Self-healing polyvinyl chloride (PVC) based on microencapsulated nucleophilic thiol-click chemistry

Polymer 69 (2015) 1e9

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Self-healing polyvinyl chloride (PVC) based on microencapsulated nucleophilic thiol-click chemistry Dong Yu Zhu a, b, Guang Sheng Cao b, Wen Lian Qiu b, Min Zhi Rong b, Ming Qiu Zhang b, * a

School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, China Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, GD HPPC Lab, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 February 2015 Received in revised form 9 May 2015 Accepted 29 May 2015 Available online 1 June 2015

Polyvinyl chloride (PVC), a typical commodity thermoplastic polymer, is enabled for self-healing using nucleophilic thiol-click chemistry. A healing agent consisting of bifunctional monomer glycidyl methacrylate (GMA), polythiol pentaerythritol tetrakis (3-mercaptopropionate) (PETMP) and basic catalyst 2,4,6-tris(dimethylaminomethyl) phenol (DMP-30) shows good wetting of the PVC matrix, fast reaction kinetics, a high resistance to thermal processing, and tolerance to oxygen. The PVC composite containing microencapsulated healant was fabricated by hot compression molding and exhibited autonomic selfhealability characterized by the recovery of mechanical strength within 2e3 h at room temperature in air. Both the C]C and epoxide group of GMA participate in the reaction with polythiol, but the thiol-ene “click” reaction proceeds faster than the thiol-epoxy reaction. The outcomes of the present study contribute to developing new healing agents and expand the family of self-healing thermoplastics. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Self-healing Thiol-click chemistry Microcapsules

1. Introduction Self-healing polymers containing microencapsulated healing agent have been a focal point of self-healing materials over the past decade [1e8]. Compared with most intrinsic self-healing approaches [9e11], extrinsic self-healing based on embedded healants does not modify the macromolecular structure. Therefore, it is suitable for application to polymers produced commercially at a large scale. However, the majority of research activities in this aspect have focused on thermosets represented by cured epoxy [1,12e16] rather than thermoplastics, which are also traded and used in great quantities. Only a few thermoplastics have been studied, such as poly(ethylene-co-methacrylic acid) (EMAA) [17], polymethyl methacrylate (PMMA) [18e21], polystyrene (PS) [22e24] and PMMA/PS blends [25]. The earlier efforts to introduce self-healability to thermoplastic polymers were in the development of healing systems qualified for re-bonding cracks [18e22]. Subsequently, the robustness of the healing capsules to withstand the thermal forming of thermoplastics became an important topic [23,26]. More recently, a heat-

* Corresponding author. Materials Science Institute, Sun Yat-sen University, Guangzhou 510275, China. E-mail address: [email protected] (M.Q. Zhang). http://dx.doi.org/10.1016/j.polymer.2015.05.052 0032-3861/© 2015 Elsevier Ltd. All rights reserved.

resisting and air-insensitive healing agent was fabricated [24]. Although research goals are climbing towards applicable products, there are still many unsolved problems. For example, the healing speed of reported systems is not fast enough. One day or even longer time had to be spent for obtaining the highest healing efficiency [18e22]. Evidently, this duration fails to satisfy the requirement that cracks should be recovered soon after their initiation to prevent unstable propagation. Although ultrafast healing systems have been available for epoxy composites [27e30], they are incompatible with thermoplastics. Thus, novel healing agents or healing chemistries should be studied with the aim of reducing the healing times in self-healing thermoplastics. By surveying the possible solutions, “click chemistry” [31] attracted our attention because of its simplicity, mild conditions, high speed, and insensitivity to oxygen and moisture. Pilot studies showed that the azide/alkyne “click” reaction is feasible for selfhealing [32e34]. In this study, nucleophilic thiol-click chemistry [35] with a basic catalyst is evaluated, which does not need a Cu(I) catalyst like that used in azide/alkyne “click” reactions nor light or heat stimuli as used in radical-mediated thiol-ene click chemistry [36]. According to this design, glycidyl methacrylate (GMA), which possesses both an electron-deficient C]C bond and epoxide group, and highly active pentaerythritol tetrakis (3-mercaptopropionate)

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sodium styrene-maleate copolymer (Mn ¼ 12500, Mw/Mn ¼ 2.84) with the content of maleic anhydride (MA) ¼ 49.5 wt.% was prepared as described in Ref. [38]. PVC (S-700) used as the matrix of self-healing thermoplastic composite was provided by Qilu Petrochemical Co., China. Methyltin mercaptide served as the thermal stabilizer of PVC, and was purchased from Crompton Corporation, USA. The control materials, PS (666D) and poly(butylene succinate) (PBS; HX-B601), which were chosen to examine the suitability of the designed healing agent for other thermoplastics, were supplied by SINOPEC Beijing Yanshan Company, China, and Anqing Hexing Chemical Co., Ltd., China, respectively.

