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Effect of failure modes on healing behavior and multiple healing capability of self-healing polyurethanes
Construction and Building Materials 186 (2018) 1212–1219
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effect of failure modes on healing behavior and multiple healing capability of self-healing polyurethanes Libang Feng a,⇑, Zhengyang Yu a, Yaohui Bian a, Yanping Wang a, Yanhua Zhao b, Liuxiaohui Gou a a b
School of Materials Science and Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China School of Civil Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Polyurethane with excellent thermal
The as-prepared polyurethanes (PU-DA) exhibit outstanding self-healing performance based on actions of thermo-reversible Diels-Alder reaction and molecular movement. PU-DA fragments can recombine together as a whole even after destroyed by tearing, indicating PU-DA exhibits outstanding reprocessing capability. The reprocessed PU-DA wins good mechanical property when torn fragments are treated at 120 °C for 30 min followed by 24h at 60 °C.
reversibility and self-healing performance is developed. Healing behavior is based on thermoreversible Diels-Alder reaction and molecular movement. Different failure modes have unalike repairing effect. Self-healing polyurethane presents outstanding reprocessing performance. The as-prepared polyurethane has multiple healing capability.
a r t i c l e
i n f o
Article history: Received 8 September 2017 Received in revised form 7 August 2018 Accepted 10 August 2018
Keywords: Polyurethane Self-healing Diels-Alder reaction Failure modes
⇑ Corresponding author. E-mail address: [email protected] (L. Feng). https://doi.org/10.1016/j.conbuildmat.2018.08.048 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
a b s t r a c t Materials can suffer from different damages in actual application. Consequently, diversified failure modes can be caused, while different failures may have unalike repairing effect. Thereupon, it is quite important of exploring the effect of failure modes on the repairing behavior of self-healing materials. Herein, a kind of thermoplastic polyurethane containing Diels-Alder bonds (PU-DA) has been synthesized successfully. Based on the dual actions of thermo-reversible Diels-Alder reaction and molecular movement, the asprepared polyurethane exhibits excellent self-healing performance. The repairing behavior of polyurethanes with different failure modes has been investigated both qualitatively and quantitatively. Results show that polyurethanes with various cracks can be repaired. However, PU-DA film with shallower crack exhibits a high healing efficiency as compared to that with deeper crack at the similar heat treatment. Meanwhile, polyurethane has multiple healing capability and cracks can be caused and repaired repeatedly. Additionally, PU-DA fragments can recombine together as a whole even after destroyed by tearing, indicating PU-DA also exhibits outstanding reprocessing capability. The research
L. Feng et al. / Construction and Building Materials 186 (2018) 1212–1219
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can lay foundation for realizing the favorable repairing of various failure modes, and further promote selfhealing materials and technology applied into practice. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction Polyurethane is a kind of polymer that has typical hard/soft segmental dual-component structure [1–3]. Because of its microphase separated structure as well as characteristic property, polyurethane has been used in a wide range of industrial applications, for instance, paints, coatings, adhesives, sealants, and insulation foams, and so on [4,5]. Once exposed to variable environmental conditions, such as temperature, moisture, chemicals, and radiation, micro-cracks may be caused in polyurethanes [6,7]. These micro-cracks can induce structure fragmentation, and which further leads to the reduction of mechanical properties such as strength, stiffness and dimensional stability [8]. Inspired by the self-healing phenomenon of biological organization [9], polyurethane with self-healing property has been put forward. Selfhealing materials are able to heal themselves either autonomically or in response to external stimuli such as changes in pH, heat, or light, etc. [10–12]. The thermal reversibility of Diels–Alder reaction (DA reaction) and the mild conditions of its retro reaction make it particularly desirable for preparing self-healing materials [13–15]. DA reaction is a [4 + 2] cycloaddition between a diene and a dienophile, which takes place at 55–80 °C. Consequently, an addition product (namely, DA adduct) will be resulted. DA adducts can be cleaved at 100–130 °C by opposite retro Diels-Alder reaction (r-DA reaction) [16–28]. The DA reaction and r-DA reaction can be repeatedly for many times by treating materials at different temperatures. Self-healing polyurethanes based on thermoreversible DA reaction has been developed and studied by several research groups [6,19,23,28]. Dolci [6] synthesized a thermoresponsive crosslinked polyurethane by cyclocarbonate/amine polymerization. Turkenburg [19] prepared a polyester-based polyurethane that contains various amounts of thermo-reversible bonds established by DA reaction between furfuryl and maleimide groups. Du [23] reported a linear self-healing polyurethane based on DA reaction between furan groups and bismaleimide groups. In these researches, emphases have been placed on self-healing materials preparation, thermo-reversible DA reaction and r-DA reaction, and the self-healing capability. Moreover, only the healing behavior of completely broken polyurethanes has been investigated. However, in the real life and actual use, materials may be destroyed by all sorts of external forces, such as bending, ballistic impact, razor cut, accidental damage, scratches, compression, and tensile, and so on [7,27]. Consequently, materials with various failure modes can be caused, while different failure modes may have unalike repairing effect. Therefore, it is quite important of exploring the effect of failure modes on the repair behavior of selfhealing materials. In our previous paper [28], a thermo-reversible polyurethane containing DA bonds (PU-DA) has been synthesized successfully. It has been proved that the as-prepared PU-DA exhibits excellent self-healing performance, and cracks in PU-DA films are repaired by combined actions of thermo-reversible DA reaction and thermal movement of molecular chains. Furthermore, DA bond and its thermal reversibility contribute much to the recovery of mechanical property while thermal movement effect of molecular chains acts as an auxiliary recovering force which accelerates the whole healing process. In actual use, polyurethanes may also suffer from different modes of damages, and consequently diversified failure modes can be caused in polyurethanes. Different failure modes
may have inequable repairing effect. Therefore, it is quite necessary of investigating the healing behavior of polyurethanes with different failure modes. Herein, the healing behavior of polyurethane with various failure modes, such as different crack depths, multiple damages, and tearing damage, are investigated in depth. Meanwhile, the multiple healing capability and reprocessing performance of the self-healing polyurethanes are examined. The research and obtained results can provide important reference for developing materials with excellent self-healing performance. Furthermore, the research can lay foundation for realizing the favorable repairing of various failure modes resulted from all sorts of external forces. 2. Material and methods 2.1. Materials 4,40 -Diphenylmethane diisocyanate (MDI) was purchased from Aladdin Co. Ltd. 1,10 -(Methylenedi-4,1-phenylene) bismaleimide (BMI) was provided by Energy Chemical. Furfuryl amine (FAm) was supplied by Aladdin. Poly(propylene glycol) (PPG-2000) with a number-average molecular weight of 2000 g/mol was purchased from Jiangsu Haian Petroleum Chemical Factory, and which was dried at 120 °C for 2 h under vacuum before use. N,N-dimethylformamide (DMF) was supplied by Rionlon corporation, and it was dried with anhydrous MgSO4 before use. 2.2. PU-DA synthesis and film preparation The linear self-healing polyurethane based on DA reaction (PU-DA) was synthesized via the reaction between furan-terminated prepolymer (MPF) and BMI, while MPF was synthesized from MDI while chains were extended by PPG-2000 and FAm. Further information about the successful synthesis and film preparation of PU-DA is available in our previous work [28]. The thickness of the prepared film is 1.5 mm. 2.3. Preparation of sample with different crack depths The PU-DA films with a crack of 0.5 mm, 1.0 mm, and 1.5 mm in depth, respectively, were prepared while cracks in PU-DA films were cut with a paper knife. The crack depth is controlled by steel blocks with a standard height of 0.5 mm and 1.0 mm, while crack with 1.5 mm in depth means the film is cut off. 2.4. Preparation of multiple damaged samples The PU-DA samples for multiple scratching were prepared according to Scheme 1. Firstly, PU-DA films with different crack depths, such as 0.5 mm, 1.0 mm, and 1.5 mm, were prepared with knife cutting. Next, samples were heattreated at 120 °C for 15 min and then 60 °C for 24 h. Consequently, cracks in films were healed and 1st healed PU-DA films are obtained. A similar cut is made at the original crack site in 1st healed PU-DA films once again. And 2nd healed PU-DA films are prepared after healed by the similar heat treatment procedure as 1st healed films. The 3rd healed PU-DA films are obtained in the same way. 2.5. The torn sample preparation and the reprocessing behavior evaluation The samples for reprocessing behavior evaluation were prepared by hot compression molding as follows: the original PU-DA films were torn into fragments with dimension of 3 mm 3 mm 1 mm. The fragments were then put into a rectangle-shaped mould on a polytetrafluoroethylene (PTFE) plate. The asprepared PU-DA presents in a yellow color. In order to observe obviously, part PU-DA films are dyed with methyl red. Then the fragments were formed via hot compression molding at 120 °C for 20–40 min followed by 24 h at 60 °C. The mechanical strength of the original and reprocessed films was measured. The size of original and reprocessed PU-DA films is 35 mm 20 mm 1 mm. 2.6. Thermal reversibility measurements The thermal reversibility of PU-DA was investigated by Fourier Transform Infrared Spectra (FT-IR, FTS-3000 spectrometer, DIGILAB), Differential Scanning Calorimetry (DSC, DSC-60, Shimadzu), and viscosity test (NDJ-8S, Shanghai Nirun
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Scheme 1. Sample preparation and heat treatment procedure for multiple damages. Intelligent Technology Co. Ltd.). The heat treatment procedure of PU-DA as well as the resultant different films for FT-IR and DSC measurements was performed as Scheme 2. Firstly, the as-prepared PU-DA film (which is labeled as PU-DA0) was heat-treated at 120 °C for 30 min. The reversible DA reaction takes place upon 120 °C of heat treatment, and the resultant film is denoted as r-PU-DA. Second, the r-PU-DA film was treated at 60 °C for 24 h and the resulted film is labeled as PU-DA1. The FT-IR and DSC of PU-DA0, r-PU-DA, and PU-DA1 films are measured, respectively. In additional, the viscosity change of PU-DA in DMF (20 wt%) is examined at 60 °C as soon as PU-DA in DMF is heat-treated at 120 °C for 20 min. After the test finished, PU-DA in DMF (20 wt%) is treated at 120 °C for another 20 min and the 2nd viscosity measurement is carried out. The viscosity change is measured for three times according to the similar procedure. 2.7. Self-healing property evaluation The self-healing property was evaluated both qualitatively and quantitatively. The qualitative observation was carried out by observing the crack evolution with a polarizing optical microscope (POM, Zeiss Axioskop 2 plus) at 120 °C. The quantitative analysis was examined by measuring the recovery of tensile strength after healing process, namely, treated at 120 °C for 15 min followed at 60 °C for 24 h. The self-healing efficiency was calculated by the recovery of tensile strength using the following equation:
g¼
rhealed rcut roriginal rcut
ð1Þ
where rhealed denotes the tensile strength of the healed sample, and rcut represents the tensile strength of sample after cut without healing, while roriginal is the tensile strength of original sample.
