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The dynamic chain effect on healing performance and thermo-mechanical properties of a polyurethane network
Journal Pre-proof The dynamic chain effect on healing performance and thermomechanical properties of a polyurethane network
Lei Zhang, Haiqing Wang, Zenghui Dai, Zhongxiang Zhao, Feiya Fu, Xiangdong Liu PII:
S1381-5148(19)30846-6
DOI:
https://doi.org/10.1016/j.reactfunctpolym.2019.104444
Reference:
REACT 104444
To appear in:
Reactive and Functional Polymers
Received date:
15 August 2019
Revised date:
3 December 2019
Accepted date:
3 December 2019
Please cite this article as: L. Zhang, H. Wang, Z. Dai, et al., The dynamic chain effect on healing performance and thermo-mechanical properties of a polyurethane network, Reactive and Functional Polymers (2019), https://doi.org/10.1016/ j.reactfunctpolym.2019.104444
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© 2019 Published by Elsevier.
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The Dynamic Chain Effect on Healing Performance and Thermo-Mechanical Properties of a Polyurethane Network Lei Zhang, Haiqing Wang, Zenghui Dai, Zhongxiang Zhao, Feiya Fu, and Xiangdong Liu*
School of Materials Science and Engineering, Zhejiang Sci-Tech University, Xiasha Higher Education Zone, Hangzhou 310018, China.
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E-mail address: [email protected]
Abstract: Dynamic covalent chains that composed of three or five dynamic polyurea bonds are
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introduced into cross-linked polyurethane to overcome the classical dilemma between healing rate
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and thermo-mechanical properties. Comparing with dynamic urea bond, the incorporation of the
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dynamic polyurea chains with multiple leads to signific antly improved thermo-mechanical
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performance and healing rate/efficiency on the network. Remarkably, several folds’ increases in
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tensile and adhesion shear strength as well as 53 o C enhancement in glass transition temperature are achieved without sacrificing the material properties of the polyurethane network. We attribute the improvements to the decomposed polyurea chain segments, which have higher diffusivity and larger quantity than the dynamic urea bond that tied in the control network. This study provides a concept of dynamic covalent chain to address the classical dilemma between healing performances and thermo-mechanical properties among healable thermosets.
Keywords: Dynamic polyurea chain; Healing; Thermo-mechanical properties; Dilemma
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Introduction
Healable polymers have been intensively studied during the past decades to elongate the service life by preventing failure originated from defects growth.1-6 Up to date, healability can be objectified through extrinsic healing and intrins ic healing,
7-9
while the latter enables multiple
healing at the same position through dynamic linkages. 10-18 Various interactions have been applied 19-23
hydrogen bonding,
24-26
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for intrinsic healing so far, such as metal-ligand coordination,
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supramolecular affinity27-29 and dynamic covalent bonding, 30-32 which show attractive structure
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design flexibility.
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Among these interactions, healing based on dynamic covalent bond (DCB) is focused
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recently due to its simple preparation and possibility to offer acceptable mechanical properties and chemical resistance. Till now, several DCB species, including Diels -Alder cycloaddition, 33
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thiol-disulfide exchange, 34-37 disulfide exchange reaction, 38-39 transesterification, 40-43 amine-ketone
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exchange, 44 urea exchange, 45-49 olefin metathesis 50-52 and pinacol based C-C bond reversion, 53
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have been applied in various healable thermosets. Remarkably, regardless of the diverse reaction types, the reactive moieties at the ruptured surface require effective collision to perform the healing reactions, which is highly influenced by polymer chain mobility. 54 However, since DCB is welded in the polymer network, sacrificing chain rigidity or cross -link density becomes the final solution to achieve sufficient chain mobility. Obviously, this would cause severe reduction in glass transition temperature (T g ), mechanical modulus and strength. This is the dilemma that shadows the field of healable polymers. In fact, although new intrinsic healable thermosets are reported continuously, many of them are feeble to exhibit adequate mechanical strength. This is a very
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acute problem as a large amount of thermosets are utilized for structural applications. Polyurethane has been widely used as a thermoset. Although having received intensive focus, healable polyurethane remains stuck between healing and material properties. A solution to deal with the dilemma is urgently required.
Thermodynamically, material healing can be considered as a non-equilibrium to equilibrium 5
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process, which is driven by entropy increase. Polymer chains with high flexibility possess more
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conformations, thus the entropy is higher. 55 Meanwhile, reptation model suggests that the healing
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time is determined by the duration of conformation changes to the Gaussian state, during which
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structures with smaller molecular weight are preferred. 54,56 Based on these previous studies, this
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manuscript proposes a covalent chain containing multiple dynamic bonds, named as dynamic covalent chain (DCC). Although both DCB and DCC are based on reversible chemical reaction,
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DCC is less restricted by polymer chain mobility. DCC can split into many highly reactive and
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mobile fragments to diffuse and split at the ruptured surface, resulting in new DCC and healing. In
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contrast, DCB is linked to the polymer network with one end. Confined by the bulk chains, its mobility is suppressed, which further causes reactivity and healing rate reduction. This is the reason that most healable polymers based on DCB are developed by sacrificing material properties. In addition, during our submission period, a DCC structure was published and applied for material healing. Unfortunately, neither the advantages of DCC over DCB, nor the underlying mechanisms are not concerned. This study provides a systematic understanding of DCC and studies the effect of the numbers of dynamic covalent bonds on DCC to the healing performance. Importantly, DCC is not limited to polyurethane. As corresponding DCC is synthesized and introduced, it can also help other healable polymers get rid of the dilemma. 3
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Experimental
Materials. Isophorone diisocyanate (IPDI, 99%), hexamethylene diisocyanate (HDI, 99%), piperazine anhydrous (PA, 99%), N-aminoethylpiperazine (N-AEP, 99%), dibutyltin dilaurate (DBTDL, 99%), pentaerythritol (97%), dibutylamine (99%) and bromocresol green (99.9%) were
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purchased from Aladdin Reagent (Shanghai) Co.,Ltd. and used as received. Polyether polyol (HSH305, hydroxyl value: 320-355 mg KOH/g) was obtained from Haian Petrochemical and
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dehydrated by distillation under reduced pressure. N,N-dimethylformamide (DMF, 99%) was
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provided by Hangzhou Mike Chemical Agents Co., Ltd. and dehydrated by molecular sieve (4 Å).
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1-methylpiperazine (99%) was purchased from Meryer Chemical Technology Co., Ltd.
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Synthesis of dynamic covalent polyurea trimer (DPT) and pentamer (DPP) chains. Synthesis
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of DPT and DPP follows the reaction in Scheme S1. Specifically, 0.86g (10 mmol) of PA in 15 mL
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of chloroform was slowly introduced in 30 mL of chloroform containing 4.44g (20 mmol) of IPDI and stirred under ice-water bath for 2h. The solution was later evaporated until turbid followed by adding 40mL ethyl acetate to generate white precipitates, filtered and dried under 80℃ vacuum for 12h to obtain 5.04g of crude DPT. Yield=95%. The content of isocyanate in polyurea trimer was confirmed by dibutylamine titration. (Found 13.6 wt.%; Calc. 15.8 wt.%). DPP was prepared in a similar except changing the molar ratio of PA and IPDI to 2:3. Yield=96%. Isocynate content: Found 8.5 wt.%, Calc. 10 wt.%.
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Preparation of cross-linked polyurethanes. Cross-linked polyurethane PU, PU3 and PU5 based on different dynamic structures are sketched in Scheme S2. The specific preparation methods are listed as below.
