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Solvent-free thermo-reversible and self-healable crosslinked polyurethane with dynamic covalent networks based on phenol-carbamate bonds
Polymer 181 (2019) 121788
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Solvent-free thermo-reversible and self-healable crosslinked polyurethane with dynamic covalent networks based on phenol-carbamate bonds
T
Jiaxin Shi, Tianze Zheng, Baohua Guo, Jun Xu∗ Advanced Materials Laboratory of Ministry of Education (MOE), Department of Chemical Engineering, Tsinghua University, Beijing, 100084, China
H I GH L IG H T S
phenol-carbamate bonds synthesized from bisphenol-S and diisocyanate are dynamic covalent bonds. • The parameters of phenol-carbamate bond association and activation energy of bond exchange are calculated. • Thermodynamic • The polyurethane with dynamic phenol-carbamate bonds demonstrates excellent self-healing and reprocessing properties.
A R T I C LE I N FO
A B S T R A C T
Keywords: Polyurethane Dynamic covalent network Self-healing
Polyurethanes with dynamic covalent networks have the potential of self-healing, reprocessing and recycling, however, how to improve the efficiency of self-healing and reprocessing, and reduce the use of solvent and costs, are still problems to solve. In this work, we synthesized a cross-linked polyurethane containing dynamic phenolcarbamate bonds from low-cost raw materials without use of solvent. The dynamic polyurethane possesses excellent self-healing and reprocessing properties, which exhibits low relaxation activation energy (92.7 kJ/mol) and can be almost completely healed at 80 °C for 2 h. After remolded for 4 times, the material shows almost little change in mechanical properties. Further studies on the phenol-carbamate bonds give the quantitative reaction equilibrium constants, thermodynamic parameters and dissociation activation energy. In viewpoint of the dynamic properties and easy synthesis of phenol-carbamates, various types of cross-linked polymers containing such dynamic bonds can be further tailor designed.
1. Introduction As a family of widely used polymeric materials, polyurethanes (PUs) play an important role in industrial applications and daily life. The properties of PUs can be tuned in a wide range using different types of polyols and isocyanates. Urethane groups with the ability to form hydrogen bonds act as the physical cross-linkages in the material [1], but the low modulus and creep of linear PUs limits their applications. Chemical cross-linking can improve mechanical properties and thermostability of PUs, however, the traditional chemically cross-linked polymers are difficult to recycle and reprocess, and their forming processes are limited [2,3]. Meanwhile, due to the increasingly serious resource and environmental problems, self-healable, thermo-reversible and recyclable cross-linked polymers are receiving widespread attention. Among them, the cross-linked polymers with dynamic covalent bonds show the advantages of self-healing and ease of reprocessing. The network structure of the cross-linked polymers based on
∗
dynamic covalent bonds can be divided into two types: dissociative and associative dynamic covalent networks (DCNs) [2,4]. Associative DCNs maintain the crosslinking degree at elevated temperatures, but the chemical bond exchange rate increases. These DCNs exhibit permanent cross-linking and significant viscoelastic behavior at high temperatures. Typical examples are transesterification DCNs, first prepared by Leibler et al. and named “vitrimers” [5–10]. In addition, dynamic urethanes catalyzed by dibutyltin dilaurate (DBTDL) [11–13], disulfides [14–20] and anion-catalyzed dynamic siloxane [21,22] have been utilized to build associative DCNs. Although polymers containing associative DCNs can be remolded at solid to semi-solid state in a wide processing temperature window, the degree of crosslinking does not change much, causing insufficient fluidity during processing and thus rough surface of the products. In contrast, for dissociative DCNs, the initial dynamic covalent bonds dissociate and exchange to form new network structure, so the crosslinking degree changes during the heating process and the materials can be remolded in solid, semi-solid and liquid state. Diels-
Corresponding author. E-mail address: [email protected] (J. Xu).
https://doi.org/10.1016/j.polymer.2019.121788 Received 13 July 2019; Received in revised form 3 September 2019; Accepted 8 September 2019 Available online 09 September 2019 0032-3861/ © 2019 Published by Elsevier Ltd.
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Scheme 1. Synthesis of Phen-CPU.
Fig. 1. FTIR spectra of 3HDI and Phen-CPU at room temperature.
Fig. 2. DSC heating curve of Phen-CPU after stored at 0 °C for 2 h.
Alder adducts, usually produced by the reaction between furan and maleimide, are a kind of widely adopted dissociative DCNs [23–26]. However, the synthesis becomes more complex when furan and maleimide groups are incorporated. In addition, maleimide and furan groups are prone to oxidation, causing the polymers turn into darker color, typically from yellow to brown. Recently, another type of reaction for constructing dissociative DCNs, the reversible addition of isocyanates and active hydrogen compounds, has received extensive attention. Such dynamic covalent bonds include hindered urea [27–30] and oxime-urethane [31–33], among which the reversible structure of the polyurethane/polyurea will inject new vitality into the general polyurethane and polyurea materials. As isocyanate blocking agents, phenolic compounds have a long history of application. The produced phenol-carbamates are stable at room temperatures and can dissociate at elevated temperatures to regenerate free isocyanate groups [34]. Based on this concept, phenolcarbamates can be incorporated into PUs to construct dissociative DCNs. According to the blocked isocyanate chemistry, both the type of isocyanate and the electronic effect of the substituent on the benzene ring affect the dissociation temperature of the phenol-carbamates. As for phenolic compounds, the stronger the electron-withdrawing effect of the substituent on the benzene ring is, the lower the dissociation temperature is [34]。 Since the reaction of phenol with isocyanate is slow, it is necessary to add a catalyst to accelerate the reaction. DBTDL is a frequently used catalyst, but it strongly catalyzes the side reaction of isocyanate at high
Fig. 3. Storage modulus and dissipation factor tan δ of Phen-CPU measured by DMA from 25 °C to 100 °C, at a heating rate of 3 °C min−1 and a frequency of 1 Hz. The samples were stored at room temperature rather than 0 °C to prevent crystallization of PEG moiety.
