Synthesis, Thermal and Mechanical Properties of Fully Biobased Poly (hexamethylene succinate-co-2,5-furandicarboxylate) Copolyesters

Poly (hexamethylene succinate) (PHS) is a biobased and biodegradable polyester. In this research, two fully biobased high-molecular-weight poly (hexamethylene succinate-co-2,5-furandicarboxylate) (PHSF) copolyesters with low hexamethylene furandicarboxylate (HF) unit contents (about 5 and 10 mol%) were successfully synthesized through a two-step transesterification/esterification and polycondensation method. The basic thermal behavior, crystal structure, isothermal crystallization kinetics, melting behavior, thermal stability, and tensile mechanical property of PHSF copolyesters were studied in detail and compared with those of PHS. PHSF showed a decrease in the melt crystallization temperature, melting temperature, and equilibrium melting temperature while showing a slight increase in the glass transition temperature and thermal decomposition temperature. PHSF copolyesters displayed the same crystal structure as PHS. Compared with PHS, PHSF copolyesters showed the improved mechanical property. The presence of about 10 mol% of HF unit increased the tensile strength from 12.9 ± 0.9 MPa for PHS to 39.2 ± 0.8 MPa; meanwhile, the elongation at break also increased from 498.5 ± 4.78% to 1757.6 ± 6.1%.


Introduction
Poly (hexamethylene succinate) (PHS) is a biodegradable aliphatic polyester, which can be synthesized from succinic acid (SA) and 1,6-hexanediol (HDO) through a two-stage melt polycondensation method. In recent years, PHS has been investigated in detail [1][2][3][4]. Yang et al. studied the basic thermal behavior of PHS. They reported that PHS had a glass transition temperature (T g ) of −47.4 • C. PHS exhibited double-melting endotherms due to the melting, recrystallization, and remelting mechanisms [5]. Gesti et al. studied the crystal structure and enzymatic degradation behavior of PHS. The crystal structure of PHS was determined as a monoclinic unit cell, with a = 1.612 nm, b = 1.464 nm, c = 1.440 nm, and β = 38.6 • C. They also studied the relationship between enzymatic degradation and the crystal domain of PHS [3]. Franco et al. studied the crystallization kinetics of PHS. Based on the Lauritzen and Hoffman secondary crystallization theory, the nucleation rate parameters (K g III and K g II ) and the transition temperature of the crystallization regime were calculated [4,6]. In addition, some PHS-based copolyesters and nanocomposites have also been reported in the literature [5,[7][8][9][10][11][12]. For instance, in a previous work, we synthesized poly (hexamethylene succinate-co-ethylene succinate) with 3 mol% of ethylene succinate unit (PHSE3) copolyesters with different molecular weights (12,000 and 58,000 g/mol) and studied the influence of the molecular weight on the crystallization behavior of PHSE3 [5]. Due to the lower melt viscosity and the higher mobility of the polymer chains, the crystallization of low-molecular-weight PHSE3 occurred more easily than that with high molecular weight. Therefore, the melting temperature (T m ) and melt crystallization temperature (T cc ) of the high-molecular-weight polyester decreased during 1.48 (g) ppm belonged to the methylene of the HF unit. For the PHSF copolyesters, all the signals from the two homopolymers were observed. Moreover, the intensities of the signal from proton a gradually decreased with an increase in the HF unit content. Therefore, all the above results indicated that the PHSF copolyesters were successfully synthesized. In addition, the actual composition of the HS unit was calculated via the following equation: where I h and I a are the integrations of the peaks belonging to the protons (h) and (a), respectively. As demonstrated in Table 1, the ratio of HS/HF was very close to the feed ratio of SA/DMFD, indicating the successful synthesis of PHSF copolyesters. was from SA, while the peaks appearing at 4.08 (b), 1.64 (c), and 1.38 (d) ppm were attributed to the methylene of the hexamethylene succinate (HS) unit in PHS. For PHF, the peak at 7.19 ppm (h) was attributed to the furan ring, while the peaks at 4.34 (e), 1.79 (f), and 1.48 (g) ppm belonged to the methylene of the HF unit. For the PHSF copolyesters, all the signals from the two homopolymers were observed. Moreover, the intensities of the signal from proton a gradually decreased with an increase in the HF unit content. Therefore, all the above results indicated that the PHSF copolyesters were successfully synthesized. In addition, the actual composition of the HS unit was calculated via the following equation: where Ih and Ia are the integrations of the peaks belonging to the protons (h) and (a), respectively. As demonstrated in Table 1, the ratio of HS/HF was very close to the feed ratio of SA/DMFD, indicating the successful synthesis of PHSF copolyesters. The average molecular weights of the PHS and PHSF copolyesters were measured. From Table 1, all samples showed relatively high molecular weights, with the Mn values being higher than 40,000 g/mol and the Mw values being higher than 67,000 g/mol. In addition, the PDI values were close to each other. The average molecular weights of the PHS and PHSF copolyesters were measured. From Table 1, all samples showed relatively high molecular weights, with the M n values being higher than 40,000 g/mol and the M w values being higher than 67,000 g/mol. In addition, the PDI values were close to each other. (220), (040), and (240) planes of the PHS crystals, respectively [3]. In addition, the PHSF copolyesters showed similar WAXD patterns as PHS, demonstrating that the presence of the HF unit did not change the crystal structure. Therefore, the HF unit should reside in the amorphous region and be expelled from the crystal lattice of the PHS crystals. From Figure 2, the crystallinity values of PHS, PHSF5, and PHSF10 were measured to be about 28%, 25%, and 20%, respectively, indicating that the presence of a small amount of HF unit decreased the crystallinity of PHS to some extent.  Figure 2 displays the wide-angle X-ray diffraction (WAXD) profiles of the PHS and PHSF copolyesters after crystallizing at 32 °C for 10 h. For the PHS homopolyester, three main diffraction peaks appearing at 2θ = 21.3°, 24.2°, and 30.1° were attributed to the (220), (040), and (240) planes of the PHS crystals, respectively [3]. In addition, the PHSF copolyesters showed similar WAXD patterns as PHS, demonstrating that the presence of the HF unit did not change the crystal structure. Therefore, the HF unit should reside in the amorphous region and be expelled from the crystal lattice of the PHS crystals. From Figure 2, the crystallinity values of PHS, PHSF5, and PHSF10 were measured to be about 28%, 25%, and 20%, respectively, indicating that the presence of a small amount of HF unit decreased the crystallinity of PHS to some extent.