3. Results and discussion 3.1. Thermal reversibility of PU-DA The thermoplastic polyurethane containing DA bonds was synthesized according to the reaction between furan-terminated prepolymer (MPF) and bismaleimide (BMI) [28]. In order to confirm the thermo-reversible DA bonds have been introduced into polyurethane successfully and PU-DA has been endowed with the thermal reversibility, the chemical structure, thermal behavior, and viscosity of PU-DA before and after heat treatment are investigated by FT-IR, DSC, and viscosity test, as shown in Figs. 1 and 2. The thermal reversibility of PU-DA is investigated firstly by FTIR (see as Fig. 1a). The characteristic peak at 1774 cm1 in the spectrum of PU-DA0 confirms that DA bonds have been introduced into PU-DA successfully [29]. Peaks at 1717, 1599, and 1107 cm1 are ascribed to AC@O, ANAH, and ACAO adsorption, respectively, and which remain unchanged upon heat treatment and can be used as internal standard to allow quantification [30]. By contrast, the characteristic peak of DA bonds which presents at 1774 cm1 disappears after heat-treated at 120 °C for 30 min because of the retro DA reaction (r-DA reaction), see as the spectrum of r-PUDA. The characteristic peak of DA bonds appears again after r-PU-DA is heat-treated at 60 °C for 24 h (see as the spectrum of PU-DA1), indicating DA bonds regenerate upon 60 °C treatment. The reason that leads to this result is that DA bonds in PU-DA0
are broken because of r-DA reaction at 120 °C while DA bonds regenerate when treated at 60 °C for 24 h again. The results confirm that thermo-reversible DA bonds have been introduced into PU-DA successfully and PU-DA exhibits thermal reversibility. The thermal reversibility of PU-DA is further investigated by DSC technique, and DSC traces are shown in Fig. 1b. PU-DA0 exhibits a significant endothermic peak around 120 °C while r-PU-DA does not. The endothermic peak results from the cleavage of DA bonds due to r-DA reaction. The endothermic peak re-appears when r-PU-DA is treated at 60 °C for 24 h (see as DSC trace of PU-DA1), manifesting that DA bonds regenerate owing to DA reaction upon heat treatment at 60 °C. The results show PU-DA exhibits good thermal reversibility indeed. Furthermore, the thermal reversibility of PU-DA is verified by viscosity change upon heat treatment at 60 °C as well. The viscosity of PU-DA in DMF (20 wt%) is recorded at 60 °C after heattreated at 120 °C for 20 min, as shown in Fig. 2. It is obvious that PU-DA solution presents a very low viscosity after heat-treated at 120 °C for 20 min. The viscosity of the solution increases gradually when PU-DA solution is treated at 60 °C, and it reach a very large value as 1324 Pas after 90 min. The viscosity of the solution increases continuously until a PU-DA gel is resulted. The reason leads to the results can be explained as follows: r-DA reaction takes place at 120 °C and the long PU-DA chains are broken into MPF and BMI molecule. As a result, the molecular weight of PU-DA decreases and the viscosity reduces. By contrast, MPF and BMI connect again because of DA reaction and the long PU-DA chains regenerate upon heat-treated at 60 °C. So the viscosity of the PUDA solution grows gradually with 60 °C of treatment. More importantly, the obtained PU-DA gel can return back to solution with a low viscosity for the second time when the gel is treated at 120 °C for 20 min again. Similarly, the viscosity of the PU-DA solution increases gradually again when PU-DA is treated at 60 °C once more, and PU-DA can form gel for the second time upon 60 °C treatment for about 90 min. The phenomena of viscosity change upon heat treatment at 120 °C and then 60 °C can be repeated for at least three times, indicating that PU-DA exhibits excellent thermal reversibility. All of results above confirm PU-DA has the excellent thermal reversibility due to DA reaction and r-DA reaction, and which highlights that PU-DA can be used as self-healing materials [23,25]. Our previous paper [28] has confirmed that the as-prepared thermoplastic thermo-reversible polyurethane exhibits excellent self-healing performance, and cracks in PU-DA films can be repaired by dual actions of thermo-reversible DA reaction and thermal movement of molecular chains. Furthermore, DA bond and its thermal reversibility contribute much to the recovery of mechanical property while thermal movement effect of molecular chains acts as an auxiliary recovering force which accelerates the whole healing process. 3.2. Effect of crack depth on healing behavior of PU-DA
Scheme 2. Heat-treated procedure for thermal reversibility of PU-DA.
Various crack depths in polyurethane films can be caused in actual use. Different crack depths may have unalike repairing
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Fig. 1. Changes of chemical structure (a) and thermal property (b) of various PU-DAs.
Fig. 2. Viscosity upon heat treatment at 60 °C for three cycles.
effect. So the healing behavior of PU-DA with different crack depths upon heat treatment is investigated by POM and tensile test, as shown in Fig. 3 and Table 1. The self-healing behavior of PU-DA with different crack depths, i.e., 0.5 mm, 1.0 mm, and 1.5 mm, upon heat treatment at 120 °C is firstly investigated by observing the crack evolution with POM qualitatively, as shown in Fig. 3. It can be observed that the width of all cracks reduces gradually with the extending of heat treatment time, indicating that all PU-DA films have self-healing performance. However, the healing degree is different for three films at the same heat treatment time. In specific, cracks in PU-DA film with a crack depth of 0.5 mm and 1.0 mm have disappeared completely when the heat treatment time reaches 15 min and 20 min, respectively, as shown in Fig. 3a and b. By contrast, crack in PU-DA film with a crack depth of 1.5 mm disappears partly until 20 min of heat treatment at 120 °C, manifesting that the self-healing process has not finished at this moment. The results show that the repair efficiency is unequal for PU-DA with different crack depths. The reason that causes these results can be analyzed as follows: upon heat treatment at 120 °C, the long PU-DA chains will break into MPF prepolymer and BMI gradually because of r-DA reaction. The resultant short chains and molecules can move and diffuse across cracks under the thermal action. As a result, cracks in PU-DA films will be filled by polymer chains and BMI.
So cracks can be repaired gradually upon heat treatment. As the polyurethane is of elastic material, it is easy to deform upon heat treatment or subjected to external force. Thus the width of crack may enhance with the increase of crack depth as well, and which will lead to more time is needed to repair for PU-DA films with deeper cracks. Thereupon, more time is needed to repair the crack for PU-DA with deeper cracks as compared to that with shallower cracks. In short, a longer heat treatment time at 120 °C is required to fill and renovate the crack completely for PU-DA films with deeper cracks as compared to PU-DA films with shallower cracks. The self-healing behavior is further examined quantitatively via investigating the tensile property of PU-DA films with different crack depths before and after heat-treated at 120 °C for different time followed at 60 °C for 24 h. The effect of heat treatment time at 120 °C on the tensile strength of PU-DA with different crack depths is examined, and the investigated tensile data and healing efficiency are listed in Table 1. Meanwhile, in order to compare the tensile strength of original (as-prepared) and cut (with different crack depths) PU-DA films are incorporated in Table 1 as well. It is obvious that the tensile strength of the original PU-DA film is 6.55 MPa, and it decreases to 2.05 MPa and 1.07 MPa, respectively, when films are cut and cracks of 0.5 mm and 1.0 mm in depth are resulted. The thickness of the PU-DA film is 1.5 mm, so PU-DA film with 1.5 mm of crack in depth has been cut off. Thereupon, the tensile strength of the cut film with a crack of 1.5 mm in depth is zero. It is obvious that heat treatment time at 120 °C has marked influence on the tensile properties. The tensile strength of all PU-DA with cracks increases gradually with the extension of heat treatment time at 120 °C. It reaches 5.23 MPa, 3.77 MPa, and 2.31 MPa in turn for PU-DA films with a crack depth of 0.5 mm, 1.0 mm, and 1.5 mm when the heat treatment time extends to 15 min. However, the tensile strength begins to decrease with the heat treatment time further extending to 20 min. The reason can be illustrated by the following explanations: upon heat treatment at 120 °C, r-DA reaction takes place and long PU-DA chains break into short polymer segments such as MPF and BMI gradually. The short segments can move and diffuse from one side to another side of cracks under thermal action. Consequently, cracks will be filled by short PU-DA chains and BMI. When further treated at 60 °C, DA reaction between MPF and BMI takes place again and long PU-DA chains regenerate. As a result, cracks in films can be remended by the filled chains, and the strength can be restored gradually with the heat treatment time extending. However, too long time of treatment at 120 °C can lead to the side reaction. Moreover, that not all MPF and BMI can take part in the DA
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Fig. 3. POM photographs of crack evolution in PU-DA films from different crack depths during the heat treatment at 120 °C: (a) 0.5 mm, (b) 1.0 mm, and (c) 1.5 mm.
