Startseite Naturwissenschaften Recyclable polytriazole resins with high performance based on Diels-Alder dynamic covalent crosslinking
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Recyclable polytriazole resins with high performance based on Diels-Alder dynamic covalent crosslinking

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Veröffentlicht/Copyright: 31. Dezember 2023
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e-Polymers
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Abstract

N,N-Di-2-propyn-1-yl-2-furanmethanamine (DPFA) was synthesized from 2-furanemethylamine and 3-chloropropyne. Then, furan-containing polytriazoles were made from DPFA and diazide compounds. The Diels-Alder (DA) reaction between 4,4′-bismaleimidodiphenylmethane and furan-containing polytriazoles was used to prepare recyclable polytriazole resins. The effects of the main chain structures on the reversible processes and mechanical properties of the resins were investigated. The results show that the flexibility of the chain structures could be regulated by introducing different contents of azide-terminated polyethylene glycol (PEG) in the polymerization process. The PEG segments could not only promote the degree of the DA reaction but also play a role in reinforcing and/or toughening the recyclable polytriazole resins. RFPTA-5 resin film displays a tensile strength of 107.2 MPa and RFPTA-20 resin film shows an elongation at break of 224.8%. Moreover, the resin films demonstrate high recyclability.

Graphical abstract

Keywords: recyclable polytriazole; novel recyclable thermosetting resin; Diels-Alder reaction; thermally reversible cross-linked polymer; green polymer

1 Introduction

Recycling of thermosetting resins has become one of the hotspots in the field of composite materials (1,2). Polytriazole resins have been employed in the aerospace industry due to their advantageous mechanical characteristics, heat resistance, and solvent resistance (3,4,5). Exploring recyclable polytriazole resins for the expective green development is an interesting subject with great significance.

Extensive research has been conducted on the introduction of reversible bonds for the preparation of recyclable thermosetting resins (6,7,8,9,10,11). The Diels-Alder (DA) reaction is one of the most used approaches, which is a [4 + 2] cycloaddition reaction between an electron-rich conjugated diene and an electron-poor dienophile, and the DA reaction has the advantages of moderate reaction conditions and catalyst-free requirements. The furan-maleimide DA reaction can be used to insert DA crosslinks into polymer networks, which is a popular technique for creating reversible cross-linked resins. The thermally reversible resins manufactured possess remarkable recyclability, reprocessing properties, shape memory, and self-healing capabilities.

Some thermally reversible resins have been reported in recent years (7,8,12,13). The behavior and features of the thermally reversible resins developed have been investigated. Wudl and his colleagues (14) pioneered the heat-healable dynamic covalent polymers in 2002, which were constructed based on the thermally reversible furan-maleimide DA reaction. Sun and co-workers (12) synthesized highly strong and tough polyurethanes that are cross-linked by DA adducts. The as-obtained polyurethanes not only show a high tensile strength of ∼50.3 MPa but also possess high ductility with an elongation at break of ∼304%. Lorero et al. (15) prepared a series of epoxy resins with reversible cross-linked structures using the DA reaction and investigated the effect of the DA crosslinking ratios on the resin properties. Their results show that the addition of DA crosslinks to the thermoset leads to a decrease in mechanical and thermo-mechanical properties. Guo et al. (8) designed a dynamic hybrid cross-linked polyurethane (FPU) elastomer based on hydrogen bonding, disulfide bonding, and DA bonding. The elastomer displays excellent self-healing ability and recyclability at low temperatures. Zhou et al. (16) reported a newly designed dynamic cross-linked network based on the DA reaction as a matrix for fully recyclable aramid fabric-reinforced composites. The thermomechanical properties of the materials were enhanced by the introduction of the aromatic amide groups, which is attributed to the hydrogen bonding interactions generated between the amide groups. The matrix resin (CN64) with the optimized composition exhibited the highest interfacial shear strength of 31.5 MPa, which is comparable to that of a commercially used epoxy resin E51 (31.6 MPa), demonstrating good potential for industrial applications.

Currently, polymeric materials with reversible cross-linked structures have good mechanical properties, yet they remain inadequate for substituting conventional thermosetting resins (7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31). The research of reversible cross-linked materials based on polytriazole resins is uncommon, but the transalkylation of polytriazoles with haloalkanes has been studied in our lab (32). Zhang et al. (33) prepared shape memory polytriazole resin films with reversible cross-linked structures by the alkylation of 1,2,3-triazole rings with binary halogenated hydrocarbons. The reversible cross-linked polytriazole resin films with different molecular weights exhibited different glass transition temperatures (T g) and water absorption, and the shape memory program of the bilayer films could be controlled by the dual regulation of water and temperature. The reversible cross-linked polytriazole resin films also demonstrated good recyclability.