Basic Thermal Parameters Study
The effect of the HF unit on the basic thermal behavior of PHS was investigated with differential scanning calorimetry (DSC) in this section. Figure 3 depicts the thermal behavior of PHS and its copolyesters, including glass transition, melting behavior, and melt crystallization behavior. From Figure 3a, the Tcc of PHS was 26.1 °C. With the increasing HF unit content, the Tcc significantly decreased to 15.9 °C for PHSF5 and 2.4 °C for PHSF10, respectively. Moreover, the melt crystallization enthalpy (ΔHcc) decreased from 66.2 J/g for PHS to 49.4 J/g for PHSF5 and 37.6 J/g for PHSF10, respectively, indicating again that the crystallinity of PHS was suppressed by the presence of the HF unit. From Figure 3b, PHS displayed a Tm of 53.4 °C, with a ∆Hm of 68.3 J/g. However, the Tm and ∆Hm of the PHSF copolyesters gradually decreased with an increasing HF unit content because the regularity of the molecular chain of PHS was disturbed by the HF unit. Moreover, as shown in Figure 3b, double-melting endothermic peaks were observed for all samples during the heating process. With an increasing HF unit content, double-melting endothermic peaks became more obvious. Such a phenomenon may well be explained by the melting, recrystallization, and remelting mechanism [27][28][29]. Figure 3c depicts the enlarged glass transition region. From Figure 3c, PHS had a very low Tg of −48.8 °C, while the Tg of the PHSF copolyesters slightly increased because the presence of the furan ring decreased the chain mobility. The relevant basic thermal parameters of the PHS and PHSF copolyesters are listed in Table 2 for comparison.

