A Comparative Study on the Melt Crystallization of Biodegradable Poly(butylene succinate-co-terephthalate) and Poly(butylene adipate-co-terephthalate) Copolyesters

As an important biodegradable and partially biobased copolyester, poly(butylene succinate-co-terephthalate) (PBST) possesses comparable thermal and mechanical properties and superior gas barrier performance when compared with poly(butylene adipate-co-terephthalate) (PBAT), but it was found to display poorer melt processability during pelletizing and injection molding. To make clear its melt crystallization behavior under rapid cooling, PBST48 and PBST44 were synthesized, and their melt crystallization was investigated comparatively with PBAT48. PBST48 showed a PBAT48-comparable melt crystallization performance at a cooling rate of 10 °C/min or at isothermal conditions, but it showed a melt crystallization ability at a cooling rate of 40 °C/min which was clearly poorer. PBST44, which has the same mass composition as PBAT48, completely lost its melt crystallization ability under the rapid cooling. The weaker chain mobility of PBST, resulting from its shorter succinate moiety, is responsible for its inferior melt crystallization ability and processability. In comparison with PBAT48, PBST48 displayed higher tensile modulus, and both PBST48 and PBST44 showed higher light transmittance. The findings in this study deepen the understanding of PBST’s properties and will be of guiding significance for improving PBST’s processability and application development.

Poly(butylene succinate-co-terephthalate) (PBST) is another biodegradable aliphaticaromatic copolyester, with a structure and properties similar to PBAT [15][16][17][18].After extensive studies on synthetic technology [19][20][21] and process optimization [22,23], PBST was industrialized at Sinopec Yizheng Chemical Fiber Co., Ltd., Yizheng, China in 2020.It is a partially biobased polymer because SA is a biobased monomer [24,25].In fact, biobased SA has been commercially produced at a cost comparable to petroleum-based SA.As the cost of biobased succinic acid gradually declines, the cost of PBST will become increasingly competitive, although it is currently higher than that of PBAT.Furthermore, we found recently that PBST displays 3-4 times more oxygen and carbon dioxide and a 1.6 times stronger water vapor barrier performance when compared with PBAT.This demonstrates that the two fewer methylene groups in the succinate moiety of PBST result in a smaller free volume fraction and a slower gas diffusion [26].The advantages of PBST's gas barrier have been confirmed by other subsequent reports [27].The biobased nature and higher gas barrier make PBST more promising in demanding applications such as food package materials.In fact, the R&D of a variety of differentiated products based on PBST has been reported recently [28][29][30][31][32][33].
Although PBST has advantages in sustainability and the gas barrier, its melt processability seems inferior to that of PBAT.During PBST's industrial development, it was found that it is harder to cool it down in time for pelleting after melt polycondensation or for demolding in injection molding.In comparison, PBAT is clearly easier to pelletize and inject under the same conditions.The poorer melt processability of PBST suggests inferior melt crystallizability.However, in most reports on PBST crystallization in the literature [16,34,35], PBST displayed melt crystallization behavior similar to PBAT, with the exception of the findings of Heidarzadeh et al. [36].Heidarzadeh et al. reported that PBST melt crystallization demonstrates a higher degree of super-cooling than PBAT.As the cooling rate (mostly, 10 • C/min) used in these DSC studies of PBST [16,34,35] is much slower than that in the real melt-processing process, these DSC results do not reflect the real melt crystallizability of PBST in the industrial process.Therefore, it is necessary to evaluate the melt crystallizability at a rapid cooling rate.On the other hand, the fact that PBST possesses a significantly higher gas barrier than PBAT [26] reminds us that the tiny difference in chain structure between them may also result in a clear difference in melt crystallization and even in some crystallization-related macroscopic properties.Although the physical and mechanical properties of PBST and PBAT have been extensively studied [5][6][7][15][16][17][18][19][20][21], a comparative study is still missing.
In this study, PBAT 48 , PBST 48, and PBST 44 copolyesters with the same molar (48 mol% BT) or mass (50 wt% BT) composition as commercial PBAT resin were synthesized, and their melt crystallization behaviors at normal cooling, rapid cooling, and isothermal condition were studied comparatively.Their light transmittance performance and tensile properties were also studied.