Preparation of PU: Pentaerythritol (3.40g, 2.5 mmol), IPDI (1.11g, 5 mmol), HDI (1.68g, 10 mmol), HSH305 (3.40g, 20 mmol hydroxyl) and a drop of DBTDL (catalyst) were dissolved in
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8mL of DMF successively. The solution was put into a mold (120mm×80mm) at room
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temperature for 0.5h under vacuum, followed by treating at 60℃/2h, 90℃/10h and additional 4h
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under vacuum. A pale yellow and transparent polymer was obtained.Preparation of PU3: PU3
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was prepared in a similar way as PU except further introducing DPT (3.23g, 10.5 mmol isocyanate
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moiety), appeared as a pale yellow and transparent polymer.
Preparation of PU5: PU5 was prepared in a similar way as PU except further introducing DPP
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(5.25g, 10.5 mmol isocyanate moiety), appeared as a pale yellow and transparent polymer.
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Preparation of PU1: PU1 was prepared in a two-step method (Scheme S3) to prevent gelation before molding. N-aminoethyl piperazine with 1 equiv. amount was introduced in the second step and a pale yellow transparent polymer was obtained.
[Figure 1]
The highly reversible bonds between isocyanate and piperazine within the polyurethanes are in ratio: PU/PU1/PU3/PU5=0/1/2/4.
Characterization
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FT-IR spectra were recorded by a Nicolet 5700 Fourier transform infrared spectrometer (FT-IR, Nicolet, USA). 1 H NMR spectra were obtained by an Avance AV-400 Bruker 400 MHz nuclear magnetic resonance (NMR) spectrometer in deuterated chloroform (CDCl3 ). DSC thermograms were obtained
from
a DSC-1
differential scanning
Mettler-Toledo, Switzerland) at a heating rate of 10 K·min
-1
calorimeter
(DSC,
under nitrogen flow rate of 45 mL
-1
min and sample weight around 5 mg. Thermal weight loss was measured by a thermogravimetric
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analyzer (TGA, Mettler-Toledo, Switzerland) at a heating rate of 10 K·min -1 under nitrogen flow
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rate of 45 mL min-1 and sample weight around 10 mg. Dynamic mechanical analys is (DMA) was
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conducted on DMA1 (Mettler-Toledo, Switzerland) using shear mode under a heating rate of 3 K·min from -15 to 200 ℃ at a frequency of 1 Hz in nitrogen. The specimen feature was 10 x 10 x
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-1
1.2 mm.
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Cut healing efficiency: Samples were completely separated by a clean razor blade cut. The two
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fragments were put together, placed between two glass slides and fixed with clips. They were
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placed in a 120 ℃ oven and the healing process was monitored by an Olympus optical microscope. The variation in width was compared with the initial values at t=0 min. The apparent healing efficiency can be calculated
Tensile test: Samples (60 x 10 x 1 mm) were fully cut before healing at 120 ℃ for various times. After complete cooling, these samples were tested on a Instron 3367 tensile machine. Three samples were tested for every material to check the reproducibility.
Lap-shear test: Test samples were prepared by placing cured polyurethane (20 x 15 x 1 mm) between aluminum plates (100 x 20 x 2mm) with the same overlap area. They were placed in a 6
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Thermal deformation: Samples (Φ20 x 3 mm) were sandwiched between a coin (Φ20.5 x 1.7 mm) and a perforated metal (Φin 4mm, Φout 20 mm, thickness 1 mm) and fixed before thermally o
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treated at 120 C for 10 minutes. Same clips are applied to ensure identical forces during healing.
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After cooling, the extrusive polymer volume and the sample thickness are checked and compared.
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Viscosity: High molecular weight polyurea is dissolved in DMF solvent with a weight percentage
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of 14.29% to facilitate viscosity measurement and addition of isocyanates. Two bottles, each
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containing 6g (857 mg of high molecular weight of polyurea) of the DMF solution are marked as A1 and A2. Meanwhile, IPDI (62 mg) and DPP (292 mg) are mixed with 2 g of the above solution,
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respectively, marked as B1 and B2. The samples of same numbers are mixed together and tested
o
o
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under a Physica MCR 301 Senior rotary rheometer with a constant shear rate of 100 s -1 at 40o C,
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60 C and 80 C, respectively. For comparison, blank sample prepared by mixing 6 g of DMF solution with 2 g of DMF solution is tested by the same method as well. The evolution in viscosity is recorded for analysis.
Results and Discussions
Thermo-deformability and cut-healing capability
[Figure 2]
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Dynamic covalent polyurea trimer (DPT) and pentamer (DPP) chains were prepared from isocyanate and piperazine by generating urea bonds. The chemical structures were first verified by FT-IR spectra, as shown in Fig. 2. The peaks near 2900 cm-1 confirm the methyl and methylene -1
groups in DPT and DPP. The peaks at 1533 cm indicate the vibration of C-N bond. The peaks at -1
3359 and 1625 cm are assigned to the vibration of N-H and C=O structures, which prove the -1
formation of urea bond. They are not urethane bonds due to the fact that peak around 1730 cm is
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not found. Meanwhile, strong peak at 2250 cm-1 is corresponding to the isocyanate, which existing
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at the chain end. After normalizing by peak at 1625 cm -1 , the difference in transmittance intensity
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at 2250 cm-1 can be found. DPT shows higher peak intensity than DPP, which is corresponding to
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its higher isocyanate/urea ratio. Thus, the molecular weight of DPT is smaller.
[Figure 3]
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To further confirm the oligomer structures and calculate the molecular weight, 1 H NMR spectra are involved (Fig. 3). The resonance peak at 3.43 ppm is assigned to the protons of
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piperazine while the peaks at 1.08 and 0.96 ppm are corresponding to the protons of the methyl groups in IPDI. The peaks within the range of 1.15 ppm and 2.25 ppm are assigned to the protons of the isophorone rings while the peak around 3.0 ppm is corresponding to the protons of the alkyl groups in isophorone adjacent to nitrogen. By proton integration, the repeat unit n (in Scheme S1) of DPT is 0.92 (calc. 1.0) and DPP is 2.31 (cal. 2.0), which are close to the theoretical values. Ying et al.
45
had already proved that urea bonds prepared from piperazine derivatives and
isocyanate are dynamic at low temperature. Containing multiple urea bond with similar structures, it is expected that DPT and DPP are reversible, which means that the oligomer can rupture at
8
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elevated temperature and reform after cooling. When incorporated in polyurethanes, their characteristics enable the materials to be healable and deformable. To verify this, polyurethane PU3 (containing DPT) and PU5 (containing DPP) were developed. Meanwhile, PU1 (containing N-aminoethyl piperazine) and PU (without any urea structure) were prepared for comparison. The chemical structures are summarized in Scheme 1 and the dynamic urea bond is bolded. All these
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materials were fabricated based on the description in the Experimental section. The chemical
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structures of the polyurethanes were characterized by FT-IR (Fig. 4). For all prepared
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polyurethane polymers, no residue peak is found at 2250 cm-1, which indicates the complete
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consumption of isocyanate group. The generation of urethane bond is verified by the intensive -1
-1
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peaks at 1730 cm . The peaks at 1100 cm signify the C-O-C bond, which can be another evidence to prove the formation of urethane bond. DPP and DPT are successfully incorporated
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into the network, which are proved by the high peaks at 1625 cm -1 of PU3 and PU5. The
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cross-linked characteristics of the polyurethanes are proved by introducing PU5 (as an example)
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into DMF solvent and kept for 24 h. The material was swelled but did not show any dissolution (Fig. S1). This was also evidenced by the rubbery plateau showed in DMA curves.