(PETMP) are selected as the polymerizable component and polythiol, respectively, to compose the healing agent. In addition, 2,4,6tris(dimethylaminomethyl) phenol (DMP-30) serves as the catalyst. By taking advantage of thiol-ene/thiol-epoxy “click” reaction mechanisms, the reactive C]C bond and epoxide group of GMA can take part in polymerization when all the components of the healing agent are in close proximity (Fig. 1). As a result, highly crosslinked networks can be created within a short time, leading to a highstrength bonding material for crack repair. Moreover, all these liquid chemicals have high boiling temperatures (GMA, 189
C; PETMP, 275
C; and DMP-30, 250
C) and should be capable of withstanding the thermal processing of conventional thermoplastics. To verify the healing performance of the above system, GMA and PETMP are microencapsulated separately (DMP-30 accompanies PETMP in the same capsule), and compounded with polyvinyl chloride (PVC) to produce a composite material. The self-healing function is expected to follow a similar process as that of other composites based on dual healing microcapsules [12,13]. PVC is the third-most widely produced plastic, after polyethylene and polypropylene. PVC is used in construction for pipes, doors and windows, and in non-food packaging, imitation leather, and inflatable products. The polarity of the GMA/PETMP/DMP-30 healing agent matches that of PVC and favors interfacial compatibility, which helps to improve the wetting between healing agent and polymer matrix [37]. To the best of our knowledge, there is no report of introducing a self-healing ability to PVC. Therefore, the objective of this work is to examine whether nucleophilic thiolclick chemistry can be applied in the self-healing of PVC at fast healing speeds. Thus the family of thermo-moldable self-healing commodity plastics and healing agents can be expanded.

2.2. Preparation of microencapsulated healing agent The microcapsules containing GMA were prepared with poly(melamine-formaldehyde) (PMF) shell according to the method reported previously [26,39], and PETMP was also encapsulated by PMF shell by a similar procedure described elsewhere [12]. The PETMP-loaded microcapsules were uniformly dispersed in DMP-30 at 40
C for 24 h to allow infiltration of the catalyst DMP30. Afterwards, the microcapsules were rinsed with ethyl ether, and dried at room temperature. The core content of the microcapsules was determined by extraction method as follows. Dried microcapsules (weighed as M) were ground, and sealed in a filter paper bag. The sample bag was accurately weighed as M1, and extracted by acetone in a Soxhlet apparatus for 72 h to remove the core material. Having been dried in vacuum, the sample bag was weighed again as M2. The core content of microcapsules, Mcore, was calculated from: (M1  M2)/ M  100%. For preparing GMA capsules, melamine (25 g, 0.198 mol), formaldehyde (37 wt.%, 45 g, 0.554 mol), and 50 ml deionized water were mixed in a 250-ml three-neck round-bottom flask with a magnetic stirrer. Two drops of triethanolamine were added to the mixture. The system was stirred at 70
C for 15 min until a transparent MF prepolymer solution was obtained. The solution was cooled to room temperature. In a three-necked round-bottom flask (1000 ml) equipped with a mechanical stirring paddle, 320 ml surfactant aqueous solution containing 0.2 wt.% SDBS and 0.1 wt.% PVA-124 was added. The flask was placed into an oil bath of 35
C and continually stirred before the MF prepolymer solution and 80 g GMA were added. A 16-ml amount of 5.0 wt.% acetic acid was dropped into the system to adjust the pH value to 4.8, while maintaining the system at 35
C for at least 30 min to form a stable oil-in-water emulsion. The reaction temperature was then