The copolyesters were melt-processed by a heat press (GT-7014-A50C, Taiwan, China) under about 15 MPa at 165 • C to prepare film samples with a thickness of about 400 µm.The films were directly used for WAXD observation and optical property measurement.Dumbbell-shaped standard specimens (2 × 25 × 0.4 mm 3 ) were prepared from the films by a cutter and used for the tensile test.

Characterization
The copolyesters (0.1250 g) were dissolved in chloroform.The resulting solution (0.005 g/mL) was used for an intrinsic viscosity (IV) measurement at 25 • C using a IVS300 semi-automatic viscometer tester (Hangzhou Zhongwang Co., Hangzhou, China) equipped with an Ubbelohde viscometer (inner diameter 0.36 mm). 1 H NMR spectra were recorded with a Bruker AC-80 spectroscopy instrument (400 M).Deuterated chloroform was used as the solvent and tetramethylsilane was used as the internal reference.The pulse sequence, temperature, and pulse duration were one pulse, 25 • C, and 1 s, respectively.
Thermal transition behaviors of PBAT and PBST were recorded with differential scanning calorimetry (DSC, Q200, TA Instrument Co., New Castle, DE, USA) under nitrogen flow.For nonisothermal crystallization and melting, the samples (6-8 mg) were heated at 10 • C/min from room temperature to 220 • C, kept at this temperature for 5 min, then cooled at 10 • C/min or 40 • C/min to −80 • C, kept at this temperature for another 5 min, and finally heated to 220 • C at 10 • C/min.For the isothermal crystallization and melting, the samples were heated at 10 • C/min from room temperature to 220 • C, kept at this temperature for 5 min, then quenched to the predetermined melt crystallization temperature (T c ), kept at this temperature for 30 min, then cooled at 10 • C/min to −80 • C, and finally heated to 220 • C at 10 • C/min.
Wide-angle X-ray diffraction (WAXD) patterns of PBAT and PBST were recorded with the X-ray diffractometer (PANalytical B.V., X-pert-Powder, Almelo, The Netherlands) with a CuKα radiation (1.54 Å), working at 40 KV and 40 mA.The sample was scanned from 2θ = 5 • to 2θ = 80 • with a step size of 0.026 • and an acquisition time of 30 s per step.
The transmittance and haze of the copolyesters were determined using a CS-821 N desktop spectrophotometric colorimeter (Hangzhou CHNSpec Technology Co. Ltd., Hangzhou, China).All of the tests were carried out in the visible wavelength range 400-800 nm.
The tensile properties of the copolyesters were measured with a Zwick/Roell Z020 (Zwick Co., Ulm, Germany) universal testing machine at a tensile speed of 50 mm/min.The value of the load cell and preload were 500 N and 0 N, respectively.All of the specimens were kept at 25 • C and 50% relative humidity for at least 48 h before testing.At least five specimens were tested for each sample.

Results and Discussion
As the BT unit content in commercial PBAT is usually about 47-48 mol% or 50-51 wt%, one PBAT (PBAT 48 ) and two PBST (PBST 44 and PBST 48 ) samples with the same molar (~48 mol%, PBAT 48 and PBST 48 ) or mass percentage (~50 wt%, PBAT 48 and PBST 44 ) of BT unit were synthesized and used for this study.The 1 H NMR characterization and calculation of the chemical composition, average sequence length, and randomness degree are listed in the supporting information (Figure S1).The copolymer molar and mass compositions, the average sequence length, the randomness degree, and the intrinsic viscosity of the copolyesters are listed in Table 1.Random copolyesters with high-enough intrinsic viscosity (>1.0 dL/g, chloroform as solvent) and expected copolymer composition were successfully synthesized.When the resin melt was discharged out of the bottom outlet of the 2.5 L reactor with the aid of nitrogen pressure, stretched into thin strips and cooled by tap-water in a 2-m long tank, and then cut into pellets by a pelletizer, it was found that PBAT 48 was easily cut off but that PBST 48 was not.The PBST pellets were connecting with each other, as shown in Figure 1.PBST 44 was too soft after water cooling and so more difficult than PBST 48 to be cut off under the same condition.The result implies that the melt crystallization of these copolyesters under rapid cooling was quite different.Although there are extensive studies on the melt crystallization of PBST [16,34,35] at a conventional cooling rate, research on melt crystallization in the case of rapid cooling is still lacking.So, the isothermal melt crystallization and the nonisothermal melt crystallization of PBAT and PBST at rapid and conventional cooling rates are comparatively investigated in this work.