[Figure 4]
Thermo-deformability gives a preliminary view on the diverse reversible reaction rates within the materials, which was performed by sandwiching the polyurethane samples between metal plates (Fig. 5a). After heating at elevated temperature, more polyurethane is squeezed out in PU5 than PU3 and PU1 (Fig. 5b), while no apparent extrudate is observed in PU sample. Meanwhile, reduction in sample thickness takes place simultaneous ly (Fig. 5c). PU sample almost maintains
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its thickness after the heating treatment. PU1 shows 19% reduction of its original thickness. With N-AEP, PU1 contains dynamic urea bond, which facilitate macroscopic deformation. Remarkably, when dynamic covalent polyurea chains are added, 24% (PU3) and 34% (PU5) decrease in thickness are found. The results provide perceptual knowledge that the dynamic covalent chains have advantage over dynamic covalent bonds in promoting chain mobility.
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[Figure 5]
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[Table 1]
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[Scheme 1]
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To illustrate the influence of chain mobility on healing, cut polyurethane samples are placed
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with same separated width (Fig. 5d and 5e). The healing efficiency is calculated based on residue width (Fig. 5d). After 30 min, PU and PU1 show reduction in cut width though the residue gap is
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still conspicuous. Meanwhile, almost complete healing can be found in PU3 and PU5. In fact,
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more obvious distinction is observed at the initial healing stage. When PU3 and PU5 already show
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healing efficienc ies above 80%, the healing efficiency PU and PU1 remain below 50%. It is worth to note that, since the T g of PU is quite low (23o C) (Table 1 and Fig. S2), chain diffusion in PU should be easier for physical chain entanglement at the crack interface, shown as cut-healing. However, such healing is not sufficient. 90% recovery at equilibrium is observed in PU while completely healing is found in PU3 and PU5 (Fig. 5d). Meanwhile, with DCB and DCC, T g s of the samples become higher, especially when DCC is introduced. Specifically, incorporating DPT results in 28o C increase in T g . As DCC becomes longer, further enhancement in Tg is found, as o
evidenced by the high T g of PU5 (80 C). When DCC is incorporated, improvements in tensile modulus and thermal stability than dynamic covalent bonds (Table 1). The storage moduli of PU3 10
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and PU5, (11.9 MPa and 40.4 MPa), which are orders of magnitude larger than PU1 (1.0 MPa) and PU (0.5 MPa). The thermal decomposition temperatures T d10 of PU3 (297 o C) and PU5 (295 o
C) are also higher than PU1 (283 o C) and PU (264 o C) (Table 1 and Fig. S3). These results
confirm that incorporating DCC enables higher healing efficiency and T g than DCB, which seems to overcome the classical dilemma. This encourages us to perform further in-depth investigation.
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It is worth to mention that healing based on visualizing surface recovery provides only an
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intuitive view on healing efficiency. In fact, even though the material was optically healed, the
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properties of the healed part can be quite different from the origin. In this study, mechanical
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healing is evaluated by cyclic cut-heal tests with different cut positions (Fig. 6a and 6b). Since
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every part of the sample suffers equivalent stress, tensile test is ideal to evaluate the healing quality inside the material. Herein, healing efficiencies of all samples are evaluated by comparing
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the original tensile strength variation (Fig. 6c and Fig. S4). Polyurethanes containing DCC (PU3
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and PU5) are able to sustain a 500 g loading even after three cycles (Fig. 6b). Polyurethanes with
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DCB (PU1) can only hold a 100 g loading while PU cannot even withstand a 100 g loading (not included in the figure). Remarkably, pristine polyurethanes with DCC have tensile strengths of 5.4 MPa (PU5) and 4.9 MPa (PU3), which are several folds larger than PU1 (1.2 MPa) and PU (0. 8 MPa). The tensile strengths are smaller than conventional PU due to the usage of chemicals containing flexible chains like hexamethylene diisocyanate and polyether polyol. Even after three cycles’ cut-heal tests, the remaining strength of PU5 (2.9 MPa) and PU3 (2.0 MPa) are still larger than pristine PU1 and PU. The healing efficiencies of PU5 are also (94%/1 cycle, 54%/3 cycles) better than PU1 (57% /1 cycle, 53%/3 cycles) and PU (29%/1 cycle, 19%/3 cycles). Furthermore, cyclic cut-heal is performed at the same cut position to show multiple healing capability (Fig. 6d). 11
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Load capability, tensile strength and healing efficiency (Fig. 6e, 6f and Fig. S5) similar to former experiments were achieved. Therefore, healing performance depends on the healing cycles while is independent from the healing positions of the samples. [Figure 6] Adhes ion is not only related to the material itself, but also highly influenced by the interface
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quality. Herein, polyurethanes were placed in between aluminum plates for measuring adhesion
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strength (Fig. 6g). The healing rates were compared based on the shear strength after diverse
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healing durations (Fig. 6h). PU5 and PU3 containing DCC reach their maximum adhesion
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strengths after healing for 25 min and 40 min while PU1 still not reach equilibrium up to 60 min.
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The adhesion strengths at equilibrium are 1.9 ± 0.3 MPa (PU5) and 1.1 ± 0.1 MPa (PU3), respectively. Meanwhile, the maximum adhesion strength of PU1 is 0.9±0.1 MPa (at 60 min). PU
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has no shear strength even though the healing time is elongated up to 60 min. Overall, the results
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evidence that higher adhesion strength, healing rate and healing efficiency are obtained when DCC
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is incorporated, especially if the DCC chain is longer. Long DCC structure has greater rheological behavior, which increases the actual bonding area and eliminates interface gaps between polymer and aluminum plates (Fig. 6g). Based on numerous urea bonds, DCC can collapse into small fragments to increase wettability and interaction with aluminum plate. On the other hand, healing of PU sample greatly depends on the chain diffusion and physical entanglements. However, it is hard to fill the interface voids and even impossible to generate physical entanglements between polymer and aluminum plate. In addition, cyclic shear-heal tests prove that polyurethanes with DCC almost maintains the adhesion strength even after 10 cycles (Fig. 6i). The adhesion strength of PU5 after 10 cycles is 1.5±0.1 MPa while PU1 is only 1.0±0.05 MPa. Consequently, cyclic 12
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adhesion tests also evidence the capability of DCC to overcome the classical dilemma between material properties and healing performances.
[Figure 7]
We also carried out a survey of recent publications (from 2016 to present) to show the advantages of DCC. To ensure comparability in polymer species, referential materials based on
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polyurethane with dynamic structures are summarized (Fig. 7). Although many of them have been reported with high healing rate and efficiency, they are achieved by low T g to ensure sufficient
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chain mobility. Our study shows the highest Tg among all the references, together with very
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attractive healing rate and healing effciency. It is necessary to mention that the DCC concept is not
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only applicable to polyurethane. DCC based on other appropriate reversible bonds can be applied
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to various thermosets, such as epoxy, unsaturated polyester and polybenzoxazine etc.
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[Figure 8]
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High molecular weight polyureas with or without additional reagents (IPDI or DPP) are applied to understand the healing enhancement with DPP (Fig. 8 and Fig. S6). The reversibility of urea bond can be dominated by association-dissociation or exchange reaction, depending on the urea/isocyanate-end ratio. 45 Meanwhile, since the rates of the two reactions are quite different, variation in molecular weight takes place, which is perceived as viscosity change. Therefore, comparing the viscosity change rate can reveal the reactivity and benefit the comprehension of DPP in healing enhancement. In this study, no apparent change in viscosity is observed in all o
systems at 40 C, which implies the temperature is insufficient to initiate reactions. As temperature further increases to above 60 o C, viscosity of the solution decreases, which is attributed to the 13
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dissociation of the urea bonds and reaction takes place. Since the viscosity change is highly related to reaction degree, it is clear that increasing reaction temperature can accelerate the reaction rates and material containing DPP shows higher reactivity than IPDI and PUrea systems.