2. Experimental 2.1. Materials GMA was purchased from Shanghai Yuanji Chemical Co., China. PETMP was purchased from Fluka Chemie AG (Buchs, Switzerland). DMP-30 was supplied by Shanghai Medical Group Reagent Co., China. The raw materials of the microcapsule shell, melamine (M, A.R.) and formaldehyde (F, A.R., 37 wt.%), were also supplied by Shanghai Medical Group Reagent Co. The emulsifier, sodium dodecyl benzenesulfonate (SDBS, A.R.), was obtained from Sinopharm Chemical Reagent Co. Ltd., China. The stabilizer, polyvinyl alcohol (PVA-124, 87e89% hydrolyzed, Mw ¼ 88000e97000), was supplied by Guangzhou FBEST Trade Co., China. Another emulsifier,

+

HS HS

S

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OH O CH3 S C O C C CH2 H2 H

O

O

O

RT

GMA

OHH O CH3 2 C O C C CH2 H S

O

O O

Cat. (DMP-30)

O H2 H2C C C O C CH3

O

O

SH

O

O

SH

PETMP

O

O

O

O H2 OH H2C CH C O C CH3 S

O

O

S

O

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O OH H C C C O C H2 H2 CH3

Fig. 1. Polymerization scheme of the proposed healing agent.

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D.Y. Zhu et al. / Polymer 69 (2015) 1e9

gradually raised to 68
C within 40 min and maintained for another 200 min under continuous agitation. Once the isothermal reaction was completed, the flask was removed from the oil bath and cooled down to ambient temperature. The slurry of microcapsules was poured into a large amount of water to remove the emulsifier. The deposits were filtered through a mesh sieve, rinsed with deionized water several times, and dried at room temperature in a fume cupboard. Fig. 2(a) shows Fourier transform infrared (FTIR) spectra of the microcapsules and related materials. For the synthesis of PETMP capsules, the same MF prepolymer solution used to create GMA microcapsules was used. Eighty grams PETMP was added into a 400-g aqueous solution containing 2 wt.%

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of emulsifier sodium styrene-maleate copolymer. The mixture was vigorously stirred at 500 rpm for 30 min, and then 2 drops of 1octanol were added to eliminate any surface bubbles of the PETMP emulsion. Subsequently, the MF prepolymer solution was added to the PETMP emulsion at 35
C with continuous agitation for 1 h while a pH value of 3 was maintained by adding citric acid. After stirring for another 3 h at 60
C, the reaction mixture was cooled down to room temperature, and the deposit of microcapsules was separated through a Buchner funnel, rinsed with deionized water, and vacuum-dried. FTIR spectra of the microcapsules and related materials are shown in Fig. 2(b). 2.3. Preparation of self-healing PVC composite and control samples

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3380cm

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1640cm

Powdered PVC was premixed with the thermal stabilizer methyltin mercaptide (2 wt.%) using an MB-KS0601 blender. GMA loaded microcapsules (diameter: 105 mm, see Fig. 3; core content: 80.1 wt.%) and PETMP/DMP-30-loaded microcapsules (diameter: 85 mm, see Fig. 3; core content: 87.5 wt.%) were added. The selfhealing PVC composite was made by compression molding the compounds at 160
C under 6 MPa for 15 min. The unfilled PVC used as a reference was fabricated following the same procedures without the addition of microcapsules. The unfilled PS and PBS samples used for examining the suitability of the healing agent were also compression molded under 6 MPa for 15 min at processing temperatures of 150
C for PS and 120
C for PBS.

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908 cm

GMA

Extracted PMF shell

GMA-loaded microcapsules

2.4. Characterization 3500

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To monitor the kinetics of the “click” reactions, both differential scanning calorimetry (DSC) and rheological methods were used. DSC isothermal curves were recorded at 25
C using a TA Q10 Instrument, and the rheological experiments were conducted on a strain-controlled TA ARES/RFS rheometer with 8 mm parallel-plate geometry. When the top and bottom rheometer plates were fixed, a suitable amount of healant (i.e. premixed PETMP/GMA at stoichiometric ratio with a measured content of DMP-30) was injected onto the bottom plate. Subsequently, time sweep experiments were performed at a frequency of 10 rad/s and strain of 1% within the range from 0 to 3 h at 25
C. Furthermore, micro-Raman technology was adopted to investigate the reaction mechanism. Time-dependent Raman spectra of the healant system were recorded using a laser micro-Raman

500

-1

Wavenumber [cm ]

(b) Fig. 2. (a) FTIR spectrum of GMA-loaded microcapsules in comparison with those of extracted PMF shell and core material (GMA). (b) FTIR spectrum of PETMP-loaded microcapsules in comparison with those of extracted PMF shell and core material (PETMP). Note: Although both the GMA-loaded microcapsules and PETMP-loaded microcapsules are encapsulated with PMF, the preparation conditions and emulsifier systems are slightly different because of the different properties of the core materials. Moreover, PETMP may react with the shell during encapsulation [38]. Accordingly, the microstructures of the resultant PMF shells of the two types of microcapsules are not identical, leading to different appearances of FTIR spectra of the extracted shell as shown above.