a Molar percentage of TPA in diacid feed; b molar percentage of BT unit in copolyesters, calculated from 1 H NMR results; c mass percentage of BT unit in copolyesters calculated from ϕBT; d,e number-average length of BX (X = A or S) and BT sequences; f degree of randomness; and g intrinsic viscosity measured at 25 °C using chloroform as solvent.
When the resin melt was discharged out of the bottom outlet of the 2.5 L reactor with the aid of nitrogen pressure, stretched into thin strips and cooled by tap-water in a 2-m long tank, and then cut into pellets by a pelletizer, it was found that PBAT48 was easily cut off but that PBST48 was not.The PBST pellets were connecting with each other, as shown in Figure 1.PBST44 was too soft after water cooling and so more difficult than PBST48 to be cut off under the same condition.The result implies that the melt crystallization of these copolyesters under rapid cooling was quite different.Although there are extensive studies on the melt crystallization of PBST [16,34,35] at a conventional cooling rate, research on melt crystallization in the case of rapid cooling is still lacking.So, the isothermal melt crystallization and the nonisothermal melt crystallization of PBAT and PBST at rapid and conventional cooling rates are comparatively investigated in this work.

Nonisothermal Melt Crystallization and Melting
First, the thermal transition behaviors of PBAT48, PBST48, and PBST44 were studied with DSC at a cooling/heating rate of 10 °C/min, which is widely used in DSC studies of PBST in the literature [16,34,35] After erasing the heat history, all of the copolyesters showed melt crystallization during cooling at 10 °C/min (Figure 2a), then glass transition and melting occurred during the 2nd heating at 10 °C/min (Figure 2b).The melt crystallization and 2nd melting peaks are attributed to the BT sequence.The BA and BS sequences did not crystallize from melt under the testing condition.But melting peaks of BA and BS sequences were indeed observed in the 1st heating scan (see Figure S2 in supporting information).It has been reported that, although the BS crystal of PBST can not be detected by X-ray diffraction, it can indeed be formed slowly after long-time storage at room temperature, and therefore can be observed in the first DSC scan [35].The results in Figure S2 provide evidence for a similar conclusion regarding the copolyesters in this study.
At the cooling rate of 10 °C/min, PBAT48 and PBST48 showed almost the same melt crystallization behavior, with almost the same temperature, enthalpy, and half period of melt crystallization (Tc 78 °C, ΔHc 19.8-20.4J/g, t1/2 43-45 s, see Table 2).No cold crystallization was observed in the secondary heating curve, suggesting that the melt crystallization of them occurred sufficiently.In comparison with the thermal transition properties of commercial PBAT [37,38], the PBAT48 sample displayed almost the same transition temperatures (Tg, Tc, Tm) but higher ΔHc and ΔHm.The existence of branched or chain extender structures in the macromolecular chain of commercial PBAT may account for its lower transition enthalpies.In comparison, PBST44 clearly showed poorer melt crys-

Nonisothermal Melt Crystallization and Melting
First, the thermal transition behaviors of PBAT 48 , PBST 48 , and PBST 44 were studied with DSC at a cooling/heating rate of 10 • C/min, which is widely used in DSC studies of PBST in the literature [16,34,35] After erasing the heat history, all of the copolyesters showed melt crystallization during cooling at 10 • C/min (Figure 2a), then glass transition and melting occurred during the 2nd heating at 10 • C/min (Figure 2b).The melt crystallization and 2nd melting peaks are attributed to the BT sequence.The BA and BS sequences did not crystallize from melt under the testing condition.But melting peaks of BA and BS sequences were indeed observed in the 1st heating scan (see Figure S2 in supporting information).It has been reported that, although the BS crystal of PBST can not be detected by X-ray diffraction, it can indeed be formed slowly after long-time storage at room temperature, and therefore can be observed in the first DSC scan [35].The results in Figure S2 provide evidence for a similar conclusion regarding the copolyesters in this study.
