Thermal-Driven Self-Healing and Recyclable Waterborne

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Thermal-driven self-healing and recyclable waterborne polyurethane films based on reversible covalent interaction Yuanlai Fang, Xiaosheng Du, Yuxu Jiang, Zongliang Du, Peiting Pan, Xu Cheng, and Haibo Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03151 • Publication Date (Web): 26 Sep 2018 Downloaded from http://pubs.acs.org on September 27, 2018

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ACS Sustainable Chemistry & Engineering

Thermal-driven

self-healing

and

recyclable

waterborne

polyurethane films based on reversible covalent interaction Yuanlai Fang†, Xiaosheng Du†, Yuxu Jiang‡, Zongliang Du†, Peiting Pan†, Xu Cheng†*, Haibo Wang†* † Textile Institute, College of Light Industry, Textile and Food Science Engineering, Sichuan University, Chengdu 610065, China ‡Chengdu Textile College, Chengdu 611731, China

ABSTRACT: By introducing novel stimuli-responsive Diels-Alder (DA) diol into polymer chains, a series of environmentally friendly, self-healing and recyclable waterborne polyurethane based on DA/retro-DA reactions (WPU-DA-x) were successfully prepared through a modified acetone process. The properties and appearance of the resultant WPU-DA-x latex were analyzed by considering particle size and Zeta potential which shown these dispersions with an outstanding storage stability. Simultaneously, the molecular mechanism of the self-healing behavior was extensively investigated via 1H NMR, FT-IR and UV-vis spectroscopies exhibiting effective reversibility of the DA/retro-DA chemistry. Moreover, WPU-DA-x films with outstanding mechanical performance show an excellent self-healing capability due to the couple/decouple process of the DA reactions, and the self-healing process was qualitatively and quantitatively studied. At the same time, the networked WPU-DA-x could be recycled via hotpressing and solution casting. At last, the thermal stability of WPU-DA-x films was distinctly

ACS Paragon Plus Environment

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enhanced shown by the TGA results. The fruitful outcomes indicate that WPU-DA-x exhibited a greatly potential application as a smart material.

KEYWORDS: Waterborne polyurethane (WPU), Self-healing, Recycling, Diels-Alder (DA) chemistry INTRODUCTION Waterborne polyurethane (WPU) has attracted broadly attentions because it does not cause a considerable release of volatile organic compounds (VOC).1-3 The versatile environmentally friendly WPU has a wide range of commercial applications such as leather finishing, coating, textile laminating, adhesives, and other consumer products.4-6 However, over the duration of operation and utilization of WPU materials, micro notches or breaks may be generated from the impact, abrasion and fatigue alone or in combination which may result in catastrophic failure and shorten the lifespan of the WPU. Nevertheless, to the best of our knowledge, almost no work has been done to give WPU a self-healing function. Therefore, to extend the service lifespan, strength the safety and reduce the contamination of environment, it is greatly significant to endow WPU materials excellent self-healing and recycling abilities by carefully molecular design.7-8 Self-healing polymeric materials are an important class of smart materials which can repair the cracks and restore the comprehensive performances, and it has attracted much attention in recent decades.9-11 Intrinsically stimuli-responsive self-healing systems based on reversible interactions typically involve external factors (heat, light, pH, etc) to trigger the healing progress, which is accomplished through a bond-breaking and bond-rebuilding process.12-14 These reversible chemistries include dynamic covalent bonds like DA reactions, ester exchange, disulfide exchange, boroxine/boronic acid equilibrium process and etc.15-17 On the other aspect,


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noncovalent interactions like macromolecular rearrangement, hydrogen bonds, metal-ligand coordination and π-π stacking which have a poor bonding energy are often used in some flexible and soft self-repairing polymers.18-19 For different application cases, different mechanisms are chosen to prepare diverse self-healing materials. Heat-triggered intrinsic self-healing polymers grounded on the DA/retro-DA reversible reactions are probably the most well-known mechanism for self-repairing target.20-21 [4+2] cycloaddition DA reactions involve diene (usually furan) and dienophile (usually maleimide) as precursors at a moderate temperature, which results in cyclohexene derivatives. Certainly, the DA products (cyclohexene derivatives) are thermally unstable which carry out retro-DA reactions to regenerate the corresponding diene and dinophile at an elevated temperature.22 Self-healing systems built on DA/retro-DA chemistry are simple, effective, mild reaction condition, and minimal side reactions.23-24 Many efforts have been done to endow multiple functional materials with self-healing ability. For example, Sodano et al. have prepared a rigid and self-healable polyurethane based on DA reactions, which shown ideal self-repairing performance.15 Bowman et al. discovered scaffold self-healing polyurethane networks which contained a permanent crosslinked framework to sustain the matrix shape and the reversible DA products as the liquid healing agents to repair the cracks.12 Xia et al. introduced the polysiloxane and DA bonds into polyurethane to construct selfhealable, shape memorial and biocompatible smart materials.25 Mandal’s group made a thermoreversible and partially fluorinated crosslinked polymer built on dynamic DA bonds which exhibited both of outstanding self-healing and lower surface energy properties.26 On the basis of DA reactions, Singha et al. have prepared polyurethane and polyhedral oligomeric silsesquioxanes

hybrid

materials

which

owing

ultrahydrophobic

and

self-repairing

performances.27 Li et al. have synthesized a novel self-healing and flexible conductive composite