Basic Thermal Parameters Study
The effect of the HF unit on the basic thermal behavior of PHS was investigated with differential scanning calorimetry (DSC) in this section. Figure 3 depicts the thermal behavior of PHS and its copolyesters, including glass transition, melting behavior, and melt crystallization behavior. From Figure 3a, the T cc of PHS was 26.1 • C. With the increasing HF unit content, the T cc significantly decreased to 15.9 • C for PHSF5 and 2.4 • C for PHSF10, respectively. Moreover, the melt crystallization enthalpy (∆H cc ) decreased from 66.2 J/g for PHS to 49.4 J/g for PHSF5 and 37.6 J/g for PHSF10, respectively, indicating again that the crystallinity of PHS was suppressed by the presence of the HF unit. From Figure 3b, PHS displayed a T m of 53.4 • C, with a ∆H m of 68.3 J/g. However, the T m and ∆H m of the PHSF copolyesters gradually decreased with an increasing HF unit content because the regularity of the molecular chain of PHS was disturbed by the HF unit. Moreover, as shown in Figure 3b, double-melting endothermic peaks were observed for all samples during the heating process. With an increasing HF unit content, double-melting endothermic peaks became more obvious. Such a phenomenon may well be explained by the melting, recrystallization, and remelting mechanism [27][28][29]. Figure 3c depicts the enlarged glass transition region. From Figure 3c, PHS had a very low T g of −48.8 • C, while the T g of the PHSF copolyesters slightly increased because the presence of the furan ring decreased the chain mobility. The relevant basic thermal parameters of the PHS and PHSF copolyesters are listed in Table 2 for comparison. Table 2. Basic thermal behavior data of PHS and PHSF copolyesters.

Isothermal Crystallization Kinetics Study
The isothermal melt crystallization kinetics study of the PHS and PHSF copolyesters was further investigated with DSC. Figure 4 shows the variation in relative crystallinity (X t ) with crystallization time (t) for PHS and PHSF5 at different T c values. As illustrated in Figure 4, for the same sample, the crystallization time prolonged with increasing T c , indicating a slow crystallization rate. At the same T c of 32.5 • C, the crystallization time of PHSF5 was remarkably longer than that of PHS, indicating that the copolyester crystallized more slowly than PHS. The above results implied that both the small degree of supercooling and the slightly increased HF unit were the two main reasons for the longer crystallization time. PHSF10 showed a similar variation trend, as illustrated in Figure S1 in the Supporting Information.

Samples
Td

Isothermal Crystallization Kinetics Study
The isothermal melt crystallization kinetics study of the PHS and PHSF copolyesters was further investigated with DSC. Figure 4 shows the variation in relative crystallinity (Xt) with crystallization time (t) for PHS and PHSF5 at different Tc values. As illustrated in Figure 4, for the same sample, the crystallization time prolonged with increasing Tc, indicating a slow crystallization rate. At the same Tc of 32.5 °C, the crystallization time of PHSF5 was remarkably longer than that of PHS, indicating that the copolyester crystallized more slowly than PHS. The above results implied that both the small degree of supercooling and the slightly increased HF unit were the two main reasons for the longer crystallization time. PHSF10 showed a similar variation trend, as illustrated in Figure S1 in the Supporting Information.   The well-known Avrami equation was utilized to analyze the overall isothermal melt crystallization kinetics of the PHS and PHSF copolyesters. The relationship between X t and t may be described by the Avrami equation as follows: where k is the crystallization rate constant related to both nucleation and growth-rate parameters, and n is the Avrami exponent depending on the nature of nucleation and the growth geometry of the crystals [30,31]. Figure 5 displays the related Avrami plots for PHS and PHSF5, while the Avrami plots of PHSF10 are illustrated in Figure S2 for brevity. The Avrami parameters n and k were calculated from the slopes and intercepts of the corresponding plots and are summarized in Table 3. The n values slightly varied from 2.3 to 2.7 for both PHS and PHSF copolyesters, indicating that the crystallization mechanism remained unchanged for all samples. The crystallization mechanism of the PHS and PHSF copolyesters may correspond to an athermal nucleation and three-dimensional spherulite growth [32], which needs further investigation on the basis of optical microscopy observation. On the one hand, the k values gradually decreased with an increase in T c for both PHS and PHSF copolyesters, suggesting again that the decreased degree of supercooling reduced the crystallization rate. On the other hand, the k values became smaller in the copolyesters with an increase in the HF unit content at the same T c , confirming again that the higher the HF unit content, the slower the crystallization rate. In addition, crystallization half-time (t 0.5 ), the time to achieve 50% of the final crystallinity, was calculated by the following equation: The influence of T c and the HF unit content on the crystallization rate may further be concluded from the t 0.5 data listed in Table 2, similar to those from the k values.
The well-known Avrami equation was utilized to analyze the overall isothermal melt crystallization kinetics of the PHS and PHSF copolyesters. The relationship between Xt and t may be described by the Avrami equation as follows: 1 − X t = e -kt n (2) where k is the crystallization rate constant related to both nucleation and growth-rate parameters, and n is the Avrami exponent depending on the nature of nucleation and the growth geometry of the crystals [30,31]. Figure 5 displays the related Avrami plots for PHS and PHSF5, while the Avrami plots of PHSF10 are illustrated in Figure S2 for brevity. The Avrami parameters n and k were calculated from the slopes and intercepts of the corresponding plots and are summarized in Table 3. The n values slightly varied from 2.3 to 2.7 for both PHS and PHSF copolyesters, indicating that the crystallization mechanism remained unchanged for all samples. The crystallization mechanism of the PHS and PHSF copolyesters may correspond to an athermal nucleation and three-dimensional spherulite growth [32], which needs further investigation on the basis of optical microscopy observation. On the one hand, the k values gradually decreased with an increase in Tc for both PHS and PHSF copolyesters, suggesting again that the decreased degree of supercooling reduced the crystallization rate. On the other hand, the k values became smaller in the copolyesters with an increase in the HF unit content at the same Tc, confirming again that the higher the HF unit content, the slower the crystallization rate. In addition, crystallization half-time (t0.5), the time to achieve 50% of the final crystallinity, was calculated by the following equation: The influence of Tc and the HF unit content on the crystallization rate may further be concluded from the t0.5 data listed in Table 2, similar to those from the k values.

