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Preparation and properties of self-healing cross-linked polyurethanes based on blocking and deblocking reaction
Reactive and Functional Polymers 144 (2019) 104347
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Preparation and properties of self-healing cross-linked polyurethanes based on blocking and deblocking reaction ⁎
Feilong Han, Sayyed Asim Ali Shah, Xin Liu, Fugui Zhao, Bowen Xu, Junying Zhang , Jue Cheng
T
⁎
Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Polyurethane Self-healing Cross-linked Block and deblock polymer Thermal reversible
Self-healing polyurethane via incorporating reversible bonds obtains great recognition. However, a reasonable design of polyurethane containing reversible bonds with high self-healing efficiency, facile preparing and low costs is still a challenge. Here, we report a novel self-healing polyurethane (Sh-C-PU) containing reversible urethane bonds (eNHCOOe), which was conducted by the reaction of diphenol (tetrabromobisphenol A, TBBPA) and 4, 4′-diphenylmethane diisocyanate (MDI). TBBPA can block the eNCO groups in MDI at 60 °C, and eNCO groups can be deblocked at the temperature higher than 90 °C, which gave Sh-C-PU self-healing property. FTIR analysis demonstrated thermal reversibility of Sh-C-PU by monitoring the deblocking and reblocking reaction in thermal cycles. In addition, qualitative self-healing phenomenon of cracks was observed under polarizing optical microscopy, and quantitative evaluation of self-healing Sh-C-PU via mechanical properties confirmed the great healing property of Sh-C-PU with healing efficiency by 97.82%, 86.38% and 55.04% for three times self-healing cycles, respectively.
1. Introduction Polyurethanes have been widely used in elastomers, adhesives, and coatings due to its great regulatory property, serving as a critical role in our daily life [1–3]. However, polyurethanes are liable to be damaged in the service process and create much microcracks, which cannot be detected and repaired easily [4]. Because of these cracks, the lifespan of polyurethanes is greatly shortened [5,6], resulting in a lot of waste of resource. The concept of self-healing was put forward in the 1980s, which is effective to solve the damage issue [7,8], receiving more and more attention [1,3,9–13]. Up to now, there are some strategies to achieve self-healing process [14]. Among them, the extrinsic self-healing mechanism can conduct reparation without external intervention [15]. But this healing mechanism requires loading additional healing agent and other additives in the materials, which may degrade the mechanical properties of the materials, and the healing operation can be carried out only once. Comparing with extrinsic self-healing mechanism, intrinsic self-healing mechanism is quite attractive for its multiple responsive characteristics and no healing agent required [16–19]. In 1969, Craven reported the thermally reversible polymer with the Diels-Alder (D-A) bonded crosslinking network for the first time [20]. D-A reaction is thermally reversible [4 + 2] cycloaddition reaction, which reacts between diene
⁎
and dienophile such as furan groups and maleimide groups [14, 21,22]. When it is heated to higher temperature, the thermal reversible reaction takes place and the initial bond will fracture [23]. When the temperature goes down, the cycloaddition reaction occurs again and the covalent bond will form again. Kennedy and Wagener were the pioneer of a thermally reversible reaction, particularly the D-A reaction for the cross-linking and linear polymers [24,25]. Recently, Du et al. [26]. designed a novel self-healing polyurethane which was extended by maleimide pendant and crosslinked by furan. The materials exhibited great mechanical properties but high healing temperature. This D-A reaction self-healing system is characterized as mild reaction condition, high yield, and low side-reactions [27–29]. Although D-A reaction system shows great potential for healing property, the economic drawback of scaling up synthesis would outweigh the benefit of healing, at least for industrial application. Furthermore, reversible disulfide bonds system, introduced in selfhealing materials, is another way to implement self-healing property [30,31]. Disulfide bonds can be cleaved into two thiol groups in the presence of reducing agent, which can reduce the molecular weight and enhance the kinetic ability of the molecular chain. And then the molecular chains can diffuse and fuse at the damaged site. In the presence of an oxidizing agent, the thiol groups can reform into disulfide bonds, increasing the molecular weight and repairing the damaged part of the
Corresponding authors. E-mail addresses: [email protected] (J. Zhang), [email protected] (J. Cheng).
https://doi.org/10.1016/j.reactfunctpolym.2019.104347 Received 1 June 2019; Received in revised form 16 August 2019; Accepted 25 August 2019 Available online 26 August 2019 1381-5148/ © 2019 Elsevier B.V. All rights reserved.
Reactive and Functional Polymers 144 (2019) 104347
F. Han, et al.
OCN
NCO
+
HO
O
MDI
H n
PTMG-1000 DMF
O
H N
OCN
60 o C, 1 h
O
On
O
NCO
N H
Prepolymer Br
Br
HO
DMF
OH Br
70 oC, 1.5 h
Br
TBBPA Br
Prepolymer*
Br
Br
O
O
Br
Prepolymer
Br
Br
O x
O
Br
Prepolymer
Br
Ex-PU OH HO
OH
70 oC, 0.5h
GL
Br O OCN
N H
Br
O
O Br
Br
Br
O N H
NCO
Br
HO
OH Br
OCN
Br
NCO
Scheme 1. The synthetic process of Sh-C-PU
restored after several break/heal cycles [37]. However, the materials incorporated hydrogen bonds demonstrated low tensile strength after experiencing thermal cycles. Likewise, the thermal endurance and solvent resistance properties of the hydrogen-bond system in polyurethanes are poor relative to other materials. In this work, we designed a novel self-healing network via incorporating reversible urethane bonds into polyurethane materials. The key element of this self-healing strategy is found in the blocking and deblocking reaction of diisocyanate in urethane bond [38,39]. Normally, blocking agents of monophenol, such as phenol and bromine generation of phenol, are widely used in industry and research work to
material [30]. Because of its characteristic healing mechanism, materials can finish the healing progress without heating progress like D-A reaction system. Rekondo et al. [32] reported that a cross-linked polyurethane based on disulfide bonds can be healed at room temperature and exhibited great healing efficiency of 97%. Whereas, disulfide bonds system demonstrates poor mechanical and thermal properties, and the synthesis process is also complicated. Introducing hydrogen bonds into polyurethanes also display their self-healing capacity since hydrogen bonds are described as reversible bonds [33–36]. Cordier and co-workers first reported hydrogen-bonding thermoplastic elastomer at 2008. The material's tensile property was completely 2
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3. Characterization
protect isocyanates [40–42]. Inspired by this block and deblock chemistry, bisphenol compounds were selected to use as chain extenders to prepare self-healing polyurethanes. Meanwhile, bisphenol compounds with electron withdrawing substituents were preferred because of the relatively low deblocking temperature [38,43,44]. TBBPA is such a kind of bisphenol compound, which was used as blocking agents and chain extender. At high temperature (from 90 °C), urethane bonds, formed by the reaction of TBBPA and MDI, can cleave into isocyanate and hydroxyl groups so as to reduce the molecular weight, enhance the kinetic ability of the molecular chain, and restore the damage. At low temperature, isocyanate and hydroxyl groups will reform urethane bonds again. As compared to the other self-healing strategies mentioned above, the self-healing polyurethanes in this work illustrate the merits as following: first, it can be easily achieved by the reaction of diisocyanate and bisphenol chain extender without involvement of additional moieties; second, Sh-C-PU can maintains the excellent comprehensive properties of polyurethane to the greatest extent, due to structural similarity of Sh-C-PU with neat polyurethane; third, Sh-C-PU exhibits a high self-healing efficiency after the samples experienced three healing cycles; finally, the industrialization costs of this strategy is lower than those of D-A reaction system, disulfide system and hydrogen-based system.
