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Sustainable elastomer of triazolinedione-modified Eucommia ulmoides gum with enhanced elasticity and shape memory capability
Polymer 184 (2019) 121904
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Sustainable elastomer of triazolinedione-modified Eucommia ulmoides gum with enhanced elasticity and shape memory capability Hengchen Zhang a, Cuihong Ma a, Ruyi Sun a, Xiaojuan Liao a, Jianhua Wu b, **, Meiran Xie a, b, * a b
School of Chemistry and Molecular Engineering, East China Normal University, Shanghai, 200241, China National & Local United Engineering Laboratory of Integrative Utilization Technology of Eucommia Ulmoides, Jishou University, Jishou, Hunan, 416000, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Eucommia ulmoides gum Elasticity Shape memory capability
Sustainable Eucommia ulmoides gum (EUG) was a biomaterial with the molecular structure of trans-1,4-poly isoprene and poor elasticity, and was usually considered as a hard rubber with a limited applications in rubber products. Triazolinedione (TAD)-based Alder-ene reaction was used to modify EUG to precisely tune the me chanical properties and effectively improve the elasticity by reducing crystallinity, introducing urazole groups, and forming hydrogen bonds between EUG molecules. By adjusting TAD feed, the modified EUG could become an excellent tough elastomer with combined high strength, elongation, and toughness; or a unique rubber with superhigh elongation, low Young’s modulus, and good elastic recovery rate. Furthermore, the modified EUG displayed enhanced shape memory capability with fast shape recovery speed, high cyclic shape recovery rate after 10-cycle deformations, and large shape memory recoverable deformation. Therefore, these elastomers may have extended applications in tires and smart materials.
1. Introduction Elastomers are a class of polymeric materials with important indus trial applications in tires, seals, shock absorbers, shape memory devices, and insulators [1–4]. As the most widely used elastomer, natural rubber (NR) with the molecular structure of cis-1,4-polyisoprene has the ad vantages of high elasticity, good mechanical property, and wear resis tance [5]. However, the output of NR is limited by the inadequate plant resource of Hevea brasiliensis in the world especially in China, and NR can not meet the increasing needs of practical applications [6,7]. The synthetic elastomers [3,5,8–10], such as isoprene rubber (IR), styrene-butadiene-styrene triblock copolymers (SBS), trans- polyisoprene (TPI), polynorbornene (PNB), and polyurethane (PU), are mostly unsustainable petroleum-based products, which can not replace NR in engineering tire manufacturing and some other applications. As an isomer of NR, Eucommia ulmoides gum (EUG) is a natural semi-crystalline biomaterial extracted from the sustainable resource of Eucommia ulmoides oliver with wide planting areas in China, which is the second largest natural gum source in the world and expected to compensate for the shortage of NR that can only grow in tropical areas [11,12]. Although EUG possesses the advantages of non-toxicity, high
mechanical strength, electrical insulation, chemical resistance, and easy processing owing to its molecular structure of trans-1,4-polyisoprene and the unique duality of rubber and plasticity [13], the high elasticity brought by the flexible molecular chains could not exhibited at room temperature because of the existence of crystalline in EUG [11]. EUG was used as the toughener for plastics, or as an auxiliary component in rubber products, while the required amount of EUG was usually less than 20%, which greatly restricted its applications in the field of elas tomers, and of course it was hardly considered as a single component rubber like as the conventional rubber or NR [12,13]. Some researches on changing EUG into elastomers by chemical modification such as chlorination and epoxidation were carried out, which could effectively reduce the crystallinity and increase elasticity of EUG to some extent, but the elongation at break of modified EUG was sharply decreased at the same time, resulting in the deteriorated toughness [13–15]. In addition, these methods are usually poor controllable to the modifica tion efficiency, impeding the mass-production and up-scale application of EUG-based elastomers with stable performance. Therefore, it is imperative to design a kind of modified EUG elastomers with excellent elasticity and functionality by a simple and controllable method. 1,2,4-Triazoline-3,5-dione (TAD), as an enophile reagent of
* Corresponding author. School of Chemistry and Molecular Engineering, East China Normal University, Shanghai, 200241, China. ** Corresponding author. E-mail addresses: [email protected] (J. Wu), [email protected] (M. Xie). https://doi.org/10.1016/j.polymer.2019.121904 Received 27 August 2019; Received in revised form 9 October 2019; Accepted 12 October 2019 Available online 23 October 2019 0032-3861/© 2019 Elsevier Ltd. All rights reserved.
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environmental friendly click chemistry, can react rapidly and efficiently with diene-based polyolefins at room temperature without the require ment of catalyst and no release of hazardous substance or any byprod ucts during the reaction process, and has been used for the modification of unsaturated polymers [16–19]. For example, the changeable prop erties of diene-based polymers through TAD-chemistry were reported [16,20], and the modification imparted to the thermoplastics, namely, greatly improved the mechanical strength and elongation at break to these materials. Particularly, the modified 1,4-polybutadiene rubber exhibited a remarkably high elongation at break of 1782% and tensile strength of 2.8 MPa, while cis-1,4-polyisoprene was difficult to work with and either solubilization was never achieved or the films were weak and tacky after 1% of TAD modification [20]. The mechanical and dy namic properties of TAD-modified styrene-butadiene rubber were researched, and the urazole pendants-produced hydrogen bonds could improve the mechanical strength and broaden the range of damping properties [21]. The TAD-based click chemistry of the electrospun SBS fibers with tunable mechanical properties were investigated, and found that the cross-linking reaction of the SBS fibers by bis-TAD allowed to adjust the elongation at break from about 90% to 700%, whereas the modulus of the fibers covered a range from 11 MPa to over 130 MPa [22]. Importantly, the systematic research on TAD reversible click chemistry was presented, and provided a series of modified polymers with notable properties, including shape memory polymers, reversible cross-linked elastomers, and high modulus self-healing polymers [23–25]. Herein, EUG was modified by the TAD-based Alder-ene chemistry, a reliable and efficient tool, to provide a family of high-performance modified EUG elastomers with widely tunable mechanical properties, including the excellent tough elastomer with combined high strength, elongation, and toughness, or the outstanding rubber with good roomtemperature elasticity and elastic recovery. After TAD-modification, the urazole groups in the side chain of modified EUG could form the intra- and intermolecular hydrogen bonds [16,20], which acted as the physical cross-linking points to form a network structure, change the related physical and mechanical properties, and enable the elongation at break to significantly increase, although the tensile strength gradually decrease, devoting to the high toughness. In addition, the urazole groups endowed EUG with a certain extent of polarity, so that the modified EUG had some additional performances, such as variable hardness, unique room-temperature damping, good oil resistance, and enhanced shape memory property. Consequently, it is envisioned that the simple and effective TAD-modification could make the modified EUG elastomer or rubber be possible for up-scale practical applications to meeting the increasing demand of elastic materials in current industry [6,8,9].
properties, variable contents of bTAD were used to react with EUG (the feed molar ratio of bTAD to the double bonds in EUG ¼ 0.01:1, 0.03:1, 0.05:1, 0.1:1, 0.15:1, and 0.2:1). After EUG was sufficiently dissolved in CHCl3, the bTAD solution in CHCl3 was added into the EUG solution, and the mixture was stirred for 6 h at room temperature. The modified EUG was precipitated into methanol, cut the solid into strips, and dried in a vacuum oven at 40 � C to a constant weight. After molding compression at 80 � C under a constant applied stress of 10 MPa [14], the samples of modified EUG were obtained, which were designated as TbxEUG, where x indicates the molar ratio of bTAD contents to the double bonds of EUG. 2.3. Characterization 1 H NMR spectrum was recorded on a Bruker DRX500 (500 MHz) spectrometer using tetramethylsilane as an internal standard. FT-IR spectra of polymer thin films were recorded on a Thermo Nicolet iS50 in the region of 4000–400 cm 1 for attenuated total reflectance (ATR) measurements. Gel-permeation chromatography (GPC) was used to calculate the relative molecular weight and polydispersity equipped with a Waters 1515 Isocratic HPLC pump, a Waters 2414 refractive index detector, and a set of Waters Styragel columns (7.8 � 300 mm with particle size of 5 mm; pore diameter of 103, 104, and 105 Å). The mea surement was carried out at 40 � C, the concentration of polymer solution was approximately 0.2 wt%, and the flow rate was set at 1.0 mL min 1 in all the measurements. Differential scanning calorimetry (DSC) was performed on a Q2000 DSC instrument in a nitrogen atmosphere. The sample firstly heated from 30 to 100 � C and held 3 min at this tem perature, then cooled to 100 � C and heated again from 100 to 100 � C, and the heating or cooling rate was 10 � C min 1. X-ray diffraction (XRD) was performed on a Shimadzu NERCN-TC-007 instrument, and the scanning range was 5� –80� and scanning rate was 10� min 1 in this experiment. Tensile tests were carried out on a Shimadzu AG-Xplus electronic tensile tester in accordance to GB/T 528–2009 at an extend ing rate of 200 mm min 1 during the process. The dumbbell-shaped samples with an effective gauge length of 12 mm, a width of 2 mm, and a thickness of 1 mm were prepared by using a hand operated cutting press. In general, five samples were obtained for testing the values of tensile strength and elongation at break. A predefined strain of 100% was used in the loading-unloading cycle test with extending rate of 50 mm min 1. The hydrodynamic diameter was determined by means of dynamic light scattering (DLS) analysis using a Malvern Zeta sizer NanoZS light scattering apparatus (Malvern Instruments, U.K.) with a He–Ne laser (633 nm, 4 mW), and the Nano ZS instrument incorporates noninvasive backscattering (NIBS) optics with a detection angle of 173� . Hardness test was carried out using a XY-1 Shore A hardness meter under room temperature in accordance to GB/T 531–2008. The thick ness of test samples was more than 6 mm. Oil resistance tests were performed in the paraffin oil solvent at room temperature in accordance to GB/T 539–2008. Dynamic mechanical analysis (DMA) was measured with a PerkinElmer DMA 8000 instrument, the test was carried out at the tension condition with frequency of 1 Hz and strain of 0.2%, and the scanning temperature was ranged from 100 to 100 � C at a heating rate of 3 � C min 1. Shape memory test of samples was conducted in a 60 � C water bath.