These results can be understood from their different reaction mechanisms (Fig. 9a). For PUrea system,
viscosity
decrease is
attributed
to
the reduction
of
molecular
weight
by 45
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association-dissociation of the urea bond. Since the rate constant of the dissociation is small
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while the moieties are still bonding to the large polymer chain, the reaction rate of PUrea system is
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quite mild.
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[Figure 9]
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Meanwhile, when IPDI is introduced (IPDI system), exchange reaction also carries out
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besides association-dissociation reaction. The mobility of small molecular IPDI is much higher
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than polymer chain and exchange reaction has lower energy barrier than the dissociation
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reaction. 45 These factors conspire to accelerate the reaction rate of IPDI system. DPP contains isocyanate moieties at the chain end as well. When the same molar amount is introduced, the reaction rate of DPP system would be similar to IPDI system if DPP is not dynamic. This is contradictory with the experimental results that reaction rate of DPP system is much higher. In other words, DPP is dynamic and can undergo chain segmentation, during which small molecular segments are generated and participate in the exchange reaction. The fragmentation of DPP is also confirmed by characterizing the extrudate of PU5 (Fig. 5b) through FT-IR (Fig. S7). The peaks at 1730 cm-1 and 1625 cm-1 are assigned to the urethane bond and urea bond, respectively. Comparing with PU5, the extrudate contains larger amount of urea bond and much less amount of 14
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urethane bond, which implies more DPP in the extrudate. Since DPP is covalently bonded to the network, without collapse into small free fragments, it will be fixed to the network and would not induce components difference in the extrudate. Overall, the advantages of DCC is the consequence of high segments diffusivity and more reactive moieties. These two factors contribute to the high healability to overcome the classical dilemma.
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Cartoon figures are further applied to facilitate understanding the different healing process of
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DCC and DCB (Fig. 9b). Conventional healing based on DCB requires diffusion and collision of
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the reactive groups at the chain end, which is highly determined by the polymer chain mobility.
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Comparing with small molecules, the movement of polymer chain is quite slow. Thus higher
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temperature, more flexible chain and longer healing time are required in order to obtain a tolerable healing rate and efficiency. This is probably one reason that many past researches are based on low
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T g materials. On the other hand, after DCC collapse, the generated small fragments have high
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mobility and reactivity to easily diffuse to the damaged region and perform healing by forming
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DCC again. The process is not restricted by polymer chains and cross-link density, thus higher healing rate and healing efficiency can be acquired. The endothermic curves of DSC prove that dynamic covalent chain undergoes reversible urea exchange reactions (Fig. S8)
Conclusions
We propose the dynamic covalent chain has better performance than dynamic covalent bond to develop healable polymers. Attributing to its capability to split into segments and recovery after healing, dynamic covalent chains show advantages over dynamic covalent bonds. As demonstrated 15
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in the polyurethane network, with higher dynamic covalent bond numbers, dynamic covalent chain offers much enhanced thermos-mechanical properties and healing rate/efficiency in polyurethane than dynamic covalent bond. In particular, with dynamic polyurea chain o
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incorporation, T g increases from 23 C to 80 C, which is the highest Tg according to the recent publication survey. Dynamic covalent chain can be a potential way to overcome the classical
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dilemma between high healability and material properties.
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Supporting Information
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DMA, TGA, FT-IR, DSC and tensile curves of polyurethane samples, FT-IR of extrudate and
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Acknowledgements
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viscosity evolution of polyurea are given in SI.
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This work was financially supported by the National Natural Science Foundation of China (51573167 and 51873195) and Zhejiang Provincial Natural Science Foundation of China under Grant No. LQ18E030004.
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Scheme 1. The chemical structures of PU, PU1, PU3 and PU5. The urea bonds prepared from
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isocyanate and piperazine are different in the polymers.
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Table 1. Thermal and mechanical properties of the polyurethanes. Sample
E’/MPa
T g /o Ca
T d10 /o Cb
PU
0.48
27
264
PU1
0.96
32
283
PU3
11.88
55
297
PU5
40.41
80
295
b
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Temperature of tanδ peak; Temperature at 10 wt.% loss.
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Figure 1. From left to right: PU1 polymer before and after healing; PU3 polymer before and after
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healing; PU5 polymer before and after healing.
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Figure 2. Fourier transform infrared spectroscopy of DPP and DPT.
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Figure 3. 1 H NMR spectra of DPT and DPP.
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Figure 4. Fourier transform infrared spectroscopy of PU, PU1, PU3 and PU5.
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Figure 5. Thermo-deformability and cut-healing capability of the polyurethanes. (a) Test set of
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thermo-deformability; (b) Appearance of polyurethanes after compression at elevated temperature;
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(c) Residue thickness of polyurethanes after compression deformation; (d) Healing efficiency of
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polyurethanes; (e) Appearances of original cut samples and samples after healing.
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Figure 6. Mechanical properties of polyurethanes without additive or with N-AEP and dynamic covalent chains. (a) Cut-heal-load cycles of polyurethanes at different positions; (b) Photographs of samples under tests; (c) Tensile strength and healing efficiency of pristine and tested samples; (d) Cut-heal-load cycles of polyurethanes at the same positions; (e) Photographs of samples under tests; (f) Tensile strength and healing efficiency of pristine and tested samples; (g) Shear adhesion strength test set; (h) Shear adhesion strength of samples after specific healing duration; (i) Shear adhesion strength of samples after different healing events. Tensile curves are summarized in Fig. 34
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S7 and S8.
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The
reference
No.
1-11
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polyurethanes.
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Figure 7. Research survey on the Tg of recent publications (2016-present) related to healable
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[(46)],[(58)]-[64],[66],[68],[(69)].
36
are
corresponding
to
references
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Figure 8. Relative viscosity changes of PUrea, IPDI and DPP systems at the temperature of (a) o
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temperatures.
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40 C, (b) 60 C and (c) 80 C; (d) The decrease rates of PUrea, IPDI and DPP systems vs.
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Figure 9. (a) Reaction mechanism of dynamic covalent chain in the polymer systems; (b) Difference of healing processes of polyurethanes with dynamic covalent bond or dynamic covalent chain.
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L. Zhang wrote the manuscript; H. Wang collected the main data in the work; Z. Dai conducted a part of the FT-IR and NMR tests; Z. Zhao conducted the viscosity test; F. Fu helped the experiment conduction and X. Liu designed the experiments and helped the data analysis.
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Journal Pre-proof Declaration of interests
☒ 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.
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may be considered as potential competing interests:
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☐The authors declare the following financial interests/personal relationships which
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High healing rate, healing efficiency and glass transition temperature can be achieved if a dynamic polyurea chain with more dynamic bond is introduced.
Dynamic chain segmentation facilitates functional terminal diffusion and exchange
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reaction, leading to higher reaction rate and healing efficiency.