Table 1 Mechanical property and healing efficiency of PU-DA with various cracks in depth during the different heat treatment time at 120 °C. Heat treatment time (min) at 120 °C
Crack depth (mm)
Tensile strength (MPa)
Elongation at break (%)
Young’s modulus (MPa)
Healing efficiency (%)
Original
/
6.55 ± 0.36
444 ± 43
4.53 ± 0.28
/
Cut
0.5 1.0
2.05 ± 0.13 1.07 ± 0.11
92 ± 22 61 ± 17
3.08 ± 0.19 4.10 ± 0.21
/ /
5
0.5 1.0 1.5
3.56 ± 0.17 2.47 ± 0.27 0.65 ± 0.18
188 ± 29 223 ± 33 58 ± 17
1.75 ± 0.29 1.62 ± 0.21 0.84 ± 0.17
34 26 10
10
0.5 1.0 1.5
4.29 ± 0.23 3.36 ± 0.15 1.65 ± 0.30
239 ± 35 225 ± 30 100 ± 21
1.97 ± 0.30 1.89 ± 0.22 1.58 ± 0.24
50 42 25
15
0.5 1.0 1.5
5.23 ± 0.15 3.77 ± 0.23 2.31 ± 0.26
304 ± 32 246 ± 25 231 ± 27
2.11 ± 0.22 2.07 ± 0.25 1.78 ± 0.27
71 49 35
20
0.5 1.0 1.5
5.00 ± 0.12 3.49 ± 0.22 2.09 ± 0.28
263 ± 34 249 ± 29 209 ± 23
1.92 ± 0.25 1.80 ± 0.22 1.55 ± 0.18
66 44 32
reaction again at 60 °C.i.e., most of MPF and BMI can take part in the DA reaction again while a small quantity of MPF and BMI has not participated the DA reaction for the 2nd time [31,32]. As a result, part DA bonds and DA adducts have not regenerated for the 2nd treatment. Therefore, the amount of DA bonds will decrease. Both the side reaction and the loss of DA bonds can debate the mechanical strength of PU-DA films. Additionally, results reveal that the healing effect decreases with the increase of crack depth. Under the same heat treatment condition as 120 °C/15 min and then 60 °C/24 h, the healing efficiency can reach 71%, 49%, and 35%, respectively, for PU-DA films with a crack depth of 0.5 mm, 1.0 mm, and 1.5 mm in turn. The results are in accordance with what have been observed by POM (see as Fig. 3). Since more time may be needed to repair the deeper cracks than do for the shallower cracks, PU-DA film with deeper crack exhibits a low healing efficiency as compared to PU-DA film with shallower crack. Therefore, it can be concluded that all cracks with various depths in polyurethane films can be repaired. However, PU-DA film with deeper crack exhibits a low healing efficiency as compared to
that with shallower crack upon the similar heat treatment. Furthermore, the healing efficiency enhances gradually with the extending of the heat treatment time at 120 °C for all polyurethane films with different crack depths. 3.3. Effect of multiple damages on healing behavior and multiple healing efficiency of PU-DA The multiple healing ability is very important as materials may be damaged for many times in actual use. In our research, samples of multiple damages and repairs are prepared by cutting at the same crack in PU-DA and then treated upon 120 °C/15 min and then 60 °C/24 h repeatedly, see as Scheme 1. The effect of multiple damages on healing behavior of PU-DA is investigated by tensile test, and results are shown in Table 2. It can be found that after the first healing process, the tensile strength and elongation at break of PU-DAs with different crack depths increase while Young’s modulus decreases as compared to the corresponding cut films, respectively. For the same sample while damaged and healed for different times, the healing
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L. Feng et al. / Construction and Building Materials 186 (2018) 1212–1219 Table 2 Mechanical property and multiple healing efficiency of PU-DA with different crack depths. PU-DA sample
Tensile strength (MPa)
Elongation at break (%)
Young’s modulus (MPa)
Healing efficiency (%)
Original Cut 1st Healed 2nd Healed 3rd Healed
6.55 ± 0.36 2.05 ± 0.13 5.23 ± 0.15 4.45 ± 0.24 3.78 ± 0.21
444 ± 43 92 ± 22 304 ± 32 308 ± 29 283 ± 25
4.53 ± 0.28 3.08 ± 0.19 2.11 ± 0.22 1.76 ± 0.17 1.19 ± 0.19
/ / 71 53 38
1.0 mm
Cut 1st Healed 2nd Healed 3rd Healed
1.07 ± 0.11 3.77 ± 0.23 2.84 ± 0.21 2.20 ± 0.14
61 ± 17 246 ± 25 192 ± 21 178 ± 19
4.10 ± 0.21 2.07 ± 0.25 1.59 ± 0.14 1.32 ± 0.15
/ 49 32 21
1.5 mm
1st Healed 2nd Healed 3rd Healed
2.31 ± 0.26 1.63 ± 0.18 0.83 ± 0.13
231 ± 27 115 ± 14 75 ± 16
1.78 ± 0.27 1.42 ± 0.19 0.87 ± 0.16
35 25 13
0.5 mm
efficiency decreases gradually with the increase of cut and heat treatment numbers. However, even after the third damage and heat treatment process, the healing efficiency of PU-DAs with cracks of 0.5 mm, 1.0 mm, and 1.5 mm in depth can still reach 38%, 21%, and 13%, respectively, indicating that PU-DA has multiple healing capability and cracks can be repaired repeatedly. Similarly, PU-DA film with deeper crack exhibits a low healing efficiency after every damage and repair cycles as compared to PU-DA film with shallower crack. The reason that results in the decrease of healing efficiency with the increase of damage/heat treatment cycles is owing to both the side reaction and the loss of DA bonds as well [31,32]. Thus, results above show that cracks in polyurethane films can be caused and repaired for more than three times. The healing efficiency decreases gradually with the increase of cut and heat treatment numbers because of the side reaction and the incomplete formation of DA bonds. However, even after the third cycles of damage and heat treatment, a high healing efficiency can be obtained, indicating the as-prepared polyurethanes have multiple healing capability. 3.4. Effect of tearing damage on healing behavior and reprocessing performance of PU-DA Polyurethanes may also suffer from tearing damage in actual use, and the healing behavior and reprocessing performance of torn PU-DA based on DA reaction and r-DA reaction are examined. The self-healing behavior and the reprocessing procedure of torn PU-DAs upon heat treatment are shown in Fig. 4. Firstly, both the yellow and red PU-DA films are torn into fragments and
blended uniformly. Then the blended fragments are put into a rectangle-shaped mould and then treated by hot compression molding at 120 °C for different time (20 min, 30 min, and 40 min, respectively) followed by 24 h at 60 °C. Results show that PU-DA fragments with different colors can recombine together as a whole. The reason can be deduced as follows: the long PU-DA chains will break into short chains and molecules, for instance, MPF and BMI, at 120 °C because of r-DA reaction. The resultant short chains and molecules can move and diffuse among adjacent fragments under the thermal action. Consequently, the fragments will fit together by polyurethane chains. After then cooled down to 60 °C, DA reaction between MPF and BMI happens again and long polyurethane chains regenerate. As a result, the torn PU-DA fragments recombine together via DA bonds. In addition, it is obvious from reprocessing sample colors in Fig. 4 that fragments distribute more and more uniformly with the extending of the treatment time at 120 °C, indicating that the recombination among different fragments becomes better and better. Additionally, it is worth noting that the fracture appearance after stretching of the reprocessed PU-DA treated at 120 °C for 20 min is very irregular, as shown in Fig. 4. By contrast, the fracture appearances of both reprocessed PU-DA films treated at 120 °C for 30 min and 40 min are regular. These indicate that 20 min of treatment at 120 °C is not enough for r-DA reaction. Consequently, part PU-DA chains have not broken into MPF and BMI molecules, and which is unfavorable for chains and molecules moving and diffusing. As a result, it is inadequate of moving and diffusing for chains and molecules among adjacent fragments. It is easy to separate for fragments with incomplete combination during the stretching process. On the contrary, it is difficult to break for fragments with complete
Fig. 4. Reprocessing procedure and performance of torn PU-DAs.
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treatment time will bring about side reaction and loss of DA bonds [31,32]. Therefore, the reprocessed PU-DA with the optimal strength can be obtained when the torn fragments are treated at 120 °C for 30 min followed by 24 h at 60 °C. It can be concluded that the as-prepared PU-DA exhibits good self-healing capability even after tearing damage.
4. Conclusions
Fig. 5. Representative stress-strain curves of original and reprocessed PU-DAs.
combination. Thereupon, the irregular fracture is resulted upon drawing for the reprocessed PU-DA treated at 120 °C for 20 min. By contrast, when the heat treatment is enough, adequate molecules can move and diffuse among different fragments. Accordingly, gaps and spaces between fragments have been filled by the molecules and polymer chains. Furthermore, DA bonds create again and long PU-DA chains regenerate by treating at 60 °C. Therefore, the regular fracture is obtained when stretching. For further investigating the reprocessing performance quantitatively, tensile test is carried out and strength recovery ratio is calculated. Fig. 5 shows the representative stress-strain curves of original (as-prepared) and healed (heat-treated at 120 °C for 20 min, 30 min, and 40 min followed by 24 h at 60 °C, respectively) PU-DA films. The tensile test data and the strength recovery rate are listed in Table 3. It can be found from Fig. 5 and Table 3 that the tensile strength of the original PU-DA film can reach 6.53 MPa. After torn and reprocessed, the tensile strength of all resultant PU-DA films decreases as compared to that of the original one. The resultant PU-DA films reprocessed at 120 °C for different time have diverse strengths. The tensile strengths of the reprocessed PU-DA treated at 120 °C for 20 min, 30 min, and 40 min achieve 1.44 MPa, 3.06 MPa, and 2.23 MPa, respectively. Meanwhile, the tensile strength recovery rates can reach 22%, 47%, and 34% one by one accordingly. This can be explained by DA and r-DA reactions as well as thermal movement of molecular chains. The r-DA reaction takes place upon treated at 120 °C, and PU-DA chains are broken into MPF and BMI. MPF and BMI can move and diffuse among the adjacent fragments via thermal movement. Consequently, gaps and spaces among fragments can be filled by polymer chains upon heat treatment. As a result, PU-DA fragments recombine together as a whole. However, too short heat treatment time at 120 °C will lead to incomplete combination of PU-DA fragments while too long heat
A thermoplastic polyurethane containing DA bonds (PU-DA) has been synthesized successfully. The resultant PU-DA exhibits good thermal reversibility, and which has been confirmed by FT-IR, DSC, and viscosity measurements. Grounded on the excellent thermal reversibility of DA bonds and thermal movement effect of molecular chains, the as-synthesized polyurethane exhibits excellent self-healing and reprocessing performance. Polyurethanes may suffer from different modes of damages, and consequently diversified failure modes can be caused when polyurethanes are used in actual application environments. Results show that different failure modes have unalike repairing effect, and which has been proved by POM observation and tensile test. Under the same heat treatment conditions as 120 °C/15 min and then 60 °C/24 h, the healing efficiency can reach 71%, 49%, and 35%, respectively, for PU-DA films with a crack depth of 0.5 mm, 1.0 mm, and 1.5 mm in turn. Moreover, the as-prepared polyurethanes have multiple healing capability, and cracks in polyurethane films can be caused and repaired repeatedly. Even after the third damage and heat treatment process, the healing efficiency of PU-DAs with cracks of 0.5 mm, 1.0 mm, and 1.5 mm in depth can still reach 38%, 21%, and 13% in turn. Additionally, torn fragments from as-synthesized PU-DA films can be reprocessed, and torn fragments can recombine together as a whole upon hot compression molding. The tensile strength recovery can reach 22%, 47%, and 34% when the torn fragments of PU-DA are treated at 120 °C for 20 min, 30 min, and 40 min, respectively. So it can be concluded that the PU-DA exhibits good self-healing capability even after tearing damage. The reprocessed PU-DA with the optimal strength can be obtained when the torn fragments are treated at 120 °C for 30 min followed by 24 h at 60 °C. The research and obtained results can provide important reference for developing materials with excellent self-healing performance. Furthermore, the research can lay foundation for realizing the favorable repairing of various failure modes.