In order to obtain sufficient reversibility and optimum mechanical properties, this work focuses on the preparation of recyclable polytriazole resins based on a furan-containing alkyne and a bismaleimide, and the effects of different contents of flexible segments in a highly DA cross-linked polymer network on properties are studied.

In detail, a novel N,N-di-2-propyn-1-yl-2-furanmethanamine (DPFA) was synthesized from furfurylamine, and a series of furan-functionalized polytriazoles were prepared by using DPFA. An azide-terminated polyethylene glycol (ATPEG2000) was added during the preparation of the furan-functionalized polytriazoles. Thus, the thermally reversible cross-linked networks based on the DA reaction of the furan-functionalized polytriazoles with a bismaleimide were obtained. Then, we investigated the recyclability and mechanical properties of these thermally reversible cross-linked polytriazoles.

2 Materials and methods

2.1 Materials

3-Chloropropyne and furfurylamine (FA) were purchased from Shanghai Adamas Reagent Co. Ltd, China. 4,4′-Bismaleimidodiphenylmethane (BDM) was supplied by Shanghai Meryer Chemical Technology Co. Ltd, China. 4,4′-Bis(azidomethyl)-1,1′-biphenyl (BAMBP) and diazide-terminated polyethylene glycol (ATPEG2000) were prepared in our laboratory. In addition, DPFA was synthesized in our laboratory (the synthetic route in Scheme S1). Sodium hydroxide, ethyl acetate, anhydrous sodium sulfate, N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), chloroform-d, and DMSO-d6 were provided from Shanghai Titan Scientific Co. Ltd, China. All purchased reagents and solvents were used directly without further treatment.

2.2 Characterization

The FT-IR spectrum was recorded on the Nicolet iS10 infrared spectrometer (Thermo Scientific, Waltham, USA). The scanning range is from 4,000 to 400 cm−1 with 4 cm−1 resolution and 32 scans. The proton Nuclear Magnetic Resonance (1H-NMR) spectrum was measured on an AVANCE III 400 spectrometer (Bruker, Massachusetts, USA). DMSO-d6 and CDCl3 were used as solvents, and tetramethylsilane was used as the standard. The Gel Permeation Chromatography (GPC) measurement was performed using a PL/PL-GPC50 instrument with a differential refractive index detector (Agilent, California, USA) at 50°C with DMF as an eluent and polystyrene as the standard. The Differential Scanning Calorimetry (DSC) tests were carried out with a Q2000 analyzer (TA, Delaware, USA), and the ramp was settled from −30°C to 200°C with a heating rate of 5 or 10°C·min−1. The Dynamic Mechanical Analysis (DMA) tests were carried out on TA Instruments Q800 (TA, New Castle, USA) over the temperature range from −30°C to 180°C at a frequency of 1 Hz and a heating rate of 5°C·min−1. Tensile performance tests were carried out at ambient temperature on a universal testing machine CMT2503 (MTS Systems, Shanghai, China) according to GB-T 1040.3-2006 by using a 1 kN load cell with a tensile speed of 10 mm·min−1. The swelling capacity test of cross-linked polytriazole resin films was performed by swelling the samples in DMF at room temperature for 7 days and drying them in a vacuum oven at 70°C for 24 h (Figure S4).

2.3 Preparation of furan-functionalized polytriazole (FPTA) resins

The synthetic route to FPTA (in detail, named as FPTA-j; j is the molar percentage of ATPEG2000 to the total azide monomer, j = 0, 5, 10, 15, and 20) is shown in Scheme 1. The FPTA resins were prepared by a catalyst-free two-step ontogenetic polymerization, and the feeding ratios of the dialkynyl monomer to the diazidyl monomer are shown in Table S1. The following procedure shows the preparation process: First, DPFA and ATPEG2000 were added to a 50 mL three-neck flask equipped with a condenser, stirrer, and thermometer. The mixture in the flask was heated to 70°C and kept at 70°C for 3 h under stirring. Thereafter, BAMBP was added, and the reaction continued at 70°C for 3 h. Afterwards, the reactants were transferred to an oven at 110°C for 12 h to obtain FPTA resins.

Scheme 1 The synthesis route of FPTA and RFPTA resins.
Scheme 1

The synthesis route of FPTA and RFPTA resins.

