Influence of the Epoxy/Acid Stoichiometry on the Cure Behavior and Mechanical Properties of Epoxy Vitrimers

Bisphenol A epoxy resin cured with a mixture of dimerized and trimerized fatty acids is the first epoxy vitrimer and has been extensively studied. However, the cure behavior and thermal and mechanical properties of this epoxy vitrimer depend on the epoxy/acid stoichiometry. To address these issues, epoxy vitrimers with three epoxy/acid stoichiometries (9:11, 1:1 and 11:9) were prepared and recycled four times. Differential scanning calorimetry (DSC) was used to study the cure behavior of the original epoxy vitrimers. The dynamic mechanical properties and mechanical performance of the original and recycled epoxy vitrimers were investigated by using dynamic mechanical analysis (DMA) and a universal testing machine. Furthermore, the reaction mechanism of epoxy vitrimer with different epoxy/acid stoichiometry was interpreted. With an increase in the epoxy/acid ratio, the reaction rate, swelling ratio, glass transition temperature and mechanical properties of the original epoxy vitrimers decreased, whereas the gel content increased. The recycling decreased the swelling ratio and elongation at break of the original epoxy vitrimers. Moreover, the elongation at break of the recycled epoxy vitrimers decreased with the epoxy/acid ratio at the same recycling time. However, the gel content, tensile strength and toughness of the original epoxy vitrimers increased after the recycling. The mechanical properties of epoxy vitrimers can be tuned with the variation in the epoxy/acid stoichiometry.


Introduction
Epoxy resin, one of the most important and popular classes of thermosetting polymers, is firmly rooted in many areas, both in our daily lives and in industry [1][2][3][4][5]. Due to its outstanding mechanical strength, dimensional and thermal stability, as well as creep, chemical and electrical insulation resistance, epoxy resin has been widely used in construction, adhesives, electronic and electrical devices, coatings, composites and so on [1,[6][7][8][9][10]. However, due to their insoluble and infusible nature, the irreversible covalent networks restrict epoxy resin from being recycled and reprocessed. Thus, most of the epoxy wastes are disposed of by landfill or incineration, which has caused not only a waste of resources but also environmental pollution. To solve these problems, a thrust to develop a novel class of thermosetting polymers with recyclable, healable and reprocessable features has been carried out during the last two decades [11,12].
In 2011, Leibler et al. developed a new class of polymers, called vitrimer, based on associative covalent adaptable networks of epoxy resins cured with a mixture of dimerized and trimerized fatty acids through transesterification reactions [13]. Vitrimers contain crosslinked networks constituted by dynamic covalent bonds, which can undergo exchangeable reactions without changing the crosslink density. In this case, vitrimers behave Table 1. Chemical structures and molecular weights (MWs) of reagents.

Epoxy oligomer
Bisphenol A epoxy oligomer (trade name of 0164) was obtained from Nantong Xingchen Synthetic Material Co., Ltd. (Nantong, China). A mixed fatty acid with about 23 wt% dimerized acid and 77 wt% trimerized acid was supplied by Shanghai Zhipu Chemical Co., Ltd. (Shanghai, China). Zinc acetate (Zn(Ac)2) was purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Table 1 lists detailed information about molecular weights and chemical structures of each reagent for epoxy vitrimers.

Preparation of Epoxy Vitrimer
Zinc acetate was mixed with the mixture of fatty acids to prepare a masterbatch of curing agents in a 500 mL beaker at 900 rpm at 180 °C for 2 h. The content of Zn(Ac)2 was 10 mol% of the COOH groups. The epoxy oligomer was introduced into the masterbatch and stirred at 1500 rpm for 3 min at 120 °C. Finally, the mixture was quickly poured into a PTFE (polytetrafluoroethylene) mold with a diameter of 100 mm and a height of 5 mm and placed in an oven at 120 °C for 4 h. Figure 1 presents a schematic illustration for the preparation of epoxy vitrimer. The stoichiometries of the epoxy to acid of epoxy vitrimers were 9:11, 1:1 and 11:9 and the respective samples were named as EV45, EV50 and EV50. Co., Ltd. (Shanghai, China). Zinc acetate (Zn(Ac)2) was purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Table 1 lists detailed information about molecular weights and chemical structures of each reagent for epoxy vitrimers.

