Journal of Colloid and Interface Science 430 (2014) 61–68
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Self-healable interfaces based on thermo-reversible Diels–Alder reactions in carbon fiber reinforced composites W. Zhang, J. Duchet ⇑, J.F. Gérard UMR 5223 CNRS IMP, Université de Lyon, INSA Lyon, F 69621 Villeurbanne, France
a r t i c l e
i n f o
Article history: Received 27 March 2014 Accepted 9 May 2014 Available online 23 May 2014 Keywords: Self-healing Interphase Diels–Alder Carbon fiber Composite
a b s t r a c t Thermo-reversible Diels–Alder (DA) bonds formed between maleimide and furan groups have been used to generate an interphase between carbon fiber surface and an epoxy matrix leading to the ability of interfacial self-healing in carbon:epoxy composite materials. The maleimide groups were grafted on an untreated T700 carbon fiber from a three step surface treatment: (i) nitric acid oxidization, (ii) tetraethylenepentamine amination, and (iii) bismaleimide grafting. The furan groups were introduced in the reactive epoxy system from furfuryl glycidyl ether. The interface between untreated carbon fiber and epoxy matrix was considered as a reference. The interfacial shear strength (IFSS) was evaluated by single fiber micro-debonding test. The debonding force was shown to have a linear dependence with embedded length. The highest healing efficiency calculated from the debonding force was found to be about 82% more compared to the value for the reference interface. All the interphases designed with reversible DA bonds have a repeatable self-healing ability. As after the fourth healing, they can recover a relatively high healing efficiency (58% for the interphase formed by T700-BMI which is oxidized for 60 min during the first treatment step). Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction In a polymer composite material, the interfacial region, denoted as ‘interphase’ between reinforcing material (fiber or filler) and matrix is a key issue. Its main function is to sustain stress transfer from the polymer matrix to the reinforcing material and to ensure durability of composite material. In fact, stress concentration and micro-crack formation usually occur in this region and tend to propagate leading to failure of composite material. These cracks are usually unpredictable and difficult to be repaired. Therefore, avoiding or repairing these micro-cracks became a main objective in the field of composite materials. In 1996, Zhou [1] advised that the self-healing ability of living organs can be taken as reference in the design and processing of polymer composite materials. As a consequence, the self-healing within polymer composite materials was considered by many researchers. So far, two conceptions of self-healing can be taken into account for this type of materials. The first concept is based on the use, in the failure zone, of a healing agent that has the capacity to react with one component of the matrix or to fill the crack with the material resulting from its polymerization. The first route deals with the embedding of microcap⇑ Corresponding author. Fax: +33 4 72 44 85 27. E-mail address: [email protected] (J. Duchet). http://dx.doi.org/10.1016/j.jcis.2014.05.007 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
sules or hollow fibers into the polymer matrix, which are previously filled with a liquid healing agent, i.e. a monomer [2– 10]. When a micro-crack grows and crosses the hollow structures, the liquid healing agent is released and fills the crack. The released healing agent gets in touch with still unreacted functional groups available in the matrix and heals the crack. The limit of these healing filled objects is their unique capacity of micro-cracks healing at the same location. For this reason, Toohey [11] and Williams [12] proposed another new healing route based on a microvascular network, similar to human blood circulatory system, which is filled with a low viscosity healing agent. As micro-cracks are generated in the matrix, those ones extend until crossing the microvascular network. As a consequence, the healing agent will be delivered continuously into cracks through capillary driving force. For most of the proposed healing routes based on the reaction of an encapsulated or displayable liquid healing agent which could react to polymerize, the interaction with a catalyst well dispersed in the polymer matrix is required. Even though this system can heal micro-cracks several times at the same location, the healing efficiency will depend on the consumption of catalyst within the matrix. More recently, another concept of polymer matrix selfhealing was proposed. This one is based on the introduction of reversible bonds into polymer matrix architecture: (i) supramolecular bonds such as (multi-)hydrogen bonds [13] or host-guest
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structures as metal ligands [14] and (ii) thermo-reversible Diels– Alder (DA) bonds [15–21]. Thanks to the thermal reversible character of DA bonds, the micro-cracks can be healed as many times as required from a thermal heating and cooling cycle which allows to reform DA bonds. As, for composite materials, the interphase is a privileged place for stress and micro-crack formation [22], any of the methods mentioned above takes into account this interfacial region, but only the matrix. Only the work of Peterson et al. [23] has focused on the introduction thermal-reversible Diels–Alder adduct into the interphase of glass fiber-reinforced epoxy matrix composite materials. A healing efficiency of 41% could be achieved within the interphase. However, for the carbon fiber/epoxy composite, this concept should be a more difficult challenge because of the extremely inert carbon fiber surface with a relatively small diameter compared with glass fiber. In this paper, a new approach is proposed to introduce thermal reversible Diels–Alder bonds formed between maleimide groups grafted on carbon fiber surface and furan groups introduced within the epoxy–amine matrix in the interfacial region, i.e. in the interphase of carbon fiber reinforced composites. From such location of reversible bonds in the weakest part of the composite material, the carbon/epoxy interphases can display a self-healing ability compared to conventional interphases encountered in usual composite materials. The self-healing behavior is demonstrated by focusing on the fiber/matrix interface via micromechanical analysis using single fiber micro-droplet pull-out test [24].
2. Experimental
2.2. Carbon fiber surface treatment The graphitic structure of the T700-U surface is very inert and no functional groups are present for direct grafting. Thus, a surface treatment based on oxidization reactions was performed to render the fiber surface active from the generation of polar groups, such as carboxylic or carbonyl groups [26–28]. At the first step, T700-U was displayed in refluxed acetone for 1 h to remove surface impurities. An oxidization treatment using nitric acid HNO3 solution (concentration: 69 vol%) was carried out at 115 °C for 30, 60, and 90 min to increase the surface quantity of acidic functional groups. The fiber after oxidization will be denoted as T700-HNO3. After oxidization, the T700-HNO3 fiber was reacted with tetraethylenepentamine, TEPA, at 190 °C for 17 h to graft amino groups which are required for the further ‘Michael’ addition reaction with maleimides [29]. After the amination step, the fiber is denoted as T700-TEPA. Then, the T700-TEPA fiber was immerged into BMI:N,N-Dimethylformamide (DMF) solution at 80 °C for 2 h. For such a reaction, the two maleimide groups of BMI molecule have the same reactivity, i.e. the two groups may react at the same time (back-bitting). As maleimide groups are required for further coupling with reactive groups on the matrix side in order to obtain a maleimide-rich surface, saturated solution of BMI was used. After the different treatments, the fibers were washed with DMF for 2 h to remove the excess of unreacted BMI that is just physically absorbed on fiber surface. The last washing procedure consisted in a rinsing with acetone. In the final step, the washed fibers were dried at 80 °C for 24 h under vacuum. The maleimide-grafted carbon fibers are denoted as T700-BMI. The different steps of a fiber surface treatment are summarized in Fig. 1.