Acknowledgements This research is supported by National Natural Science Foundation of China (Grant No. 51463010).
Conflict of interest statements We declare there are no conflicts of interest.
Table 3 Mechanical property and healing efficiency of reprocessed PU-DAs with different heat treatment time at 120 °C. PU-DA sample
Heat treatment time (min) at 120 °C
Tensile Strength (MPa)
Elongation at break (%)
Young’s modulus (MPa)
Strength recovery rate (%)
Original
/
6.53 ± 0.36
448 ± 43
4.52 ± 0.25
/
Reprocessed
20 30 40
1.44 ± 0.18 3.06 ± 0.20 2.23 ± 0.17
145 ± 25 235 ± 23 198 ± 21
2.44 ± 0.17 3.68 ± 0.26 2.66 ± 0.22
22 47 34
L. Feng et al. / Construction and Building Materials 186 (2018) 1212–1219
References [1] S. Oprea, V.O. Potolinca, C.D. Varganici, Synthesis and properties of polyurethane urea with pyridine-2,6-dicarboxamide moieties in their structure, RSC Adv. 6 (2016) 106904–106913. [2] M. Garrido, J.R. Correia, T. Keller, Effect of service temperature on the shear creep response of rigid polyurethane foam used in composite sandwich floor panels, Constr. Build. Mater. 118 (2016) 235–244. [3] Y.R. Xu, D.J. Chen, A novel self-healing polyurethane based on disulfide bonds, Macromol. Chem. Phys. 217 (2016) 1191–1196. [4] K. Holzworth, Z. Jia, A.V. Amirkhizi, J. Qiao, S.N. Nasser, Effect of isocyanate content on thermal and mechanical properties of polyurea, Polymer 54 (2013) 3079–3085. [5] J.L. Gadley, R.J. Andrade, J.M. Maia, Effect of soft-to-hard segment ratio on viscoelastic behavior of model thermoplastic polyurethanes during phase transitions, Macromol. Mater. Eng. 301 (2016) 953–963. [6] E. Dolci, V. Froidevaux, G. Michaud, F. Simon, R. Auvergne, S. Fouquay, S. Caillol, Thermoresponsive crosslinked isocyanate-free polyurethanes by Diels-Alder polymerization, J. Appl. Polym. Sci. 134 (2017) 44408. [7] F. Awaja, S.N. Zhang, M. Tripathi, A. Nikiforov, N. Pugno, Cracks, microcracks and fracture in polymer structures: formation, detection, autonomic repair, Prog. Mater. Sci 83 (2016) 536–573. [8] J.W. Pang, I.P. Bond, A hollow fiber reinforced polymer composite encompassing self-healing and enhanced damage visibility, Compos. Sci. Technol. 65 (2005) 1791–1799. [9] P. Martin, Wound healing-aiming for perfect skin regeneration, Science 276 (1997) 75–81. [10] J.L. Wietor, R.P. Sijbesma, A self-healing elastomer, Angew. Chem. Int. Ed. 47 (2008) 8161–8163. [11] S. Burattini, B.W. Greenland, D. Chappell, H.M. Colquhoun, W. Hayes, Healable polymeric materials: a tutorial review, Chem. Soc. Rev 39 (2010) 1973–1985. [12] Y.L. Yang, X. Lu, W.W. Wang, A tough polyurethane elastomer with self-healing ability, Mater. Des. 127 (2017) 30–36. [13] Y.L. Liu, T.W. Chou, Self-healing polymers based on thermally reversible DielsAlder chemistry, Polym. Bull. 4 (2014) 2194–2205. [14] A. Duval, G. Couture, S. Caillol, L. Avérous, Biobased and aromatic reversible thermoset networks from condensed tannins via the Diels-Alder reaction, ACS Sustainable Chem. Eng. 5 (2017) 1199–1207. [15] S. Boufi, M.N. Belgacem, J. Quillerou, A. Gandini, Urethanes and polyurethanes bearing furan moieties. 4. Synthesis, kinetics and characterization of linear polymers, Macromolecules 26 (1993) 6706–6717. [16] C.D. Varganici, O. Ursache, C. Gaina, V. Gaina, D. Rosu, B.C. Simionescu, Synthesis and characterization of a new thermoreversible polyurethane network, Ind. Eng. Chem. Res. 52 (2013) 5287–5295. [17] P.F. Du, M.Y. Wu, X.X. Liu, Z. Zheng, X.L. Wang, T. Joncheray, Y.F. Zhang, DielsAlder-based crosslinked self-healing polyurethane/urea from polymeric methylene diphenyl diisocyanate, J. Appl. Polym. Sci. 131 (2014) 1017–1022.
1219
[18] X.J. Cheng, C. Peng, D.H. Zhang, S.Z. Liu, A.Q. Zhang, H. Huang, J.S. Lian, A facile method for the preparation of thermally remendable cross-linked polyphosphazenes, J. Polym. Sci. Polym. Chem. 51 (2013) 1205–1214. [19] D.H. Turkenburg, H.V. Bracht, B. Funke, M. Schmider, D. Janke, H.R. Fischer, Polyurethane adhesives containing Diels-Alder based thermoreversible bonds, J. Appl. Polym. Sci. 134 (2017) 44972. [20] L. Irusta, J.F.B. Ma, J. Aizpurua, Polyurethanes based on isophorone diisocyanate trimer and polypropylene glycol crosslinked by thermal reversible diels alder reactions, J. Appl. Polym. Sci. 133 (2016) 44543. [21] C. Lakatos, K. Czifrak, J.K. Kocsis, L. Daroczi, M. Zsuga, S. Keki, Shape memory crosslinked polyurethanes containing thermoreversible Diels-Alder couplings, J. Appl. Polym. Sci. 133 (2016) 44145. [22] P.F. Du, H.Y. Jia, Q.H. Chen, Z. Zheng, X.L. Wang, D.L. Chen, Slightly crosslinked polyurethane with Diels-Alder adducts from trimethylolpropane, J. Appl. Polym. Sci. 133 (2016) 43971. [23] P.F. Du, X.X. Liu, Z. Zheng, X.L. Wang, T. Joncheray, Y.F. Zhang, Synthesis and characterization of linear self-healing polyurethane based on thermally reversible Diels-Alder reaction, RSC Adv. 3 (2013) 15475–15482. [24] Z.Y. Yu, L.B. Feng, C.S. Chai, Y.P. Wang, Y.H. Liu, X.H. Qiang, Structure and healing behavior of self-healing polyurethane based on Diels-Alder reaction, Acta Polym. Sin. 11 (2016) 1579–1586. [25] P.F. Du, M.Y. Wu, X.X. Liu, Z. Zheng, X.L. Wang, P.Y. Sun, T. Joncheray, Y.F. Zhang, Synthesis of linear polyurethane bearing pendant furan and crosslinked healable polyurethane containing Diels-Alder bonds, New J. Chem. 38 (2014) 770–776. [26] S. Yu, R.C. Zhang, Q. Wu, T.H. Chen, P.C. Sun, Bio-inspired high-performance and recyclable cross-linked polymers, Adv. Mater. 25 (2013) 4912–4917. [27] P. Zhang, G. Li, Advances in healing-on-demand polymers and polymer composites, Prog. Polym. Sci. 57 (2016) 32–63. [28] L.B. Feng, Z.Y. Yu, Y.H. Bian, J.S. Lu, X.T. Shi, C.S. Chai, Self-healing behavior of polyurethanes based on dual actions of thermo-reversible Diels-Alder reaction and thermal movement of molecular chains, Polymer 124 (2017) 48–59. [29] K. Adachi, A.K. Achimuthu, Y. Chujo, Synthesis of organic-inorganic polymer hybrids controlled by Diels-Alder reaction, Macromolecules 37 (2004) 9793– 9797. [30] Y.L. Liu, C.Y. Hsieh, Crosslinked epoxy materials exhibiting thermal remendablility and removability from multifunctional maleimide and furan compounds., J. Polym. Sci. Part A: Polym. Chem. 44 (2006) 905–913. [31] G. Postiglione, S. Turri, M. Levi, Effect of the plasticizer on the self-healing properties of a polymer coating based on the thermoreversible Diels-Alder reaction, Prog. Org. Coat. 78 (2015) 526–531. [32] G. Rivero, L.T.T. Nguyen, X.K. Hillewaere, F.E. Prez, One-pot thermoremendable shape memory polyurethanes, Macromolecules 47 (2014) 2010– 2018.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effect of failure modes on healing behavior and multiple healing capability of self-healing polyurethanes Libang Feng a,⇑, Zhengyang Yu a, Yaohui Bian a, Yanping Wang a, Yanhua Zhao b, Liuxiaohui Gou a a b
School of Materials Science and Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China School of Civil Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Polyurethane with excellent thermal
The as-prepared polyurethanes (PU-DA) exhibit outstanding self-healing performance based on actions of thermo-reversible Diels-Alder reaction and molecular movement. PU-DA fragments can recombine together as a whole even after destroyed by tearing, indicating PU-DA exhibits outstanding reprocessing capability. The reprocessed PU-DA wins good mechanical property when torn fragments are treated at 120 °C for 30 min followed by 24h at 60 °C.
