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A novel strategy for the synthesis of self-healing capsule and its application
Accepted Manuscript A novel strategy for the synthesis of self-healing capsule and its application Tao Sun, Xuejing Shen, Chong Peng, Hongyu Fan, Minjing Liu, Zhanjun Wu PII:
S0266-3538(18)32132-8
DOI:
https://doi.org/10.1016/j.compscitech.2018.12.006
Reference:
CSTE 7497
To appear in:
Composites Science and Technology
Received Date: 3 September 2018 Revised Date:
3 December 2018
Accepted Date: 6 December 2018
Please cite this article as: Sun T, Shen X, Peng C, Fan H, Liu M, Wu Z, A novel strategy for the synthesis of self-healing capsule and its application, Composites Science and Technology (2019), doi: https://doi.org/10.1016/j.compscitech.2018.12.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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A novel strategy for the synthesis of self-healing capsule and its application
a
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Tao Sun a, Xuejing Shen a, Chong Peng,a Hongyu Fan b∗, Minjing Liu a, Zhanjun Wu a∗
School of Aeronautics and Astronautics, State Key Laboratory of Structural Analysis for
Industrial Equipment, Dalian University of Technology, Dalian 116024, PR China
School of Physics and Materials Engineering, Dalian Nationalities University, Dalian 116600,
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b
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PR China
Abstract: Phenol formaldehyde (PF) resin capsules containing dicyclopentadiene (DCPD) as core materials are rationally designed and fabricated. The synthesis consists of preparation of polystyrene (PS) sphere, PF coating on PS sphere, followed by removal of PS core, amination modification and importing of DCPD. Solution phase switchable transport trough PF shell layer
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is key for the synthesis of DCDP@PF capsules. The resultant DCDP@PF capsules have a diameter of ~500 nm, shell thickness of ~50 nm, and core content of ~45 wt%. The results show that DCDP@PF capsules have outstanding thermal stability with initial evaporation temperature
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(defined at 5% of weight loss), increased by ~30 oC compared with that of pure DCPD, and good
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resistance to acetone. Preliminary results indicated that the prepared DCPD@PF capsules can effectivelly improve the mechanical properties of epoxy matrix as well as impart it self-healing properties. When 15 wt% DCPD@PF capsules were inttrouducd into epoxy matrix, 81.4% incresement in fracture toughness, 30.8% incresment in tensiles strength and 91.8% recovery in fracture toughness can be obtained. This work provides a new insight into the investigation of the fabraction of self-healing capsules.
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Keywords: A. Functional composites; B. Mechanical properties; Self-healing capsules; A. Polymer-matrix composites (PMCs) 1. Introduction
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Carbon fibre reinforced polymer composites (CFRPC) have been widely used in aircraft, automobiles, marine industries and wind energy generation due to the possibility to combine high strength and stiffness with improved fatigue resistance and lightweight
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[1-7]. However, CFRPC are susceptible to some mechanical damages in their service lives. Among of them, micro-crack is caused by seasonal temperature variation, external
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restraint, residual stress or accidental overloads [8-9]. The generation and development of micro-cracks adversely affect CFRPC’s mechanical behaviour and even induces catastrophic material failure [10]. In addition, as a result of composites intrinsic heterogeneity, it is difficult to rapidly identify and repair cracks [11].
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An important area of recent development has thus been to explore the possibility of proposing smart composite materials that are able to autonomously detect and heal microcracks, thus improving composites’ safe application and extending their lifetime [12-15].
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So far, a lot of approaches have been developed. According to repairing mechanism, these approaches can be divided into two main families. One is extrinsic systems, where
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external healing agent is introduced into the composite [16-20]. Another is intrinsic systems, where the self-healing behaviour is an intrinsic property of the matrix [21-24]. For extrinsic systems, the healing agent is usually stored in reservoirs (e.g. capsules, pipelines or microvascular networks), which are prior to embed into polymer matrix. When crack emerges and propagates through the matrix, these reservoirs will open, release healing agent, fill the crack and repair it. These systems, which are autonomous,
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can heal cracks with relatively large distances and exhibit healing performance close to full recovery of the virgin properties of the composite, provided that the damage extent was below a certain threshold.
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Compared with other approaches, healing-agent-loaded capsules can be easily incorporated into polymer matrix using existing blending techniques and no need to change the molecular structure of matrix. Therefore, self-healing technology based on
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capsule is more likely to be commercialized in future. So far, various methods, such as insitu polymerization, interface proliferation and orifice method, have been developed to
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prepare self-healing capsules. Many kinds of capsules have been prepared. According to special demands (type of matrix or service conditions), different healing agents were selected, such as epoxy, siloxane, 2-octylcyanoacrylate, glycidyl methacrylate, and dibutylphthalate, et al. Many polymers were used as shell materials (e.g. poly (melamine-
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formaldehyde), polyurea or polyurethane) [25-29].
Although, self-healing technology based on capsule has made great progress, it is still challenging to prepare self-healing polymer matrix with excellent properties. This is
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because that self-healing capsules prepared by existing method are commonly in micro scale and with a wide size distribution. Incorporation of nonuniform, micron size capsules
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will reduce the mechanical properties of polymer matrix. Therefore, how to produce uniform capsules with submicron or nanometre scale in large quantity still remain to be further investigated [30].
Here, we have demonstrated a rational design of self-healing capsules via a combination hydrothermal synthesis method with solution-phase switchable delivery technology. Dicyclopentadiene was used as core material and healing agent. Phenol formaldehyde
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(PF) resin was elected as shell material. The results showed that incorporating the formed microcapsules not only provide self-healing properties to the epoxy matrix but also can improve its mechanical performance. When 15 wt% self-healing capsules were
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introduced, tensile strength and fracture toughness of the formed epoxy composite were increased by 30.8% and 81.4%, respectively. The fracture toughness of the self-healing epoxy composite is 91.8% of that of pure resin.
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2. Experiment section 2.1 Materials
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Styrene, potassium persulfate, phenol, hexamethylenetetramine (HMT), silane coupling agent (KH-550), tetrahydrofuran (THF), dichloromethane (CH2Cl2), acetone, and toluene were purchased from Sinopharm Chemical Reagent Co, China. Tungsten hexachloride (WCl6) was obtained from Energy Chemical Technology Co, Shanghai, China.
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Dicyclopentadiene (DCPD) was obtained from TCI Shanghai Chemical Industrial Development Co, China. Epoxy resin (E-51) with an epoxide value of 0.51 and 4,4Diaminodiphenyl methane (DDM, curing agent) were purchased from Nantong Xingchen
purification.