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based on DA chemistry and three-dimensional graphene.28 By taking the advantages of the decouple/couple process of the DA connects, polyurethane has been equipped with excellent self-healing capability. More important thing is that heat-triggered DA/retro-DA reactions are not only used for self-healing materials application, but also have the potential to act a dynamic crosslinker for recycling of thermosetting materials.29-31 It is known that the covalent crosslinked structure can effectively enhance the mechanical, solvent resistance and heat resistance performances of the polymers. However, these superior stiff materials usually sacrifice the recycling, reshaping and reprocessing merits. If the DA bonds are introduced into the polymers’ backbone, the materials would acquire both of excellent mechanical property and environmentally friendly recycling capability.30,

31

Sun et al. have

synthesized recyclable polyurethane networks with extraordinary mechanical performance. The reprocessed specimens via solution casting and hot-pressing shown no decrease of the high stiffness, strength and toughness.30 However, some self-healing materials or composites grounded on the DA chemistry did not exhibit any recyclable property as the presence of nonreversibly physical or chemical crosslinking structure.32-33 Hence, this work plans to introduce the DA conjunctions into the WPU’s backbone which are going to cleave into small segments at an elevated temperature. Surely, the decomposed small molecules are easier to move and dissolve which are the essential of the recyclable functionality. Herein, in this present work, we reported a facile route to design and prepare self-healing and recycling waterborne polyurethane based on reversible DA/retro-DA reactions (WPU-DA-x). At the beginning, a novel diol containing DA bonds is going to be prepared from the N-(2hydroxyethyl)-maleimide and furfuryl alcohol. Following, the WPU-DA-x latex will be prepared through a polymerization of DA diol, polypropylene glycol, 1,4-butanediol and isophorone


4

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diisocyanate, and the emulsification of the WPU-DA-x prepolymer with distilled water. The properties of the synthesized emulsions will be well studied via considering the particle size and Zeta potential. More importantly, the mechanism of the reversible DA chemistry and the selfhealing performance of the WPU-DA-x films will be systematically investigated. At the same time, the recycling property of the WPU-DA-x will be studied. It is expected that this novel WPU-DA-x will exhibit an excellent self-healing and recycling performances. EXPERIMENTAL SECTION Synthesis of the 1-(Hydroxymethyl)-10-oxatricuclo [5.2.1.02,6] dec-8-ene-3,5-dione-2aminoethanol (DA diol) The synthesized process of DA diol was illustrated in Scheme S1. N-(2-hydroxyethyl)maleimide (HEMI, 8.00 g, 57 mmol, synthesized according to the previous work and the details were exhibited in supporting information15, 25) was dissolved with 20 mL toluene in a 50 mL round-bottomed flask. Then the furfuryl alcohol (5.56 g, 57 mmol) was added to the former solution at an equal molar ratio. After the mixture was stirred under magnetic stirring at 75 °C for 24 h, the white solid product was precipitated. DA diol was obtained after the crude precipitate was filtrated under vacuum, washed with diethyl ether and dried in vacuum oven. The final yield was 90.8% (12.37 g, 51.75 mmol) and its 1H NMR spectrum was shown in Figure S4 (400 MHz, d6-DMSO, δ). 6.500 (br, 2H, -CH=CH-), 5.056 (d, 1H, J=1.6 Hz, CH-O-CH), 4.901 (t, 1H, J=5.6 Hz, -OH), 4.729 (t, 1H, J=5.6 Hz, -OH), 4.021 (q, 1H, J=6Hz, -C-CH2-OH), 3.678 (q, 1H, J=5.2Hz, -C-CH2-OH), 3.386 (s, 4H, HO-CH2-CH2-N), 3.026 (d, 1H, J=6.4 Hz, -CO-CH-CH) and 2.864 (d, 1H, J=6.4 Hz, -CO-CH-C-). Preparation of DA chemistry modified waterborne polyurethane emulsion (WPU-DA-x)


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The WPU-DA-x was synthesized following a modified acetone methods by three steps, where the acetone was substituted by MEK to reduce the viscosity of the PU prepolymer. The formulations of the WPU-DA-x are presented in Table 1. The molar ratio of isocyanate groups with respect to hydroxyl groups was maintained at 1.3. According to the Scheme 1, the synthesis procedure of WPU-DA-x was illustrated as following by taking WPU-DA-4 as an example. In particular, firstly, a stoichiometric mixture of dehydrated IPDI (21.14 g, 95.22 mmol), PPG2000 (31.20 g, 15.60 mmol), DMPA (3.00 g, 22.39 mmol), DA diol (2.40 g, 10.04 mmol) and four drops of DBTDL were added into a 250 mL four-necked round-bottom glass reactor equipped with a mechanical stirrer, nitrogen inlet and condenser with drying tube. The mixture was reacted for 3 h at 85 °C to get linear and -NCO groups terminated oligomer. After that, when the system was cooled to 75 °C, a predetermined amount of BDO (2.27 g, 25.22 mmol) was dropped into the reaction mixture to extend the intermediated products. During the chain extending process, an appropriate amount of MEK was added occasionally to reduce the viscosity of the reaction mixture. After reacting for another 2 h, the system was cooled to 35 °C and TEA (2.26 g, 22.39 mmol) was added into the system to neutralize the ionic centers of DMPA for 30 min. Finally, the resultant mixture was dispersed by deionized water under vigorous stirring for 1 h to get the final WPU-DA-4 emulsion. In emulsification process, the residual -NCO of the prepolymer reacted with water to form urea and biuret derivatives eventually. The MEK solvent was removed by using a rotary evaporator in a 50 °C water bath under vacuum. The ultimate WPUDA-4 emulsion was slightly transparent and weakly yellow with a solid content about 30 wt%. In the same way, other WPU-DA-x samples with different DA-diol content (wt%) were synthesized. The structure characterization of the WPU-DA-x was confirmed by FT-IR and 1H NMR spectra.


