The e ﬀ ect of isothermal crystallization on mechanical properties of poly ( ethylene 2,5 - furandicarboxylate

: The e ﬀ ects of isothermal crystallization tem -perature/time on mechanical properties of bio - based polyester poly ( ethylene 2,5 - furandicarboxylate ) ( PEF ) were investigated. The intrinsic viscosity, crystallization proper ties, thermal properties, and microstructure of PEF were characterized using ubbelohde viscometer, X - ray di ﬀ rac tion, polarizing optical microscope, di ﬀ erential scanning calorimetry, and scanning electron microscopy. The PEF sample isothermal crystallized at various temperatures for various times was denoted as PEF - T - t . The results showed that the isothermal crystallization temperature a ﬀ ected the mechanical properties of PEF - T - 30 by simultaneously a ﬀ ecting its crystallization properties and intrinsic viscosity. The isothermal crystallization time only a ﬀ ected the crystal lization properties of PEF - 110 - t . The crystallinity of PEF - 110 40 was 17.1%. With small crystal size, poor regularity, and α ′ - crystal, PEF - 110 - 40 can absorb the energy generated in the tensile process to the maximum extent. Therefore, the best mechanical properties can be obtained for PEF - 110 - 40 with the tensile strength of 43.55 MPa, the tensile modulus of 1,296 MPa, and the elongation at a break of 13.36%.

Nevertheless, the drawbacks of PEF, such as poor toughness, slow crystallization rate, dark color, and low molecular weight, have limited its industrial application, so it has not been commercialized yet to date. Esterification polycondensation (5)(6)(7)(8) and transesterification polycondensation (9)(10)(11)(12)(13) are the main routes to synthesize PEF. The current research mainly focuses on the study of synthesis process parameters (7,13), copolymerization (8,(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25), and nanocomposite (26)(27)(28)(29) to improve molecular weight, reduce the chroma of the product, control the crystallization properties, and improve the mechanical properties. Knoop et al. (9) prepared high molecular weight furan dicarboxylic acid-based polyester via melt polymerization followed by solid-state polycondensation and studied the crystallization properties of high molecular weight PEF. The results showed that the molecular weight reached 83,000 g·mol −1 , crystallization occurs between 110°C and 200°C, and the crystallization rate was the fastest between 150°C and 160°C. Wu et al. (12) prepared PEF via transesterification polycondensation using organic nonmetal 1,8-diazabicylo [5.4.0]undec-7-ene (DBU) as a catalyst and studied its isothermal crystallization and nonisothermal crystallization kinetics. The results showed that PEF exhibited rapidly melt crystallization behavior in both nonisothermal and isothermal manners. When the cooling rate of nonisothermal crystallization was 10°C·min −1 , the melt crystallization rate was the fastest at 160°C with a minimum melt crystallization half-life (1.6 min). After complete isothermal crystallization at 160-210°C, the crystallinity was 27-34%. Stoclet et al. (30) studied the relationship between isothermal crystallization temperature and crystal form of PEF. The results showed that the transformation of crystal form occurred near 170°C. When the crystallization temperature T c was below 170°C, the irregular α′-crystal was formed. When the crystallization temperature T c was above 170°C, the regular α-crystal could be formed. Furthermore, Tsanaktsis et al. (31) showed that there existed three types of crystals: α, α′, which were formed during melt or coldcrystallization at various temperature ranges, and β-crystal type that was formed upon solution crystallization of PEF. Mao et al. (32) and Maini et al. (33) deeply determined the crystal structure of PEF using fiber and powder patterns, respectively. Lotti et al. (27) in situ prepared PEF nanocomposites with multi-walled carbon nanotubes (MWCNTs) and graphene oxide (GO) via melt polycondensation. The effects of crystallization mode and nanoparticle type on the crystal form of PEF were investigated. The results showed that the pure PEF produced β-crystal after solvent treatment, while α-crystal formed after melt or cold-crystallization, but PEF nanocomposites could singly form α-crystal. Wang et al. (14,34) used 2,2,4,4-tetramethyl-1,3-cyclobutanediol (CBDO) as copolymerization modifier to study the effect of modifier dosage on the properties of PEF. The results showed that CBDO could improve the transparency and mechanical properties of PEF, but the barrier property decreased.
In this paper, PEF was prepared by the esterification polycondensation method. The relationship between crystallization temperature/time and crystallization behavior of PEF, such as crystallization rate, crystal form, crystal morphology, and crystallinity, as well as the relationship between crystallization behavior and mechanical properties was investigated. The results can provide a method and theoretical reference for the modification and postprocessing of PEF. Ethylene glycol (EG, 99%) and phenol (C 6 H 5 OH, 99%) were purchased from Guangdong Guanghua Technology Co., Ltd. (China). Dibutyltin oxide (DBTO, 98%) was purchased from Acros organizations. 1,1,2,2-tetrachloroethane (C 2 H 2 Cl 4 , 99%) was purchased from Chengdu Kelong Chemical Co., Ltd. (China). All the chemicals were used as received.