3.1. FTIR analysis FTIR spectra were recorded on Bruker Alpha FTIR spectrometer, ranging from 4000 to 400 cm−1, which was applied to monitor the thermally reversible deblocking and reblocking behavior and analyze the structure of Sh-C-PU. In order to monitor the thermally reversible deblocking and reblocking behavior preferably, the mixture of Ex-PU and GL dissolved in DMF was laid evenly between two KBr tablets, and then cured it in vacuum oven at 70 °C for 1 h. The spectrum of the cured Sh-C-PU heated at 80 °C was named “Contrast”. The cured Sh-C-PU was heated for 15 min at 90 °C, 100 °C, 110 °C, 120 °C, 130 °C, 140 °C and 150 °C respectively to study the effect of the heating temperature on deblocking reaction; the cured Sh-C-PU was heated at 130 °C from 5 min to 30 min to study the effect of the heating time on deblocking reaction; the cured Sh-C-PU which was heated at 130 °C for 15 min was cooled at 60 °C to study the effect of cooling time on reblocking reaction; the cured Sh-C-PU was experienced three heating (130 °C, 15 min) and cooling (60 °C, 15 min) cycles to study its the thermal reversible behavior. 3.2. Swelling regularity analysis The Sh-C-PU films were divided into three groups. The samples were weighed and followed by soaking in DMF solvent in bottles, and subjected to an oil-bath for 5 h at 50 °C, 90 °C and 130 °C, respectively. After that, the samples were filtered with 800-mesh and weighed, and followed by drying in an oven and weighed again. Then the swelling ratio and the dissolution ratio of Sh-C-PU were calculated according to the following formulas.
2. Experimental 2.1. Reagents and materials MDI (99%) was purchased from Wanhua Chemical Group Co.,Ltd. PTMG-1000 (Polytetrahydrofuran glycol, Mn = 1000) was provided by Adamas and used after 2 h drying under vacuum at 125 °C. DMF (N, Ndimethylformamide) was purchased from 3A chemicals and was used after being dried with CaH2 and purified by vacuum distillation. TBBPA was supplied by Heowns. Glycerine (GL) was purchased from China National Medicines. BDO (1,4-butanediol, 98%) was purchased from China National Medicines.
Swelling ratio = (Ms − Ms′)/ Ms′ ∗ 100%
(1)
Dissolution ratio = (Ms′ − Mr )/Ms′ ∗ 100%
(2)
Ms = the mass of the soaked sample Ms′ = the mass of the sample before soaking
2.2. Synthesis of self-healing cross-linked polyurethane (Sh-C-PU)
Mr = the mass of the dried sample
Solution of MDI (4.00 g, 0.016 mol) and PTMG-1000 (8.00 g, 0.008 mol) dissolved in DMF were charged into a 100 mL three-necked flask equipped with mechanical stirring and dropping funnel. The reaction was carried out under a nitrogen atmosphere at 60 °C for 1 h. Then TBBPA (3.4816 g, 0.0064 mol) was dissolved in DMF (20 mL) and added dropwise into the isocyanate end-capped prepolymer (Prepolymer) solution with the continuous mechanical stirring. After that, the temperature was increased to 70 °C and kept for 1.5 h. GL (0.1012 g, 0.0011 mol) was dissolved in DMF (10 mL) and added to the isocyanate terminated linear polyurethane (Ex-PU) under the intensely stirring and the temperature was kept at 70 °C. A half an hour later, the whole mixture was poured into polytetrafluoroethylene mold and the mixture was put in a vacuum oven at 70 °C for 5 days to gain Sh-C-PU film. The synthetic process of this reaction is exhibited as followed (Scheme 1).
3.3. Self-healing property analysis A crack in the Sh-C-PU film was observed under polarizing optical microscopy (POM, Shanghai Changfang Optical instrument Co.,Ltd. XP500E). And then Sh-C-PU was heated for several hours in the vacuum oven at 130 °C to evaluate the self-healing property of Sh-C-PU. The mechanical properties of the healed films and the original films, cut into a dumbbell shape (20 × 4 × 1–2 mm), were characterized by Electronic universal material (Model CMT4204) testing machine at the stretching rate of 20 mm/min. 4. Results and discussion 4.1. Synthesis of prepolymer, Ex-PU and Sh-C-PU The synthetic process of Sh-C-PU was monitored by FTIR, and the spectra were shown in Fig. 1. It can be seen from the spectrum of Prepolymer in Fig. 1(a) that the strong peak at 2270.61 cm−1 was assigned to the characteristic eNCO groups in prepolymer. The absorptions observed at 1730.21 cm−1 and 3280.63 cm−1 were C]O bond absorption and NeH bond stretching vibration [43], which demonstrated the successful preparation of prepolymer via the reaction of MDI and PTMG-1000. Fig. 1b showed the characteristic peaks of Ex-PU. Peaks at 3271.75 cm−1, 2272.61 cm−1, 1726.79 cm−1 correspond to NeH, eNCO, C]O bonds, respectively [43]. The eNCO specific
2.3. Synthesis of cross-linked polyurethane (BDO-C-PU) The cross-linked polyurethane synthesized by BDO instead of TBBPA was named BDO-C-PU, which was used as a control. The synthesis process of BDO-C-PU was similar to that of Sh-C-PU, except that BDO acted as a chain extender after the reaction between MDI and PTMG-1000 finished. And BDO-C-PU films were also prepared in the same way as Sh-C-PU films.
3
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which deduced the partial dissolution of the deblocked moiety in DMF. When the sample, soaked in DMF, was heated to 130 °C for 5 h, as shown in Fig. 2(c), (c′), the sample was all dissolved. It is obvious that the deblocking reaction of urethane occurred in Sh-C-PU products, and as the reaction proceeded, the cross-linked network structure was slowly destroyed to form linear chains. Therefore, the solubility experiment confirmed the formation of cross-linked structure and deblocking performance of as-prepared Sh-C-PU.
4.3. Thermal reversible property analysis In order to study the deblocking and reblocking property of Sh-CPU, we used the FTIR to monitor the deblocking and reblocking reaction by directly measuring the regeneration and disappearance of the eNCO groups [40,42,43]. FTIR spectra were collected at different temperatures with the same heating time (15 min), and the results were shown in Fig. 3(a). From the spectrum of Sh-C-PU at 80 o C (contrast), the eNCO peak at 2270 cm−1 was not observed, which suggested that the initial deblocking temperature was higher than 80 °C. Yet, a weak absorption peak at 3530 cm−1 (eOH groups) was found at 80 °C because of a small amount of excessive crosslinker. The intensity of the characteristic absorptions of eNCO groups and eOH groups increased with the increase of temperature from 80 °C to 150 °C, as shown in Fig. 3(a), and the variation of the released eNCO groups with temperatures during the deblocking process can be observed from Fig. 3(b). The eC6H6 groups which did not participate in the deblocking reaction were selected as the reference group, and the characteristic peak height of the eNCO groups was quantitatively calculated, denoted as eNCO/eC6H6, as shown in Fig. 3(b). It can be seen from Fig. 3(b) that as the heating temperature increased, the deblocking rate gradually increased and the degree of deblocking also increased. However, when the temperature increased from 130 °C to 140 °C, the eNCO/eC6H6 peak was decreased which might be due to some side reactions. With the above analysis, the temperature range 90–130 °C was considered to be the most suitable deblocking temperature. Sequentially, the effect of heating time on deblocking reaction of ShC-PU sample at 130 °C was investigated, as shown in Fig. 4(a), (b). The peak of eNCO groups was observed when heated for 5 min, and the height of peak became higher with the passage of time as shown in Fig. 4(a). With the heating time increasing the peak of the eNCO groups at 2270 cm−1 increased continuously. At the same time, the characteristic peak of eOH groups at 3530 cm−1 appeared and increased gradually along with eNCO groups. In order to observe the changes of eNCO groups with the passage of time, eNCO/eC6H6 was calculated, as shown in Fig. 4(b). As the heating time going on, the deblocking rate and deblocking degree also increased. The degree of deblocking attained the maximum point at 10 min and the degree of deblocking reaction declined after 15 min because free eNCO groups at high temperature maybe react with residual cross-linker glycerol in the samples. To study the reblocking regularity of Sh-C-PU, FTIR spectra were collected with interval 1 min during the cooling process and initial heating temperature was programmed at 130 °C. The FTIR spectra in the cooling process were exhibited in Fig. 5(a). It can be seen from the spectra that the characteristic absorptions of both of eNCO groups at 2270 cm−1 and eOH groups at 3530 cm−1 gradually declined during the cooling process, which illustrated that eNCO groups reappeared via deblocking reaction can participate in reblocking reaction again. Fig. 5(b) showed the relationship between the degree of reblocking and cooling time. In order to obtain the relationship, the residual fraction (A) of eNCO groups named and calculated by using Eq. (3), and the calculated results are shown in Fig. 5(b).