temperature, resulting in permanent crosslinking [35]. Meanwhile, the dissociation temperature has a positive correlation with the reactivity of the isocyanate [36]. Therefore, in order to depress the side reaction, phenol, isocyanate and catalyst should be carefully selected in the phenol-carbamate DCNs so that the properties of the material can remain stable in multiple reprocesses. Cao et al. [37] used poly(tetramethylene ether glycol) (PTMEG), 1,2,3-trichlorobenzene and toluene2
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avoid problems caused by DBTDL. As for BPS, the sulfone group between the two benzene rings has a strong electron-withdrawing ability, which further lowers the carbamate dissociation temperature, improves the self-healing rate and reduces the temperature of reprocessing. The results show that Phen-CPU has an excellent self-healing property and can achieve almost complete recovery of mechanical properties by remolding at 100 °C, 5 MPa for only 1 min. Further studies reveal the mechanism that BPS-HDI adducts can be exchanged or dissociated at moderate temperatures. Furthermore, the equilibrium constants at different temperatures and the activation energy of bond dissociation are calculated. 2. Experimental section 2.1. Materials Polyethylene glycol (PEG Mn = 2000, for synthesis) was provided by Shanghai Dongda Chemistry Co., Ltd. Hexamethylene diisocyanate (HDI, 99%), bisphenol S (BPS, 98%), 1,4-Diazabicyclo[2.2.2]octane (DABCO, 99%), 1,2-propanediol (99%) were purchased from AdmasBeta. HDI trimer (Wannate HT100) with 21.8 wt % NCO content was provided by Wannate Chemistry Co., Ltd. and used as received. PEG was dried under high vacuum at 120 °C for 2 h in a vacuum oven before used. 2.2. Instrumentation Differential scanning calorimetry (DSC) measurement was conducted on a Shimadzu TA 60-WS with a heating and cooling rate of 10 °C/min under nitrogen atmosphere. Dynamic mechanical analysis (DMA) was carried out on an Anton Paar MCR301 with stress/strain controlled rheometer in a torsional mode at temperatures ranging from 25 to 100 °C with a heating rate of 3 °C/min. The maximum strain was 0.01% and the frequency was 1 Hz. Stress-relaxation curves were also obtained using Anton Paar MCR301 in a parallel-plate mode. Circular specimens with diameter of 25 mm were punched out of the sample sheets. Each sample was first equilibrated at a set temperature for 15 min, then it was initially compressed by a strain of 5% and the strain was maintained. Tensile tests were conducted on a JinJian UTM-1432 tensile tester equipped with a 500 N load cell. The samples were directly stretched to failure at a constant cross-head speed of 50 mm/min. Dumbbell-shaped specimens were cut from cast sheets using a standard bench-top die according to GB/T-528. Temperature variable Fourier transform infrared (FTIR) spectra were collected on a Nicolet 6700 FT-IR spectrophotometer equipped with a deuterated triglycine sulfate detector and a hot stage. To prepare the IR samples, the uncured reactants mixture was cast onto a KBr plate to obtain a thin film, on top of which a second KBr plate was placed. Then the KBr sandwiched sample was cured in an oven at 60 °C for 24 h. The transmission spectra were collected with an average of 16 scans for each run at a resolution of 4 cm−1 in the range of 4000−500 cm−1 after the sandwiched sample was equilibrated at the desired temperature for 5 min. The materials were reprocessed on a lab press instrument (Carver Model C laboratory press) at 100 °C under 5 MPa for 1 min. After remolded, the sample was annealed at 60 °C for 3 h, 40 °C for 3 h and then at room temperature for 3 h in an oven with desiccant to fully equilibriate the reaction of phenol-carbamates. Optical micrographs were obtained with Olympus BX41P (Japan) microscope equipped with a Linkam T95-PE temperature controller (England).
Fig. 4. (a) Variable temperature FTIR spectroscopy of Phen-CPU (b) Van't Hoff plot for Phen-CPU.
2,4-diisocyanate (TDI) to synthesize cross-linked PUs containing phenol-carbamate under DBTDL catalysis. It was found that phenolcarbamates could dissociate a small amount of free isocyanate at a really high temperature so as to achieve self-healability, but its initial dissociating temperature was a bit high, above 120 °C. In addition, the infrared spectrum showed that the isocyanate peak intensity rose first and then decreased during the heating process, illustrating that significant side reactions occurred, making it difficult to achieve complete reprocessing and self-healing ability. In this work, we report a solvent-free, low-cost, reprocessable and self-healable cross-linked polyurethane with reversible covalent bond phenol-carbamates (Phen-CPU). Phen-CPU was produced from low-cost industrial feedstocks: polyethylene glycol (PEG) as a polyol, hexamethylene diisocyanate (HDI) as a diisocyanate, bisphenol S (BPS) as a chain extender, HDI trimer (3HDI) as a crosslinker, and 1,4-diazabicyclo [2.2.2] octane (DABCO) as a catalyst. PEG can dissolve the other components, thereby avoids the use of solvents and improves environmental friendliness. At the same time, low-molecular weight PEG has a reduced ability to crystallize in the highly cross-linked polymers, which helps the material to maintain elasticity at room temperature. HDI and 3-HDI, as aliphatic isocyanates, show good light stability and are more difficult to produce side reactions than aromatic isocyanates, such as TDI and MDI. DABCO was chosen as a catalyst because of its strong ability for catalyzing the reaction between hydroxyl and isocyanate, and its weak ability for catalyzing side reactions [1] so as to
2.3. Sample preparation To synthesize Phen-CPU, a charge of 10.00 g (5 mmol) PEG, 1.68 g HDI (10 mmol) and 0.05 g DABCO (0.5 wt% of PEG) was added into a 3
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Fig. 5. Stress relaxation of Phen-CPU and the corresponding figures for calculating the activation energy. (a) Stress relaxation curves of Phen-CPU. The dashed line indicates the characteristic relaxation time where the normalized modulus decreases to 1/e. (b) Arrhenius analysis of τ* versus 1000/T. (c) Arrhenius analysis of τ*/ε versus 1000/T. (d) Reaction conversion obtained by FTIR analysis. The dashed line indicates the gel point conversion calculated by Flory−Stockmayer theory.
