High Interlaminar Shear Strength Enhancement of Carbon Fiber

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High Interlaminar Shear Strength Enhancement of Carbon Fiber/Epoxy Composite through Fiber- and Matrix-Anchored Carbon Nanotube Networks Yilei Wang, Suresh Kumar Raman Pillai, Jianfei Che, and Mary B Chan-Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13197 • Publication Date (Web): 21 Feb 2017 Downloaded from http://pubs.acs.org on February 23, 2017

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ACS Applied Materials & Interfaces

High Interlaminar Shear Strength Enhancement of Carbon Fiber/Epoxy Composite through Fiber- and Matrix-Anchored Carbon Nanotube Networks

Yilei Wang1†*, Suresh Kumar Raman Pillai1†, Jianfei Che2 and Mary B. Chan-Park 1*

1

School of Chemical and Biomedical Engineering, Nanyang Technological University,

Singapore 637459, Singapore 2

Key Laboratory of Soft Chemistry and Functional Materials, Ministry of Education,

Nanjing University of Science and Technology, Nanjing, P.R. China. *e-mail: [email protected]

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Abstract To improve the interlaminar shear strength (ILSS) of carbon fiber reinforced epoxy composite, networks of multi-walled carbon nanotubes (MWNTs) were grown on micron-sized carbon fibers and single-walled carbon nanotubes (SWNTs) were dispersed into the epoxy matrix so that these two types of carbon nanotubes entangle at the carbon fiber (CF)/epoxy matrix interface. The MWNTs on the CF fiber (CFMWNTs) were grown by chemical vapor deposition (CVD) while the single-walled carbon nanotubes (SWNTs) were finely dispersed in the epoxy matrix precursor with the aid of a dispersing agent -- polyimide-graft-bisphenol A diglyceryl acrylate (PIBDA) copolymer. Using vacuum assisted resin transfer molding, the SWNT-laden epoxy matrix precursor was forced into intimate contact with the “hairy” surface of the CF-MWNT fiber. The tube density and the average tube length of the MWNT layer on CF was controlled by the CVD growth time. The ILSS of the CFMWNT/epoxy resin composite was examined using the short beam shear test. With addition of MWNTs onto the CF surface as well as SWNTs into the epoxy matrix, the ILSS of CF/epoxy resin composite was 47.59±2.26 MPa, which represented a ~103% increase compared with the composite made with pristine CF and pristine epoxy matrix (without any SWNT filler). FESEM established that the enhanced composite did not fail at the CF/epoxy matrix interface. Keywords: carbon fiber, carbon nanotube, epoxy matrix, composite, ILSS

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Introduction The superior properties of carbon fiber (CF) reinforced epoxy composites have attracted much interest and they remain the composites of choice in many applications such as aerospace parts and sports equipment.1-5 For example, they possess high modulus, high tensile strength, high toughness, low density and low susceptibility to corrosion. However, the poor interaction between unmodified and fairly inert CF surface and epoxy matrix results in low interlaminar shear strength (ILSS), which limits the use of CF/epoxy composite in some high-performance applications such as structural parts in military aircraft. To improve the ILSS, fiber surface treatments such as plasma treatment, chemical functionalization and nanomaterial grafting have been explored.6-8 However, the increase has been usually modest. Modifications which increase the surface area and reactivity of reinforcing filler fibers are known to improve the mechanical properties of composites.9-10 Surface modification methods of CF include (1) wet chemical methods such as acidic modification and electrochemical modification and (2) dry surface modifications such as plasma surface modification, high energy irradiation and thermal modification.11 Recently, MWNTs have been attached onto the surface of CF to enhance the interaction with epoxy matrix by processes such as chemical modification, chemical vapor deposition (CVD) method and electrodeposition.10,

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Fan et al. used

electrochemical treatment of CF to achieve highly uniform catalyst nanoparticle distribution prior to CVD growth of MWNTs and found ~17% improvement in composite ILSS.13 Both multi-walled carbon nanotubes (MWNTs) and single-walled

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carbon nanotubes (SWNTs) have been deposited on CF surface with electrodeposition method;14 the ILSS of composites prepared in this way reached up to 48.5 MPa (~30% increase). Rong et al. grafted carbon nanotubes onto CF by (1) CVD method and (2) chemical method.15 The CVD CF modification approach resulted in ~11% higher tensile strength than chemical grafting of MWNTs to CF.