2.4 Preparation of recyclable polytriazole (RFPTA) resin films based on DA reaction

The synthesis route of thermally reversible cross-linked polytriazole (RFPTA) resins is shown in Scheme 1, and the formulations of FPTA and BDM for the synthesis of RFPTA resin films are listed in Table S2. The following procedure shows the preparation process: First, FPTA was dissolved in 10 mL of DMF, and BDM was added after FPTA was completely dissolved. Second, the well-mixed solution was poured into a smooth horizontal dish, and DMF was slowly evaporated in an oven at 70°C for 24 h. Third, the dish with cross-linked polytriazole resins was slowly cooled at room temperature. Some deionized water was added to the dishes, and the resin films were separated from the dishes. Finally, the films would be placed in a vacuum oven at 70°C for 6 h to remove residual water and solvent to gain dry resin films. The obtained resins or resin films from FPTA-j are named as RFPTA-j (j = 0, 5, 10, 15, and 20)

2.5 Recycling of RFPTA resin films

The recycling of RFPTA was achieved by swelling the clipped RFPTA resin films in DMF for 24 h and then heating them at 110°C to obtain a solution of RFPTA resins, followed by a slow removal of the solvent in a surface dish at 70°C for 24 h. Some deionized water was added to the dishes, and the recycled resin films were separated from the dishes. At the end, the films would be placed in a vacuum oven at 70°C for 6 h to remove residual water and solvent.

3 Results and discussion

3.1 Characterizations and properties of FPTA resins

3.1.1 FT-IR spectroscopy of FPTA resins

The FT-IR spectrum of FPTA is shown in Figure 1. As shown in Figure 1, the absorption peak at 1,112 cm−1 belongs to the stretching vibration peak of –C–O–C– of PEG, and the wide absorption peak at 2,881 cm−1 belongs to the stretching vibration peak of –CH2– of PEG. Besides, the absorption peak observed at 1,501 cm−1 was attributed to the –C═C– group of furan. The overlapping peaks of the stretching vibrations of –C≡C– and –N3 at 2,100 cm−1 indicate the presence of residual alkynyl and azide groups.

Figure 1 FT-IR spectra of FPTA resins.
Figure 1

FT-IR spectra of FPTA resins.

3.1.2 GPC and solubility test results of FPTA resins

The GPC curves of FPTA resins are depicted in Figure 2, while the GPC analysis results of FPTA resins are presented in Table 1. The results show that all the FPTA resins have high relative molecular masses and narrow distributions. With the increase in ATPEG2000 content, the M̅n of FPTA increases from 1.20 × 104 to 1.70 × 104, and the M̅w increases from 2.03 × 104 to 3.67 × 104.

Figure 2 GPC spectra of FPTA resins.
Figure 2

GPC spectra of FPTA resins.

Table 1

GPC analysis results of the FPTA resins

FPTA resins M̅n (g·mol−1) M̅w (g·mol−1) PDI
FPTA-0 1.20 × 104 2.03 × 104 1.69
FPTA-5 1.25 × 104 2.77 × 104 2.22
FPTA-10 1.38 × 104 2.50 × 104 1.82
FPTA-15 1.42 × 104 2.56 × 104 1.80
FPTA-20 1.70 × 104 3.67 × 104 2.15

Additionally, solubility tests were conducted on FPTA resins and the results are shown in Table S3. PEG chain segments added to the FPTA resins improve the solubility of these resins.

3.1.3 DSC analysis of FPTA resins

Figure 3 shows the DSC analysis results for FPTA resins. As shown in the figure, the glass transition temperatures of FPTA resins decline as the ATPEG2000 content rises, largely because of the introduction of the flexible PEG structure.

Figure 3 DSC curves of FPTA resins.
Figure 3

DSC curves of FPTA resins.

3.2 Structure characterization and properties of RFPTA resins

3.2.1 In-situ FT-IR spectra of a mixture of FPTA resins and BDM

In-situ FT-IR spectroscopy was used to study the DA reaction between FPTA and BDM. The FPTA and BDM with an equivalent amount of functional groups were mixed and dissolved in DMSO solvent at room temperature, then the solution was cast on a KBr pellet, and finally, the solvent was removed with a freeze drier to form a film. The infrared tests were immediately carried out under heating, and some test time points were selected for comparison. The FT-IR spectra are shown in Figure 4. The benzene ring (in BDM) absorption peak (1,510 cm−1) is unchanged during the reaction and is chosen as the reference peak. As the reaction proceeds, the strength of the absorption peak for the double bond of maleimide (692 cm−1) decreases, indicating a DA reaction between BDM and the furan ring in FPTA. In the end, the absorption peak of the double bond of maleimide diminishes significantly, indicating that the reaction had reached a high extent. At the same time, an enhancement of the absorption peak at 1,775 cm−1 which belongs to the DA adduct can be observed. In addition, the residual overlapping peaks of alkyne and azide at 2,100 cm−1 gradually diminished at 70°C, indicating that the residual alkyne and azide groups continue to react with each other.