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41
Lei Zhang, Haiqing Wang, Zenghui Dai, Zhongxiang Zhao, Feiya Fu, Xiangdong Liu PII:
S1381-5148(19)30846-6
DOI:
https://doi.org/10.1016/j.reactfunctpolym.2019.104444
Reference:
REACT 104444
To appear in:
Reactive and Functional Polymers
Received date:
15 August 2019
Revised date:
3 December 2019
Accepted date:
3 December 2019
Please cite this article as: L. Zhang, H. Wang, Z. Dai, et al., The dynamic chain effect on healing performance and thermo-mechanical properties of a polyurethane network, Reactive and Functional Polymers (2019), https://doi.org/10.1016/ j.reactfunctpolym.2019.104444
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© 2019 Published by Elsevier.
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The Dynamic Chain Effect on Healing Performance and Thermo-Mechanical Properties of a Polyurethane Network Lei Zhang, Haiqing Wang, Zenghui Dai, Zhongxiang Zhao, Feiya Fu, and Xiangdong Liu*
School of Materials Science and Engineering, Zhejiang Sci-Tech University, Xiasha Higher Education Zone, Hangzhou 310018, China.
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E-mail address: [email protected]
Abstract: Dynamic covalent chains that composed of three or five dynamic polyurea bonds are
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introduced into cross-linked polyurethane to overcome the classical dilemma between healing rate
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and thermo-mechanical properties. Comparing with dynamic urea bond, the incorporation of the
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dynamic polyurea chains with multiple leads to signific antly improved thermo-mechanical
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performance and healing rate/efficiency on the network. Remarkably, several folds’ increases in
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tensile and adhesion shear strength as well as 53 o C enhancement in glass transition temperature are achieved without sacrificing the material properties of the polyurethane network. We attribute the improvements to the decomposed polyurea chain segments, which have higher diffusivity and larger quantity than the dynamic urea bond that tied in the control network. This study provides a concept of dynamic covalent chain to address the classical dilemma between healing performances and thermo-mechanical properties among healable thermosets.
Keywords: Dynamic polyurea chain; Healing; Thermo-mechanical properties; Dilemma
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Introduction
Healable polymers have been intensively studied during the past decades to elongate the service life by preventing failure originated from defects growth.1-6 Up to date, healability can be objectified through extrinsic healing and intrins ic healing,
7-9
while the latter enables multiple
healing at the same position through dynamic linkages. 10-18 Various interactions have been applied 19-23
hydrogen bonding,
24-26
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for intrinsic healing so far, such as metal-ligand coordination,
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supramolecular affinity27-29 and dynamic covalent bonding, 30-32 which show attractive structure
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design flexibility.
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Among these interactions, healing based on dynamic covalent bond (DCB) is focused
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recently due to its simple preparation and possibility to offer acceptable mechanical properties and chemical resistance. Till now, several DCB species, including Diels -Alder cycloaddition, 33
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thiol-disulfide exchange, 34-37 disulfide exchange reaction, 38-39 transesterification, 40-43 amine-ketone
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exchange, 44 urea exchange, 45-49 olefin metathesis 50-52 and pinacol based C-C bond reversion, 53
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have been applied in various healable thermosets. Remarkably, regardless of the diverse reaction types, the reactive moieties at the ruptured surface require effective collision to perform the healing reactions, which is highly influenced by polymer chain mobility. 54 However, since DCB is welded in the polymer network, sacrificing chain rigidity or cross -link density becomes the final solution to achieve sufficient chain mobility. Obviously, this would cause severe reduction in glass transition temperature (T g ), mechanical modulus and strength. This is the dilemma that shadows the field of healable polymers. In fact, although new intrinsic healable thermosets are reported continuously, many of them are feeble to exhibit adequate mechanical strength. This is a very
2
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acute problem as a large amount of thermosets are utilized for structural applications. Polyurethane has been widely used as a thermoset. Although having received intensive focus, healable polyurethane remains stuck between healing and material properties. A solution to deal with the dilemma is urgently required.
Thermodynamically, material healing can be considered as a non-equilibrium to equilibrium 5
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process, which is driven by entropy increase. Polymer chains with high flexibility possess more
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conformations, thus the entropy is higher. 55 Meanwhile, reptation model suggests that the healing
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time is determined by the duration of conformation changes to the Gaussian state, during which
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structures with smaller molecular weight are preferred. 54,56 Based on these previous studies, this
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manuscript proposes a covalent chain containing multiple dynamic bonds, named as dynamic covalent chain (DCC). Although both DCB and DCC are based on reversible chemical reaction,
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DCC is less restricted by polymer chain mobility. DCC can split into many highly reactive and
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mobile fragments to diffuse and split at the ruptured surface, resulting in new DCC and healing. In
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contrast, DCB is linked to the polymer network with one end. Confined by the bulk chains, its mobility is suppressed, which further causes reactivity and healing rate reduction. This is the reason that most healable polymers based on DCB are developed by sacrificing material properties. In addition, during our submission period, a DCC structure was published and applied for material healing. Unfortunately, neither the advantages of DCC over DCB, nor the underlying mechanisms are not concerned. This study provides a systematic understanding of DCC and studies the effect of the numbers of dynamic covalent bonds on DCC to the healing performance. Importantly, DCC is not limited to polyurethane. As corresponding DCC is synthesized and introduced, it can also help other healable polymers get rid of the dilemma. 3
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Experimental
Materials. Isophorone diisocyanate (IPDI, 99%), hexamethylene diisocyanate (HDI, 99%), piperazine anhydrous (PA, 99%), N-aminoethylpiperazine (N-AEP, 99%), dibutyltin dilaurate (DBTDL, 99%), pentaerythritol (97%), dibutylamine (99%) and bromocresol green (99.9%) were
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purchased from Aladdin Reagent (Shanghai) Co.,Ltd. and used as received. Polyether polyol (HSH305, hydroxyl value: 320-355 mg KOH/g) was obtained from Haian Petrochemical and
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dehydrated by distillation under reduced pressure. N,N-dimethylformamide (DMF, 99%) was
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provided by Hangzhou Mike Chemical Agents Co., Ltd. and dehydrated by molecular sieve (4 Å).
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1-methylpiperazine (99%) was purchased from Meryer Chemical Technology Co., Ltd.
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Synthesis of dynamic covalent polyurea trimer (DPT) and pentamer (DPP) chains. Synthesis
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of DPT and DPP follows the reaction in Scheme S1. Specifically, 0.86g (10 mmol) of PA in 15 mL
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of chloroform was slowly introduced in 30 mL of chloroform containing 4.44g (20 mmol) of IPDI and stirred under ice-water bath for 2h. The solution was later evaporated until turbid followed by adding 40mL ethyl acetate to generate white precipitates, filtered and dried under 80℃ vacuum for 12h to obtain 5.04g of crude DPT. Yield=95%. The content of isocyanate in polyurea trimer was confirmed by dibutylamine titration. (Found 13.6 wt.%; Calc. 15.8 wt.%). DPP was prepared in a similar except changing the molar ratio of PA and IPDI to 2:3. Yield=96%. Isocynate content: Found 8.5 wt.%, Calc. 10 wt.%.
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Preparation of cross-linked polyurethanes. Cross-linked polyurethane PU, PU3 and PU5 based on different dynamic structures are sketched in Scheme S2. The specific preparation methods are listed as below.
Preparation of PU: Pentaerythritol (3.40g, 2.5 mmol), IPDI (1.11g, 5 mmol), HDI (1.68g, 10 mmol), HSH305 (3.40g, 20 mmol hydroxyl) and a drop of DBTDL (catalyst) were dissolved in
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8mL of DMF successively. The solution was put into a mold (120mm×80mm) at room
oo
temperature for 0.5h under vacuum, followed by treating at 60℃/2h, 90℃/10h and additional 4h
pr
under vacuum. A pale yellow and transparent polymer was obtained.Preparation of PU3: PU3
e-
was prepared in a similar way as PU except further introducing DPT (3.23g, 10.5 mmol isocyanate
Pr
moiety), appeared as a pale yellow and transparent polymer.