2.1. Materials 2.3. Fiber surface analysis Untreated and unsized ex-polyacrylonitrile (ex-PAN) based carbon fiber, denoted as T700-U, was kindly provided by TORAY Inc. Its average diameter is about 7 lm. The tensile strength and tensile modulus are 2.45 GPa and 125 GPa, respectively. All the reactants used in this study for surface treatment of carbon fibers, i.e. nitric acid (aqueous solution: 69 vol%), tetraethylenepentamine (TEPA, technique grade), 1,10 -(methylenedi-4,1-phenylene)bismaleimide (BMI, purity: 95%), and furfuryl glycidyl ether (FGE, purity: 96%) were purchased from Sigma–Aldrich. The epoxy–amine system that was used to design the epoxy matrix was based on a diglycidyl ether of bisphenol-A epoxy pre ¼ 0:15Þ (LY556, Huntsman), denoted as DGEBA and a polymer ðn cycloaliphatic diamine (isophorone diamine), purchased from Sigma–Aldrich, denoted as IPD. The curing of the DGEBA/IPD system was performed at 80 °C for 2 h followed by a post-curing at 160°C for 2 h. The glass transition temperature, Tg, of the resulting DGEBA-IPD epoxy–amine network is about 160 °C as expected from previous works [25]. In order to introduce furan groups into the epoxy matrix network architecture, furan epoxy monomer, FGE, was introduced into the reactive system. Thus, the furan-functionalized epoxy matrix was prepared by mixing DGEBA and FGE with a mass ratio of 6:4. The hardener, i.e. IPD, was added with a stoichiometric ratio, amino hydrogen-to-epoxy equals to 1. As this monomer is mono-functional (one epoxy group available for reaction with IPD), FGE acts as chain extender between crosslinks, i.e. contributes to decrease the Tg of the final network. The glass transition temperature of the DGEBA/FGE-IPD network was measured to be 63 °C (see Table 1). The furan-functionalization of the DGEBA-FGE/IPD polymer network was checked by Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy, ATR-FTIR, using a Nicolet iS10 Thermo Scientific spectrometer with a Ge crystal at room temperature. The spectra were recorded at 4 cm1 resolution, and 32 scans were collected for each sample.
X-ray photoelectron spectrometry (XPS) was used to characterize the functional groups created by oxidization on the carbon fiber surface. The XPS spectra were collected on an AXIS ULTRADLD (KRATOS ANALYTICAL) X-ray photoelectron spectrometer. Data were collected using Slot (300 700 lm) mode with the different pass energy of 160 eV (full range) and 20 eV (region). The X-ray source was Al monochromatic (1486.6 eV, 150 W) and the energy scale of the spectrometer was 0–1200 eV. The C1s binding energy (BE) of the graphitic peak was fixed at 284.6 eV for calibration. Atomic Force Microscopy (AFM) was used to analyze fiber surface topography at each treatment step. AFM surface patterns were acquired in air at room temperature using a Nanoscope IIIa Multimode (Digital Instruments/Brucker, CA). Tapping mode was performed at a scan rate of 1 Hz with uncoated silicon probes which have a resonance frequency between 280 and 405 kHz, a spring constant between 20 and 80 N/m, length between 115 and 135 lm, and width between 30 and 40 lm. The surface topography images and the average roughness were analyzed by Nanoscope 6.14R1 software. 2.4. Micromechanical analysis of interphase from micro-debonding test 2.4.1. Processing single fiber/droplet specimens The two ends of T700-BMI mono-fibers were stuck on a metallic frame using a commercial epoxy glue (ÒAraldite) after applying a pre-stress. After mixing the epoxy monomers (DGEBA and FGE) with amine comonomer, this reactive mixture was deposited using a thin copper wire to process micro-droplets of matrix on the single fibers (Fig. 2, left). Then, the epoxy–amine droplets were cured following the thermal cycle mentioned previously. As the viscosity of the reactive system is very low which is not suitable for depositing on single fibers, the epoxy–amine system was pre-cured at
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W. Zhang et al. / Journal of Colloid and Interface Science 430 (2014) 61–68 Table 1 Chemical formula of reagents used for surface grafting of carbon fibers and for epoxy–amine matrix synthesis. Reagent
Chemical formula H N
TEPA H2N
BMI
H N
N H
NH2 O
O
N
N
O
O
DGEBA
H C
H2C O
CH3 CH3
O
C H2
H C
OH
CH3 C O H2
C CH3
O
C H2
H C
CH2 O
H2N
IPD H3C
NH2 CH3
FGE
C
C O H2
O
CH3
O O
50 °C for 1 h to increase the viscosity before depositing the droplets. In addition, the Diels–Alder reaction is a thermo-reversible reaction which tends to form Diels–Alder adducts below 60 °C and separates into reactants (furan and maleimide) above 90 °C. Between them, it is an equilibrium reaction [23]. After curing at so high temperature, the most part of DA adducts are separated. If the temperature decreases to room temperature rapidly, these separated furan and maleimide groups will not reconnect in time (reto-Diels–Alder reaction) which will dramatically influence the interfacial shear strength. Thus, after curing, the samples were left at 60 °C for 2 h to reform DA adducts. Then, the single fibers were cut in segments about 5 mm in length, at the middle of which only one droplet was deposited. One end of the fiber segment was glued on a triangle shape paper support to be clamped easily on tensile machine (Fig 2, right). All the droplets were observed by optical microscopy and only axisymmetric droplets were kept for testing. For evaluating the self-healing ability, the initial carbon fiber T700U was also considered to prepare samples as reference interface.
2.4.2. Microdroplet single fiber pull-out test The debonding test was carried out on a tensile testing machine using a 10 N maximum load cell with a sensitivity of 0.001 N. The displacement rate and the data collection rate were 0.1 mm/min and 100 Hz respectively. The interphase was tested by clamping the micro-droplet between two razor blades (Fig. 3). The distance between the two blades can be adjusted for each of the microdroplets. During debonding test, the force was recorded as a function of displacement. When droplet debonding occurred, the tensile force decreased suddenly. Since the displacement speed was very slow, the maximum tensile force was considered as the force required for debonding. This force is denoted as F1max. After debonding, the droplet slid along the fiber and a frictional force was marked as f1. After the first debonding, the debonded micro-droplet samples were heated at 90 °C for 1 h and cooled down slowly in furnace until to the room temperature and left at room temperature for 24 h. This process was performed in order to reform the DA adducts in the interphase, i.e. in the fractured interfacial zone
Fig. 1. Protocol for grafting maleimide groups on T700-U carbon fiber surface.
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Fig. 2. Preparation of samples for micro-debonding test.
and to allow self-healing of the interphase. In the second cycle, these healed samples were tested again using the same micromechanical test to repeat above-mentioned debonding procedure. The debonding force and sliding friction during the second cycle were denoted as F2max and f2, respectively. Such a protocol of debonding via microdroplet test followed by a healing thermal treatment was repeated several times. Fig. 4 reports load–displacement trace during debonding tests repeated several times on a same specimen (the healing treatment was applied between each debonding test). For quantifying the efficiency of self-healing, denoted as g, the ratio between the force for debonding of the healed interphase, i.e. after the healing step of the epoxy microdroplet on carbon fiber, and the initial debonding force was evaluated by:
g¼
F ði þ 1Þmax F1max
ði ¼ 1; 2; 3 . . .Þ
3. Results and discussion 3.1. Interphase formation Fig. 5 shows the process of the formation of an interphase containing thermo-reversible bonds generated from the reaction of maleimide groups grafted on the carbon fiber surface and the furan groups from FGE which was combined with DGEBA to react with isophorone diamine for processing the epoxy–amine matrix. In fact, after curing, the most part of maleimide groups on fiber surface will be consumed to form DA adducts with FGE. As the ratio of FGE respect to DGEBA is high (DGEBA:FGE = 6:4 wt.%), many furan groups are left unreacted in the fiber/epoxy matrix interphase as well as in the bulk matrix. As debonding occurs from
Fig. 3. Schema of micro-droplet debonding test and equipment.