Fig. 1. FTIR spectra of (a) Prepolymer, (b) Ex-PU, (c) Sh-C-PU.
absorption became weaker and 1759.73 cm−1 revealed new peak of C]O bonds due to urethane linkage made by TBBPA and MDI [45]. Analogously, Fig. 1(c) showed the spectrum of the final product Sh-CPU: specific absorptions at 3286.07 cm−1 was attributed to NeH bonds stretching vibration, and strong peak at 1730.21 cm−1 was C]O bonds absorption [43]. With the addition of GL, C]O stretching at 1730.31 cm−1 became much stronger, and the peak of isocyanate group at 2270 cm−1 disappeared. These phenomena all confirmed the successful synthesis of Sh-C-PU.
4.2. Swelling regularity analysis In order to further research and ascertain formation of the threedimensional network and the thermal deblocking phenomenon of Sh-CPU, the swelling property representation of Sh-C-PU samples was investigated [46,47]. The swelling ratio and dissolution ratio of Sh-C-PU samples were calculated and presented in Table 1 and Fig. 2. It can be seen from Fig. 2(a) and (a′) the first group of Sh-C-PU samples did not dissolve in DMF after being incubated at 50 °C for 5 h, but only swelling. And the swelling ratio was 603.13% after filtration (as shown in Table 1). After the sample was collected and dried in oven, the determined dissolution ratio was 21.88%, which might be deduced by the loss in collecting process and small amount dissolution of molecular chains. In general, the as-prepared product is cross-linked three-dimensional network. Continuously, the second group of Sh-C-PU samples was soaked in DMF at 90 °C. After 5 h, the samples also did not dissolve except for swollen. After filtering, the swelling ratio and dissolution ratio were found to be 536.00% and 24.00% respectively (shown in Table 1). And the shape of the soaked sample was changed from rectangle to irregular shape, as shown in Fig. 2(b), (b′). The Sh-C-PU samples, soaked at 90 °C for 5 h, should absorb more solvent and exhibit greater swelling. Therefore, the swelling ratio should increase by increasing the temperature due to easy deformation of molecular chains. However, the swelling ratio at 90 °C was lower than that at 50 °C because the deblock reaction at 90 °C occurred and the network was damaged slightly, Table 1 Test result of Sh-C-PU samples' swelling regularity. Test temperature (°C)
Ms' (g)
Ms (g)
Mr (g)
Swelling ratio (%)
Dissolution ratio (%)
50 90 130
0.32 0.25 0.27
2.25 1.59 –
0.25 0.19 –
603.13 536.00 –
21.88 24.00 –
A = ( NCO/ C6 H6 )t /(NCO/ C6 H6 )0 4
(3)
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Fig. 2. Swelling regularity of Sh-C-PU samples at different temperature with DMF as a solvent: (a) sample before soaking at 50 °C, (a′) sample after soaked at 50 °C; (b) sample before soaking at 90 °C, (b′) sample after soaked at 90 °C; (c) sample before soaking at 130 °C, (c′) sample after soaked at 130 °C.
Fig. 3. (a) FTIR spectra of deblocking reaction of Sh-C-PU and (b) the relationship of eNCO/C6H6 versus same heating time at different temperature.
deblocking reaction that fragile urethane group fractured to become isocyanate and phenolic hydroxyl groups. In the meantime, the characteristic peak of eNH at 3300 cm−1 became weaker as the deblocking reaction progresses. The heated sample at 130o C was subsequently experienced the first cooling process (cool-1) at 60 °C for 15 min to ensure the reblocking reaction and eNCO groups almost disappeared in the cooling process. The second heating and cooling cycle (heat-2 and cool-2) proceeded at the same temperature like the first cycle, and the characteristic peaks of eNCO groups again appeared and disappeared. After the second cycle, the samples proceeded the third heating and cooling cycle (heat-3 and cool-3) immediately as shown in Fig. 6(a), and these can confirm thermal reversible property of Sh-C-PU with three processes of heating and cooling. Through qualitative analysis of the above FTIR spectra, it is known that the as-synthesized Sh-C-PU exhibited really thermal reversible property. In order to more accurately analyze the thermal reversible behavior of Sh-C-PU, a quantitative analysis which had been introduced above was utilized. The eC6H6 groups which did not participate in the reaction was selected as the internal reference group and the characteristic peak height of the eNCO groups at 2270 cm−1 was quantitatively calculated by using eNCO/
(NCO/ C6 H6 )t = the height of NCO absorption peak relative to the absorption peak of C6 H6 at time (NCO/ C6 H6 )0 = the height of NCO absorption peak relative to the absorption peak of C6 H6 at initial time As shown in Fig. 5(b), with the cooling time extending, the amount of residual eNCO groups first decreased sharply and then slowly, ant it almost disappeared in about 8 min. From the analysis mentioned above, eNCO groups were released by deblocking reaction at a higher temperature and reacted with TBBPA again at a lower temperature. Therefore, the reversible behavior of ShC-PU samples can be reflected by observing the specific absorption of eNCO groups during the heating and cooling cycles, which was shown in Fig. 6(a). In the first heating process (heat-1), the appearance of the peaks at 2270 cm−1 and 3530 cm−1 clarified that free eNCO and eOH groups were released after heating at 130 °C for 15 min, which owed to 5
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Fig. 4. (a) FTIR spectra of deblocking reaction of Sh-C-PU and (b) The relationship of eNCO/C6H6 versus deblocking time at 130 °C.
PU before and after thermal treatment were observed via POM as shown in Fig. 7(b) and (b′) respectively. It can be seen from Fig. 7(b′) that the crack in the BDO-C-PU did not disappear during the heating process. Therefore, we can be inferred that Sh-C-PU showed an exceptional self-healing property. The reason for these phenomena is that BDO-C-PU is cross-linked polyurethane whose chains cannot move freely to repair the damage. By contrast, during the heating process, the cross-linking network of Sh-C-PU can be degraded to be linear chains gradually with deblocking reaction taking place during the heating process [49,50]. Thus, during the heating process, cracks in Sh-C-PU will be filled and healed quickly. Healing efficiency, a quantitative analysis, was determined by recovery of tensile strength after healed, which provided significant information to investigate the healing property of Sh-C-PU samples [14,51–53]. The healing progress of Sh-C-PU was shown in Fig. 8. In order to judge the appropriate healing time, the sample Sh-C-PU was broken at tensile instrument, and then healed in vacuum oven at 130 °C for 1 h, 2 h, 3 h respectively (sample named Healed-1 h, Healed-2 h, Healed-3 h), as shown in Fig. 9 and Table 2. The stress of the original sample of Sh-C-PU was 3.67 MPa. After heating 1 h, the stress of the sample Healed-1 h could back to 3.59 MPa and healing efficiency of the first cycle was 97.82%. The stress of the sample Healed-2 h was 2.43 MPa and healing efficiency was 66.21%, and the stress of the
C6H6 value. The calculated results were shown in Fig. 6(b). It can be found from Fig. 6(b) that after experiencing the first heating process, the eNCO/C6H6 value reached 0.161, and the height of eNCO groups became weaker, 0.121 the second cycle and 0.101 the third cycle as the sample experienced more heating and cooling cycles. Besides, after finished cooling process, the eNCO groups can be blocked almost and the peak height was only 0.004 for the first cycle, 0.003 for the second cycle, and 0.004 for the third cycle. 4.4. Self-healing property analysis The self-healing property of Sh-C-PU was primitively investigated by observing the changes of cracks via POM qualitatively [26,48]. The surface of the samples was scratched cross crack with a scalpel, and the samples were heated at 130o C for 1 h to observe the evolution of cross crack, as shown in Fig. 7. As we can see from Fig. 7(a), the cross crack of Sh-C-PU can be observed clearly via POM when it was scratched. As the heating time increased, the crack disappeared ultimately after 1 h as shown in Fig. 7(a′). In order to better contrast this phenomenon, a cross crack was scratched on the surface of cross-linked polyurethane BDO-C-PU which was synthesized without TBBPA but BDO as a chain extender, then followed by heating at 130 °C for 1 h, and the surfaces of BDO-C-
Fig. 5. (a) FTIR spectra of reblocking reaction of Sh-C-PU and (b) the relationship of degree of reblocking and cooling time. 6
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Fig. 6. (a) FTIR spectra of heating and cooling process of Sh-C-PU and (b) specific absorption relative height of eNCO group via heating and cooling process.