Preparation of PU5: PU5 was prepared in a similar way as PU except further introducing DPP
rn
al
(5.25g, 10.5 mmol isocyanate moiety), appeared as a pale yellow and transparent polymer.
Jo u
Preparation of PU1: PU1 was prepared in a two-step method (Scheme S3) to prevent gelation before molding. N-aminoethyl piperazine with 1 equiv. amount was introduced in the second step and a pale yellow transparent polymer was obtained.
[Figure 1]
The highly reversible bonds between isocyanate and piperazine within the polyurethanes are in ratio: PU/PU1/PU3/PU5=0/1/2/4.
Characterization
5
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FT-IR spectra were recorded by a Nicolet 5700 Fourier transform infrared spectrometer (FT-IR, Nicolet, USA). 1 H NMR spectra were obtained by an Avance AV-400 Bruker 400 MHz nuclear magnetic resonance (NMR) spectrometer in deuterated chloroform (CDCl3 ). DSC thermograms were obtained
from
a DSC-1
differential scanning
Mettler-Toledo, Switzerland) at a heating rate of 10 K·min
-1
calorimeter
(DSC,
under nitrogen flow rate of 45 mL
-1
min and sample weight around 5 mg. Thermal weight loss was measured by a thermogravimetric
oo
f
analyzer (TGA, Mettler-Toledo, Switzerland) at a heating rate of 10 K·min -1 under nitrogen flow
pr
rate of 45 mL min-1 and sample weight around 10 mg. Dynamic mechanical analys is (DMA) was
e-
conducted on DMA1 (Mettler-Toledo, Switzerland) using shear mode under a heating rate of 3 K·min from -15 to 200 ℃ at a frequency of 1 Hz in nitrogen. The specimen feature was 10 x 10 x
Pr
-1
1.2 mm.
al
Cut healing efficiency: Samples were completely separated by a clean razor blade cut. The two
rn
fragments were put together, placed between two glass slides and fixed with clips. They were
Jo u
placed in a 120 ℃ oven and the healing process was monitored by an Olympus optical microscope. The variation in width was compared with the initial values at t=0 min. The apparent healing efficiency can be calculated
Tensile test: Samples (60 x 10 x 1 mm) were fully cut before healing at 120 ℃ for various times. After complete cooling, these samples were tested on a Instron 3367 tensile machine. Three samples were tested for every material to check the reproducibility.
Lap-shear test: Test samples were prepared by placing cured polyurethane (20 x 15 x 1 mm) between aluminum plates (100 x 20 x 2mm) with the same overlap area. They were placed in a 6
Journal Pre-proof 120 ℃ oven for various times before cooling and testing on a Heson shear tester with a 5 mm·min-1 stretch rate. After test, the separated fragments were carefully placed as origin, healed and re-tested to show the multiple healing capability.
Thermal deformation: Samples (Φ20 x 3 mm) were sandwiched between a coin (Φ20.5 x 1.7 mm) and a perforated metal (Φin 4mm, Φout 20 mm, thickness 1 mm) and fixed before thermally o
f
treated at 120 C for 10 minutes. Same clips are applied to ensure identical forces during healing.
oo
After cooling, the extrusive polymer volume and the sample thickness are checked and compared.
pr
Viscosity: High molecular weight polyurea is dissolved in DMF solvent with a weight percentage
e-
of 14.29% to facilitate viscosity measurement and addition of isocyanates. Two bottles, each
Pr
containing 6g (857 mg of high molecular weight of polyurea) of the DMF solution are marked as A1 and A2. Meanwhile, IPDI (62 mg) and DPP (292 mg) are mixed with 2 g of the above solution,
al
respectively, marked as B1 and B2. The samples of same numbers are mixed together and tested
o
o
rn
under a Physica MCR 301 Senior rotary rheometer with a constant shear rate of 100 s -1 at 40o C,
Jo u
60 C and 80 C, respectively. For comparison, blank sample prepared by mixing 6 g of DMF solution with 2 g of DMF solution is tested by the same method as well. The evolution in viscosity is recorded for analysis.
Results and Discussions
Thermo-deformability and cut-healing capability
[Figure 2]
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Dynamic covalent polyurea trimer (DPT) and pentamer (DPP) chains were prepared from isocyanate and piperazine by generating urea bonds. The chemical structures were first verified by FT-IR spectra, as shown in Fig. 2. The peaks near 2900 cm-1 confirm the methyl and methylene -1
groups in DPT and DPP. The peaks at 1533 cm indicate the vibration of C-N bond. The peaks at -1
3359 and 1625 cm are assigned to the vibration of N-H and C=O structures, which prove the -1
formation of urea bond. They are not urethane bonds due to the fact that peak around 1730 cm is
oo
f
not found. Meanwhile, strong peak at 2250 cm-1 is corresponding to the isocyanate, which existing
pr
at the chain end. After normalizing by peak at 1625 cm -1 , the difference in transmittance intensity
e-
at 2250 cm-1 can be found. DPT shows higher peak intensity than DPP, which is corresponding to
Pr
its higher isocyanate/urea ratio. Thus, the molecular weight of DPT is smaller.
[Figure 3]
rn
al
To further confirm the oligomer structures and calculate the molecular weight, 1 H NMR spectra are involved (Fig. 3). The resonance peak at 3.43 ppm is assigned to the protons of
Jo u
piperazine while the peaks at 1.08 and 0.96 ppm are corresponding to the protons of the methyl groups in IPDI. The peaks within the range of 1.15 ppm and 2.25 ppm are assigned to the protons of the isophorone rings while the peak around 3.0 ppm is corresponding to the protons of the alkyl groups in isophorone adjacent to nitrogen. By proton integration, the repeat unit n (in Scheme S1) of DPT is 0.92 (calc. 1.0) and DPP is 2.31 (cal. 2.0), which are close to the theoretical values. Ying et al.
45
had already proved that urea bonds prepared from piperazine derivatives and
isocyanate are dynamic at low temperature. Containing multiple urea bond with similar structures, it is expected that DPT and DPP are reversible, which means that the oligomer can rupture at
8
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elevated temperature and reform after cooling. When incorporated in polyurethanes, their characteristics enable the materials to be healable and deformable. To verify this, polyurethane PU3 (containing DPT) and PU5 (containing DPP) were developed. Meanwhile, PU1 (containing N-aminoethyl piperazine) and PU (without any urea structure) were prepared for comparison. The chemical structures are summarized in Scheme 1 and the dynamic urea bond is bolded. All these
f
materials were fabricated based on the description in the Experimental section. The chemical
oo
structures of the polyurethanes were characterized by FT-IR (Fig. 4). For all prepared
pr
polyurethane polymers, no residue peak is found at 2250 cm-1, which indicates the complete
e-
consumption of isocyanate group. The generation of urethane bond is verified by the intensive -1
-1
Pr
peaks at 1730 cm . The peaks at 1100 cm signify the C-O-C bond, which can be another evidence to prove the formation of urethane bond. DPP and DPT are successfully incorporated
al
into the network, which are proved by the high peaks at 1625 cm -1 of PU3 and PU5. The
rn
cross-linked characteristics of the polyurethanes are proved by introducing PU5 (as an example)
Jo u
into DMF solvent and kept for 24 h. The material was swelled but did not show any dissolution (Fig. S1). This was also evidenced by the rubbery plateau showed in DMA curves.