At the cooling rate of 10 • C/min, PBAT 48 and PBST 48 showed almost the same melt crystallization behavior, with almost the same temperature, enthalpy, and half period of melt crystallization (T c 78 • C, ∆H c 19.8-20.4J/g, t 1/2 43-45 s, see Table 2).No cold crystallization was observed in the secondary heating curve, suggesting that the melt crystallization of them occurred sufficiently.In comparison with the thermal transition properties of commercial PBAT [37,38], the PBAT 48 sample displayed almost the same transition temperatures (T g , T c , T m ) but higher ∆H c and ∆H m .The existence of branched or chain extender structures in the macromolecular chain of commercial PBAT may account for its lower transition enthalpies.In comparison, PBST 44 clearly showed poorer melt crystallizability.The T c , ∆H c , and t 1/2 of PBST 44 were 47 • C, 17.3 J/g, and 98 s, respectively.Its melt crystallization took place at a higher supercooling degree at a much slower rate (t 1/2 over 2 times).Due to insufficient melt crystallization, weak but still obvious cold crystallization (∆H cc = 1.2 J/g) can be observed during the 2nd heating.In summary, at the conventional DSC cooling rate of 10 • C/min, PBST 48 has excellent melt crystallizability comparable to that of PBAT 48 and better than that of commercial PBAT [37,38].Its thermal transition properties are also consistent with those reported for PBST 45-50 in the literature [16,34,35].But the PBST 44 with lower ϕ BT but the same ϕ w,BT shows significantly poorer crystallizability than PBAT 48 .It seems that the molar composition is the determining factor for the melt crystallization of these copolyesters.
tallizability.The Tc, ΔHc, and t1/2 of PBST44 were 47 °C, 17.3 J/g, and 98 s, respectively.Its melt crystallization took place at a higher supercooling degree at a much slower rate (t1/2 over 2 times).Due to insufficient melt crystallization, weak but still obvious cold crystallization (ΔHcc = 1.2 J/g) can be observed during the 2nd heating.In summary, at the conventional DSC cooling rate of 10 °C/min, PBST48 has excellent melt crystallizability comparable to that of PBAT48 and better than that of commercial PBAT [37,38].Its thermal transition properties are also consistent with those reported for PBST45-50 in the literature [16,34,35].But the PBST44 with lower ϕBT but the same ϕw,BT shows significantly poorer crystallizability than PBAT48.It seems that the molar composition is the determining factor for the melt crystallization of these copolyesters.    .
Cooling at 10 As the cooling rate during practical melt processing is much faster than the conventional DSC cooling rate, the above DSC results do not represent the melt crystallization behavior under real processing conditions.To make clear the melt crystallization of PBST and PBAT under real processing conditions, the DSC cooling rate was raised to 40 • C/min (the maximum controllable cooling rate of the DSC instrument) and other conditions were kept unchanged.The DSC curves are shown in Figure 2a ′ ,b ′ and the results are summarized in Table 2 too.
