insert here Fig. 2 also reveals the surface roughness of different samples. It can be clearly seen that the surface roughness increases linearly with the concentration of D400. The surface roughness increase of CF/D400 is attributed to physical absorption of D400 by Van der Waals' force, which is strong enough to prevent D400 from undesired falling off upon washing. Compared with physical absorption, the deposition amount of D400 on carbon fiber in chemical grafting is larger, resulting higher surface roughness. When the D400 concentration is 2×10-4 mol/L, D400 deposits on carbon fibers uniformly (Fig. 2 e) and fully covers the carbon fiber surface (Ra=95.9 nm). When the D400 concentration is 10-3 mol/L, the surplus D400 results in a dramatic increase in the surface roughness (Ra=95.9 nm). This surplus D400 is more noticeable and the surface roughness is the maximum with 255 nm at D400 concentration of 5×10-3 mol/L. The untreated CF fiber profile is subtracted as a background from the Z displacements presented in Fig. S1 (the picture is given in the Supporting Information). We can see that the height difference and amplitude of variation between the modified carbon fibers curves and untreated CF curve increase from top to bottom. It also suggests that surface roughness of ACF/D400 increases with increase of D400 concentration. We
believe the increased roughness is beneficial to improve the mechanical interlocking between carbon fiber and epoxy. 3.2. Surface characteristics of Carbon Fiber Fig. 3 insert here XPS is performed to determine the surface element composition of carbon fiber, and a quantitative analysis is carried out. The wide-scan and high resolution spectra of samples are shown in Fig. 3. The untreated CF surface consists mainly of carbon and oxygen (Fig. 3 a). After acid treatment, a significant increase of oxygen content can be found (Fig. 3 b). Fig. 3 c is the XPS spectrum of CF/D400. The existence of N 1s confirms the absorption of D400 on carbon fiber. The high resolution spectrum of C 1s (Fig. 3 d) shows the relative content of C-O (286.2) and C-N (285.6 eV), owing to ether and amino groups in D400. Moreover, from the high resolution spectrum of N 1s, only one binding energy peak is found at 399.1 eV (Fig. 3 e), which is typical for C-N bonds in amino groups, indicating that D400 was physical absorbed on the carbon fiber via Van der Waals interactions. Compared with CF/D400 (Fig. 3 c), the wide-scan spectrum of ACF/D400 (Fig. 3 f) shows a significant increase of nitrogen content from 11.9% to 24.5%. A new binding energy peak at 287.9 eV is also found from the high resolution spectrum of C 1s (Fig. 3 g), which is attributed to new generated bond (-N-C=O), corresponding to chemical bonds formed between carboxyl groups on the surface of ACF and the amino groups of D400. The reaction is also confirmed by tracing N1s spectrum, as shown in Fig. 3 h,
where a new peak (400.1 eV) assigned as amide (-N-C=O) [10] appears. These results indicate amino groups of D400 react with carboxyl groups of ACF. 3.3. Wettability of Carbon Fiber Fig. 4 insert here The functionalization of D400 changes the surface free energy of carbon fiber. The wettability of carbon fiber is investigated using advancing dynamic contact angle test. The advancing contact angle (θ), the surface energy (γ), dispersion component (γ d) and polar component (γp) of untreated CF, ACF, CF/D400 and ACF/D400 are shown in Fig. 4. After D400 grafting treatment, the contact angles of both water and diiodomethane decrease, while the surface energy of carbon fiber increases. The increased polar component of surface energy can be interpreted from the increase on polar
amine
groups
in
D400.
Therefore,
the
wettability
between
the
amino-functionalized carbon fiber and the polar polymer matrix should be significantly improved [4]. In addition, the increased dispersion component, caused by the increased roughness, also contributes to the increase of surface energy [27]. Meanwhile, the surface free energy of carbon fiber becomes higher as the D400 concentration increases. In summary, the change of the fiber surface free energy confirms the successful deposition of D400 on carbon fiber surface. 3.4. Interfacial Property Testing of Carbon Fiber/Epoxy Composite Fig. 5 a and b demonstrate the images of micro-droplet before and after interfacial debonding. Fig. 5 c shows IFSS results of untreated CF, ACF, CF/D400 and ACF/D400.