Epoxy oligomer 192
Mixture of fatty acid 296 Zinc acetate 183.48

Preparation of Epoxy Vitrimer
Zinc acetate was mixed with the mixture of fatty acids to prepare a masterbatch of curing agents in a 500 mL beaker at 900 rpm at 180 °C for 2 h. The content of Zn(Ac)2 was 10 mol% of the COOH groups. The epoxy oligomer was introduced into the masterbatch and stirred at 1500 rpm for 3 min at 120 °C. Finally, the mixture was quickly poured into a PTFE (polytetrafluoroethylene) mold with a diameter of 100 mm and a height of 5 mm and placed in an oven at 120 °C for 4 h. Figure 1 presents a schematic illustration for the preparation of epoxy vitrimer. The stoichiometries of the epoxy to acid of epoxy vitrimers were 9:11, 1:1 and 11:9 and the respective samples were named as EV45, EV50 and EV50.

Preparation of Epoxy Vitrimer
Zinc acetate was mixed with the mixture of fatty acids to prepare a masterbatch of curing agents in a 500 mL beaker at 900 rpm at 180 °C for 2 h. The content of Zn(Ac)2 was 10 mol% of the COOH groups. The epoxy oligomer was introduced into the masterbatch and stirred at 1500 rpm for 3 min at 120 °C. Finally, the mixture was quickly poured into a PTFE (polytetrafluoroethylene) mold with a diameter of 100 mm and a height of 5 mm and placed in an oven at 120 °C for 4 h. Figure 1 presents a schematic illustration for the preparation of epoxy vitrimer. The stoichiometries of the epoxy to acid of epoxy vitrimers were 9:11, 1:1 and 11:9 and the respective samples were named as EV45, EV50 and EV50.

Preparation of Epoxy Vitrimer
Zinc acetate was mixed with the mixture of fatty acids to prepare a masterbatch of curing agents in a 500 mL beaker at 900 rpm at 180 • C for 2 h. The content of Zn(Ac) 2 was 10 mol% of the COOH groups. The epoxy oligomer was introduced into the masterbatch and stirred at 1500 rpm for 3 min at 120 • C. Finally, the mixture was quickly poured into a PTFE (polytetrafluoroethylene) mold with a diameter of 100 mm and a height of 5 mm and placed in an oven at 120 • C for 4 h. Figure 1 presents a schematic illustration for the preparation of epoxy vitrimer. The stoichiometries of the epoxy to acid of epoxy vitrimers were 9:11, 1:1 and 11:9 and the respective samples were named as EV45, EV50 and EV50.

Recycling of Epoxy Vitrimers
Cured epoxy vitrimer film was cut into small pieces and then reprocessed to bulk material by the hot press at 180 °C and 10 MPa for 20 min. Then, the recycled sample was cut into small pieces and reprocessed again. The recycling process was repeated three times at a 10 °C increase in temperature each time. The reprocessed samples for the first, second, third and fourth times were abbreviated as 1R, 2R, 3R and 4R, respectively. Figure  2 represents a schematic illustration for the preparation and recycling of epoxy vitrimer.

Recycling of Epoxy Vitrimers
Cured epoxy vitrimer film was cut into small pieces and then reprocessed to bulk material by the hot press at 180 • C and 10 MPa for 20 min. Then, the recycled sample was cut into small pieces and reprocessed again. The recycling process was repeated three times at a 10 • C increase in temperature each time. The reprocessed samples for the first, second, third and fourth times were abbreviated as 1R, 2R, 3R and 4R, respectively. Figure 2 represents a schematic illustration for the preparation and recycling of epoxy vitrimer.

Recycling of Epoxy Vitrimers
Cured epoxy vitrimer film was cut into small pieces and then reprocessed to bulk material by the hot press at 180 °C and 10 MPa for 20 min. Then, the recycled sample was cut into small pieces and reprocessed again. The recycling process was repeated three times at a 10 °C increase in temperature each time. The reprocessed samples for the first, second, third and fourth times were abbreviated as 1R, 2R, 3R and 4R, respectively. Figure  2 represents a schematic illustration for the preparation and recycling of epoxy vitrimer.