In comparison with the results for the cooling rate of 10 • C/min, PBAT 48 showed lower T c (63 • C), shorter t 1/2 (15 s), but constant ∆H c (20.1 J/g) at a cooling rate of 40 • C/min, and no cold crystallization was observed during the second heating.Obviously, PBAT 48 still manifested an excellent melt crystallization performance, though a higher supercooling degree was observed.Under the same cooling rate, the T c , ∆H c, and t 1/2 values of PBST 48 were 51 • C, 16.9 J/g, and 30 s, respectively.The lower T c , smaller ∆H c" and longer t 1/2 of PBST 48 shown in Figure 3a indicate that it clearly has a poorer melt crystallizability than PBAT 48 under rapid cooling conditions.An obvious cold crystallization (∆H cc = 4.7 J/g) in the 2nd heating was observed, indicating that the melt crystallization of PBST 48 was insufficient.In order to compare the effect of the cooling rate on the melt crystallization of PBAT 48 and PBST 48 more clearly, the difference between the melt crystallization temperature and enthalpy (∆T c = T c,10 − T c,40 , ∆ (∆H c ) = ∆H c,10 − ∆H c,40 ) of PBAT 48 and PBST 48 are defined and compared in Figure 3b.It can be seen that the ∆T c and ∆ (∆H c ) values of PBST 48 are clearly higher than those of PBAT 48 .This indicates that PBST 48 was affected more remarkably than PBAT 48 by the cooling rate in melt crystallization and exhibited weakened melt crystallizability at a cooling rate of 40 • C/min.As for PBST 44 , no obvious melt crystallization peak was observed during the rapid cooling, and there was a big cold crystallization peak (∆H cc 15.4 J/g) in the 2nd heating.This means that PBST 44 almost completely lost its melt crystallization ability.The above results indicate that the melt crystallizability of PBST 48 and PBST 44 is clearly inferior to that of PBAT 48 under rapid cooling, regardless of having the same molar or mass composition.This is the reason why PBST is more difficult to melt-process.

PBST44
nd nd a : Heat history was erased at 220 °C for 5 min; b melt crystallization temperature; c melt crystallization enthalpy; d melt crystallization half period-namely, the time taken for the relative crystallinity reaching 50%; e cold crystallization enthalpy during 2nd heating; f melting enthalpy during 2nd heating; g commercial PBAT with Mn of 39,700 g/mol and melt flow index 3.5 g/10 min, reported in ref. [37,38].
As the cooling rate during practical melt processing is much faster than the conventional DSC cooling rate, the above DSC results do not represent the melt crystallization behavior under real processing conditions.To make clear the melt crystallization of PBST and PBAT under real processing conditions, the DSC cooling rate was raised to 40 °C/min (the maximum controllable cooling rate of the DSC instrument) and other conditions were kept unchanged.The DSC curves are shown in Figure 2a′,b′ and the results are summarized in Table 2 too.
In comparison with the results for the cooling rate of 10 °C/min, PBAT48 showed lower Tc (63 °C), shorter t1/2 (15 s), but constant ΔHc (20.1 J/g) at a cooling rate of 40 °C/min, and no cold crystallization was observed during the second heating.Obviously, PBAT48 still manifested an excellent melt crystallization performance, though a higher supercooling degree was observed.Under the same cooling rate, the Tc, ΔHc, and t1/2 values of PBST48 were 51 °C, 16.9 J/g, and 30 s, respectively.The lower Tc, smaller ΔHc,, and longer t1/2 of PBST48 shown in Figure 3a indicate that it clearly has a poorer melt crystallizability than PBAT48 under rapid cooling conditions.An obvious cold crystallization (ΔHcc = 4.7 J/g) in the 2nd heating was observed, indicating that the melt crystallization of PBST48 was insufficient.In order to compare the effect of the cooling rate on the melt crystallization of PBAT48 and PBST48 more clearly, the difference between the melt crystallization temperature and enthalpy (ΔTc = Tc,10-Tc,40, Δ (ΔHc) = ΔHc,10-ΔHc,40) of PBAT48 and PBST48 are defined and compared in Figure 3b.It can be seen that the ΔTc and Δ (ΔHc) values of PBST48 are clearly higher than those of PBAT48.This indicates that PBST48 was affected more remarkably than PBAT48 by the cooling rate in melt crystallization and exhibited weakened melt crystallizability at a cooling rate of 40 °C/min.As for PBST44, no obvious melt crystallization peak was observed during the rapid cooling, and there was a big cold crystallization peak (ΔHcc 15.4 J/g) in the 2nd heating.This means that PBST44 almost completely lost its melt crystallization ability.The above results indicate that the melt crystallizability of PBST48 and PBST44 is clearly inferior to that of PBAT48 under rapid cooling, regardless of having the same molar or mass composition.This is the reason why PBST is more difficult to melt-process.The inferior melt crystallization ability of PBST under rapid cooling can be attributed to its weaker chain mobility due to its shorter aliphatic diacid moiety (2 CH 2 in succinate vs. 4 CH 2 in adipate).The weaker chain mobility of PBST has been demonstrated by a smaller free volume fraction and a smaller gas diffusion coefficient in our previous study [26].At a relatively slow cooling rate of 10 • C/min, the chain mobility of PBST 48 is still high enough to meet the need of the chain rearrangement during melt crystallization.However, under rapid cooling conditions, PBST 48 cannot provide enough chain mobility to rapidly rearrange its chain into the lattice.As a result, it needs a higher supercooling degree to start and a longer time to complete melt crystallization under rapid cooling.In comparison, PBAT has sufficient chain mobility to meet the demands of melt crystallization even under rapid cooling.