2. Experimental 2.1. Materials EUG (>94%) was provided by Xiangxi Laodie Biotechnology Co. Ltd., Hunan Province, China. Butyl isocyanate, ethyl carbazate, and triethylenediamine were purchased from Energy Chemical Technology Co. Ltd. Toluene, hydrochloric acid, tetrahydrofuran (THF), and chlo roform (CHCl3) were purchased from Shanghai Chemical Reagent Co. Ltd., Shanghai, China. Potassium hydroxide (KOH) and bromine (Br2) were purchased from Sinopharm Chemical Reagent Co. Ltd., Shanghai, China. 4-Butyl-l,2,4-triazoline-3,5-dione (bTAD) was synthesized ac cording to the literature procedure [24]. Dichloromethane (CH2Cl2) and methanol were purchased from Shanghai Wohua Chemical Co. Ltd., Shanghai, China. The reagent grade solvents were distilled over drying agents prior to use.
3. Results and discussion 3.1. Preparation and characterization of TAD-modified EUG When EUG was reacted with bTAD via the Alder-ene reaction (Scheme S1), the isomerization of double bond on the EUG backbone did happen, accompanying with the migration of a σ-bonded hydrogen atom from the initial olefin to the TAD molecule to form a new N–H bond on the urazole side group [20], and meanwhile, the formation of a new C–N bond connecting the modifier bTAD to EUG generated the modified EUG, TbxEUG. EUG and the representative Tb20EUG were distinguished
2.2. Modification of EUG In order to get a variety of modified EUG with different elastic 2
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were indicative of occurred modification reaction. The original EUG had a high Mn value of 170.2 kDa. With increase in the degree of TADmodification, the apparent Mn, reflected by the hydrodynamic volume, of TbxEUG tended to decrease gradually from 106.3 kDa for Tb1EUG to 18.8 kDa for Tb18EUG, and further heavily down to 14.3 kDa for Tb20EUG when the feed amount of bTAD was up to 20% (Fig. 2a and Table S1). The hydrodynamic volume of TbxEUG in THF was corre spondingly reduced, due to the interaction of intramolecular N–H⋯N hydrogen bonds originating from the urazole moieties, resulting in a decrease in the GPC-measured Mn value [23]. The formed intra molecular N–H⋯N hydrogen bond between the pendant urazole groups of TbxEUG was verified by the blue-shift of the absorption peak from –O 1704 to 1685 cm 1 in Fig. S1, ascribed to the strengthened C– stretching vibrations as the increased bTAD content and enhanced hydrogen bond interaction [19,21]. Therefore, the results of GPC indi cated that the TAD-modification has an obvious effect on the tested Mn value of TbxEUG. The influence of bTAD-modification on the glass-transition temper ature (Tg), melting point (Tm), and crystallinity (Xc) of TbxEUG was demonstrated using differential scanning calorimetry (DSC), as shown in Fig. 2b. Because of the flexible molecular chains, the trans-1,4-poly isoprene structure of EUG displayed a low Tg of 68.1 � C. However, the EUG molecular chains were regularly arranged, and displayed the semicrystalline property with Xc of 22.5% and Tm of 56.1 � C, so that EUG did show good low-temperature resistance, but appeared hard plasticity at room temperature rather than soft elasticity [11]. As the bTAD feed increased from 1% to 10%, the Xc of TbxEUG sharply decreased from 22.5% to 2.6%, their Tm also decreased from 56.1 � C to 30.9 � C, while their Tg raised from 68.1 � C to 46.8 � C. When the feed amount of bTAD further increased to 15%, the crystallinity of Tb15EUG dis appeared entirely and no melting peak was observed, making it become an amorphous material accompanying with the Tg up to 25.5 � C, which was much lower than room temperature, and it was accordingly an elastomer with a good low-temperature resistance. Finally, the Tg of Tb20EUG raised to 10.5 � C if 20% of bTAD was loaded. In addition to decrease in crystallinity, the mobility of polymer chains was limited by the interaction of intramolecular N–H⋯N hydrogen bonds between the urazole groups, resulting in the increase in Tg [22,25]. It was believed that the enhanced intramolecular and intermolecular interactions reduced the free volume, caused the rotation of segments to require higher temperature to overcome the energy barriers, and thereby devoted to an increased Tg [27,28]. Especially, when the incorporated bTAD content was 3%, the Xc of Tb3EUG reduced to about a half of EUG, making it become a lower crystalline material. At this point, the Tg of Tb3EUG was still below 60 � C, so it displayed the excellent properties of thermoplastic elastomer. The TGA curves demonstrated the thermal
Fig. 1. 1H NMR spectra of EUG (a) and TAD-modified EUG (b).
by 1H NMR spectroscopy (Fig. 1), and the aforementioned change in chemical structure could be verified. As shown in Fig. 1a, the signal peak at 5.12 ppm was ascribed to the proton (Ha) on the double bond of EUG backbone, and the signal peaks at 2.06 ppm and 1.98 ppm belonged to the protons (Hb,c) on the two different methylene groups. The strong signal peak at 1.60 ppm was related to the protons (Hd) on the methyl group of trans-1,4-polyisoprene structure [26]. Since the Alder-ene re action only transferred the double bond to the next carbon atom of one trans-isoprene unit, the degree of unsaturation was retained during the modification process, while most of the proton signals hardly changed, and only a signal peak, representing the methenyl proton (Hf) on the Tb20EUG backbone connected to the nitrogen atom, appeared at 3.78 ppm (Fig. 1b). The signal peak at 9.84 ppm was assigned to the proton (He) on the nitrogen atom of urazole group, and other signal peaks at 3.54 ppm, 1.28 ppm, and 0.86 ppm were designated as the protons (Hg,h,i) on the butyl group of urazole pendent [20,23,26]. The truly grafting efficiency of bTAD onto the Tb20EUG backbone could be evaluated by the peak area ratio of methenyl proton (Hf) to that of double bond proton (Ha) from the corresponding 1H NMR spectrum, and was calculated to be 22% by the peak area ratio (Sf:Sa) of 0.22:1, which was agreed well with the feed ratio of bTAD to EUG (0.20:1). Overall, the Alder-ene reaction of bTAD with EUG was quite efficient to generate TbxEUG. In addition, the variation of the number-average molecular weight (Mn) of TbxEUG compared with that of EUG was revealed by gelpermeation chromatography (GPC) measurements, and the results
Fig. 2. (a) GPC traces and (b) DSC curves of TbxEUG with varied amount of bTAD. 3
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Fig. 3. (a) Stress-strain curves and (b) tensile loading-unloading cycles of EUG, NR, and TbxEUG with different bTAD contents.
stability of TbxEUG was slightly lower than that of EUG (Fig. S2). Although the thermal decomposition temperature of urazole groups was about 300 � C, which was lower than that of EUG backbone, a slight decrease in thermal stability did not affect its use as an elastomer [23]. After TAD-modification, a number of urazole moieties were randomly suspended on the side chains of EUG backbone, hindered the alignment of the modified-EUG chains, and led to the decreased Xc values of TbxEUG with the increase in bTAD content, which were also confirmed by X-ray diffraction (XRD) technique (Fig. S3) and higher than those by DSC measurement (Table S1).
from 772% to 1130%, then decreased slightly, and finally reached to over 1800%. This is because that after modification of EUG with bTAD, the introduced urazole moieties affected the orderly arrangement of EUG chains and reduced its crystallinity, and thus decreased the tensile strength. On the other hand, the urazole groups could form a hydrogen-bond network via the intramolecular and intermolecular in teractions, which corresponded to the physical cross-linking function, resulted in the locally order but overall disorder, and increased the elongation at break of TbxEUG [19–21,30]. It could be obtained from the above change of mechanical properties that the Young’s modulus monotonously decreased from 72.20 MPa for EUG to 41.0–0.53 MPa for different TbxEUG due to the weakened tensile strength, while the toughness value of 99.78 MJ m 3 for EUG was first up to 122.89 and 124.78 MJ m 3, and then gradually descended to 11.35 MJ m 3 for TbxEUG, which were all higher than the toughness of 9.14 MJ m 3 for NR. Especially, owning to the hard and tough feature, Tb3EUG illus trated the maximum toughness of 124.98 MJ m 3 by the optimal com bination of high tensile strength of 27.62 MPa and elongation at break of 1130% (Fig. S4), which was a unique high-performance tough elastomer with combined high elasticity, strength, and toughness simultaneously, and even better than the mechanical performance of the most conven tional PU thermoplastic elastomer [9]. Tb5EUG with the tensile strength of 15.06 MPa, elongation at break of 1046%, and toughness of 64.86 MJ m 3 was also a good tough elastomer. Furthermore, Tb18EUG demonstrated the tensile strength of 2.42 MPa, which were very close to that of NR, the elongation at break of 1280% and the toughness of 14.44 MJ m 3 were better than those of NR, just its Young’s modulus of 1.51 MPa was deviated from that of NR. Therefore, Tb18EUG has the potential to become a new material matched well with the conventional rubbers such as NR [15,31]. Finally, Tb20EUG exhibited an extremely high elongation at break of 1852%, while a low tensile strength of 1.27 MPa, which was only about half that of NR, and possessed the soft and weak feature with the toughness of 11.35 MJ m 3 and Young’s modulus of 0.53 MPa. The specific mechanical features of EUG, TbxEUG, and NR were listed in Table S2. The elastic recovery rate of materials is another important parameter for evaluating the properties of elastomers or rubbers. The elastic re covery of TbxEUG was changed after modification, which was analyzed by tensile loading-unloading cycle test, and the results were listed in Table S3. It was found that the elastic recovery rate of materials increased from 33% for EUG to near 60% for TbxEUG as the increase of bTAD amount (Fig. 3b), indicating the improved room-temperature elasticity of TbxEUG [9]. However, there have been still a certain gap of the elastic recovery rate between Tb18EUG (53%) or Tb20EUG (57%) with NR (87%), although Tb18EUG shared very closed mechanical
3.2. Mechanical and physical properties of modified EUG The mechanical properties of EUG could be finely tuned by the modification with varied contents of bTAD, which is beneficial to improve the room-temperature elasticity and expand the scope of application of EUG. It was known that EUG possessed high tensile strength of 37.12 MPa and moderate elongation at break of 582%, as depicted in its stress-strain curve (Fig. 3a). The Young’s modulus and toughness of materials were also obtained from their stress-strain curves, the Young’s modulus was the ratio of stress to the correspond ing strain in the initial elastic deformation stage of stress-strain curve, and the toughness was the integral area of stress-strain curve [29]. As a result, EUG displayed good toughness of 99.78 MJ m 3 and high Young’s modulus of 72.20 MPa, but poor room-temperature elasticity, and therefore it was usually considered as a hard rubber. Because of the flexible but regular-arranged molecular chains, EUG molecules pro duced a folded chain conformation with ordered crystallinity, and exhibited the hard and strong thermoplastic elastomer property with the stress yielding phenomenon when subjected to tensile force [28]. In contrast, NR has the prominent elasticity with low tensile strength of 2.50 MPa, large elongation at break of 712%, poor toughness of 9.14 MJ m 3, and low Young’s modulus of 0.58 MPa; because NR owned a random coiled conformation composed of flexible and irregular-arranged molecular chains, it displayed soft and weak feature when subjected to tensile force, and its stress increased linearly with increasing strain during stretching, similar to an ideal elastomer [29]. In view of the wide application of NR, EUG was envisioned to have the similar or closed mechanical properties with NR after chemical modi fication. As expected, when EUG was modified by various contents of bTAD, the resulting TbxEUG exhibited a wide range of mechanical properties including different tensile strength and elongation at break, which were indicated by the stress-strain curves (Fig. 3a). As increase in bTAD content, the tensile strength of TbxEUG gradually decreased from 34.51 MPa to 1.27 MPa, while the elongation at break first increased 4