[Figure 4]
Thermo-deformability gives a preliminary view on the diverse reversible reaction rates within the materials, which was performed by sandwiching the polyurethane samples between metal plates (Fig. 5a). After heating at elevated temperature, more polyurethane is squeezed out in PU5 than PU3 and PU1 (Fig. 5b), while no apparent extrudate is observed in PU sample. Meanwhile, reduction in sample thickness takes place simultaneous ly (Fig. 5c). PU sample almost maintains
9
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its thickness after the heating treatment. PU1 shows 19% reduction of its original thickness. With N-AEP, PU1 contains dynamic urea bond, which facilitate macroscopic deformation. Remarkably, when dynamic covalent polyurea chains are added, 24% (PU3) and 34% (PU5) decrease in thickness are found. The results provide perceptual knowledge that the dynamic covalent chains have advantage over dynamic covalent bonds in promoting chain mobility.
pr
[Figure 5]
oo
[Table 1]
f
[Scheme 1]
e-
To illustrate the influence of chain mobility on healing, cut polyurethane samples are placed
Pr
with same separated width (Fig. 5d and 5e). The healing efficiency is calculated based on residue width (Fig. 5d). After 30 min, PU and PU1 show reduction in cut width though the residue gap is
al
still conspicuous. Meanwhile, almost complete healing can be found in PU3 and PU5. In fact,
rn
more obvious distinction is observed at the initial healing stage. When PU3 and PU5 already show
Jo u
healing efficienc ies above 80%, the healing efficiency PU and PU1 remain below 50%. It is worth to note that, since the T g of PU is quite low (23o C) (Table 1 and Fig. S2), chain diffusion in PU should be easier for physical chain entanglement at the crack interface, shown as cut-healing. However, such healing is not sufficient. 90% recovery at equilibrium is observed in PU while completely healing is found in PU3 and PU5 (Fig. 5d). Meanwhile, with DCB and DCC, T g s of the samples become higher, especially when DCC is introduced. Specifically, incorporating DPT results in 28o C increase in T g . As DCC becomes longer, further enhancement in Tg is found, as o
evidenced by the high T g of PU5 (80 C). When DCC is incorporated, improvements in tensile modulus and thermal stability than dynamic covalent bonds (Table 1). The storage moduli of PU3 10
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and PU5, (11.9 MPa and 40.4 MPa), which are orders of magnitude larger than PU1 (1.0 MPa) and PU (0.5 MPa). The thermal decomposition temperatures T d10 of PU3 (297 o C) and PU5 (295 o
C) are also higher than PU1 (283 o C) and PU (264 o C) (Table 1 and Fig. S3). These results
confirm that incorporating DCC enables higher healing efficiency and T g than DCB, which seems to overcome the classical dilemma. This encourages us to perform further in-depth investigation.
f
It is worth to mention that healing based on visualizing surface recovery provides only an
oo
intuitive view on healing efficiency. In fact, even though the material was optically healed, the
pr
properties of the healed part can be quite different from the origin. In this study, mechanical
e-
healing is evaluated by cyclic cut-heal tests with different cut positions (Fig. 6a and 6b). Since
Pr
every part of the sample suffers equivalent stress, tensile test is ideal to evaluate the healing quality inside the material. Herein, healing efficiencies of all samples are evaluated by comparing
al
the original tensile strength variation (Fig. 6c and Fig. S4). Polyurethanes containing DCC (PU3
rn
and PU5) are able to sustain a 500 g loading even after three cycles (Fig. 6b). Polyurethanes with
Jo u
DCB (PU1) can only hold a 100 g loading while PU cannot even withstand a 100 g loading (not included in the figure). Remarkably, pristine polyurethanes with DCC have tensile strengths of 5.4 MPa (PU5) and 4.9 MPa (PU3), which are several folds larger than PU1 (1.2 MPa) and PU (0. 8 MPa). The tensile strengths are smaller than conventional PU due to the usage of chemicals containing flexible chains like hexamethylene diisocyanate and polyether polyol. Even after three cycles’ cut-heal tests, the remaining strength of PU5 (2.9 MPa) and PU3 (2.0 MPa) are still larger than pristine PU1 and PU. The healing efficiencies of PU5 are also (94%/1 cycle, 54%/3 cycles) better than PU1 (57% /1 cycle, 53%/3 cycles) and PU (29%/1 cycle, 19%/3 cycles). Furthermore, cyclic cut-heal is performed at the same cut position to show multiple healing capability (Fig. 6d). 11
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Load capability, tensile strength and healing efficiency (Fig. 6e, 6f and Fig. S5) similar to former experiments were achieved. Therefore, healing performance depends on the healing cycles while is independent from the healing positions of the samples. [Figure 6] Adhes ion is not only related to the material itself, but also highly influenced by the interface
f
quality. Herein, polyurethanes were placed in between aluminum plates for measuring adhesion
oo
strength (Fig. 6g). The healing rates were compared based on the shear strength after diverse
pr
healing durations (Fig. 6h). PU5 and PU3 containing DCC reach their maximum adhesion
e-
strengths after healing for 25 min and 40 min while PU1 still not reach equilibrium up to 60 min.
Pr
The adhesion strengths at equilibrium are 1.9 ± 0.3 MPa (PU5) and 1.1 ± 0.1 MPa (PU3), respectively. Meanwhile, the maximum adhesion strength of PU1 is 0.9±0.1 MPa (at 60 min). PU
al
has no shear strength even though the healing time is elongated up to 60 min. Overall, the results
rn
evidence that higher adhesion strength, healing rate and healing efficiency are obtained when DCC
Jo u
is incorporated, especially if the DCC chain is longer. Long DCC structure has greater rheological behavior, which increases the actual bonding area and eliminates interface gaps between polymer and aluminum plates (Fig. 6g). Based on numerous urea bonds, DCC can collapse into small fragments to increase wettability and interaction with aluminum plate. On the other hand, healing of PU sample greatly depends on the chain diffusion and physical entanglements. However, it is hard to fill the interface voids and even impossible to generate physical entanglements between polymer and aluminum plate. In addition, cyclic shear-heal tests prove that polyurethanes with DCC almost maintains the adhesion strength even after 10 cycles (Fig. 6i). The adhesion strength of PU5 after 10 cycles is 1.5±0.1 MPa while PU1 is only 1.0±0.05 MPa. Consequently, cyclic 12
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adhesion tests also evidence the capability of DCC to overcome the classical dilemma between material properties and healing performances.
[Figure 7]
We also carried out a survey of recent publications (from 2016 to present) to show the advantages of DCC. To ensure comparability in polymer species, referential materials based on
oo
f
polyurethane with dynamic structures are summarized (Fig. 7). Although many of them have been reported with high healing rate and efficiency, they are achieved by low T g to ensure sufficient
pr
chain mobility. Our study shows the highest Tg among all the references, together with very
e-
attractive healing rate and healing effciency. It is necessary to mention that the DCC concept is not
Pr
only applicable to polyurethane. DCC based on other appropriate reversible bonds can be applied
al
to various thermosets, such as epoxy, unsaturated polyester and polybenzoxazine etc.
rn
[Figure 8]
Jo u
High molecular weight polyureas with or without additional reagents (IPDI or DPP) are applied to understand the healing enhancement with DPP (Fig. 8 and Fig. S6). The reversibility of urea bond can be dominated by association-dissociation or exchange reaction, depending on the urea/isocyanate-end ratio. 45 Meanwhile, since the rates of the two reactions are quite different, variation in molecular weight takes place, which is perceived as viscosity change. Therefore, comparing the viscosity change rate can reveal the reactivity and benefit the comprehension of DPP in healing enhancement. In this study, no apparent change in viscosity is observed in all o
systems at 40 C, which implies the temperature is insufficient to initiate reactions. As temperature further increases to above 60 o C, viscosity of the solution decreases, which is attributed to the 13
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dissociation of the urea bonds and reaction takes place. Since the viscosity change is highly related to reaction degree, it is clear that increasing reaction temperature can accelerate the reaction rates and material containing DPP shows higher reactivity than IPDI and PUrea systems.