Isothermal Crystallization and Melting Behavior
Li FX et al. studied the isothermal melt crystallization of PBST 70 [39], but the isothermal melt crystallization of PBST 44-48 has not been reported in the literature.Therefore, the isothermal melt crystallization behavior of PBAT 48 , PBST 48 , and PBST 44 was further investigated in this study.The nonisothermal melt crystallization peak temperature at 10 • C/min cooling, namely, 78 • C, 78 • C, and 47 • C (see Table 2), was selected for the isothermal melt crystallization.After erasing the heat history, the melt was quenched to the isothermal temperature.As seen in Figure 4a, all of the three copolyesters displayed distinctive isothermal melt crystallization behaviors.First, two exothermic peaks appeared, and second, the peaks were very asymmetrical.Both peaks were attributed to the crystallization of the BT sequence.The smaller peak may correspond to the crystallization of the shorter BT sequences which could only crystallize more slowly to form immature crystals after the longer BT sequences had crystallized to form more perfect crystals.After 30 min of isothermal melt crystallization, the samples were cooled down further to −80 °C at 10 °C/min.Neither further melt crystallization during the cooling nor cold crystallization during the 2nd heating process were observed, indicating that they had sufficiently crystallized during the isothermal crystallization process.During the 2nd heating, they all displayed two melting peaks.The first smaller one represents the melting of the immature crystals formed at the later stage of isothermal melt crystallization, and the second/main peak represents the melting of the more perfect crystals formed at the earlier stage of isothermal melt crystallization.Although it is reported that the BS sequence in PBST50 can crystallize slowly at room temperature [35], the smaller peak in Figure 4d can not be regarded as the melting of BS or BA crystals.As the first Tm (88 °C) of From the thermal flow data in Figure 4a, the relative crystallinity (X t ) was calculated with Equation (1) and plotted in Figure 4b.From the Avrami equation shown in Equation ( 2), the Avarami plots (Equation (3)) were made and shown in Figure 4c.The linear part in the grey area corresponding to the isothermal time range 0.05-0.22min is used to calculate the crystallization rate constant K and the Avrami index n by linear fitting.The half time of isothermal melt crystallization, t 1/2 , is calculated from n and K with Equation (4).All the results are summarized in Table 3.The Avrami indexes n of all of the three copolyesters are close to three.The K and t 1/2 values of PBST 48 are equal to those of PBAT 48 , indicating that their isothermal melt crystallization rates are equal to each other.The t 1/2 value of PBST 48 (36.6 s) is obviously longer than that (11 s) reported for PBST 70 at 80 • C [39], indicating much slower melt crystallization of PBST 48 than PBST 70 .The t 1/2 value of PBST 44 is longer than that of PBST 48 (0.68 min vs. 0.61 min), indicating a slower crystallization rate.These results are similar the crystallization performance of non-isothermal melt crystallization at a cooling rate of 10 • C/min.4a,c.
After 30 min of isothermal melt crystallization, the samples were cooled down further to −80 • C at 10 • C/min.Neither further melt crystallization during the cooling nor cold crystallization during the 2nd heating process were observed, indicating that they had sufficiently crystallized during the isothermal crystallization process.During the 2nd heating, they all displayed two melting peaks.The first smaller one represents the melting of the immature crystals formed at the later stage of isothermal melt crystallization, and the second/main peak represents the melting of the more perfect crystals formed at the earlier stage of isothermal melt crystallization.Although it is reported that the BS sequence in PBST 50 can crystallize slowly at room temperature [35], the smaller peak in Figure 4d can not be regarded as the melting of BS or BA crystals.As the first T m (88 • C) of PBAT 48 is much higher than the equilibrium melting temperature of PBA (63 • C [40]), it is impossible to attribute the first melting peak of PBAT 48 to the melting of BA crystal.Therefore, in can be deduced that the first peaks of PBST 48 and PBST 44 are not the melting of BS crystals.