Fig. 7. POM photographs of evolution process of cracks in (a) Sh-C-PU and (b) BDO-C-PU films heated at 130 °C.
Fig. 8. The healing progress and the structural evolution of Sh-C-PU.
7
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Fig. 11. The stress of original and healed samples of Sh-C-PU and BDO-C-PU.
Fig. 9. Stress-strain curves of Sh-C-PU and the healed samples.
Table 4 Mechanical property and healing efficiency of Sh-C-PU and BDO-C-PU.
Table 2 Healing efficiency of samples with different healing time. Test results
Sample
Sample
Sh-C-PU
Stain (%) Stress (MPa) Healing efficiency (%)
Sh-C-PU
Healed-1 h
Healed-2 h
Healed-3 h
533.52 3.67 –
476.99 3.59 97.82
321.35 2.43 66.21
153.14 1.93 52.58
BDO-C-PU
Table 3 Healing efficiency of Sh-C-PU samples as a function of healing times.
Stain (%) Stress (MPa) Healing efficiency (%)
Sample Sh-C-PU
Healed 1st
Heating 2nd
Heating 3rd
533.52 3.67 –
476.99 3.59 97.82
428.24 3.17 86.38
250.65 2.02 55.04
Stain (%)
Healing efficiency (%)
3.67 3.59 9.02 0.43
533.52 476.99 186.65 16.84
97.82 4.77
the height of absorption peak of eNCO groups decreased gradually and mildly as heating time increased, and then the amount of free eNCO groups declined because free eNCO groups at high temperature reacted with free glycerol in the matrix or side reactions of eNCO groups occurred. While deblocking reaction was still going on and the crosslinked network of Sh-C-PU material was still destroyed, and the amount of eNCO groups involved in reblocking reaction were less and less. Therefore, the most suitable heating time was considered to be 1 h. An important feature of thermally reversible self-healing elastomers was the ability to perform multiple self-healing at the same place [14,51–53]. Therefore, in this work, the healing efficiency of Sh-C-PU was evaluated for breaking and healing cycles. The Sh-C-PU was first broken at tensile instrument and then the broken sample would be healed in vacuum oven at 130 °C for 1 h as the first healed sample (named Healed 1st). Similarly, the second healed sample and the third healed sample were obtained in this way (named Healed 2nd and Healed 3rd). The stress-strain curves of these samples were shown in Fig. 10 and Table 3. As shown in Fig. 10 and Table 3, the tensile strength of the sample healed 1st reached 3.59 MPa, close to the initial sample's tensile strength of 3.67 MPa, and the healing efficiency was as high as 97.82%; the tensile strength of the sample healed 2nd was 3.17 MPa, and the healing efficiency was 86.38%; the tensile strength of sample healed 3rd was only 2.02 MPa with healing efficiency by 55.04%. It demonstrated that the tensile strength and healing efficiency of the self-healing materials reduced as the healing times increased. With the analysis of thermal reversible property of Sh-C-PU, the amount of free eNCO groups released via deblocking reaction would be decreased as thermal cycles increased and the molecular chain structure of material itself would be destroyed after many healed times, so the healing efficiency and mechanical strength of the material decreased as the healing cycles increased. Meanwhile, in order to further highlight the excellent self-healing properties of Sh-C-PU, the mechanical properties and healing efficiency of BDO-C-PU were also determined, as shown in Fig. 11 and Table 4. As we can see from Table 4, although the tensile stress of BDO-C-PU (9.02 MPa) was higher than that of Sh-C-PU (3.67 MPa), its healing
Fig. 10. Stress-strain curves of Sh-C-PU healed by different healing times.
Test results
Original Healed Original Healed
Stress (MPa)
sample Healed-3 h reached 1.93 MPa with 52.58% healing efficiency. As shown in Table 2, the healing efficiency of the healed samples was getting weaker with the heating time increasing. As depicted in Fig. 4,
8
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Fig. 12. Comparison of tensile strength (initial stress and healed stress) and healing efficiency of Sh-C-PU samples with different proportions of TBBPA and GL from 8:2 to 5:5.
ability to deblock.
Table 5 Tensile strength data and healing efficiency of Sh-C-PU samples with different proportions of TBBPA and GL. Different proportions of TBBPA and GL 8:2 7:3 6:4 5:5
Initial Healed Initial Healed Initial Healed Initial Healed
Tensile strength (MPa)
Healing efficiency (%)
3.67 3.59 2.47 1.57 7.96 3.26 10.56 2.35
97.82
5. Conclusions A new route to synthesize cross-linked self-healing polyurethane (Sh-C-PU) using TBBPA as a reversible chain extender and GL as crosslinker was analyzed by FTIR and swelling regularity analysis. The thermal reversible property of Sh-C-PU was confirmed via FTIR, which experienced three heating and cooling cycles. POM qualitatively analysis revealed that crack at material was repaired by heating process. Tensile strength measurements on the original and healed samples demonstrated great mechanical and self-healing properties, 97.82% after once healed, 86.38% after twice healed, and 55.04% after three times healed cycles. The different proportion of TBBPA and GL from 8:2 to 5:5 indicated that the degree of cross-linking, the tensile stress of ShC-PU improved but healing efficiency decreased. The high self-healing efficiency polyurethanes which can be widely used in coatings, adhesives and elastomers. Take it a step further, this strategy can also use for the recovery and reuse of thermosetting polyurethane.
63.56 32.66 29.34
efficiency was much lower than that of Sh-C-PU upon heat treatment. The healing efficiency of Sh-C-PU can reach 97.82% when heated at 130 °C for 1 h. By contrast, it only arrived 4.77% for BDO-C-PU after the same heat treatment. These results showed that BDO-C-PU exhibited poor self-healing capability since its molecular chains cannot be broken by deblocking reaction. Thereupon, it can be concluded that Sh-C-PU exhibited great self-healing performance, and cracks in Sh-C-PU can be repaired by combined actions of the reversible block and deblock reactions and thermal movement of molecular chains. Similarly, the ratio of TBBPA and GL is also an important factor to influence mechanical performance and healing efficiency. Sh-C-PU samples with different proportions of TBBPA and GL from 8:2 to 5:5 were synthesized. After experiencing the same heating treatment, the tensile strength and healing efficiency of the original Sh-C-PU and the healed Sh-C-PU samples were measured and shown in Fig. 12 and Table 5. As for original Sh-C-PU sample, the tensile stress overall exhibited a rapid upward trend with increasing the ratio of GL, however, when the proportion of TBBPA and GL was 7:3, the tensile stress suddenly dropped since tensile stress of polyurethane resulted in hard segment separation between soft and hard segments when the degree of cross-linking was relatively weaker. When the proportion of TBBPA and GL increased, the cross-linking density also increased, which led to the increase of tensile stress of Sh-C-PU. Therefore, the tensile stress of ShC-PU samples was increased as the improvement of the degree of crosslinking. However, the self-healing property of Sh-C-PU was diverse. With the proportion of TBBPA and GL increasing, the healing efficiency became weaker gradually, that means, as the smaller the amount of TBBPA, the less the amount of urethane group, and so the less the
Declaration of Competing Interest The authors declare no competing financial interest. Acknowledgments The author would like to thank the financial support from National Key Research Program of China (Project no. 2016YFB0302000) and the financial support from the National Natural Science Foundation of China under Grant No. 21476013. References [1] T.P. Galhenage, D. Hoffman, S.D. Silbert, S.J. Stafslien, J. Daniels, T. Miljkovic, J.A. Finlay, S.C. Franco, A.S. Clare, B.T. Nedved, Fouling-release performance of silicone oil-modified siloxane-polyurethane coatings, ACS Appl. Mater. Interfaces 8 (42) (2016). [2] 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 (26) (2017). [3] B. Zimmer, C. Nies, C. Schmitt, W. Possart, Chemistry, polymer dynamics and mechanical properties of a two-part polyurethane elastomer during and after crosslinking. Part I: dry conditions, Polymer 115 (2017) 77–95. [4] G. Menges, Materials Science of Polymers for Engineers 3e, (2012). [5] Y.W. Dong, S. Meure, D. Solomon, Self-healing polymeric materials: a review of
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Contents lists available at ScienceDirect
Reactive and Functional Polymers journal homepage: www.elsevier.com/locate/react
Preparation and properties of self-healing cross-linked polyurethanes based on blocking and deblocking reaction ⁎
Feilong Han, Sayyed Asim Ali Shah, Xin Liu, Fugui Zhao, Bowen Xu, Junying Zhang , Jue Cheng
T
⁎
Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Polyurethane Self-healing Cross-linked Block and deblock polymer Thermal reversible
Self-healing polyurethane via incorporating reversible bonds obtains great recognition. However, a reasonable design of polyurethane containing reversible bonds with high self-healing efficiency, facile preparing and low costs is still a challenge. Here, we report a novel self-healing polyurethane (Sh-C-PU) containing reversible urethane bonds (eNHCOOe), which was conducted by the reaction of diphenol (tetrabromobisphenol A, TBBPA) and 4, 4′-diphenylmethane diisocyanate (MDI). TBBPA can block the eNCO groups in MDI at 60 °C, and eNCO groups can be deblocked at the temperature higher than 90 °C, which gave Sh-C-PU self-healing property. FTIR analysis demonstrated thermal reversibility of Sh-C-PU by monitoring the deblocking and reblocking reaction in thermal cycles. In addition, qualitative self-healing phenomenon of cracks was observed under polarizing optical microscopy, and quantitative evaluation of self-healing Sh-C-PU via mechanical properties confirmed the great healing property of Sh-C-PU with healing efficiency by 97.82%, 86.38% and 55.04% for three times self-healing cycles, respectively.