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Scheme 1. Preparation of the WPU-DA-x emulsions Preparation of WPU-DA-x films The WPU-DA-x films with a thickness about approximately 0.5 mm were prepared. In details, a predetermined amount of WPU-DA-x emulsions were poured into the horizontally placed and freshly cleaned polytetrafluoroethylene (PTEF) mold (6 cm × 7 cm × 1 cm). After dried at room temperature for 48 hours, the majority of water evaporated. The WPU-DA-x particles got closed,


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and following the chains were reshuffled. Than the films were peeled from the mold, all water and solvents were totally removed in a vacuum oven at 60 °C for 24 h to get constant weight WPU-DA-x films. All manufactured WPU-DA-x films were stored in a desiccator containing silica gel before any characterization. Quantitative evaluation of self-healing efficiency The absolutely dried WPU-DA-x films were cracked with surgical blade, and the broken films were heated at 130 °C for 30 min to carry out the retro-DA reactions. During this process, the chains cleave into lower molecular weight segments, which have a better mobility, to fill the cracks. After that, the films were maintained at 65 °C for 24 h to reconnect the DA bonds and restore the comprehensive properties. In this work, the ultimate tensile stress was taken to calculate the self-healing efficiency following equation (1), where the δ (virgin) is the ultimate tensile stress of the virgin and unbroken sample but the δ (repaired) means the ultimate tensile stress of the corresponding repaired samples.34 Healing efficiency (%) = [δ (repaired)/δ (virgin)] × 100 %

(1)

Table 1. Formulations of WPU-DA-x Samplea

Relative mole ratio

DA diol content

(IPDI:PPG2000:DMPA:DA diol:BDO)

(wt%)

WPU-DA-0

106.03:15.60:22.39:0.00:43.57

0

WPU-DA-1

103.33:15.60:22.39:2.51:38.98

1

WPU-DA-2

100.63:15.60:22.39:5.02:34.40

2

WPU-DA-4

95.22:15.60:22.39:10.04:25.22

4


8

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WPU-DA-6 a

89.81:15.60:22.39:15.06:16.04

6

Samples here were entitled as WPU-DA-x, where the DA meant the DA crosslinked points induced into the polymers’ backbone and the x indicated the content of DA diol by weight of solid content in the waterborne polyurethane formulations.

Materials Maleic anhydride, furan and ethanolamine were purchased from Kelong Reagent Co., Ltd. (Chengdu, China), and used without further purification. Furfuryl alcohol was provided by Adamas Beta Reagent Co., Ltd. (Shanghai, China). Polypropylene glycol with number-average molecular weight of 2000 g mol-1 (PPG2000) and 2,2-dimethylol propionic acid (DMPA) (Wanhua Chemical Co., Ltd., Yantai, China) were dehydrated under vacuum at 120 °C for 2 h prior to use. Isophorone diisocyanate (IPDI) and dibutyltin dilaurate (DBTDL) were supplied by Wanhua Chemical Co., Ltd. (Yantai, China). 1,4-butanediol (BDO, dehydrated under vacuum oven at 100 °C for 2 h) and triethylamine (TEA) were purchased from Kelong Reagent Co., Ltd. (Chengdu, China). Ethyl acetate (EAC), methyl alcohol (MT), methyl ethyl ketone (MEK), toluene and acetone were supplied by Kelong Reagent Co., Ltd. (Chengdu, China) and dried with the 4 Å molecular sieves. Other solvents and chemicals were used without further purification. Characterization The particle size and Zeta potential of the WPU-DA-x emulsions were measured on the dynamic light scattering ultrafine particle analyzer (Malvern Model Zeta sizer ZEN3600) at room temperature. The concentrations of the WPU-DA-x dispersions are 5 wt% and 30 wt% for particle size and Zeta potential measurements respectively, and each testing was repeated for three times.


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Fourier transform infrared spectra (FT-IR) of the thin waterborne polyurethane films were recorded on a Nicolet 560 FT-IR Spectrum Scanner (Thermo Scientific, USA) after 32 scans with a resolution of 4 cm-1 in the region of 4000 to 400 cm-1. Proton nuclear magnetic resonance spectra (1H NMR) were acquired on a Bruker AV400 NMR spectrometer (400 MHz, Germany) at 25 °C with CDCl3 or d6-DMSO as the solvent and tetramethylsilane (TMS) as an internal reference. UV-vis spectra of the thin WPU-DA-x film fabricated via latex casting on the quartz wall were recorded on an Analytic-jena Specord S600 (Germany) over a wavenumber range from 200 to 500 nm in an absorption mode at an indoor condition. Differential scanning calorimeter (DSC) measurements were performed with the NETZSCH DSC 214 (Germany). The temperature procedure is heated from -60 °C to 180 °C with a ramping rate of 10 °C min-1 (first run), cooling to -60 °C with rate of -20 °C min-1 and reheating to 180 °C with rate of 10 °C min-1 (second run). X-ray diffractogram (XRD) was recorded by a X’ pro MPD DY129 with monochromatic CuKα radiation (λ=1.541 Å) and the generator worked at 35 kV and 30 mA. The results were taken from the thin WPU-DA-x films (approximately 0.5 mm) in the region from 5 ° to 50 °. Thermo gravimetric analysis (TGA) was performed with a NETZSCH TG-209 F3 thermo gravimetric analyzer (Germany) at a scanning rate of 10 °C min-1 under flowing nitrogen atmosphere (40 mL min-1). Results were recorded from room temperature to 600 °C. Mechanical properties were measured on dumbbell-shaped specimens (20 mm × 5 mm × 0.5 mm) with an Instron tensile testing machine (Model 5569, USA) at a crosshead speed of 100 mm