Wu et al. used 3-

aminopropyltriethoxysilane as coupling agent to attach MWNTs to CF;16 the improved bond between CF-MWNTs and epoxy matrix enhanced the ILSS by ~50%. Kepple et al. used CVD growth of MWNTs on CF cloth and found ~5% improvement in flexural modulus of the grafted CF-epoxy composite.17 With another strategy, carbon nanotubes (CNTs) were incorporated into the matrix of CF/epoxy composites, resulting in a modest increase in ILSS.18 With tip sonication instead of mechanical stirring, the MWNTs were well dispersed in the epoxy matrix, but the highest ILSS of CF/epoxy composite was only 5% higher than the control sample. Fan et al. improved the ILSS of glass fiber/epoxy composite by 33% by dispersion of MWNTs into epoxy using mechanical stirring.19 Godara et al. used the drumwinder technique to fabricate the CNT-CF/epoxy prepregs. Their study, which employed “bare” CF, did not find significant improvement in ILSS for any of the types of CNTs they used in the matrix, but the best performance was obtained with MWNTs-modified carbon fibers.20 To our knowledge, these distinct approaches, i.e. CNT modification of CF versus CNT reinforcement of matrix, to employing CNTs to strengthen CF/epoxy composites have not previously been combined. Their combination may exhibit synergies which improve the composite performance more

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than either single approach. In addition to strengthening the matrix and the matrix/filler interface, if the matrix CNTs have lengths that are not much smaller than the thickness of the CNT layer on the CF surface, interpenetration of the respective networks of CF-anchored CNTs and matrix-anchored CNTs would increase the thickness of the interphase layer around each fiber through which stress is transferred between filler and matrix. The combination of stronger matrix and thicker stress transfer layer should result in better than additive improvement in the strength of the CF/matrix interface. In this report, we employed two interpenetrating carbon nanotube networks: one anchored on the CF filler and another in the epoxy matrix to increase the ILSS of the CF/epoxy composite. We employed CVD to grow MWNTs on desized CFs (Scheme 1). The CF-MWNTs were impregnated with epoxy and layers of the epoxy/CFMWNTs were built and consolidated with a vacuum bagging process (Scheme 2). The epoxy was also reinforced with SWNTs well-dispersed with a polyimidebisphenol A diglyceryl acrylate copolymer (PI-BDA, Scheme S1). The dispersion of SWNTs, morphology of CF before and after MWNTs growth, and ILSS property of the composites were characterized. Composite made with CF-MWNTs but without SWNTs added to the epoxy matrix exhibited ILSS of 31.62±1.10 MPa, compared with 23.45±1.85 MPa for the CF/epoxy composite, representing an improvement of ~35%. Inclusion of polyimide-graft-bisphenol A diglyceryl acrylate (PI-BDA) dispersed SWNTs into the epoxy matrix precursor increased ILSS by up to ~103% compared with pristine CF/epoxy resin composite. It appears that roughening of the

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CF surface with MWNTs improves the filler/matrix bond. Combining this fiber roughening effect with adding SWNTs to the epoxy matrix further improves filler/matrix integration and dramatically improves the ILSS.