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Synthetic Material Co, China. All the materials were used as received without any further
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2.2 Preparation of self-healing capsules The preparation of self-healing capsules is consist of synthesis of polystyrene (PS) spheres, coating PS with phenol formaldehyde (PF) resin, removal of PS, amination of hollow PF spheres and importing of DCPD. The synthesis of PS spheres and coating PS with PF resins are carried out according to the processes reported in our previous work [31]. The as-synthesized composites was denoted PS@PF. PS removal and amination of
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hollow PF spheres are a continuous process. Typically, 0.5 g PS@PF composite spheres were dispersed in a mixed solution (15 mL CH2Cl2 and 15 mL THF) and then allowed to stand for 6 h at 30 oC. Subsequently, 3 mL silane coupling agent (KH-550) was added and
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sonicate for 10 min. Finally, the formed mixture was heated and refluxed for 10 h. Aminated hollow PF spheres were collected by centrifugation and washed for 3 times with CH2Cl2. The last step is importing DCPD into aminated hollow PF sphere. Briefly, 2
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mL CH2Cl2 and 20 g DCPD were mixed to form a homogeneous solution. Next, aminated hollow PF spheres was dispersed into this mixture under sonication and the formed
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suspension was stirred for 6 h at room temperature, then the reaction system was heated to 40 oC to evaporate CH2Cl2. The product was isolated by centrifugation and washed for 3 times with ethanol.
2.3 Preparation of self-healing epoxy composites
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For the preparation of self-healing epoxy composites, 100 parts E51 was firstly diluted by acetone. Then, different amount of DCPD@PF capsules and WCl6 (relative to epoxy resin) and ethanol were added into the solution by ultrasonic dispersion about 15 min.
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After removing acetone and ethanol by reduced pressure distillation and heated to 70 oC, 25 parts DDM was added to the mixture under violent stirring. After DDM is completely
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dissolved, the mixed reactants were degassed, poured into mould and cured for 4 h at 100 C and followed by 2 h at 130 oC. The cured pure epoxy sample was also synthesized
following the same procedure for comparison 2.4 Characterizations
Fourier-transform infrared (FTIR) spectra were recorded by Nicolet 6700 Flex spectrometer to identify the chemical structure of the specimens, which were prepared by
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grinding the samples with potassium bromide (KBr) or by attaching the samples to a KBr disc. Transmission electron microscope (TEM) images of the samples were carried out with TecnaiF30 electron microscope equipped with a cold field emission gun. Scanning
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electronic microscope (SEM) investigations were carried out with Nova Nano SEM 450 instrument to observe the morphologies of samples. Thermo gravimetric analyse was performed on a Q600 thermo balance. The measurement was carried out under N2 with a
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heating rate of 10 oC/min.
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Fig. 1 Tapered double-cantilever beam (TDCB) geometry [33]. Note: all dimensions in mm. A tapered double-cantilever beam (TDCB) test was used to evaluate the fracture
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behaviour. The TDCB fracture geometry, which was developed by Mostovoy et al [32], provided a crack-length-independent measure of fracture toughness, KIC =αPC
Where, PC is the critical fracture load. The geometry of sample was shown in Fig. 1. In this case, α=11.2 × 103·m-3/2 was determined experimentally as discussed by Brown et al [33]. Prior to testing, a natural pre-crack was created with a fresh razor blade into the center groove of the specimen. Subsequently, the specimen was pin loaded and tested under displacement control
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using a 0.3 mm/min displacement rate until the crack had propagated through the insert groove section of the sample where the microcapsules reside. Subsequently, the samples were unloaded, allowing the crack faces to come back into contact, and then left to heal for 24 h at room
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temperature (without any external intervention, e.g. no applied heat or pressure). Finally, the healed specimens were tested again following the above procedure. Healing efficiencies is
included five specimens to yield averaged value.
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Results and discussion
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defined as the ratio of fracture toughness, KIC, of healed and virgin materials. Each batch
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Fig. 2 Schematic illustration of the fabrication of DCPD@PF capsules. A designed synthesis of self-healing capsules is illustrated in Fig. 2. Firstly, monodisperse PS spheres with a diameter of 400 nm are prepared by emulsion polymerization. Secondly, PS
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sphere is used as seed and uniformly coated by highly cross-linked phenol formaldehyde (PF) resin via hydrothermal method. The resultant composites particle is denoted as PS@PF. The
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third step includes the swelling of PF shell layer, dissolving of PS seed and amination of formed hollow PF sphere. The last step involves introduction of healing agent (DCPD) and shrinking of PF shell. The prepared self-healing capsule is named as DCPD@N-PF. To ensure the formed PS@PF with uniform core structure, the morphology and dispersity of PS seed is crucial. Some characterizations were firstly made on PS spheres. As showed in Fig. 3a, the obtained PS sphere with a smooth surface shows good sphere morphology. Dynamic light
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scattering (DLS) curve (shown in Fig. 3b) indicates that the determined size of PS spheres is very close to that measured by SEM. In addition, the polydispersity index is approximately 3.5%, which further indicates a high uniformity in the sizes of these PS spheres. These uniform PS
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spheres are helpful to synthesis uniform PS@PF spheres and exactly control the thickness of shell. In order to obtain single-core embedded composite particles, the monodispersity and stability of PS spheres are critical. As shown in Fig. 3c, the prepared PS sphere is mondisperes.
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Fig. 3d indicates the zeta potential values are all below -30 mV in the pH range from 2-13. In generally, when its zeta potential is below -30 mV or above 30 mV, the colloidal solution of the
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particle is considered to be stable [34]. Stable PS colloidal solution is necessary to synthesize
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PS@PF composites particle with single PS core.
Fig. 3 SEM (a) and TEM (c) images, DLS (b) and zeta potential (d) curve of PS spheres. The π-π interaction between PS and phenol molecules possibly enables the formation of the phenol-containing polymer on the surface of PS sphere, further forming a PS@PF core-shell structure via polymerization between phenol and formaldehyde. The successful surface coating
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of PS seed with PF layer is confirmed by SEM observation. As shown in Fig. 4a, the obtained PS@PF sphere is very uniform and its diameter is about 500 nm. It indicates that the thickness of the PF layer is about 50 nm. However, from TEM image of PS@PF composite sphere (Fig. 4c),
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core-shell structure was not found. This may be because polystyrene and poly (phenolic resin) all are polymers and their transmittance to light is similar, so there is no obvious contrast between the pictures. To prove that PF layer is successfully coated on the surface of PS sphere, PF, PS,
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and PS@PF were characterized by FTIR spectroscopy. As seen in Fig. 4b, the characteristic peaks of PF and PS all can be found in the spectrum of PS@PF sphere. The thermal behavior of
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PS, PF, and PS@PF were also investigated by using thermogravimetry test (TG, Fig. 4d). The results showed that PS can be completely pyrolysis before 450 oC and the carbon yield of pure PF is about 54 wt% at 800 oC. For the case of PS@PF, the weight loss is about 47% before 450 o
C and final mass residue rate is about 28%. Based on the spherical size determined by SEM
followed:
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observations before and after PF coating, the mass ratio between PS and PF can be calculated as
R=mPS ÷ mPS
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=ρPS × VPS ÷ ρPF × VPS
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=[ρPS × (4/3) πrps3] ÷ [ρPF × (4/3) π(rc3-rps3)]
=[ρPS ×rps3] ÷ [ρPF × (rc3-rps3)]
Where, the densities ρ1 of PS and ρ2 of PF are ∼1.0 g⋅cm-3 and ∼1.2 g⋅cm-3, respectively [35,36]. rPS, which is radius of PS sphere, is ∼200 nm. rC, which is the radius of PS@PF composites particles, is ∼250 nm. Thus, a mass ratio between PS and PF of 46.7:53.3 can be estimated. The calculated result is in good agreement with the TG data. Thus, the results of FTIR and TGA further prove that the PS surface was coated with the PF polymer.