1. Introduction Polyurethanes have been widely used in elastomers, adhesives, and coatings due to its great regulatory property, serving as a critical role in our daily life [1–3]. However, polyurethanes are liable to be damaged in the service process and create much microcracks, which cannot be detected and repaired easily [4]. Because of these cracks, the lifespan of polyurethanes is greatly shortened [5,6], resulting in a lot of waste of resource. The concept of self-healing was put forward in the 1980s, which is effective to solve the damage issue [7,8], receiving more and more attention [1,3,9–13]. Up to now, there are some strategies to achieve self-healing process [14]. Among them, the extrinsic self-healing mechanism can conduct reparation without external intervention [15]. But this healing mechanism requires loading additional healing agent and other additives in the materials, which may degrade the mechanical properties of the materials, and the healing operation can be carried out only once. Comparing with extrinsic self-healing mechanism, intrinsic self-healing mechanism is quite attractive for its multiple responsive characteristics and no healing agent required [16–19]. In 1969, Craven reported the thermally reversible polymer with the Diels-Alder (D-A) bonded crosslinking network for the first time [20]. D-A reaction is thermally reversible [4 + 2] cycloaddition reaction, which reacts between diene
⁎
and dienophile such as furan groups and maleimide groups [14, 21,22]. When it is heated to higher temperature, the thermal reversible reaction takes place and the initial bond will fracture [23]. When the temperature goes down, the cycloaddition reaction occurs again and the covalent bond will form again. Kennedy and Wagener were the pioneer of a thermally reversible reaction, particularly the D-A reaction for the cross-linking and linear polymers [24,25]. Recently, Du et al. [26]. designed a novel self-healing polyurethane which was extended by maleimide pendant and crosslinked by furan. The materials exhibited great mechanical properties but high healing temperature. This D-A reaction self-healing system is characterized as mild reaction condition, high yield, and low side-reactions [27–29]. Although D-A reaction system shows great potential for healing property, the economic drawback of scaling up synthesis would outweigh the benefit of healing, at least for industrial application. Furthermore, reversible disulfide bonds system, introduced in selfhealing materials, is another way to implement self-healing property [30,31]. Disulfide bonds can be cleaved into two thiol groups in the presence of reducing agent, which can reduce the molecular weight and enhance the kinetic ability of the molecular chain. And then the molecular chains can diffuse and fuse at the damaged site. In the presence of an oxidizing agent, the thiol groups can reform into disulfide bonds, increasing the molecular weight and repairing the damaged part of the
Corresponding authors. E-mail addresses: [email protected] (J. Zhang), [email protected] (J. Cheng).
https://doi.org/10.1016/j.reactfunctpolym.2019.104347 Received 1 June 2019; Received in revised form 16 August 2019; Accepted 25 August 2019 Available online 26 August 2019 1381-5148/ © 2019 Elsevier B.V. All rights reserved.
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OCN
NCO
+
HO
O
MDI
H n
PTMG-1000 DMF
O
H N
OCN
60 o C, 1 h
O
On
O
NCO
N H
Prepolymer Br
Br
HO
DMF
OH Br
70 oC, 1.5 h
Br
TBBPA Br
Prepolymer*
Br
Br
O
O
Br
Prepolymer
Br
Br
O x
O
Br
Prepolymer
Br
Ex-PU OH HO
OH
70 oC, 0.5h
GL
Br O OCN
N H
Br
O
O Br
Br
Br
O N H
NCO
Br
HO
OH Br
OCN
Br
NCO
Scheme 1. The synthetic process of Sh-C-PU
restored after several break/heal cycles [37]. However, the materials incorporated hydrogen bonds demonstrated low tensile strength after experiencing thermal cycles. Likewise, the thermal endurance and solvent resistance properties of the hydrogen-bond system in polyurethanes are poor relative to other materials. In this work, we designed a novel self-healing network via incorporating reversible urethane bonds into polyurethane materials. The key element of this self-healing strategy is found in the blocking and deblocking reaction of diisocyanate in urethane bond [38,39]. Normally, blocking agents of monophenol, such as phenol and bromine generation of phenol, are widely used in industry and research work to
material [30]. Because of its characteristic healing mechanism, materials can finish the healing progress without heating progress like D-A reaction system. Rekondo et al. [32] reported that a cross-linked polyurethane based on disulfide bonds can be healed at room temperature and exhibited great healing efficiency of 97%. Whereas, disulfide bonds system demonstrates poor mechanical and thermal properties, and the synthesis process is also complicated. Introducing hydrogen bonds into polyurethanes also display their self-healing capacity since hydrogen bonds are described as reversible bonds [33–36]. Cordier and co-workers first reported hydrogen-bonding thermoplastic elastomer at 2008. The material's tensile property was completely 2
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3. Characterization
protect isocyanates [40–42]. Inspired by this block and deblock chemistry, bisphenol compounds were selected to use as chain extenders to prepare self-healing polyurethanes. Meanwhile, bisphenol compounds with electron withdrawing substituents were preferred because of the relatively low deblocking temperature [38,43,44]. TBBPA is such a kind of bisphenol compound, which was used as blocking agents and chain extender. At high temperature (from 90 °C), urethane bonds, formed by the reaction of TBBPA and MDI, can cleave into isocyanate and hydroxyl groups so as to reduce the molecular weight, enhance the kinetic ability of the molecular chain, and restore the damage. At low temperature, isocyanate and hydroxyl groups will reform urethane bonds again. As compared to the other self-healing strategies mentioned above, the self-healing polyurethanes in this work illustrate the merits as following: first, it can be easily achieved by the reaction of diisocyanate and bisphenol chain extender without involvement of additional moieties; second, Sh-C-PU can maintains the excellent comprehensive properties of polyurethane to the greatest extent, due to structural similarity of Sh-C-PU with neat polyurethane; third, Sh-C-PU exhibits a high self-healing efficiency after the samples experienced three healing cycles; finally, the industrialization costs of this strategy is lower than those of D-A reaction system, disulfide system and hydrogen-based system.
