Macromolecular Research DOI 10.1007/s13233-018-6037-9
Article www.springer.com/13233 pISSN 1598-5032 eISSN 2092-7673
Self-Healing and Shape Memory Linear Polyurethane Based on Disulfide Linkages with Excellent Mechanical Property Lei Ling1,2 Jinhui Li1 Guoping Zhang*,1,3 Rong Sun1 Ching-Ping Wong3,4
1
Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences, Shenzhen, 518055, P. R. China Nano Science and Technology Institute, University of Science and Technology of China, Suzhou, 215123, P. R. China 3 Department of Electronics Engineering, The Chinese University of Hong Kong, Hong Kong, P. R. China 4 School of Mechanical Engineering, Georgia Institute of Technology, 771 Ferst Drive, Atlanta, Georgia 30332, United States 2
Received September 28, 2017 / Revised November 14, 2017 / Accepted November 22, 2017 Abstract: Self-healing polymeric materials have attracted extensively interests due to the ability to heal the damage autonomously. The self-healing systems based on dynamic disulfide bonds have been the most promising due to the efficient healing capacity at a mild condition. However, it is still of great challenge for designing the polymer with excellent mechanical and self-healing property by a simple synthetic route. Herein, a novel series of self-healing linear polyurethanes with the disulfide linkage as the grafting point were developed. The synthetic polymers all exhibited excellent mechanical properties (breaking strength and elongation at break were as much as 31.91 MPa and 1156% for PU-A). Meanwhile, the effects of different ratios of soft/hard segments on the mechanical properties and healing efficiencies have been investigated by stress-strain tests. The results showed that with the increase of soft segments contents, the breaking strength and elongation at break of the polymer improved significantly, while the healing efficiency and Young’s modulus showed a declining trend. The self-healing polyurethane can quickly restore its over 90% of mechanical property after healing at moderate temperature for 10 min. The cyclic tensile tests also showed the dissipated efficiencies and self-recovery abilities of the polymers. Finally, the recovery capability tests verified the shape memory effect in the polymers, which can replace an external force to accelerate the healing process. Keywords: self-healing, disulfide bond, polyurethane, mechanical property, shape memory.
1. Introduction Over the past decades, there has been increasing interests in design and development of intelligent polymeric materials, which mainly included self-healing polymer (SHP),1-4 shape memory polymer (SMP)5-7 and responsive polymer.8 Self-healing materials, originating from the biological self-curing mechanism,9 possess the capability to recover the initial performance without the need of manual assistance or intervention after damage. Such ability result in the extension of lifetime and reduction of maintenance of polymeric materials. Since White10 et al. presented the first self-healing system incorporating catalyst and a healing agent encapsulated within microspheres, there were many successful systems reported, generally including encapsulation11 and reversible chemistry.12,13 Acknowledgments: This work was financially supported by NSFC-Guangdong Joint Funding (U1601202), NSFC-Shenzhen Robot Joint Funding (U1613215), Guangdong and Shenzhen Innovative Research Team Program (No. 2011D052 and KYPT20121228160843692), Key Laboratory of Guangdong Province (2014B030301014), R&D Funds for Basic Research Program of Shenzhen (Grant No. JCYJ20150401145529012, JCYJ20160331191741738, and JSGG20160229194437896) and SIAT Innovation Program for Excellent Young Researchers (Y6G015). *Corresponding Author: Guoping Zhang (
[email protected]) Macromol. Res.