Synthesis of PEF
PEF was synthesized via the esterification-polycondensation method. FDCA and EG at a molar ratio of diacid/diol = 1:1.8 were added into the reactor equipped with a mechanical stirrer, N 2 inlet, and reflux condenser, and DBTO (0.15 mol% based on FDCA) was used as a catalyst. The air in the reactor was purged with nitrogen for three times, and the esterification was carried out at 200°C under nitrogen protection. After almost all the theoretical amount of water was distilled out, gradually increasing the temperature to 230°C, reducing the reaction pressure below 100 Pa within 30 min, and keeping this condition for 4 h, PEF was prepared with an intrinsic viscosity of 0.44 dL·g −1 .

Preparation of samples
Samples were prepared by compression molding, the molding temperature, pressure, and time were 190°C, 10 MPa, and 2 min, respectively. Then, the samples were transferred rapidly to the water-cooling plate of the molding machine to obtain amorphous samples (named PEF, 0.36 dL·g −1 ).
The intrinsic viscosity of the crystallized samples did not decrease obviously until the crystallization temperature reached 190°C (PEF-190-30, 0.32 dL·g −1 ).

Characterization
The tensile properties of PEF with various crystallization temperatures and times were tested according to ISO 527-2:1993, with a tensile rate of 10 mm·min −1 . The tensile specimens were dumbbell-shaped with the dimension of 1 mm in thickness and 2 mm in neck width. All samples were conditioned at room temperature for at least 48 h before testing.
A scanning electron microscope (SEM) was used to characterize the micromorphology of the tensile sections of the samples. The accelerating voltage was 10 kV, and the sections were sprayed with gold before SEM analysis.
The intrinsic viscosity of samples was tested by Ubbelohde viscometer at 25 ± 2°C according to GB/T 14190-2008. Phenol/1,1,2,2-tetrachloroethane with a mass ratio of 1:1 was used as the mixed solvent.
The thermal and crystallization characteristics of PEF samples with various crystallization temperatures and time were measured by differential scanning calorimetry (DSC) and calibrated by indium and zinc standards. About 5 mg of the sample was put into an aluminum pot and heated from 30°C to 250°C at a heating rate of 10°C·min −1 under the N 2 protection (40 mL·min −1 ). The crystallinity was calculated according to Eq. 1: where ΔH m is the enthalpy of melting, ΔH cc is the enthalpy of cold crystallization, and ΔH m 0 is the theoretical melting enthalpy of 100% crystalline PEF, ΔH m 0 = 137 J·g −1 (35).
Furthermore, isothermal crystallization of PEF from the melt was studied at a crystallization temperature range from 90°C to 190°C. The temperature was raised from 30°C to 250°C at the rate of 10°C·min −1 , kept for 2 min to eliminate the heat history, then rapidly lowered to the isothermal crystallization temperature (T c ) at the rate of 50°C·min −1 for a certain period of time. X-ray diffraction (XRD) was used to test the crystallinity and crystal form of the samples, with Cu Kα radiation (λ = 0.154 nm) in the angle 2θ range from 5°to 40°, tube voltage of 45 kV, and tube current of 200 mA. The crystal morphology and quantity of PEF were observed by polarized optical microscopy (POM) with X-8 hot stage and the magnification of 200×.  of crystallization temperature, and α′-crystal transforms to α-crystal at 150°C. Figure A1 shows that there is no crystal in the untreated sample ( Figure A1a) and a few small spherulites appear after crystallization at 110°C ( Figure A1b). With the increase of crystallization temperature, the number of spherulites increases significantly, especially after crystallization at 150-190°C. PEF-170-30 has the largest number of spherulites. The crystal size is so small that the change of size is not obvious when it is magnified 200 times. Papageorgiou et al. (35) also reported that PEF exhibits a large nucleation density and a small spherulite size even at high crystallization temperatures of 210°C.