Figure 4 FT-IR spectra of the mixture of FPTA and BDM at 70°C with different holding times.
Figure 4

FT-IR spectra of the mixture of FPTA and BDM at 70°C with different holding times.

In order to investigate the optimal reaction temperature, the reaction extent of the mixture of FPTA and BDM for RFPTA-0 resin at different temperatures (40°C, 50°C, 60°C, 70°C, and 80°C) was investigated by in-situ FT-IR spectroscopy. Since the absorption peak of the benzene ring (1,510 cm−1) remains unchanged, it is selected as the reference peak. As the DA reaction progresses, the maleimide reacts with the furan to create a DA adduct, and the maleimide’s peak gradually diminishes. The intensity of the absorption peak of maleimide (692 cm−1) can be used to calculate the conversion of the maleimide group, which in turn determines the DA reaction degree. The formula for calculating the DA reaction degree is as follows:

(1) DA reaction degree = 1 − ( I / I ref ) t ( I / I ref ) t 0 × 100 %

where I and I ref represent the peak intensity at 692 cm−1 and the peak intensity at the reference peak (1,512 cm−1), respectively, t is the test time, and t 0 is the initial time.

The DA reaction degree of FPTA-0 and BDM for RFPTA-0 resin at various temperatures is shown in Figure 5a. It can be seen that the DA reaction degree increases rapidly and reaches a high value within 1 h. Meanwhile, the DA reaction shows an obvious temperature dependence at the early stage of the reaction, and the higher the reaction temperature, the faster the DA reaction is. Moreover, the DA reaction degree increases slowly after 1 h, and the DA reaction reaches an equilibrium state after 12 h. At 50°C, 60°C, and 70°C, the DA reaction degree for RFPTA-0 reaches equilibrium at around 65%, whereas at 40°C and 80°C, it drops to around 55%. The restricted energy input at 40°C hinders any increase in DA reaction degree in the 40°C scenario, and the decrease in DA reaction degree at 80°C can be attributed to the presence of the reverse DA (r-DA) reaction.

Figure 5 (a) DA reaction degree of FPTA-0 and BDM for RFPTA-0 resin at different temperatures and (b) DA reaction degree of FPTA-10 and BDM for RFPTA-10 resin at different temperatures.
Figure 5

(a) DA reaction degree of FPTA-0 and BDM for RFPTA-0 resin at different temperatures and (b) DA reaction degree of FPTA-10 and BDM for RFPTA-10 resin at different temperatures.

The DA reaction degree of FPTA-10 and BDM for RFPTA-10 resin at various temperatures is shown in Figure 5b. The DA reaction degree of RFPTA-10 and RFPRA-0 at different temperatures shows a similar pattern of change. Furthermore, the DA reaction degree grows steadily as the temperature rises. A lower DA reaction degree is observed at 40°C due to the lack of energy. The DA reaction reaches equilibrium at 80°C with a lower degree of DA reaction due to the appearance of the r-DA reaction. At the temperature of 70°C, a high DA reaction degree is exhibited. Thereby, 70°C is chosen as the crosslinking temperature for the preparation of thermally reversible polytriazole resins.

In particular, both RFPTA-0 and RFPTA-10 showed a decline in the DA reaction degree at 80°C after 1 h. The high concentration of free furan and maleimide structures in the early stages of the reaction provides strong support for the prevalence of the DA reaction. Moreover, the occurrence of the DA reaction leads to a decrease in the concentration of furan and maleimide, while the temperature governs the equilibrium between the DA reaction and the r-DA reaction.

To investigate the effect of PEG chain segments on the DA reaction, the DA reaction degree is calculated for RFPTA with different contents of PEG segments (j) at 70°C. The curves of DA reaction degree vs time are shown in Figure 6, and it can be seen that the initial slope of DA reaction degree vs time increases after the addition of PEG, and the DA reaction degree reaches the equilibrium value (∼90%) within 4 h, which is a significant improvement when compared with RFPTA-0 without PEG segments.

Figure 6 DA reaction degree of RFPTA resins at 70°C.
Figure 6

DA reaction degree of RFPTA resins at 70°C.