Nowadays, self-healing polymers based on dynamic chemical reversible bonds such as disulfide bond,14,15 Diels-Alder (DA) bonding,16,17 acylhydrazone bond18 and hydrogen bonding,19 etc., have been the most promising due to the reversible nature. Thereinto, disulfide bond is a dynamic covalent bond based on thiol/disulfide dynamic exchange reactions,20 which play a key role in stabilizing highly ordered structures of proteins in biological system and accomplishing the self-healing process in chemical system. There were three major advantages of introducing dynamic disulfide bond into polymer matrix to design a selfhealing material, i.e. i) response under various stimuli, such as heat,21 light22,23 and pH,24 ii) mild healing condition,25 iii) healing mechanism of one-step manner.26 By taking advantage of the unique properties of disulfide bond, many researchers designed the self-healing polymers to meet the requirements of different applications. For example, Klumperman and co-workers27 utilized the disulfide links incorporated in a rubber network to design a self-healing rubber, which could be able to restore its tensile failure strain at a moderate temperature (about 60 °C). Then Rekondo28 et al. tried to introduce an aromatic disulfide into polyurethane to obtain a cross-liked polymer and enabled it possessed the capacity of self-healing at room temperature. Chen24 et al. synthesized a self-healing hydrogel which containing both acylhydrazone and disulfide bonds and found the hydro© The Polymer Society of Korea and Springer 2018
Macromolecular Research gel was responded to pH condition to accomplish the self-healing process. And Zhang22 et al. prepared a sunlight triggered dynamic polyurethane network that contained the disulfide functionality in the main chain. However, it is still a challenge for designing an excellent stretchability self-healing polymer based on disulfide bond, which limits the applications in the future. Besides, SMPs have been attracted more attentions in scientific community for their potential applications in biomedical devices, electronics, self-healing materials smart adhesives, and deployable structures.29-32 SMP is a group of polymers that are able to memorize temporary shapes and then recover to their original geometries via external stimulation.33 There are two notable features for SMPs,34,35 one is a soft segment consists of the chains that fixed the temporary shape; and another one is a hard segment including covalent bonds or intermolecular interactions which recovered the permanent shape. Maitland34 et al. designed an amorphous network with uniform supermolecular structures to synthesize the shape memory polymer. Lately, Xie and co-workers36 prepared the cross-linked networks by thermosreversible DA adducts to obtain a SMP with highly versatile shape adaptability. According to the research of Sodano37 and Rowan,2 the shape memory effect from SMP could replace the external force to bring the two crack surfaces together, which was able to assist the self-healing process. As is well-known, the smart polymer which is fabricated by a simple route, shows excellent mechanical performance, self-healing and shape memory abilities, all these remarkable characteristics provide it as an ideal candidate for various applications. In this work, we synthesized a series of self-healing linear polyurethanes based on disulfide bonds though two-steps synthesis routes. The as-prepared self-healing polyurethanes exhibited excellent mechanical properties and could fully restore the original performances without external intervention at a moderate temperature after damaging. And we also studied the effects of contents of soft segments and disulfide bonds on mechanical properties and healing efficiencies. Besides, we found that the linear polyurethanes possessed the shape memory effect, which facilitated the self-healing process.
Scheme 1. The synthetic procedures of self-healing linear polyurethane.
routes were described briefly as follows: Firstly, MDI (2 mmol, 0.5107 g) was dissolved in 5.0 g of DMF in a 100 mL of glass flask at room temperature. PTMEG was preheated at 110 °C for 2 h under vacuum to remove the moisture, then the pretreated PTMEG (1 mmol, 2.0 g) was dissolved in 10.0 g of DMF and slowly added into the flask. The mixture was kept stirring at 80 °C for 2 h under nitrogen protection to prepare the prepolymer. Secondly, HPDS (1 mmol, 0.2554 g) was dissolved in 5.0 g of DMF with the assistance of ultrasonication, then it was introduced into the previous prepolymer as the chain-extender and the mixture was stirred under N2 at 80 °C for another 2 h. Finally, the as-prepared polyurethane polymer was poured into the molds made from Teflon and volatilized solvents under 70 °C to obtain the faint yellow transparent films, which was named PUA and the molar ratio of MDI/PTMEG/HPDS was 2/1/1. Other samples also were synthesized by the same procedure and their molar ratios of these monomers were 2.5/1/1.5 and 3/1/2, which were labeled PU-B and PU-C, respectively. And the all compositions of various MDI/PTMEG/HPDS and disulfide bonds contents polyurethane polymer were summarized in Table 1. 2.3. Characterization