Methods
Tensile tests were conducted on an Instron 3366 testing machine according to ASTM D638 (type V). The dumbbell-shaped samples were tested at room temperature. The crosshead speed during measurements was 200 mm/min. At least five samples were measured for each composition. DMA was carried out on a DMA + 450 dynamic mechanical analyzer (01 dB Metravib, France). The samples with dimensions of 30 × 20 × 2.5 mm 3 were mounted on a tension clamp and tested at a ramping rate of 3 °C/min and a frequency of 1 Hz from −25 to 125 °C. The Tg was determined as the temperature of the maximum damping factor (tan δ) value in a tan δ versus temperature curve. Isothermal and dynamic DSC analyses were performed on a DSC 1 instrument (Mettler-Toledo, Switzerland) under a nitrogen

Methods
Tensile tests were conducted on an Instron 3366 testing machine according to ASTM D638 (type V). The dumbbell-shaped samples were tested at room temperature. The crosshead speed during measurements was 200 mm/min. At least five samples were measured for each composition. DMA was carried out on a DMA + 450 dynamic mechanical analyzer (01 dB Metravib, France). The samples with dimensions of 30 × 20 × 2.5 mm 3 were mounted on a tension clamp and tested at a ramping rate of 3 • C/min and a frequency of 1 Hz from −25 to 125 • C. The T g was determined as the temperature of the maximum damping factor (tan δ) value in a tan δ versus temperature curve. Isothermal and dynamic DSC analyses were performed on a DSC 1 instrument (Mettler-Toledo, Switzerland) under a nitrogen flow of 20 mL/min. For isothermal curing, the uncured sample (~20 mg) was heated rapidly to 120 • C and kept at that temperature until the heat flow leveled off to the baseline. The isothermal heat of reaction (∆H i ) was calculated by the integration of the exothermal curve in time. After the isothermal scan, the sample was cooled rapidly to 20 • C and then reheated to 300 • C at a heating rate of 10 • C/min to determine the heat of reaction during dynamic curing (∆H t ). The swelling ratio (SR) and gel content (GC) of cured networks were tested in toluene at room temperature for 48 h. After wiping the solvent, the swollen sample was weighed and dried at 80 • C for 24 h in a ventilated oven to remove the toluene. The SR and GC were calculated as follows: where w s is the weight of the swollen sample, w 0 is the initial weight and w d is the weight of the dried sample.