characteristic band of hydrogen bonded N–H (3300 cm−1) of polyurethane as well as the C–N skeleton stretching in 3HDI (767 cm−1) [38] exists in Phen-CPU. Although a low reactive phenolic hydroxyl group and aliphatic isocyanates were used in the synthesis, the addition of DABCO could effectively improve the efficiency of polyurethane synthesis. It should be noted that organotin catalysts such as DBTDL should not be used. Although organotin catalysts can strongly catalyze the reaction of hydroxyl groups with isocyanates, they also catalyze the side reaction of isocyanates at high temperatures, which is harmful to maintenance of DCNs. In order to determine the crystallization ability of the PEG moiety in the highly cross-linked Phen-CPU, DSC tests were carried out. The thermal history of the sample was removed at 60 °C, then the temperature was held at 0 °C for 2 h, followed by cooling to −10 °C and heated again at a heating rate of 10 °C/min. The DSC curve of Phen-CPU shows a Tm of 29 °C attributed to crystal melting of the PEG segments (Fig. 2). The Tm is close to room temperature and much lower than the melting point of pure PEG 2000 (~50 °C). Obviously, the PEG segment mobility is limited due to the rather high crosslinking degree so that the crystallization ability decreases. Consequently, Phen-CPU can maintain the amorphous state for a long period of time at room temperature without crystallization of PEG. The CPU shows a similar DSC curve (Fig. S6), further indicating that the high crosslinking degree suppresses the crystallization of PEG segments. In order to study the DCNs of Phen-CPU, we use DMA to measure the change of modulus with increasing temperature. The DMA curve (Fig. 3) shows that the storage modulus and tan δ are relatively stable from room temperature to 40 °C, but they start to change rapidly at temperatures higher than 40 °C. Unlike conventional thermosets, PhenCPU exhibits a distinctive flow behavior at higher temperatures, consistent with the presence of the thermally activated dynamic phenolcarbamate bonds. The flow behavior reflects the steady dissociation of the network and decrease of the crosslinking degree. In order to explain the phenomenon in the DMA curves, we further studied the dissociation of phenol-carbamates by variable temperature
250 ml three-necked flask with a Teflon mechanical agitator, then the temperature was raised to 80 °C and held for an hour under stirring. After the prepolymer was synthesized, 2.50 g BPS (10 mmol) was added into the prepolymer at 80 °C. After the BPS was completely dissolved, 1.93 g (3.33 mmol) 3HDI was added (see Scheme 1). After 1 min of stirring and eliminating the bubbles under vacuum conditions, the mixture was poured into a Teflon plate and further cured at 60 °C for 24 h in a dry condition. After curing, the samples were cooled to room temperature and stored in a desiccator. The control sample CPU was prepared in a similar manner except that 0.76 g 1,2-propylene glycol (10 mmol) was used instead of BPS. 2.4. Self-healing test In the scratch self-healing test, the sample with the thickness of ca. 1 mm was initially cast on a glass slide and then scratched by a razor. To observe the self-healing ability of Phen-CPU, the sample was heated at 80 °C for a period of time and the images under microscope were taken every 20 min. In the uniaxial self-healing test, the dumbbell-shaped specimen was first elongated to failure on tensile tester, then the two pieces were gently brought into contact for 10 s and put into an oven with desiccant to heal under 80 °C without external force. After self-healing, the sample was cooled to room temperature and stored for 3 h. At last, the healed specimen was tested again on tensile tester. 3. Results and discussion Since the reaction between phenol and isocyanate is slow, it is necessary to check whether the isocyanate was completely reacted under the experimental conditions or not. As shown in the FTIR curve of 3HDI in Fig. 1, the absorbance band at 2270 cm−1 indicative of the isocyanate stretching vibrations disappears in the FTIR curve of Phen-CPU, indicating that the reaction between the phenol and the isocyanate is complete under the experimental condition. Meanwhile, the 4
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area measured in the IR spectrum of 3HDI, and [BPS] is the molar concentration of BPS in Phen-CPU. At low temperatures, the mobility of the network and chemical bonds is limited, the values of Keq is difficult to obtain, so the Keq of Phen-CPU at temperatures from 70 °C to 150 °C are calculated. At 70 °C, p ~94%, and Keq ~230 M−1, while at 150 °C, p ~77.5%, Keq ~13 M−1. From Van't Hoff plot we obtain ΔHr0 = −42.1 kJ/mol and ΔSr0 = −78.1 J/(mol·K). One of the macroscopic performances of dynamic networks is the complete stress relaxation at temperatures where the dynamic bonds can exchange. We can see from Fig. 5 (a) that the stress can relax almost to zero at the test temperatures not lower than 40 °C, indicating that the network could fully exchange. In contrast, the control sample CPU shows incomplete stress relaxation (Fig. S2), due to absence of DCNs in CPU. The stress relaxation curve of Phen-CPU can be divided into two stages: in the first stage, the modulus drops sharply, mainly due to the dissociation of hydrogen bonds and segment relaxation [39], etc. This stage can also be found in the stress relaxation curve of CPU at 80 °C (Fig. S2). In the second stage, the modulus decreases gradually, which is mainly attributed to dynamic exchange of the phenol-carbamate bonds in the networks. According to the Maxwell model, the characteristic relaxation time (τ*) can be extracted as the time when the normalized residual stress (G/G0) is 1/e. If the stress relaxation is determined by dynamic bonds exchange, the relaxation time-temperature relationship should show an Arrhenius-like dependence. The activation energy of stress relaxation is calculated by Arrhenius equation, to be 92.7 kJ/mol. The dissociation degree of DCN and the content of free –NCO is very low at 40 °C, but there is still complete stress relaxation, indicating obvious exchange of chemical bonds. As the temperature increases, the lifetime of dynamic covalent bonds decreases (due to the higher forward and backward reaction rates) and the equilibrium shifts to higher degree of dissociation so that the crosslinking degree is lowered [27]. Both effects will facilitate the network rearrangement. Due to the bond exchange reaction (BER), the stress relaxation behavior of polymers with DCNs is different from that of the polymers with permanent crosslinks. Researchers have carried out some theoretical and experimental studies on the conformational relaxation and BER of such polymers [40–44]. For example, a theoretical framework for dealing with a transient polymer network undergoing small deformations was proposed by Terentjev et al. [40]. In addition, a thermoviscoelastic constitutive model based on the Bernstein-Kearsley-Zapas (BKZ) theory was established by Yu et al. [41] to separate the contributions of BERs from those of chain conformational relaxation. Semenov and Rubinstein (S-R) theory for associating polymer gel networks [42] has been successfully used to study polymers with dynamic covalent networks [27,40,43,44]. The S-R theory predicts that relaxation is highly dependent on the reaction conversion (the fraction of the associated adducts) [42,45]. According to S-R theory, the mean time during which the initial phenol-carbamate adduct will find a new partner is the same as the phenol-carbamate bond lifetime [42]. When the conversion is higher than the gel point, stress relaxation is determined by the dynamic bond lifetime rather than the diffusion of the segments. As a result, the exchange of dynamic bonds can be studied at the temperatures when the conversion is higher than the gel point, and we can use S-R theory to analyse the stress relaxation behavior above the gel point to calculate the activation energy of bond exchange. The theoretical gel point conversion of Phen-CPU, is calculated to be 0.82 using the Flory−Stockmayer equation, as indicated by the dashed line in Fig. 5(d). It can be found that the conversion (the fraction of the associated adducts) is always higher than the gel point below 130 °C. According to S-R theory, the relaxation at the temperature range is mainly determined by the exchange of dynamic bonds. The S-R theory establishes a relationship between the actual relaxation time and the lifetime of the dynamic adduct, when the conversion is above the gel point, as follows:
Fig. 6. Self-healing behavior of Phen-CPU. a) Optical images of repairing behavior of scratched Phen-CPU healed at 80 °C. b) Stress–strain curves of PhenCPU healed at 80 °C for 2 h.