2. Experimental 2.1. Materials 4,4’-Methylenebis(phenyl isocyanate) (MDI, 98%) was purchased from J&K Chemical Beijing and used as received; Poly(tetramethylene glycol) (PTMEG, Mn2,000) was supplied by Aladdin and used after 2 h of drying under vacuum at 110 °C; Bis(4-hydroxyphenyl) disulfide (HPDS, 98%) was provided by TCI Shanghai; N,N-dimethylformamide (DMF, 99%) was purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. and dried with the molecular sieves for 24 h and freshly distilled before use. 2.2. Preparation of self-healing polyurethanes A typical procedure for preparing the self-healing linear polyurethane was presented in Scheme 1. The two-step synthesis © The Polymer Society of Korea and Springer 2018
Fourier transform infrared (FTIR) spectra were monitored by a Bruker Vertex 70 spectrometer (Bruker Optik GmbH, Ettlingen, Germany) with the range of 4000-400 cm-1. The Raman spectra were recorded with a LabRAM HR Raman Spectrometer (HORIBA Jobin-Yvon, France) using a laser excitation wavelength of 633 nm and 15.7 mW power irradiation. The scattered light was detected with a thermoelectric cooled charge coupled device detector (CCD) and the collection time for a spectrum was 60 s. Differential scanning calorimetry (DSC) was Table 1. Recipes for the preparation of the polyurethanes with different molar ratios of three monomers Sample
MDI
PTMEG2000
HPDS
PU-A
2
1
1
Content (S-S) 25%
PU-B
2.5
1
1.5
30%
PU-C
3
1
2
33.3% Macromol. Res.
Macromolecular Research measured with 5-10 mg of sample by a Q20-1173 DSC thermal system (TA instruments, New Castle, USA) under Nitrogen gas was purged at a flow rate of about 50 mL min-1. Before the DSC measurement, the samples were preheated to 100 °C to eliminate the effects of thermal history. Subsequently, they were cooled down to -80 °C and then measured from -80 °C to 100 °C using a heating rate of 10 °C min-1. Thermogravimetry analysis (TGA) was made on a TA SDTQ600 thermogravimetric analyzer, the samples of about 10 mg were placed into 70 mL alumina pans and heated under a nitrogen flow from 30 °C to 800 °C at a rate of 10 K min-1. Scanning electronic micrographs (SEM) were recorded with a Nova NanoSEM 450. 2.4. Tensile tests Tensile tests were carried out on a RGM-4100 (Reger Instrument Co., Ltd., China) at room temperature with a crosshead speed of 50 mm min-1, the specimens were dumbbell-shaped tensile bars (approximately 35 mm×6 mm×0.1-0.4 mm). The neck regions of neat specimens were scratched perpendicular to the tensile axis with a clean razor blade. The scratch depth was approximately over 90% thickness of the original tensile bar. Then the samples were healed to 100 °C for 10 min in a microwave oven. The reason for selecting microwave as the heater is that the samples can be heated from inside to outside by the molecular vibration, resulting in the high heating efficiency to ensure the healing process completed. The reliable results were obtained by averaging the at least ten measured data.
3. Results and discussion The procedure of the synthesis of the self-healing polyurethanes based on disulfide bonds was shown in Scheme 1. Briefly, the HPDS was introduced into the polyurethane prepolymer as a chain-extender to synthesis the linear polyurethane resulting the main chains containing disulfide bonds. Besides, Scheme 1 indicated the self-healing mechanism originated from the disulfide exchange reaction. The whole self-healing mechanism was presented in Figure 1, when the crack occurred and then subjected a gentle pressure to bring the fracture contacted together. Through the thermal stimulation, the system reached the bond dissociation energy of disulfide bonds and these bonds were dissociated to form the radical, which would attack the adjacent and resulted in the reformation of new S-S bonds. The sample could restore nearly the original mechanical properties though the collaboration of disulfide exchange reaction and shape memory effect after heating to the specific temperature for some periods. 3.1. Characterization of self-healing polymers The successful synthesis of PU-A, PU-B and PU-C was confirmed by FTIR as shown in Figure 2(a). Comparing with the spectrum of MDI (Figure S1), the characteristic absorption peak of -NCO groups at 2260 cm-1 were completely disappeared in the spectra of PU-A, PU-B, and PU-C. The result indicated that all the -NCO groups were totally expended after the synthesis of these polyurethane polymers. Furthermore, the new bonds at 1549 cm-1 were observed which corresponded to the coupling of the
Figure 1. Illustration of the healing process of the self-healing polyurethane.