DSC analysis of PEF-T-30
The DSC heating curves of PEF-T-30 are shown in Figure 2, and the thermal properties are summarized in Table A1 (in Appendix). It shows that with the increase of isothermal crystallization temperature, the glass transition temperature (T g ) of PEF-T-30 moves to the high-temperature region, the crystallization peak weakens, and the melting peak temperature (T m ) does not change significantly. The glass transition and crystallization peak disappear when the crystallization temperature exceeds 110°C, indicating high crystallinity. When the crystallization temperature reaches 150°C, a smaller melting peak of the secondary crystal appears. With the increase of the crystallization temperature, the melting peak of the secondary crystal shifts linearly to the high-temperature direction, and the strength remains unchanged. Combined with Figure 1, it is indicated that the crystallinity of PEF increases with the increase of crystallization temperature and then tends to be flat, but the crystallization temperature has little effect on the melting temperature. When the crystallinity increases to a certain extent, the secondary crystal will occur. The secondary crystal lamella is formed between the primary crystal lamella, and the physical hindrance in the formation process makes these crystal lamellae thinner, resulting in the melting temperature of the secondary crystal being lower than that of the primary crystal. The same observations were reported in the studies of Stoclet et al. (30,37) and van Berkel et al. (38).

Mechanical properties of PEF-T-30
The stress-strain curves of PEF-T-30 are shown in Figure 3, and the mechanical properties are shown in Table A2. It shows that the PEF-T-30 samples do not yield during the tensile process, which is a typical brittle fracture. The tensile strength, modulus, and elongation at break of PEF-T-30 first increase and then decrease with the increase of crystallization temperature. The tensile strength and elongation at the break of PEF-110-30 are the highest, which is twice as high as that of non-isothermal crystallization PEF. Combined with Figures 1 and 2, it is shown that the mechanical properties of PEF-T-30 are closely related to crystallinity, crystal regularity. The crystals allow the initiation and propagation of shear deformation more easily and then enhance the energy dissipating processing, and the mechanical properties are improved effectively with the increase of crystallinity. However, when the crystallinity reaches a certain degree, a large number of crystals tend to produce stress concentration during the tensile process, resulting in the decline of mechanical properties. The crystallinity of PEF-110-30 is 14.1% with small crystal size, poor regularity, mainly α′-crystal, which can absorb the energy generated in the tensile process to the maximum extent, so it gets the best mechanical properties. Compared with PEF-170-30 and PEF-190-30, although the crystallinity is   Figure A2. The tensile cross-section of PEF without isothermal crystallization ( Figure A2a) is relatively regular, the irregularity of tensile section of PEF-110-30 ( Figure A2b) with crystallization at 110°C increases, and uniform deformation traces appear. The tensile section of PEF-130-30 ( Figure A2c) tends to be flat and deformed locally. PEF-150-30 ( Figure A2d) shows a flat section, and a large number of particles and micro pits with a diameter of about 1 μm appear on the surface which may be caused by the separation of the crystalline phase from the amorphous region. These changes in surface fracture correlate with the mechanical properties results.

Effect of crystallization time on PEF properties 3.2.1 XRD and POM analysis of PEF-110-t
To analyze the effects of crystallization time on crystal form, crystallinity, and crystal morphology, the PEF-110-t with various crystallization times at 110°C was measured by XRD and POM. The results are shown in Figure 4 and Figure A3. It can be seen from Figure 4 that the XRD pattern of PEF shows amorphous peaks. Weak characteristic diffraction peaks appear in the XRD pattern of PEF-110-20. With the increase of crystallization time, the intensity of diffraction peaks enhance, there are obvious diffraction peaks in PEF-110-720 pattern. However, there is no characteristic diffraction peak near 19.1°which belongs to α-crystal. The results show that the α′-crystal with poor regularity is mainly formed during isothermal crystallization at 110°C. Figure A3 shows that a small number of small spherulites appear after crystallization for 40 min ( Figure A3b). With the increase of crystallization time, spherulites size and the number increase slowly, and a large number of spherulites appear in PEF-110-720 ( Figure A3d). The results indicate that PEF crystallizes slowly at 110°C.