3.2.2 1H-NMR spectra of a mixture of FPTA resins and BDM

The mixture of FPTA resins and BDM has a reversible chemical structure by the DA reaction, and the reversible process can be identified by 1H-NMR spectroscopy. FPTA-10 and BDM were dissolved in DMSO-d6 in a NMR tube at room temperature and the 1H-NMR spectrum of FPTA-10 and BDM for RFPTA-10 at room temperature was obtained, and then the NMR tube with the mixture of FPTA resins and BDM was treated at 70°C for 24 h to obtain the 1H-NMR spectra of RFPTA-10 after DA reaction, and finally the NMR tube was heated at 110°C for 4 h to obtain the 1H-NMR spectra of RFPTA-10 after r-DA reaction. Figure 7 shows the NMR analysis results. As shown in Figure 7(a), the initial a, b, and c correspond to H of furan, and d is the peak of the double bond on the BDM. After the DA reaction of furan and maleimide, the formation of the DA adduct structure produces new peak positions: a′, b′, c′, d′exo, and d′endo as shown in Figure 7(b). In addition, a peak of d′exo close to the H2O is covered. After the occurrence of the r-DA reaction, the peaks a′, b′, c′, and d′ belonging to the DA adduct disappear (Figure 7(c)). Thereby, the structural reversibility of the RFPTA resins is demonstrated.

Figure 7 1H-NMR spectra of reactants and products in DMSO-d6. (a) The mixture of FPTA-10 and BDM at room temperature; (b) the reaction product of FPTA-10 and BDM at 70°C for 24 h; and (c) the reaction product of FPTA-10 and BDM at 70°C for 24 h and then 110°C for 4 h.
Figure 7

1H-NMR spectra of reactants and products in DMSO-d6. (a) The mixture of FPTA-10 and BDM at room temperature; (b) the reaction product of FPTA-10 and BDM at 70°C for 24 h; and (c) the reaction product of FPTA-10 and BDM at 70°C for 24 h and then 110°C for 4 h.

3.2.3 Thermal analyses of the mixture of FPTA resins and BDM for RFPTA resins

In order to reveal the temperatures of the DA reaction between FPTA resins and BDM, DSC analyses were conducted. The mixtures were obtained by successively dissolving FPTA resins and BDM in DMSO solvent and then removing the solvent using a freeze dryer. The DSC curves for the mixtures are shown in Figure 8. It can be seen that an exothermic peak attributed to the DA reaction occurs between 40°C and 100°C, and the top peak temperatures arrive at around 75°C, while an endothermic peak due to the r-DA reaction occurs between 100°C and 150°C with the top peak at about 130°C. Besides, a sharp endothermic peak at around 140°C appears due to the co-melting point of BDM with FPTA resins, which is close to the melting point of pure BDM (158.3°C, Figure S5).

Figure 8 DSC curves of the mixtures of FPTA resins and BDM (5°C·min−1).
Figure 8

DSC curves of the mixtures of FPTA resins and BDM (5°C·min−1).

3.2.4 Thermal reversibility of RFPTA resin films

In addition, the r-DA reaction of the RFPTA resins was measured by DSC analyses to study the effect of different contents of PEG segments on the thermal properties of the resins. The results are shown in Figure 9. As shown in Figure 9, the RFPTA resins have a significant endothermic peak due to which the r-DA reaction occurs between 80°C and 150°C, and the top peak temperatures reach around 110°C. Compared with the RFPTA-0, the peak of the r-DA reaction of the resins with the addition of PEG shows a decrease of about 5°C. This indicates that the presence of PEG segments is beneficial to the r-DA reaction. Moreover, the increasing proportion of PEG in the RFPTA resins leads to a decreasing proportion of the DA adduct, and the endothermic peak intensity of the r-DA reaction correspondingly decreases.

Figure 9 DSC curves of RFPTA resins for r-DA reaction (10°C·min−1).
Figure 9

DSC curves of RFPTA resins for r-DA reaction (10°C·min−1).

DMA tests of RFPTA resin films were carried out and DMA diagrams are shown in Figure 10. It can be observed that the RFPTA resin films exhibit a rapid decrease in modulus at above 80°C due to the glass transition. In particular, the modulus of RFPTA-20 at 80°C is quite low because of its low glass transition temperature. As the content of PEG segments increases, the temperature at which the reversible resins have a modulus of 0 MPa gradually decreases, and all resins are completely decross-linked at around 130°C due to the r-DA reaction.

Figure 10 DMA analysis of RFPTA resin films’ (a) storage modulus and (b) tan δ.
Figure 10

DMA analysis of RFPTA resin films’ (a) storage modulus and (b) tan δ.