Figure 2. (a) FTIR spectra and (b) Raman spectra of PU-A, PU-B, and PU-C. Macromol. Res.
© The Polymer Society of Korea and Springer 2018
Macromolecular Research C-N stretching vibration with the CNH deformation.38 Besides, the peak at 1715 cm-1 ascribing to the oxygen-containing group C=O bond, which also confirmed the PU polymers were successfully developed as expected. In addition, Raman spectroscopy is very useful for monitoring the presence of disulfide and thiol functionalities. The results of the Raman spectroscopy measurements successfully proved the existence of S-S bond in the synthesized samples as shown in Figure 2(b). The characteristic peaks of S-S bond at around 498 cm-1 and C-S bond at 640 cm-1 were clearly observed in the Raman spectroscopy of PU-A, PU-B, and PU-C, which indicating the existence of disulfide bonds in the samples.39 Meanwhile, Figure 2(b) also showed the evolution of this peak area at 498 cm-1 with increasing the disulfide bond content. 3.2. Thermal analysis of the polymers As is well known that if the glass transition temperature (Tg) of soft segments is within the appropriate temperature ranges decided the shape memory effect in polymers. And the Tg of the as-prepared materials deduced from DSC experiments were presented in Figure 3(a), which depicted the thermal behaviors of PU-A, PU-B, and PU-C that were measured between -80 °C and 50 °C by a DSC cycle. The glass transition processes of PUA, PU-B, and PU-C could be clearly observed from the curves and the Tg was around -62.0 °C, -59.6 °C, and -56.8 °C, respectively. The results also indicated the Tg rose slightly with decreasing the contents of soft segments, which were in correspondence with the compositions of these samples.40 Moreover, the thermal stability and weight loss of PU-A, PU-B, and PU-C was assessed by TGA. The TGA curves of samples under a nitrogen atmosphere between 30 °C and 800 °C were shown in Figure 3(b), which indicated the decomposition of the PUs were occurred in a single step and started at about 280 °C. As can be seen from the inset figure, the temperatures at 5% weight loss (T5) of PU-A, PU-B, and PU-C were around 295 °C, 288 °C, and 281 °C, respectively. The results illustrated the T5 decreased slightly with reducing the contents of soft segments in the systems. The reason is that hard segments of the polyurethanes are more prone to thermal decomposition than soft segments.41 It also can be seen that with decreasing the soft segments the char yields at 800 °C increased slightly. In short, all these self-heal-
ing linear polyurethanes exhibit excellent thermal stability and maintain the T5 higher than 280 °C. 3.3. Mechanical properties of the polymers The mechanical properties were investigated for the PU-A, PU-B, and PU-C by the strain-stress curves that were shown in Figure 4(a). From the typical curves, a remarkable improvement in strain-at-break and stress-at-break simultaneously can be seen by increasing the soft segment contents in the systems (adding the molecular weight of the polymer). Furthermore, the primary mechanical parameters of the polymers and the standard deviations of each property were summarized in Table 2. As the content of PTMEG increased from 16 to 25 mol%, the strain-atbreak and stress-at-break all increased by about 1.5-fold which were from 846±31 to 1156±109% and from 22.90± 1.72 to 31.91±1.08 MPa respectively. However, the Young’s modulus of these samples exhibited an around 1.5-fold decrease from 5.61±0.53 to 4.04±0.31 MPa. In addition, the PU-A exhibited the highest work of fracture among the samples. The results were due to the intrinsic flexibility and the existence of de-entanglement of chains occurs during extension in a longer-chain polymer. In general, our self-healing polyurethanes exhibited excellent mechanical properties compared with other self-healing systems based on disulfide bond. Because the existence of abundant phenyl groups in the long-chain polymers enhanced the mechanical properties of the systems. The charts for compare the various self-healing polymers based on disulfide bond ware presented in Figure 4(b). In terms of stress-at-break and strainat-break, our self-healing linear polyurethanes outperformed most systems. For example, the reported cross-linked epoxidized polysulfides,15 semicrystalline covalently cross-linked networks2 and dynamic physically cross-liked polyurethane network,22 the stress-at-break and strain-at-break were below 20 MPa and 800%, respectively. The poly(urea-urethane) thermoset elastomer with aromatic disulfide crosslinks28 exhibited an elongation of 3000%, however the tensile strength (0.8 MPa) was relatively low. In conclusion, the as-prepared self-healing linear polyurethanes with disulfide linkages in our work integrated high tensile strength and elongation at break, which would be more promising in the practical application.