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Fig. 4 SEM (a) and TEM (c) images of PS@PF spheres; FTIR (b) and TG (d) curves of PS, PF and PS@PF sphere.
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In order to obtain hollow PF ball, it is necessary to remove PS core without destroying PF shell. A swelling-dissolving-removing- shrinking approach was then designed. As shown in Fig. 5a and 5b, the volume of PS@PF sphere bulk is significantly expanded after immersing in
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CH2Cl2. It demonstrates that PF has excellent swelling property. The same phenomenon has been reported in previous literature [37]. The molecular segment distance in the swollen PF will
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become large, and some compound with small molecule can pass through it. In addition, the suspension of PS spheres disperse in H2O is non-transparent, white emulsion (Fig. 5c). In contrast, when it was dispersed in THF, a transparent solution was obtained (Fig. 5d). It suggests that PS can be dissolved in THF. Therefore, starting from the PS@PF ball, it is theoretically feasible to prepare a hollow PF ball via swelling-dissolving-removing-shrinking (SDRS) approach using a mixture of THF and CH2Cl2
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Fig. 5 Photo of PS@PF sphere before (a) and after (b) soaking in CH2Cl2; PS sphere
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dispersed in H2O (c) and THF (d).
After drying the mixture of CH2Cl2 and THF used to soak PS@PF sphere, a white and translucent film was obtained (Fig. 6a). FTIR spectra of the white films are consistent with that of PS (Fig. 6b). Fig. 6c shows the TEM image of PS@PF spheres after SDRS treatment. It clearly displays that obtained sample is hollow and highly dispersible spheres with uniform
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diameter (ca. 500 nm), which is in good agreement with the result obtained from SEM observation (Fig. 3a). It showed that SDRS treatment is reversible and does not cause permanent deformation of the material. The high contrast of PF shell relative to the core indicates that the
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spheres are hollow. The resulting PF spheres have a 360 nm hollow cavity in diameter. It is smaller than that of the original PS seed (ca. 400 nm). It is believed to be the result of PS
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fragment residual. In addition, the resulting hollow sphere is not regular. A certain deformation is observed. Its hollow cavities are like a crystal with high-index facets (Fig. 6d). This may originate from the combination of diffusion impact of dissolved PS sphere and the inhomogeneity of PF swelling and retraction. In addition, the weight of the sample was reduced after the SDRS treatment. The amount of reduction was consistent with the previous
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thermogravimetric results (Fig. 4d). The above results show that SDRS approach is feasible to
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prepare hollow PF spheres from PS@PF composite particle.
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Fig. 6 Photo (a) and FTIR spectra (b) of the residue via drying the mixed solution (THF and CH2Cl2) used to soak the PS@PF; low (c) and high (d) resolution TEM images of PS@PF sphere after SDRS treatment.
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To improve the interfacial adhesion between self-healing capsules and epoxy resin matrix, amiantion modification was employed using (3-Aminopropyl) triethoxysilane
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(KH550) as agent (Fig. 7). KH550 has siloxane (-Si-OC2H5) and amine (-NH2) groups. Under certain reaction conditions, the Si-OC2H5 group will firstly hydrolyzed and product silanol (Si-OH) (Fig. 7, equation (1)). Then, a condensation reaction will occur between the hydroxyl of silanol and that of PF (Fig. 7, equation (2)). Thus, the amine group was incorporated to the surface of hollow PF sphere via chemical bonding. The presence of amine group can further improve the disparity of PF microcapsules via charge repulsion.
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Furthermore, its active hydrogen can react with the epoxy group of epoxy resin and lead
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to covalently linked interface epoxy matrix and microcapsules (Fig. 7, equation (3)).
Fig. 7 Schematic diagram of amination modification of hollow PF spheres. FTIR was employed to distinguish the difference between virgin and aminated hollow
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PF spheres. As shown in Fig. 8, the peaks at 1000 cm-1 and 3380 cm-1 are belonged to hydroxyl group. The peaks at 2900 cm-1 and 2840 cm-1 are C-H stretch. The perks at 1610 cm-1 and 1480 cm-1 are C=C aromatic. The perk at 1230 cm-1 is assigned to C-O group.
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They all are the characteristic spectrum of PF. Compared with the FTIR spectra of PF microcapsules, new peaks at 3400 cm-1 and 1215 cm-1 were observed on that of modified
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PF sphere. They are typical stretching vibration peaks of N-H and C-H. Moreover, the other new peaks at 1396 cm-1, 1150 cm-1, 1030 cm-1 and 895 cm-1, which can be assigned to Si-OH, Si-O-C and Si-C groups, respectively. It proves the present of silanol. The great change of adsorption peak at 900-1100 cm-1 may be attributed to the bonding manner’s change of O-H group. All these changes suggest that silane coupling agent is bind to the surface of PF capsules. The PF mentioned later is all aminated, if not specifically stated.
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Fig. 8 FTIR spectra of hollow PF spheres before and after modified with KH550. Through the same solvent swell process, DCPD was firstly imported into the interior
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cavity of hollow PF spheres. Then, the temperature of the system was increased to the boiling point of CH2Cl2. CH2Cl2 with low boiling point was evaporated and lead to the shrinkage of PF shell. Thus, liquid DCPD was entrapped inside the hollow PF spheres and the formed composites sphere was denoted as DCDP@PF. Its interface between
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hollow cavity and PF shell is much less obvious (Fig. 9a). Meanwhile, the rough surface of the hollow cavity becomes smooth. It may be caused by the depositing of DCPD. This also provides indirect evidence that the method to introduce DCPD to hollow PF spheres
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is effective. Because the viscosities of epoxy resins are commonly large, it is necessary to use diluting agent to decrease viscosity of system when dispersing DCPD@PF in them.