Cure Behavior
DSC is a powerful tool for monitoring the cure reaction of epoxy resins [36,37]. To determine the cure behavior of epoxy vitrimers, the uncured sample was cured by isothermal curing at 120 • C followed by dynamic curing. As shown in Figure 3, all epoxy vitrimers exhibit an exothermic peak during the isothermal curing and dynamic curing. The peak area of dynamic curing increases with the epoxy/acid ratio. The cure reaction of epoxy resins can be divided into two distinct stages: chemical control and diffusion control [38]. Chemical control reaction occurs at the beginning of curing and dominates until the appearance of vitrification, at which the cure reaction becomes very slow and finally stops, known as a diffusion-controlled reaction. In this circumstance, the heat flow levels out during the isothermal curing ( Figure 3a). However, during the subsequent dynamic curing, the higher temperature results in the continuing reaction of epoxy. Thus, an exothermic peak appears ( Figure 3b). The cure reactions of epoxides with acids in epoxy vitrimers are presented in Figure  4. Five main reactions are considered [21,25,39,40]: the polyaddition of epoxides and acids, forming the characteristic β-hydroxyl ester of epoxy vitrimer (1), ring-opening polymerization (ROP) via hydroxyl groups (2), condensation-esterification of acids and hydroxyl groups (3), catalytic ROP between epoxides, forming ether bonds (4) and transesterification of β-hydroxyl ester (5). For the epoxy vitrimer with a 1:1 epoxy/acid ratio, main Reactions (1, 2 and 5) take place. In addition, Reactions (2 and 3) or Reaction (4) occurs with the excess of acids or epoxides. It is worth noting that Reaction (4) generally takes place at an elevated temperature [41]. In addition, the steric hindrance of long-chain curing agents limits the reaction of epoxy resins [42,43]. Therefore, EV55 exhibits the lowest reaction rate and conversion during the isothermal curing among all epoxy vitrimers, as shown in Figure 3. For EV45, Reaction (2) is more pronounced than EV50 due to the excess of acid and the existence of the catalyst. In this case, EV45 exhibits the highest reaction rate and conversion among all epoxy vitrimers. It is believed that the instantaneous reaction rate (dα/dt) is proportional to the heat flow (dH/dt) during a cure reaction where α is the conversion (extent of reaction) and ∆H i + ∆H d is the total heat generated during isothermal and dynamic curings. The conversion is given by where ∆H t is the heat generated at a certain time in an isothermal DSC run. As shown in Figure 3a, all epoxy vitrimers exhibit an autocatalytic reaction during isothermal curing. Moreover, the heat generated at the dynamic curing increases with the epoxy/acid ratio ( Figure 3b). The conversion of epoxy vitrimers begins to decrease with the epoxy/acid ratio after 10 min curing at 120 • C, as shown in Figure 3c. When curing at 120 • C for 50 min, the conversions of EV45, EV50 and EV55 are 0.76, 0.72 and 0.61, respectively, indicating that the conversion and reaction rate of the epoxy vitrimers decreases with the epoxy/acid ratio.
The cure reactions of epoxides with acids in epoxy vitrimers are presented in Figure 4. Five main reactions are considered [21,25,39,40]: the polyaddition of epoxides and acids, forming the characteristic β-hydroxyl ester of epoxy vitrimer (1), ring-opening polymerization (ROP) via hydroxyl groups (2), condensation-esterification of acids and hydroxyl groups (3), catalytic ROP between epoxides, forming ether bonds (4) and transesterification of β-hydroxyl ester (5). For the epoxy vitrimer with a 1:1 epoxy/acid ratio, main Reactions (1, 2 and 5) take place. In addition, Reactions (2 and 3) or Reaction (4) occurs with the excess of acids or epoxides. It is worth noting that Reaction (4) generally takes place at an elevated temperature [41]. In addition, the steric hindrance of long-chain curing agents limits the reaction of epoxy resins [42,43]. Therefore, EV55 exhibits the lowest reaction rate and conversion during the isothermal curing among all epoxy vitrimers, as shown in Figure 3. For EV45, Reaction (2) is more pronounced than EV50 due to the excess of acid and the existence of the catalyst. In this case, EV45 exhibits the highest reaction rate and conversion among all epoxy vitrimers. of time at 120 °C of the original epoxy vitrimers.
The cure reactions of epoxides with acids in epoxy vitrimers are presented in Figure  4. Five main reactions are considered [21,25,39,40]: the polyaddition of epoxides and acids, forming the characteristic β-hydroxyl ester of epoxy vitrimer (1), ring-opening polymerization (ROP) via hydroxyl groups (2), condensation-esterification of acids and hydroxyl groups (3), catalytic ROP between epoxides, forming ether bonds (4) and transesterification of β-hydroxyl ester (5). For the epoxy vitrimer with a 1:1 epoxy/acid ratio, main Reactions (1, 2 and 5) take place. In addition, Reactions (2 and 3) or Reaction (4) occurs with the excess of acids or epoxides. It is worth noting that Reaction (4) generally takes place at an elevated temperature [41]. In addition, the steric hindrance of long-chain curing agents limits the reaction of epoxy resins [42,43]. Therefore, EV55 exhibits the lowest reaction rate and conversion during the isothermal curing among all epoxy vitrimers, as shown in Figure 3. For EV45, Reaction (2) is more pronounced than EV50 due to the excess of acid and the existence of the catalyst. In this case, EV45 exhibits the highest reaction rate and conversion among all epoxy vitrimers.