FTIR spectra. Fig. 4(a) demonstrates that only a very small –NCO peak (2270 cm−1) can be observed at 30 °C, namely there is no significant dissociation of the phenol-carbamate bonds at this temperature. Upon heating, the free –NCO absorption increases as the reaction equilibrium shifts, with some phenol-carbamate bonds decompose to free –NCO. Combined with the DMA curve showing a decrease in G′ at temperatures higher than ~40 °C after a rubber platform, the FTIR spectra prove that the decrease in crosslink density leads to this result. As for the control sample CPU, because of the stability of the permanent network, there is no significant decrease in the storage modulus during heating. By observing the characteristic absorption peak of the C–N skeleton stretching in 3HDI (767 cm−1), the percentage of phenol-carbamates dissociated at different temperatures can be calculated according to the following equation. Furthermore, based on the amount of feeding ratio and the density of Phen-CPU (~1.1 g/cm2), the percentage of the associated phenol-carbamates and the equilibrium constant of reaction between phenol in BPS and isocyanate in HDI/3HDI can be calculated.
p=1−
A2270, t A767, t 2 ∗ A2270,0
(1)
A767,0
K eq =
[Phen − carbamate] [Phenolic ][Isocynate]
=
2 ∗ [BPS ] ∗ p (2 ∗ [BPS ] ∗ (1 − p))2
(2)
where p is the percentage of the associated phenol-carbamates, A is the integral area of the band, t is the test temperature, A0 is the absorbance 5
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Fig. 7. Figures showing the reprocessability of Phen-CPU. (a) Photograph of Phen-CPU pieces (left) and that after reprocessing (right) by compression molding (100 °C, 5 MPa, 1 min). (b) Stress–strain curves of the original and remolded Phen-CPU.
ln
τ∗ ε
( ) ~lnτ
b
= lnA0 +
Ea, r RT
Fig. 7 (a), the reprocessing can be achieved very quickly at this temperature, indicating the potential of recycling. We repeatedly molded the same sample under the above conditions for 4 times and carried out the tensile test after annealing. The stress-strain curves of the initial sample and the remoled samples are really similar (Fig. 7 (b)), showing almost no change in the material performance after reprocessing.
(3)
where τ* is the actual relaxation time, τb is the lifetime of the adduct, Ea,r is the activation energy of the exchange of the adduct, ε is the relative distance to the gel point which equal to p/pg-1. We use Arrhenius law to calculate the exchange activation energy of the dynamic phenolcarbamate bond, Ea,r = 80.9 kJ/mol. The above studies have proved that the BPS-HDI structure has quite strong dynamic properties. Therefore, the synthesized Phen-CPU is supposed to have good self-healing and reprocessing ability. According to the results of stress relaxation, the relaxation time at 80 °C is only 29 s. Therefore, the sample is expected to achieve efficient self-healing or remolding at this temperature. In the scratch self-healing test at 80 °C (Fig. 6 (a)), the smaller scratches could be healed within 20 min, and the deeper scratches disappear completely within 2 h. In the uniaxial tensile test of the selfhealed samples (Fig. 6 (b)), the fracture stress could reach 98% of the initial value after healing at 80 °C for 2 h, demonstrating excellent selfhealing property. Compared with Phen-CPU, the control sample CPU does not show self-healing property (Fig. S3), which can be attributed to the fact that the permanent cross-links in the control sample CPU severely restrict the movement of chains. At the same time, the fracture cross-section of the self-healed Phen-CPU becomes inconspicuous, and it is also strong enough to withstand considerable tension (Fig. S4). The only difference between Phen-CPU and CPU is that there are phenolcarbamate bonds in Phen-CPU but not in CPU, so the key to the selfhealing performance is the dynamic phenol-carbamate bonds rather than hydrogen bonding or other factors. In the reprocessing test, we first cut the sample into small pieces, and then remolded them at 100 °C under 5 MPa for 1 min. As shown in
4. Conclusion In summary, a new polyurethane elastomer with dynamic covalent networks based on phenol-carbamate bonds was prepared from polyethylene glycol, hexamethylene diisocyanate, bisphenol S and HDI trimer (3HDI) in a quick and easy manner without use of solvent. Through FTIR and stress relaxation analysis, we prove that the phenolcarbamate bonds possess obvious dynamic properties, and start to dissociate at 50–60 °C. The activation energy of stress relaxation (92.7 kJ/ mol) and that of phenol-carbamate exchange (80.9 kJ/mol) are relatively low, leading to excellent self-healing and reprocessing performance. Heated at 80 °C for 2 h, Phen-CPU can self-heal almost completely in scratch and uniaxially tensile test. Phen-CPU can also be reprocessed relatively quick at 100 °C with little change in the mechanical properties, demonstrating the potential for recycling. Since polyurethanes with phenol-carbamate bonds can be easily synthesized from phenols and isocyanates under suitable conditions with catalyst, the dynamic properties and the final mechanical properties can be adjusted by changing the molecular structure of phenols and isocyanates, the feeding ratio and the structure of reactants. We expect that the dynamically cross-linked polyurethanes based on phenolic compounds will find broad applications in transportation, machinery, construction, electronics and other fields requiring elastomers. 6
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Acknowledgements
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The authors are indebted to the National Natural Science Foundation of China (Grant No. 51673110, U1862205) for financial support. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.polymer.2019.121788. References [1] E. Delebecq, J.-P. Pascault, B. Boutevin, F. Ganachaud, On the versatility of urethane/urea bonds: reversibility, blocked isocyanate, and non-isocyanate polyurethane, Chem. Rev. 113 (2013) 80–118, https://doi.org/10.1021/cr300195n. [2] W. Denissen, J.M. Winne, F.E. Du Prez, Vitrimers: permanent organic networks with glass-like fluidity, Chem. Sci. 7 (2016) 30–38, https://doi.org/10.1039/ c5sc02223a. [3] W. Zou, J. Dong, Y. Luo, Q. Zhao, T. Xie, Dynamic covalent polymer networks: from old chemistry to modern day innovations, Adv. Mater. 29 (2017) 1606100, https:// doi.org/10.1002/adma.201606100. [4] C.J. Kloxin, T.F. Scott, B.J. Adzima, C.N. Bowman, Covalent adaptable networks (CANs): a unique paradigm in cross-linked polymers, Macromolecules 43 (2010) 2643–2653, https://doi.org/10.1021/ma902596s. [5] D. Montarnal, M. Capelot, F. Tournilhac, L. Leibler, Silica-like malleable materials from permanent organic networks, Science 334 (2011) 965–968, https://doi.org/ 10.1126/science.1212648. [6] M. Capelot, M.M. Unterlass, F. Tournilhac, L. Leibler, Catalytic control of the vitrimer glass transition, ACS Macro Lett. 1 (2012) 789–792, https://doi.org/10. 1021/mz300239f. [7] M. Capelot, D. Montarnal, F. Tournilhac, L. Leibler, Metal-catalyzed transesterification for healing and assembling of thermosets, J. Am. Chem. Soc. 134 (2012) 7664–7667, https://doi.org/10.1021/ja302894k. [8] J. Han, T. Liu, C. Hao, S. Zhang, B. Guo, J. Zhang, A catalyst-free epoxy vitrimer system based on multifunctional hyperbranched polymer, Macromolecules 51 (2018) 6789–6799, https://doi.org/10.1021/acs.macromol.8b01424. [9] Y. Yang, E.M. Terentjev, Y. Wei, Y. Ji, Solvent-assisted programming of flat polymer sheets into reconfigurable and self-healing 3D structures, Nat. Commun. 