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Therefore, the stability of DCPD@PF in solvent is very important [38]. Here toluene and acetone were chose as typical solvents. DCPD@PF capsules were firstly dispersed in them for a time and then were centrifuged. The obtained supernatant was investigated by FTIR. Fig. 9c shows the FTIR spectra of DCDP, pure toluene and that used to soak DCPD@PF (denoted as toluene-DCPD@PF). It can be seen that the spectra of tolueneDCPD@PF both display the characteristic peaks of toluene and the characteristic peaks of
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DCPD, which indicates that DCPD diffused out from capsules in soaking process. In contrast, the spectrum of acetone-DCPD@PF is same as that of pure acetone (Fig. 9d). It indicates that DCPD@PF capsule has good stability, which is better than that reported in
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the literature [38]. Therefore, acetone was chose as dispersant to prepare DCDP@PF capsules suspension. Dispersing DCPD@PF into resin and curing epoxy resin, DCPD@PF has to experience heat impact. Therefore, their thermal stability is also very
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important. The thermal behavior of the samples of PF, DCPD and DCPD@PF were investigated by thermogravimetry test (Fig. 9b). In the case of PF, two main weight loss
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stages were observed, and 46% weight loss was reached at 800 oC. For DCPD, it shows a 100% weight loss in the temperature range of 160-210 oC, which is closed to the boiling point of DCPD (170 oC). The TG curve of PS@PF exhibits four main stages of weight loss for the DCPD@PF capsules within the concerned temperature range. The first stage
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(2% weight loss), which is in the range of 25-150 oC, should be due to of the physically adsorbed water. The second stage (46% weight loss), which is in the range of 150-225 oC, can be assigned to the evaporation of DCPD. The third and fourth weight loss temperature
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ranges are 220-470 oC and 470-600 oC, respectably. They are originates from the evaporation of chemically adsorbed water and thermal degradation of PF. It indicated that
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the DCPD@PF capsules were stable up to the rupture and release of the vaporized healing agent (DCPD).
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Fig. 9 TEM image of DCPD@PF spheres (a); TGA curves of PF, DCPD and DCDP@PF capsules (b); FTIR spectra of DCPD, pure toluene, toluene that used to soak DCPD@PF
capsules (d).
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(c); FTIR spectra ofDCPD, pure acetone and acetone that used to soak DCPD@PF
Incorporation of additives into a polymer would inevitably affect its intrinsic properties. For an ideal self-healing capsule, its introduction should impart self-healing
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properties to the material matrix while improve the mechanical properties of matrix.
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Therefore, the tensile strengths of cured pure epoxy resin and epoxy composites were investigated by a unidirectional tensile test. The strain-stress curves of cured composites with different loading of DCPD@PF capsules are shown in Fig. 10a. Corresponding tensile strength (the maximum stress in the stress-strain curve) with error bars is shown in Fig. 10b. It shows the favorable effects of doping DCPD@PF on the reinforcement of cured epoxy composites. The largest average tensile strength of cured epoxy composites is 102.6 MPa with a DCPD@PF loading of 8 wt%. It is 33.3% higher than that of cured
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pure epoxy (76.5 MPa). For the cured epoxy composites with a DCPD@PF loading of 2, 5, 8 and 15 wt%, the tensile strength is observed to be 10.9 (84.9 MPa), 18.8 (90.86 MPa), 33.3 (102.6 MPa), and 26.6 (96.8 MPa) higher than that of cured pure epoxy (74
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MPa), respectively.
Fig. 10 Stress-strain curves of epoxy composites filled with different loadings of
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DCPD@PF capsules (a); and tensile strength with error bars of epoxy composites filled with different loadings of DCPD@PF capsules (b), WCl6 (c), 10 wt% WCl6 and different
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loadings of DCPD@PF (d).
As shown in Fig. 10c, the tensile strength of epoxy composites decreases with the increase of WCl6 loading. This is mainly because WCl6, which not only does not participate but also hinder the reaction between epoxy resins and curing agent, lead to the formed epoxy matrix with low crosslinking density and degree of cure. Li et al has found that there are little increase for healing efficiency when the WCl6 concentration beyond 10 wt% [39]. In order to obtain high healing efficiency and virgin tensile strength, a WCl6
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loading of 10 wt% was thus chosen. Fig. 10d shows the tensile strengths of the epoxy composites, which contained 10 wt% WCl6, increase firstly and then decrease with the increase of the DCPD@PF. When the content of the DCPD@PF is 8 wt%, the tensile
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strength of epoxy composite reaches the maximum (88.2 MPa).
Fig. 11 The effect of DCPD@PF loading on the virgin and healed fracture toughness of
epoxy composite with 10 wt% WCl6 (a); SEM image of the healed crack on the self-
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healing specimens containing 15 wt% DCPD@PF capsules (b). Autonomic healing efficiency is evaluated on the basis of the ability to recover fracture toughness of epoxy composites. A tapered double-cantilever beam test method
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has been used to assess the fracture toughness of epoxy matrix before and after selfhealing. As displayed in Fig. 11a, at a constant weight ratio of catalyst/epoxy composite
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of 1:10, the fracture toughness of epoxy composite drastically increases with the increase of DCPD@PF capsules loading from 0 to 15 wt%.The fracture toughness of epoxy matrix containing 15 wt% DCPD@PF capsule increased by 81.4% compared with that of the cured pure epoxy resin. Clearly, the healing effect is correlative to the healing agent quantity offered by the broken capsules on the fracture planes. Therefore, the self-healing properties of epoxy composites also increased with the increase of DCPD@PF loading. The fracture toughness of epoxy matrix containing 15wt% DCPD@PF recovers 91.8 of
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that of cured pure epoxy resin. In addition, it takes only one day for epoxy matrix containing DCPD@PF capsule to self-heal. It is much shorter than the 15 days reported in previous literature [40]. However, according to the results of tensile test, tensile properties
Therefore,
the
self-healing
properties
of
epoxy
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of the obtained composite materials decreased with 15 wt% DCPD@PF capsules. composites
containing
larger
concentrations of DCPD@PF capsules have not been tested. SEM test was employed to
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investigate the healed cracks on epoxy resin containing DCPD@PF capsule. Taking epoxy resin containing 15% DCPD@PF capsule as an example (Fig. 11b), a healed crack
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with a width of about 500 nm can be observed. In the crack, the filler can be clearly observed. It is shown that DCPD@PF capsules were broken when the crack passes through the region contains them during the crack initiation and propagation process. DCPD will flows out of the capsule and flows along the crack direction. In this process,
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the polymerization reaction takes place when DCPD meet WCl6 embedded in the matrix. Thus the reparation of crack repair can be realized. 4. Conclusions
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In summary, DCPD@PF capsules has been rationally designed and fabricated via surface coating, swelling, dissolving and solution-phase switchable transport of DCPD.
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This method can simply control the morphology of DCPD@PF capsules. The prepared DCPD@PF capsule exhibits a uniform spherical shapes and narrow size distribution. The introduction of DCPD@PF capsules not only gives self-healing ability to the resulting composite, but also improves its mechanical properties. When 15 wt% of the DCPD@PF capsules were introduced, the fracture toughness of the resulting epoxy composite were increased by 81.4%. At the same time, after complete fracture, the fracture strength of the
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composites can be restored to 91.8% of that of cured pure epoxy resin. The proposed strategy in this paper is potential to be developed to an efficient and effective route for encapsulation of various substances in different fields of application.