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overall disordered entangled structure due to the dual function of the flexible molecular chains of TbxEUG and the hydrogen bonds between urazole groups, and the change of the entangled structure of TbxEUG during stretching process was shown in Scheme 1. The intermolecular interaction between the urazole groups caused a portion of TbxEUG molecular chains to aggregate to form the stable segment clusters, as indicated by the circular dotted lines. When subjected to external force, most of the flexible molecular chains of TbxEUG extended, but the formed molecular clusters hindered the development of the elongation process [28,29,36]. It could be known that the formation of segment clusters enable TbxEUG to have high elongation at break and definite tensile strength, and the number of molecular clusters determined the mechanical properties of TbxEUG. The segment clusters of TbxEUG with different number-average hydrodynamic diameter (Dh) were proved by dynamic light scattering (DLS) measurement (Fig. S5 and Table S4), there was one peak with a small Dh corresponding to a single molecule, and another peak with a large Dh corresponding to the aggregates was observed. It could be seen from Fig. S5a that the Dh sizes of single polymer molecule at 0.5 mg mL 1 in THF solution first increased slightly from 7.7 nm for EUG to 7.8 nm for Tb3EUG, and then slowly decreased to 7.6 nm for Tb10EUG and 6.6 nm for Tb20EUG as the TAD contents increased to 3, 10, and 20%, which were also used as the positive evi dence to confirm the gradually decreased apparent Mn of TbxEUG by GPC. On the one hand, low grafting content of TAD increased the Dh size of single molecular chain; On the other hand, the formed intramolecular hydrogen bonds between the pendant urazole groups of TbxEUG decreased their Dh sizes, and the influence of intramolecular hydrogen bond forces was stronger when the urazole content was higher. In addition, the aggregates of TbxEUG with large Dh size were also observed, which exhibited a reduced trend at 90, 87, and 76 nm suc cessively as the increase in TAD contents, indicating that a certain number of the intermolecular hydrogen bonds between the pendant urazole groups were existed in the TbxEUG molecules. If the concen tration of polymers increased to 25 mg mL 1 in CHCl3 solution, which was the same as that for preparing the film for the measurement of mechanical properties, the Dh sizes of EUG and TbxEUG displayed a similar varying trend while more larger compared with those of poly mers at low concentration of 0.5 mg mL 1 in THF solution, which were 32 nm for EUG, 22 nm for Tb3EUG, 14 nm for Tb10EUG, and 12 nm for Tb20EUG, as well as 200, 145, and 115 nm for the aggregates of TbxEUG as the TAD contents increased to 3, 10, and 20% correspondingly, as shown in Fig. S5b. It was worthy to note that there was no obvious ag gregates appeared in solution of EUG even at high concentration, and on the contrary, the appearance of aggregates with large size verified the existence of hydrogen bonds between the pendant urazole groups in the
Fig. 4. Toughness versus elongation at break of TbxEUG and other types of polymer materials.
Scheme 1. Variation of network structure of TbxEUG before (a) and after (b) stretching.
properties to NR, implying that it is necessary to adjust the elastic re covery properties of TbxEUG elastomer further. By comparing EUG and TbxEUG with other types of commonly used polymer materials [1,7–10, 32–35], it was revealed from the toughness versus elongation at break that EUG is a typical tough elastomer. After modification, TbxEUG could be changed from an excellent tough elastomer, such as Tb3EUG, to an outstanding rubber, such as Tb18EUG (Fig. 4). Therefore, TbxEUG seemed to become a new type of high-performance elastic materials with a combination of high strength and toughness or large elongation and low Young’s modulus, which will enable the application range of EUG to be greatly expanded, especially its added amount in rubber matrix likely increased a lot, and expected to be used as a main component of con ventional rubber products. As described above, the resulting TbxEUG has a locally ordered but
Fig. 5. Dependence of (a) E0 and (b) tanδ on temperature for TbxEUG with different bTAD contents. 5
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TbxEUG molecules. The hardness of TbxEUG was changed after modification, the selected representatives of Tb3EUG, Tb10EUG, and Tb20EUG in different elastic regions of the full elastic-spectrum (Supplementary In formation, Figs. S6 and S7) were analyzed by Shore A tester, and the results were displayed in Table S3. It was found that Shore A hardness was reduced from 94 to near 70 as the bTAD feed increased from 0% to 20%, indicating the tunable hardness [15]. In view of the fact that rubbers or elastomers with Shore A hardness of about 70 were more suitable for processing technology, TbxEUG was easier to process than EUG. The oil resistance was also tested by the volume change of samples in the paraffinic oil solvent. As shown in Table S3, the volume change (0.05–3%) of TbxEUG was significantly lower than that (13.47%) of EUG, suggesting that the oil resistance of TbxEUG increased obviously compared with EUG because of the polarity introduced from TAD chemistry. Overall, the TbxEUG elastomer had excellent oil resistance and convenient processing properties, due to the adjustable low melting point (30–50 � C) and tunable hardness (74-89� ).
irreversible, so the mechanical energy is stored [49]. It could be seen from the IR spectra of Tb20EUG at room-temperature, 60 � C, and 80 � C – O stretching vibration at 1685 cm 1 that the absorption peak of the C– was not move obviously (Fig. S1), indicating that the hydrogen bonds existed in TbxEUG elastomers were stable in film state [50–52] and have no contribution to the reversible shape deformation. Therefore, the mechanism for shape memory property of EUG and TbxEUG was based on the transition of crystals or microcrystals phase by heating or cooling instead of the dissociation and reconstruction of hydrogen bonds with temperature. The shape memory behavior of TbxEUG with different bTAD contents was demonstrated by the curves of shape recovery against temperature and cycle number. The shape recovery angle (θ) was measured by the angle between the straight ends of the bent spec imen, and the shape recovery rate (Rr) was calculated from the following equation [49,53]: Rr ¼ (θ/180) � 100% (0� � θ � 180� ) The results of shape recovery rate against temperature and cycle number in a hot water bath were shown in Fig. S8. It was clearly seen that the Rr values of all samples increased rapidly as the temperature varied from 35 � C to 60 � C, and the maximal Rr values of EUG, Tb3EUG, Tb10EUG, and Tb20EUG reached up to 74, 91, 99, and 82% at about 60 � C (Fig. S8a), respectively, which was about 0–30 � C above the Ttrans, so the cyclic shape recovery testing for EUG and TbxEUG was conducted in a water bath at 60 � C. As a whole, TbxEUG exhibited higher Rr than that of EUG, and Tb10EUG displayed the highest Rr of 99% among these samples. What is more important, Tb3EUG, Tb10EUG, and Tb20EUG performed almost a steady variation with Rr values of 85, 95, and 80% at 60 � C (Fig. S8b), respectively, even the number of deformation cycles increased to 10, while the Rr of EUG decreased sharply to 32% at the fourth deformation cycles, indicating the poor cyclic shape memory property of EUG and the superior repetitive shape memory capabilities of TbxEUG, especially the Tb10EUG elastomer showed the best cyclic shape memory property, which could be reused for many times and had excellent potential application prospects. In addition, the shape memory behavior of EUG, Tb3EUG, Tb10EUG, and Tb20EUG elastomers was illustrated by the photos of dumbbell specimens in a 60 � C water bath (Fig. S9), and the whole shape recovery process was conducted in 10 s, indicating the high shape recovery rate and the fast shape recovery speed of TbxEUG [54,55]. Since EUG had creep phenomenon due to uncross-linking, it could not be completely relaxed to the original un stressed state after stress was relieved, whereas TbxEUG could be stably realized multiple times of shape recovery based on the fixed action of physical cross-linking from hydrogen bonds. However, the amorphous Tb20EUG with higher modification degree exhibited downward trend of these performances because the crystalline reversible phase dis appeared, which was similar to those of vulcanized rubbers without shape memory ability [56–58]. In brief, because the TbxEUG elastomers have lower Ttrans and superior cyclic shape memory properties compared to most of the shape memory polymers including conventional lightly cross-linked shape memory EUG [15,59], they can implement the cyclic shape deformations and recoveries under slightly hot conditions. In addition, because the TbxEUG elastomers have excellent elonga tion capability, they will have large shape memory recoverable de formations, and thus the shape recovery of EUG and TbxEUG elastomers under stretching was also attempted apart from bending. The heated elastomers were stretched to a certain length by the external force, and the elongated shapes were fixed at room temperature, and then shrank to a remaining length upon reheated. The most value of shape memory recoverable deformations for TbxEUG was greater than 400%, and the maximum value for Tb3EUG was greater than 600% under the condition of higher recovery rate. For example, the selected sample of Tb3EUG with a gauge length of 15 mm was stretched to 105 mm (deformed strain, ε1 ¼ 600%) in a 60 � C water bath and fixed at room temperature, and the sample shrank to a remaining length of 23 mm (residual strain,
3.3. Dynamic mechanical properties of modified EUG Elastomers are inevitably used under dynamic conditions, so the dynamic mechanical properties are critical to the description of elas tomer performance. The dynamic mechanical properties of TbxEUG with different bTAD contents were investigated by dynamic mechanical analysis (DMA), and the dependence of storage modulus (E0 ) and loss factor (tanδ) on temperature for TbxEUG was shown in Fig. 5. As tem perature went up, the E0 of TbxEUG slowly decreased with a temperature-independent rubbery plateau ranging from 40 to 40 � C, and descended when exceeding 40 � C (Fig. 5a), which was related to the melting transition of materials. With increase in amount of bTAD, the E0 of TbxEUG decreased sharply, especially at low temperature region [37]. In the tanδ curves (Fig. 5b), the changing tendency was similar with E0 curves. The tanδ of TbxEUG changed little at low temperature range but heightened a lot between 20 � C and 40 � C. In addition, the existence of Tg for EUG and the urazole moieties derived from modifier bTAD resulted in a lower tanδ (damping peak), ranging from 0.1 to 0.3. EUG exhibited a single peak at a low temperature of 50 � C on the tanδ curve, which was not helpful for damping performance. With the incorporation of bTAD into EUG backbone, the damping peak of TbxEUG gradually moved toward high temperature and had a damping peak near 20 � C, indicating that the elastomer had unique room-temperature damping property [38,39]. When EUG was modified by 1–5% of bTAD, the tanδ curves appeared double peaks, which widened the damping peak, and thus enhanced the damping perfor mance of TbxEUG [40,41]. In a word, the TAD-modification may enable EUG to be a new type of tough elastomer or soft rubber with improved elasticity and damping property, which will expand its applications like as NR and other rubbers [42,43], although the elastic recovery and hardness of TbxEUG elastomers are still not as good as those of NR, these parameters should be further optimized by precise tuning of TAD structure and other aspects. 3.4. Shape memory properties of modified EUG The thermal-induced shape memory polymers are a new class of smart materials that can replace shape memory alloys, which consist of two phases, a reversible phase that can change with external conditions and a stationary phase that provides a permanent shape to the material [3,44–49]. When EUG and TbxEUG heated above Tm (¼ transition temperature, Ttrans), they can be deformed and the temporary shape was fixed by cooling to room temperature. As a result of the restoration of the network chain conformational entropy, polymers can be restored to their original form upon reheating, and the entanglements between the chains and the hydrogen bonds from urazole groups play the role of physical cross-linking function, which prevent polymers from deforming 6
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Declaration of competing interest
Table 1 Shape memory recoverable deformation of EUG and TbxEUG. Sample [ref.]