The thermal transition properties during the 2nd heating are summarized in Table 4.For PBAT 48 , it can be seen that, although the ∆H m2 value (16.2 J/g) is lower, the total ∆H m value (∆H m,sum = ∆H m1 + ∆H m1 = 18.8 J/g) is almost the same as the ∆H m value (19.2 J/g) after nonisothermal melt crystallization at 10 • C/min (see Table 2).This result supports the conclusion that PBAT 48 has strong melt crystallizability.In other words, the BT sequences in PBAT 48 crystallized sufficiently during 10 • C/min cooling; as a result, the ∆H m,sum could not be further increased by the annealing.But for PBST 48 and PBST 44 , the ∆H m,sum values (21.9, 20.1 J/g) are clearly higher than those values (19.7, 18.8 J/g) after nonisothermal melt crystallization at 10 • C/min (see Table 2), indicating that the two PBST samples did not crystallize sufficiently during the 10 • C/min cooling, and therefore the melt enthalpies were further increased after annealing.On the other hand, the two PBST samples displayed higher ∆H m,sum values after isothermal melt crystallization than PBAT 48, regardless of weaker nonisothermal melt crystallizability.In other words, they can have slightly higher crystallinity than PBAT 48 .

Light Transmittance Performance
PBAT film is often translucent.But we find PBST film always displays better light transmittance performance than PBAT film.The light transmittance and haze of PBAT 48 , PBST 48 , and PBST 44 films with thickness of ca.425 µm were measured and compared in Table 5.It can be seen that all the films display almost the same haze but different transmittance in such an order: PBAT 48 < PBST 48 < PBST 44 .The difference in light transmittance might be related to their difference in crystallization and in the formed microscopic crystal morphology.The films were melt-processed by heat press at 165 • C and then cooled with cooling water.Due to the stronger chain mobility of PBAT 48 , it can be inferred that its crystals probably grew faster to form crystals with a bigger size than PBST 48 and PBST 44 .For PBST 48 and PBST 44 , it was difficult to finish the melt crystallization quickly under such a rapid cooling process, so the crystallization occurred mostly at lower temperatures.As lower temperature is known to favor nucleation over crystal growth, PBST 48 and PBST 44 crystals would have smaller size and consequently higher transmittance than PBAT 48 .However, the authors failed to demonstrate this point using polarized light optical microscopy (POM).Although clear spherulites can be observed for commercial PBAT [41] and PBST 70 [39], there is in fact no report of the spherulite observation of PBST 44-48 in the literature.In Lee SH et al.'s report, they discuss how PBST 10-20 can form clear spherulites during a supercooling of 30 • C, but the spherulites structure of PBST 30 is not clear and the typical Maltese-cross is not clearly seen, and for PBST 40 , no spherulite can be observed at all [18].Zheng C et al. [35] demonstrated the existence of PBST 50 spherulites with POM, but the spherulite boundary was blurred.As the average length of BT sequences in the PBST and PBAT copolyesters is very small (close to two, Table 1), the polymers lack enough chain regularity, so there might be a lot of defects in the formed crystals.Possibly for this reason, it is difficult to form a sufficiently ordered crystal structure with a big enough size.In fact, it was reported that the spherulite size of PBST 50 determined by light scattering pattern is only 11.4 um in diameter [35].Tensile modulus; b stress at yielding; c stress at break; d strain at yielding; e strain at break; and f commercial PBAT with M n of 39,700 g/mol and melt flow index 3.5 g/10 min, reported in ref. [37].