reversibility and self-healing performance is developed. Healing behavior is based on thermoreversible Diels-Alder reaction and molecular movement. Different failure modes have unalike repairing effect. Self-healing polyurethane presents outstanding reprocessing performance. The as-prepared polyurethane has multiple healing capability.
a r t i c l e
i n f o
Article history: Received 8 September 2017 Received in revised form 7 August 2018 Accepted 10 August 2018
Keywords: Polyurethane Self-healing Diels-Alder reaction Failure modes
⇑ Corresponding author. E-mail address: [email protected] (L. Feng). https://doi.org/10.1016/j.conbuildmat.2018.08.048 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
a b s t r a c t Materials can suffer from different damages in actual application. Consequently, diversified failure modes can be caused, while different failures may have unalike repairing effect. Thereupon, it is quite important of exploring the effect of failure modes on the repairing behavior of self-healing materials. Herein, a kind of thermoplastic polyurethane containing Diels-Alder bonds (PU-DA) has been synthesized successfully. Based on the dual actions of thermo-reversible Diels-Alder reaction and molecular movement, the asprepared polyurethane exhibits excellent self-healing performance. The repairing behavior of polyurethanes with different failure modes has been investigated both qualitatively and quantitatively. Results show that polyurethanes with various cracks can be repaired. However, PU-DA film with shallower crack exhibits a high healing efficiency as compared to that with deeper crack at the similar heat treatment. Meanwhile, polyurethane has multiple healing capability and cracks can be caused and repaired repeatedly. Additionally, PU-DA fragments can recombine together as a whole even after destroyed by tearing, indicating PU-DA also exhibits outstanding reprocessing capability. The research
L. Feng et al. / Construction and Building Materials 186 (2018) 1212–1219
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can lay foundation for realizing the favorable repairing of various failure modes, and further promote selfhealing materials and technology applied into practice. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction Polyurethane is a kind of polymer that has typical hard/soft segmental dual-component structure [1–3]. Because of its microphase separated structure as well as characteristic property, polyurethane has been used in a wide range of industrial applications, for instance, paints, coatings, adhesives, sealants, and insulation foams, and so on [4,5]. Once exposed to variable environmental conditions, such as temperature, moisture, chemicals, and radiation, micro-cracks may be caused in polyurethanes [6,7]. These micro-cracks can induce structure fragmentation, and which further leads to the reduction of mechanical properties such as strength, stiffness and dimensional stability [8]. Inspired by the self-healing phenomenon of biological organization [9], polyurethane with self-healing property has been put forward. Selfhealing materials are able to heal themselves either autonomically or in response to external stimuli such as changes in pH, heat, or light, etc. [10–12]. The thermal reversibility of Diels–Alder reaction (DA reaction) and the mild conditions of its retro reaction make it particularly desirable for preparing self-healing materials [13–15]. DA reaction is a [4 + 2] cycloaddition between a diene and a dienophile, which takes place at 55–80 °C. Consequently, an addition product (namely, DA adduct) will be resulted. DA adducts can be cleaved at 100–130 °C by opposite retro Diels-Alder reaction (r-DA reaction) [16–28]. The DA reaction and r-DA reaction can be repeatedly for many times by treating materials at different temperatures. Self-healing polyurethanes based on thermoreversible DA reaction has been developed and studied by several research groups [6,19,23,28]. Dolci [6] synthesized a thermoresponsive crosslinked polyurethane by cyclocarbonate/amine polymerization. Turkenburg [19] prepared a polyester-based polyurethane that contains various amounts of thermo-reversible bonds established by DA reaction between furfuryl and maleimide groups. Du [23] reported a linear self-healing polyurethane based on DA reaction between furan groups and bismaleimide groups. In these researches, emphases have been placed on self-healing materials preparation, thermo-reversible DA reaction and r-DA reaction, and the self-healing capability. Moreover, only the healing behavior of completely broken polyurethanes has been investigated. However, in the real life and actual use, materials may be destroyed by all sorts of external forces, such as bending, ballistic impact, razor cut, accidental damage, scratches, compression, and tensile, and so on [7,27]. Consequently, materials with various failure modes can be caused, while different failure modes may have unalike repairing effect. Therefore, it is quite important of exploring the effect of failure modes on the repair behavior of selfhealing materials. In our previous paper [28], a thermo-reversible polyurethane containing DA bonds (PU-DA) has been synthesized successfully. It has been proved that the as-prepared PU-DA exhibits excellent self-healing performance, and cracks in PU-DA films are repaired by combined actions of thermo-reversible DA reaction and thermal movement of molecular chains. Furthermore, DA bond and its thermal reversibility contribute much to the recovery of mechanical property while thermal movement effect of molecular chains acts as an auxiliary recovering force which accelerates the whole healing process. In actual use, polyurethanes may also suffer from different modes of damages, and consequently diversified failure modes can be caused in polyurethanes. Different failure modes
may have inequable repairing effect. Therefore, it is quite necessary of investigating the healing behavior of polyurethanes with different failure modes. Herein, the healing behavior of polyurethane with various failure modes, such as different crack depths, multiple damages, and tearing damage, are investigated in depth. Meanwhile, the multiple healing capability and reprocessing performance of the self-healing polyurethanes are examined. The research and obtained results can provide important reference for developing materials with excellent self-healing performance. Furthermore, the research can lay foundation for realizing the favorable repairing of various failure modes resulted from all sorts of external forces. 2. Material and methods 2.1. Materials 4,40 -Diphenylmethane diisocyanate (MDI) was purchased from Aladdin Co. Ltd. 1,10 -(Methylenedi-4,1-phenylene) bismaleimide (BMI) was provided by Energy Chemical. Furfuryl amine (FAm) was supplied by Aladdin. Poly(propylene glycol) (PPG-2000) with a number-average molecular weight of 2000 g/mol was purchased from Jiangsu Haian Petroleum Chemical Factory, and which was dried at 120 °C for 2 h under vacuum before use. N,N-dimethylformamide (DMF) was supplied by Rionlon corporation, and it was dried with anhydrous MgSO4 before use. 2.2. PU-DA synthesis and film preparation The linear self-healing polyurethane based on DA reaction (PU-DA) was synthesized via the reaction between furan-terminated prepolymer (MPF) and BMI, while MPF was synthesized from MDI while chains were extended by PPG-2000 and FAm. Further information about the successful synthesis and film preparation of PU-DA is available in our previous work [28]. The thickness of the prepared film is 1.5 mm. 2.3. Preparation of sample with different crack depths The PU-DA films with a crack of 0.5 mm, 1.0 mm, and 1.5 mm in depth, respectively, were prepared while cracks in PU-DA films were cut with a paper knife. The crack depth is controlled by steel blocks with a standard height of 0.5 mm and 1.0 mm, while crack with 1.5 mm in depth means the film is cut off. 2.4. Preparation of multiple damaged samples The PU-DA samples for multiple scratching were prepared according to Scheme 1. Firstly, PU-DA films with different crack depths, such as 0.5 mm, 1.0 mm, and 1.5 mm, were prepared with knife cutting. Next, samples were heattreated at 120 °C for 15 min and then 60 °C for 24 h. Consequently, cracks in films were healed and 1st healed PU-DA films are obtained. A similar cut is made at the original crack site in 1st healed PU-DA films once again. And 2nd healed PU-DA films are prepared after healed by the similar heat treatment procedure as 1st healed films. The 3rd healed PU-DA films are obtained in the same way. 2.5. The torn sample preparation and the reprocessing behavior evaluation The samples for reprocessing behavior evaluation were prepared by hot compression molding as follows: the original PU-DA films were torn into fragments with dimension of 3 mm 3 mm 1 mm. The fragments were then put into a rectangle-shaped mould on a polytetrafluoroethylene (PTFE) plate. The asprepared PU-DA presents in a yellow color. In order to observe obviously, part PU-DA films are dyed with methyl red. Then the fragments were formed via hot compression molding at 120 °C for 20–40 min followed by 24 h at 60 °C. The mechanical strength of the original and reprocessed films was measured. The size of original and reprocessed PU-DA films is 35 mm 20 mm 1 mm. 2.6. Thermal reversibility measurements The thermal reversibility of PU-DA was investigated by Fourier Transform Infrared Spectra (FT-IR, FTS-3000 spectrometer, DIGILAB), Differential Scanning Calorimetry (DSC, DSC-60, Shimadzu), and viscosity test (NDJ-8S, Shanghai Nirun
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Scheme 1. Sample preparation and heat treatment procedure for multiple damages. Intelligent Technology Co. Ltd.). The heat treatment procedure of PU-DA as well as the resultant different films for FT-IR and DSC measurements was performed as Scheme 2. Firstly, the as-prepared PU-DA film (which is labeled as PU-DA0) was heat-treated at 120 °C for 30 min. The reversible DA reaction takes place upon 120 °C of heat treatment, and the resultant film is denoted as r-PU-DA. Second, the r-PU-DA film was treated at 60 °C for 24 h and the resulted film is labeled as PU-DA1. The FT-IR and DSC of PU-DA0, r-PU-DA, and PU-DA1 films are measured, respectively. In additional, the viscosity change of PU-DA in DMF (20 wt%) is examined at 60 °C as soon as PU-DA in DMF is heat-treated at 120 °C for 20 min. After the test finished, PU-DA in DMF (20 wt%) is treated at 120 °C for another 20 min and the 2nd viscosity measurement is carried out. The viscosity change is measured for three times according to the similar procedure. 2.7. Self-healing property evaluation The self-healing property was evaluated both qualitatively and quantitatively. The qualitative observation was carried out by observing the crack evolution with a polarizing optical microscope (POM, Zeiss Axioskop 2 plus) at 120 °C. The quantitative analysis was examined by measuring the recovery of tensile strength after healing process, namely, treated at 120 °C for 15 min followed at 60 °C for 24 h. The self-healing efficiency was calculated by the recovery of tensile strength using the following equation:
g¼
rhealed rcut roriginal rcut
ð1Þ
where rhealed denotes the tensile strength of the healed sample, and rcut represents the tensile strength of sample after cut without healing, while roriginal is the tensile strength of original sample.