Fig. 3. Size distribution of GMA-loaded and PETMP-loaded microcapsules.

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spectrometer (Renishaw inVia, UK) equipped with a Leica microscope by a 514.5-nm laser line at a resolution of 1 cm1. Fourier transform infrared (FTIR) spectra were recorded by a Bruker TENSOR27 spectrometer. Sizes of the microcapsules were determined using a Malvern MasterSizer 2000 particle size analyzer. Thermogravimetric analyses (TGA) of the microcapsules were carried out using a TA Q50 instrument. The samples were heated from 40 to 800
C at a rate of 10
C/min under the protection of nitrogen flow. To evaluate the self-healing ability of the materials, the method proposed by Jones et al. [40] was employed. Impact testing was conducted at 25
C on an Izod notched specimen (52.7  12.7  10.1 mm3) according to ASTM D265-034 using a JJ-20 impact tester produced by Changchun Research Institute for Testing Machines Co. Ltd., China. The composite specimen containing dual healant microcapsules was impacted to failure, and then the fractured surfaces were allowed to come back into contact with each other in air under a gentle pressure of ~0.2 MPa at room temperature. The healed specimen was impacted to failure again. Healing efficiency was defined as the ratio of impact strengths of healed and virgin specimens with a given concentration of healing microcapsules. Each batch included five specimens to yield an average value. For comparison, the specimens of neat PVC, PS, and PBS were also impacted to failure, and then the premixed healing agent (15 mL) was injected onto the fractured planes. Subsequently, the failed specimens were healed and characterized under the same conditions as described above. The morphological observation and energy dispersive X-ray spectroscopy (EDS) analysis of the fractured surfaces of self-healing specimen were conducted by scanning electron microscopy (SEM, Hitachi S-4800). 3. Results and discussion As mentioned in the Introduction, thiol-ene and thiol-epoxy “click” reactions of GMA with polythiol in the presence of a basic catalyst (DMP-30) are proposed to act as the healing chemistry in the present study. Because healing speed is a main objective, the reaction kinetics of the system are presented first. The DSC isothermal scanning curves (Fig. 4(a)) show that the mixture of GMA, PETMP, and DMP-30 releases a large amount of heat very quickly. The expected polymerization is exothermic and fast, which facilitates healing at lower temperatures without external heating. The plots in Fig. 4(a) also indicate that there are two peaks on each curve because both C]C and the epoxide group of GMA have reacted with thiols. With increasing DMP-30 content, the two peaks become sharper and the time required to complete the reaction decreases. The reactivities of the two functional groups of GMA are different, and this disparity widens because of the increased polymerization rate (viscosity growth), which is shown in the case of a higher basic catalyst concentration. Similar results are seen in the rheological measurements (Fig. 4(b) and (c)). At first, the complex viscosity, h*, of the system fluctuates with time like that of a liquid (Fig. 4(b)). After 20e30 min, h* drastically increases and approaches equilibrium within an additional 30e40 min. In addition, both the storage shear modulus, G0 , and loss shear modulus, G00 , exhibit similar trends with time (Fig. 4(c)). The behavior agrees with the characteristics of a “click” reaction. Although initiation may take some time, the reaction proceeds and terminates very fast upon starting. A careful observation of Fig. 4(c) shows that the G0 values at the equilibrium state are much higher than G00 . This result is attributed to the formation of crosslinked networks, which are able to store elastic energy. Eventually, the elasticity of the polymerized system has the dominant role.

Fig. 4. (a) DSC isothermal scanning, (b) time dependence of complex viscosity, h*, and (c) time dependences of storage shear modulus, G0 , and loss shear modulus, G00 , of stoichiometric mixture of GMA/PETMP with different contents of DMP-30. Temperature: 25
C. The content of DMP-30 is expressed by weight percentage with respect to the total amount of GMA and PETMP.