Results and Discussion The morphology of CF surface after CVD was characterized by FESEM. As shown in Figure 1a, the diameter of bare carbon fibers is ~8 µm and the fiber surfaces are relatively smooth with apparent shallow longitudinal striations and there is significant clear space between the fibers. In order to grow CNTs uniformly on each carbon fiber, the carbon fibers (CFs) were pulled out from CF cloth and desized by Soxhlet extraction with acetone as solvent. The carbon fibers were placed inside the CVD furnace such that the fibers were not allowed to bundle together during CNT growth process. This procedure helps the carbon source (m-xylene) and catalyst precursor (ferrocene) to interact with surface of each carbon fiber and grow MWNTs uniformly on the fiber surface. The CNTs were uniform at the interface of each carbon fiber. After desizing the carbon fibers by Soxhlet extraction, no breakage was found along the fibers. In Figure 1b, MWNTs have densely grown from the CF surfaces to form a tangled mat on each fiber that fills a significant portion of the interfiber spaces. The thickness of the MWNT mat on the CF fibers can be roughly measured in this image and was found to be ~1 µm. (The actual MWNT lengths may be considerably longer than this in view of their convoluted conformation on the CF surface.) Individual MWNT diameters were observed to be a few tens of nm. To

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confirm the attachment of MWNTs on the CFs, the Raman spectrum of bare CF and hairy MWNTs grown CF were taken in the range of 300-3200 cm-1 and is shown in Figure 2. From the comparation, the D peaks have a significant increase which indicates the hairy MWNTs grown on the CF are rich of defects and uniformly cover the CF surface. The quality of the SWNTs dispersion in the epoxy matrix precursor may dramatically affect the mechanical properties of the product.21 The dispersion of PIBDA wrapped SWNTs in DMF was characterized by Atomic Force Microscopy (AFM).

Figure 3 shows that SWNTs are individually dispersed and uniformly

wrapped with PI-BDA. As shown in the height profile curves (Figure 3b), the diameter of PI-BDA wrapped SWNT is 4-5 nm and the diameter of an uncoated SWNT is only 1-2 nm; the thickness of the PI-BDA coating is hence about 1.5 nm. The high efficacy of PI-BDA for fine SWNT dispersion is attributed to its comb-like structure, in which the PI backbone has strong π-π interaction with SWNT sidewall and the BDA side-chains impart strong inter-SWNT repulsion via steric hindrance.22 The average length of PI-BDA wrapped SWNTs is ~2 µm which is larger than the thickness of the MWNT mat on the reinforcing fiber; the SWNTs were probably cut during the sonication process. However, the SWNTs are still long enough that their penetration, carried by flowing matrix precursor, into the void spaces in the MWNT mat promotes stress transfer between the CF and the surrounding matrix. Thermo-Gravimetric Analysis (TGA) under O2 atmosphere was employed to estimate the weight percentage of each component in the composites. TGA curves of

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CF-MWNTs and PI-BDA-wrapped-SWNTs composites containing 0.2%, 0.5% 1.0% and 2.0% SWNT/PI-BDA are shown in Figure 4. Between 50 oC to 380 oC, there is ~19% weight loss which may be attributed to the oxidation of epoxy. As the temperature rises from 380 oC to 600 oC, the MWNTs and SWNTs showed an additional 20 wt% loss as they are burnt off. Above 600 oC, the PI-BDA and CF burned off and contributed ~2 wt% and 51 wt% weight loss; above 800 oC, the residual matter (2 wt% to 8 wt%) is probably oxidized iron from the catalyst particles formed on the CF during the MWNT CVD process. Short beam shear testing (ASTM D2344) was performed to determine the ILSS of each specimen. The testing was carried out at room temperature in air with a ramp speed of 2 mm/min. The stress versus strain curve for representative samples is shown in Figure 5a (and all loading force versus central deflection curves (8 per composite formulation) are shown in the Supporting Figure S2). The curves rise gradually to the maximum loading force and thereafter drop as cracks develop in the samples. At higher CNT concentration, the samples are stiffer; the peak load has a maximum at intermediate CNT concentration. Figure 5b summarizes the ILSS values derived from the stress/strain curves of Figure 5a. The ILSS of the control composite (CF/epoxy) is 23.45±1.85 MPa. Functionalization of the CF with CVD-grown MWNTs improves ILSS by 34.8%±11.6% to 31.62±1.10 MPa. The improvement is attributable to the increase in contact area between CF and matrix due to the dense mat of MWNTs on the CF surfaces. When the epoxy matrix precursor is strengthened by addition of PI-BDA-