After acid-treatment, the IFSS of the composite has a negligible increase in comparison with untreated CF (48.8 MPa). After adsorption of D400, the IFSS of CF/D400 composite increases to 55.8 MPa which is attributed to improvement of surface
energy.
After
D400
grafting
treatment,
IFSS
of
composites
is
significantly higher than the untreated one. D400, which can not only react with the reactive groups on the carbon fiber surface, but also increase resin compatibility and react with the matrix system. While the roughness enhances mechanical interlocking between the fibers and epoxy by the introduction of D400 into the interfacial region. The changing trend of IFSS of composites shows ‘‘roof’’ shape as D400 concentration goes up, peaking (83.8 MPa) at 2×10-4 mol/L concentration of D400. After that, IFSS of decrease to 77.5 MPa and 71.4 MPa at 10-3 and 5×10-3 mol/L concentration of D400, respectively. In our understanding, these D400 molecules have exactly and completely covered the whole surface of carbon fibers at 2×10-4 mol/L concentration of D400. Gao et al. [30] reported that grafting polymer was highly effective in stress transfer in epoxy matrix. Fig. 5 insert here Fig. 6 depicts the surface morphology of carbon fibers after debonding from the epoxy matrix. For the untreated CF and ACF composites (Fig. 6 a and b), the surfaces are similarly neat, which indicates that the debonding between carbon fiber and epoxy is interfacial failure because of the weak van der Waals force between them. After adsorbed D400, a little of epoxy resin remains on the carbon fiber surface after debonding (Fig. 6 c), implying interface between the CF/D400 and epoxy become
stronger. On the other hand, for ACF/D400 composite, more epoxy debris adhere on the
carbon
fiber
after
debonding
due
to
the
strong
covalent
bonds formed between amino and carboxyl groups, which implies that the interface between the ACF/D400 and epoxy is stronger than that of CF/D400 composites. Fig. 6 d, e and f show debonding SEM images of carbon fiber in different concentrations composites. As D400 concentration goes up, there is more remaining epoxy resin. When the concentration of D400 is 2×10-4 mol/L, more resin fragments of resin are displayed than the others, which indicate that the interfacial strength between the carbon fiber and epoxy is the strongest. As D400 concentration continues to go up, for 10-3 mol/L, the large image clearly displays the obvious crevices (the green arrow pointed in Fig. 6 f) between bare carbon fiber and epoxy resin, which indicates that the interfacial strength between the carbon fiber and epoxy is weak [17]. Fig. 6 insert here The IFSS change is related to interfacial failure mode of the composite. Normally, the interfacial failure mode of composite includes adhesive and cohesive failure. We propose the failure models depending on the interfacial strength, as shown in Fig. 6 g~k. In the case of untreated CF and ACF, the failure mode is adhesive failure (Fig. 6 g), due to that the untreated CF and ACF have poor wettability and there is no chemical interaction between carbon fiber and epoxy (only Van der Waal interaction). This is consistent with the results from Fig. 6 a and b. For CF/D400 composite (Fig. 6 h), the failure mode is still adhesive failure, though there is a little epoxy resin remaining on the carbon fiber. This is because the interfacial failure may take place on
the surface of carbon fiber owing to the weak interactions (Van der Waals force) between CF and D400, although the epoxy groups could react with D400 via a nucleophilic ring opening mechanism [4]. When D400 grafted on carbon fiber surface, the greater amounts of D400 improve the wettability of carbon fiber (as refereed in Fig. 4 b). At the same time, covalent bond between CFs and D400 is stronger than that of Van der Waals force. This additional interphase area worked as an additional reinforcement can relieve the stress concentration effectively, transfer the loads from matrix to carbon fibers uniformly and change the failure mode from adhesive