Solvent Stability and Gel Content
To determine the solvent stability of the original and recycled epoxy vitrimers, samples were immersed in toluene at room temperature for 48 h. As shown in Figure 5, except for EV55, all samples are stable in toluene after being immersed in toluene for 48 h.  Figure 6 depicts the swelling ratios and gel contents of the original and recycled epoxy vitrimers. For the original epoxy vitrimers, with the increase in the epoxy/acid ratio, the swelling ratio increases and the gel content decreases, indicating that the crosslinking density of epoxy vitrimers decreases with the epoxy/acid ratio. This trend is opposite to the vitrimer system of tetrafunctional epoxy/dimerized acid [44]. After recycling, the swelling ratio of the original epoxy vitrimers, especially EV55, lowers, as shown in Figure  6a. For recycled epoxy vitrimers, the swelling ratio of the recycled epoxy vitrimers decreases with the recycling time. In addition, the swelling ratio of recycled EV55 is higher than those of recycled EV50 and EV45 at the same recycling time. In contrast to the swell-  vitrimer system of tetrafunctional epoxy/dimerized acid [44]. After recycling, the swelling ratio of the original epoxy vitrimers, especially EV55, lowers, as shown in Figure 6a. For recycled epoxy vitrimers, the swelling ratio of the recycled epoxy vitrimers decreases with the recycling time. In addition, the swelling ratio of recycled EV55 is higher than those of recycled EV50 and EV45 at the same recycling time. In contrast to the swelling ratio, the recycling increases the gel content of the original epoxy vitrimers, especially for EV55. For EV50 and EV55, the gel content increases with the recycling time. However, the recycling time has little effect on the gel content of EV45 due to their high crosslinking density. It is important to mention that the highest swelling (146%) and lowest gel content (71%) of EV55 are caused by the lowest reaction rate and occurrence of catalytic ROP between epoxides at an elevated temperature, as mentioned previously. In this case, some sol of EV55 is soluble in toluene ( Figure 5) due to its lowest crosslinking density. However, after recycling at a higher temperature, catalytic ROP continues. Thus, the swelling ratio decreases and the gel content increases with the recycling time. Furthermore, the increased crosslinking density improves the solvent stability of recycled EV55.  Figure 6 depicts the swelling ratios and gel contents of the original and recycled epoxy vitrimers. For the original epoxy vitrimers, with the increase in the epoxy/acid ratio, the swelling ratio increases and the gel content decreases, indicating that the crosslinking density of epoxy vitrimers decreases with the epoxy/acid ratio. This trend is opposite to the vitrimer system of tetrafunctional epoxy/dimerized acid [44]. After recycling, the swelling ratio of the original epoxy vitrimers, especially EV55, lowers, as shown in Figure  6a. For recycled epoxy vitrimers, the swelling ratio of the recycled epoxy vitrimers decreases with the recycling time. In addition, the swelling ratio of recycled EV55 is higher than those of recycled EV50 and EV45 at the same recycling time. In contrast to the swelling ratio, the recycling increases the gel content of the original epoxy vitrimers, especially for EV55. For EV50 and EV55, the gel content increases with the recycling time. However, the recycling time has little effect on the gel content of EV45 due to their high crosslinking density. It is important to mention that the highest swelling (146%) and lowest gel content (71%) of EV55 are caused by the lowest reaction rate and occurrence of catalytic ROP between epoxides at an elevated temperature, as mentioned previously. In this case, some sol of EV55 is soluble in toluene ( Figure 5) due to its lowest crosslinking density. However, after recycling at a higher temperature, catalytic ROP continues. Thus, the swelling ratio decreases and the gel content increases with the recycling time. Furthermore, the increased crosslinking density improves the solvent stability of recycled EV55.    Figure 7 illustrates storage modulus-temperature curves of the original and recycled epoxy vitrimers. The effect of recycling on the E of the original epoxy vitrimers depends on the epoxy/acid ratio. In the glassy state, the E of the recycled EV45 is higher than that of the original one. However, during the glass transition, an opposite trend appears. In the rubbery state, the E of the recycled EV45 is not lower than that of the original one. For EV50, the effect of recycling on the original vitrimer is complicated. After the fourth recycling, the E' of the recycled vitrimer is higher than that of the original one within the whole temperature interval. Due to the significantly increased crosslinking density after the high-temperature recycling, all recycled vitrimers of EV55 have a higher E than the original one. In addition, the E value increases with the recycling time during and after the glass transition. the rubbery state, the E′ of the recycled EV45 is not lower than that of the original one. For EV50, the effect of recycling on the original vitrimer is complicated. After the fourth recycling, the E' of the recycled vitrimer is higher than that of the original one within the whole temperature interval. Due to the significantly increased crosslinking density after the high-temperature recycling, all recycled vitrimers of EV55 have a higher E′ than the original one. In addition, the E′ value increases with the recycling time during and after the glass transition.  Figure 8 shows loss modulus-temperature curves of the original and recycled epoxy vitrimers. Loss modulus indicates the viscous response of the viscoelastic material and describes the molecular motions of the polymers [45]. All loss modulus-temperature curves exhibit a glass transition peak. For EV45, the glass transition peak shifts to a lower temperature after recycling (Figure 8a). However, EV55 shows an opposite trend ( Figure  8c). Before the glass transition, the loss moduli of EV45 and EV55 are lower than those of recycled ones. However, the loss modulus of EV45 is higher than those of recycled ones during the glass transition and at the rubbery state (Figure 8a). For EV55, the trend of loss modulus during the glass transition and at the rubbery state is the same as the one before the glass transition. The effect of recycling on the loss modulus of EV50 is complicated. Before the glass transition, the loss modulus of EV50 is the same as that of the fourth recycling, but is higher than those of first, second and third recycling (Figure 8b). During the glass transition and at the rubbery state, the loss modulus of EV50 is the same as that of the third recycling, higher than that of the first recycling and lower than those of the second and fourth recycling.  [45]. All loss modulus-temperature curves exhibit a glass transition peak. For EV45, the glass transition peak shifts to a lower temperature after recycling (Figure 8a). However, EV55 shows an opposite trend (Figure 8c). Before the glass transition, the loss moduli of EV45 and EV55 are lower than those of recycled ones. However, the loss modulus of EV45 is higher than those of recycled ones during the glass transition and at the rubbery state (Figure 8a). For EV55, the trend of loss modulus during the glass transition and at the rubbery state is the same as the one before the glass transition. The effect of recycling on the loss modulus of EV50 is complicated. Before the glass transition, the loss modulus of EV50 is the same as that of the fourth recycling, but is higher than those of first, second and third recycling (Figure 8b). During the glass transition and at the rubbery state, the loss modulus of EV50 is the same as that of the third recycling, higher than that of the first recycling and lower than those of the second and fourth recycling.  