100% Strain
200% Strain
400% Strain
600% Strain
800% Strain
EUG Tb3EUG Tb10EUG Tb20EUG cross-linked PE [60,61] cross-linked TPI [15,59] PCL [47,62] PNB [63] PU [64,65]
✓ ✓ ✓ ✓ ✓
✓ ✓ ✓ ✓ ✓
⨯ ✓ ✓ ✓ ⨯
⨯ ✓ ⨯ ⨯ ⨯
⨯ ⨯ ⨯ ⨯ ⨯
✓
✓
✓
⨯
⨯
✓ ✓ ✓
✓ ⨯ ✓
⨯ ⨯ ✓
⨯ ⨯ ⨯
⨯ ⨯ ⨯
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors thank the National Natural Science Foundation of China (No. 21574041, 21871091) and the National & Local United Engineer ing Laboratory of Integrative Utilization Technology of Eucommia Ulmoides (NLE201602) for financial supports of this work. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.polymer.2019.121904. References [1] K.S. O’Connor, A. Watts, T. Vaidya, A.M. LaPointe, M.A. Hillmyer, G.W. Coates, Controlled chain walking for the synthesis of thermoplastic polyolefin elastomers: synthesis, structure, and properties, Macromolecules 49 (2016) 6743–6751. [2] M.O. Saed, C.P. Ambulo, H. Kim, R. De, V. Raval, K. Searles, D.A. Siddiqui, J. M. Cue, M.C. Stefan, M.R. Shankar, T.H. Ware, Molecularly-engineered, 4D-printed liquid crystal elastomer actuators, Adv. Funct. Mater. 29 (2019) 1806412. [3] C. Liu, H. Qin, P.T. Mather, Review of progress in shape-memory polymers, J. Mater. Chem. 17 (2007) 1543–1558. [4] R.H. Dong, T. Zhang, X.L. Feng, Interface-assisted synthesis of 2D materials: trend and challenges, Chem. Rev. 118 (2018) 6189–6235. [5] D. Quitmann, N. Gushterov, G. Sadowski, F. Katzenberg, J.C. Tiller, Solventsensitive reversible stress-response of shape memory natural rubber, ACS Appl. Mater. Interfaces 5 (2013) 3504–3507. [6] M. Mariano, N.E. Kissi, A. Dufresne, Cellulose nanocrystal reinforced oxidized natural rubber nanocomposites, Carbohydr. Polym. 137 (2016) 174–183. [7] M. Arroyo, M.A. L� opez-Manchado, B. Herrero, Organo-montmorillonite as substitute of carbon black in natural rubber compounds, Polymer 44 (2003) 2447–2453. [8] J. Liu, S. Wang, Z.H. Tang, J. Huang, B.C. Guo, G.S. Huang, Bioinspired engineering of two different types of sacrificial bonds into chemically cross-linked cis-1,4-polyisoprene toward a high–performance elastomer, Macromolecules 49 (2016) 8593–8604. [9] G. Beniah, D.J. Fortman, W.H. Heath, W.R. Dichtel, J.M. Torkelson, Nonisocyanate polyurethane thermoplastic elastomer: amide-based chain extender yields enhanced nanophase separation and properties in polyhydroxyurethane, Macromolecules 50 (2017) 4425–4434. [10] J. Lacombe, S. Pearson, F. Pirolt, S. Norsic, F. D’Agosto, C. Boisson, C. Souli�eZiakovic, Structural and mechanical properties of supramolecular polyethylenes, Macromolecules 51 (2018) 2630–2640. [11] K.C. Yao, H.R. Nie, Y.R. Liang, D. Qiu, A.H. He, Polymorphic crystallization behaviors in cis-1,4-polyisoprene/trans-1,4-polyisoprene blends, Polymer 80 (2015) 259–264. [12] G. E, F. Kent, B. Swinney, Properties and applications of trans-1,4-polyisoprene, Ind. Eng. Chem. Prod. Res. Dev. 5 (1966) 134–138. [13] L. Xia, Y. Wang, Z.G. Ma, A.H. Du, G.X. Qiu, Z.X. Xin, Preparation of epoxidized Eucommia ulmoides gum and its application in styrene-butadiene rubber (SBR)/ silica composites, Polym. Adv. Technol. 28 (2017) 94–101. [14] Y.X. Zhao, B.C. Huang, W. Yao, H.L. Cong, H.F. Shao, A.H. Du, Epoxidation of high trans-1,4-polyisoprene and its properties, J. Appl. Polym. Sci. 107 (2008) 2986–2993. [15] T. Tsujimoto, K. Toshimitsu, H. Uyama, S. Takeno, Y. Nakazawa, Maleated trans1,4-polyisoprene from Eucommia ulmoides Oliver with dynamic network structure and its shape memory property, Polymer 55 (2014) 6488–6493. [16] G.B. Butler, Triazolinedione modified polydienes, Ind. Eng. Chem. Prod. Res. Dev. 19 (1980) 512–528. [17] Z.K. Wang, L. Yuan, N.M. Trenor, L. Vlaminck, S. Billiet, A. Sarkar, F.E. Du Prez, M. Stefik, C.B. Tang, Sustainable thermoplastic elastomers derived from plant oil and their “click-coupling” via TAD chemistry, Green Chem. 17 (2015) 3806–3818. [18] J. Chen, R.Y. Sun, X.J. Liao, H.J. Han, Y.W. Li, M.R. Xie, Tandem metathesis polymerization-induced self-assembly to nanostructured block copolymer and the controlled triazolinedione modification for enhancing dielectric properties, Macromolecules 51 (2018) 10202–10213. [19] J. Zhao, H.C. Zhang, C.H. Ma, H.J. Han, R.Y. Sun, M.R. Xie, Binary modification of Eucommia ulmoides gum toward elastomer with tunable mechanical properties and good compatibility, J. Polym. Sci., Part A: Polym. Chem. 57 (2019) 1247–1255. [20] K. Leong, G.B. Butler, Chemical reactions on polymers. II. Modification of diene polymers with triazolinediones via the ene reaction, J. Macromol. Sci.-Chem. A14 (1980) 287–319.
Fig. 6. The complicated flower blooming-like process simulated by shape memory behavior of Tb3EUG at 60 � C.
ε2 ¼ 53%) after shape recovery upon immersing it in the hot water (Fig. S10), indicating the Rr value was 91.1%, which was calculated from the following equation [55]: Rr ¼ (ε1-ε2/ε1) � 100% The value of shape memory recoverable deformation, representing shape recovery after stretching several times. For comparison, Tb3EUG has much larger shape memory recoverable deformation (600%) than that of cross-linked polyethylene (PE) (200%) [60,61], cross-linked TPI (400%) [15,59], polycaprolactone (PCL) (200%) [47,62], PNB (187%) [63], and PU (400%) [64,65]. These results could be observed from Table 1. Interestingly, a complicated flower blooming-like process (Fig. 6 and Movie S1) could be simulated by the shape memory behavior of Tb3EUG with the best mechanical properties and the most large shape memory recoverable deformation among TbxEUG elastomers, which showed that Tb3EUG has the stable and efficient shape memory properties. As a whole, the TbxEUG elastomers are a class of high-performance shape memory polymeric materials with large deformation, low stimulation temperature, cyclic shape memory ability, and simple preparation process. 4. Conclusions A facile TAD-based Alder-ene reaction was used to modify EUG, affording a series of TbxEUG elastomers with tunable mechanical properties and enhanced shape memory capability by adjusting TAD feed. The TAD-derived urazole side groups on TbxEUG could reduce crystallinity and produce the physical cross-linking network structure formed by the intramolecular and intermolecular hydrogen bonds, resulting in the variable properties of tensile strength, elongation, toughness, Young’s modulus, elastic recovery, hardness, damping, and oil resistance. In addition, the TAD-modification enabled TbxEUG to have fast shape recovery speed and high cyclic shape recovery rate. Overall, TbxEUG with improved room-temperature elasticity may become a tough elastomer or a conventional rubber, which should be used as a feedstock to share the demand for NR in a certain extent with huge potential applications in the field of tire industry, daily necessities, and smart materials. Further improvement in the elastic recovery and hardness of modified EUG is in progress. 7
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Sustainable elastomer of triazolinedione-modified Eucommia ulmoides gum with enhanced elasticity and shape memory capability Hengchen Zhang a, Cuihong Ma a, Ruyi Sun a, Xiaojuan Liao a, Jianhua Wu b, **, Meiran Xie a, b, * a b
School of Chemistry and Molecular Engineering, East China Normal University, Shanghai, 200241, China National & Local United Engineering Laboratory of Integrative Utilization Technology of Eucommia Ulmoides, Jishou University, Jishou, Hunan, 416000, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Eucommia ulmoides gum Elasticity Shape memory capability
Sustainable Eucommia ulmoides gum (EUG) was a biomaterial with the molecular structure of trans-1,4-poly isoprene and poor elasticity, and was usually considered as a hard rubber with a limited applications in rubber products. Triazolinedione (TAD)-based Alder-ene reaction was used to modify EUG to precisely tune the me chanical properties and effectively improve the elasticity by reducing crystallinity, introducing urazole groups, and forming hydrogen bonds between EUG molecules. By adjusting TAD feed, the modified EUG could become an excellent tough elastomer with combined high strength, elongation, and toughness; or a unique rubber with superhigh elongation, low Young’s modulus, and good elastic recovery rate. Furthermore, the modified EUG displayed enhanced shape memory capability with fast shape recovery speed, high cyclic shape recovery rate after 10-cycle deformations, and large shape memory recoverable deformation. Therefore, these elastomers may have extended applications in tires and smart materials.