failure involving fiber-matrix interface debonding to cohesive failure within the interphase [31]. When D400 concentration is low (10-5 mol/L), D400 molecules partially cover carbon fiber surface (Fig. 6 i). As D400 concentration goes up, like 2×10-4 mol/L, D400 fully covers carbon fiber surface (Fig. 2 e). The wettability of carbon fiber becomes better and the number of the chemical bonds increases, resulting stronger interfacial strength (Fig. 6 j), which is confirmed by the fact that a layer of epoxy remains on carbon fiber surface (Fig. 6 e). Moreover, the increased fiber/ matrix compatibility is a decisive factor for final quantification of fiber/matrix interfacial adhesion in full coverage of D400. This brings an increase in the volume fraction and crosslink density of interphase in the composites and a decrease in the effective polymer chain mobility in the interphase region [8,31,32]. However, when D400 concentration is further increased (10-3 mol/L and 5×10-3 mol/L), IFSS start falling due to the D400 over covered on carbon fiber (Fig. 6f). In this way the composite failure may take place within the D400 layer, because of the weak Van der Waals interaction among D400
molecules [17]. Therefore, the cohesive failure happens both in epoxy and D400 layer, thus the remaining epoxy becomes less and the IFSS slightly decreases, as observed in Fig. 6 k. Based on the above analysis, we can conclude that when the concentration of D400 is 2×10-4 mol/L, the strong interface would lead to cohesive failure in matrix, where the IFSS is the highest. 3.5. Tensile Strength Table. 1 insert here The tensile tests are carried out and the results are summarized in Table. 1. It can be seen that the tensile strength of all D400 treated carbon fibers are a little higher than that of the untreated CF (4.73 GPa). We also note that the tensile strength of D400 grafted carbon fiber increased slightly with the increase of concentration, which could be attributed to the increasing surface coverage of D400. The results imply that D400 grafting can not only make up the tensile strength losses due to the etching effect during the acid treatment, but also mildly enhance tensile strength of carbon fiber. 4. Conclusion In this study, a green grafting method is proposed to chemically functionalize carbon fibers by D400 in water. It is found that the IFSS of carbon fiber composites was improved significantly. When the concentration of D400 solution is 2×10-4 mol/L, the IFSS of ACF/D400 composites is improved by 69.9%. We believe the presence of D400 can increase the polarity, wettability and surface roughness of the carbon fiber, which are responsible for the enhancement of IFSS of the ACF/D400 composites. After
the surface functionalization by D400, the tensile strength of carbon fibers have not been sacrificed, but mildly enhanced. This green and simple grafting method can be applied in versatile and scalable fabrication of high-performance carbon fibers, as well as excellent carbon fiber reinforced composites. 5. Acknowledgements The authors gratefully acknowledge financial supports from the Chang Jiang Scholars Program and National Natural Science Foundation of China (51073047) 6. Refrences [1] P.C. Varelidis, R.L. McCullough, C.D. Papaspyrides. The effect on the mechanical properties of carbon/epoxy composites of polyamide coatings on the fibers. Compos Sci Technol 1999;59:1813-23. [2] M.A. Montes-Moran, R.J. Young. Raman spectroscopy study of HM carbon fibres: effect of plasma treatment on the interfacial properties of single fibre/epoxy composites. Part I: fibre characterization. Carbon 2002;40:845-855. [3] F. Li, Y. Liu, C.B. Qu, H.M. Xiao, S.Y. Fu. Enhanced mechanical properties of short carbon fiber reinforced polyethersulfone composites by graphene oxide coating. Polymer 2015;59:155-165. [4] L.C. Ma, Y.D. Huang. Improving the interfacial properties of carbon fiber-reinforced epoxy composites by grafting of branched polyethyleneimine on carbon fiber surface in supercritical methanol. Compos Sci Technol 2015;114:64-71.