Tensile Properties
The tensile properties of PBAT 48 , PBST 48, and PBST 44 were further investigated.Typical tensile curves are shown in Figure 5.As semi-crystalline polymers, they all exhibited typical ductile-tensile behavior, displaying elastic deformation, yielding, high elastic deformation, and strain hardening in sequence with increasing tensile strain.In comparison with PBAT 48 , PBST 48 displayed the same tensile stress at break but a clearly higher tensile modulus (E, 144 MPa vs. 107 MPa) and yield strength (σ y , 9.9 MPa vs. 7.7 MPa) because of its lower chain flexibility and higher crystallinity, demonstrated by the nonisothermal melt crystallization results.PBST 44 has less BT molar content but the same BT mass content and similar crystallinity to PBAT 48 , so it exhibited E and σ y comparable to PBAT 48 , and slightly lower tensile stress at break (49 MPa vs. 55 MPa).As PBAT is too soft for many applications, the higher modulus and yielding strength of the PBST copolyesters are very desirable properties for practical applications.In addition, when compared with commercial PBAT reported in the literature [37], the PBST copolyesters exhibited not only a much higher modulus (107-144 MPa vs. 46 MPa [37]), but also a much higher strength (49-56 MPa vs. 29 MPa [37]).This result also supports the conclusion that the PBST copolyesters used in this study have a sufficiently high molecular weight.

Tensile Properties
The tensile properties of PBAT48, PBST48, and PBST44 were further investigated.Typical tensile curves are shown in Figure 5.As semi-crystalline polymers, they all exhibited typical ductile-tensile behavior, displaying elastic deformation, yielding, high elastic deformation, and strain hardening in sequence with increasing tensile strain.In comparison with PBAT48, PBST48 displayed the same tensile stress at break but a clearly higher tensile modulus (E, 144 MPa vs. 107 MPa) and yield strength (σy, 9.9 MPa vs. 7.7 MPa) because of its lower chain flexibility and higher crystallinity, as demonstrated by the nonisothermal melt crystallization results.PBST44 has less BT molar content but the same BT mass content and similar crystallinity to PBAT48, so it exhibited E and σy comparable to PBAT48, and slightly lower tensile stress at break (49 MPa vs. 55 MPa).As PBAT is too soft for many applications, the higher modulus and yielding strength of the PBST copolyesters are very desirable properties for practical applications.In addition, when compared with commercial PBAT reported in the literature [37], the PBST copolyesters exhibited not only a much higher modulus (107-144 vs. 46 MPa [37]), but also a much higher strength (49-56 vs. 29 MPa [37]).This result also supports the conclusion that the PBST copolyesters used in this study have a sufficiently high molecular weight.

Conclusions
In this study, the melt crystallization of PBST48 and PBST44 was investigated in nonisothermal and isothermal manners and compared with PBAT48.PBST48 showed a PBAT48-comparable melt crystallization performance at a conventional DSC cooling rate of 10 °C/min or at isothermal conditions, but showed a clearly lower melt crystallization temperature and enthalpy and a much longer crystallization time at a cooling rate of 40 °C/min.PBST44, which has the same mass percentage of BT unit as PBAT48, not only showed a much slower crystallization rate at a cooling rate of 10 °C/min, but it also completely lost its melt crystallization ability at a cooling rate of 40 °C/min.The weaker melt crystallization ability of PBSTs at the same copolymer composition is attributed to the shorter aliphatic diacid moiety in PBST than in PBAT (succinate vs. adipate) and the resultant weaker chain mobility.It is the main reason why PBST is more difficult to pelletize and injection-mold.Due to the difference in melt crystallization at rapid cooling and chain flexibility, the PBST films show higher light transmittance, and PBST48 showed an obviously higher tensile modulus and yielding strength than PBAT48.Although PBST crystallizes more slowly under rapid cooling, it has superior mechanical, optical, and gas barrier properties to PBAT and therefore appears to be a more promising biodegradable aliphatic-aromatic copolyester when it comes to more demanding applications, such as food packaging.The promotion of the melt crystallization of PBST under rapid cooling will be reported later.

Table 2 .
Thermal transition properties of PBAT48, PBST48, and PBST44 during cooling and 2nd heating DSC scans a. .

Table 3 .
Isothermal melt crystallization kinetic constants BT sequence of PBAT 48 , PBST 48 and PBST 44 *.K and n are calculated from the data in the time range from 0.05 to 0.22 min, as indicated by the grey area in Figure *
a :