Melting Behavior and Equilibrium Melting-Point Study
The subsequent melting behavior was further studied with DSC for PHS and its copolyesters after crystallizing at indicated temperatures. As shown in Figures 6 and S3 in the Supporting Information, both PHS and its copolyesters displayed similar melting behavior. Unlike the melting behavior after the nonisothermal melt crystallization (Figure 3b), only one endothermic peak was observed after the isothermal melt crystallization at the indicated temperatures. Furthermore, the T m gradually increased with the increasing of the T c . During the heating process after the isothermal melt crystallization, the melting, recrystallization, and remelting phenomenon did not occur, which should be attributed to the more perfect crystals formed through this crystallization process.
where η is related to the stability of the crystals undergoing the melting process [33].
where η is related to the stability of the crystals undergoing the melting process [33]. The classical Hoffman-Weeks equation was used to determine the equilibrium melting-point (Tm o ) values of PHS and PHSF copolyesters as follows: where η is related to the stability of the crystals undergoing the melting process [33].

Thermal Stability Study
The thermal stability of the PHS and PHSF copolyesters was studied with thermogravimetric analysis (TGA). As illustrated in Figure 8, both PHS and PHSF copolyesters displayed a one-step thermal decomposition process. From parts a and b of Figure 8, the temperature at 5% weight loss (Td) and the maximum weight-loss rate (Tmax) were read, respectively, and are listed in Table 2. All samples showed rather high Td and Tmax values, which is indicative of the high thermal stability. For instance, the Td slightly increased from 365.7 °C for PHS to 368.5 °C for PHSF5 and 369.2 °C for PHSF10, respectively, confirming that the presence of the HF unit retarded the thermal degradation and increased

Thermal Stability Study
The thermal stability of the PHS and PHSF copolyesters was studied with thermogravimetric analysis (TGA). As illustrated in Figure 8, both PHS and PHSF copolyesters displayed a one-step thermal decomposition process. From parts a and b of Figure 8, the temperature at 5% weight loss (T d ) and the maximum weight-loss rate (T max ) were read, respectively, and are listed in Table 2. All samples showed rather high T d and T max values, which is indicative of the high thermal stability. For instance, the T d slightly increased from 365.7 • C for PHS to 368.5 • C for PHSF5 and 369.2 • C for PHSF10, respectively, confirming that the presence of the HF unit retarded the thermal degradation and increased the thermal stability of PHS to some extent. However, it should be noted that the T max values varied slightly between 421.1 and 422.4 • C for all samples, indicating that the presence of the HF unit did not show an obvious influence on T max .