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wrapped-SWNTs into it, there is further improvement in the composite ILSS: with 0.5wt% SWNTs added into the epoxy matrix precursor formulation, the ILSS reaches as high as 47.59±2.26 MPa, which is an improvement of 102.9±18.7% over the control composite. The ILSS of the CF-MWNT-filled composites increases with SWNT content in the matrix from 0 wt% to its peak at 0.5 wt% and then decreases with more SWNT content beyond 0.5 wt%. With even higher SWNT content, like 2.0 wt% SWNT/PI-BDA (Figure 5b), the CF-MWNT filled composite has decreased ILSS (28.44±2.51 MPa) than that with 0 wt% SWNTs (31.62±1.10 MPa). As shown in Figure 5b (blue column), without MWNTs growth on CF, the 0.2 wt% SWNTs in epoxy matrix could also improve the composite’s performance with an increase in ILSS of 10.2% (25.85±2.49 MPa). The ILSS of composite with hairy CNT-CF and epoxy has only 34.8% increase. However, ILSS of the composite with hairy CNT-CF and 0.2 wt% SWNTs in epoxy matrix reaches 56.3%, which indicates that there is synergistic effect from two CNT networks. Incorporation of PI-BDA-wrapped-SWNTs in the epoxy matrix precursor produces a number of benefits. Firstly, it stiffens the matrix. Secondly, the SWNTs in the matrix near the CF may interlock with the MWNTs on the CF, improving stress transfer between filler and matrix. The decline in ILSS at even higher SWNT loading (>0.5 wt%) may be attributable to the increase in viscosity of the epoxy/SWNT matrix precursor. As the resin/SWNT mixture viscosity rises, it may be harder for it to flow into the void spaces in the MWNT mat on the CF, thereby hindering the integration of CF-MWNTs with the matrix.

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The effects of CF functionalization and matrix enhancement with nanotubes can be graphically seen in electron microscopy of fractured composites. Figure 6 shows the composite fracture surfaces resulting from the ILSS testing. In Figure 6a, the base CF/epoxy sample separated cleanly at the CF/matrix interface, with very little epoxy remaining on the CF; the CF/matrix bond is weaker than the matrix. In Figure 6b (CF-MWNTs/epoxy), a layer of matrix remains adhered to the CF, implying that the CF-MWNTs adhesion to the matrix material embedded within the MWNT mat on the CF is stronger than the matrix. The fracture surface is still quite clean and appears to conform to the modified fiber surface. In Figure 6c (CF-MWNT/epoxy-SWNTs), the fracture surface has a complex morphology. Flakes and filaments of matrix, some longer than the thickness of the carbon nanotube mat, radiate in all directions from the fiber axis. This suggests that the shear stress which produced the fracture was distributed further into the matrix in Figure 6c than in Figures 6a and 6b, an expected consequence of the interpenetration of the MWNTs on the CF and the SWNTs in the matrix and the greater strength of the SWNTs-enhanced matrix. The FESEM imagery results are consistent with the results of the mechanical testing. While the MWNT mats on the carbon fibers are quite dense, there are void spaces within them and a matrix precursor with sufficiently low viscosity, such as epoxy L20 used here, may infiltrate into the MWNT mat to reach the CF surface, resulting in good integration of the CF into the matrix. When the matrix precursor is itself laden with nanotubes of length comparable to or greater than the MWNT lengths, this results in a set of interpenetrating nanotube networks anchored respectively on the CF