3.1. FTIR analysis FTIR spectra were recorded on Bruker Alpha FTIR spectrometer, ranging from 4000 to 400 cm−1, which was applied to monitor the thermally reversible deblocking and reblocking behavior and analyze the structure of Sh-C-PU. In order to monitor the thermally reversible deblocking and reblocking behavior preferably, the mixture of Ex-PU and GL dissolved in DMF was laid evenly between two KBr tablets, and then cured it in vacuum oven at 70 °C for 1 h. The spectrum of the cured Sh-C-PU heated at 80 °C was named “Contrast”. The cured Sh-C-PU was heated for 15 min at 90 °C, 100 °C, 110 °C, 120 °C, 130 °C, 140 °C and 150 °C respectively to study the effect of the heating temperature on deblocking reaction; the cured Sh-C-PU was heated at 130 °C from 5 min to 30 min to study the effect of the heating time on deblocking reaction; the cured Sh-C-PU which was heated at 130 °C for 15 min was cooled at 60 °C to study the effect of cooling time on reblocking reaction; the cured Sh-C-PU was experienced three heating (130 °C, 15 min) and cooling (60 °C, 15 min) cycles to study its the thermal reversible behavior. 3.2. Swelling regularity analysis The Sh-C-PU films were divided into three groups. The samples were weighed and followed by soaking in DMF solvent in bottles, and subjected to an oil-bath for 5 h at 50 °C, 90 °C and 130 °C, respectively. After that, the samples were filtered with 800-mesh and weighed, and followed by drying in an oven and weighed again. Then the swelling ratio and the dissolution ratio of Sh-C-PU were calculated according to the following formulas.
2. Experimental 2.1. Reagents and materials MDI (99%) was purchased from Wanhua Chemical Group Co.,Ltd. PTMG-1000 (Polytetrahydrofuran glycol, Mn = 1000) was provided by Adamas and used after 2 h drying under vacuum at 125 °C. DMF (N, Ndimethylformamide) was purchased from 3A chemicals and was used after being dried with CaH2 and purified by vacuum distillation. TBBPA was supplied by Heowns. Glycerine (GL) was purchased from China National Medicines. BDO (1,4-butanediol, 98%) was purchased from China National Medicines.
Swelling ratio = (Ms − Ms′)/ Ms′ ∗ 100%
(1)
Dissolution ratio = (Ms′ − Mr )/Ms′ ∗ 100%
(2)
Ms = the mass of the soaked sample Ms′ = the mass of the sample before soaking
2.2. Synthesis of self-healing cross-linked polyurethane (Sh-C-PU)
Mr = the mass of the dried sample
Solution of MDI (4.00 g, 0.016 mol) and PTMG-1000 (8.00 g, 0.008 mol) dissolved in DMF were charged into a 100 mL three-necked flask equipped with mechanical stirring and dropping funnel. The reaction was carried out under a nitrogen atmosphere at 60 °C for 1 h. Then TBBPA (3.4816 g, 0.0064 mol) was dissolved in DMF (20 mL) and added dropwise into the isocyanate end-capped prepolymer (Prepolymer) solution with the continuous mechanical stirring. After that, the temperature was increased to 70 °C and kept for 1.5 h. GL (0.1012 g, 0.0011 mol) was dissolved in DMF (10 mL) and added to the isocyanate terminated linear polyurethane (Ex-PU) under the intensely stirring and the temperature was kept at 70 °C. A half an hour later, the whole mixture was poured into polytetrafluoroethylene mold and the mixture was put in a vacuum oven at 70 °C for 5 days to gain Sh-C-PU film. The synthetic process of this reaction is exhibited as followed (Scheme 1).
3.3. Self-healing property analysis A crack in the Sh-C-PU film was observed under polarizing optical microscopy (POM, Shanghai Changfang Optical instrument Co.,Ltd. XP500E). And then Sh-C-PU was heated for several hours in the vacuum oven at 130 °C to evaluate the self-healing property of Sh-C-PU. The mechanical properties of the healed films and the original films, cut into a dumbbell shape (20 × 4 × 1–2 mm), were characterized by Electronic universal material (Model CMT4204) testing machine at the stretching rate of 20 mm/min. 4. Results and discussion 4.1. Synthesis of prepolymer, Ex-PU and Sh-C-PU The synthetic process of Sh-C-PU was monitored by FTIR, and the spectra were shown in Fig. 1. It can be seen from the spectrum of Prepolymer in Fig. 1(a) that the strong peak at 2270.61 cm−1 was assigned to the characteristic eNCO groups in prepolymer. The absorptions observed at 1730.21 cm−1 and 3280.63 cm−1 were C]O bond absorption and NeH bond stretching vibration [43], which demonstrated the successful preparation of prepolymer via the reaction of MDI and PTMG-1000. Fig. 1b showed the characteristic peaks of Ex-PU. Peaks at 3271.75 cm−1, 2272.61 cm−1, 1726.79 cm−1 correspond to NeH, eNCO, C]O bonds, respectively [43]. The eNCO specific
2.3. Synthesis of cross-linked polyurethane (BDO-C-PU) The cross-linked polyurethane synthesized by BDO instead of TBBPA was named BDO-C-PU, which was used as a control. The synthesis process of BDO-C-PU was similar to that of Sh-C-PU, except that BDO acted as a chain extender after the reaction between MDI and PTMG-1000 finished. And BDO-C-PU films were also prepared in the same way as Sh-C-PU films.
3
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which deduced the partial dissolution of the deblocked moiety in DMF. When the sample, soaked in DMF, was heated to 130 °C for 5 h, as shown in Fig. 2(c), (c′), the sample was all dissolved. It is obvious that the deblocking reaction of urethane occurred in Sh-C-PU products, and as the reaction proceeded, the cross-linked network structure was slowly destroyed to form linear chains. Therefore, the solubility experiment confirmed the formation of cross-linked structure and deblocking performance of as-prepared Sh-C-PU.
4.3. Thermal reversible property analysis In order to study the deblocking and reblocking property of Sh-CPU, we used the FTIR to monitor the deblocking and reblocking reaction by directly measuring the regeneration and disappearance of the eNCO groups [40,42,43]. FTIR spectra were collected at different temperatures with the same heating time (15 min), and the results were shown in Fig. 3(a). From the spectrum of Sh-C-PU at 80 o C (contrast), the eNCO peak at 2270 cm−1 was not observed, which suggested that the initial deblocking temperature was higher than 80 °C. Yet, a weak absorption peak at 3530 cm−1 (eOH groups) was found at 80 °C because of a small amount of excessive crosslinker. The intensity of the characteristic absorptions of eNCO groups and eOH groups increased with the increase of temperature from 80 °C to 150 °C, as shown in Fig. 3(a), and the variation of the released eNCO groups with temperatures during the deblocking process can be observed from Fig. 3(b). The eC6H6 groups which did not participate in the deblocking reaction were selected as the reference group, and the characteristic peak height of the eNCO groups was quantitatively calculated, denoted as eNCO/eC6H6, as shown in Fig. 3(b). It can be seen from Fig. 3(b) that as the heating temperature increased, the deblocking rate gradually increased and the degree of deblocking also increased. However, when the temperature increased from 130 °C to 140 °C, the eNCO/eC6H6 peak was decreased which might be due to some side reactions. With the above analysis, the temperature range 90–130 °C was considered to be the most suitable deblocking temperature. Sequentially, the effect of heating time on deblocking reaction of ShC-PU sample at 130 °C was investigated, as shown in Fig. 4(a), (b). The peak of eNCO groups was observed when heated for 5 min, and the height of peak became higher with the passage of time as shown in Fig. 4(a). With the heating time increasing the peak of the eNCO groups at 2270 cm−1 increased continuously. At the same time, the characteristic peak of eOH groups at 3530 cm−1 appeared and increased gradually along with eNCO groups. In order to observe the changes of eNCO groups with the passage of time, eNCO/eC6H6 was calculated, as shown in Fig. 4(b). As the heating time going on, the deblocking rate and deblocking degree also increased. The degree of deblocking attained the maximum point at 10 min and the degree of deblocking reaction declined after 15 min because free eNCO groups at high temperature maybe react with residual cross-linker glycerol in the samples. To study the reblocking regularity of Sh-C-PU, FTIR spectra were collected with interval 1 min during the cooling process and initial heating temperature was programmed at 130 °C. The FTIR spectra in the cooling process were exhibited in Fig. 5(a). It can be seen from the spectra that the characteristic absorptions of both of eNCO groups at 2270 cm−1 and eOH groups at 3530 cm−1 gradually declined during the cooling process, which illustrated that eNCO groups reappeared via deblocking reaction can participate in reblocking reaction again. Fig. 5(b) showed the relationship between the degree of reblocking and cooling time. In order to obtain the relationship, the residual fraction (A) of eNCO groups named and calculated by using Eq. (3), and the calculated results are shown in Fig. 5(b).