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Acknowledgments
The project was supported by Liaoning Provincial Natural Science Foundation guiding Plan (No. 201602189) and Dalian Science and Technology Star Project (No. 2017RQ149).
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S0266-3538(18)32132-8
DOI:
https://doi.org/10.1016/j.compscitech.2018.12.006
Reference:
CSTE 7497
To appear in:
Composites Science and Technology
Received Date: 3 September 2018 Revised Date:
3 December 2018
Accepted Date: 6 December 2018
Please cite this article as: Sun T, Shen X, Peng C, Fan H, Liu M, Wu Z, A novel strategy for the synthesis of self-healing capsule and its application, Composites Science and Technology (2019), doi: https://doi.org/10.1016/j.compscitech.2018.12.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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A novel strategy for the synthesis of self-healing capsule and its application
a
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Tao Sun a, Xuejing Shen a, Chong Peng,a Hongyu Fan b∗, Minjing Liu a, Zhanjun Wu a∗
School of Aeronautics and Astronautics, State Key Laboratory of Structural Analysis for
Industrial Equipment, Dalian University of Technology, Dalian 116024, PR China
School of Physics and Materials Engineering, Dalian Nationalities University, Dalian 116600,
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b
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PR China
Abstract: Phenol formaldehyde (PF) resin capsules containing dicyclopentadiene (DCPD) as core materials are rationally designed and fabricated. The synthesis consists of preparation of polystyrene (PS) sphere, PF coating on PS sphere, followed by removal of PS core, amination modification and importing of DCPD. Solution phase switchable transport trough PF shell layer
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is key for the synthesis of DCDP@PF capsules. The resultant DCDP@PF capsules have a diameter of ~500 nm, shell thickness of ~50 nm, and core content of ~45 wt%. The results show that DCDP@PF capsules have outstanding thermal stability with initial evaporation temperature
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(defined at 5% of weight loss), increased by ~30 oC compared with that of pure DCPD, and good
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resistance to acetone. Preliminary results indicated that the prepared DCPD@PF capsules can effectivelly improve the mechanical properties of epoxy matrix as well as impart it self-healing properties. When 15 wt% DCPD@PF capsules were inttrouducd into epoxy matrix, 81.4% incresement in fracture toughness, 30.8% incresment in tensiles strength and 91.8% recovery in fracture toughness can be obtained. This work provides a new insight into the investigation of the fabraction of self-healing capsules.
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Keywords: A. Functional composites; B. Mechanical properties; Self-healing capsules; A. Polymer-matrix composites (PMCs) 1. Introduction
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Carbon fibre reinforced polymer composites (CFRPC) have been widely used in aircraft, automobiles, marine industries and wind energy generation due to the possibility to combine high strength and stiffness with improved fatigue resistance and lightweight
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[1-7]. However, CFRPC are susceptible to some mechanical damages in their service lives. Among of them, micro-crack is caused by seasonal temperature variation, external
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restraint, residual stress or accidental overloads [8-9]. The generation and development of micro-cracks adversely affect CFRPC’s mechanical behaviour and even induces catastrophic material failure [10]. In addition, as a result of composites intrinsic heterogeneity, it is difficult to rapidly identify and repair cracks [11].
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An important area of recent development has thus been to explore the possibility of proposing smart composite materials that are able to autonomously detect and heal microcracks, thus improving composites’ safe application and extending their lifetime [12-15].
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So far, a lot of approaches have been developed. According to repairing mechanism, these approaches can be divided into two main families. One is extrinsic systems, where
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external healing agent is introduced into the composite [16-20]. Another is intrinsic systems, where the self-healing behaviour is an intrinsic property of the matrix [21-24]. For extrinsic systems, the healing agent is usually stored in reservoirs (e.g. capsules, pipelines or microvascular networks), which are prior to embed into polymer matrix. When crack emerges and propagates through the matrix, these reservoirs will open, release healing agent, fill the crack and repair it. These systems, which are autonomous,
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can heal cracks with relatively large distances and exhibit healing performance close to full recovery of the virgin properties of the composite, provided that the damage extent was below a certain threshold.
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Compared with other approaches, healing-agent-loaded capsules can be easily incorporated into polymer matrix using existing blending techniques and no need to change the molecular structure of matrix. Therefore, self-healing technology based on
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capsule is more likely to be commercialized in future. So far, various methods, such as insitu polymerization, interface proliferation and orifice method, have been developed to
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prepare self-healing capsules. Many kinds of capsules have been prepared. According to special demands (type of matrix or service conditions), different healing agents were selected, such as epoxy, siloxane, 2-octylcyanoacrylate, glycidyl methacrylate, and dibutylphthalate, et al. Many polymers were used as shell materials (e.g. poly (melamine-
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formaldehyde), polyurea or polyurethane) [25-29].
Although, self-healing technology based on capsule has made great progress, it is still challenging to prepare self-healing polymer matrix with excellent properties. This is
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because that self-healing capsules prepared by existing method are commonly in micro scale and with a wide size distribution. Incorporation of nonuniform, micron size capsules
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will reduce the mechanical properties of polymer matrix. Therefore, how to produce uniform capsules with submicron or nanometre scale in large quantity still remain to be further investigated [30].
Here, we have demonstrated a rational design of self-healing capsules via a combination hydrothermal synthesis method with solution-phase switchable delivery technology. Dicyclopentadiene was used as core material and healing agent. Phenol formaldehyde
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(PF) resin was elected as shell material. The results showed that incorporating the formed microcapsules not only provide self-healing properties to the epoxy matrix but also can improve its mechanical performance. When 15 wt% self-healing capsules were
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introduced, tensile strength and fracture toughness of the formed epoxy composite were increased by 30.8% and 81.4%, respectively. The fracture toughness of the self-healing epoxy composite is 91.8% of that of pure resin.
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2. Experiment section 2.1 Materials
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Styrene, potassium persulfate, phenol, hexamethylenetetramine (HMT), silane coupling agent (KH-550), tetrahydrofuran (THF), dichloromethane (CH2Cl2), acetone, and toluene were purchased from Sinopharm Chemical Reagent Co, China. Tungsten hexachloride (WCl6) was obtained from Energy Chemical Technology Co, Shanghai, China.
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Dicyclopentadiene (DCPD) was obtained from TCI Shanghai Chemical Industrial Development Co, China. Epoxy resin (E-51) with an epoxide value of 0.51 and 4,4Diaminodiphenyl methane (DDM, curing agent) were purchased from Nantong Xingchen
purification.