The loss factor tan δ of the RFPTA materials shows that the glass transition of RFPTA resin films occurs between 70°C and 120°C. The glass transition temperature decreases with the addition of PEG segments because the chain segments move more easily with the increase in the number of flexible chain segments. Besides, it is also attributed to the slight decrease in crosslink density caused by the increase in the content of PEG segments in RFPTA resins.

Moreover, the internal friction of the resins increases sharply after 130°C, which is due to the disintegration of the cross-linked structure caused by the r-DA reaction. Especially, because of the high content of PEG segments, RFPTA-20 resin film softens significantly after the glass transition occurs, so the modulus drops to zero and Tanδ increases rapidly when the temperature is higher than 80°C.

The r-DA reaction is characterized by the sol-gel phenomenon. FPTA-0 and BDM with equal functional groups are dissolved in a small amount of DMF solution in a sample bottle, as shown in Figure 11(a). The reaction is carried out at 60°C for 48 h, and the solution becomes a yellowish gel, as shown in Figure 11(b), indicating that the DA reaction occurs and a cross-linked structure is produced.

Figure 11 Photos of RFPTA-0 in DMF solvent: (a) the solution of FPTA-0 and BDM at room temperature; (b) a gel formed at 60°C for 48 h; (c) flowing gel after heating at 110°C within 1 min; (d) a sol formed after heating at 110°C within 3 min; and (e) a gel formed again at 60°C for 48 h.
Figure 11

Photos of RFPTA-0 in DMF solvent: (a) the solution of FPTA-0 and BDM at room temperature; (b) a gel formed at 60°C for 48 h; (c) flowing gel after heating at 110°C within 1 min; (d) a sol formed after heating at 110°C within 3 min; and (e) a gel formed again at 60°C for 48 h.

After heating to 110°C, the gel starts to flow within 1 min and the cross-linked structure is disrupted to the semi-solution state shown in Figure 11(c), and the gel becomes a solution completely within 3 min, i.e., the gel with the cross-linked structure disintegrates as shown in Figure 11(d), indicating that the r-DA reaction occurs. Once again, the solution becomes a gel when it keeps at 60°C for 48 h as shown in Figure 11(e). This sol and gel formation process can be repeated many times, which demonstrates that the cross-linked resin formed is thermally reversible.

Figure 12 FT-IR spectra of RFPTA-0 before (a) and after (b) the r-DA reaction.
Figure 12

FT-IR spectra of RFPTA-0 before (a) and after (b) the r-DA reaction.

To clarify the reversible phenomenon, the RFPTA-0 resin after the r-DA reaction was characterized by IR spectra, as shown in Figure 12. With the occurrence of the r-DA reaction, the absorption peak of the maleimide double bond at 692 cm−1 is enhanced, and that of the furan at 1,150 cm−1 changes in the same way. Correspondingly, the absorption peak of the DA adduct at 1,775 cm−1 is weakened, indicating the dissociation of the DA adduct structure. This clearly demonstrates the occurrence of the r-DA reaction.

3.2.5 Mechanical properties and recyclability of RFPTA resin films

The content of PEG has a significant effect on the mechanical properties of the resins. The tensile properties of the RFPTA resin films are shown in Figure 13, and Figure 14 shows the stress–strain curves of the films. It can be seen from Figure 14 that with the increase in the content of PEG segments, both the strength and the initial slope of stress–strain curves that represent the modulus of the RFPTA first increase and then decrease, while the elongation gradually increases and has a rapid increase for RFPTA-20 resin. The elongation at break of RFPTA-20 resin arrives at 224.8%. Furthermore, when the content of PEG segments increases, so does the integral area of the stress–strain curve, which measures the material’s toughness. RFPTA-0 has a fracture toughness of 2.05 × 108 J·m−3, while RFPTA-20 has a fracture toughness of 3.12 × 109 J·m−3. As shown in Figure 13, RFPTA-0 resin exhibits low tensile strength and small elongation at break due to the rigidity of the triazole and benzene rings and the lower DA reaction degree as shown in Figure 6. With the increase in the content of PEG segments, the flexibility and the DA reaction degree of the resin increase. A significant increase in the strength of RFPTA-5 resin occurs. The strength of RFPTA-5 resin arrives at 107.2 MPa and increases by 100% as compared with that of RFPTA-0 resin.

Figure 13 Tensile properties of RFPTA resin films.
Figure 13

Tensile properties of RFPTA resin films.

Figure 14 Stress–strain curves of RFPTA resin films at room temperature.
Figure 14

Stress–strain curves of RFPTA resin films at room temperature.