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and in the matrix, which should further improve the bond between the reinforcing fibers and the matrix. The presence of MWNTs on the CF and PI-BDA-wrappedSWNTs in the matrix leads to interpenetrating networks comprising nanotubes (NTs) anchored respectively to the filler and the matrix. This has the effect of thickening the region around the filler fibers in which stresses are transferred between filler and matrix, which increases the filler/matrix shear strength. The hairy MWNTs on CF surface could effectively pinch into the epoxy matrix and interact with the PI-BDAwrapped-SWNTs, improving the composite ILSS – like a Velcro effect at the interphase. The use of PI-BDA as SWNT dispersant improves the dispersion of SWNTs in the epoxy matrix precursor and also improves the wetting of the SWNTs by the epoxy, both of which improve their integration with the matrix. Various means of depositing CNTs onto CF have been proposed, such as chemical grafting, electrodeposition (ED) and CVD, and all of which can form a “hairy” coating of MWNTs on the CF fiber surface. However, most methods of depositing CNTs on CF bundles or weave (including the ED and chemical grafting methods) suffer from the drawback that the surfaces of fibers that are in the interior of fiber bundles or woven cloth are less accessible and less likely to be heavily functionalized with CNTs by these processes and special care has to be taken to remove the sizing of the fiber bundles as we did here. Beside the CVD growth method, we also tried to deposit SWNTs onto the CF surface with electrodeposition method. The experimental details and results are given in supporting information (Section II). From the collected data, density of electrodeposited SWNTs on CF is lower than the

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CVD grown MWNTs on CF. The composite with 5 min deposition have the best ILSS (13.0±0.28 MPa) which is much lower than the results obtained through CVD method. Conclusions We report a new approach to increasing the CF/epoxy composite interlaminar shear strength (ILSS) through forming an interpenetrating MWNT/SWNT network at the CF filler/matrix interface by in-situ growth of MWNTs on the carbon fiber and finely dispersing SWNTs in the epoxy resin matrix precursor. FESEM images confirm that MWNTs are uniformly grown on the CF to form a dense mat on the surface. MWNT functionalization of the CF filler improves the composite ILSS by ~35% compared with unmodified CF, when using unmodified epoxy as the matrix precursor. ILSS of the composite was further improved by incorporation of well-dispersed SWNTs into the epoxy matrix precursor; we did this using PI-BDA as the SWNT dispersing agent. With 0.5 wt% PI-BDA-dispersed SWNTs in the epoxy matrix precursor, the resulting composite has ILSS which showed a significantly high ~103% increase compared to that of the control material. Our process can be further refined to optimize the composite ILSS through optimization of the CVD process parameter control of the MWNT mat density and thickness and, also the length of SWNTs added into the epoxy matrix itself. Our work provides a new approach to design of enhanced carbon fiber composites for various applications.

Materials and Methods

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Materials All chemicals and materials were used as received without further purification. 3,3′Dihydroxy-4,4′-diaminobiphenyl (HAB, 97%) was purchased from Tokyo Chemical Industry. α-Glycidyl terminated bisphenol A acrylate (GBA, trade name Ebecryl 3605) was purchased from UCB chemicals. 4,4′-Oxydiphthalic anhydride (ODPA, 97%), 4(dimethylamino) pyridine (DMAP, 99%), butylated hydroxytoluene (BHT, 99%), sodium hydrogen carbonate (NaHCO3, 99.5%), N,N′-dimethylacetamide (DMAc), xylene, dimethyl sulfoxide (DMSO), N,N′-dimethylformamide (DMF), and methanol were all obtained from Sigma-Aldrich. Epoxy Resin L20 and Hardener EPH 161 (Aero) were purchased from R&G Faserverbundwerkstoffe GmbH. Carbon fiber fabric (245 g/m2) was purchased from Cyntech Composite Pte Ltd.