From the intersection of the time dependences of G0 and G00 in Fig. 4(c), the gelation time are determined as 49, 38, and 35 min corresponding to the contents of DMP-30 of 1.0, 1.5, and 2.0 wt%,

D.Y. Zhu et al. / Polymer 69 (2015) 1e9

respectively. These results indicate that increasing the catalyst loading within the studied range can accelerate the polymerization reaction, but does not change the polymerization degree of the resultant polymer as shown by the stable overall G0 at the equilibrium state. This finding is useful for the subsequent formulation of healing agent to have both a high healing efficiency and healing speed. To further understand the polymerization of GMA, real-time micro-Raman spectra of the reaction system were measured at different time intervals (Fig. 5(a)). With increasing time, both the peaks of C]C and epoxide group are gradually diminished along with the consumption of eSH. Considering that the C]O group of GMA and polythiol is not involved in either reaction, its peak height can serve as an internal standard. Accordingly, the peak area ratios of C]C/C]O, epoxide group/C]O and eSH/C]O are calculated and plotted as a function of time in Fig. 5(b). The data show that the two functional groups of GMA participate in reactions at the same time, and the reaction is complete within around 60 min for the system with 1.5 wt.% DMP-30, which is consistent with the observation of isothermal DSC scanning (Fig. 4(a)) and the measured gel time (38 min, Fig. 4(c)). Thus, the thiol-ene and thiol-epoxy “click” reactions of GMA with polythiol occur quickly without heating or protection from an inert gas. Moreover, the reaction rates of C]C and epoxide group are different. The consumption of C]C

Fig. 5. (a) In-situ micro-Raman spectra of a stoichiometric mixture of GMA/PETMP containing 1.5 wt% DMP-30 recorded at different times at room temperature in air. (b) Peak area ratios estimated from (a) versus time.

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resembles that of the thiol group, which is faster than that of epoxide group. There remains a certain amount of unreacted epoxide group when nearly no reactive C]C remains as shown after the crossing point of the two curves in Fig. 5(b). This result explains the appearance of two exothermic peaks in Fig. 4(a) and their dependence on the catalyst concentration. On the basis of the above investigation, we started to include the components of the healing agent into microcapsules (refer to the Experimental for more details). When the GMA-loaded microcapsules and PETMP/DMP-30-loaded microcapsules were ground together between two glass slides, the encapsulated chemicals flowed out and turned into consolidated film after 3 h, tightly gluing the glass slides together. This finding shows that the healing agent maintains the reactivity after being microencapsulated,

Fig. 6. Thermal decomposition behaviors of (a) GMA-loaded microcapsules and (b) PETMP/DMP-30-loaded microcapsules in comparison with those of the core materials and extracted shell. Note: Although both the GMA-loaded microcapsules and PETMP/ DMP-30-loaded microcapsules are encapsulated with PMF, the preparation conditions and emulsifier systems are slightly different because of the different properties of the core materials (refer to the Experimental). Moreover, PETMP may react with the shell during encapsulation [38]. Accordingly, microstructures of the resultant PMF shells of the two types of microcapsules are not identical, leading to different pyrolytic performances of the extracted shell as shown above.

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Fig. 7. Dependence of healing efficiency of PVC composites on healing time. Total content of GMA-loaded microcapsules and PETMP/DMP-30-loaded microcapsules: 20 wt.%. To keep a stoichiometric ratio of the core materials, the weight ratio of GMAloaded microcapsules to PETMP/DMP-30-loaded microcapsules is 1:1.56. Healing temperature: 25
C.

which is critical to study the healing performance of the PVC composite hereinafter. Fig. 6 shows the thermal stability of the microcapsules by thermogravimetric analysis. For GMA-loaded microcapsules (Fig. 6(a)), the slight weight loss between 100 and 200
C results from the evaporation of (i) adsorbed moisture and (ii) formaldehyde produced by the deformaldehyde reaction of PMF [41]. The maximum rate of pyrolysis at ~220
C corresponds to the decomposition of the core material, GMA. In general, the microcapsules have good capsulation and heat resistance. In the case of PETMP/ DMP-30-loaded microcapsules, the main weight losses of both core and shell substances appear in the range from 250 to 450
C (Fig. 6(b)). The minor decomposition below 250
C is attributed to the evaporation of (i) adsorbed water, (ii) DMP-30 on the capsule surface, and (iii) free formaldehyde. Therefore, these microcapsules also have acquired reasonably high thermal stability. These results suggest that thermal processing of a polymer composite filled with these microcapsules at about 200
C would not deteriorate the healing agent. In a processing system for thermoplastics, the in situ temperature of the extruder is usually higher than the setting temperature because of mechanical heating, and the present healing system is not suitable for engineering plastics that require being processed at temperatures higher than 200
C.