Figure 3. (a) DSC curves and (b) TGA curves of PU-A, PU-B, and PU-C. © The Polymer Society of Korea and Springer 2018
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Macromolecular Research
Figure 4. (a) Stress-strain curves of PU-A, PU-B, and PU-C. (b) Stress-at-break versus Strain-at-break charts for comparison of various self-healing polymers based on disulfide bonds, including (1) cross-linked epoxidized polysulfides,15 (2) semicrystalline covalently cross-linked networks,2 (3) dynamic physically cross-liked polyurethane network,22 linear polyurethane with aliphatic disulfide linkages,42 (4) poly(urea-urethane) thermoset elastomer with aromatic disulfide crosslinks,28 (5) PU-A, PU-B, and PU-C in this work.
3.4. Cyclic tensile loading-unloading test
Ui = d
Cyclic tensile loading-unloading tests were carried out to investigate the energy dissipation and the self-recovery abilities for the self-healing polyurethanes. Figure 5(a) showed the typical stress-strain curves of the self-healing PUs with different disulfide bonds (hard segment) contents during a loading-unloading cycle at a strain of 600%. Obvious hysteresis loops can be observed during the cycles and the loops became larger with increasing the disulfide bonds contents. The energy dissipated and dissipated efficiency were calculated according to the Eqs. (1), (2), and (3).43
Ui =
max 0
U Ui
d
= --------i 100
(1) (2) (3)
During a cycle with the strain of 600%, the energy dissipated and dissipated efficiency were 13.72 MJ/m3 and 72.24% for PU-A, 24.09 MJ/m3 and 73.90% for PU-B and 28.83 MJ/m3 and 76.78% for PU-C, respectively. The larger hysteresis for PU-C should be ascribed to the disassociation of more abundant
Figure 5. (a) Loading-unloading cycles of PU-A, PU-B and PU-C at a strain of 600%, (b) self-recovery performed by cyclic tensile tests of PU-A, (c) PU-B, and (d) PU-C at a strain of 100%. Macromol. Res.
© The Polymer Society of Korea and Springer 2018
Macromolecular Research hydrogen bonding between the chains during stretching, and the enhancement of mechanical strength and toughness was related to this energy dissipation process.43 In addition, the sequential loading-unloading cycles of the samples at a strain of 100% were shown in Figure 5(b)-(d), which implied the selfrecovery abilities within the short period. After the first loadingunloading cycles of PU-A, PU-B, and PU-C, the second immediate cycles all showed the nearly same hysteresis loop profile and area. However, with the increase of soft segment contents in the systems, the hysteresis loop of the second cycle exhibited more similar behavior compared with the first cycle, indicating the excellent instant self-recovery ability. The possible reason was the competitive result between the elastic contraction of chains and the more temporarily hydrogen bonds in a short-chain polymer during the unloading process. The cyclic tensile loading-unloading tests demonstrated that the longerchain polymer exhibited more excellent elasticity.
Figure 6. Damaging and healing process of the self-healing polyurethane: (a) Cut with a clean razor blade, (b) break into two segments, (c) contact together, (d) stretching process after the thermal healing.