Fig. 4. Load–displacement curves of debonding tests performed on the same microdroplet prepared from DGEBA-FGE/IPD matrix on T700-BMI carbon fiber.
the microdroplet test, one can suppose that an interfacial crack is generated and the droplet slides to a new location on fiber surface. During the healing treatment, the unreacted furan groups and the ones generated from retro-DA reaction could react with the maleimide groups to reform DA reversible bonds in the fiber-matrix interfacial area. As a consequence, a new interphase between carbon fiber and epoxy–amine network is generated according to the formation of new DA adducts. 3.1.1. Characterization of epoxy network As mentioned previously, the introduction of furan groups within epoxy–amine networks leads to the increase in molar mass between crosslinks, i.e. decreasing the crosslink density. In fact, according to its functionality, FGE acts as chain extender and a strong decrease of the glass transition temperature, Tg, could be evidenced. The presence of furan groups is confirmed by ATR-FTIR as furanrelated absorption bands located at 1503, 1150, 1014, and 750 cm1 are evidenced for the DGEBA-FGE/IPD epoxy network (Fig. 6). 3.1.2. Fiber surface treatments 3.1.2.1. Efficiency of oxidization. Pittman [27,28] who used the same time of oxidization for the treatment of carbon fibers using HNO3 found that the quantity of surface acidic groups increased with oxidization time. However, this increase is not related to the increase in density of acidic groups per unit area, but to the increase in fiber surface area arising from corrosion effect of nitric acid. These authors observed that after 60 min, the corrosion speed increased significantly. One of the appropriate methods for evidencing the reactivity of carbon fibers after oxidization treatment is XPS. Fig. 7 reports the relative peak area of C1s binding energy (BE) at 288.8 eV, which relates to the presence of carboxyl groups. The amount of carboxyl groups increases with oxidization time, especially after 60 min in agreement with Pittman’s works [27,28]. As reported in the literature [27,28], the exposure to the nitric acid leads to a large corrosion effect which increases surface area. Fig. 8 reports the average roughness, Ra, as a function of oxidization time. Below 60 min of oxidization treatment based on HNO3, surface roughness increases linearly with oxidization time. However, the surface roughness decreases for 90 min. These results suggest that before 60 min of oxidization treatment, only the outer surface
W. Zhang et al. / Journal of Colloid and Interface Science 430 (2014) 61–68 O
O
N
o
N
O
NH 2
O
O N O O
NH 2 O
O O
O
FGE
+
IPD
N
DGEBA
O
O
O
65
O
N
O
O
O
O N
O
N O
O O
O
N
O O
O
O
O
O
O
Fig. 5. Formation of a thermo-reversible interphase: DA thermal reversible bonds, d irreversible covalent bonds and Van der Waals force.
Fig. 6. ATR-FTIR spectra of furan-functionalized epoxy (DGEBA-FGE/IPD) network and FGE monomer.
Fig. 8. Surface roughness of T700-HNO3 carbon fibers as a function of oxidization time measured by AFM.
Fig. 7. Relative area of the XPS peaks at binding energy 288.8 eV related to carboxylic groups as a function of the oxidization time.
Fig. 9. Fiber surface topography and functional groups as a function of oxidization time.
layer of carbon fiber is etched. While, for 90 min of oxidization treatment, a deeper surface layer was etched and interconnected sub-surface pores or internal voids and cracks can be created in the bulk. At the same time, the protrusion on the surface would be smoothened due to the corrosion effect, i.e. the surface roughness decreases and the acidic groups initially present on the surface are etched. The effect of the oxidization time on the surface topography can be schematically drawn in Fig. 9.
amination treatment from TEPA grafting smoothes the surface topography since the long chains of polyamine cover the fiber surface after grafting. In fact, according to TEPA amine functionality equal to 7, the molecular architecture of the resulting surface remains complex. Nevertheless, the ‘Michael’ addition reaction between bismaleimide and the primary and secondary amines leads to an increase in roughness which can be highlighted by AFM. This phenomenon confirms the grafting of bismaleimide component on carbon fiber surface as the formation of multi-layers.
3.1.2.2. Surface grafting of carbon fibers. AFM images reported in Fig. 10 display the fiber topography after each surface treatment step. After oxidization, the fiber surface became rougher (convex), due to the corrosion effect after HNO3 solution exposure. The
3.2. Interfacial debonding The interfacial adhesion in carbon fiber reinforced composites involves two types of intermolecular forces: (i) the primary forces
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Fig. 10. AFM surface profiles of carbon fiber surfaces after different surface treatments: quantification of surface roughness parameters.
Fig. 11. Interphase debonding and self-healing processes: d irreversible covalent bonds Van der Waals force.
that are covalent bonds displaying high binding energy providing from the reaction between the grafted functional groups on carbon fiber surface and the DGEBA prepolymer and (ii) the secondary forces that are physical bonds such as Van der Waals forces which exist all the time. The debonding of the micro-droplet will lead to the break of covalent bonds at the interface and/or in the interfacial region, i.e. in the interphase. For an interface that is prepared from a non-grafted carbon fiber which cannot lead to interphase containing reversible bonds, only sliding friction and Van der Waals forces can be recovered. Nevertheless, these forces are almost negligible and cannot be considered for further healing. On another hand, for interfaces considering thermally reversible DA bonds, the heating treatment performed after debonding will lead to a random recombination of furan and maleimide groups
to form a new interfacial region (Fig. 11), which would recover a large part of the interfacial debonding force. In the case of a BMI-treated carbon fiber and a furan functionalized epoxy network, i.e. for which an interphase containing reversible units is generated, the first debonding force appears to be proportional to the embedded length. The linear relationship between the debonding force for the first test and embedded length is displayed in Fig. 12. From the proposed model on the interphase generation, fracture mechanisms, and healing processes demonstrated above, the second debonding force should be directly proportional to embedded length. By plotting the force for the second debonding as a function of the embedded length, the two interfaces (with or without DA adducts) display very different behaviors as shown in
W. Zhang et al. / Journal of Colloid and Interface Science 430 (2014) 61–68
Fig. 12. Dependence of the debonding force for first debonding test with embedded length for the interface between T700-BMI carbon fiber and DGEBA-FGE/IPD epoxy matrix.
Fig. 13. Dependence of remaining debonding force with the embedded length: carbon fiber (T700U)/epoxy (6DGEBA/4FGE) interface, i.e. without DA adduct (square dots); carbon fiber (T700-BMI)/epoxy (6DGEBA/4FGE) interface, i.e. with DA adducts (circle dots).
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Fig. 15. Average self-healing efficiency after subtracting the friction force for interfaces based on DGEBA/FGE (6:4) epoxy matrix and T700-BMI carbon fibers prepared from T700-HN03 fibers oxidized at different times.
Fig. 16. Evolution of the self-healing efficiency as a function of debonding and healing cycles for DGEBA/FGE (6:4) epoxy matrix/T700-BMI carbon fiber (prepared from T700-HNO3 carbon fiber oxidized for 60 min).
with the embedded length compared to carbon/epoxy interface without DA bonds for which the required debonding force is very low and almost no longer depends on the embedded length. The very low values of debonding forces are mainly due to physical bonds, i.e. Van Der Waals force.
3.3. Efficiency of interphase self-healing
Fig. 14. Average self-healing efficiency of the interfaces based on DGEBA/FGE (6:4) epoxy matrix and T700-BMI carbon fibers prepared from T700-HN03 fibers oxidized at different times.
Fig. 13. The dependence with embedded length reflects the key role of Diels–Alder adducts which heal the interfacial fracture. For the interphase based on a DA adduct, the debonding force increases
Fig. 14 shows the results of average healing efficiency after the first healing treatment measured on four types of interfaces which differ from the oxidization time in HNO3 before grafting with TEPA and BMI. The healing efficiency of the DA modified interphases prepared from oxidized fibers is obviously higher compared to that without DA modified interphases generated from untreated carbon fibers. An optimum value is achieved by using T700-BMI carbon fibers which were oxidized during the first step for 60 min. The self-healing efficiency for interphases prepared from initially untreated fibers is 28%. This result cannot be attributed to a self-healing effect but to friction forces generated form sliding of the matrix droplet on the carbon fiber. Fig. 15 shows the average self-healing efficiency without taking account the friction force, i.e. subtracting the friction force for calculation. As a consequence, the self-healing efficiency of the system with untreated fiber is only about 8% which can be considered as negligible.