1. Introduction Elastomers are a class of polymeric materials with important indus trial applications in tires, seals, shock absorbers, shape memory devices, and insulators [1–4]. As the most widely used elastomer, natural rubber (NR) with the molecular structure of cis-1,4-polyisoprene has the ad vantages of high elasticity, good mechanical property, and wear resis tance [5]. However, the output of NR is limited by the inadequate plant resource of Hevea brasiliensis in the world especially in China, and NR can not meet the increasing needs of practical applications [6,7]. The synthetic elastomers [3,5,8–10], such as isoprene rubber (IR), styrene-butadiene-styrene triblock copolymers (SBS), trans- polyisoprene (TPI), polynorbornene (PNB), and polyurethane (PU), are mostly unsustainable petroleum-based products, which can not replace NR in engineering tire manufacturing and some other applications. As an isomer of NR, Eucommia ulmoides gum (EUG) is a natural semi-crystalline biomaterial extracted from the sustainable resource of Eucommia ulmoides oliver with wide planting areas in China, which is the second largest natural gum source in the world and expected to compensate for the shortage of NR that can only grow in tropical areas [11,12]. Although EUG possesses the advantages of non-toxicity, high
mechanical strength, electrical insulation, chemical resistance, and easy processing owing to its molecular structure of trans-1,4-polyisoprene and the unique duality of rubber and plasticity [13], the high elasticity brought by the flexible molecular chains could not exhibited at room temperature because of the existence of crystalline in EUG [11]. EUG was used as the toughener for plastics, or as an auxiliary component in rubber products, while the required amount of EUG was usually less than 20%, which greatly restricted its applications in the field of elas tomers, and of course it was hardly considered as a single component rubber like as the conventional rubber or NR [12,13]. Some researches on changing EUG into elastomers by chemical modification such as chlorination and epoxidation were carried out, which could effectively reduce the crystallinity and increase elasticity of EUG to some extent, but the elongation at break of modified EUG was sharply decreased at the same time, resulting in the deteriorated toughness [13–15]. In addition, these methods are usually poor controllable to the modifica tion efficiency, impeding the mass-production and up-scale application of EUG-based elastomers with stable performance. Therefore, it is imperative to design a kind of modified EUG elastomers with excellent elasticity and functionality by a simple and controllable method. 1,2,4-Triazoline-3,5-dione (TAD), as an enophile reagent of
* Corresponding author. School of Chemistry and Molecular Engineering, East China Normal University, Shanghai, 200241, China. ** Corresponding author. E-mail addresses: [email protected] (J. Wu), [email protected] (M. Xie). https://doi.org/10.1016/j.polymer.2019.121904 Received 27 August 2019; Received in revised form 9 October 2019; Accepted 12 October 2019 Available online 23 October 2019 0032-3861/© 2019 Elsevier Ltd. All rights reserved.
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environmental friendly click chemistry, can react rapidly and efficiently with diene-based polyolefins at room temperature without the require ment of catalyst and no release of hazardous substance or any byprod ucts during the reaction process, and has been used for the modification of unsaturated polymers [16–19]. For example, the changeable prop erties of diene-based polymers through TAD-chemistry were reported [16,20], and the modification imparted to the thermoplastics, namely, greatly improved the mechanical strength and elongation at break to these materials. Particularly, the modified 1,4-polybutadiene rubber exhibited a remarkably high elongation at break of 1782% and tensile strength of 2.8 MPa, while cis-1,4-polyisoprene was difficult to work with and either solubilization was never achieved or the films were weak and tacky after 1% of TAD modification [20]. The mechanical and dy namic properties of TAD-modified styrene-butadiene rubber were researched, and the urazole pendants-produced hydrogen bonds could improve the mechanical strength and broaden the range of damping properties [21]. The TAD-based click chemistry of the electrospun SBS fibers with tunable mechanical properties were investigated, and found that the cross-linking reaction of the SBS fibers by bis-TAD allowed to adjust the elongation at break from about 90% to 700%, whereas the modulus of the fibers covered a range from 11 MPa to over 130 MPa [22]. Importantly, the systematic research on TAD reversible click chemistry was presented, and provided a series of modified polymers with notable properties, including shape memory polymers, reversible cross-linked elastomers, and high modulus self-healing polymers [23–25]. Herein, EUG was modified by the TAD-based Alder-ene chemistry, a reliable and efficient tool, to provide a family of high-performance modified EUG elastomers with widely tunable mechanical properties, including the excellent tough elastomer with combined high strength, elongation, and toughness, or the outstanding rubber with good roomtemperature elasticity and elastic recovery. After TAD-modification, the urazole groups in the side chain of modified EUG could form the intra- and intermolecular hydrogen bonds [16,20], which acted as the physical cross-linking points to form a network structure, change the related physical and mechanical properties, and enable the elongation at break to significantly increase, although the tensile strength gradually decrease, devoting to the high toughness. In addition, the urazole groups endowed EUG with a certain extent of polarity, so that the modified EUG had some additional performances, such as variable hardness, unique room-temperature damping, good oil resistance, and enhanced shape memory property. Consequently, it is envisioned that the simple and effective TAD-modification could make the modified EUG elastomer or rubber be possible for up-scale practical applications to meeting the increasing demand of elastic materials in current industry [6,8,9].
properties, variable contents of bTAD were used to react with EUG (the feed molar ratio of bTAD to the double bonds in EUG ¼ 0.01:1, 0.03:1, 0.05:1, 0.1:1, 0.15:1, and 0.2:1). After EUG was sufficiently dissolved in CHCl3, the bTAD solution in CHCl3 was added into the EUG solution, and the mixture was stirred for 6 h at room temperature. The modified EUG was precipitated into methanol, cut the solid into strips, and dried in a vacuum oven at 40 � C to a constant weight. After molding compression at 80 � C under a constant applied stress of 10 MPa [14], the samples of modified EUG were obtained, which were designated as TbxEUG, where x indicates the molar ratio of bTAD contents to the double bonds of EUG. 2.3. Characterization 1 H NMR spectrum was recorded on a Bruker DRX500 (500 MHz) spectrometer using tetramethylsilane as an internal standard. FT-IR spectra of polymer thin films were recorded on a Thermo Nicolet iS50 in the region of 4000–400 cm 1 for attenuated total reflectance (ATR) measurements. Gel-permeation chromatography (GPC) was used to calculate the relative molecular weight and polydispersity equipped with a Waters 1515 Isocratic HPLC pump, a Waters 2414 refractive index detector, and a set of Waters Styragel columns (7.8 � 300 mm with particle size of 5 mm; pore diameter of 103, 104, and 105 Å). The mea surement was carried out at 40 � C, the concentration of polymer solution was approximately 0.2 wt%, and the flow rate was set at 1.0 mL min 1 in all the measurements. Differential scanning calorimetry (DSC) was performed on a Q2000 DSC instrument in a nitrogen atmosphere. The sample firstly heated from 30 to 100 � C and held 3 min at this tem perature, then cooled to 100 � C and heated again from 100 to 100 � C, and the heating or cooling rate was 10 � C min 1. X-ray diffraction (XRD) was performed on a Shimadzu NERCN-TC-007 instrument, and the scanning range was 5� –80� and scanning rate was 10� min 1 in this experiment. Tensile tests were carried out on a Shimadzu AG-Xplus electronic tensile tester in accordance to GB/T 528–2009 at an extend ing rate of 200 mm min 1 during the process. The dumbbell-shaped samples with an effective gauge length of 12 mm, a width of 2 mm, and a thickness of 1 mm were prepared by using a hand operated cutting press. In general, five samples were obtained for testing the values of tensile strength and elongation at break. A predefined strain of 100% was used in the loading-unloading cycle test with extending rate of 50 mm min 1. The hydrodynamic diameter was determined by means of dynamic light scattering (DLS) analysis using a Malvern Zeta sizer NanoZS light scattering apparatus (Malvern Instruments, U.K.) with a He–Ne laser (633 nm, 4 mW), and the Nano ZS instrument incorporates noninvasive backscattering (NIBS) optics with a detection angle of 173� . Hardness test was carried out using a XY-1 Shore A hardness meter under room temperature in accordance to GB/T 531–2008. The thick ness of test samples was more than 6 mm. Oil resistance tests were performed in the paraffin oil solvent at room temperature in accordance to GB/T 539–2008. Dynamic mechanical analysis (DMA) was measured with a PerkinElmer DMA 8000 instrument, the test was carried out at the tension condition with frequency of 1 Hz and strain of 0.2%, and the scanning temperature was ranged from 100 to 100 � C at a heating rate of 3 � C min 1. Shape memory test of samples was conducted in a 60 � C water bath.