The recycling experiment of the RFPTA resin films is shown in Figure 15. The fractured sample could not dissolve but swell in DMF at room temperature. When the temperature rises up to 110°C, the film could be completely dissolved within 5 min. According to the preparation process of the RFPTA resin film, the recycled film can be prepared again at 70°C.

Figure 15 Schematic diagram of the recycling experiment of the RFPTA resin films.
Figure 15

Schematic diagram of the recycling experiment of the RFPTA resin films.

The tensile test was performed on the recycled resin films (Table 2), and Figure 16 shows the stress–strain curves after recycling the RFPTA-5 resin film three times. The mechanical properties of the films after recycling once are almost the same, indicating that the RFPTA-5 resin has excellent recycling abilities. With the repetition of the recycling procedure, the tensile strength and tensile modulus of the recycled film show a decreasing trend, while the elongation at break does not change much. For the RFPTA-5 resin films, the recovery rates of tensile strength and tensile modulus after one recycle were 96.6% and 102.2%, and after three recycles were 73.3% and 56.8%, respectively. The potential deterioration of the bismaleimide and the furan rings in the resin may result in a drop in the mechanical properties.

Table 2

Tensile properties of RFPTA resin films before and after recycling

Samples Original film Recycled film (first recycle)
Tensile strength (MPa) Tensile modulus (GPa) Elongation at break (%) Tensile strength (MPa) Tensile modulus (GPa) Elongation at break (%)
RFPTA-0 53.5 0.9496 6.8 54.9 1.074 5.2
RFPTA-5 107.2 1.457 9.3 103.6 1.391 9.5
RFPTA-10 49.7 0.6442 21.2 49.8 0.6612 13.0
RFPTA-15 19.0 0.1771 100.6 22.3 0.2264 96.9
RFPTA-20 22.5 0.0851 224.8 20.6 0.0738 206.4
Figure 16 Comparison of stress–strain curves of RFPTA-5 resin films with different recycles.
Figure 16

Comparison of stress–strain curves of RFPTA-5 resin films with different recycles.

4 Conclusion

In this work, thermally reversible cross-linked RFPTA resins are successfully prepared by a DA reaction between BDM and FPTA resins. The results show that the flexible PEG segments could promote the DA reaction degree and play a role in the reinforcing and toughening of the reversible cross-linked polytriazole resins. The mechanical properties of the reversible resins could be adjusted by changing the content of PEG segments. The tensile strength of RFPTA-5 film is 107.2 MPa and is two times higher than that of RFPTA-0 resin. RFPTA-20 resin film shows an elongation at break of 224.8%. The reversible polytriazole resins demonstrate excellent recyclability. The RFPTA resin films and the recycled RFPTA resin films have comparable mechanical properties, and the recovery rates of tensile strength of the RFPTA resin films after one recycle are all above 90%.

  1. Funding information: The authors gratefully acknowledge the support of the Fundamental Research Funds for the Central Universities (no. JKD 01231701).

  2. Author contributions: Kejie Heng: investigation, writing – original draft, writing – review and editing, visualization, methodology, formal analysis, and project administration; Jun Zhang: methodology and formal analysis; Caiyun Wang: investigation and methodology; Keying Wang: methodology; Liqiang Wan: project management and supervision; Farong Huang: conceptualization, resources, funding acquisition, writing – review and editing, project administration, and supervision.

  3. Conflict of interest: The authors state no conflict of interest.

  4. Data availability statement: All data generated or analyzed in this study are included in this published article.

References

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Received: 2023-08-08
Revised: 2023-09-28
Accepted: 2023-11-15
Published Online: 2023-12-31

© 2023 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/epoly-2023-0119
Schlagwörter für diesen Artikel
recyclable polytriazole; novel recyclable thermosetting resin; Diels-Alder reaction; thermally reversible cross-linked polymer; green polymer
Creative Commons
BY 4.0