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Fig. 17. SEM images of fiber surface after 4 debonding-healing cycles: I, II, III, IV are the partial enlarged views of fiber surface after each debonding.
As shown previously, the carbon fiber oxidized in nitric acid for 90 min displays the higher quantity of surface functional groups. Nevertheless, the healing efficiency is not the highest for interfaces based on this BMI-treated carbon fiber. This phenomenon can be attributed to the fact that the acidic functional groups on this fiber surface are mainly located in pores, voids and cracks. These ‘hidden’ functional groups could not be used for further grafting reaction, thereby being useless for interfacial self-healing. For the interphase based on DGEBA/FGE (6:4) epoxy matrix and T700-BMI carbon fiber which was prepared from T700-HNO3 carbon fiber oxidized for 60 min, the self-healing can be repeated several times with a relatively high healing efficiency (Fig. 16). Park et al. [19] have mentioned that Diels–Alder bonds are weaker than other covalent bonds in a polymer backbone. They may be broken preferentially when loaded excessively. When the broken samples are heated, all the separated DA bonds will be reconnected. So, theoretically, these DA bonds present in the interphase can contribute to heal the interfacial fracture several times without any reduction in healing efficiency. But the results indicate that the healing efficiency is reduced step-by-step as the debonding-healing cycle number is increasing. It can be attributed to the reduction in furan groups on the side of matrix. During the debonding, friction, uneven forces and displacement rate etc. will lead to an asymmetrical separation of DA adducts. In this case, some of the furan groups will be left, in the form of DA adduct on fiber surface after extraction from the matrix (cohesive debonding). Near the newly formed interface (before healing), the concentration of residual furan groups which will further be used for reforming DA adducts is reduced after debonding (Fig. 11). Fig. 17 shows SEM images of the carbon fiber surface of one specimen after several debonding-healing cycles. Obviously, after each debonding, there are some pieces of epoxy matrix that remain on the fiber surface and that can contain furan groups. Until the fourth debonding, this cohesive fracture becomes unobvious. 4. Conclusion Diels–Alder adducts have been introduced into the interphase of carbon fiber reinforced epoxy composites allowing the interphase to have a thermally activated self-healing ability. The healing efficiency was evaluated by means of micro-droplet
debonding test, i.e. by proceeding to a thermal treatment after interface debonding. The highest healing efficiency (82%) was achieved for the interface based on a furan-modified epoxy matrix and a bismaleimide-grafted carbon fiber which was previously oxidized in HNO3 for 60 min, i.e. allowing introducing the higher concentration of DA adducts into interphase. The interphase of this system can be healed several times and maintains a relatively high healing efficiency. As the surface quantity of initial reactive groups for further grafting on the carbon fiber is a key parameter, future work will focus on new fiber surface treatment methods to graft more functional groups. In fact, the increase in the DA adducts quantity in interphase is an important issue to reach a higher healing efficiency. In addition, the amount of FGE in matrix strongly influences its thermal and mechanical properties as FGE acts as chain extender between crosslinks. In order to apply this concept to high Tg thermoset matrices, the introduction of furan groups will be studied in order to modify as low as possible the architecture of the matrices. The same concept will be also transferred to carbon nanotube based nano-composite materials as in this case the interphases will be emphasized due to the nanoscale of the carbon nanotubes. References [1] B.L. Zhou, Mater. Chem. Phys. 45 (2) (1996) 114–119. [2] M. Motuku, U.K. Vaidya, G.M. Janowski, Smart Mater. Struct. 8 (5) (1999) 623. [3] S.M. Bleay, C.B. Loader, V.J. Hawyes, L. Humberstone, P.T. Curtis, Composites 32 (12) (2001) 1767–1776. [4] E.N. Brown, M.R. Kessler, N.R. Sottos, S.R. White, J. Microencapsulation 20 (6) (2003) 719–730. [5] S.H. Cho, H.M. Andersson, S.R. White, N.R. Sottos, P.V. Braun, Adv. Mater. 18 (8) (2006) 997–1000. [6] M.R. Kessler, S.R. White, Composites 32 (5) (2001) 683–699. [7] J.D. Rule, N.R. Sottos, S.R. White, Polymer 48 (12) (2007) 3520. [8] T. Yin, M.Z. Rong, M.Q. Zhang, G.C. Yang, Compos. Sci. Technol. 67 (2) (2007) 201–212. [9] B.J. Blaiszik, N.R. Sottos, S.R. White, Compos. Sci. Technol. 68 (3–4) (2008) 978– 986. [10] Y.C. Yuan, M.Z. Rong, M.Q. Zhang, Polymer 49 (10) (2008) 2531–2541. [11] K.S. Toohey, N.R. Sottos, J.A. Lewis, J.S. Moore, S.R. White, Nat. Mater. 6 (2007) 581–585. [12] H.R. Williams, R.S. Trask, I.P. Bond, Smart Mater. Struct. 16 (4) (2007) 1198– 1207. [13] P. Cordier, F. Tournilhac, C. Soulie-Ziakovic, L. Leibler, Nature 451 (2008) 977– 980. [14] F.R. Kersey, D.M. Loveless, S.L. Craig, J. R. Soc. Interface 4 (13) (2007) 373–380. [15] X. Chen, F. Wudl, A.K. Mal, H. Shen, S.R. Nutt, Macromolecules 36 (6) (2003) 1802–1807. [16] A.P. Thomas, N.N. Sia, Acta Mater. 55 (2007) 5684–5696. [17] W. Ying, Self-healing polymers and composites based on thermal activation, in: Bolanos, F. Wudl, H. Thomas, K. Nathan, (Eds.), Proc. SPIE 2007, p. 6526. [18] Q. Tian, Y.C. Yuan, M.Z. Rong, M.Q. Zhang, J. Mater. Chem. 19 (2009) 1289– 1296. [19] J.S. Park, T. Darlington, A.F. Starr, K. Takahashi, J. Riendeau, H.T. Hahn, Compos. Sci. Technol. 70 (15) (2010) 2154–2159. [20] A.M. Peterson, R.E. Jensen, G.R. Palmese, ACS Appl. Mater. Interfaces 2 (4) (2010) 1141–1149. [21] N. Yoshie, M. Watanabe, H. Araki, K. Ishida, Polym. Degrad. Stabil. 95 (5) (2010) 826–829. [22] B. Chen, D. Dillard, J. Dillard, R. Clark, Int. J. Fract. 114 (2) (2002) 167–190. [23] A.M. Peterson, R.E. Jensen, G.R. Palmese, Compos. Sci. Technol. 71 (5) (2011) 586–592. [24] U. Gaur, B. Miller, Compos. Sci. Technol. 34 (1) (1989) 35–51. [25] V. Bellenger, W. Dahoui, J. Verdu, J. Galy, Y.G. Won, J.P. Pascault, Polymer 30 (1989). 2013-1018. [26] S.D. Gardner, C.S.K. Singamsetty, Z. Wu, C.U. Pittman Jr., Surf. Interface Anal. 24 (5) (1996) 311–320. [27] C.U. Pittman Jr., Z. Wu, W. Jiang, G.R. He, B. Wu, W. Li, S. Gardner, Carbon 35 (7) (1997) 929–943. [28] C.U. Pittman Jr., G.R. He, B. Wu, S. Gardner, Carbon 35 (3) (1997) 317–331. [29] J.L. Hopewell, G.A. George, D.J.T. Hill, Polymer 41 (23) (2000) 8221–8229.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Self-healable interfaces based on thermo-reversible Diels–Alder reactions in carbon fiber reinforced composites W. Zhang, J. Duchet ⇑, J.F. Gérard UMR 5223 CNRS IMP, Université de Lyon, INSA Lyon, F 69621 Villeurbanne, France
a r t i c l e
i n f o
Article history: Received 27 March 2014 Accepted 9 May 2014 Available online 23 May 2014 Keywords: Self-healing Interphase Diels–Alder Carbon fiber Composite