2. Experimental 2.1. Materials EUG (>94%) was provided by Xiangxi Laodie Biotechnology Co. Ltd., Hunan Province, China. Butyl isocyanate, ethyl carbazate, and triethylenediamine were purchased from Energy Chemical Technology Co. Ltd. Toluene, hydrochloric acid, tetrahydrofuran (THF), and chlo roform (CHCl3) were purchased from Shanghai Chemical Reagent Co. Ltd., Shanghai, China. Potassium hydroxide (KOH) and bromine (Br2) were purchased from Sinopharm Chemical Reagent Co. Ltd., Shanghai, China. 4-Butyl-l,2,4-triazoline-3,5-dione (bTAD) was synthesized ac cording to the literature procedure [24]. Dichloromethane (CH2Cl2) and methanol were purchased from Shanghai Wohua Chemical Co. Ltd., Shanghai, China. The reagent grade solvents were distilled over drying agents prior to use.
3. Results and discussion 3.1. Preparation and characterization of TAD-modified EUG When EUG was reacted with bTAD via the Alder-ene reaction (Scheme S1), the isomerization of double bond on the EUG backbone did happen, accompanying with the migration of a σ-bonded hydrogen atom from the initial olefin to the TAD molecule to form a new N–H bond on the urazole side group [20], and meanwhile, the formation of a new C–N bond connecting the modifier bTAD to EUG generated the modified EUG, TbxEUG. EUG and the representative Tb20EUG were distinguished
2.2. Modification of EUG In order to get a variety of modified EUG with different elastic 2
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were indicative of occurred modification reaction. The original EUG had a high Mn value of 170.2 kDa. With increase in the degree of TADmodification, the apparent Mn, reflected by the hydrodynamic volume, of TbxEUG tended to decrease gradually from 106.3 kDa for Tb1EUG to 18.8 kDa for Tb18EUG, and further heavily down to 14.3 kDa for Tb20EUG when the feed amount of bTAD was up to 20% (Fig. 2a and Table S1). The hydrodynamic volume of TbxEUG in THF was corre spondingly reduced, due to the interaction of intramolecular N–H⋯N hydrogen bonds originating from the urazole moieties, resulting in a decrease in the GPC-measured Mn value [23]. The formed intra molecular N–H⋯N hydrogen bond between the pendant urazole groups of TbxEUG was verified by the blue-shift of the absorption peak from –O 1704 to 1685 cm 1 in Fig. S1, ascribed to the strengthened C– stretching vibrations as the increased bTAD content and enhanced hydrogen bond interaction [19,21]. Therefore, the results of GPC indi cated that the TAD-modification has an obvious effect on the tested Mn value of TbxEUG. The influence of bTAD-modification on the glass-transition temper ature (Tg), melting point (Tm), and crystallinity (Xc) of TbxEUG was demonstrated using differential scanning calorimetry (DSC), as shown in Fig. 2b. Because of the flexible molecular chains, the trans-1,4-poly isoprene structure of EUG displayed a low Tg of 68.1 � C. However, the EUG molecular chains were regularly arranged, and displayed the semicrystalline property with Xc of 22.5% and Tm of 56.1 � C, so that EUG did show good low-temperature resistance, but appeared hard plasticity at room temperature rather than soft elasticity [11]. As the bTAD feed increased from 1% to 10%, the Xc of TbxEUG sharply decreased from 22.5% to 2.6%, their Tm also decreased from 56.1 � C to 30.9 � C, while their Tg raised from 68.1 � C to 46.8 � C. When the feed amount of bTAD further increased to 15%, the crystallinity of Tb15EUG dis appeared entirely and no melting peak was observed, making it become an amorphous material accompanying with the Tg up to 25.5 � C, which was much lower than room temperature, and it was accordingly an elastomer with a good low-temperature resistance. Finally, the Tg of Tb20EUG raised to 10.5 � C if 20% of bTAD was loaded. In addition to decrease in crystallinity, the mobility of polymer chains was limited by the interaction of intramolecular N–H⋯N hydrogen bonds between the urazole groups, resulting in the increase in Tg [22,25]. It was believed that the enhanced intramolecular and intermolecular interactions reduced the free volume, caused the rotation of segments to require higher temperature to overcome the energy barriers, and thereby devoted to an increased Tg [27,28]. Especially, when the incorporated bTAD content was 3%, the Xc of Tb3EUG reduced to about a half of EUG, making it become a lower crystalline material. At this point, the Tg of Tb3EUG was still below 60 � C, so it displayed the excellent properties of thermoplastic elastomer. The TGA curves demonstrated the thermal
Fig. 1. 1H NMR spectra of EUG (a) and TAD-modified EUG (b).
by 1H NMR spectroscopy (Fig. 1), and the aforementioned change in chemical structure could be verified. As shown in Fig. 1a, the signal peak at 5.12 ppm was ascribed to the proton (Ha) on the double bond of EUG backbone, and the signal peaks at 2.06 ppm and 1.98 ppm belonged to the protons (Hb,c) on the two different methylene groups. The strong signal peak at 1.60 ppm was related to the protons (Hd) on the methyl group of trans-1,4-polyisoprene structure [26]. Since the Alder-ene re action only transferred the double bond to the next carbon atom of one trans-isoprene unit, the degree of unsaturation was retained during the modification process, while most of the proton signals hardly changed, and only a signal peak, representing the methenyl proton (Hf) on the Tb20EUG backbone connected to the nitrogen atom, appeared at 3.78 ppm (Fig. 1b). The signal peak at 9.84 ppm was assigned to the proton (He) on the nitrogen atom of urazole group, and other signal peaks at 3.54 ppm, 1.28 ppm, and 0.86 ppm were designated as the protons (Hg,h,i) on the butyl group of urazole pendent [20,23,26]. The truly grafting efficiency of bTAD onto the Tb20EUG backbone could be evaluated by the peak area ratio of methenyl proton (Hf) to that of double bond proton (Ha) from the corresponding 1H NMR spectrum, and was calculated to be 22% by the peak area ratio (Sf:Sa) of 0.22:1, which was agreed well with the feed ratio of bTAD to EUG (0.20:1). Overall, the Alder-ene reaction of bTAD with EUG was quite efficient to generate TbxEUG. In addition, the variation of the number-average molecular weight (Mn) of TbxEUG compared with that of EUG was revealed by gelpermeation chromatography (GPC) measurements, and the results
Fig. 2. (a) GPC traces and (b) DSC curves of TbxEUG with varied amount of bTAD. 3
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Fig. 3. (a) Stress-strain curves and (b) tensile loading-unloading cycles of EUG, NR, and TbxEUG with different bTAD contents.
stability of TbxEUG was slightly lower than that of EUG (Fig. S2). Although the thermal decomposition temperature of urazole groups was about 300 � C, which was lower than that of EUG backbone, a slight decrease in thermal stability did not affect its use as an elastomer [23]. After TAD-modification, a number of urazole moieties were randomly suspended on the side chains of EUG backbone, hindered the alignment of the modified-EUG chains, and led to the decreased Xc values of TbxEUG with the increase in bTAD content, which were also confirmed by X-ray diffraction (XRD) technique (Fig. S3) and higher than those by DSC measurement (Table S1).
from 772% to 1130%, then decreased slightly, and finally reached to over 1800%. This is because that after modification of EUG with bTAD, the introduced urazole moieties affected the orderly arrangement of EUG chains and reduced its crystallinity, and thus decreased the tensile strength. On the other hand, the urazole groups could form a hydrogen-bond network via the intramolecular and intermolecular in teractions, which corresponded to the physical cross-linking function, resulted in the locally order but overall disorder, and increased the elongation at break of TbxEUG [19–21,30]. It could be obtained from the above change of mechanical properties that the Young’s modulus monotonously decreased from 72.20 MPa for EUG to 41.0–0.53 MPa for different TbxEUG due to the weakened tensile strength, while the toughness value of 99.78 MJ m 3 for EUG was first up to 122.89 and 124.78 MJ m 3, and then gradually descended to 11.35 MJ m 3 for TbxEUG, which were all higher than the toughness of 9.14 MJ m 3 for NR. Especially, owning to the hard and tough feature, Tb3EUG illus trated the maximum toughness of 124.98 MJ m 3 by the optimal com bination of high tensile strength of 27.62 MPa and elongation at break of 1130% (Fig. S4), which was a unique high-performance tough elastomer with combined high elasticity, strength, and toughness simultaneously, and even better than the mechanical performance of the most conven tional PU thermoplastic elastomer [9]. Tb5EUG with the tensile strength of 15.06 MPa, elongation at break of 1046%, and toughness of 64.86 MJ m 3 was also a good tough elastomer. Furthermore, Tb18EUG demonstrated the tensile strength of 2.42 MPa, which were very close to that of NR, the elongation at break of 1280% and the toughness of 14.44 MJ m 3 were better than those of NR, just its Young’s modulus of 1.51 MPa was deviated from that of NR. Therefore, Tb18EUG has the potential to become a new material matched well with the conventional rubbers such as NR [15,31]. Finally, Tb20EUG exhibited an extremely high elongation at break of 1852%, while a low tensile strength of 1.27 MPa, which was only about half that of NR, and possessed the soft and weak feature with the toughness of 11.35 MJ m 3 and Young’s modulus of 0.53 MPa. The specific mechanical features of EUG, TbxEUG, and NR were listed in Table S2. The elastic recovery rate of materials is another important parameter for evaluating the properties of elastomers or rubbers. The elastic re covery of TbxEUG was changed after modification, which was analyzed by tensile loading-unloading cycle test, and the results were listed in Table S3. It was found that the elastic recovery rate of materials increased from 33% for EUG to near 60% for TbxEUG as the increase of bTAD amount (Fig. 3b), indicating the improved room-temperature elasticity of TbxEUG [9]. However, there have been still a certain gap of the elastic recovery rate between Tb18EUG (53%) or Tb20EUG (57%) with NR (87%), although Tb18EUG shared very closed mechanical
3.2. Mechanical and physical properties of modified EUG The mechanical properties of EUG could be finely tuned by the modification with varied contents of bTAD, which is beneficial to improve the room-temperature elasticity and expand the scope of application of EUG. It was known that EUG possessed high tensile strength of 37.12 MPa and moderate elongation at break of 582%, as depicted in its stress-strain curve (Fig. 3a). The Young’s modulus and toughness of materials were also obtained from their stress-strain curves, the Young’s modulus was the ratio of stress to the correspond ing strain in the initial elastic deformation stage of stress-strain curve, and the toughness was the integral area of stress-strain curve [29]. As a result, EUG displayed good toughness of 99.78 MJ m 3 and high Young’s modulus of 72.20 MPa, but poor room-temperature elasticity, and therefore it was usually considered as a hard rubber. Because of the flexible but regular-arranged molecular chains, EUG molecules pro duced a folded chain conformation with ordered crystallinity, and exhibited the hard and strong thermoplastic elastomer property with the stress yielding phenomenon when subjected to tensile force [28]. In contrast, NR has the prominent elasticity with low tensile strength of 2.50 MPa, large elongation at break of 712%, poor toughness of 9.14 MJ m 3, and low Young’s modulus of 0.58 MPa; because NR owned a random coiled conformation composed of flexible and irregular-arranged molecular chains, it displayed soft and weak feature when subjected to tensile force, and its stress increased linearly with increasing strain during stretching, similar to an ideal elastomer [29]. In view of the wide application of NR, EUG was envisioned to have the similar or closed mechanical properties with NR after chemical modi fication. As expected, when EUG was modified by various contents of bTAD, the resulting TbxEUG exhibited a wide range of mechanical properties including different tensile strength and elongation at break, which were indicated by the stress-strain curves (Fig. 3a). As increase in bTAD content, the tensile strength of TbxEUG gradually decreased from 34.51 MPa to 1.27 MPa, while the elongation at break first increased 4