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min-1. Measurements were performed on five replicates of each sample, and the average values (strain and tensile stress at break) were shown. The healing progress of the scratches (made by surgical blade) was monitored by polarizing optical microscope (POM, Nikon Eclipse LV100D, Japan) equipped with a heating stage. The micrographs were taken at different time intervals during the heating process from room temperature to 130 °C with a ramping rate of 10 °C min-1. RESULTS AND DISCUSSION Chemical structure of DA diol and WPU-DA-x The DA diol was synthesized through [4+2] cycloddition reaction at mild condition and it’s chemical structure was analyzed by 1H NMR spectroscopy as shown in Figure S4. The ratio of the peak areas of 6.50 ppm (labeled c, 2H, -CH=CH-, originated from furfuryl alcohol) respect to the total of 3.02 ppm and 2.86 ppm (labeled e1 and e2, 2H, -C-CH-CH-C-, originated from HEMI) is 1:1, manifesting the successful preparation of DA diol. The WPU-DA-x containing DA linkages were prepared via polyaddtion reactions between -NCO and -OH through a modified acetone process. The FT-IR and 1H NMR spectroscopies were taken to verify the chemical structure of WPU-DA-x and the corresponding spectra were plotted in Figure S5 and Figure S6 respectively. A broad absorption band at 3331 cm-1 is related to the urethane N-H stretching vibration and its deformation vibration emerge at 1531 cm-1. The absorption band at 1708 cm-1 is corresponded to the stretching vibration of C=O groups of urethane. These special absorption bands indicate that urethane linkages have been formed from -NCO and –OH groups. In addition, all of WPU-DA-x samples do not appear the -NCO’s absorption band which located in ~2270 cm-1. From the FT-IR analyzing, it is known that the WPU-DA-x was synthesized


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successfully.35-36 Furthermore, a weak absorption peak at 1772 cm-1 emerged which is related to DA adducts. As presented by the insert of Figure S7, the signal of 1772 cm-1 become more obvious when the content of DA diol increased by 6 wt%, which is accordant with the formulation presented in Table 1. The increased signal of DA products proves that the DA bonds were successfully introduced into WPU- DA-x backbones. Besides, from the 1H NMR spectrum of WPU-DA-6 (Figure S6), the peaks at 6.40 ppm (labeled g) and 4.91 ppm (labeled h) emerged which related to DA bonds, proving that the DA diol was incorporated into WPU-DA-x chains. Properties of WPU-DA-x emulsions The resultant WPU and WPU-DA-x emulsions were analyzed by Zeta-size analyzer to evaluate the storage stability. The particle size and distribution of the suspended waterborne polyurethane particles are shown in Figure 1 (a). The average sizes are 47.4 nm, 39.6 nm, 43.2 nm, 41.4 nm and 37.2 nm and the polydispersity indexes (PDI) are 0.22, 0.17, 0.13, 0.15 and 0.11 for WPUDA-0, WPU-DA-1, WAPU-DA-2, WAPU-DA-4 and WAPU-DA-6 respectively. The particle size is almost identical and the distribution is uniform for five WPU-DA-x dispersions. It indicates that DA bonds-embedded WPU-DA-x prepolymers could be well dispersed into deionized water to get the final WPU-DA-x latexes. Furthermore, Zeta potential is another important indicator to evaluate the stability of the latexes. The Zeta potential is exhibited in Figure 1 (b), and values are -33.8 mV, -30.5 mV, -28.2 mV, 29.3 mV and -28.5 mV for WPU-DA-1, WAPU-DA-2, WAPU-DA-4 and WAPU-DA-6 respectively. The high absolute values of Zeta potential illustrate that abundant electrostatic repulsive energy is presented on the surface of waterborne particles.37-38 On the other hand, the introduction of DA diols into WPU-DA-x chains negligibly impacts the values of Zeta potential,


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suggesting the incorporation of DA bonds hardly influences the stability of the latexes. Besides, the WPU-DA-x dispersions were stored in the hermetic glass bottles under the indoor condition which did not emerged any precipitates at the bottom even through it is placed for more than 6 months. In summary, WPU-DA-x dispersions are well stable and the introduction of DA bonds into the backbone caused little effect on the appearance.