a b s t r a c t Thermo-reversible Diels–Alder (DA) bonds formed between maleimide and furan groups have been used to generate an interphase between carbon fiber surface and an epoxy matrix leading to the ability of interfacial self-healing in carbon:epoxy composite materials. The maleimide groups were grafted on an untreated T700 carbon fiber from a three step surface treatment: (i) nitric acid oxidization, (ii) tetraethylenepentamine amination, and (iii) bismaleimide grafting. The furan groups were introduced in the reactive epoxy system from furfuryl glycidyl ether. The interface between untreated carbon fiber and epoxy matrix was considered as a reference. The interfacial shear strength (IFSS) was evaluated by single fiber micro-debonding test. The debonding force was shown to have a linear dependence with embedded length. The highest healing efficiency calculated from the debonding force was found to be about 82% more compared to the value for the reference interface. All the interphases designed with reversible DA bonds have a repeatable self-healing ability. As after the fourth healing, they can recover a relatively high healing efficiency (58% for the interphase formed by T700-BMI which is oxidized for 60 min during the first treatment step). Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction In a polymer composite material, the interfacial region, denoted as ‘interphase’ between reinforcing material (fiber or filler) and matrix is a key issue. Its main function is to sustain stress transfer from the polymer matrix to the reinforcing material and to ensure durability of composite material. In fact, stress concentration and micro-crack formation usually occur in this region and tend to propagate leading to failure of composite material. These cracks are usually unpredictable and difficult to be repaired. Therefore, avoiding or repairing these micro-cracks became a main objective in the field of composite materials. In 1996, Zhou [1] advised that the self-healing ability of living organs can be taken as reference in the design and processing of polymer composite materials. As a consequence, the self-healing within polymer composite materials was considered by many researchers. So far, two conceptions of self-healing can be taken into account for this type of materials. The first concept is based on the use, in the failure zone, of a healing agent that has the capacity to react with one component of the matrix or to fill the crack with the material resulting from its polymerization. The first route deals with the embedding of microcap⇑ Corresponding author. Fax: +33 4 72 44 85 27. E-mail address: [email protected] (J. Duchet). http://dx.doi.org/10.1016/j.jcis.2014.05.007 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
sules or hollow fibers into the polymer matrix, which are previously filled with a liquid healing agent, i.e. a monomer [2– 10]. When a micro-crack grows and crosses the hollow structures, the liquid healing agent is released and fills the crack. The released healing agent gets in touch with still unreacted functional groups available in the matrix and heals the crack. The limit of these healing filled objects is their unique capacity of micro-cracks healing at the same location. For this reason, Toohey [11] and Williams [12] proposed another new healing route based on a microvascular network, similar to human blood circulatory system, which is filled with a low viscosity healing agent. As micro-cracks are generated in the matrix, those ones extend until crossing the microvascular network. As a consequence, the healing agent will be delivered continuously into cracks through capillary driving force. For most of the proposed healing routes based on the reaction of an encapsulated or displayable liquid healing agent which could react to polymerize, the interaction with a catalyst well dispersed in the polymer matrix is required. Even though this system can heal micro-cracks several times at the same location, the healing efficiency will depend on the consumption of catalyst within the matrix. More recently, another concept of polymer matrix selfhealing was proposed. This one is based on the introduction of reversible bonds into polymer matrix architecture: (i) supramolecular bonds such as (multi-)hydrogen bonds [13] or host-guest
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structures as metal ligands [14] and (ii) thermo-reversible Diels– Alder (DA) bonds [15–21]. Thanks to the thermal reversible character of DA bonds, the micro-cracks can be healed as many times as required from a thermal heating and cooling cycle which allows to reform DA bonds. As, for composite materials, the interphase is a privileged place for stress and micro-crack formation [22], any of the methods mentioned above takes into account this interfacial region, but only the matrix. Only the work of Peterson et al. [23] has focused on the introduction thermal-reversible Diels–Alder adduct into the interphase of glass fiber-reinforced epoxy matrix composite materials. A healing efficiency of 41% could be achieved within the interphase. However, for the carbon fiber/epoxy composite, this concept should be a more difficult challenge because of the extremely inert carbon fiber surface with a relatively small diameter compared with glass fiber. In this paper, a new approach is proposed to introduce thermal reversible Diels–Alder bonds formed between maleimide groups grafted on carbon fiber surface and furan groups introduced within the epoxy–amine matrix in the interfacial region, i.e. in the interphase of carbon fiber reinforced composites. From such location of reversible bonds in the weakest part of the composite material, the carbon/epoxy interphases can display a self-healing ability compared to conventional interphases encountered in usual composite materials. The self-healing behavior is demonstrated by focusing on the fiber/matrix interface via micromechanical analysis using single fiber micro-droplet pull-out test [24].
2. Experimental
2.2. Carbon fiber surface treatment The graphitic structure of the T700-U surface is very inert and no functional groups are present for direct grafting. Thus, a surface treatment based on oxidization reactions was performed to render the fiber surface active from the generation of polar groups, such as carboxylic or carbonyl groups [26–28]. At the first step, T700-U was displayed in refluxed acetone for 1 h to remove surface impurities. An oxidization treatment using nitric acid HNO3 solution (concentration: 69 vol%) was carried out at 115 °C for 30, 60, and 90 min to increase the surface quantity of acidic functional groups. The fiber after oxidization will be denoted as T700-HNO3. After oxidization, the T700-HNO3 fiber was reacted with tetraethylenepentamine, TEPA, at 190 °C for 17 h to graft amino groups which are required for the further ‘Michael’ addition reaction with maleimides [29]. After the amination step, the fiber is denoted as T700-TEPA. Then, the T700-TEPA fiber was immerged into BMI:N,N-Dimethylformamide (DMF) solution at 80 °C for 2 h. For such a reaction, the two maleimide groups of BMI molecule have the same reactivity, i.e. the two groups may react at the same time (back-bitting). As maleimide groups are required for further coupling with reactive groups on the matrix side in order to obtain a maleimide-rich surface, saturated solution of BMI was used. After the different treatments, the fibers were washed with DMF for 2 h to remove the excess of unreacted BMI that is just physically absorbed on fiber surface. The last washing procedure consisted in a rinsing with acetone. In the final step, the washed fibers were dried at 80 °C for 24 h under vacuum. The maleimide-grafted carbon fibers are denoted as T700-BMI. The different steps of a fiber surface treatment are summarized in Fig. 1.