Because the reactivity and thermal stability of the microencapsulated healing agent have been confirmed, self-healing PVC composites containing the healing capsules were fabricated and characterized. Fig. 7 illustrates the dependence of the healing efficiency of the composites on healing time at room temperature. The PVC composite is self-healable because of the embedded healing agent. The restoration of mechanical strength is realized without any manual intervention. The designed dual-microcapsule system works successfully, and the reactivity is not affected during the manufacturing of the composites. The healing capsules can maintain their integrity after the compression molding (Fig. 8). More importantly, the healing speed is greatly increased as compared with those of other self-healing thermoplastics. For example, selfhealing PS based on atom transfer radical polymerization required 48 h to yield the highest healing efficiency [23]. Our system requires 2 h at 25
C to attain a healing efficiency of 96.8%. The equilibrium maximum value of 100% is reached after 3 h. Because the main components of the healing agent, GMA, and PETMP, are small-molecule fluids before polymerization, they may exert a solvent effect on the damaged PVC specimen. In other words, the physical interaction represented by chain interdiffusion and entanglement across the crack interface [37] contributes to the damage rehabilitation. To clarify this issue, a stoichiometric mixture of GMA and PETMP was manually injected into the impactfractured surface of an unfilled PVC specimen to induce healing (refer to the Experimental). Because the basic catalyst, DMP-30, is not present, the “click” reactions cannot proceed. The healing efficiency, 25.3%, measured from the healed specimens is entirely the result of a solvent effect. Evidently, GMA/PETMP is not a good solvent for PVC because the solvent effect offers a low level of crack healing. When DMP-30 was added, the situation was completely different. For the unfilled PVC specimen, the healing efficiency of the manually healed version is 104.1%, indicating that the polymerization of the healing agent rather than the solvent effect is mainly responsible for crack healing. However, partial swelling of PVC by the healing agent molecules at the beginning of the process is necessary for the subsequent interlocking between the polymerized healing agent and the matrix, which ensures a high healing efficiency. The influence of microcapsule content on healing efficiency is studied in Fig. 9. For the convenience of comparison, a conservative healing period of 3 h is used for specimens to ensure sufficient healing under ambient conditions. At each data point, the weight ratio of GMA-loaded microcapsules to PETMP/DMP-30-loaded microcapsules is 1:1.56, which retains the stoichiometric ratio of the core materials. The healing efficiency increases with capsule concentration from 0 to 20 wt.%, and gradually levels off with capsule

Fig. 8. SEM images of the fractured surfaces of PVC composite containing (a) 10 wt.% and (b) 20 wt.% dual microcapsules. Weight ratio of GMA-loaded microcapsules to PETMP/ DMP-30-loaded microcapsules is 1:1.56. The released core material was removed by ethanol immediately after the impact test.

D.Y. Zhu et al. / Polymer 69 (2015) 1e9

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Fig. 9. Influence of healing capsule concentration on impact strength and healing efficiency of PVC composites. Weight ratio of GMA-loaded microcapsules to PETMP/ DMP-30-loaded microcapsules is 1:1.56 in all the self-healing specimens. Healing of the fractured specimens was conducted at 25
C for 3 h.