Figure 1. (a), particle size and distribution of waterborne polyurethane particles; (b), Zeta potential of waterborne polyurethane emulsions. Thermal-driven reversibility of DA diol and WPU-DA-x films The characteristic reversible DA/retro-DA reactions were investigated by 1H NMR spectroscopy. A 0.1 mol L-1 solution of DA diol in d6-DMSO was stored in a scew-top NMR tube. After every couple or decouple process of the DA diols, the solutions were frozen in a refrigerator, which could avoid any DA/retro-DA reactions at a mild condition before getting their 1H NMR spectra. The 1H NMR spectrum of DA diol is shown in Figure 2 (a) named 1-DA which exhibits characteristic peaks of DA products at 6.50 (A), 5.06 (C), 3.03 (B2) and 2.86 (B1). However, after the 1-DA was heated at 130 °C for 30 min, these peaks attributed to DA adducts were


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sharply weakened and some new peaks at 6.99 (e) assigned to maleimide and 7.55 (d), 6.37 (c) and 6.25 (b) assigned to furan appear, as shown by the 1-Retro-DA. Afterwards, in order to explore the reversibility of the DA/retro-DA reactions, the sample of 1-Retro-DA was maintained at 65 °C for 5 h to carry out [4+2] cycloaddition DA reactions. It can be observed that these peaks belonged to DA adducts get strong. In contrast, the peaks ascribed to maleimide and furan groups become poor as exhibited by 2-DA. At last, the sample of 2-DA was reheated at 130 °C to cleave the DA bonds again. Accordingly, the most of characteristic peaks for DA adducts are disappear form the spectrum of 2-Retro-DA. From the above 1H NMR spectroscopy analysis of DA diols, it is proved that the DA/retro-DA reactions have a thermal-driven reversibility but the reversible degree cannot be 100%.39 According to calculation from the peak area of the A (DA adducts) and e (maleimide), the reversible degree is approximately ~70% which is high enough to prepare the self-healing waterborne polyurethane. The maintenance of the thermal-driven reversibility of DA chemistry on polymers level is critical for self-healing and recycling process. Thus, the reversibility of this characteristic reaction of WPU-DA-6 was monitored through FT-IR spectroscopy. Especially, maleimide and DA adducts have distinctive absorption bands at 696 cm-1 and 1772 cm-1 respectively.40 The reversible nature of WPU-DA-6 was monitored via FT-IR spectroscopy in terms of repeated heating/cooling procedure. As shown in Figure 2 (b), the initial WPU-DA-6 (1-DA) exhibited special absorption band of DA adducts at 1772 cm-1 (left insert in Figure 2 (b)) but do not appear the particular peaks for maleimide at 696 cm-1 (right insert in Figure 2 (b)) and 827 cm-1. In contrast, after the DA bonds connected sample was heated at 130 °C for 30 min, the retro-DA reactions were verified by the appearance of 696 cm-1 of maleimide and the disappearance of special peak for the DA adducts at 1772 cm-1 as presented of 1-Retro-DA in Figure 2 (b).


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Following, maintaining the 1-Retro-DA at 65 °C for 5 h for DA connecting reactions, the peak at 1772 cm-1 emerged distinctly, but the absorption bands at 696 cm-1 and 827 cm-1 decreased visibly as presented in 2-DA, which demonstrated that the DA products were formed again from the reversed maleimide and furan groups. At last, the maleimide and furan derivatives could be regenerated one more time upon higher temperature stimulation as depicted by 2-Retro-DA in Figure 2 (b). Hence, from the FT-IR results, the reversible nature of the DA/retro-DA reaction in WPU-DA-x is successfully justified for several times in response to heating stimulation.

Figure 2. (a), 1H NMR spectra of different samples, 1-DA was the DA diol (cross-linking DA reaction), 1-Retro-DA means heated up to 130 °C for 30 min (cleaving, retro DA reaction), 2-DA refers to cooled down to 65 °C for 5h (cross-linking DA reaction) and 2-Retro-DA related to heated up to 130 °C for 30min (cleaving, retro DA reaction); (b), the FT-IR spectra of different samples, 1-DA was the initial WPU-DA-6 (cross-linking DA reaction), 1-Retro-DA means heating at 130 °C for 30 min (cleaving, retro DA reaction), 2-DA refers to cooled down to 65 °C for 5h (cross-linking, DA reaction) and 2-Retro-DA related to heating at 130 °C for 30min (cleaving, retro DA reaction).


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The retro-DA process in the WPU-DA-6 film at 130 °C was also verified through UV-vis spectroscopy. It is known that maleimide functionalities exhibit a special absorption band at ~287 nm which was arisen from the conjugated effect of its π-π* (C=C) and n-π* (C=O) chromophore excitation.25 The thin film of WPU-DA-6 was fabricated by casting the WPU-DA6 latex on the quartz wall and dried in a vacuum oven at 60 °C with a few minutes. The UV-vis spectra were collected at different time intervals (1, 2, 5, 10, 15, 20, 25 and 30 min) at 130 °C which were presented in in Figure 3 (a). The increasing absorption intensity of maleimide chromophore proved that the DA bonds could be disconnected into maleimide and furan groups at a high temperature (130 °C). WPU-DA-x exhibited thermal reversibility, and this process could be observed directly by solgel transition performance. As shown in Figure 3 (b), when the fragments of WPU-DA-6 were immersed in N,N-dimethylformamide (DMF, 70 wt%) at room temperature, the specimens could not be dissolved but only be swollen as presented in Figure 3 (b)-(2). However, the specimens were dissolved totally after being heated at 130 °C for 30 min as shown Figure 3 (b)-(3), which illustrated that the DA bonds could be cleaved at a higher temperature and the WPU-DA-6 chains were broken into soluble segments. When the DMF was evaporated from Figure 3 (b)-(3), the WPU-DA-6 was a viscous liquid as exhibited in Figure 3 (b)-(4). Afterwards, maintaining the viscous liquid at 65 °C for 5 h, the DA bonds connected again and a monolithic WPU-DA-6 elastomer was formed which could not flow freely as shown in Figure 3 (b)-(5). The sol-gel transition process could be repeated as presented by Figure 3 (b)-(6) and Figure 3 (b)-(7). Thus, the above analysis through phase transition experimental justified that the DA bonds were successfully introduced into backbone of WPU-DA-x and it exhibited an integral thermal-driven reversibility.