2.1. Materials 2.3. Fiber surface analysis Untreated and unsized ex-polyacrylonitrile (ex-PAN) based carbon fiber, denoted as T700-U, was kindly provided by TORAY Inc. Its average diameter is about 7 lm. The tensile strength and tensile modulus are 2.45 GPa and 125 GPa, respectively. All the reactants used in this study for surface treatment of carbon fibers, i.e. nitric acid (aqueous solution: 69 vol%), tetraethylenepentamine (TEPA, technique grade), 1,10 -(methylenedi-4,1-phenylene)bismaleimide (BMI, purity: 95%), and furfuryl glycidyl ether (FGE, purity: 96%) were purchased from Sigma–Aldrich. The epoxy–amine system that was used to design the epoxy matrix was based on a diglycidyl ether of bisphenol-A epoxy pre ¼ 0:15Þ (LY556, Huntsman), denoted as DGEBA and a polymer ðn cycloaliphatic diamine (isophorone diamine), purchased from Sigma–Aldrich, denoted as IPD. The curing of the DGEBA/IPD system was performed at 80 °C for 2 h followed by a post-curing at 160°C for 2 h. The glass transition temperature, Tg, of the resulting DGEBA-IPD epoxy–amine network is about 160 °C as expected from previous works [25]. In order to introduce furan groups into the epoxy matrix network architecture, furan epoxy monomer, FGE, was introduced into the reactive system. Thus, the furan-functionalized epoxy matrix was prepared by mixing DGEBA and FGE with a mass ratio of 6:4. The hardener, i.e. IPD, was added with a stoichiometric ratio, amino hydrogen-to-epoxy equals to 1. As this monomer is mono-functional (one epoxy group available for reaction with IPD), FGE acts as chain extender between crosslinks, i.e. contributes to decrease the Tg of the final network. The glass transition temperature of the DGEBA/FGE-IPD network was measured to be 63 °C (see Table 1). The furan-functionalization of the DGEBA-FGE/IPD polymer network was checked by Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy, ATR-FTIR, using a Nicolet iS10 Thermo Scientific spectrometer with a Ge crystal at room temperature. The spectra were recorded at 4 cm1 resolution, and 32 scans were collected for each sample.
X-ray photoelectron spectrometry (XPS) was used to characterize the functional groups created by oxidization on the carbon fiber surface. The XPS spectra were collected on an AXIS ULTRADLD (KRATOS ANALYTICAL) X-ray photoelectron spectrometer. Data were collected using Slot (300 700 lm) mode with the different pass energy of 160 eV (full range) and 20 eV (region). The X-ray source was Al monochromatic (1486.6 eV, 150 W) and the energy scale of the spectrometer was 0–1200 eV. The C1s binding energy (BE) of the graphitic peak was fixed at 284.6 eV for calibration. Atomic Force Microscopy (AFM) was used to analyze fiber surface topography at each treatment step. AFM surface patterns were acquired in air at room temperature using a Nanoscope IIIa Multimode (Digital Instruments/Brucker, CA). Tapping mode was performed at a scan rate of 1 Hz with uncoated silicon probes which have a resonance frequency between 280 and 405 kHz, a spring constant between 20 and 80 N/m, length between 115 and 135 lm, and width between 30 and 40 lm. The surface topography images and the average roughness were analyzed by Nanoscope 6.14R1 software. 2.4. Micromechanical analysis of interphase from micro-debonding test 2.4.1. Processing single fiber/droplet specimens The two ends of T700-BMI mono-fibers were stuck on a metallic frame using a commercial epoxy glue (ÒAraldite) after applying a pre-stress. After mixing the epoxy monomers (DGEBA and FGE) with amine comonomer, this reactive mixture was deposited using a thin copper wire to process micro-droplets of matrix on the single fibers (Fig. 2, left). Then, the epoxy–amine droplets were cured following the thermal cycle mentioned previously. As the viscosity of the reactive system is very low which is not suitable for depositing on single fibers, the epoxy–amine system was pre-cured at
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W. Zhang et al. / Journal of Colloid and Interface Science 430 (2014) 61–68 Table 1 Chemical formula of reagents used for surface grafting of carbon fibers and for epoxy–amine matrix synthesis. Reagent
Chemical formula H N
TEPA H2N
BMI
H N
N H
NH2 O
O
N
N
O
O
DGEBA
H C
H2C O
CH3 CH3
O
C H2
H C
OH
CH3 C O H2
C CH3
O
C H2
H C
CH2 O
H2N
IPD H3C
NH2 CH3
FGE
C
C O H2
O
CH3
O O
50 °C for 1 h to increase the viscosity before depositing the droplets. In addition, the Diels–Alder reaction is a thermo-reversible reaction which tends to form Diels–Alder adducts below 60 °C and separates into reactants (furan and maleimide) above 90 °C. Between them, it is an equilibrium reaction [23]. After curing at so high temperature, the most part of DA adducts are separated. If the temperature decreases to room temperature rapidly, these separated furan and maleimide groups will not reconnect in time (reto-Diels–Alder reaction) which will dramatically influence the interfacial shear strength. Thus, after curing, the samples were left at 60 °C for 2 h to reform DA adducts. Then, the single fibers were cut in segments about 5 mm in length, at the middle of which only one droplet was deposited. One end of the fiber segment was glued on a triangle shape paper support to be clamped easily on tensile machine (Fig 2, right). All the droplets were observed by optical microscopy and only axisymmetric droplets were kept for testing. For evaluating the self-healing ability, the initial carbon fiber T700U was also considered to prepare samples as reference interface.
2.4.2. Microdroplet single fiber pull-out test The debonding test was carried out on a tensile testing machine using a 10 N maximum load cell with a sensitivity of 0.001 N. The displacement rate and the data collection rate were 0.1 mm/min and 100 Hz respectively. The interphase was tested by clamping the micro-droplet between two razor blades (Fig. 3). The distance between the two blades can be adjusted for each of the microdroplets. During debonding test, the force was recorded as a function of displacement. When droplet debonding occurred, the tensile force decreased suddenly. Since the displacement speed was very slow, the maximum tensile force was considered as the force required for debonding. This force is denoted as F1max. After debonding, the droplet slid along the fiber and a frictional force was marked as f1. After the first debonding, the debonded micro-droplet samples were heated at 90 °C for 1 h and cooled down slowly in furnace until to the room temperature and left at room temperature for 24 h. This process was performed in order to reform the DA adducts in the interphase, i.e. in the fractured interfacial zone
Fig. 1. Protocol for grafting maleimide groups on T700-U carbon fiber surface.
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Fig. 2. Preparation of samples for micro-debonding test.
and to allow self-healing of the interphase. In the second cycle, these healed samples were tested again using the same micromechanical test to repeat above-mentioned debonding procedure. The debonding force and sliding friction during the second cycle were denoted as F2max and f2, respectively. Such a protocol of debonding via microdroplet test followed by a healing thermal treatment was repeated several times. Fig. 4 reports load–displacement trace during debonding tests repeated several times on a same specimen (the healing treatment was applied between each debonding test). For quantifying the efficiency of self-healing, denoted as g, the ratio between the force for debonding of the healed interphase, i.e. after the healing step of the epoxy microdroplet on carbon fiber, and the initial debonding force was evaluated by:
g¼
F ði þ 1Þmax F1max
ði ¼ 1; 2; 3 . . .Þ
3. Results and discussion 3.1. Interphase formation Fig. 5 shows the process of the formation of an interphase containing thermo-reversible bonds generated from the reaction of maleimide groups grafted on the carbon fiber surface and the furan groups from FGE which was combined with DGEBA to react with isophorone diamine for processing the epoxy–amine matrix. In fact, after curing, the most part of maleimide groups on fiber surface will be consumed to form DA adducts with FGE. As the ratio of FGE respect to DGEBA is high (DGEBA:FGE = 6:4 wt.%), many furan groups are left unreacted in the fiber/epoxy matrix interphase as well as in the bulk matrix. As debonding occurs from
Fig. 3. Schema of micro-droplet debonding test and equipment.