DSC analysis of PEF-110-t
The DSC heating curves of PEF-110-t with various crystallization times at 110°C are shown in Figure 5, and the thermal properties are summarized in Table A3. It shows that the crystallization peak of PEF-110-t weakens, while the melting enthalpy (ΔH m ) increases with the extension of crystallization time, indicating that the crystallinity is improved. There is no crystallization peak on the heating curve of PEF-110-720, indicating that the crystallization reaches saturation, and the saturation crystallinity is 45.4%. A small melting peak also appears around 130°C when the crystallization time exceeds 60 min. Combined with Figure 4, it is proved that this peak is the melting peak of the secondary crystal.

Mechanical properties of PEF-110-t
The stress-strain curves of PEF-110-t are shown in Figure 6, and the mechanical properties are shown in Table A4. It shows that brittle fracture occurs in PEF-110-t. With the extension of crystallization time, the tensile strength and elongation at break first increase and then decrease, and   Figure A4. The tensile section of PEF-110-20 ( Figure A4b) shows coarser fracture than that of PEF, and large deformation traces appear locally. PEF-110-40 ( Figure A4c) shows the largest deformation traces and obvious stress whitening. The regularity of PEF-110-60 ( Figure A4d) is improved with reduced deformation. The results show that with the extension of crystallization time, the irregularity of the PEF fracture surfaces increases first and then decreases, which is consistent with the results of mechanical properties.

Isothermal crystallization of PEF
DSC isothermal crystallization curves at different crystallization temperatures are shown in Figure 7. It shows that there is no obvious crystallization before isothermal crystallization temperature below 130°C within 1 h. When the isothermal crystallization temperature ranges from 130°C to 190°C, the crystallization peak appears, and PEF crystallizes fastest at 170°C. To investigate whether PEF crystallizes at 110°C, the crystallization time is extended to 720 min, as shown in the embedded figure, and the crystallization peak appears in the curve. It indicates that PEF can be crystallized at 110°C, but the crystallization rate is extremely slow.

Conclusion
PEF was prepared via the direct esterification method, and the effects of isothermal crystallization temperature and time on the properties of PEF were investigated. The results reveal that the isothermal crystallization temperature affects the mechanical properties of PEF-T-30 by affecting its crystallization properties and intrinsic viscosity. With the increase of crystallization temperature, the crystallinity of PEF-T-30 increases with the increase of crystallization temperature, the regularity of crystal structure increases too, and α′-crystal with poor regularity transforms to α-crystal with good regularity at 150°C. Only small spherulites of PEF are obtained by isothermal crystallization at a temperature range from 90°C to 190°C. The isothermal crystallization time only affects the crystallization properties of PEF-110-t. With the extension of crystallization time, the crystal number and size of PEF-110-t increase, and the saturated crystallinity of PEF with crystallization temperature of 110°C is 45.4%. The secondary crystallization of PEF occurs with either increasing crystallization temperature or prolonging crystallization time. With small crystal size, poor regularity, α′-crystal, and the crystallinity of 17.1%, PEF-110-40 can absorb the energy generated in the tensile process to the maximum extent. Therefore, the best mechanical properties can be obtained for PEF-110-40 with the tensile strength of 43.55 MPa, the tensile modulus of 1,296 MPa, and the elongation at a break of 13.36%.
Acknowledgments: Thank the teachers and students in group 1102 for guiding the operation of the equipment during my experiments.   Author contributions: Wei Zhang: writingoriginal draft, writingreview and editing, methodology, data curation, validationverification, formal analysis; Shaoying Liu: writingoriginal draft, writingreview and editing, supervision, project administration; Qingyin Wang: project administration, supervision; Gongying Wang: resources, funding acquisition. Table A1: Thermal properties of PEF-T-30 T m1 : melting temperature of the secondary crystal; b T m2 : melting temperature of the primary crystal; c ND is non peak.   Isothermal crystallization and properties of PEF  9 Figure