9 (2018) 1–7, https://doi.org/10.1038/s41467-018-04257-x. [10] Q. Chen, Y. Wei, Y. Ji, Photo-responsive liquid crystalline vitrimer containing oligoanilines, Chin. Chem. Lett. 28 (2017) 2139–2142, https://doi.org/10.1016/j. cclet.2017.09.011. [11] B. Jin, H. Song, R. Jiang, J. Song, Q. Zhao, T. Xie, Programming a crystalline shape memory polymer network with thermo- and photo-reversible bonds toward a single-component soft robot, Sci. Adv. 4 (2018), https://doi.org/10.1126/sciadv. aao3865 eaao3865. [12] N. Zheng, Z. Fang, W. Zou, Q. Zhao, T. Xie, Thermoset shape-memory polyurethane with intrinsic plasticity enabled by transcarbamoylation, Angew. Chem. Int. Ed. 55 (2016) 11421–11425, https://doi.org/10.1002/anie.201602847. [13] P. Yan, W. Zhao, L. Jiang, B. Wu, K. Hu, Y. Yuan, J. Lei, Reconfiguration and shape memory triggered by heat and light of carbon nanotube–polyurethane vitrimer composites, J. Appl. Polym. Sci. 135 (2018) 45784, https://doi.org/10.1002/app. 45784. [14] J.H. Chen, D.D. Hu, Y.D. Li, J. Zhu, A.K. Du, J.B. Zeng, Castor oil-based high performance and reprocessable poly(urethane urea) network, Polym. Test. 70 (2018) 174–179, https://doi.org/10.1016/j.polymertesting.2018.07.004. [15] A. Ruiz De Luzuriaga, R. Martin, N. Markaide, A. Rekondo, G. Cabañero, J. Rodríguez, I. Odriozola, Epoxy resin with exchangeable disulfide crosslinks to obtain reprocessable, repairable and recyclable fiber-reinforced thermoset composites, Mater. Horizons. 3 (2016) 241–247, https://doi.org/10.1039/c6mh00029k. [16] T. Li, Z. Xie, J. Xu, Y. Weng, B.H. Guo, Design of a self-healing cross-linked polyurea with dynamic cross-links based on disulfide bonds and hydrogen bonding, Eur. Polym. J. 107 (2018) 249–257, https://doi.org/10.1016/j.eurpolymj.2018.08.005. [17] M. Chen, L. Zhou, Y. Wu, X. Zhao, Y. Zhang, Rapid stress relaxation and moderate temperature of malleability enabled by the synergy of disulfide metathesis and carboxylate transesterification in epoxy vitrimers, ACS Macro Lett. 8 (2019) 255–260, https://doi.org/10.1021/acsmacrolett.9b00015. [18] B.T. Michal, C.A. Jaye, E.J. Spencer, S.J. Rowan, Inherently photohealable and thermal shape-memory polydisulfide networks, ACS Macro Lett. 2 (2013) 694–699, https://doi.org/10.1021/mz400318m. [19] H. Xiang, M. Rong, M. Zhang, Intrinsic self-healing and solid-state recycling of vulcanized natural rubber, Acta Polym. Sin. 28 (2017) 1130–1140, https://doi.org/ 10.11777/j.issn1000-3304.2017.17016. [20] L. Tian, L. Yang, Z. Wang, H. Xia, Room-temperature self-healable and reprocessable polyurethane elastomers combined diels-alder bond and disulfide bond, Acta Polym. Sin. 5 (2019) 510–515, https://doi.org/10.11777/j.issn1000-3304. 2019.19021.
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Solvent-free thermo-reversible and self-healable crosslinked polyurethane with dynamic covalent networks based on phenol-carbamate bonds
T
Jiaxin Shi, Tianze Zheng, Baohua Guo, Jun Xu∗ Advanced Materials Laboratory of Ministry of Education (MOE), Department of Chemical Engineering, Tsinghua University, Beijing, 100084, China
H I GH L IG H T S
phenol-carbamate bonds synthesized from bisphenol-S and diisocyanate are dynamic covalent bonds. • The parameters of phenol-carbamate bond association and activation energy of bond exchange are calculated. • Thermodynamic • The polyurethane with dynamic phenol-carbamate bonds demonstrates excellent self-healing and reprocessing properties.
A R T I C LE I N FO
A B S T R A C T
Keywords: Polyurethane Dynamic covalent network Self-healing
Polyurethanes with dynamic covalent networks have the potential of self-healing, reprocessing and recycling, however, how to improve the efficiency of self-healing and reprocessing, and reduce the use of solvent and costs, are still problems to solve. In this work, we synthesized a cross-linked polyurethane containing dynamic phenolcarbamate bonds from low-cost raw materials without use of solvent. The dynamic polyurethane possesses excellent self-healing and reprocessing properties, which exhibits low relaxation activation energy (92.7 kJ/mol) and can be almost completely healed at 80 °C for 2 h. After remolded for 4 times, the material shows almost little change in mechanical properties. Further studies on the phenol-carbamate bonds give the quantitative reaction equilibrium constants, thermodynamic parameters and dissociation activation energy. In viewpoint of the dynamic properties and easy synthesis of phenol-carbamates, various types of cross-linked polymers containing such dynamic bonds can be further tailor designed.
1. Introduction As a family of widely used polymeric materials, polyurethanes (PUs) play an important role in industrial applications and daily life. The properties of PUs can be tuned in a wide range using different types of polyols and isocyanates. Urethane groups with the ability to form hydrogen bonds act as the physical cross-linkages in the material [1], but the low modulus and creep of linear PUs limits their applications. Chemical cross-linking can improve mechanical properties and thermostability of PUs, however, the traditional chemically cross-linked polymers are difficult to recycle and reprocess, and their forming processes are limited [2,3]. Meanwhile, due to the increasingly serious resource and environmental problems, self-healable, thermo-reversible and recyclable cross-linked polymers are receiving widespread attention. Among them, the cross-linked polymers with dynamic covalent bonds show the advantages of self-healing and ease of reprocessing. The network structure of the cross-linked polymers based on
∗
dynamic covalent bonds can be divided into two types: dissociative and associative dynamic covalent networks (DCNs) [2,4]. Associative DCNs maintain the crosslinking degree at elevated temperatures, but the chemical bond exchange rate increases. These DCNs exhibit permanent cross-linking and significant viscoelastic behavior at high temperatures. Typical examples are transesterification DCNs, first prepared by Leibler et al. and named “vitrimers” [5–10]. In addition, dynamic urethanes catalyzed by dibutyltin dilaurate (DBTDL) [11–13], disulfides [14–20] and anion-catalyzed dynamic siloxane [21,22] have been utilized to build associative DCNs. Although polymers containing associative DCNs can be remolded at solid to semi-solid state in a wide processing temperature window, the degree of crosslinking does not change much, causing insufficient fluidity during processing and thus rough surface of the products. In contrast, for dissociative DCNs, the initial dynamic covalent bonds dissociate and exchange to form new network structure, so the crosslinking degree changes during the heating process and the materials can be remolded in solid, semi-solid and liquid state. Diels-
Corresponding author. E-mail address: [email protected] (J. Xu).
https://doi.org/10.1016/j.polymer.2019.121788 Received 13 July 2019; Received in revised form 3 September 2019; Accepted 8 September 2019 Available online 09 September 2019 0032-3861/ © 2019 Published by Elsevier Ltd.
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Scheme 1. Synthesis of Phen-CPU.
Fig. 1. FTIR spectra of 3HDI and Phen-CPU at room temperature.