3.5. Self-healing process and healing efficiencies The processes of damaging and healing were shown in Figure 6. Firstly, the specimens were cut completely into two segments with a clean razor blade, then the two sections were immediately stuck together, subjected to a gentle pressure and stored in a 100 °C microwave oven for only 10 min without any exoteric forces. The healed samples could nearly restore their original mechanical properties. Besides, from the SEM images of the sample after damaging and healing (Figure S2), it was obvious that the scratch on the sample almost disappeared completely within just 10 min. To quantify the healing efficiency, which is defined as the ratio of recovered breaking strength to the original strength, the mechanical properties of PU-A, PU-B and PUC after healing were investigated by the tensile tests. The original samples were carried out on the tensile experiments at the speed of 50 mm min-1 and the data were summarized in Table 2. According to the works of Zhang22 and Chen,42 the self-healing mechanism involved in the similar systems should be primarily ascribed to the disulfide exchange reaction rather than hydrogen bonds. When the fracture occurred, the plastic deformation of film produced. Upon heating, the polymer could recovery its original shape and brought the crack surfaces connective. Aromatic disulfide crosslinks would be cleaved in a hemolytic manner through having the lowest bond dissociation energy.44 Then such cleaved species rearranged to form new disulfides with adjacent chains. And the healing efficiency was calculated using Eq. (4).37 Healing efficiency (%)=
Breaking strengthHealed ×100 Breaking strengthOriginal
(4)
Figure 7(a)-(c) showed the characteristic strain-stress curves of original and healed polymer samples. Meanwhile, the mechani-
cal properties after healing for PU-A, PU-B and PU-C and the standard deviations of each property were also summarized in Table 3. It indicated the recovery of strain-at-break and Young’s modulus for all samples were nearly over 90% to the original (Figure S3). Here, the healing efficiency was represented by the ratio of the healed breaking strength to the original tensile strength according to the Eq. (4). Figure 7(d) illustrated the healing efficiencies and error bars for the self-healing polymers with different disulfide bonds contents. The average healing efficiencies estimated from the experiments were 73.83% for PU-A, 80.27% for PU-B, and 91.36% for PU-C, respectively. And the error bars were used to indicate the standard deviation of multiple specimens. It was obvious that with increasing the disulfide bonds contents in the chains, the self-healing polymers showed the higher and more stable healing efficiency. The reason should be that the addition of S-S bonds contents dramatically promoted the disulfide exchange reaction efficiency at the identical healing condition. The measured healing efficiencies were approximately 70-95% falling below 100%. There were two primary reasons, the first one was a result of the efficiency of the disulfide exchange reaction and the second reason was related to the contact of the fracture surfaces when healing. Therefore, when the contents of disulfide bonds were around 33.3 mol% in the self-healing systems, the healing efficiency could reach at 91.36%, which illustrated the high-efficiency and stable self-healing capacity in the polymers. And the examples of various self-healing systems and their healing efficiencies were also summarized in Table S1. Compared with other reported systems, the results proved the measured healing efficiency in this work reached the same level. However, it’s worth noting that the healing process taken less time (only 10 min) by
Table 2. Summary of the mechanical properties of the samples. The average values were obtained from more than 5 samples. Sample
Strain-at-break (%)
Stress-at-break (MPa)
Young’s modulus (MPa)
PU-A
1156 ± 109
31.91 ± 1.08
4.04 ± 0.31
PU-B
997 ± 60
25.85 ± 1.79
4.91 ± 0.25
PU-C
846 ± 31
22.90 ± 1.72
5.61 ± 0.53
© The Polymer Society of Korea and Springer 2018
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Figure 7. Stress-strain curves of PU-A, PU-B, and PU-C films before and after thermal healing: (a) PU-A, (b) PU-B, (c) PU-C, (d) healing efficiencies of the samples. Table 3. Summary of the mechanical properties of the samples after healing. The average values were obtained from more than 5 samples Sample
Strain-at-break (%)
Stress-at-break (MPa)
Young’s modulus (MPa)
PU-A (Healed)
1076 ± 101
23.56 ± 2.67
3.90 ± 0.63
PU-B (Healed)
929 ± 81
20.75 ± 2.13
4.37 ± 0.26
PU-C (Healed)
837 ± 37
20.92 ± 2.65
5.27 ± 0.28