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Synthetic Material Co, China. All the materials were used as received without any further
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2.2 Preparation of self-healing capsules The preparation of self-healing capsules is consist of synthesis of polystyrene (PS) spheres, coating PS with phenol formaldehyde (PF) resin, removal of PS, amination of hollow PF spheres and importing of DCPD. The synthesis of PS spheres and coating PS with PF resins are carried out according to the processes reported in our previous work [31]. The as-synthesized composites was denoted PS@PF. PS removal and amination of
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hollow PF spheres are a continuous process. Typically, 0.5 g PS@PF composite spheres were dispersed in a mixed solution (15 mL CH2Cl2 and 15 mL THF) and then allowed to stand for 6 h at 30 oC. Subsequently, 3 mL silane coupling agent (KH-550) was added and
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sonicate for 10 min. Finally, the formed mixture was heated and refluxed for 10 h. Aminated hollow PF spheres were collected by centrifugation and washed for 3 times with CH2Cl2. The last step is importing DCPD into aminated hollow PF sphere. Briefly, 2
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mL CH2Cl2 and 20 g DCPD were mixed to form a homogeneous solution. Next, aminated hollow PF spheres was dispersed into this mixture under sonication and the formed
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suspension was stirred for 6 h at room temperature, then the reaction system was heated to 40 oC to evaporate CH2Cl2. The product was isolated by centrifugation and washed for 3 times with ethanol.
2.3 Preparation of self-healing epoxy composites
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For the preparation of self-healing epoxy composites, 100 parts E51 was firstly diluted by acetone. Then, different amount of DCPD@PF capsules and WCl6 (relative to epoxy resin) and ethanol were added into the solution by ultrasonic dispersion about 15 min.
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After removing acetone and ethanol by reduced pressure distillation and heated to 70 oC, 25 parts DDM was added to the mixture under violent stirring. After DDM is completely
o
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dissolved, the mixed reactants were degassed, poured into mould and cured for 4 h at 100 C and followed by 2 h at 130 oC. The cured pure epoxy sample was also synthesized
following the same procedure for comparison 2.4 Characterizations
Fourier-transform infrared (FTIR) spectra were recorded by Nicolet 6700 Flex spectrometer to identify the chemical structure of the specimens, which were prepared by
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grinding the samples with potassium bromide (KBr) or by attaching the samples to a KBr disc. Transmission electron microscope (TEM) images of the samples were carried out with TecnaiF30 electron microscope equipped with a cold field emission gun. Scanning
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electronic microscope (SEM) investigations were carried out with Nova Nano SEM 450 instrument to observe the morphologies of samples. Thermo gravimetric analyse was performed on a Q600 thermo balance. The measurement was carried out under N2 with a
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heating rate of 10 oC/min.
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Fig. 1 Tapered double-cantilever beam (TDCB) geometry [33]. Note: all dimensions in mm. A tapered double-cantilever beam (TDCB) test was used to evaluate the fracture
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behaviour. The TDCB fracture geometry, which was developed by Mostovoy et al [32], provided a crack-length-independent measure of fracture toughness, KIC =αPC
Where, PC is the critical fracture load. The geometry of sample was shown in Fig. 1. In this case, α=11.2 × 103·m-3/2 was determined experimentally as discussed by Brown et al [33]. Prior to testing, a natural pre-crack was created with a fresh razor blade into the center groove of the specimen. Subsequently, the specimen was pin loaded and tested under displacement control
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using a 0.3 mm/min displacement rate until the crack had propagated through the insert groove section of the sample where the microcapsules reside. Subsequently, the samples were unloaded, allowing the crack faces to come back into contact, and then left to heal for 24 h at room
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temperature (without any external intervention, e.g. no applied heat or pressure). Finally, the healed specimens were tested again following the above procedure. Healing efficiencies is
included five specimens to yield averaged value.
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Results and discussion
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defined as the ratio of fracture toughness, KIC, of healed and virgin materials. Each batch
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Fig. 2 Schematic illustration of the fabrication of DCPD@PF capsules. A designed synthesis of self-healing capsules is illustrated in Fig. 2. Firstly, monodisperse PS spheres with a diameter of 400 nm are prepared by emulsion polymerization. Secondly, PS
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sphere is used as seed and uniformly coated by highly cross-linked phenol formaldehyde (PF) resin via hydrothermal method. The resultant composites particle is denoted as PS@PF. The
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third step includes the swelling of PF shell layer, dissolving of PS seed and amination of formed hollow PF sphere. The last step involves introduction of healing agent (DCPD) and shrinking of PF shell. The prepared self-healing capsule is named as DCPD@N-PF. To ensure the formed PS@PF with uniform core structure, the morphology and dispersity of PS seed is crucial. Some characterizations were firstly made on PS spheres. As showed in Fig. 3a, the obtained PS sphere with a smooth surface shows good sphere morphology. Dynamic light
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scattering (DLS) curve (shown in Fig. 3b) indicates that the determined size of PS spheres is very close to that measured by SEM. In addition, the polydispersity index is approximately 3.5%, which further indicates a high uniformity in the sizes of these PS spheres. These uniform PS
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spheres are helpful to synthesis uniform PS@PF spheres and exactly control the thickness of shell. In order to obtain single-core embedded composite particles, the monodispersity and stability of PS spheres are critical. As shown in Fig. 3c, the prepared PS sphere is mondisperes.
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Fig. 3d indicates the zeta potential values are all below -30 mV in the pH range from 2-13. In generally, when its zeta potential is below -30 mV or above 30 mV, the colloidal solution of the
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particle is considered to be stable [34]. Stable PS colloidal solution is necessary to synthesize
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PS@PF composites particle with single PS core.
Fig. 3 SEM (a) and TEM (c) images, DLS (b) and zeta potential (d) curve of PS spheres. The π-π interaction between PS and phenol molecules possibly enables the formation of the phenol-containing polymer on the surface of PS sphere, further forming a PS@PF core-shell structure via polymerization between phenol and formaldehyde. The successful surface coating
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of PS seed with PF layer is confirmed by SEM observation. As shown in Fig. 4a, the obtained PS@PF sphere is very uniform and its diameter is about 500 nm. It indicates that the thickness of the PF layer is about 50 nm. However, from TEM image of PS@PF composite sphere (Fig. 4c),
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core-shell structure was not found. This may be because polystyrene and poly (phenolic resin) all are polymers and their transmittance to light is similar, so there is no obvious contrast between the pictures. To prove that PF layer is successfully coated on the surface of PS sphere, PF, PS,
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and PS@PF were characterized by FTIR spectroscopy. As seen in Fig. 4b, the characteristic peaks of PF and PS all can be found in the spectrum of PS@PF sphere. The thermal behavior of
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PS, PF, and PS@PF were also investigated by using thermogravimetry test (TG, Fig. 4d). The results showed that PS can be completely pyrolysis before 450 oC and the carbon yield of pure PF is about 54 wt% at 800 oC. For the case of PS@PF, the weight loss is about 47% before 450 o
C and final mass residue rate is about 28%. Based on the spherical size determined by SEM
followed:
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observations before and after PF coating, the mass ratio between PS and PF can be calculated as
R=mPS ÷ mPS
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=ρPS × VPS ÷ ρPF × VPS
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=[ρPS × (4/3) πrps3] ÷ [ρPF × (4/3) π(rc3-rps3)]
=[ρPS ×rps3] ÷ [ρPF × (rc3-rps3)]
Where, the densities ρ1 of PS and ρ2 of PF are ∼1.0 g⋅cm-3 and ∼1.2 g⋅cm-3, respectively [35,36]. rPS, which is radius of PS sphere, is ∼200 nm. rC, which is the radius of PS@PF composites particles, is ∼250 nm. Thus, a mass ratio between PS and PF of 46.7:53.3 can be estimated. The calculated result is in good agreement with the TG data. Thus, the results of FTIR and TGA further prove that the PS surface was coated with the PF polymer.