[5] H.L. Tekinalp, V. Kunc, G.M. Velez-garcia, C.E. Duty, L.J. Love, A.K. Naskar, C.A. Blue, and S. Ozcan. Highly oriented carbon fiber-polymer composites via additive manufacturing. Compos Sci Technol 2014;105:144-150. [6] X.Q. Zhang, X.Y. Fan. Interfacial Microstructure and Properties of Carbon Fiber Composites Modified with Graphene Oxide. ACS Appl Mater Inter 2012;4: 1543-1552. [7] M. Ashuri, F. Moztarzadeh, and N. Nezafati. Development of a composite based on hydroxyapatite and magnesium and zinc‐ containing sol-gel-derived bioactive glass for bone substitute applications. Mater Sci Eng C 2012;32:2330-2339. [8] M. Sharma, S. Gao, E. Mäder, H. Sharma, L. Yew, and J. Bijwe. Carbon fiber surfaces and composite interphases. Compos Sci Technol 2014;102:35-50. [9] F. Zhao, Y. Huang, L. Liu, Y. Bai, and L. Xu. Formation of a carbon fiber/polyhedral oligomeric silsesquioxane/carbon nanotube hybrid reinforcement and its effect on the interfacial properties of carbon fiber/epoxy composites. Carbon 2011;49:2624-2632. [10] A.Y. Boroujeni, M. Tehrani, A.J. Nelson, and M. Al-haik. Hybrid carbon nanotube – carbon fiber composites with improved in-plane mechanical properties. Composites : Part B 2014;66:475-483. [11] J.Q. Zhao, Q.Y. Li, X.F. Zhang, M.J. Xiao, W. Zhang, and C. Lu. Grafting of polyethylenimine onto cellulose nanofibers for interfacial enhancement in their epoxy nanocomposites. Carbohydr Polym 2017;157:1419-1425.
[12] T. Ramanathan1, A.A. Abdala, S. Stankovich, D. A. Dikin1, M. Herrera-Alonso R.K. Prud'Homme, L.C. Brinson. Functionalized graphene sheets for polymer nanocomposites. Nat Nanotechnol 2008;3:327-331. [13] Y. Mo, M. Yang, Z. Lu, and F. Huang. Preparation and tribological performance of chemically-modified reduced graphene oxide/polyacrylonitrile composites. Composites: Part A 201;54:153-158. [14] H. Abdizadeh, M. Ashuri, P. Tavakoli, A. Nouribahadory, and H. Reza. Improvement in physical and mechanical properties of aluminum/zircon composites fabricated by powder metallurgy method. Mater Des 2011;32:4417-4423. [15] M. Ashuri and L.L. Shaw. Silicon as a potential anode material for Li-ion batteries: where size, geometry and structure matter. Nanoscale 2016;8:74-103. [16] G.S. Wu, L.C. Ma, L. Liu, and Y.D. Huang. Interfacial properties and impact toughness of methylphenylsilicone resin composites by chemically grafting POSS and tetraethylenepentamine onto carbon fibers. Composites: Part A 2016;84:1-8. [17] Q. Peng, Y. Li, X. He, H. Lv, P. Hu, Y. Shang, C. Wang, R. Wang, T. Sritharan, and S. Du. Interfacial enhancement of carbon fiber composites by poly(amido amine) functionalization. Compos Sci Technol 2013;74:37-42. [18] S. Chen, Y. Cao, and J. Feng. Polydopamine as an efficient and robust platform to functionalize carbon fiber for high-performance polymer composites. ACS Appl Mater Interfaces 2014;6:349-356.
[19] S. Ahmad, A. Najafabadi, M. Ashuri, and M. Tahriri. Surface Modification of Castor Oil-Based Polyurethane by Polyacrylic Acid Graft using a Two-Step Plasma Treatment for Biomedical Applications. Adv Polym Technol 2014;33:1-8. [20] S.A.A. Najafabadi, H. Keshvari, Y. Ganji, M. Tahriri and M. Ashur. Chitosan/heparin surface modified polyacrylic acid grafted polyurethane film by two step plasma treatment. Surface Engineering 2012;28:710-714. [21] W. Wan, L. Li, Z. Zhao, H. Hu, X. Hao, D. A. Winkler, L. Xi, T. C. Hughes, and J. Qiu. Ultrafast fabrication of covalently cross-linked multifunctional graphene oxide monoliths. Adv Funct Mater 2014;24:4915-4921. [22]
T.Y.
Lee,
J.
Carioscia,
Thiol-allylether-methacrylate
ternary
Z.
Smith,
systems.
and
Evolution
C.N.