Fig. 2. DSC heating curve of Phen-CPU after stored at 0 °C for 2 h.
Alder adducts, usually produced by the reaction between furan and maleimide, are a kind of widely adopted dissociative DCNs [23–26]. However, the synthesis becomes more complex when furan and maleimide groups are incorporated. In addition, maleimide and furan groups are prone to oxidation, causing the polymers turn into darker color, typically from yellow to brown. Recently, another type of reaction for constructing dissociative DCNs, the reversible addition of isocyanates and active hydrogen compounds, has received extensive attention. Such dynamic covalent bonds include hindered urea [27–30] and oxime-urethane [31–33], among which the reversible structure of the polyurethane/polyurea will inject new vitality into the general polyurethane and polyurea materials. As isocyanate blocking agents, phenolic compounds have a long history of application. The produced phenol-carbamates are stable at room temperatures and can dissociate at elevated temperatures to regenerate free isocyanate groups [34]. Based on this concept, phenolcarbamates can be incorporated into PUs to construct dissociative DCNs. According to the blocked isocyanate chemistry, both the type of isocyanate and the electronic effect of the substituent on the benzene ring affect the dissociation temperature of the phenol-carbamates. As for phenolic compounds, the stronger the electron-withdrawing effect of the substituent on the benzene ring is, the lower the dissociation temperature is [34]。 Since the reaction of phenol with isocyanate is slow, it is necessary to add a catalyst to accelerate the reaction. DBTDL is a frequently used catalyst, but it strongly catalyzes the side reaction of isocyanate at high
Fig. 3. Storage modulus and dissipation factor tan δ of Phen-CPU measured by DMA from 25 °C to 100 °C, at a heating rate of 3 °C min−1 and a frequency of 1 Hz. The samples were stored at room temperature rather than 0 °C to prevent crystallization of PEG moiety.
temperature, resulting in permanent crosslinking [35]. Meanwhile, the dissociation temperature has a positive correlation with the reactivity of the isocyanate [36]. Therefore, in order to depress the side reaction, phenol, isocyanate and catalyst should be carefully selected in the phenol-carbamate DCNs so that the properties of the material can remain stable in multiple reprocesses. Cao et al. [37] used poly(tetramethylene ether glycol) (PTMEG), 1,2,3-trichlorobenzene and toluene2
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avoid problems caused by DBTDL. As for BPS, the sulfone group between the two benzene rings has a strong electron-withdrawing ability, which further lowers the carbamate dissociation temperature, improves the self-healing rate and reduces the temperature of reprocessing. The results show that Phen-CPU has an excellent self-healing property and can achieve almost complete recovery of mechanical properties by remolding at 100 °C, 5 MPa for only 1 min. Further studies reveal the mechanism that BPS-HDI adducts can be exchanged or dissociated at moderate temperatures. Furthermore, the equilibrium constants at different temperatures and the activation energy of bond dissociation are calculated. 2. Experimental section 2.1. Materials Polyethylene glycol (PEG Mn = 2000, for synthesis) was provided by Shanghai Dongda Chemistry Co., Ltd. Hexamethylene diisocyanate (HDI, 99%), bisphenol S (BPS, 98%), 1,4-Diazabicyclo[2.2.2]octane (DABCO, 99%), 1,2-propanediol (99%) were purchased from AdmasBeta. HDI trimer (Wannate HT100) with 21.8 wt % NCO content was provided by Wannate Chemistry Co., Ltd. and used as received. PEG was dried under high vacuum at 120 °C for 2 h in a vacuum oven before used. 2.2. Instrumentation Differential scanning calorimetry (DSC) measurement was conducted on a Shimadzu TA 60-WS with a heating and cooling rate of 10 °C/min under nitrogen atmosphere. Dynamic mechanical analysis (DMA) was carried out on an Anton Paar MCR301 with stress/strain controlled rheometer in a torsional mode at temperatures ranging from 25 to 100 °C with a heating rate of 3 °C/min. The maximum strain was 0.01% and the frequency was 1 Hz. Stress-relaxation curves were also obtained using Anton Paar MCR301 in a parallel-plate mode. Circular specimens with diameter of 25 mm were punched out of the sample sheets. Each sample was first equilibrated at a set temperature for 15 min, then it was initially compressed by a strain of 5% and the strain was maintained. Tensile tests were conducted on a JinJian UTM-1432 tensile tester equipped with a 500 N load cell. The samples were directly stretched to failure at a constant cross-head speed of 50 mm/min. Dumbbell-shaped specimens were cut from cast sheets using a standard bench-top die according to GB/T-528. Temperature variable Fourier transform infrared (FTIR) spectra were collected on a Nicolet 6700 FT-IR spectrophotometer equipped with a deuterated triglycine sulfate detector and a hot stage. To prepare the IR samples, the uncured reactants mixture was cast onto a KBr plate to obtain a thin film, on top of which a second KBr plate was placed. Then the KBr sandwiched sample was cured in an oven at 60 °C for 24 h. The transmission spectra were collected with an average of 16 scans for each run at a resolution of 4 cm−1 in the range of 4000−500 cm−1 after the sandwiched sample was equilibrated at the desired temperature for 5 min. The materials were reprocessed on a lab press instrument (Carver Model C laboratory press) at 100 °C under 5 MPa for 1 min. After remolded, the sample was annealed at 60 °C for 3 h, 40 °C for 3 h and then at room temperature for 3 h in an oven with desiccant to fully equilibriate the reaction of phenol-carbamates. Optical micrographs were obtained with Olympus BX41P (Japan) microscope equipped with a Linkam T95-PE temperature controller (England).
Fig. 4. (a) Variable temperature FTIR spectroscopy of Phen-CPU (b) Van't Hoff plot for Phen-CPU.
2,4-diisocyanate (TDI) to synthesize cross-linked PUs containing phenol-carbamate under DBTDL catalysis. It was found that phenolcarbamates could dissociate a small amount of free isocyanate at a really high temperature so as to achieve self-healability, but its initial dissociating temperature was a bit high, above 120 °C. In addition, the infrared spectrum showed that the isocyanate peak intensity rose first and then decreased during the heating process, illustrating that significant side reactions occurred, making it difficult to achieve complete reprocessing and self-healing ability. In this work, we report a solvent-free, low-cost, reprocessable and self-healable cross-linked polyurethane with reversible covalent bond phenol-carbamates (Phen-CPU). Phen-CPU was produced from low-cost industrial feedstocks: polyethylene glycol (PEG) as a polyol, hexamethylene diisocyanate (HDI) as a diisocyanate, bisphenol S (BPS) as a chain extender, HDI trimer (3HDI) as a crosslinker, and 1,4-diazabicyclo [2.2.2] octane (DABCO) as a catalyst. PEG can dissolve the other components, thereby avoids the use of solvents and improves environmental friendliness. At the same time, low-molecular weight PEG has a reduced ability to crystallize in the highly cross-linked polymers, which helps the material to maintain elasticity at room temperature. HDI and 3-HDI, as aliphatic isocyanates, show good light stability and are more difficult to produce side reactions than aromatic isocyanates, such as TDI and MDI. DABCO was chosen as a catalyst because of its strong ability for catalyzing the reaction between hydroxyl and isocyanate, and its weak ability for catalyzing side reactions [1] so as to
2.3. Sample preparation To synthesize Phen-CPU, a charge of 10.00 g (5 mmol) PEG, 1.68 g HDI (10 mmol) and 0.05 g DABCO (0.5 wt% of PEG) was added into a 3
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Fig. 5. Stress relaxation of Phen-CPU and the corresponding figures for calculating the activation energy. (a) Stress relaxation curves of Phen-CPU. The dashed line indicates the characteristic relaxation time where the normalized modulus decreases to 1/e. (b) Arrhenius analysis of τ* versus 1000/T. (c) Arrhenius analysis of τ*/ε versus 1000/T. (d) Reaction conversion obtained by FTIR analysis. The dashed line indicates the gel point conversion calculated by Flory−Stockmayer theory.