3. Results and discussion 3.1. Thermal reversibility of PU-DA The thermoplastic polyurethane containing DA bonds was synthesized according to the reaction between furan-terminated prepolymer (MPF) and bismaleimide (BMI) [28]. In order to confirm the thermo-reversible DA bonds have been introduced into polyurethane successfully and PU-DA has been endowed with the thermal reversibility, the chemical structure, thermal behavior, and viscosity of PU-DA before and after heat treatment are investigated by FT-IR, DSC, and viscosity test, as shown in Figs. 1 and 2. The thermal reversibility of PU-DA is investigated firstly by FTIR (see as Fig. 1a). The characteristic peak at 1774 cm1 in the spectrum of PU-DA0 confirms that DA bonds have been introduced into PU-DA successfully [29]. Peaks at 1717, 1599, and 1107 cm1 are ascribed to AC@O, ANAH, and ACAO adsorption, respectively, and which remain unchanged upon heat treatment and can be used as internal standard to allow quantification [30]. By contrast, the characteristic peak of DA bonds which presents at 1774 cm1 disappears after heat-treated at 120 °C for 30 min because of the retro DA reaction (r-DA reaction), see as the spectrum of r-PUDA. The characteristic peak of DA bonds appears again after r-PU-DA is heat-treated at 60 °C for 24 h (see as the spectrum of PU-DA1), indicating DA bonds regenerate upon 60 °C treatment. The reason that leads to this result is that DA bonds in PU-DA0
are broken because of r-DA reaction at 120 °C while DA bonds regenerate when treated at 60 °C for 24 h again. The results confirm that thermo-reversible DA bonds have been introduced into PU-DA successfully and PU-DA exhibits thermal reversibility. The thermal reversibility of PU-DA is further investigated by DSC technique, and DSC traces are shown in Fig. 1b. PU-DA0 exhibits a significant endothermic peak around 120 °C while r-PU-DA does not. The endothermic peak results from the cleavage of DA bonds due to r-DA reaction. The endothermic peak re-appears when r-PU-DA is treated at 60 °C for 24 h (see as DSC trace of PU-DA1), manifesting that DA bonds regenerate owing to DA reaction upon heat treatment at 60 °C. The results show PU-DA exhibits good thermal reversibility indeed. Furthermore, the thermal reversibility of PU-DA is verified by viscosity change upon heat treatment at 60 °C as well. The viscosity of PU-DA in DMF (20 wt%) is recorded at 60 °C after heattreated at 120 °C for 20 min, as shown in Fig. 2. It is obvious that PU-DA solution presents a very low viscosity after heat-treated at 120 °C for 20 min. The viscosity of the solution increases gradually when PU-DA solution is treated at 60 °C, and it reach a very large value as 1324 Pas after 90 min. The viscosity of the solution increases continuously until a PU-DA gel is resulted. The reason leads to the results can be explained as follows: r-DA reaction takes place at 120 °C and the long PU-DA chains are broken into MPF and BMI molecule. As a result, the molecular weight of PU-DA decreases and the viscosity reduces. By contrast, MPF and BMI connect again because of DA reaction and the long PU-DA chains regenerate upon heat-treated at 60 °C. So the viscosity of the PUDA solution grows gradually with 60 °C of treatment. More importantly, the obtained PU-DA gel can return back to solution with a low viscosity for the second time when the gel is treated at 120 °C for 20 min again. Similarly, the viscosity of the PU-DA solution increases gradually again when PU-DA is treated at 60 °C once more, and PU-DA can form gel for the second time upon 60 °C treatment for about 90 min. The phenomena of viscosity change upon heat treatment at 120 °C and then 60 °C can be repeated for at least three times, indicating that PU-DA exhibits excellent thermal reversibility. All of results above confirm PU-DA has the excellent thermal reversibility due to DA reaction and r-DA reaction, and which highlights that PU-DA can be used as self-healing materials [23,25]. Our previous paper [28] has confirmed that the as-prepared thermoplastic thermo-reversible polyurethane exhibits excellent self-healing performance, and cracks in PU-DA films can be repaired by dual actions of thermo-reversible DA reaction and thermal movement of molecular chains. Furthermore, DA bond and its thermal reversibility contribute much to the recovery of mechanical property while thermal movement effect of molecular chains acts as an auxiliary recovering force which accelerates the whole healing process. 3.2. Effect of crack depth on healing behavior of PU-DA
Scheme 2. Heat-treated procedure for thermal reversibility of PU-DA.
Various crack depths in polyurethane films can be caused in actual use. Different crack depths may have unalike repairing
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Fig. 1. Changes of chemical structure (a) and thermal property (b) of various PU-DAs.
Fig. 2. Viscosity upon heat treatment at 60 °C for three cycles.
effect. So the healing behavior of PU-DA with different crack depths upon heat treatment is investigated by POM and tensile test, as shown in Fig. 3 and Table 1. The self-healing behavior of PU-DA with different crack depths, i.e., 0.5 mm, 1.0 mm, and 1.5 mm, upon heat treatment at 120 °C is firstly investigated by observing the crack evolution with POM qualitatively, as shown in Fig. 3. It can be observed that the width of all cracks reduces gradually with the extending of heat treatment time, indicating that all PU-DA films have self-healing performance. However, the healing degree is different for three films at the same heat treatment time. In specific, cracks in PU-DA film with a crack depth of 0.5 mm and 1.0 mm have disappeared completely when the heat treatment time reaches 15 min and 20 min, respectively, as shown in Fig. 3a and b. By contrast, crack in PU-DA film with a crack depth of 1.5 mm disappears partly until 20 min of heat treatment at 120 °C, manifesting that the self-healing process has not finished at this moment. The results show that the repair efficiency is unequal for PU-DA with different crack depths. The reason that causes these results can be analyzed as follows: upon heat treatment at 120 °C, the long PU-DA chains will break into MPF prepolymer and BMI gradually because of r-DA reaction. The resultant short chains and molecules can move and diffuse across cracks under the thermal action. As a result, cracks in PU-DA films will be filled by polymer chains and BMI.
So cracks can be repaired gradually upon heat treatment. As the polyurethane is of elastic material, it is easy to deform upon heat treatment or subjected to external force. Thus the width of crack may enhance with the increase of crack depth as well, and which will lead to more time is needed to repair for PU-DA films with deeper cracks. Thereupon, more time is needed to repair the crack for PU-DA with deeper cracks as compared to that with shallower cracks. In short, a longer heat treatment time at 120 °C is required to fill and renovate the crack completely for PU-DA films with deeper cracks as compared to PU-DA films with shallower cracks. The self-healing behavior is further examined quantitatively via investigating the tensile property of PU-DA films with different crack depths before and after heat-treated at 120 °C for different time followed at 60 °C for 24 h. The effect of heat treatment time at 120 °C on the tensile strength of PU-DA with different crack depths is examined, and the investigated tensile data and healing efficiency are listed in Table 1. Meanwhile, in order to compare the tensile strength of original (as-prepared) and cut (with different crack depths) PU-DA films are incorporated in Table 1 as well. It is obvious that the tensile strength of the original PU-DA film is 6.55 MPa, and it decreases to 2.05 MPa and 1.07 MPa, respectively, when films are cut and cracks of 0.5 mm and 1.0 mm in depth are resulted. The thickness of the PU-DA film is 1.5 mm, so PU-DA film with 1.5 mm of crack in depth has been cut off. Thereupon, the tensile strength of the cut film with a crack of 1.5 mm in depth is zero. It is obvious that heat treatment time at 120 °C has marked influence on the tensile properties. The tensile strength of all PU-DA with cracks increases gradually with the extension of heat treatment time at 120 °C. It reaches 5.23 MPa, 3.77 MPa, and 2.31 MPa in turn for PU-DA films with a crack depth of 0.5 mm, 1.0 mm, and 1.5 mm when the heat treatment time extends to 15 min. However, the tensile strength begins to decrease with the heat treatment time further extending to 20 min. The reason can be illustrated by the following explanations: upon heat treatment at 120 °C, r-DA reaction takes place and long PU-DA chains break into short polymer segments such as MPF and BMI gradually. The short segments can move and diffuse from one side to another side of cracks under thermal action. Consequently, cracks will be filled by short PU-DA chains and BMI. When further treated at 60 °C, DA reaction between MPF and BMI takes place again and long PU-DA chains regenerate. As a result, cracks in films can be remended by the filled chains, and the strength can be restored gradually with the heat treatment time extending. However, too long time of treatment at 120 °C can lead to the side reaction. Moreover, that not all MPF and BMI can take part in the DA
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Fig. 3. POM photographs of crack evolution in PU-DA films from different crack depths during the heat treatment at 120 °C: (a) 0.5 mm, (b) 1.0 mm, and (c) 1.5 mm.