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Fig. 4 SEM (a) and TEM (c) images of PS@PF spheres; FTIR (b) and TG (d) curves of PS, PF and PS@PF sphere.
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In order to obtain hollow PF ball, it is necessary to remove PS core without destroying PF shell. A swelling-dissolving-removing- shrinking approach was then designed. As shown in Fig. 5a and 5b, the volume of PS@PF sphere bulk is significantly expanded after immersing in
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CH2Cl2. It demonstrates that PF has excellent swelling property. The same phenomenon has been reported in previous literature [37]. The molecular segment distance in the swollen PF will
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become large, and some compound with small molecule can pass through it. In addition, the suspension of PS spheres disperse in H2O is non-transparent, white emulsion (Fig. 5c). In contrast, when it was dispersed in THF, a transparent solution was obtained (Fig. 5d). It suggests that PS can be dissolved in THF. Therefore, starting from the PS@PF ball, it is theoretically feasible to prepare a hollow PF ball via swelling-dissolving-removing-shrinking (SDRS) approach using a mixture of THF and CH2Cl2
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Fig. 5 Photo of PS@PF sphere before (a) and after (b) soaking in CH2Cl2; PS sphere
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dispersed in H2O (c) and THF (d).
After drying the mixture of CH2Cl2 and THF used to soak PS@PF sphere, a white and translucent film was obtained (Fig. 6a). FTIR spectra of the white films are consistent with that of PS (Fig. 6b). Fig. 6c shows the TEM image of PS@PF spheres after SDRS treatment. It clearly displays that obtained sample is hollow and highly dispersible spheres with uniform
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diameter (ca. 500 nm), which is in good agreement with the result obtained from SEM observation (Fig. 3a). It showed that SDRS treatment is reversible and does not cause permanent deformation of the material. The high contrast of PF shell relative to the core indicates that the
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spheres are hollow. The resulting PF spheres have a 360 nm hollow cavity in diameter. It is smaller than that of the original PS seed (ca. 400 nm). It is believed to be the result of PS
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fragment residual. In addition, the resulting hollow sphere is not regular. A certain deformation is observed. Its hollow cavities are like a crystal with high-index facets (Fig. 6d). This may originate from the combination of diffusion impact of dissolved PS sphere and the inhomogeneity of PF swelling and retraction. In addition, the weight of the sample was reduced after the SDRS treatment. The amount of reduction was consistent with the previous
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thermogravimetric results (Fig. 4d). The above results show that SDRS approach is feasible to
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prepare hollow PF spheres from PS@PF composite particle.
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Fig. 6 Photo (a) and FTIR spectra (b) of the residue via drying the mixed solution (THF and CH2Cl2) used to soak the PS@PF; low (c) and high (d) resolution TEM images of PS@PF sphere after SDRS treatment.
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To improve the interfacial adhesion between self-healing capsules and epoxy resin matrix, amiantion modification was employed using (3-Aminopropyl) triethoxysilane
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(KH550) as agent (Fig. 7). KH550 has siloxane (-Si-OC2H5) and amine (-NH2) groups. Under certain reaction conditions, the Si-OC2H5 group will firstly hydrolyzed and product silanol (Si-OH) (Fig. 7, equation (1)). Then, a condensation reaction will occur between the hydroxyl of silanol and that of PF (Fig. 7, equation (2)). Thus, the amine group was incorporated to the surface of hollow PF sphere via chemical bonding. The presence of amine group can further improve the disparity of PF microcapsules via charge repulsion.
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Furthermore, its active hydrogen can react with the epoxy group of epoxy resin and lead
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to covalently linked interface epoxy matrix and microcapsules (Fig. 7, equation (3)).
Fig. 7 Schematic diagram of amination modification of hollow PF spheres. FTIR was employed to distinguish the difference between virgin and aminated hollow
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PF spheres. As shown in Fig. 8, the peaks at 1000 cm-1 and 3380 cm-1 are belonged to hydroxyl group. The peaks at 2900 cm-1 and 2840 cm-1 are C-H stretch. The perks at 1610 cm-1 and 1480 cm-1 are C=C aromatic. The perk at 1230 cm-1 is assigned to C-O group.
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They all are the characteristic spectrum of PF. Compared with the FTIR spectra of PF microcapsules, new peaks at 3400 cm-1 and 1215 cm-1 were observed on that of modified
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PF sphere. They are typical stretching vibration peaks of N-H and C-H. Moreover, the other new peaks at 1396 cm-1, 1150 cm-1, 1030 cm-1 and 895 cm-1, which can be assigned to Si-OH, Si-O-C and Si-C groups, respectively. It proves the present of silanol. The great change of adsorption peak at 900-1100 cm-1 may be attributed to the bonding manner’s change of O-H group. All these changes suggest that silane coupling agent is bind to the surface of PF capsules. The PF mentioned later is all aminated, if not specifically stated.
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Fig. 8 FTIR spectra of hollow PF spheres before and after modified with KH550. Through the same solvent swell process, DCPD was firstly imported into the interior
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cavity of hollow PF spheres. Then, the temperature of the system was increased to the boiling point of CH2Cl2. CH2Cl2 with low boiling point was evaporated and lead to the shrinkage of PF shell. Thus, liquid DCPD was entrapped inside the hollow PF spheres and the formed composites sphere was denoted as DCDP@PF. Its interface between
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hollow cavity and PF shell is much less obvious (Fig. 9a). Meanwhile, the rough surface of the hollow cavity becomes smooth. It may be caused by the depositing of DCPD. This also provides indirect evidence that the method to introduce DCPD to hollow PF spheres
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is effective. Because the viscosities of epoxy resins are commonly large, it is necessary to use diluting agent to decrease viscosity of system when dispersing DCPD@PF in them.