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Figure 3. (a) UV-vis spectra of WPU-DA-6 thin film at different time interval at 130 °C. (b) Dissolution behavior of the WPU-DA-6 elastomer: (1) initial WPU-DA-6 elastomer in the DMF; (2) elastomer was swollen at RT for 10 h; (3) elastomer was dissolved at 130 °C for 30 min. And the sol-gel transition process of WPU-DA-6 elastomer: (4) retro-DA cleaved sol after evaporating the DMF from (3) solution; (5) DA bonds connected gel at 65 °C for 5 h; (6) retroDA cleaved sol at 130 °C for 30 min; (7) DA bonds connected gel at 65 °C for 5 h. For further investigating the reversibility of WPU-DA-x films built on DA chemistry, the DSC traces were recorded from -20 °C to 200 °C with the heating rate of 10 °C min-1 and the results were presented in Figure S8 (a). The curve of the WPU-DA-6 for the first heating program exhibited a special endothermic peak at ~140 °C which assigned to the retro-DA reaction. However, the traces for both of the WPU-DA-6 for the second heating program and the WPUDA-0 do not emerge any endothermic peak at the same temperature region. Moreover, XRD pattern of the WPU-DA-6 film was shown in Figure S8 (b) which presented a broad and strong peak at 2θ=19° assign to the crystal of the hard microdomain of the polyurethane. Correspondingly, the first heating program of the DSC thermograms of WPU-DA-0 and WPUDA-6 shows a melting process for the crystal of the hard microdomain, but the second heating


17


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program do not appear this broad endothermic peak because of the poor crystallization upon the cooling process with a rate of -20 °C min-1.41 Therefore, from the DSC results, it was known that the synthesized WPU-DA-x had the thermal reversibility. Self-healing behaviors of WPU-DA-x films The self-healing processes of the WPU-DA-x films were systematically investigated. From the macroscopic self-healing behavior in Figure 4 (a), the restoration capability of the mechanical performance was studied. In details, the rectangular film of WPU-DA-6 (50 mm × 5 mm × 1 mm) was cut into two pieces and the fresh surfaces were brought into contact immediately with an external intervention. Following, it was processed by thermally treatment at 130 °C for 30 min and 65 °C for 24 h to carry out the retro-DA reactions and DA reactions respectively. After the repairing procedure was completed, the healed film could be stretched and bended without break at the contact location. It illustrated that the mechanical properties of the broken films could be restored based on the retro-DA/DA reactions, rearrangement of polymer chains and abundant hydrogen bonds. Apart from macroscopical analysis, the self-healing process was also recorded microscopically by the POM. As exhibited in Figure 4 (b), a crack about 0.3 mm in depth was made on the WPUDA-6 film using a surgical blade. The cracked film was put on a heating stage, and heated from room temperature to 130 °C with a heating speed of 10 °C min-1.20 It could be observed that the crack closed slightly in the heating progress. More clearly, when the temperature was kept at 130 °C for 5 min, the crack almost diminished, and after being heated for 10 min at this temperature, the crack disappeared completely. According to the synthetic process, the DA bonds were introduced into the backbone of the waterborne polyurethane, therefore, when the retro-DA


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reactions occurred, the polymer chains were cleaved into small segments. Because the broken segments have a better mobility, the cracks were filled through the diffusion and rearrangement of the cleaved chains. However, the control sample WPU-DA-0 without DA connects could not repair the crack as WPU-DA-6 in the same self-healing condition. As shown in Figure S9, after the damaged WPU-DA-0 film was heated at 130 °C for 10 min, the crack could not be disappeared but just diminished somewhat as the thermo-induced expansion. Summarily, the retro-DA reactions were the previously potential requirement for the whole self-healing process for WPU-DA-x.

Figure 4. Pictures of the self-healing behavior, (a) healed WPU-DA-6 could be bent or stretched; (b) POM images of the self-healing process of the crack on the WPU-DA-6 film. Additionally, the self-healing process of the WPU-DA-x was evaluated by a tensile measurement in a more quantitative way, and the healing efficiencies were calculated from ratio of the ultimate tensile stress of the repaired over the virgin specimens.42 The load-displacement curves of the