Table 1 Mechanical property and healing efficiency of PU-DA with various cracks in depth during the different heat treatment time at 120 °C. Heat treatment time (min) at 120 °C
Crack depth (mm)
Tensile strength (MPa)
Elongation at break (%)
Young’s modulus (MPa)
Healing efficiency (%)
Original
/
6.55 ± 0.36
444 ± 43
4.53 ± 0.28
/
Cut
0.5 1.0
2.05 ± 0.13 1.07 ± 0.11
92 ± 22 61 ± 17
3.08 ± 0.19 4.10 ± 0.21
/ /
5
0.5 1.0 1.5
3.56 ± 0.17 2.47 ± 0.27 0.65 ± 0.18
188 ± 29 223 ± 33 58 ± 17
1.75 ± 0.29 1.62 ± 0.21 0.84 ± 0.17
34 26 10
10
0.5 1.0 1.5
4.29 ± 0.23 3.36 ± 0.15 1.65 ± 0.30
239 ± 35 225 ± 30 100 ± 21
1.97 ± 0.30 1.89 ± 0.22 1.58 ± 0.24
50 42 25
15
0.5 1.0 1.5
5.23 ± 0.15 3.77 ± 0.23 2.31 ± 0.26
304 ± 32 246 ± 25 231 ± 27
2.11 ± 0.22 2.07 ± 0.25 1.78 ± 0.27
71 49 35
20
0.5 1.0 1.5
5.00 ± 0.12 3.49 ± 0.22 2.09 ± 0.28
263 ± 34 249 ± 29 209 ± 23
1.92 ± 0.25 1.80 ± 0.22 1.55 ± 0.18
66 44 32
reaction again at 60 °C.i.e., most of MPF and BMI can take part in the DA reaction again while a small quantity of MPF and BMI has not participated the DA reaction for the 2nd time [31,32]. As a result, part DA bonds and DA adducts have not regenerated for the 2nd treatment. Therefore, the amount of DA bonds will decrease. Both the side reaction and the loss of DA bonds can debate the mechanical strength of PU-DA films. Additionally, results reveal that the healing effect decreases with the increase of crack depth. Under the same heat treatment condition as 120 °C/15 min and then 60 °C/24 h, the healing efficiency can reach 71%, 49%, and 35%, respectively, for PU-DA films with a crack depth of 0.5 mm, 1.0 mm, and 1.5 mm in turn. The results are in accordance with what have been observed by POM (see as Fig. 3). Since more time may be needed to repair the deeper cracks than do for the shallower cracks, PU-DA film with deeper crack exhibits a low healing efficiency as compared to PU-DA film with shallower crack. Therefore, it can be concluded that all cracks with various depths in polyurethane films can be repaired. However, PU-DA film with deeper crack exhibits a low healing efficiency as compared to
that with shallower crack upon the similar heat treatment. Furthermore, the healing efficiency enhances gradually with the extending of the heat treatment time at 120 °C for all polyurethane films with different crack depths. 3.3. Effect of multiple damages on healing behavior and multiple healing efficiency of PU-DA The multiple healing ability is very important as materials may be damaged for many times in actual use. In our research, samples of multiple damages and repairs are prepared by cutting at the same crack in PU-DA and then treated upon 120 °C/15 min and then 60 °C/24 h repeatedly, see as Scheme 1. The effect of multiple damages on healing behavior of PU-DA is investigated by tensile test, and results are shown in Table 2. It can be found that after the first healing process, the tensile strength and elongation at break of PU-DAs with different crack depths increase while Young’s modulus decreases as compared to the corresponding cut films, respectively. For the same sample while damaged and healed for different times, the healing
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L. Feng et al. / Construction and Building Materials 186 (2018) 1212–1219 Table 2 Mechanical property and multiple healing efficiency of PU-DA with different crack depths. PU-DA sample
Tensile strength (MPa)
Elongation at break (%)
Young’s modulus (MPa)
Healing efficiency (%)
Original Cut 1st Healed 2nd Healed 3rd Healed
6.55 ± 0.36 2.05 ± 0.13 5.23 ± 0.15 4.45 ± 0.24 3.78 ± 0.21
444 ± 43 92 ± 22 304 ± 32 308 ± 29 283 ± 25
4.53 ± 0.28 3.08 ± 0.19 2.11 ± 0.22 1.76 ± 0.17 1.19 ± 0.19
/ / 71 53 38
1.0 mm
Cut 1st Healed 2nd Healed 3rd Healed
1.07 ± 0.11 3.77 ± 0.23 2.84 ± 0.21 2.20 ± 0.14
61 ± 17 246 ± 25 192 ± 21 178 ± 19
4.10 ± 0.21 2.07 ± 0.25 1.59 ± 0.14 1.32 ± 0.15
/ 49 32 21
1.5 mm
1st Healed 2nd Healed 3rd Healed
2.31 ± 0.26 1.63 ± 0.18 0.83 ± 0.13
231 ± 27 115 ± 14 75 ± 16
1.78 ± 0.27 1.42 ± 0.19 0.87 ± 0.16
35 25 13
0.5 mm
efficiency decreases gradually with the increase of cut and heat treatment numbers. However, even after the third damage and heat treatment process, the healing efficiency of PU-DAs with cracks of 0.5 mm, 1.0 mm, and 1.5 mm in depth can still reach 38%, 21%, and 13%, respectively, indicating that PU-DA has multiple healing capability and cracks can be repaired repeatedly. Similarly, PU-DA film with deeper crack exhibits a low healing efficiency after every damage and repair cycles as compared to PU-DA film with shallower crack. The reason that results in the decrease of healing efficiency with the increase of damage/heat treatment cycles is owing to both the side reaction and the loss of DA bonds as well [31,32]. Thus, results above show that cracks in polyurethane films can be caused and repaired for more than three times. The healing efficiency decreases gradually with the increase of cut and heat treatment numbers because of the side reaction and the incomplete formation of DA bonds. However, even after the third cycles of damage and heat treatment, a high healing efficiency can be obtained, indicating the as-prepared polyurethanes have multiple healing capability. 3.4. Effect of tearing damage on healing behavior and reprocessing performance of PU-DA Polyurethanes may also suffer from tearing damage in actual use, and the healing behavior and reprocessing performance of torn PU-DA based on DA reaction and r-DA reaction are examined. The self-healing behavior and the reprocessing procedure of torn PU-DAs upon heat treatment are shown in Fig. 4. Firstly, both the yellow and red PU-DA films are torn into fragments and
blended uniformly. Then the blended fragments are put into a rectangle-shaped mould and then treated by hot compression molding at 120 °C for different time (20 min, 30 min, and 40 min, respectively) followed by 24 h at 60 °C. Results show that PU-DA fragments with different colors can recombine together as a whole. The reason can be deduced as follows: the long PU-DA chains will break into short chains and molecules, for instance, MPF and BMI, at 120 °C because of r-DA reaction. The resultant short chains and molecules can move and diffuse among adjacent fragments under the thermal action. Consequently, the fragments will fit together by polyurethane chains. After then cooled down to 60 °C, DA reaction between MPF and BMI happens again and long polyurethane chains regenerate. As a result, the torn PU-DA fragments recombine together via DA bonds. In addition, it is obvious from reprocessing sample colors in Fig. 4 that fragments distribute more and more uniformly with the extending of the treatment time at 120 °C, indicating that the recombination among different fragments becomes better and better. Additionally, it is worth noting that the fracture appearance after stretching of the reprocessed PU-DA treated at 120 °C for 20 min is very irregular, as shown in Fig. 4. By contrast, the fracture appearances of both reprocessed PU-DA films treated at 120 °C for 30 min and 40 min are regular. These indicate that 20 min of treatment at 120 °C is not enough for r-DA reaction. Consequently, part PU-DA chains have not broken into MPF and BMI molecules, and which is unfavorable for chains and molecules moving and diffusing. As a result, it is inadequate of moving and diffusing for chains and molecules among adjacent fragments. It is easy to separate for fragments with incomplete combination during the stretching process. On the contrary, it is difficult to break for fragments with complete
Fig. 4. Reprocessing procedure and performance of torn PU-DAs.
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treatment time will bring about side reaction and loss of DA bonds [31,32]. Therefore, the reprocessed PU-DA with the optimal strength can be obtained when the torn fragments are treated at 120 °C for 30 min followed by 24 h at 60 °C. It can be concluded that the as-prepared PU-DA exhibits good self-healing capability even after tearing damage.
4. Conclusions
Fig. 5. Representative stress-strain curves of original and reprocessed PU-DAs.
combination. Thereupon, the irregular fracture is resulted upon drawing for the reprocessed PU-DA treated at 120 °C for 20 min. By contrast, when the heat treatment is enough, adequate molecules can move and diffuse among different fragments. Accordingly, gaps and spaces between fragments have been filled by the molecules and polymer chains. Furthermore, DA bonds create again and long PU-DA chains regenerate by treating at 60 °C. Therefore, the regular fracture is obtained when stretching. For further investigating the reprocessing performance quantitatively, tensile test is carried out and strength recovery ratio is calculated. Fig. 5 shows the representative stress-strain curves of original (as-prepared) and healed (heat-treated at 120 °C for 20 min, 30 min, and 40 min followed by 24 h at 60 °C, respectively) PU-DA films. The tensile test data and the strength recovery rate are listed in Table 3. It can be found from Fig. 5 and Table 3 that the tensile strength of the original PU-DA film can reach 6.53 MPa. After torn and reprocessed, the tensile strength of all resultant PU-DA films decreases as compared to that of the original one. The resultant PU-DA films reprocessed at 120 °C for different time have diverse strengths. The tensile strengths of the reprocessed PU-DA treated at 120 °C for 20 min, 30 min, and 40 min achieve 1.44 MPa, 3.06 MPa, and 2.23 MPa, respectively. Meanwhile, the tensile strength recovery rates can reach 22%, 47%, and 34% one by one accordingly. This can be explained by DA and r-DA reactions as well as thermal movement of molecular chains. The r-DA reaction takes place upon treated at 120 °C, and PU-DA chains are broken into MPF and BMI. MPF and BMI can move and diffuse among the adjacent fragments via thermal movement. Consequently, gaps and spaces among fragments can be filled by polymer chains upon heat treatment. As a result, PU-DA fragments recombine together as a whole. However, too short heat treatment time at 120 °C will lead to incomplete combination of PU-DA fragments while too long heat
A thermoplastic polyurethane containing DA bonds (PU-DA) has been synthesized successfully. The resultant PU-DA exhibits good thermal reversibility, and which has been confirmed by FT-IR, DSC, and viscosity measurements. Grounded on the excellent thermal reversibility of DA bonds and thermal movement effect of molecular chains, the as-synthesized polyurethane exhibits excellent self-healing and reprocessing performance. Polyurethanes may suffer from different modes of damages, and consequently diversified failure modes can be caused when polyurethanes are used in actual application environments. Results show that different failure modes have unalike repairing effect, and which has been proved by POM observation and tensile test. Under the same heat treatment conditions as 120 °C/15 min and then 60 °C/24 h, the healing efficiency can reach 71%, 49%, and 35%, respectively, for PU-DA films with a crack depth of 0.5 mm, 1.0 mm, and 1.5 mm in turn. Moreover, the as-prepared polyurethanes have multiple healing capability, and cracks in polyurethane films can be caused and repaired repeatedly. Even after the third damage and heat treatment process, the healing efficiency of PU-DAs with cracks of 0.5 mm, 1.0 mm, and 1.5 mm in depth can still reach 38%, 21%, and 13% in turn. Additionally, torn fragments from as-synthesized PU-DA films can be reprocessed, and torn fragments can recombine together as a whole upon hot compression molding. The tensile strength recovery can reach 22%, 47%, and 34% when the torn fragments of PU-DA are treated at 120 °C for 20 min, 30 min, and 40 min, respectively. So it can be concluded that the PU-DA exhibits good self-healing capability even after tearing damage. The reprocessed PU-DA with the optimal strength can be obtained when the torn fragments are treated at 120 °C for 30 min followed by 24 h at 60 °C. The research and obtained results can provide important reference for developing materials with excellent self-healing performance. Furthermore, the research can lay foundation for realizing the favorable repairing of various failure modes.