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Therefore, the stability of DCPD@PF in solvent is very important [38]. Here toluene and acetone were chose as typical solvents. DCPD@PF capsules were firstly dispersed in them for a time and then were centrifuged. The obtained supernatant was investigated by FTIR. Fig. 9c shows the FTIR spectra of DCDP, pure toluene and that used to soak DCPD@PF (denoted as toluene-DCPD@PF). It can be seen that the spectra of tolueneDCPD@PF both display the characteristic peaks of toluene and the characteristic peaks of
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DCPD, which indicates that DCPD diffused out from capsules in soaking process. In contrast, the spectrum of acetone-DCPD@PF is same as that of pure acetone (Fig. 9d). It indicates that DCPD@PF capsule has good stability, which is better than that reported in
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the literature [38]. Therefore, acetone was chose as dispersant to prepare DCDP@PF capsules suspension. Dispersing DCPD@PF into resin and curing epoxy resin, DCPD@PF has to experience heat impact. Therefore, their thermal stability is also very
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important. The thermal behavior of the samples of PF, DCPD and DCPD@PF were investigated by thermogravimetry test (Fig. 9b). In the case of PF, two main weight loss
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stages were observed, and 46% weight loss was reached at 800 oC. For DCPD, it shows a 100% weight loss in the temperature range of 160-210 oC, which is closed to the boiling point of DCPD (170 oC). The TG curve of PS@PF exhibits four main stages of weight loss for the DCPD@PF capsules within the concerned temperature range. The first stage
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(2% weight loss), which is in the range of 25-150 oC, should be due to of the physically adsorbed water. The second stage (46% weight loss), which is in the range of 150-225 oC, can be assigned to the evaporation of DCPD. The third and fourth weight loss temperature
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ranges are 220-470 oC and 470-600 oC, respectably. They are originates from the evaporation of chemically adsorbed water and thermal degradation of PF. It indicated that
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the DCPD@PF capsules were stable up to the rupture and release of the vaporized healing agent (DCPD).
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Fig. 9 TEM image of DCPD@PF spheres (a); TGA curves of PF, DCPD and DCDP@PF capsules (b); FTIR spectra of DCPD, pure toluene, toluene that used to soak DCPD@PF
capsules (d).
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(c); FTIR spectra ofDCPD, pure acetone and acetone that used to soak DCPD@PF
Incorporation of additives into a polymer would inevitably affect its intrinsic properties. For an ideal self-healing capsule, its introduction should impart self-healing
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properties to the material matrix while improve the mechanical properties of matrix.
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Therefore, the tensile strengths of cured pure epoxy resin and epoxy composites were investigated by a unidirectional tensile test. The strain-stress curves of cured composites with different loading of DCPD@PF capsules are shown in Fig. 10a. Corresponding tensile strength (the maximum stress in the stress-strain curve) with error bars is shown in Fig. 10b. It shows the favorable effects of doping DCPD@PF on the reinforcement of cured epoxy composites. The largest average tensile strength of cured epoxy composites is 102.6 MPa with a DCPD@PF loading of 8 wt%. It is 33.3% higher than that of cured
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pure epoxy (76.5 MPa). For the cured epoxy composites with a DCPD@PF loading of 2, 5, 8 and 15 wt%, the tensile strength is observed to be 10.9 (84.9 MPa), 18.8 (90.86 MPa), 33.3 (102.6 MPa), and 26.6 (96.8 MPa) higher than that of cured pure epoxy (74
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MPa), respectively.
Fig. 10 Stress-strain curves of epoxy composites filled with different loadings of
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DCPD@PF capsules (a); and tensile strength with error bars of epoxy composites filled with different loadings of DCPD@PF capsules (b), WCl6 (c), 10 wt% WCl6 and different
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loadings of DCPD@PF (d).
As shown in Fig. 10c, the tensile strength of epoxy composites decreases with the increase of WCl6 loading. This is mainly because WCl6, which not only does not participate but also hinder the reaction between epoxy resins and curing agent, lead to the formed epoxy matrix with low crosslinking density and degree of cure. Li et al has found that there are little increase for healing efficiency when the WCl6 concentration beyond 10 wt% [39]. In order to obtain high healing efficiency and virgin tensile strength, a WCl6
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loading of 10 wt% was thus chosen. Fig. 10d shows the tensile strengths of the epoxy composites, which contained 10 wt% WCl6, increase firstly and then decrease with the increase of the DCPD@PF. When the content of the DCPD@PF is 8 wt%, the tensile
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strength of epoxy composite reaches the maximum (88.2 MPa).
Fig. 11 The effect of DCPD@PF loading on the virgin and healed fracture toughness of
epoxy composite with 10 wt% WCl6 (a); SEM image of the healed crack on the self-
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healing specimens containing 15 wt% DCPD@PF capsules (b). Autonomic healing efficiency is evaluated on the basis of the ability to recover fracture toughness of epoxy composites. A tapered double-cantilever beam test method
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has been used to assess the fracture toughness of epoxy matrix before and after selfhealing. As displayed in Fig. 11a, at a constant weight ratio of catalyst/epoxy composite
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of 1:10, the fracture toughness of epoxy composite drastically increases with the increase of DCPD@PF capsules loading from 0 to 15 wt%.The fracture toughness of epoxy matrix containing 15 wt% DCPD@PF capsule increased by 81.4% compared with that of the cured pure epoxy resin. Clearly, the healing effect is correlative to the healing agent quantity offered by the broken capsules on the fracture planes. Therefore, the self-healing properties of epoxy composites also increased with the increase of DCPD@PF loading. The fracture toughness of epoxy matrix containing 15wt% DCPD@PF recovers 91.8 of
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that of cured pure epoxy resin. In addition, it takes only one day for epoxy matrix containing DCPD@PF capsule to self-heal. It is much shorter than the 15 days reported in previous literature [40]. However, according to the results of tensile test, tensile properties
Therefore,
the
self-healing
properties
of
epoxy
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of the obtained composite materials decreased with 15 wt% DCPD@PF capsules. composites
containing
larger
concentrations of DCPD@PF capsules have not been tested. SEM test was employed to
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investigate the healed cracks on epoxy resin containing DCPD@PF capsule. Taking epoxy resin containing 15% DCPD@PF capsule as an example (Fig. 11b), a healed crack
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with a width of about 500 nm can be observed. In the crack, the filler can be clearly observed. It is shown that DCPD@PF capsules were broken when the crack passes through the region contains them during the crack initiation and propagation process. DCPD will flows out of the capsule and flows along the crack direction. In this process,
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the polymerization reaction takes place when DCPD meet WCl6 embedded in the matrix. Thus the reparation of crack repair can be realized. 4. Conclusions
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In summary, DCPD@PF capsules has been rationally designed and fabricated via surface coating, swelling, dissolving and solution-phase switchable transport of DCPD.
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This method can simply control the morphology of DCPD@PF capsules. The prepared DCPD@PF capsule exhibits a uniform spherical shapes and narrow size distribution. The introduction of DCPD@PF capsules not only gives self-healing ability to the resulting composite, but also improves its mechanical properties. When 15 wt% of the DCPD@PF capsules were introduced, the fracture toughness of the resulting epoxy composite were increased by 81.4%. At the same time, after complete fracture, the fracture strength of the
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composites can be restored to 91.8% of that of cured pure epoxy resin. The proposed strategy in this paper is potential to be developed to an efficient and effective route for encapsulation of various substances in different fields of application.
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Acknowledgments
The project was supported by Liaoning Provincial Natural Science Foundation guiding Plan (No. 201602189) and Dalian Science and Technology Star Project (No. 2017RQ149).
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