characteristic band of hydrogen bonded N–H (3300 cm−1) of polyurethane as well as the C–N skeleton stretching in 3HDI (767 cm−1) [38] exists in Phen-CPU. Although a low reactive phenolic hydroxyl group and aliphatic isocyanates were used in the synthesis, the addition of DABCO could effectively improve the efficiency of polyurethane synthesis. It should be noted that organotin catalysts such as DBTDL should not be used. Although organotin catalysts can strongly catalyze the reaction of hydroxyl groups with isocyanates, they also catalyze the side reaction of isocyanates at high temperatures, which is harmful to maintenance of DCNs. In order to determine the crystallization ability of the PEG moiety in the highly cross-linked Phen-CPU, DSC tests were carried out. The thermal history of the sample was removed at 60 °C, then the temperature was held at 0 °C for 2 h, followed by cooling to −10 °C and heated again at a heating rate of 10 °C/min. The DSC curve of Phen-CPU shows a Tm of 29 °C attributed to crystal melting of the PEG segments (Fig. 2). The Tm is close to room temperature and much lower than the melting point of pure PEG 2000 (~50 °C). Obviously, the PEG segment mobility is limited due to the rather high crosslinking degree so that the crystallization ability decreases. Consequently, Phen-CPU can maintain the amorphous state for a long period of time at room temperature without crystallization of PEG. The CPU shows a similar DSC curve (Fig. S6), further indicating that the high crosslinking degree suppresses the crystallization of PEG segments. In order to study the DCNs of Phen-CPU, we use DMA to measure the change of modulus with increasing temperature. The DMA curve (Fig. 3) shows that the storage modulus and tan δ are relatively stable from room temperature to 40 °C, but they start to change rapidly at temperatures higher than 40 °C. Unlike conventional thermosets, PhenCPU exhibits a distinctive flow behavior at higher temperatures, consistent with the presence of the thermally activated dynamic phenolcarbamate bonds. The flow behavior reflects the steady dissociation of the network and decrease of the crosslinking degree. In order to explain the phenomenon in the DMA curves, we further studied the dissociation of phenol-carbamates by variable temperature
250 ml three-necked flask with a Teflon mechanical agitator, then the temperature was raised to 80 °C and held for an hour under stirring. After the prepolymer was synthesized, 2.50 g BPS (10 mmol) was added into the prepolymer at 80 °C. After the BPS was completely dissolved, 1.93 g (3.33 mmol) 3HDI was added (see Scheme 1). After 1 min of stirring and eliminating the bubbles under vacuum conditions, the mixture was poured into a Teflon plate and further cured at 60 °C for 24 h in a dry condition. After curing, the samples were cooled to room temperature and stored in a desiccator. The control sample CPU was prepared in a similar manner except that 0.76 g 1,2-propylene glycol (10 mmol) was used instead of BPS. 2.4. Self-healing test In the scratch self-healing test, the sample with the thickness of ca. 1 mm was initially cast on a glass slide and then scratched by a razor. To observe the self-healing ability of Phen-CPU, the sample was heated at 80 °C for a period of time and the images under microscope were taken every 20 min. In the uniaxial self-healing test, the dumbbell-shaped specimen was first elongated to failure on tensile tester, then the two pieces were gently brought into contact for 10 s and put into an oven with desiccant to heal under 80 °C without external force. After self-healing, the sample was cooled to room temperature and stored for 3 h. At last, the healed specimen was tested again on tensile tester. 3. Results and discussion Since the reaction between phenol and isocyanate is slow, it is necessary to check whether the isocyanate was completely reacted under the experimental conditions or not. As shown in the FTIR curve of 3HDI in Fig. 1, the absorbance band at 2270 cm−1 indicative of the isocyanate stretching vibrations disappears in the FTIR curve of Phen-CPU, indicating that the reaction between the phenol and the isocyanate is complete under the experimental condition. Meanwhile, the 4
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area measured in the IR spectrum of 3HDI, and [BPS] is the molar concentration of BPS in Phen-CPU. At low temperatures, the mobility of the network and chemical bonds is limited, the values of Keq is difficult to obtain, so the Keq of Phen-CPU at temperatures from 70 °C to 150 °C are calculated. At 70 °C, p ~94%, and Keq ~230 M−1, while at 150 °C, p ~77.5%, Keq ~13 M−1. From Van't Hoff plot we obtain ΔHr0 = −42.1 kJ/mol and ΔSr0 = −78.1 J/(mol·K). One of the macroscopic performances of dynamic networks is the complete stress relaxation at temperatures where the dynamic bonds can exchange. We can see from Fig. 5 (a) that the stress can relax almost to zero at the test temperatures not lower than 40 °C, indicating that the network could fully exchange. In contrast, the control sample CPU shows incomplete stress relaxation (Fig. S2), due to absence of DCNs in CPU. The stress relaxation curve of Phen-CPU can be divided into two stages: in the first stage, the modulus drops sharply, mainly due to the dissociation of hydrogen bonds and segment relaxation [39], etc. This stage can also be found in the stress relaxation curve of CPU at 80 °C (Fig. S2). In the second stage, the modulus decreases gradually, which is mainly attributed to dynamic exchange of the phenol-carbamate bonds in the networks. According to the Maxwell model, the characteristic relaxation time (τ*) can be extracted as the time when the normalized residual stress (G/G0) is 1/e. If the stress relaxation is determined by dynamic bonds exchange, the relaxation time-temperature relationship should show an Arrhenius-like dependence. The activation energy of stress relaxation is calculated by Arrhenius equation, to be 92.7 kJ/mol. The dissociation degree of DCN and the content of free –NCO is very low at 40 °C, but there is still complete stress relaxation, indicating obvious exchange of chemical bonds. As the temperature increases, the lifetime of dynamic covalent bonds decreases (due to the higher forward and backward reaction rates) and the equilibrium shifts to higher degree of dissociation so that the crosslinking degree is lowered [27]. Both effects will facilitate the network rearrangement. Due to the bond exchange reaction (BER), the stress relaxation behavior of polymers with DCNs is different from that of the polymers with permanent crosslinks. Researchers have carried out some theoretical and experimental studies on the conformational relaxation and BER of such polymers [40–44]. For example, a theoretical framework for dealing with a transient polymer network undergoing small deformations was proposed by Terentjev et al. [40]. In addition, a thermoviscoelastic constitutive model based on the Bernstein-Kearsley-Zapas (BKZ) theory was established by Yu et al. [41] to separate the contributions of BERs from those of chain conformational relaxation. Semenov and Rubinstein (S-R) theory for associating polymer gel networks [42] has been successfully used to study polymers with dynamic covalent networks [27,40,43,44]. The S-R theory predicts that relaxation is highly dependent on the reaction conversion (the fraction of the associated adducts) [42,45]. According to S-R theory, the mean time during which the initial phenol-carbamate adduct will find a new partner is the same as the phenol-carbamate bond lifetime [42]. When the conversion is higher than the gel point, stress relaxation is determined by the dynamic bond lifetime rather than the diffusion of the segments. As a result, the exchange of dynamic bonds can be studied at the temperatures when the conversion is higher than the gel point, and we can use S-R theory to analyse the stress relaxation behavior above the gel point to calculate the activation energy of bond exchange. The theoretical gel point conversion of Phen-CPU, is calculated to be 0.82 using the Flory−Stockmayer equation, as indicated by the dashed line in Fig. 5(d). It can be found that the conversion (the fraction of the associated adducts) is always higher than the gel point below 130 °C. According to S-R theory, the relaxation at the temperature range is mainly determined by the exchange of dynamic bonds. The S-R theory establishes a relationship between the actual relaxation time and the lifetime of the dynamic adduct, when the conversion is above the gel point, as follows:
Fig. 6. Self-healing behavior of Phen-CPU. a) Optical images of repairing behavior of scratched Phen-CPU healed at 80 °C. b) Stress–strain curves of PhenCPU healed at 80 °C for 2 h.