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overall disordered entangled structure due to the dual function of the flexible molecular chains of TbxEUG and the hydrogen bonds between urazole groups, and the change of the entangled structure of TbxEUG during stretching process was shown in Scheme 1. The intermolecular interaction between the urazole groups caused a portion of TbxEUG molecular chains to aggregate to form the stable segment clusters, as indicated by the circular dotted lines. When subjected to external force, most of the flexible molecular chains of TbxEUG extended, but the formed molecular clusters hindered the development of the elongation process [28,29,36]. It could be known that the formation of segment clusters enable TbxEUG to have high elongation at break and definite tensile strength, and the number of molecular clusters determined the mechanical properties of TbxEUG. The segment clusters of TbxEUG with different number-average hydrodynamic diameter (Dh) were proved by dynamic light scattering (DLS) measurement (Fig. S5 and Table S4), there was one peak with a small Dh corresponding to a single molecule, and another peak with a large Dh corresponding to the aggregates was observed. It could be seen from Fig. S5a that the Dh sizes of single polymer molecule at 0.5 mg mL 1 in THF solution first increased slightly from 7.7 nm for EUG to 7.8 nm for Tb3EUG, and then slowly decreased to 7.6 nm for Tb10EUG and 6.6 nm for Tb20EUG as the TAD contents increased to 3, 10, and 20%, which were also used as the positive evi dence to confirm the gradually decreased apparent Mn of TbxEUG by GPC. On the one hand, low grafting content of TAD increased the Dh size of single molecular chain; On the other hand, the formed intramolecular hydrogen bonds between the pendant urazole groups of TbxEUG decreased their Dh sizes, and the influence of intramolecular hydrogen bond forces was stronger when the urazole content was higher. In addition, the aggregates of TbxEUG with large Dh size were also observed, which exhibited a reduced trend at 90, 87, and 76 nm suc cessively as the increase in TAD contents, indicating that a certain number of the intermolecular hydrogen bonds between the pendant urazole groups were existed in the TbxEUG molecules. If the concen tration of polymers increased to 25 mg mL 1 in CHCl3 solution, which was the same as that for preparing the film for the measurement of mechanical properties, the Dh sizes of EUG and TbxEUG displayed a similar varying trend while more larger compared with those of poly mers at low concentration of 0.5 mg mL 1 in THF solution, which were 32 nm for EUG, 22 nm for Tb3EUG, 14 nm for Tb10EUG, and 12 nm for Tb20EUG, as well as 200, 145, and 115 nm for the aggregates of TbxEUG as the TAD contents increased to 3, 10, and 20% correspondingly, as shown in Fig. S5b. It was worthy to note that there was no obvious ag gregates appeared in solution of EUG even at high concentration, and on the contrary, the appearance of aggregates with large size verified the existence of hydrogen bonds between the pendant urazole groups in the
Fig. 4. Toughness versus elongation at break of TbxEUG and other types of polymer materials.
Scheme 1. Variation of network structure of TbxEUG before (a) and after (b) stretching.
properties to NR, implying that it is necessary to adjust the elastic re covery properties of TbxEUG elastomer further. By comparing EUG and TbxEUG with other types of commonly used polymer materials [1,7–10, 32–35], it was revealed from the toughness versus elongation at break that EUG is a typical tough elastomer. After modification, TbxEUG could be changed from an excellent tough elastomer, such as Tb3EUG, to an outstanding rubber, such as Tb18EUG (Fig. 4). Therefore, TbxEUG seemed to become a new type of high-performance elastic materials with a combination of high strength and toughness or large elongation and low Young’s modulus, which will enable the application range of EUG to be greatly expanded, especially its added amount in rubber matrix likely increased a lot, and expected to be used as a main component of con ventional rubber products. As described above, the resulting TbxEUG has a locally ordered but
Fig. 5. Dependence of (a) E0 and (b) tanδ on temperature for TbxEUG with different bTAD contents. 5
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TbxEUG molecules. The hardness of TbxEUG was changed after modification, the selected representatives of Tb3EUG, Tb10EUG, and Tb20EUG in different elastic regions of the full elastic-spectrum (Supplementary In formation, Figs. S6 and S7) were analyzed by Shore A tester, and the results were displayed in Table S3. It was found that Shore A hardness was reduced from 94 to near 70 as the bTAD feed increased from 0% to 20%, indicating the tunable hardness [15]. In view of the fact that rubbers or elastomers with Shore A hardness of about 70 were more suitable for processing technology, TbxEUG was easier to process than EUG. The oil resistance was also tested by the volume change of samples in the paraffinic oil solvent. As shown in Table S3, the volume change (0.05–3%) of TbxEUG was significantly lower than that (13.47%) of EUG, suggesting that the oil resistance of TbxEUG increased obviously compared with EUG because of the polarity introduced from TAD chemistry. Overall, the TbxEUG elastomer had excellent oil resistance and convenient processing properties, due to the adjustable low melting point (30–50 � C) and tunable hardness (74-89� ).
irreversible, so the mechanical energy is stored [49]. It could be seen from the IR spectra of Tb20EUG at room-temperature, 60 � C, and 80 � C – O stretching vibration at 1685 cm 1 that the absorption peak of the C– was not move obviously (Fig. S1), indicating that the hydrogen bonds existed in TbxEUG elastomers were stable in film state [50–52] and have no contribution to the reversible shape deformation. Therefore, the mechanism for shape memory property of EUG and TbxEUG was based on the transition of crystals or microcrystals phase by heating or cooling instead of the dissociation and reconstruction of hydrogen bonds with temperature. The shape memory behavior of TbxEUG with different bTAD contents was demonstrated by the curves of shape recovery against temperature and cycle number. The shape recovery angle (θ) was measured by the angle between the straight ends of the bent spec imen, and the shape recovery rate (Rr) was calculated from the following equation [49,53]: Rr ¼ (θ/180) � 100% (0� � θ � 180� ) The results of shape recovery rate against temperature and cycle number in a hot water bath were shown in Fig. S8. It was clearly seen that the Rr values of all samples increased rapidly as the temperature varied from 35 � C to 60 � C, and the maximal Rr values of EUG, Tb3EUG, Tb10EUG, and Tb20EUG reached up to 74, 91, 99, and 82% at about 60 � C (Fig. S8a), respectively, which was about 0–30 � C above the Ttrans, so the cyclic shape recovery testing for EUG and TbxEUG was conducted in a water bath at 60 � C. As a whole, TbxEUG exhibited higher Rr than that of EUG, and Tb10EUG displayed the highest Rr of 99% among these samples. What is more important, Tb3EUG, Tb10EUG, and Tb20EUG performed almost a steady variation with Rr values of 85, 95, and 80% at 60 � C (Fig. S8b), respectively, even the number of deformation cycles increased to 10, while the Rr of EUG decreased sharply to 32% at the fourth deformation cycles, indicating the poor cyclic shape memory property of EUG and the superior repetitive shape memory capabilities of TbxEUG, especially the Tb10EUG elastomer showed the best cyclic shape memory property, which could be reused for many times and had excellent potential application prospects. In addition, the shape memory behavior of EUG, Tb3EUG, Tb10EUG, and Tb20EUG elastomers was illustrated by the photos of dumbbell specimens in a 60 � C water bath (Fig. S9), and the whole shape recovery process was conducted in 10 s, indicating the high shape recovery rate and the fast shape recovery speed of TbxEUG [54,55]. Since EUG had creep phenomenon due to uncross-linking, it could not be completely relaxed to the original un stressed state after stress was relieved, whereas TbxEUG could be stably realized multiple times of shape recovery based on the fixed action of physical cross-linking from hydrogen bonds. However, the amorphous Tb20EUG with higher modification degree exhibited downward trend of these performances because the crystalline reversible phase dis appeared, which was similar to those of vulcanized rubbers without shape memory ability [56–58]. In brief, because the TbxEUG elastomers have lower Ttrans and superior cyclic shape memory properties compared to most of the shape memory polymers including conventional lightly cross-linked shape memory EUG [15,59], they can implement the cyclic shape deformations and recoveries under slightly hot conditions. In addition, because the TbxEUG elastomers have excellent elonga tion capability, they will have large shape memory recoverable de formations, and thus the shape recovery of EUG and TbxEUG elastomers under stretching was also attempted apart from bending. The heated elastomers were stretched to a certain length by the external force, and the elongated shapes were fixed at room temperature, and then shrank to a remaining length upon reheated. The most value of shape memory recoverable deformations for TbxEUG was greater than 400%, and the maximum value for Tb3EUG was greater than 600% under the condition of higher recovery rate. For example, the selected sample of Tb3EUG with a gauge length of 15 mm was stretched to 105 mm (deformed strain, ε1 ¼ 600%) in a 60 � C water bath and fixed at room temperature, and the sample shrank to a remaining length of 23 mm (residual strain,
3.3. Dynamic mechanical properties of modified EUG Elastomers are inevitably used under dynamic conditions, so the dynamic mechanical properties are critical to the description of elas tomer performance. The dynamic mechanical properties of TbxEUG with different bTAD contents were investigated by dynamic mechanical analysis (DMA), and the dependence of storage modulus (E0 ) and loss factor (tanδ) on temperature for TbxEUG was shown in Fig. 5. As tem perature went up, the E0 of TbxEUG slowly decreased with a temperature-independent rubbery plateau ranging from 40 to 40 � C, and descended when exceeding 40 � C (Fig. 5a), which was related to the melting transition of materials. With increase in amount of bTAD, the E0 of TbxEUG decreased sharply, especially at low temperature region [37]. In the tanδ curves (Fig. 5b), the changing tendency was similar with E0 curves. The tanδ of TbxEUG changed little at low temperature range but heightened a lot between 20 � C and 40 � C. In addition, the existence of Tg for EUG and the urazole moieties derived from modifier bTAD resulted in a lower tanδ (damping peak), ranging from 0.1 to 0.3. EUG exhibited a single peak at a low temperature of 50 � C on the tanδ curve, which was not helpful for damping performance. With the incorporation of bTAD into EUG backbone, the damping peak of TbxEUG gradually moved toward high temperature and had a damping peak near 20 � C, indicating that the elastomer had unique room-temperature damping property [38,39]. When EUG was modified by 1–5% of bTAD, the tanδ curves appeared double peaks, which widened the damping peak, and thus enhanced the damping perfor mance of TbxEUG [40,41]. In a word, the TAD-modification may enable EUG to be a new type of tough elastomer or soft rubber with improved elasticity and damping property, which will expand its applications like as NR and other rubbers [42,43], although the elastic recovery and hardness of TbxEUG elastomers are still not as good as those of NR, these parameters should be further optimized by precise tuning of TAD structure and other aspects. 3.4. Shape memory properties of modified EUG The thermal-induced shape memory polymers are a new class of smart materials that can replace shape memory alloys, which consist of two phases, a reversible phase that can change with external conditions and a stationary phase that provides a permanent shape to the material [3,44–49]. When EUG and TbxEUG heated above Tm (¼ transition temperature, Ttrans), they can be deformed and the temporary shape was fixed by cooling to room temperature. As a result of the restoration of the network chain conformational entropy, polymers can be restored to their original form upon reheating, and the entanglements between the chains and the hydrogen bonds from urazole groups play the role of physical cross-linking function, which prevent polymers from deforming 6
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Declaration of competing interest
Table 1 Shape memory recoverable deformation of EUG and TbxEUG. Sample [ref.]