Figure 9 presents the damping factor (tan δ)-temperature curves of the original and recycled epoxy vitrimers. In a tan δ versus temperature curve, the peak indicates the occurrence of glass transition. The temperature at the maximum tan δ is often defined as the glass transition temperature [46][47][48]. The tan δ-temperature curves of EV50 and EV55 exhibit only one tan δ peak at ~25 °C, indicating that these vitrimers have a single glass transition from glassy to rubbery state, also known as major transition or α transition due to the gradual chain movements. Apart from the similar glass transition to EV50 and EV55 at ~25 °C, it is interesting to note that another broad peak appears between 40 °C and 100 °C in the tan δ-temperature curve of EV45. In a polymer blend, this shoulder indicates another glass transition and two distinct glass transitions indicate the immiscibility between two components [49,50]. As shown in Figure 8a, no β glass transition peak appears after the α transition. Therefore, the original and recycled EV45 have another α transition,  Figure 9 presents the damping factor (tan δ)-temperature curves of the original and recycled epoxy vitrimers. In a tan δ versus temperature curve, the peak indicates the occurrence of glass transition. The temperature at the maximum tan δ is often defined as the glass transition temperature [46][47][48]. The tan δ-temperature curves of EV50 and EV55 exhibit only one tan δ peak at~25 • C, indicating that these vitrimers have a single glass transition from glassy to rubbery state, also known as major transition or α transition due to the gradual chain movements. Apart from the similar glass transition to EV50 and EV55 at Molecules 2022, 27, 6335 9 of 14 25 • C, it is interesting to note that another broad peak appears between 40 • C and 100 • C in the tan δ-temperature curve of EV45. In a polymer blend, this shoulder indicates another glass transition and two distinct glass transitions indicate the immiscibility between two components [49,50]. As shown in Figure 8a, no β glass transition peak appears after the α transition. Therefore, the original and recycled EV45 have another α transition, which may be attributed to the ring-opening polymerization via hydroxyl groups (Reaction 2), as discussed in Section 3.1. Figure 9 presents the damping factor (tan δ)-temperature curves of the original and recycled epoxy vitrimers. In a tan δ versus temperature curve, the peak indicates the occurrence of glass transition. The temperature at the maximum tan δ is often defined as the glass transition temperature [46][47][48]. The tan δ-temperature curves of EV50 and EV55 exhibit only one tan δ peak at ~25 °C, indicating that these vitrimers have a single glass transition from glassy to rubbery state, also known as major transition or α transition due to the gradual chain movements. Apart from the similar glass transition to EV50 and EV55 at ~25 °C, it is interesting to note that another broad peak appears between 40 °C and 100 °C in the tan δ-temperature curve of EV45. In a polymer blend, this shoulder indicates another glass transition and two distinct glass transitions indicate the immiscibility between two components [49,50]. As shown in Figure 8a, no β glass transition peak appears after the α transition. Therefore, the original and recycled EV45 have another α transition, which may be attributed to the ring-opening polymerization via hydroxyl groups (Reaction 2), as discussed in Section 3.1.  Figure 10 depicts the Tgs of the original and recycled epoxy vitrimers. The Tg of the original epoxy vitrimers decreases with the epoxy/acid ratio. A similar trend was reported in the tetrafunctional epoxy/dimerized acid vitrimer [44]. The variation in the Tg with the epoxy/acid ratio can be attributed to the crosslink density of epoxy vitrimers [33]. As discussed in Section 3.1, EV45 exhibits the fastest reaction rate, resulting in the highest crosslink density. On the contrary, the slowest reaction rate of EV55 leads to the lowest crosslink density. In this case, the Tg of the original epoxy vitrimers decreases with the epoxy/acid ratio.  Figure 10 depicts the T g s of the original and recycled epoxy vitrimers. The T g of the original epoxy vitrimers decreases with the epoxy/acid ratio. A similar trend was reported in the tetrafunctional epoxy/dimerized acid vitrimer [44]. The variation in the T g with the epoxy/acid ratio can be attributed to the crosslink density of epoxy vitrimers [33]. As discussed in Section 3.1, EV45 exhibits the fastest reaction rate, resulting in the highest crosslink density. On the contrary, the slowest reaction rate of EV55 leads to the lowest crosslink density. In this case, the T g of the original epoxy vitrimers decreases with the epoxy/acid ratio. For EV45, the recycling decreases the Tg of the original vitrimer. In addition, except for the fourth recycling, the Tg of recycled vitrimers slightly decreases with the recycling time. However, EV55 shows an opposite trend since the occurrence of the catalytic ringopening polymerization between epoxides occurs at a high recycling temperature, resulting in an increase in the crosslink density of the original epoxy vitrimer. For the recycled EV55, the Tg increases in the recycling time since the gradual increase in the recycling temperature brings about more catalytic ring-opening polymerization and, thus, a further increase in the crosslink density.  Figure 11 depicts the stress-strain curves of the original and recycled epoxy vitrimers obtained from the tensile tests. All samples behave as ductile materials with high strains but low stresses. In stress-strain curves, tensile strength is the maximum stress that a material can withstand before breaking while the stain at break is also called elongation at break [51]. For EV45, the recycling decreases the T g of the original vitrimer. In addition, except for the fourth recycling, the T g of recycled vitrimers slightly decreases with the recycling time. However, EV55 shows an opposite trend since the occurrence of the catalytic ring-opening polymerization between epoxides occurs at a high recycling temperature, resulting in an increase in the crosslink density of the original epoxy vitrimer. For the recycled EV55, the T g increases in the recycling time since the gradual increase in the recycling temperature brings about more catalytic ring-opening polymerization and, thus, a further increase in the crosslink density. Figure 11 depicts the stress-strain curves of the original and recycled epoxy vitrimers obtained from the tensile tests. All samples behave as ductile materials with high strains but low stresses. In stress-strain curves, tensile strength is the maximum stress that a material can withstand before breaking while the stain at break is also called elongation at break [51].  Figure 11 depicts the stress-strain curves of the original and recycled epoxy vitrimers obtained from the tensile tests. All samples behave as ductile materials with high strains but low stresses. In stress-strain curves, tensile strength is the maximum stress that a material can withstand before breaking while the stain at break is also called elongation at break [51].    Figure 12 presents the tensile strength of the original and recycled epoxy vitrimers. The tensile strength of the original vitrimers decreases with the epoxy/acid ratio. However, a contrary trend was shown in the tetrafunctional epoxy/dimerized acid vitrimers [44]. It is known that both thermal and mechanical properties of epoxy resins depend on crosslink density [52]. Higher crosslink density indicates both higher T g and tensile strength. Therefore, the decrease in the tensile strength is attributed to the decrease in the crosslink density of the original epoxy vitrimers with the epoxy/acid ratio. The recycling significantly increases the tensile strength of the original vitrimers except for the first recycling. For the tetrafunctional epoxy/dimerized acid vitrimers, an opposite trend was shown [44]. For the recycled epoxy vitrimers, the tensile strength exhibits the same trend as the original epoxy vitrimers after the first and second recycling. However, the tensile strength of the recycled epoxy vitrimers declines in the following sequence: EV50 > EV55 > EV45 after the third and fourth recycling. Figure 13 shows the elongation at break of the original and recycled epoxy vitrimers. Like the tensile strength, the elongation at break of the original epoxy vitrimers decreases with the epoxy/acid ratio. However, different from the tensile strength, the recycling lowers the elongation at break of the original epoxy vitrimers. A similar trend was reported in the tetrafunctional epoxy/dimerized acid vitrimers [44]. At the same recycling time, the elongation at break of recycled epoxy vitrimers also decreases with the epoxy/acid ratio.