Fig. 1. FTIR spectra of (a) Prepolymer, (b) Ex-PU, (c) Sh-C-PU.
absorption became weaker and 1759.73 cm−1 revealed new peak of C]O bonds due to urethane linkage made by TBBPA and MDI [45]. Analogously, Fig. 1(c) showed the spectrum of the final product Sh-CPU: specific absorptions at 3286.07 cm−1 was attributed to NeH bonds stretching vibration, and strong peak at 1730.21 cm−1 was C]O bonds absorption [43]. With the addition of GL, C]O stretching at 1730.31 cm−1 became much stronger, and the peak of isocyanate group at 2270 cm−1 disappeared. These phenomena all confirmed the successful synthesis of Sh-C-PU.
4.2. Swelling regularity analysis In order to further research and ascertain formation of the threedimensional network and the thermal deblocking phenomenon of Sh-CPU, the swelling property representation of Sh-C-PU samples was investigated [46,47]. The swelling ratio and dissolution ratio of Sh-C-PU samples were calculated and presented in Table 1 and Fig. 2. It can be seen from Fig. 2(a) and (a′) the first group of Sh-C-PU samples did not dissolve in DMF after being incubated at 50 °C for 5 h, but only swelling. And the swelling ratio was 603.13% after filtration (as shown in Table 1). After the sample was collected and dried in oven, the determined dissolution ratio was 21.88%, which might be deduced by the loss in collecting process and small amount dissolution of molecular chains. In general, the as-prepared product is cross-linked three-dimensional network. Continuously, the second group of Sh-C-PU samples was soaked in DMF at 90 °C. After 5 h, the samples also did not dissolve except for swollen. After filtering, the swelling ratio and dissolution ratio were found to be 536.00% and 24.00% respectively (shown in Table 1). And the shape of the soaked sample was changed from rectangle to irregular shape, as shown in Fig. 2(b), (b′). The Sh-C-PU samples, soaked at 90 °C for 5 h, should absorb more solvent and exhibit greater swelling. Therefore, the swelling ratio should increase by increasing the temperature due to easy deformation of molecular chains. However, the swelling ratio at 90 °C was lower than that at 50 °C because the deblock reaction at 90 °C occurred and the network was damaged slightly, Table 1 Test result of Sh-C-PU samples' swelling regularity. Test temperature (°C)
Ms' (g)
Ms (g)
Mr (g)
Swelling ratio (%)
Dissolution ratio (%)
50 90 130
0.32 0.25 0.27
2.25 1.59 –
0.25 0.19 –
603.13 536.00 –
21.88 24.00 –
A = ( NCO/ C6 H6 )t /(NCO/ C6 H6 )0 4
(3)
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Fig. 2. Swelling regularity of Sh-C-PU samples at different temperature with DMF as a solvent: (a) sample before soaking at 50 °C, (a′) sample after soaked at 50 °C; (b) sample before soaking at 90 °C, (b′) sample after soaked at 90 °C; (c) sample before soaking at 130 °C, (c′) sample after soaked at 130 °C.
Fig. 3. (a) FTIR spectra of deblocking reaction of Sh-C-PU and (b) the relationship of eNCO/C6H6 versus same heating time at different temperature.
deblocking reaction that fragile urethane group fractured to become isocyanate and phenolic hydroxyl groups. In the meantime, the characteristic peak of eNH at 3300 cm−1 became weaker as the deblocking reaction progresses. The heated sample at 130o C was subsequently experienced the first cooling process (cool-1) at 60 °C for 15 min to ensure the reblocking reaction and eNCO groups almost disappeared in the cooling process. The second heating and cooling cycle (heat-2 and cool-2) proceeded at the same temperature like the first cycle, and the characteristic peaks of eNCO groups again appeared and disappeared. After the second cycle, the samples proceeded the third heating and cooling cycle (heat-3 and cool-3) immediately as shown in Fig. 6(a), and these can confirm thermal reversible property of Sh-C-PU with three processes of heating and cooling. Through qualitative analysis of the above FTIR spectra, it is known that the as-synthesized Sh-C-PU exhibited really thermal reversible property. In order to more accurately analyze the thermal reversible behavior of Sh-C-PU, a quantitative analysis which had been introduced above was utilized. The eC6H6 groups which did not participate in the reaction was selected as the internal reference group and the characteristic peak height of the eNCO groups at 2270 cm−1 was quantitatively calculated by using eNCO/
(NCO/ C6 H6 )t = the height of NCO absorption peak relative to the absorption peak of C6 H6 at time (NCO/ C6 H6 )0 = the height of NCO absorption peak relative to the absorption peak of C6 H6 at initial time As shown in Fig. 5(b), with the cooling time extending, the amount of residual eNCO groups first decreased sharply and then slowly, ant it almost disappeared in about 8 min. From the analysis mentioned above, eNCO groups were released by deblocking reaction at a higher temperature and reacted with TBBPA again at a lower temperature. Therefore, the reversible behavior of ShC-PU samples can be reflected by observing the specific absorption of eNCO groups during the heating and cooling cycles, which was shown in Fig. 6(a). In the first heating process (heat-1), the appearance of the peaks at 2270 cm−1 and 3530 cm−1 clarified that free eNCO and eOH groups were released after heating at 130 °C for 15 min, which owed to 5
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Fig. 4. (a) FTIR spectra of deblocking reaction of Sh-C-PU and (b) The relationship of eNCO/C6H6 versus deblocking time at 130 °C.
PU before and after thermal treatment were observed via POM as shown in Fig. 7(b) and (b′) respectively. It can be seen from Fig. 7(b′) that the crack in the BDO-C-PU did not disappear during the heating process. Therefore, we can be inferred that Sh-C-PU showed an exceptional self-healing property. The reason for these phenomena is that BDO-C-PU is cross-linked polyurethane whose chains cannot move freely to repair the damage. By contrast, during the heating process, the cross-linking network of Sh-C-PU can be degraded to be linear chains gradually with deblocking reaction taking place during the heating process [49,50]. Thus, during the heating process, cracks in Sh-C-PU will be filled and healed quickly. Healing efficiency, a quantitative analysis, was determined by recovery of tensile strength after healed, which provided significant information to investigate the healing property of Sh-C-PU samples [14,51–53]. The healing progress of Sh-C-PU was shown in Fig. 8. In order to judge the appropriate healing time, the sample Sh-C-PU was broken at tensile instrument, and then healed in vacuum oven at 130 °C for 1 h, 2 h, 3 h respectively (sample named Healed-1 h, Healed-2 h, Healed-3 h), as shown in Fig. 9 and Table 2. The stress of the original sample of Sh-C-PU was 3.67 MPa. After heating 1 h, the stress of the sample Healed-1 h could back to 3.59 MPa and healing efficiency of the first cycle was 97.82%. The stress of the sample Healed-2 h was 2.43 MPa and healing efficiency was 66.21%, and the stress of the
C6H6 value. The calculated results were shown in Fig. 6(b). It can be found from Fig. 6(b) that after experiencing the first heating process, the eNCO/C6H6 value reached 0.161, and the height of eNCO groups became weaker, 0.121 the second cycle and 0.101 the third cycle as the sample experienced more heating and cooling cycles. Besides, after finished cooling process, the eNCO groups can be blocked almost and the peak height was only 0.004 for the first cycle, 0.003 for the second cycle, and 0.004 for the third cycle. 4.4. Self-healing property analysis The self-healing property of Sh-C-PU was primitively investigated by observing the changes of cracks via POM qualitatively [26,48]. The surface of the samples was scratched cross crack with a scalpel, and the samples were heated at 130o C for 1 h to observe the evolution of cross crack, as shown in Fig. 7. As we can see from Fig. 7(a), the cross crack of Sh-C-PU can be observed clearly via POM when it was scratched. As the heating time increased, the crack disappeared ultimately after 1 h as shown in Fig. 7(a′). In order to better contrast this phenomenon, a cross crack was scratched on the surface of cross-linked polyurethane BDO-C-PU which was synthesized without TBBPA but BDO as a chain extender, then followed by heating at 130 °C for 1 h, and the surfaces of BDO-C-
Fig. 5. (a) FTIR spectra of reblocking reaction of Sh-C-PU and (b) the relationship of degree of reblocking and cooling time. 6
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Fig. 6. (a) FTIR spectra of heating and cooling process of Sh-C-PU and (b) specific absorption relative height of eNCO group via heating and cooling process.