Acknowledgements This research is supported by National Natural Science Foundation of China (Grant No. 51463010).
Conflict of interest statements We declare there are no conflicts of interest.
Table 3 Mechanical property and healing efficiency of reprocessed PU-DAs with different heat treatment time at 120 °C. PU-DA sample
Heat treatment time (min) at 120 °C
Tensile Strength (MPa)
Elongation at break (%)
Young’s modulus (MPa)
Strength recovery rate (%)
Original
/
6.53 ± 0.36
448 ± 43
4.52 ± 0.25
/
Reprocessed
20 30 40
1.44 ± 0.18 3.06 ± 0.20 2.23 ± 0.17
145 ± 25 235 ± 23 198 ± 21
2.44 ± 0.17 3.68 ± 0.26 2.66 ± 0.22
22 47 34
L. Feng et al. / Construction and Building Materials 186 (2018) 1212–1219
References [1] S. Oprea, V.O. Potolinca, C.D. Varganici, Synthesis and properties of polyurethane urea with pyridine-2,6-dicarboxamide moieties in their structure, RSC Adv. 6 (2016) 106904–106913. [2] M. Garrido, J.R. Correia, T. Keller, Effect of service temperature on the shear creep response of rigid polyurethane foam used in composite sandwich floor panels, Constr. Build. Mater. 118 (2016) 235–244. [3] Y.R. Xu, D.J. Chen, A novel self-healing polyurethane based on disulfide bonds, Macromol. Chem. Phys. 217 (2016) 1191–1196. [4] K. Holzworth, Z. Jia, A.V. Amirkhizi, J. Qiao, S.N. Nasser, Effect of isocyanate content on thermal and mechanical properties of polyurea, Polymer 54 (2013) 3079–3085. [5] J.L. Gadley, R.J. Andrade, J.M. Maia, Effect of soft-to-hard segment ratio on viscoelastic behavior of model thermoplastic polyurethanes during phase transitions, Macromol. Mater. Eng. 301 (2016) 953–963. [6] E. Dolci, V. Froidevaux, G. Michaud, F. Simon, R. Auvergne, S. Fouquay, S. Caillol, Thermoresponsive crosslinked isocyanate-free polyurethanes by Diels-Alder polymerization, J. Appl. Polym. Sci. 134 (2017) 44408. [7] F. Awaja, S.N. Zhang, M. Tripathi, A. Nikiforov, N. Pugno, Cracks, microcracks and fracture in polymer structures: formation, detection, autonomic repair, Prog. Mater. Sci 83 (2016) 536–573. [8] J.W. Pang, I.P. Bond, A hollow fiber reinforced polymer composite encompassing self-healing and enhanced damage visibility, Compos. Sci. Technol. 65 (2005) 1791–1799. [9] P. Martin, Wound healing-aiming for perfect skin regeneration, Science 276 (1997) 75–81. [10] J.L. Wietor, R.P. Sijbesma, A self-healing elastomer, Angew. Chem. Int. Ed. 47 (2008) 8161–8163. [11] S. Burattini, B.W. Greenland, D. Chappell, H.M. Colquhoun, W. Hayes, Healable polymeric materials: a tutorial review, Chem. Soc. Rev 39 (2010) 1973–1985. [12] Y.L. Yang, X. Lu, W.W. Wang, A tough polyurethane elastomer with self-healing ability, Mater. Des. 127 (2017) 30–36. [13] Y.L. Liu, T.W. Chou, Self-healing polymers based on thermally reversible DielsAlder chemistry, Polym. Bull. 4 (2014) 2194–2205. [14] A. Duval, G. Couture, S. Caillol, L. Avérous, Biobased and aromatic reversible thermoset networks from condensed tannins via the Diels-Alder reaction, ACS Sustainable Chem. Eng. 5 (2017) 1199–1207. [15] S. Boufi, M.N. Belgacem, J. Quillerou, A. Gandini, Urethanes and polyurethanes bearing furan moieties. 4. Synthesis, kinetics and characterization of linear polymers, Macromolecules 26 (1993) 6706–6717. [16] C.D. Varganici, O. Ursache, C. Gaina, V. Gaina, D. Rosu, B.C. Simionescu, Synthesis and characterization of a new thermoreversible polyurethane network, Ind. Eng. Chem. Res. 52 (2013) 5287–5295. [17] P.F. Du, M.Y. Wu, X.X. Liu, Z. Zheng, X.L. Wang, T. Joncheray, Y.F. Zhang, DielsAlder-based crosslinked self-healing polyurethane/urea from polymeric methylene diphenyl diisocyanate, J. Appl. Polym. Sci. 131 (2014) 1017–1022.
1219
[18] X.J. Cheng, C. Peng, D.H. Zhang, S.Z. Liu, A.Q. Zhang, H. Huang, J.S. Lian, A facile method for the preparation of thermally remendable cross-linked polyphosphazenes, J. Polym. Sci. Polym. Chem. 51 (2013) 1205–1214. [19] D.H. Turkenburg, H.V. Bracht, B. Funke, M. Schmider, D. Janke, H.R. Fischer, Polyurethane adhesives containing Diels-Alder based thermoreversible bonds, J. Appl. Polym. Sci. 134 (2017) 44972. [20] L. Irusta, J.F.B. Ma, J. Aizpurua, Polyurethanes based on isophorone diisocyanate trimer and polypropylene glycol crosslinked by thermal reversible diels alder reactions, J. Appl. Polym. Sci. 133 (2016) 44543. [21] C. Lakatos, K. Czifrak, J.K. Kocsis, L. Daroczi, M. Zsuga, S. Keki, Shape memory crosslinked polyurethanes containing thermoreversible Diels-Alder couplings, J. Appl. Polym. Sci. 133 (2016) 44145. [22] P.F. Du, H.Y. Jia, Q.H. Chen, Z. Zheng, X.L. Wang, D.L. Chen, Slightly crosslinked polyurethane with Diels-Alder adducts from trimethylolpropane, J. Appl. Polym. Sci. 133 (2016) 43971. [23] P.F. Du, X.X. Liu, Z. Zheng, X.L. Wang, T. Joncheray, Y.F. Zhang, Synthesis and characterization of linear self-healing polyurethane based on thermally reversible Diels-Alder reaction, RSC Adv. 3 (2013) 15475–15482. [24] Z.Y. Yu, L.B. Feng, C.S. Chai, Y.P. Wang, Y.H. Liu, X.H. Qiang, Structure and healing behavior of self-healing polyurethane based on Diels-Alder reaction, Acta Polym. Sin. 11 (2016) 1579–1586. [25] P.F. Du, M.Y. Wu, X.X. Liu, Z. Zheng, X.L. Wang, P.Y. Sun, T. Joncheray, Y.F. Zhang, Synthesis of linear polyurethane bearing pendant furan and crosslinked healable polyurethane containing Diels-Alder bonds, New J. Chem. 38 (2014) 770–776. [26] S. Yu, R.C. Zhang, Q. Wu, T.H. Chen, P.C. Sun, Bio-inspired high-performance and recyclable cross-linked polymers, Adv. Mater. 25 (2013) 4912–4917. [27] P. Zhang, G. Li, Advances in healing-on-demand polymers and polymer composites, Prog. Polym. Sci. 57 (2016) 32–63. [28] L.B. Feng, Z.Y. Yu, Y.H. Bian, J.S. Lu, X.T. Shi, C.S. Chai, Self-healing behavior of polyurethanes based on dual actions of thermo-reversible Diels-Alder reaction and thermal movement of molecular chains, Polymer 124 (2017) 48–59. [29] K. Adachi, A.K. Achimuthu, Y. Chujo, Synthesis of organic-inorganic polymer hybrids controlled by Diels-Alder reaction, Macromolecules 37 (2004) 9793– 9797. [30] Y.L. Liu, C.Y. Hsieh, Crosslinked epoxy materials exhibiting thermal remendablility and removability from multifunctional maleimide and furan compounds., J. Polym. Sci. Part A: Polym. Chem. 44 (2006) 905–913. [31] G. Postiglione, S. Turri, M. Levi, Effect of the plasticizer on the self-healing properties of a polymer coating based on the thermoreversible Diels-Alder reaction, Prog. Org. Coat. 78 (2015) 526–531. [32] G. Rivero, L.T.T. Nguyen, X.K. Hillewaere, F.E. Prez, One-pot thermoremendable shape memory polyurethanes, Macromolecules 47 (2014) 2010– 2018.