Fig. 4. Load–displacement curves of debonding tests performed on the same microdroplet prepared from DGEBA-FGE/IPD matrix on T700-BMI carbon fiber.
the microdroplet test, one can suppose that an interfacial crack is generated and the droplet slides to a new location on fiber surface. During the healing treatment, the unreacted furan groups and the ones generated from retro-DA reaction could react with the maleimide groups to reform DA reversible bonds in the fiber-matrix interfacial area. As a consequence, a new interphase between carbon fiber and epoxy–amine network is generated according to the formation of new DA adducts. 3.1.1. Characterization of epoxy network As mentioned previously, the introduction of furan groups within epoxy–amine networks leads to the increase in molar mass between crosslinks, i.e. decreasing the crosslink density. In fact, according to its functionality, FGE acts as chain extender and a strong decrease of the glass transition temperature, Tg, could be evidenced. The presence of furan groups is confirmed by ATR-FTIR as furanrelated absorption bands located at 1503, 1150, 1014, and 750 cm1 are evidenced for the DGEBA-FGE/IPD epoxy network (Fig. 6). 3.1.2. Fiber surface treatments 3.1.2.1. Efficiency of oxidization. Pittman [27,28] who used the same time of oxidization for the treatment of carbon fibers using HNO3 found that the quantity of surface acidic groups increased with oxidization time. However, this increase is not related to the increase in density of acidic groups per unit area, but to the increase in fiber surface area arising from corrosion effect of nitric acid. These authors observed that after 60 min, the corrosion speed increased significantly. One of the appropriate methods for evidencing the reactivity of carbon fibers after oxidization treatment is XPS. Fig. 7 reports the relative peak area of C1s binding energy (BE) at 288.8 eV, which relates to the presence of carboxyl groups. The amount of carboxyl groups increases with oxidization time, especially after 60 min in agreement with Pittman’s works [27,28]. As reported in the literature [27,28], the exposure to the nitric acid leads to a large corrosion effect which increases surface area. Fig. 8 reports the average roughness, Ra, as a function of oxidization time. Below 60 min of oxidization treatment based on HNO3, surface roughness increases linearly with oxidization time. However, the surface roughness decreases for 90 min. These results suggest that before 60 min of oxidization treatment, only the outer surface
W. Zhang et al. / Journal of Colloid and Interface Science 430 (2014) 61–68 O
O
N
o
N
O
NH 2
O
O N O O
NH 2 O
O O
O
FGE
+
IPD
N
DGEBA
O
O
O
65
O
N
O
O
O
O N
O
N O
O O
O
N
O O
O
O
O
O
O
Fig. 5. Formation of a thermo-reversible interphase: DA thermal reversible bonds, d irreversible covalent bonds and Van der Waals force.
Fig. 6. ATR-FTIR spectra of furan-functionalized epoxy (DGEBA-FGE/IPD) network and FGE monomer.
Fig. 8. Surface roughness of T700-HNO3 carbon fibers as a function of oxidization time measured by AFM.
Fig. 7. Relative area of the XPS peaks at binding energy 288.8 eV related to carboxylic groups as a function of the oxidization time.
Fig. 9. Fiber surface topography and functional groups as a function of oxidization time.
layer of carbon fiber is etched. While, for 90 min of oxidization treatment, a deeper surface layer was etched and interconnected sub-surface pores or internal voids and cracks can be created in the bulk. At the same time, the protrusion on the surface would be smoothened due to the corrosion effect, i.e. the surface roughness decreases and the acidic groups initially present on the surface are etched. The effect of the oxidization time on the surface topography can be schematically drawn in Fig. 9.
amination treatment from TEPA grafting smoothes the surface topography since the long chains of polyamine cover the fiber surface after grafting. In fact, according to TEPA amine functionality equal to 7, the molecular architecture of the resulting surface remains complex. Nevertheless, the ‘Michael’ addition reaction between bismaleimide and the primary and secondary amines leads to an increase in roughness which can be highlighted by AFM. This phenomenon confirms the grafting of bismaleimide component on carbon fiber surface as the formation of multi-layers.
3.1.2.2. Surface grafting of carbon fibers. AFM images reported in Fig. 10 display the fiber topography after each surface treatment step. After oxidization, the fiber surface became rougher (convex), due to the corrosion effect after HNO3 solution exposure. The
3.2. Interfacial debonding The interfacial adhesion in carbon fiber reinforced composites involves two types of intermolecular forces: (i) the primary forces
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Fig. 10. AFM surface profiles of carbon fiber surfaces after different surface treatments: quantification of surface roughness parameters.
Fig. 11. Interphase debonding and self-healing processes: d irreversible covalent bonds Van der Waals force.
that are covalent bonds displaying high binding energy providing from the reaction between the grafted functional groups on carbon fiber surface and the DGEBA prepolymer and (ii) the secondary forces that are physical bonds such as Van der Waals forces which exist all the time. The debonding of the micro-droplet will lead to the break of covalent bonds at the interface and/or in the interfacial region, i.e. in the interphase. For an interface that is prepared from a non-grafted carbon fiber which cannot lead to interphase containing reversible bonds, only sliding friction and Van der Waals forces can be recovered. Nevertheless, these forces are almost negligible and cannot be considered for further healing. On another hand, for interfaces considering thermally reversible DA bonds, the heating treatment performed after debonding will lead to a random recombination of furan and maleimide groups
to form a new interfacial region (Fig. 11), which would recover a large part of the interfacial debonding force. In the case of a BMI-treated carbon fiber and a furan functionalized epoxy network, i.e. for which an interphase containing reversible units is generated, the first debonding force appears to be proportional to the embedded length. The linear relationship between the debonding force for the first test and embedded length is displayed in Fig. 12. From the proposed model on the interphase generation, fracture mechanisms, and healing processes demonstrated above, the second debonding force should be directly proportional to embedded length. By plotting the force for the second debonding as a function of the embedded length, the two interfaces (with or without DA adducts) display very different behaviors as shown in
W. Zhang et al. / Journal of Colloid and Interface Science 430 (2014) 61–68
Fig. 12. Dependence of the debonding force for first debonding test with embedded length for the interface between T700-BMI carbon fiber and DGEBA-FGE/IPD epoxy matrix.
Fig. 13. Dependence of remaining debonding force with the embedded length: carbon fiber (T700U)/epoxy (6DGEBA/4FGE) interface, i.e. without DA adduct (square dots); carbon fiber (T700-BMI)/epoxy (6DGEBA/4FGE) interface, i.e. with DA adducts (circle dots).
67
Fig. 15. Average self-healing efficiency after subtracting the friction force for interfaces based on DGEBA/FGE (6:4) epoxy matrix and T700-BMI carbon fibers prepared from T700-HN03 fibers oxidized at different times.
Fig. 16. Evolution of the self-healing efficiency as a function of debonding and healing cycles for DGEBA/FGE (6:4) epoxy matrix/T700-BMI carbon fiber (prepared from T700-HNO3 carbon fiber oxidized for 60 min).
with the embedded length compared to carbon/epoxy interface without DA bonds for which the required debonding force is very low and almost no longer depends on the embedded length. The very low values of debonding forces are mainly due to physical bonds, i.e. Van Der Waals force.
3.3. Efficiency of interphase self-healing
Fig. 14. Average self-healing efficiency of the interfaces based on DGEBA/FGE (6:4) epoxy matrix and T700-BMI carbon fibers prepared from T700-HN03 fibers oxidized at different times.
Fig. 13. The dependence with embedded length reflects the key role of Diels–Alder adducts which heal the interfacial fracture. For the interphase based on a DA adduct, the debonding force increases
Fig. 14 shows the results of average healing efficiency after the first healing treatment measured on four types of interfaces which differ from the oxidization time in HNO3 before grafting with TEPA and BMI. The healing efficiency of the DA modified interphases prepared from oxidized fibers is obviously higher compared to that without DA modified interphases generated from untreated carbon fibers. An optimum value is achieved by using T700-BMI carbon fibers which were oxidized during the first step for 60 min. The self-healing efficiency for interphases prepared from initially untreated fibers is 28%. This result cannot be attributed to a self-healing effect but to friction forces generated form sliding of the matrix droplet on the carbon fiber. Fig. 15 shows the average self-healing efficiency without taking account the friction force, i.e. subtracting the friction force for calculation. As a consequence, the self-healing efficiency of the system with untreated fiber is only about 8% which can be considered as negligible.