Mechanical Properties
Tensile toughness, the area under the stress-strain curve, indicates the energy absorption up to the material's failure [53][54][55]. The tensile toughness of the original and recycled epoxy vitrimers is shown in Figure 14. Similar to the tensile strength and elongation at break, the tensile toughness of the original epoxy vitrimers decreases with the epoxy/acid ratio. Except for the first recycling of EV50 and EV55, the recycling increases the tensile toughness of the original epoxy vitrimers due to the increase in both the tensile strength and elongation at break. The tensile toughness of the recycled epoxy vitrimers increases with the recycling time, except for the fourth recycling of EV45. crosslink density of the original epoxy vitrimers with the epoxy/acid ratio. The recycling significantly increases the tensile strength of the original vitrimers except for the first recycling. For the tetrafunctional epoxy/dimerized acid vitrimers, an opposite trend was shown [44]. For the recycled epoxy vitrimers, the tensile strength exhibits the same trend as the original epoxy vitrimers after the first and second recycling. However, the tensile strength of the recycled epoxy vitrimers declines in the following sequence: EV50 > EV55 > EV45 after the third and fourth recycling.  Figure 13 shows the elongation at break of the original and recycled epoxy vitrimers. Like the tensile strength, the elongation at break of the original epoxy vitrimers decreases with the epoxy/acid ratio. However, different from the tensile strength, the recycling lowers the elongation at break of the original epoxy vitrimers. A similar trend was reported in the tetrafunctional epoxy/dimerized acid vitrimers [44]. At the same recycling time, the elongation at break of recycled epoxy vitrimers also decreases with the epoxy/acid ratio.    Figure 13 shows the elongation at break of the original and recycled epoxy vitrimers. Like the tensile strength, the elongation at break of the original epoxy vitrimers decreases with the epoxy/acid ratio. However, different from the tensile strength, the recycling lowers the elongation at break of the original epoxy vitrimers. A similar trend was reported in the tetrafunctional epoxy/dimerized acid vitrimers [44]. At the same recycling time, the elongation at break of recycled epoxy vitrimers also decreases with the epoxy/acid ratio.  Tensile toughness, the area under the stress-strain curve, indicates the energy absorption up to the material's failure [53][54][55]. The tensile toughness of the original and recycled epoxy vitrimers is shown in Figure 14. Similar to the tensile strength and elongation at break, the tensile toughness of the original epoxy vitrimers decreases with the epoxy/acid ratio. Except for the first recycling of EV50 and EV55, the recycling increases the tensile toughness of the original epoxy vitrimers due to the increase in both the tensile strength and elongation at break. The tensile toughness of the recycled epoxy vitrimers increases with the recycling time, except for the fourth recycling of EV45.