Growth of MWNTs on CF with CVD method Before MWNTs growth, CF tows (~ 9 m long) were pulled out from the CF cloth and desized by Soxhlet extraction using acetone for 24 h to remove the chemicals on CF surface. As shown in Scheme 1, ferrocene was used as catalyst and m-xylene as carbon source for MWNT growth. 10 wt% ferrocene was directly dissolved into mxylene and the solution was prep-heated to 80 oC. The growth of MWNTs on CF was conducted at 800 oC in a tubular oven at ambient pressure. A portion of CF tow (~ 30 cm long) was placed in the quartz tube. The temperature was increased from 20 oC to 800 oC at 10 oC/min under argon flow of 85 sccm. When the temperature reached 800 o

C, 20 sccm hydrogen gas was flowed through the quartz tube. After the hydrogen

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flow was stabilized, 15 sccm argon was bubbled in the m-xylene/ferrocene solution and flowed through the tube for 30 min to deliver both the catalyst and carbon source. At the end of synthesis, the carbon source/catalyst flow was turned off and the oven cooled to 100 oC under argon flow. The MWNTs growth on carbon fibers of the CF tow during every CVD process cycle is around 30 cm. After every CVD process cycle, MWNT grown CFs were pulled out through one end of the furnace tube allowing the next 30 cm CFs to place inside the furnace for MWNT growth. This procedure was repeated until the 9 m CFs were covered with uniform hairy MWNT layer. After MWNTs growth, small pieces of CF were randomly cut from the 9 m of MWNTsgraft CF tow for characterization. About 30 m of CF-MWNTs cloth strips were prepared by repeating the CVD process.

PI-BDA synthesis and preparation of PI-BDA wrapped SWNTs/Epoxy L20 mixture Per the procedure of Yuan et al.22 2.160 g 3,3’-dihydroxy-4,4’-diaminobiphenyl was dissolved in 80 mL DMAc in a round-bottom flask under Ar protection. The solution was chilled in ice-bath for 15 min after which 3.100 g 4,4’-oxydiphthalic anhydride was added. The solution was warmed to ambient temperature and magnetically stirred for 24 hrs under argon atmosphere to form polyamic acid (PAA). Then 80 mL mxylene was added and the solution was refluxed at 160 oC for 3 h. After imidization, the PI was collected by precipitating in methanol. 1.176 g PI and 2.376 g Glycidyl terminated bisphenol A acrylate were dissolved in 100 mL dry DMSO and the

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solution was refluxed at 100 oC in argon environment for 2 days. The mixture was added into bulk methanol and the precipitate was filtered and washed several times with DI water. 100 mg SWNTs and 100 mg PI-BDA were put into 100 mL DMF. The mixture was sonicated with a tip sonicator for 30 min to produce a homogeneous solution with a SWNT concentration of 1 mg/mL. To prepare 1 wt% PI-BDA wrapped SWNTs in Epoxy L20, 50 mL SWNTs solution was added into 5 g Epoxy L20 and the mixture was vigorously stirred for 30 min. The DMF was removed by warming the epoxy/SWNTs in a vacuum oven at 80 oC overnight. The sticky PI-BDA wrapped SWNTs/Epoxy L20 was collected for composite preparation.

Composite fabrication About 30m CF-MWNTs cloth strip was wrapped into a coil on a roller spinning at 10 rpm with horizontal translation of 10 mm/min (Scheme 2). PI-BDA wrapped SWNTs/Epoxy L20 was applied to the CF-MWNTs during the winding process to hold the coil together after winding. Vacuum assisted resin transfer molding (VARTM) method was employed to prepare composite samples made with CFMWNTs (or CF for control) and Epoxy/SWNTs. The CF/CF-MWNTs coil was placed inside the vacuum bag, which consisted of several fabric layers including breath fabric and release cloth. The bag was sealed with sealant tape to prevent leakage of air and resin. The inlet was put into the Epoxy container and the outlet was connected to a vacuum pump through a buffer tank. The matrix precursor (epoxy or

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PI-BDA wrapped SWNTs/epoxy) was mixed with hardener for 5 min and transferred to the resin container. Then the vacuum pump was turned on and the pressure reached almost -0.1 MPa. The vacuum drove the resin flow into filler material and pressed both into the mold, after which the inlet and outlet were closed. After 24 h, the composite was removed from the vacuum bag and cured for 12h in a 60 oC oven.

Characterization The ILSSs of the fabricated composites were investigated by three-point bending test using a 5960 Dual Column Tabletop Testing System (Instron Company) according to ASTM

D2344.