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Fig. 17. SEM images of fiber surface after 4 debonding-healing cycles: I, II, III, IV are the partial enlarged views of fiber surface after each debonding.
As shown previously, the carbon fiber oxidized in nitric acid for 90 min displays the higher quantity of surface functional groups. Nevertheless, the healing efficiency is not the highest for interfaces based on this BMI-treated carbon fiber. This phenomenon can be attributed to the fact that the acidic functional groups on this fiber surface are mainly located in pores, voids and cracks. These ‘hidden’ functional groups could not be used for further grafting reaction, thereby being useless for interfacial self-healing. For the interphase based on DGEBA/FGE (6:4) epoxy matrix and T700-BMI carbon fiber which was prepared from T700-HNO3 carbon fiber oxidized for 60 min, the self-healing can be repeated several times with a relatively high healing efficiency (Fig. 16). Park et al. [19] have mentioned that Diels–Alder bonds are weaker than other covalent bonds in a polymer backbone. They may be broken preferentially when loaded excessively. When the broken samples are heated, all the separated DA bonds will be reconnected. So, theoretically, these DA bonds present in the interphase can contribute to heal the interfacial fracture several times without any reduction in healing efficiency. But the results indicate that the healing efficiency is reduced step-by-step as the debonding-healing cycle number is increasing. It can be attributed to the reduction in furan groups on the side of matrix. During the debonding, friction, uneven forces and displacement rate etc. will lead to an asymmetrical separation of DA adducts. In this case, some of the furan groups will be left, in the form of DA adduct on fiber surface after extraction from the matrix (cohesive debonding). Near the newly formed interface (before healing), the concentration of residual furan groups which will further be used for reforming DA adducts is reduced after debonding (Fig. 11). Fig. 17 shows SEM images of the carbon fiber surface of one specimen after several debonding-healing cycles. Obviously, after each debonding, there are some pieces of epoxy matrix that remain on the fiber surface and that can contain furan groups. Until the fourth debonding, this cohesive fracture becomes unobvious. 4. Conclusion Diels–Alder adducts have been introduced into the interphase of carbon fiber reinforced epoxy composites allowing the interphase to have a thermally activated self-healing ability. The healing efficiency was evaluated by means of micro-droplet
debonding test, i.e. by proceeding to a thermal treatment after interface debonding. The highest healing efficiency (82%) was achieved for the interface based on a furan-modified epoxy matrix and a bismaleimide-grafted carbon fiber which was previously oxidized in HNO3 for 60 min, i.e. allowing introducing the higher concentration of DA adducts into interphase. The interphase of this system can be healed several times and maintains a relatively high healing efficiency. As the surface quantity of initial reactive groups for further grafting on the carbon fiber is a key parameter, future work will focus on new fiber surface treatment methods to graft more functional groups. In fact, the increase in the DA adducts quantity in interphase is an important issue to reach a higher healing efficiency. In addition, the amount of FGE in matrix strongly influences its thermal and mechanical properties as FGE acts as chain extender between crosslinks. In order to apply this concept to high Tg thermoset matrices, the introduction of furan groups will be studied in order to modify as low as possible the architecture of the matrices. The same concept will be also transferred to carbon nanotube based nano-composite materials as in this case the interphases will be emphasized due to the nanoscale of the carbon nanotubes. References [1] B.L. Zhou, Mater. Chem. Phys. 45 (2) (1996) 114–119. [2] M. Motuku, U.K. Vaidya, G.M. Janowski, Smart Mater. Struct. 8 (5) (1999) 623. [3] S.M. Bleay, C.B. Loader, V.J. Hawyes, L. Humberstone, P.T. Curtis, Composites 32 (12) (2001) 1767–1776. [4] E.N. Brown, M.R. Kessler, N.R. Sottos, S.R. White, J. Microencapsulation 20 (6) (2003) 719–730. [5] S.H. Cho, H.M. Andersson, S.R. White, N.R. Sottos, P.V. Braun, Adv. Mater. 18 (8) (2006) 997–1000. [6] M.R. Kessler, S.R. White, Composites 32 (5) (2001) 683–699. [7] J.D. Rule, N.R. Sottos, S.R. White, Polymer 48 (12) (2007) 3520. [8] T. Yin, M.Z. Rong, M.Q. Zhang, G.C. Yang, Compos. Sci. Technol. 67 (2) (2007) 201–212. [9] B.J. Blaiszik, N.R. Sottos, S.R. White, Compos. Sci. Technol. 68 (3–4) (2008) 978– 986. [10] Y.C. Yuan, M.Z. Rong, M.Q. Zhang, Polymer 49 (10) (2008) 2531–2541. [11] K.S. Toohey, N.R. Sottos, J.A. Lewis, J.S. Moore, S.R. White, Nat. Mater. 6 (2007) 581–585. [12] H.R. Williams, R.S. Trask, I.P. Bond, Smart Mater. Struct. 16 (4) (2007) 1198– 1207. [13] P. Cordier, F. Tournilhac, C. Soulie-Ziakovic, L. Leibler, Nature 451 (2008) 977– 980. [14] F.R. Kersey, D.M. Loveless, S.L. Craig, J. R. Soc. Interface 4 (13) (2007) 373–380. [15] X. Chen, F. Wudl, A.K. Mal, H. Shen, S.R. Nutt, Macromolecules 36 (6) (2003) 1802–1807. [16] A.P. Thomas, N.N. Sia, Acta Mater. 55 (2007) 5684–5696. [17] W. Ying, Self-healing polymers and composites based on thermal activation, in: Bolanos, F. Wudl, H. Thomas, K. Nathan, (Eds.), Proc. SPIE 2007, p. 6526. [18] Q. Tian, Y.C. Yuan, M.Z. Rong, M.Q. Zhang, J. Mater. Chem. 19 (2009) 1289– 1296. [19] J.S. Park, T. Darlington, A.F. Starr, K. Takahashi, J. Riendeau, H.T. Hahn, Compos. Sci. Technol. 70 (15) (2010) 2154–2159. [20] A.M. Peterson, R.E. Jensen, G.R. Palmese, ACS Appl. Mater. Interfaces 2 (4) (2010) 1141–1149. [21] N. Yoshie, M. Watanabe, H. Araki, K. Ishida, Polym. Degrad. Stabil. 95 (5) (2010) 826–829. [22] B. Chen, D. Dillard, J. Dillard, R. Clark, Int. J. Fract. 114 (2) (2002) 167–190. [23] A.M. Peterson, R.E. Jensen, G.R. Palmese, Compos. Sci. Technol. 71 (5) (2011) 586–592. [24] U. Gaur, B. Miller, Compos. Sci. Technol. 34 (1) (1989) 35–51. [25] V. Bellenger, W. Dahoui, J. Verdu, J. Galy, Y.G. Won, J.P. Pascault, Polymer 30 (1989). 2013-1018. [26] S.D. Gardner, C.S.K. Singamsetty, Z. Wu, C.U. Pittman Jr., Surf. Interface Anal. 24 (5) (1996) 311–320. [27] C.U. Pittman Jr., Z. Wu, W. Jiang, G.R. He, B. Wu, W. Li, S. Gardner, Carbon 35 (7) (1997) 929–943. [28] C.U. Pittman Jr., G.R. He, B. Wu, S. Gardner, Carbon 35 (3) (1997) 317–331. [29] J.L. Hopewell, G.A. George, D.J.T. Hill, Polymer 41 (23) (2000) 8221–8229.