Artikel in diesem Heft

  1. Research Articles
  2. Chitosan nanocomposite film incorporating Nigella sativa oil, Azadirachta indica leaves’ extract, and silver nanoparticles
  3. Effect of Zr-doped CaCu3Ti3.95Zr0.05O12 ceramic on the microstructure, dielectric properties, and electric field distribution of the LDPE composites
  4. Effects of dry heating, acetylation, and acid pre-treatments on modification of potato starch with octenyl succinic anhydride (OSA)
  5. Loading conditions impact on the compression fatigue behavior of filled styrene butadiene rubber
  6. Characterization and compatibility of bio-based PA56/PET
  7. Study on the aging of three typical rubber materials under high- and low-temperature cyclic environment
  8. Numerical simulation and experimental research of electrospun polyacrylonitrile Taylor cone based on multiphysics coupling
  9. Experimental investigation of properties and aging behavior of pineapple and sisal leaf hybrid fiber-reinforced polymer composites
  10. Influence of temperature distribution on the foaming quality of foamed polypropylene composites
  11. Enzyme-catalyzed synthesis of 4-methylcatechol oligomer and preliminary evaluations as stabilizing agent in polypropylene
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  16. Thermal decomposition reaction kinetics and storage life prediction of polyacrylate pressure-sensitive adhesive
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  18. Doping silver nanoparticles into reverse osmosis membranes for antibacterial properties
  19. Melt-blended PLA/curcumin-cross-linked polyurethane film for enhanced UV-shielding ability
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  33. Preparation and properties of modified ammonium polyphosphate synergistic with tris(2-hydroxyethyl) isocynurate for flame-retardant LDPE
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  42. Effect of different tin neodecanoate and calcium–zinc heat stabilizers on the thermal stability of PVC
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  46. Fiber-reinforced polyvinyl alcohol hydrogel via in situ fiber formation
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  48. Synthesis of cadmium(ii) ion-imprinted composite membrane with a pyridine functional monomer and characterization of its adsorption performance
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  50. Comprehensive study of the radiation shielding feature of polyester polymers impregnated with iron filings
  51. Preparation and characterization of polymeric cross-linked hydrogel patch for topical delivery of gentamicin
  52. Mechanical properties of rCB-pigment masterbatch in rLDPE: The effect of processing aids and water absorption test
  53. Pineapple fruit residue-based nanofibre composites: Preparation and characterizations
  54. Effect of natural Indocalamus leaf addition on the mechanical properties of epoxy and epoxy-carbon fiber composites
  55. Utilization of biosilica for energy-saving tire compounds: Enhancing performance and efficiency
  56. Effect of capillary arrays on the profile of multi-layer micro-capillary films
  57. A numerical study on thermal bonding with preheating technique for polypropylene microfluidic device
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  63. Synthesis of low-VOC unsaturated polyester coatings for electrical insulation
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  70. Enhancing the therapeutic efficacy of Krestin–chitosan nanocomplex for cancer medication via activation of the mitochondrial intrinsic pathway
  71. Variation in tungsten(vi) oxide particle size for enhancing the radiation shielding ability of silicone rubber composites
  72. Damage accumulation and failure mechanism of glass/epoxy composite laminates subjected to repeated low velocity impacts
  73. Gamma-ray shielding analysis using the experimental measurements for copper(ii) sulfate-doped polyepoxide resins
  74. Numerical simulation into influence of airflow channel quantities on melt-blowing airflow field in processing of polymer fiber
  75. Cellulose acetate oleate-reinforced poly(butylene adipate-co-terephthalate) composite materials
  76. Radiation shielding capability and exposure buildup factor of cerium(iv) oxide-reinforced polyester resins
  77. Recyclable polytriazole resins with high performance based on Diels-Alder dynamic covalent crosslinking
  78. Adsorption and recovery of Cr(vi) from wastewater by Chitosan–Urushiol composite nanofiber membrane
  79. Comprehensive performance evaluation based on electromagnetic shielding properties of the weft-knitted fabrics made by stainless steel/cotton blended yarn
  80. Review Articles
  81. Preparation and application of natural protein polymer-based Pickering emulsions
  82. Wood-derived high-performance cellulose structural materials
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  84. Polymer-based nanocarriers for biomedical and environmental applications
  85. A review on semi-crystalline polymer bead foams from stirring autoclave: Processing and properties
  86. Rapid Communication
  87. Preparation and characterization of magnetic microgels with linear thermosensitivity over a wide temperature range
  88. Special Issue: Biodegradable and bio-based polymers: Green approaches (Guest Editors: Kumaran Subramanian, A. Wilson Santhosh Kumar, and Venkatajothi Ramarao)
  89. Synthesis and characterization of proton-conducting membranes based on bacterial cellulose and human nail keratin
  90. Fatigue behaviour of Kevlar/carbon/basalt fibre-reinforced SiC nanofiller particulate hybrid epoxy composite
  91. Effect of citric acid on thermal, phase morphological, and mechanical properties of poly(l-lactide)-b-poly(ethylene glycol)-b-poly(l-lactide)/thermoplastic starch blends
  92. Dose-dependent cytotoxicity against lung cancer cells via green synthesized ZnFe2O4/cellulose nanocomposites
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