These results can be understood from their different reaction mechanisms (Fig. 9a). For PUrea system,
viscosity
decrease is
attributed
to
the reduction
of
molecular
weight
by 45
f
association-dissociation of the urea bond. Since the rate constant of the dissociation is small
oo
while the moieties are still bonding to the large polymer chain, the reaction rate of PUrea system is
pr
quite mild.
e-
[Figure 9]
Pr
Meanwhile, when IPDI is introduced (IPDI system), exchange reaction also carries out
al
besides association-dissociation reaction. The mobility of small molecular IPDI is much higher
rn
than polymer chain and exchange reaction has lower energy barrier than the dissociation
Jo u
reaction. 45 These factors conspire to accelerate the reaction rate of IPDI system. DPP contains isocyanate moieties at the chain end as well. When the same molar amount is introduced, the reaction rate of DPP system would be similar to IPDI system if DPP is not dynamic. This is contradictory with the experimental results that reaction rate of DPP system is much higher. In other words, DPP is dynamic and can undergo chain segmentation, during which small molecular segments are generated and participate in the exchange reaction. The fragmentation of DPP is also confirmed by characterizing the extrudate of PU5 (Fig. 5b) through FT-IR (Fig. S7). The peaks at 1730 cm-1 and 1625 cm-1 are assigned to the urethane bond and urea bond, respectively. Comparing with PU5, the extrudate contains larger amount of urea bond and much less amount of 14
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urethane bond, which implies more DPP in the extrudate. Since DPP is covalently bonded to the network, without collapse into small free fragments, it will be fixed to the network and would not induce components difference in the extrudate. Overall, the advantages of DCC is the consequence of high segments diffusivity and more reactive moieties. These two factors contribute to the high healability to overcome the classical dilemma.
f
Cartoon figures are further applied to facilitate understanding the different healing process of
oo
DCC and DCB (Fig. 9b). Conventional healing based on DCB requires diffusion and collision of
pr
the reactive groups at the chain end, which is highly determined by the polymer chain mobility.
e-
Comparing with small molecules, the movement of polymer chain is quite slow. Thus higher
Pr
temperature, more flexible chain and longer healing time are required in order to obtain a tolerable healing rate and efficiency. This is probably one reason that many past researches are based on low
al
T g materials. On the other hand, after DCC collapse, the generated small fragments have high
rn
mobility and reactivity to easily diffuse to the damaged region and perform healing by forming
Jo u
DCC again. The process is not restricted by polymer chains and cross-link density, thus higher healing rate and healing efficiency can be acquired. The endothermic curves of DSC prove that dynamic covalent chain undergoes reversible urea exchange reactions (Fig. S8)
Conclusions
We propose the dynamic covalent chain has better performance than dynamic covalent bond to develop healable polymers. Attributing to its capability to split into segments and recovery after healing, dynamic covalent chains show advantages over dynamic covalent bonds. As demonstrated 15
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in the polyurethane network, with higher dynamic covalent bond numbers, dynamic covalent chain offers much enhanced thermos-mechanical properties and healing rate/efficiency in polyurethane than dynamic covalent bond. In particular, with dynamic polyurea chain o
o
incorporation, T g increases from 23 C to 80 C, which is the highest Tg according to the recent publication survey. Dynamic covalent chain can be a potential way to overcome the classical
oo
f
dilemma between high healability and material properties.
pr
Supporting Information
e-
DMA, TGA, FT-IR, DSC and tensile curves of polyurethane samples, FT-IR of extrudate and
rn
Acknowledgements
al
Pr
viscosity evolution of polyurea are given in SI.
Jo u
This work was financially supported by the National Natural Science Foundation of China (51573167 and 51873195) and Zhejiang Provincial Natural Science Foundation of China under Grant No. LQ18E030004.
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Scheme 1. The chemical structures of PU, PU1, PU3 and PU5. The urea bonds prepared from
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isocyanate and piperazine are different in the polymers.
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Table 1. Thermal and mechanical properties of the polyurethanes. Sample
E’/MPa
T g /o Ca
T d10 /o Cb
PU
0.48
27
264
PU1
0.96
32
283
PU3
11.88
55
297
PU5
40.41
80
295
b
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Temperature of tanδ peak; Temperature at 10 wt.% loss.
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Figure 1. From left to right: PU1 polymer before and after healing; PU3 polymer before and after
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healing; PU5 polymer before and after healing.
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Figure 2. Fourier transform infrared spectroscopy of DPP and DPT.
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Figure 3. 1 H NMR spectra of DPT and DPP.
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Figure 4. Fourier transform infrared spectroscopy of PU, PU1, PU3 and PU5.
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Figure 5. Thermo-deformability and cut-healing capability of the polyurethanes. (a) Test set of
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thermo-deformability; (b) Appearance of polyurethanes after compression at elevated temperature;
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(c) Residue thickness of polyurethanes after compression deformation; (d) Healing efficiency of
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polyurethanes; (e) Appearances of original cut samples and samples after healing.
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Figure 6. Mechanical properties of polyurethanes without additive or with N-AEP and dynamic covalent chains. (a) Cut-heal-load cycles of polyurethanes at different positions; (b) Photographs of samples under tests; (c) Tensile strength and healing efficiency of pristine and tested samples; (d) Cut-heal-load cycles of polyurethanes at the same positions; (e) Photographs of samples under tests; (f) Tensile strength and healing efficiency of pristine and tested samples; (g) Shear adhesion strength test set; (h) Shear adhesion strength of samples after specific healing duration; (i) Shear adhesion strength of samples after different healing events. Tensile curves are summarized in Fig. 34
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S7 and S8.
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The
reference
No.
1-11
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polyurethanes.
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Figure 7. Research survey on the Tg of recent publications (2016-present) related to healable
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[(46)],[(58)]-[64],[66],[68],[(69)].
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are
corresponding
to
references
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Figure 8. Relative viscosity changes of PUrea, IPDI and DPP systems at the temperature of (a) o
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temperatures.
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40 C, (b) 60 C and (c) 80 C; (d) The decrease rates of PUrea, IPDI and DPP systems vs.
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Figure 9. (a) Reaction mechanism of dynamic covalent chain in the polymer systems; (b) Difference of healing processes of polyurethanes with dynamic covalent bond or dynamic covalent chain.
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Journal Pre-proof Author Statement
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L. Zhang wrote the manuscript; H. Wang collected the main data in the work; Z. Dai conducted a part of the FT-IR and NMR tests; Z. Zhao conducted the viscosity test; F. Fu helped the experiment conduction and X. Liu designed the experiments and helped the data analysis.
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Journal Pre-proof Declaration of interests
☒ 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.
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may be considered as potential competing interests:
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☐The authors declare the following financial interests/personal relationships which
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Journal Pre-proof Highlights
High healing rate, healing efficiency and glass transition temperature can be achieved if a dynamic polyurea chain with more dynamic bond is introduced.
Dynamic chain segmentation facilitates functional terminal diffusion and exchange
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reaction, leading to higher reaction rate and healing efficiency.
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