FTIR spectra. Fig. 4(a) demonstrates that only a very small –NCO peak (2270 cm−1) can be observed at 30 °C, namely there is no significant dissociation of the phenol-carbamate bonds at this temperature. Upon heating, the free –NCO absorption increases as the reaction equilibrium shifts, with some phenol-carbamate bonds decompose to free –NCO. Combined with the DMA curve showing a decrease in G′ at temperatures higher than ~40 °C after a rubber platform, the FTIR spectra prove that the decrease in crosslink density leads to this result. As for the control sample CPU, because of the stability of the permanent network, there is no significant decrease in the storage modulus during heating. By observing the characteristic absorption peak of the C–N skeleton stretching in 3HDI (767 cm−1), the percentage of phenol-carbamates dissociated at different temperatures can be calculated according to the following equation. Furthermore, based on the amount of feeding ratio and the density of Phen-CPU (~1.1 g/cm2), the percentage of the associated phenol-carbamates and the equilibrium constant of reaction between phenol in BPS and isocyanate in HDI/3HDI can be calculated.
p=1−
A2270, t A767, t 2 ∗ A2270,0
(1)
A767,0
K eq =
[Phen − carbamate] [Phenolic ][Isocynate]
=
2 ∗ [BPS ] ∗ p (2 ∗ [BPS ] ∗ (1 − p))2
(2)
where p is the percentage of the associated phenol-carbamates, A is the integral area of the band, t is the test temperature, A0 is the absorbance 5
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Fig. 7. Figures showing the reprocessability of Phen-CPU. (a) Photograph of Phen-CPU pieces (left) and that after reprocessing (right) by compression molding (100 °C, 5 MPa, 1 min). (b) Stress–strain curves of the original and remolded Phen-CPU.
ln
τ∗ ε
( ) ~lnτ
b
= lnA0 +
Ea, r RT
Fig. 7 (a), the reprocessing can be achieved very quickly at this temperature, indicating the potential of recycling. We repeatedly molded the same sample under the above conditions for 4 times and carried out the tensile test after annealing. The stress-strain curves of the initial sample and the remoled samples are really similar (Fig. 7 (b)), showing almost no change in the material performance after reprocessing.
(3)
where τ* is the actual relaxation time, τb is the lifetime of the adduct, Ea,r is the activation energy of the exchange of the adduct, ε is the relative distance to the gel point which equal to p/pg-1. We use Arrhenius law to calculate the exchange activation energy of the dynamic phenolcarbamate bond, Ea,r = 80.9 kJ/mol. The above studies have proved that the BPS-HDI structure has quite strong dynamic properties. Therefore, the synthesized Phen-CPU is supposed to have good self-healing and reprocessing ability. According to the results of stress relaxation, the relaxation time at 80 °C is only 29 s. Therefore, the sample is expected to achieve efficient self-healing or remolding at this temperature. In the scratch self-healing test at 80 °C (Fig. 6 (a)), the smaller scratches could be healed within 20 min, and the deeper scratches disappear completely within 2 h. In the uniaxial tensile test of the selfhealed samples (Fig. 6 (b)), the fracture stress could reach 98% of the initial value after healing at 80 °C for 2 h, demonstrating excellent selfhealing property. Compared with Phen-CPU, the control sample CPU does not show self-healing property (Fig. S3), which can be attributed to the fact that the permanent cross-links in the control sample CPU severely restrict the movement of chains. At the same time, the fracture cross-section of the self-healed Phen-CPU becomes inconspicuous, and it is also strong enough to withstand considerable tension (Fig. S4). The only difference between Phen-CPU and CPU is that there are phenolcarbamate bonds in Phen-CPU but not in CPU, so the key to the selfhealing performance is the dynamic phenol-carbamate bonds rather than hydrogen bonding or other factors. In the reprocessing test, we first cut the sample into small pieces, and then remolded them at 100 °C under 5 MPa for 1 min. As shown in
4. Conclusion In summary, a new polyurethane elastomer with dynamic covalent networks based on phenol-carbamate bonds was prepared from polyethylene glycol, hexamethylene diisocyanate, bisphenol S and HDI trimer (3HDI) in a quick and easy manner without use of solvent. Through FTIR and stress relaxation analysis, we prove that the phenolcarbamate bonds possess obvious dynamic properties, and start to dissociate at 50–60 °C. The activation energy of stress relaxation (92.7 kJ/ mol) and that of phenol-carbamate exchange (80.9 kJ/mol) are relatively low, leading to excellent self-healing and reprocessing performance. Heated at 80 °C for 2 h, Phen-CPU can self-heal almost completely in scratch and uniaxially tensile test. Phen-CPU can also be reprocessed relatively quick at 100 °C with little change in the mechanical properties, demonstrating the potential for recycling. Since polyurethanes with phenol-carbamate bonds can be easily synthesized from phenols and isocyanates under suitable conditions with catalyst, the dynamic properties and the final mechanical properties can be adjusted by changing the molecular structure of phenols and isocyanates, the feeding ratio and the structure of reactants. We expect that the dynamically cross-linked polyurethanes based on phenolic compounds will find broad applications in transportation, machinery, construction, electronics and other fields requiring elastomers. 6
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Acknowledgements
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