Mechanical Property Study
The tensile mechanical property study should be of great importance from a practical application viewpoint. Figure 9 presents the stress-strain curves of PHS, PHSF5, and PHSF10. All samples showed an obvious yield behavior at the small strain region. From Figure 9, the relevant tensile mechanical property data, such as the Young's modulus (E t ), yield strength (σ y ), tensile strength (σ b ), and elongation at break (ε b ), were determined and are summarized in Table 4. The E t values decreased slightly from 394.7 ± 7.6 MPa for PHS to 378.2 ± 24.2 MPa for PHSF5 and 305.0 ± 6.5 MPa for PHSF10. The σ y of PHS was 16.9 ± 1.1 MPa. The σ y of PHSF5 was 16.9 ± 1.5 MPa, while the σ y of PHSF5 was only 11.0 ± 0.1 MPa. Compared with those of PHS, the σ b and ε b values of the PHSF copolyesters significantly increased. The σ b and ε b values of PHS were only 12.9 ± 0.9 MPa and 498.5 ± 4.78%, respectively. In the presence of about 5 mol% of HF unit, PHSF5 displayed a σ b of 33.4 ± 1.4 MPa and an ε b of 1228.4 ± 94.3%, respectively, while with further increasing of the HF unit to about 10 mol%, PHSF10 displayed a σ b of 39.2 ± 0.8 MPa and an ε b of 1757.6 ± 6.1%, respectively. Such an improvement in the mechanical property should be related to the presence of the rigid HF unit and the decrease in crystallinity of the copolymers. As a result, the tensile mechanical property of PHS may be remarkably improved through the copolymerization method in this research, which should be of great help and importance for its further practical application as a packaging material.

Mechanical Property Study
The tensile mechanical property study should be of great importance from a practical application viewpoint. Figure 9 presents the stress-strain curves of PHS, PHSF5, and PHSF10. All samples showed an obvious yield behavior at the small strain region. From Figure 9, the relevant tensile mechanical property data, such as the Young's modulus (Et), yield strength (σy), tensile strength (σb), and elongation at break (εb), were determined and are summarized in Table 4. The Et values decreased slightly from 394.7 ± 7.6 MPa for PHS to 378.2 ± 24.2 MPa for PHSF5 and 305.0 ± 6.5 MPa for PHSF10. The σy of PHS was 16.9 ± 1.1 MPa. The σy of PHSF5 was 16.9 ± 1.5 MPa, while the σy of PHSF5 was only 11.0 ± 0.1 MPa. Compared with those of PHS, the σb and εb values of the PHSF copolyesters significantly increased. The σb and εb values of PHS were only 12.9 ± 0.9 MPa and 498.5 ± 4.78%, respectively. In the presence of about 5 mol% of HF unit, PHSF5 displayed a σb of 33.4 ± 1.4 MPa and an εb of 1228.4 ± 94.3%, respectively, while with further increasing of the HF unit to about 10 mol%, PHSF10 displayed a σb of 39.2 ± 0.8 MPa and an εb of 1757.6 ± 6.1%, respectively. Such an improvement in the mechanical property should be related to the presence of the rigid HF unit and the decrease in crystallinity of the copolymers. As a result, the tensile mechanical property of PHS may be remarkably improved through the copolymerization method in this research, which should be of great help and importance for its further practical application as a packaging material.    305.0 ± 6.5 11.0 ± 0.1 39.2 ± 0.8 1757.6 ± 6.1

Conclusions
In this research, two fully biobased PHSF copolyesters with about 5 and 10 mol% of HF unit were successfully synthesized through a two-step melt polycondensation process. Both the chemical structure and the HF composition were analyzed and determined by 1 H NMR. PHS and PHSF copolyesters had rather high M w values of 6.75 × 10 4 to 9.30 × 10 4 g/mol. The basic thermal behavior, crystal structure, isothermal crystallization kinetics, melting behavior, thermal stability, and tensile mechanical property of the PHSF copolyesters were studied in detail and compared with those of PHS. Due to the presence of a small amount of HF unit, PHSF showed a decrease in the melt crystallization temperature, melting temperature, and equilibrium melting temperature, while showing a slight increase in the glass transition temperature and thermal decomposition temperature. PHSF copolyesters displayed the same crystal structure as PHS, indicating that the HF unit should be expelled into the amorphous region of the PHS crystals. The crystallization rate became slower with an increasing crystallization temperature for both PHS and PHSF copolyesters; moreover, PHSF copolyesters crystallized more slowly than PHS. Within the investigated temperature range, the crystallization mechanism remained unchanged. In addition, PHSF copolyesters showed an improved mechanical property compared with that of PHS. For example, PHSF10 displayed a σ b of 39.2 ± 0.8 MPa and an ε b of 1757.6 ± 6.1%, while those of PHS were only 12.9 ± 0.9 MPa and 498.5 ± 4.78%, respectively. In brief, through an easy copolymerization method, the thermal and mechanical properties of PHS may be tuned to meet practical application requirements from a sustainable viewpoint.