19


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repaired and the virgin specimens for different WPU-DA-x formulations were exhibited in Figure 5 and the details were summarized in Table 2. The self-healing efficiencies are 39.9%, 51.3%, 86.3% and 92.5% for WPU-DA-1, WPU-DA-2, WPU-DA-4 and WPU-DA-6 respectively. As a general tendency, the healing efficiencies increased with more DA-diol contained. This is resulted from the synthesis characteristic that the DA bonds were introduced into the backbone of the WPU-DA-x. If the density of DA bonds was not high enough, the cleaved WPU-DA-x chains remain keep a high molecular weight which own a relatively poor mobility like WPU-DA-1 and WPU-DA-2. By contrasted, since WPU-DA-4 and WPU-DA-6 owned a higher content of DA bonds, the polymer chains were broken into lower molecular weight segments which had an excellent mobility to fill the cracks. The well mobility makes the polymer chains entangle more sufficiently, thus the functionalities of maleimide and furan conjugated more easily which tremendously benefit to the self-healing ability. At last, the following maintenance at 65 °C for 24 h ensured that the DA bonds were reconnected and the integral properties were recovered. At the same time, the abundant hydrogen bonds originated from urea linkages of WPU-DA-x chains paly an auxiliary effect in the self-healing progress, benefiting to satisfying healing performance somewhat. Scheme S2 exhibits the assistance of hydrogen bonds during the self-healing process based on DA chemistry. In short, the increasing healing-efficiencies respect to higher DA-diol contents imply that the thermally reversible DA bonds play a significant role in the self-healing process of WPU-DA-x films.


20

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Figure 5. The load-displacement curves of virgin WPU-DA-x specimens and the repaired WPUDA-x specimens (named WPU-DA-x-r). (a) WPU-DA-1, (b) WPU-DA-2, (c) WPU-DA-4 and (d) WPU-DA-6. Table 2. The details of tensile results and the healing efficiency of each WPU-DA-x samples.

Samples

a

Stress (MPa)

Strain (%)

Healing

Virgin a

Repaired b

Virgin a

Repaired b

efficiency (%)

WPU-DA-1

17.3±1.1

6.9±1.2

936±30

212±28

39.9

WPU-DA-2

15.6±0.9

8.0±0.7

830±26

331±32

51.3

WPU-DA-4

13.1±0.4

11.3±1.8

742±50

606±51

86.3

WPU-DA-6

9.3±0.2

8.6±0.4

607±28

535±26

92.5

‘Virgin’ means the samples of WPU-DA-x before being cracked.


21


b

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‘Repaired’ means the samples of WPU-DA-x were cracked and repaired.

The self-healing mechanism of the micro notch on the WPU-DA-x films is depicted in Figure 6. At the beginning, micro scratches are caused by some damages. The following heating process at 130 °C for 30 min makes sure that the molecular chains of the WPU-DA-x were cleaved into small molecular weight fragments via retro-DA reactions. At the same time, the notch is filled with the decomposed small chains, because cleaved fragments own a good mobility at 130 °C. Lastly, the following maintenance of the cleaved WPU-DA-x films at 65 °C for 24 h allows the major of dissociative maleimide and furan moieties reconnect again via DA reaction to restore the mechanical performance with the assistance of hydrogen bonds surrounding the cracked location. According to the self-healing progress, it can be readily concluded that the repaired process of the micro scratch on the WPU-DA-x films contains the following three steps: (1) the cleavage of the DA connected points by retro-DA reactions at a higher temperature; (2) the diffusion of the decomposed small fragments to disappear the scratch; (3) the reconnection of DA bonds and the exchange equilibrium of hydrogen bonds to restore the integrally comprehensive properties of WPU-DA-x.

Figure 6. Schematic illustration of the self-healing mechanism of WPU-DA-x


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The recyclability of WPU-DA-x films As the reversible nature of the DA bonds, the networked WPU-DA-x films had another advantage that it could be recycled by conventional polymer processing methods.25, 30 The one is hot-pressing molding. As presented in Figure 7 (a) (upper), the rectangular WPU-DA-6 films were cut into small fragments and remolded into a circular shape by hot-pressing at 130 °C for 30 min under a pressure of 5 MPa. After that, the reshaped specimen was maintained at 65 °C for 24 h to reconnect the DA bonds. Another approach was solution casting. As shown in Figure 7 (a) (bottom), the solid rectangular WPU-DA-6 film was dissolved into DMF at 130 °C for 30 min through retro-DA reactions. Then the clear solution was casting into a freshly clean and circular PTEF mold to remove the solvent. Following, sustaining the samples at 65 °C for 24 h to conjugate the DA crosslinking, the circularly reshaped WPU-DA-6 was obtained. For both of recycled samples, it was found that the monolithic samples could be formed. The loaddisplacement curves of the recycled materials were presented in Figure 7 (b). It is observed that the stress-strain curves are nearly identical to the virgin WPU-DA-6 with a little decrease of the mechanical performance. Thus, the fruitful results illustrate that the excellent thermal-driven recyclability was acquired for WPU-DA-x built on thermal-triggered reversibility of the DA chemistry.