Conclusions
This work investigated the effect of epoxy/acid stoichiometry on the cure behavior

Conclusions
This work investigated the effect of epoxy/acid stoichiometry on the cure behavior and mechanical properties of epoxy vitrimers. Epoxy vitrimers with three epoxy/acid stoichiometries from the deficit to the excess of epoxy oligomers were prepared and recycled. Because of the catalytic ring-opening polymerization at elevated temperature as well as the steric effect of long-chain fatty acids, the epoxy vitrimer with the excess of epoxy oligomers shows the lowest reaction rate and conversion during the isothermal curing. The swelling ratio of the original epoxy oligomers decreases with the epoxy/acid ratio, whereas the gel content shows a contrary trend. The recycling decreases the swelling ratio but increases the gel content in the original epoxy vitrimers. The swelling ratio of the recycled epoxy vitrimers decreases with the recycling time, while the gel content shows an opposite trend. For the epoxy vitrimer with an excess of epoxy oligomers, the recycling increases the storage modulus. The T g of the original epoxy vitrimers decreases with the epoxy/acid ratio. The recycling decreases the T g of the original epoxy vitrimer with the deficit epoxy oligomers. However, the T g of the original epoxy vitrimer with the excess of epoxy oligomers increases after the recycling. In addition, the T g of the recycled epoxy vitrimer with an excess of epoxy oligomers increases with the recycling time. Both the tensile strength and toughness of the original vitrimers increase with the increase in the epoxy/acid ratio and increase after the recycling. However, the elongation at break shows a contrary trend. Both the tensile strength and toughness of the recycled epoxy vitrimers with the stoichiometry and the excess of epoxy oligomers increase with the recycling time.