The

specimen

dimensions

were

nominally

10.5 mm × 28.0 mm × 3.5 mm. Measurements were performed at room temperature with an indenter speed of 2 mm/min. Evidence of sample embrittlement or fiber fracture terminated each test. The values of ILSS for the composites were calculated from the relation: ‫= ܵܵܮܫ‬

ଷ௉೘ ସ௕௛

(1)

where Pm is the maximum compression load at fracture (N), b is the measured specimen width (mm) and h is the measured specimen thickness (mm). Eight specimens were measured for each composite formulation. The weight percentages of each component in CF-MWNTs/PI-BDA-wrappedSWNTs/Epoxy L20 composites were measured by TGA (TGA-Mettler Toledo TGA/DSC, Japan) at a heating rate of 20 °C min−1 from 30 °C to 800 °C under O2 atmosphere at a flow rate of 40 mL min−1. CF surfaces and composite fracture

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surfaces were observed by FE-SEM (JEOL, JSM-6701 FESEM, USA). The samples were sputter-coated with a thin conducting layer of gold before observation.

Supporting Information Section I: scheme of synthesis process, FESEM images of MWNT mat at different position of CF, loading force versus extension curves and dimensional data of each sample. Section II: the experiment and result of composite with hairy SWNT-graft-CF using electrodeposition method. The Supporting Information is available free of charge on the ACS Publications website.

Author Information Corresponding authors

*E-mail: [email protected] (M.B.C.P.) *E-mail: [email protected] (Y.W.) Author contributions

†The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Y.W. and S.K.R.P. contributed equally. Funding Sources

This work was financially supported by Temasek Laboratories @ NTU (9012103512). Notes

The authors declare no competing financial interest.

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Acknowledgements We thank Milo Shaffer (Imperial College) and Alexander Bismarck (Vienna University) for helpful discussions.

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(10) Qian, H.; Greenhalgh, E. S.; Shaffer, M. S. P.; Bismarck, A., Carbon NanotubeBased Hierarchical Composites: A Review. J. Mater. Chem. 2010, 20, 47514762. (11) Sharma, M.; Gao, S. L.; Mader, E.; Sharma, H.; Wei, L. Y.; Bijwe, J., Carbon Fiber Surfaces and Composite Interphases. Compos. Sci. Technol. 2014, 102, 3550. (12) Zhao, J. O.; Liu, L.; Guo, Q. G.; Shi, J. L.; Zhai, G. T.; Song, J. R.; Liu, Z. J., Growth of Carbon Nanotubes on the Surface of Carbon Fibers. Carbon 2008, 46, 380-383. (13) Fan, W. X.; Wang, Y. X.; Wang, C. G.; Chen, J. Q.; Wang, Q. F.; Yuan, Y.; Niu, F. X., High Efficient Preparation of Carbon Nanotube-Grafted Carbon Fibers with the Improved Tensile Strength. Appl. Surf. Sci. 2016, 364, 539-551. (14) Bekyarova, E.; Thostenson, E. T.; Yu, A.; Kim, H.; Gao, J.; Tang, J.; Hahn, H. T.; Chou, T. W.; Itkis, M. E.; Haddon, R. C., Multiscale Carbon Nanotube-Carbon Fiber Reinforcement for Advanced Epoxy Composites. Langmuir 2007, 23, 3970-3974. (15) Rong, H. P.; Dahmen, K. H.; Garmestani, H.; Yu, M. H.; Jacob, K. I., Comparison of Chemical Vapor Deposition and Chemical Grafting for Improving the Mechanical Properties of Carbon Fiber/Epoxy Composites with Multi-Wall Carbon Nanotubes. J. Mater. Sci. 2013, 48, 4834-4842. (16) Wu, G. S.; Ma, L. C.; Liu, L.; Wang, Y. W.; Xie, F.; Zhong, Z. X.; Zhao, M.; Jiang, B.; Huang, Y. D., Interfacially Reinforced Methylphenylsilicone Resin Composites by Chemically Grafting Multiwall Carbon Nanotubes onto Carbon Fibers. Compos. Part B-Eng. 2015, 82, 50-58. (17) Kepple, K. L.; Sanborn, G. P.; Lacasse, P. A.; Gruenberg, K. M.; Ready, W. J., Improved Fracture Toughness of Carbon Fiber Composite Functionalized with Multi Walled Carbon Nanotubes. Carbon 2008, 46, 2026-2033. (18) Chandrasekaran, V. C. S.; Advani, S. G.; Santare, M. H., Role of Processing on Interlaminar Shear Strength Enhancement of Epoxy/Glass Fiber/Multi-Walled Carbon Nanotube Hybrid Composites. Carbon 2010, 48, 3692-3699. (19) Fan, Z. H.; Santare, M. H.; Advani, S. G., Interlaminar Shear Strength of Glass Fiber Reinforced Epoxy Composites Enhanced with Multi-Walled Carbon Nanotubes. Compos. Part A-Appl. S. 2008, 39, 540-554. (20) Godara, A.; Mezzo, L.; Luizi, F.; Warrier, A.; Lomov, S. V.; van Vuure, A. W.; Gorbatikh, L.; Moldenaers, P.; Verpoest, I., Influence of Carbon Nanotube Reinforcement on the Processing and the Mechanical Behaviour of Carbon Fiber/Epoxy Composites. Carbon 2009, 47, 2914-2923. (21) Gonzalez-Dominguez, J. M.; Anson-Casaos, A.; Diez-Pascual, A. M.; Ashrafi, B.; Naffakh, M.; Backman, D.; Stadler, H.; Johnston, A.; Gomez, M.; Martinez, M. T., Solvent-Free Preparation of High-Toughness Epoxy-SWNT Composite Materials. ACS Appl. Mater. Inter. 2011, 3, 1441-1450. (22) Yuan, W.; Feng, J. L.; Judeh, Z.; Dai, J.; Chan-Park, M. B., Use of Polyimidegraft-Bisphenol A Diglyceryl Acrylate as a Reactive Noncovalent Dispersant of Single-Walled Carbon Nanotubes for Reinforcement of Cyanate Ester/Epoxy 19