Bowman.
mechanism
of
polymerization-induced shrinkage stress and mechanical properties. Macromolecules 2007;40:1473-1479. [23] L.C. Ma, L.H. Meng, and Y.D. Huang. Interfacial enhancement of carbon fiber composites by generation 1–3 dendritic hexamethylenetetramine functionalization. Appl Surf Sci 2014;296:61-68. [24] A.P. Mouritzas, K.H. Leongb, and I. Herszbergc. A review of the effect of stitching on the in-plane mechanical properties of fibre-reinforced polymer composites. Composites: Part A 1997;97:979-991. [25] H.B. Xu, X.Q. Zhang, D. Hui, and Y. Zhu. Cyclomatrix-type polyphosphazene coating: Improving interfacial property of carbon fiber/epoxy composites and preserving fiber tensile strength. Composites: Part B 2016;93:244-251.
[26] T700S data sheet, Torayca. http://www.toraycfa.com/pdfs/T700SDataSheet. pdf [27] C.F. Wang, J. Li, S.F. Sun, and Y.D. Huang Electrophoretic Deposition of Graphene Oxide on Continuous Carbon Fibers for Reinforcement of both Tensile and Interfacial Strength Compos Sci Technol 2016;135:46-53. [28] ASTM D3379-75. Standard test method for tensile strength and Young's Modulus for high-modulus single-filament materials. 1989. [29] W. Weibull, J. Appl. Mech. 1951;18:293-297. [30] B. Gao, R.L. Zhang, and H.Z. Cui. Interfacial Microstructure and Enhanced Mechanical Properties of Carbon Fiber Composites Caused by Growing Generation 1-4 Dendritic Poly(amidoamine) on a Fiber Surface. Langmuir 2016;32:8339-8349. [31] F. Zhao and Y.D. Huang. Grafting of polyhedral oligomeric silsesquioxanes on a carbon fiber surface: novel coupling agents for fiber/polymer matrix composites. J Mater Chem 2011;21:3695-3703. [32] J.F. Che, W. Yuan, G.H. Jiang, J. Dai, S.Y. Lim, and M.B. Chan-park. Epoxy Composite Fibers Reinforced with Aligned Single-Walled Carbon Nanotubes Functionalized with Generation 0 - 2 Dendritic Poly ( amidoamine ). Chem Mater 2009;11:1471-1479.
7. Figure Captions
Fig. 1 Schematic illustration of D400 grafted to carbon fiber surface in water.
Fig. 2 SEM, AFM images and their Z displacement of respective AFM images of (a) untreated CF, (b) ACF, (c) CF/D400, and D400 grafted carbon fiber in (d) 4×10-5, (e) 2×10-4, (f) 10-3 mol/L and (g) 5×10-3 mol/L D400/water solution.
Fig. 3 Wide-scan XPS spectra, C 1s and N 1s high-resolution XPS element spectra of (a) untreated CF, (b) ACF, (c-e) CF/D400 and (f-h) ACF/D400
Fig. 4 (a) Contact angles of different carbon fiber samples and (b) surface free energies of different carbon fiber samples.
Fig. 5 SEM image of micro-droplet. (a) Before debonding, (b) after debonding and (c) interfacial shear strength of the carbon fiber/epoxy composites.
Fig. 6 Surface morphologies of after debonding samples and sketch of failure mode: (a,g) untreated CF, (b,g) ACF, (c,h) CF/D400, and D400 grafted carbon fiber in (d,i) 4×10-5, (e,j) 2×10-4 and (f,k) 10-3 mol/L D400/water solution (d~f insert high-magnification pictures).
8. Tables Table. 1 Single fiber tensile strength of the samples. Samples
Number of samples
R2
Weibull shape
Expectation (GPa)
paramenter (m) Untreated CF
100
0.96
4.97
4.73
ACF
100
0.98
4.65
4.54
CF/D400
100
0.96
5.03
4.78
ACF/D400-4×10-5
100
0.95
5.24
4.95
ACF/D400-2×10-4
100
0.96
5.28
5.01
ACF/D400-10-3
100
0.94
5.41
5.12
ACF/D400-5×10-3
100
0.94
5.43
5.15