100% Strain
200% Strain
400% Strain
600% Strain
800% Strain
EUG Tb3EUG Tb10EUG Tb20EUG cross-linked PE [60,61] cross-linked TPI [15,59] PCL [47,62] PNB [63] PU [64,65]
✓ ✓ ✓ ✓ ✓
✓ ✓ ✓ ✓ ✓
⨯ ✓ ✓ ✓ ⨯
⨯ ✓ ⨯ ⨯ ⨯
⨯ ⨯ ⨯ ⨯ ⨯
✓
✓
✓
⨯
⨯
✓ ✓ ✓
✓ ⨯ ✓
⨯ ⨯ ✓
⨯ ⨯ ⨯
⨯ ⨯ ⨯
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors thank the National Natural Science Foundation of China (No. 21574041, 21871091) and the National & Local United Engineer ing Laboratory of Integrative Utilization Technology of Eucommia Ulmoides (NLE201602) for financial supports of this work. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.polymer.2019.121904. References [1] K.S. O’Connor, A. Watts, T. Vaidya, A.M. LaPointe, M.A. Hillmyer, G.W. Coates, Controlled chain walking for the synthesis of thermoplastic polyolefin elastomers: synthesis, structure, and properties, Macromolecules 49 (2016) 6743–6751. [2] M.O. Saed, C.P. Ambulo, H. Kim, R. De, V. Raval, K. Searles, D.A. Siddiqui, J. M. Cue, M.C. Stefan, M.R. Shankar, T.H. Ware, Molecularly-engineered, 4D-printed liquid crystal elastomer actuators, Adv. Funct. Mater. 29 (2019) 1806412. [3] C. Liu, H. Qin, P.T. Mather, Review of progress in shape-memory polymers, J. Mater. Chem. 17 (2007) 1543–1558. [4] R.H. Dong, T. Zhang, X.L. Feng, Interface-assisted synthesis of 2D materials: trend and challenges, Chem. Rev. 118 (2018) 6189–6235. [5] D. Quitmann, N. Gushterov, G. Sadowski, F. Katzenberg, J.C. Tiller, Solventsensitive reversible stress-response of shape memory natural rubber, ACS Appl. Mater. Interfaces 5 (2013) 3504–3507. [6] M. Mariano, N.E. Kissi, A. Dufresne, Cellulose nanocrystal reinforced oxidized natural rubber nanocomposites, Carbohydr. Polym. 137 (2016) 174–183. [7] M. Arroyo, M.A. L� opez-Manchado, B. Herrero, Organo-montmorillonite as substitute of carbon black in natural rubber compounds, Polymer 44 (2003) 2447–2453. [8] J. Liu, S. Wang, Z.H. Tang, J. Huang, B.C. Guo, G.S. Huang, Bioinspired engineering of two different types of sacrificial bonds into chemically cross-linked cis-1,4-polyisoprene toward a high–performance elastomer, Macromolecules 49 (2016) 8593–8604. [9] G. Beniah, D.J. Fortman, W.H. Heath, W.R. Dichtel, J.M. Torkelson, Nonisocyanate polyurethane thermoplastic elastomer: amide-based chain extender yields enhanced nanophase separation and properties in polyhydroxyurethane, Macromolecules 50 (2017) 4425–4434. [10] J. Lacombe, S. Pearson, F. Pirolt, S. Norsic, F. D’Agosto, C. Boisson, C. Souli�eZiakovic, Structural and mechanical properties of supramolecular polyethylenes, Macromolecules 51 (2018) 2630–2640. [11] K.C. Yao, H.R. Nie, Y.R. Liang, D. Qiu, A.H. He, Polymorphic crystallization behaviors in cis-1,4-polyisoprene/trans-1,4-polyisoprene blends, Polymer 80 (2015) 259–264. [12] G. E, F. Kent, B. Swinney, Properties and applications of trans-1,4-polyisoprene, Ind. Eng. Chem. Prod. Res. Dev. 5 (1966) 134–138. [13] L. Xia, Y. Wang, Z.G. Ma, A.H. Du, G.X. Qiu, Z.X. Xin, Preparation of epoxidized Eucommia ulmoides gum and its application in styrene-butadiene rubber (SBR)/ silica composites, Polym. Adv. Technol. 28 (2017) 94–101. [14] Y.X. Zhao, B.C. Huang, W. Yao, H.L. Cong, H.F. Shao, A.H. Du, Epoxidation of high trans-1,4-polyisoprene and its properties, J. Appl. Polym. Sci. 107 (2008) 2986–2993. [15] T. Tsujimoto, K. Toshimitsu, H. Uyama, S. Takeno, Y. Nakazawa, Maleated trans1,4-polyisoprene from Eucommia ulmoides Oliver with dynamic network structure and its shape memory property, Polymer 55 (2014) 6488–6493. [16] G.B. Butler, Triazolinedione modified polydienes, Ind. Eng. Chem. Prod. Res. Dev. 19 (1980) 512–528. [17] Z.K. Wang, L. Yuan, N.M. Trenor, L. Vlaminck, S. Billiet, A. Sarkar, F.E. Du Prez, M. Stefik, C.B. Tang, Sustainable thermoplastic elastomers derived from plant oil and their “click-coupling” via TAD chemistry, Green Chem. 17 (2015) 3806–3818. [18] J. Chen, R.Y. Sun, X.J. Liao, H.J. Han, Y.W. Li, M.R. Xie, Tandem metathesis polymerization-induced self-assembly to nanostructured block copolymer and the controlled triazolinedione modification for enhancing dielectric properties, Macromolecules 51 (2018) 10202–10213. [19] J. Zhao, H.C. Zhang, C.H. Ma, H.J. Han, R.Y. Sun, M.R. Xie, Binary modification of Eucommia ulmoides gum toward elastomer with tunable mechanical properties and good compatibility, J. Polym. Sci., Part A: Polym. Chem. 57 (2019) 1247–1255. [20] K. Leong, G.B. Butler, Chemical reactions on polymers. II. Modification of diene polymers with triazolinediones via the ene reaction, J. Macromol. Sci.-Chem. A14 (1980) 287–319.
Fig. 6. The complicated flower blooming-like process simulated by shape memory behavior of Tb3EUG at 60 � C.
ε2 ¼ 53%) after shape recovery upon immersing it in the hot water (Fig. S10), indicating the Rr value was 91.1%, which was calculated from the following equation [55]: Rr ¼ (ε1-ε2/ε1) � 100% The value of shape memory recoverable deformation, representing shape recovery after stretching several times. For comparison, Tb3EUG has much larger shape memory recoverable deformation (600%) than that of cross-linked polyethylene (PE) (200%) [60,61], cross-linked TPI (400%) [15,59], polycaprolactone (PCL) (200%) [47,62], PNB (187%) [63], and PU (400%) [64,65]. These results could be observed from Table 1. Interestingly, a complicated flower blooming-like process (Fig. 6 and Movie S1) could be simulated by the shape memory behavior of Tb3EUG with the best mechanical properties and the most large shape memory recoverable deformation among TbxEUG elastomers, which showed that Tb3EUG has the stable and efficient shape memory properties. As a whole, the TbxEUG elastomers are a class of high-performance shape memory polymeric materials with large deformation, low stimulation temperature, cyclic shape memory ability, and simple preparation process. 4. Conclusions A facile TAD-based Alder-ene reaction was used to modify EUG, affording a series of TbxEUG elastomers with tunable mechanical properties and enhanced shape memory capability by adjusting TAD feed. The TAD-derived urazole side groups on TbxEUG could reduce crystallinity and produce the physical cross-linking network structure formed by the intramolecular and intermolecular hydrogen bonds, resulting in the variable properties of tensile strength, elongation, toughness, Young’s modulus, elastic recovery, hardness, damping, and oil resistance. In addition, the TAD-modification enabled TbxEUG to have fast shape recovery speed and high cyclic shape recovery rate. Overall, TbxEUG with improved room-temperature elasticity may become a tough elastomer or a conventional rubber, which should be used as a feedstock to share the demand for NR in a certain extent with huge potential applications in the field of tire industry, daily necessities, and smart materials. Further improvement in the elastic recovery and hardness of modified EUG is in progress. 7
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