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Figure 7. (a), recyclability of WPU-DA-6 elastomer through the hot-press molding and the solution casting; (b), load-displacement curves of WPU-DA-6 and the correspondingly recycled samples by solution casting and hot-press molding. Investigation the thermal stability of WPU-DA-x films In order to study the influence of DA bonds on the thermal stabilities of WPU-DA-x films, a thermal gravimetric analysis (TGA) was carried out from room temperature to 600 °C in a nitrogen atmosphere. Figure 8 (a) and (b) presented typical TGA and first derivative of weight loss thermograms (DTG) curves respectively, and the characteristic thermal degradation dates were summarized in Table 3. From the TGA curves, it could be observed that the thermal stability was enhanced while more BDO was substituted by DA diol. The temperature of 50 % weight loss (Td50%) was increased from 333.7 °C to 357.3 °C when the DA diol content was increased from 0 to 6 wt%. There were two distinct degradation stages of WPU-DA-x films in region of 265 ~ 420 °C which assigned to the hard microdomain and soft microdomain. When the DA diol was introduced into the WPU-DA-x, the first degradation stage was more affected than soft microdomain (the second stage). The temperature of the first peaks of DTG curves were 333.7 °C, 340.0 °C, 343.9 °C, 349.5 °C and 357.5 °C for WPU-DA-0, WPU-DA-1, WPU-DA-2, WPU-DA-4 and WPU-DA-6 respectively. In fact, because of the relatively weaker bond energy of urethane bonds, the hard microdomains had a poor thermal stability, which decomposed into primary amines (or secondary amines), olefins and carbon dioxide. In contrast, polyether had a better thermal stability which degraded in the temperature range of 350 ~ 420 °C. Significant enhancement of thermal stability might result from the rigid multiple rings structure of DA diols which were more stable than smooth linear structure of BDO.43 In short, introduction of the DA


24

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bonds into the waterborne polyurethanes’ backbone not only endowed it with self-healing and recycling abilities but also strengthened its thermal stability.

Figure 8. (a) TGA curves of different WPU-DA-x films; (b) DTG curves of all WPU-DA-x films. Table 3. Particular thermal degradation dates of different WPU-DA-x samples.

Samples

Td50%a (°C)

Tmax1b (°C)

Tmax2b (°C)

WPU

333.7

314.5

381.0

WPU-DA-1

340.0

317.0

383.2

WPU-DA-2

343.9

330.6

384.7

WPU-DA-4

349.5

342.2

385.4

WPU-DA-6

357.3

351.9

386.7

a

Td50% means the temperature as the samples underwent 50 % weight loss.

b

Tmax represents the temperature when the degradation rate at the maximum (peaks of the DTG

curves).


25


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CONCLUSIONS By introducing the DA diol into polymer’s backbone, a new kind of thermal-driven self-healing and recyclable waterborne polyurethane film based on DA/retro DA reactions (WPU-DA-x) was design and synthesized. WPU-DA-x films exhibited excellent thermal reversibility and selfhealing capability. From the optical microscopic and tensile measurement results, it is known that the excellent comprehensive performance of WPU-DA-x could be restored after the cracks were healed at 130 °C for 30 min and 65 °C for 24 h. The healing efficiency could reach as high as 92.5 % for WPU-DA-6, and the WPU-DA-x films could be recycled through hot-pressing and solution casting. Besides, the thermal stability was enhanced after the DA bonds were incorporated. From the healing process, it could be implied that the cracks on the films were healed following three steps: firstly, the WPU chains cleaved into small segments through the retro-DA reaction; secondly, the diffusion and entanglement of the cleaved chains; and lastly, the reconnection of DA bonds to restore the excellent properties. This study enables the redemonstration of general points about thermal reversibility of DA bonds on materials already known, as well as presentation of self-healing and recycling capabilities of environmentally friendly waterborne polyurethane. ASSOCIATED CONTENT Supporting Information. Synthesized process of DA diol (Scheme S1), 1H NMR spectra of Furan-A, HEMI-A and HEMI (Figures S1−S3), 1H NMR spectrum of DA diol (Figure S4), FTIR and 1H NMR spectra of WPU-DA-6 (Figure S5 and Figure S6), FT-IR spectrum of different WPU-DA-x (Figure S7), DSC and XRD results of WPU-DA-0 and WPU-DA-6 (Figure S8),


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POM images of the cracks of WPU-DA-0 (Figure S9), mechanism of hydrogen bonds assisted self-healing process (Scheme S2). AUTHOR INFORMATION *Corresponding Author: E-mail: [email protected] (H. Wang); [email protected](X. Cheng) Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGEMENTS This work was funded by National Natural Science Foundation of China (NO. 51773129, 51503130), Support Plan of Science and Technology Department of Sichuan Province, China (2017GZ0129, 2018SZ0174), International Science and Technology Cooperation Program of Chengdu (2017-GH02-00068-HZ), Postdoctoral research foundation of Sichuan University (2018SCU12049) and Supported by Graduate Student’s Research and Innovation Fund of Sichuan University (2018YJSY084). REFERENCES (1) Wang, T.; Zhou, C.; Zhang, X.; Xu, D. Waterborne polyurethanes prepared from benzophenone derivatives with delayed fluorescence and room-temperature phosphorescence. Polym. Chem. 2018, 9 (11), 1303-1308, DOI 10.1039/c7py01995e. (2) Wang, J.; Zhang, H.; Miao, Y.; Qiao, L.; Wang, X. A whole-procedure solvent-free route to CO2-based waterborne polyurethane by an elevated-temperature dispersing strategy. Green Chem. 2017, 19 (9), 21942200, DOI 10.1039/C7GC00726D.


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Page 32 of 33

For Table of Contents Use Only

Table of Contents (TOC) synopsis Green waterborne polyurethane was equipped with self-healing and recycling capabilities extending lifespan of materials and conserving natural resource based on Diels-Alder chemistry.


32

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Environmentally and friendly waterborne polyurethane films are endowed with self-healing and recycling capabilities built on reversible covalent chemistry. 49x28mm (300 x 300 DPI)


Thermal-Driven Self-Healing and Recyclable Waterborne

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