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Composite. Chem. Mater. 2010, 22, 6542-6554.

Figure 1. FESEM images of bare CF (a) after desize by Soxhlet extraction and (b) hairy MWNT-graft-CF grown by CVD method.

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Figure 2. Raman spectrum of bare CF (black) and hairy MWNT-graft-CF using laser wavelength of 633 nm

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Figure 3. AFM image of PI-BDA wrapped SWNTs after dispersion in DMF using tip sonication. (a) AFM images of PI-BDA wrapped SWNTs which are deposited on SiO2 wafer. (b) The corresponding height curves of the individual PI-BDA wrapped SWNTs which are marked by blue and red cross.

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Figure 4. TGA curves of CF-MWNTs/ PI-BDA-wrapped-SWNTs composites which are crushed into powders. (Weight percentage of PI-BDA/wrapped SWNTs in matrix precursor of each composite is indicated in the inset.)

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Figure 5. (a) Strain-Stress curves and (b) ILSS of CF/Epoxy composite, hairy CNTgraft-CF/Epoxy composite and hairy CNT-graft-CF/Epoxy with various weight percentage PI-BDA-wrapped SWNTs in matrix composites.

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Figure 6. FESEM images of carbon/epoxy composite after ILSS testing. (a) Composite with “bare” CFs and epoxy. (b) Composite with hairy-CNTs-graft-CF and epoxy. (c) Composite with hairy-CNTs-graft-CF and PI-BDA-wrapped SWNTs epoxy. (Scale bar is 10 μm.)

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Scheme 1. Scheme of MWNT growth on carbon fiber (CF) using chemical vapor deposition (CVD) method.


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Scheme 2. Schematic process of CF-MWNTs/PI-BDA-wrapped-SWNTs/Epoxy composite



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High Interlaminar Shear Strength Enhancement of Carbon Fiber

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