heating with the assistance of microwave and the polyurethane elastomer still retained excellent stretchability after healing. 3.6. Shape recovery test Finally, the shape memory effect of the polymers was evaluated by the recovery capability tests37 in different manners and the integrated shape recovery processes was shown in Figure 8. Figure 8(a) showed the original shape of the film (35 mm× 5.0 mm×0.3 mm), after heating the specimen to 100 °C for 10 min and twisting immediately, then cooling it down to room temperature to fix the temporary shape (Figure 8(b)). When reheating the film to 100 °C for 5 min, it could recover the permanent shape (35 mm×5.0 mm×0.3 mm), which is shown in Figure 8(c). And Figure 8(d)-(f) showed the similar shape recovery process. The SEM images in Figure 9 also showed the changes of microstructure of the polymers under different states. Figure 9(b) and 9(e) indicated clearly that the wrinkle and ripple appeared on the surface when the polymers were fixed as temporary shape by folding or stretching. After recovering the permanent shape, the microstructure (Figure 9(c), (f)) could restore to the original states (Figure 9(a), (d)). It was a typical thermally induced one-way dual-shape memory cycle.35 The original shape repreMacromol. Res.
Figure 8. Images of the shape memory of the self-healing polymer by the recovery capability tests; (a) permanent shape (35 mm × 5.0 mm × 0.3 mm), (b) temporary shape after heating and twisting, (c) permanent shape after recovering, (d) permanent shape (35 mm × 5.2 mm × 0.3 mm), (e) temporary shape after heating and stretching, and (f) recovered permanent shape.
sented the permanent shape at room temperature. When heating above 100 °C defined as the shape memory transition temperature (Ttrans), which can be the glass transition temperature (Tg) © The Polymer Society of Korea and Springer 2018
Macromolecular Research
Figure 9. SEM images of the polymer under different states. (a) Permanent shape, (b) temporary shape after twisting, (c) permanent shape after recovering, (d) permanent shape, (e) temporary shape after stretching, and (f) recovered permanent shape.
or melting temperature (Tm), the mobility of molecular chains in polymers increased significantly. Therefore, heating resulted in the easy deformation with applying an external force. And the macroscopical deformation was related to changes of the molecular chains conformation or entropy. Then cooling the temperature below the Ttrans to room temperature, the deformation generated in the polymer can be maintained after removing the external force. The reason should be that the freezing of the molecular chains, which locked the conformation of deformed chains, or stabilized the storage of entropic energy in the system.35 When re-heating the polymer above Ttrans without any stress, the molecular mobility was activated and released the entropic energy to return to the highest state, which resulted the polymer recovery to its permanent shape at macroscopical scale. The above tests testified that the self-healing polyurethane possess the shape memory effect as the replacement of an external pressure during the healing process.
4. Conclusions In summary, we have successfully developed a series of linear polyurethane with disulfide bonds links, which exhibited excellent mechanical property, self-healing ability as well as shape memory property. The disulfide exchange reaction in the systems at a moderate temperature was responsible for the selfhealing process and the shape memory effect replacing an external force. Through the tensile tests, all the polymers showed excellent mechanical properties. Additionally, with increasing the soft segment contents, the braking strength and elongation at the break of the polymer improved significantly, while the Young’s modulus showed a declining trend. In contrast, the healing efficiency increased with the increase of the contents of disulfide bonds (hard segment). And the polymers exhibited © The Polymer Society of Korea and Springer 2018
the excellent thermally healable capacity with the maximal efficiency of 91.36% in view of the breaking strength. The cyclic tensile tests also indicated the superior self-recovery ability and lower dissipated efficiency in the longer-chains polymers with high molecular weight. Finally, the recovery capability tests verified the shape memory effect in the polymers, which can replace an external force to accelerate the healing process. Supporting information: Information is available regarding the characterization, SEM images and healing efficiencies. The materials are available via the Internet at http://www.springer. com/13233.
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