Fig. 7. POM photographs of evolution process of cracks in (a) Sh-C-PU and (b) BDO-C-PU films heated at 130 °C.
Fig. 8. The healing progress and the structural evolution of Sh-C-PU.
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Fig. 11. The stress of original and healed samples of Sh-C-PU and BDO-C-PU.
Fig. 9. Stress-strain curves of Sh-C-PU and the healed samples.
Table 4 Mechanical property and healing efficiency of Sh-C-PU and BDO-C-PU.
Table 2 Healing efficiency of samples with different healing time. Test results
Sample
Sample
Sh-C-PU
Stain (%) Stress (MPa) Healing efficiency (%)
Sh-C-PU
Healed-1 h
Healed-2 h
Healed-3 h
533.52 3.67 –
476.99 3.59 97.82
321.35 2.43 66.21
153.14 1.93 52.58
BDO-C-PU
Table 3 Healing efficiency of Sh-C-PU samples as a function of healing times.
Stain (%) Stress (MPa) Healing efficiency (%)
Sample Sh-C-PU
Healed 1st
Heating 2nd
Heating 3rd
533.52 3.67 –
476.99 3.59 97.82
428.24 3.17 86.38
250.65 2.02 55.04
Stain (%)
Healing efficiency (%)
3.67 3.59 9.02 0.43
533.52 476.99 186.65 16.84
97.82 4.77
the height of absorption peak of eNCO groups decreased gradually and mildly as heating time increased, and then the amount of free eNCO groups declined because free eNCO groups at high temperature reacted with free glycerol in the matrix or side reactions of eNCO groups occurred. While deblocking reaction was still going on and the crosslinked network of Sh-C-PU material was still destroyed, and the amount of eNCO groups involved in reblocking reaction were less and less. Therefore, the most suitable heating time was considered to be 1 h. An important feature of thermally reversible self-healing elastomers was the ability to perform multiple self-healing at the same place [14,51–53]. Therefore, in this work, the healing efficiency of Sh-C-PU was evaluated for breaking and healing cycles. The Sh-C-PU was first broken at tensile instrument and then the broken sample would be healed in vacuum oven at 130 °C for 1 h as the first healed sample (named Healed 1st). Similarly, the second healed sample and the third healed sample were obtained in this way (named Healed 2nd and Healed 3rd). The stress-strain curves of these samples were shown in Fig. 10 and Table 3. As shown in Fig. 10 and Table 3, the tensile strength of the sample healed 1st reached 3.59 MPa, close to the initial sample's tensile strength of 3.67 MPa, and the healing efficiency was as high as 97.82%; the tensile strength of the sample healed 2nd was 3.17 MPa, and the healing efficiency was 86.38%; the tensile strength of sample healed 3rd was only 2.02 MPa with healing efficiency by 55.04%. It demonstrated that the tensile strength and healing efficiency of the self-healing materials reduced as the healing times increased. With the analysis of thermal reversible property of Sh-C-PU, the amount of free eNCO groups released via deblocking reaction would be decreased as thermal cycles increased and the molecular chain structure of material itself would be destroyed after many healed times, so the healing efficiency and mechanical strength of the material decreased as the healing cycles increased. Meanwhile, in order to further highlight the excellent self-healing properties of Sh-C-PU, the mechanical properties and healing efficiency of BDO-C-PU were also determined, as shown in Fig. 11 and Table 4. As we can see from Table 4, although the tensile stress of BDO-C-PU (9.02 MPa) was higher than that of Sh-C-PU (3.67 MPa), its healing
Fig. 10. Stress-strain curves of Sh-C-PU healed by different healing times.
Test results
Original Healed Original Healed
Stress (MPa)
sample Healed-3 h reached 1.93 MPa with 52.58% healing efficiency. As shown in Table 2, the healing efficiency of the healed samples was getting weaker with the heating time increasing. As depicted in Fig. 4,
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Fig. 12. Comparison of tensile strength (initial stress and healed stress) and healing efficiency of Sh-C-PU samples with different proportions of TBBPA and GL from 8:2 to 5:5.
ability to deblock.
Table 5 Tensile strength data and healing efficiency of Sh-C-PU samples with different proportions of TBBPA and GL. Different proportions of TBBPA and GL 8:2 7:3 6:4 5:5
Initial Healed Initial Healed Initial Healed Initial Healed
Tensile strength (MPa)
Healing efficiency (%)
3.67 3.59 2.47 1.57 7.96 3.26 10.56 2.35
97.82
5. Conclusions A new route to synthesize cross-linked self-healing polyurethane (Sh-C-PU) using TBBPA as a reversible chain extender and GL as crosslinker was analyzed by FTIR and swelling regularity analysis. The thermal reversible property of Sh-C-PU was confirmed via FTIR, which experienced three heating and cooling cycles. POM qualitatively analysis revealed that crack at material was repaired by heating process. Tensile strength measurements on the original and healed samples demonstrated great mechanical and self-healing properties, 97.82% after once healed, 86.38% after twice healed, and 55.04% after three times healed cycles. The different proportion of TBBPA and GL from 8:2 to 5:5 indicated that the degree of cross-linking, the tensile stress of ShC-PU improved but healing efficiency decreased. The high self-healing efficiency polyurethanes which can be widely used in coatings, adhesives and elastomers. Take it a step further, this strategy can also use for the recovery and reuse of thermosetting polyurethane.
63.56 32.66 29.34
efficiency was much lower than that of Sh-C-PU upon heat treatment. The healing efficiency of Sh-C-PU can reach 97.82% when heated at 130 °C for 1 h. By contrast, it only arrived 4.77% for BDO-C-PU after the same heat treatment. These results showed that BDO-C-PU exhibited poor self-healing capability since its molecular chains cannot be broken by deblocking reaction. Thereupon, it can be concluded that Sh-C-PU exhibited great self-healing performance, and cracks in Sh-C-PU can be repaired by combined actions of the reversible block and deblock reactions and thermal movement of molecular chains. Similarly, the ratio of TBBPA and GL is also an important factor to influence mechanical performance and healing efficiency. Sh-C-PU samples with different proportions of TBBPA and GL from 8:2 to 5:5 were synthesized. After experiencing the same heating treatment, the tensile strength and healing efficiency of the original Sh-C-PU and the healed Sh-C-PU samples were measured and shown in Fig. 12 and Table 5. As for original Sh-C-PU sample, the tensile stress overall exhibited a rapid upward trend with increasing the ratio of GL, however, when the proportion of TBBPA and GL was 7:3, the tensile stress suddenly dropped since tensile stress of polyurethane resulted in hard segment separation between soft and hard segments when the degree of cross-linking was relatively weaker. When the proportion of TBBPA and GL increased, the cross-linking density also increased, which led to the increase of tensile stress of Sh-C-PU. Therefore, the tensile stress of ShC-PU samples was increased as the improvement of the degree of crosslinking. However, the self-healing property of Sh-C-PU was diverse. With the proportion of TBBPA and GL increasing, the healing efficiency became weaker gradually, that means, as the smaller the amount of TBBPA, the less the amount of urethane group, and so the less the
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