content from 20 to 30 wt.%. The variation is closely related to the amount of available healing agent released by the broken capsules on the fracture planes. However, when the capsule loading is high enough, 20 wt.% in the present case, further increases of the capsule loading do not improve the healing efficiency because the healing agent has covered almost the entire fractured surface. The increase of healing efficiency with an increase in content of the healing capsules and the highest healing efficiency (100%) that is close to the ceiling value (104.1%) measured from the above simulated healing experiment demonstrate again that most of the incorporated microcapsules are strong enough to resist the manufacturing process of the composite. Fig. 9 shows that the impact strength of the PVC composites decreases with increasing capsule content. This decline in ductility has also been reported for self-healing composites filled with microcapsules containing fluidic healing agent [22,23]. Comparatively, the reduction in the present study is not prominent. At the total microcapsule content of 20 wt.%, for example, the impact strength is 11.5% lower than that of unfilled PVC. In the case of solution-cast self-healing PS [22], however, the impact strength of the composite with 20 wt.% healing capsules is only about 60% of the value of unfilled PS. The higher retention of strength is promising for the engineering application of the composite. Further study to improve the adhesive capability of the cured healing agent would lead to a reduced amount of healing capsules required and hence a higher strength of the self-healing composite. Fig. 10 shows the fractured surfaces of the self-healing PVC composite. Many broken microcapsules left traces as characterized by cavities (Fig. 10(a)). By comparing Fig. 10(a) and (b), thin membranes covering the fracture plane can be identified (Fig. 10(b)), which are the polymerized healing agent. Further observation of an enlarged broken microcapsule on the fracture surface (Fig. 10(c)) confirms that the healing agent released from ruptured microcapsules was polymerized and is tightly bound to the matrix. Prior to the polymerization, GMA has the role of a small molecule solvent, promoting low-degree swelling and chain entanglement as revealed by the above mentioned solvent effect. Subsequently, GMA is polymerized into crosslinked macromolecules and the portions that have diffused into the matrix polymer reconnect the cracked PVC through mechanical interlocking inside the opposite sub-surfaces. The analysis of the healing mechanism is supported by EDS results of the re-fractured surface of a healed specimen (Fig. 11). In the region covered by a polymerized healing agent (i.e., EDS1 in

Fig. 10. SEM images of the fractured surfaces of PVC composite containing 10 wt.% dual microcapsules. Weight ratio of GMA-loaded microcapsules to PETMP/DMP-30-loaded microcapsules is 1:1.56. (a) The released core material was removed by ethanol immediately after the impact test. (b) The released core material was not removed after the impact test. (c) Enlarged view of a broken microcapsule from (b).

Fig. 11), the percentage of sulfur is much higher than that of chlorine. However, in the PVC matrix (i.e., EDS2 in Fig. 11), chlorine is the major element. Additionally, the rough appearance of the healing membrane originates from the cohesive failure of the healing membrane during the impact test. To check whether the proposed healing system is suitable for other thermoplastics, PS and poly(butylene succinate) (PBS) were

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Fig. 11. EDS analysis of the fracture surface of a healed PVC composite specimen, which contains 10 wt.% dual microcapsules. Weight ratio of GMA-loaded microcapsules to PETMP/ DMP-30-loaded microcapsules is 1:1.56.

evaluated following the same procedures of the simulative healing experiments of unfilled PVC via a manual injection of the healing agent on the fractured surfaces of the specimens. The measured healing efficiencies are 70.2% for PS and 33.3% for PBS, respectively, which are lower than the value of PVC (104.1%). Thus, the performance of the healing agent is dependent on the polymer. In general, the following three issues should be considered when designing a healing agent for a specific thermoplastic polymer [20e24]. First, the self-healing agent should be miscible with the target polymer and to allow wetting. As a result, the healing agent molecules infiltrate the matrix upon cracking of the healing capsules, swell the polymer, and induce a physical entanglement of matrix polymer across the crack interface, resulting in minor crack repair. Second, the fluidic healing agent should be polymerizable under the healing conditions, and the mechanical strength of the polymerized version should be higher than or comparable with that of the matrix, which is critical for obtaining a high repair efficiency. The healing agent molecules that have penetrated into the matrix polymer would be gradually polymerized. The formed macromolecular chains fill the crack volume and are firmly rooted in the sub-surface of the crack by entangling both the polymerized healing agent and the matrix. Consequently, the cracks are rebonded and the mechanical strength of the material is restored. Third, the encapsulated self-healing agent should be thermally and mechanically stable enough to survive the thermal processing of thermoplastics. Compared with the above criteria, we can see that the proposed healing system (GMA/PETMP/DMP-30) is suitable for PVC because of the good miscibility originating from the strong interaction between the “CeCl” groups in PVC and “-SH” in the healing agent. In contrast, the miscibility between PS or PBS and the healing agent is lower than that of PVC and thus a lower healing efficiency is produced. 4. Conclusions Nucleophilic thiol-click chemistry based on both “thiol-ene” and “thiol-epoxy” reactions is successfully applied to introduce commercially available PVC with self-healability. The healing process proceeds at room temperature in air and takes much less time to achieve the maximum healing effect as compared with other microcapsule-aided self-healing thermoplastics [18e22]. The encapsulated healing agent can withstand the compression molding of the PVC composite. Moreover, the reaction kinetics and

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