Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Unique Isodimorphism of Poly(decamethylene succinate-randecamethylene fumarate): Large Pseudoeutectic Region and Fantastic Crystallization/Melting Behavior Hai-Mu Ye,* Jing Wang, Cai-Shui Wang, and Hong-Fang Li State Key Laboratory of Heavy Oil Processing and Department of Materials Science and Engineering, China University of Petroleum, Beijing 102249, P. R. China Macromolecules Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 02/04/19. For personal use only.
S Supporting Information *
ABSTRACT: Poly(decamethylene succinate-ran-decamethylene fumarate)s (PDSFs) were synthesized, and their structure and crystallization behavior were systematically studied. All polyesters possess high crystallizability irrespective of composition and display the widest pseudoeutectic region that has been reported to date. Nevertheless, both melting point− and crystallization temperature−composition relationships demonstrate PDSFs display off-V-shaped behavior, indicating particular pseudoeutectic behavior. PDSF57 and PDSF37 located in the pseudoeutectic region are studied in detail. DF-rich crystals form first due to the assembly of fumarate-related preordered structure in the PDSF57 melt; then DS-rich crystals appear through plausible epitaxial crystallization. PDSF37 is proven to be located in the boundary of the pseudoeutectic region. DF-rich ordered structure forms first but cannot crystallize itself, which plays the role of the “soft epitaxial” substrate for DS-rich crystal formation; meanwhile, the DF-rich form crystallizes with the assistance of DS-rich crystals. The particular crystallization process of PDSF37 induces tightly attached structures between the two types of crystals and results in a fantastic and abrupt melting point jump.
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INTRODUCTION Aliphatic polyesters make up a promising class of biodegradable polymers. Because their mechanical performances are comparable to those of traditional fossil-based polymers and because of their excellent environmentally friendly properties, they have attracted an increasing level of interest.1−3 The thermal behaviors, mechanical properties, biodegradation rates, and practical applications of polyesters are strongly correlated to crystal structure and morphology.4,5 Therefore, copolymerization, a versatile way to regulate the crystallization behavior of homopolyesters, is widely utilized to prepare polyester materials with desired and controllable properties by introducing comonomeric units into a chain architecture. When two types of crystallizable units are employed to constitute random copolymers, different crystallization processes occur depending on the miscibility and ability to share crystal lattices between copolymerized units. If there is only one crystalline structure containing both types of comonomeric units in the entire composition range and crystal modification remains the same as that of homopolymers, the phenomenon is identified as isomorphism, i.e., poly(butylene succinate-ran-butylene fumarate) and poly(ε-caprolactone-ranω-pentadecalactone).6,7 An almost linear relationship for melting point−composition dependence is a direct feature for isomorphism.6−9 On the other hand, when double-crystal structures depending on composition are found, the phenomenon is defined as isodimorphism. While both comonomeric © XXXX American Chemical Society
units can be accommodated together in at least one type of homopolymer crystal lattice, the corresponding minor component is less favored and the concentration included in the crystalline structure as the major one. The minor comonomeric units are usually considered as defects in lattices, resulting in the depression of the melting point and crystallinity. Thus, a eutectic type of (i.e., V-shaped) melting point−composition relationship is recognized as being typical of isodimorphism.4,5 Most random copolyesters are isodimorphic, such as poly(butylene succinate-ran-butylene azelate), poly(butylene succinate-ran-butylene adipate), and poly(γbutyrolactone-ran-ε-caprolactone).10−13 Pan and Inoue4 and Pérez-Camargo et al.5 comprehensively reviewed the literature on isomorphism and isodimorphism, respectively. In addition, a new crystal structure might be induced in a random copolyester in some cases, for example, poly(hexamethylene sebacate-ran-hexamethylene adipate).14 For an isodimorphic copolyester, a pseudoeutectic point/ region appears within a particular composition range where double-crystal structures form together. Although the exact definition of pseudoeutectic has not been finalized, two sufficient conditions for determining pseudoeutectic of random copolymers can be summarized. First, the two crystalline Received: August 28, 2018 Revised: January 17, 2019
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DOI: 10.1021/acs.macromol.8b01848 Macromolecules XXXX, XXX, XXX−XXX
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Table 1. Compositions, Average Molecular Weights, Thermal Properties, and Atacticities of All Synthesized Polyesters DS/DF molar ratio sample
feeding ratio
NMR ratio
Mw (g/mol)
PDI
Tg (°C)
Tc (°C)
Tm (°C)
ΔHm (J/g)
R
PDS PDSF20 PDSF37 PDSF57 PDSF76 PDF
1/0 0.8/0.2 0.6/0.4 0.4/0.6 0.2/0.8 0/1
1/0 0.80/0.20 0.63/0.37 0.43/0.57 0.24/0.76 0/1
7.49 7.12 7.95 6.31 5.80 6.08
× × × × × ×
1.55 1.65 2.21 1.55 1.65 1.71
−49.2 −45.5 −41.4 −37.3 −33.6 −29.4
50.4 48.5 50.5 49.4 55.1 62.2
71.1 68.8 72.6 71.5 80.2 88.8
91.1 85.5 80.0 63.6 65.4 67.7
− 1.00 0.99 1.01 1.01 −
104 104 104 104 104 104
feeding molar ratio was set steadily at 1/1.15. Analytical grade SA, FA, DDO, tetra-n-butyl-titanate (TBT, catalyst), and p-hydroxyanisole (pHA, free radical inhibitor) were all purchased from Shanghai Aladdin Reagent Co. and used as received. The synthesized polyester was dissolved in chloroform (AR grade), centrifuged to remove impurities, and then precipitated in an excess of cold methanol (AR grade). The precipitates were collected and dried under vacuum at 50 °C for 2 days before use. Characterization of the Chemical Structure. The molecular weights and polydispersity indices of copolyesters were determined by a Viscotek-M302 TDA multiple-test gel chromatography system. The measurements were taken at 40 °C with a Shimadzu GPC-804C column at a flow rate of 1.0 mL/min, and chloroform (HPLC grade) was used as the eluent. Polystyrene standards (Fluka) were used to obtain the calibration curve. The compositions of polyesters were determined with the 1H nuclear magnetic resonance (NMR) spectrometer (JEOL, ECA-300M) with chloroform-d as the solvent and tetramethylsilane as the reference. The decamethylene succinate (DS) content was determined by comparing the integration of signals at δ = 2.62 ppm and δ = 6.84 ppm assigned to CH2−CH2 protons in succinate and CHCH protons in fumarate, respectively (see Figure S1 and eq S1). 13C NMR spectra were employed to calculate the sequence distribution and atacticities [R (see Figure S2 and eq S2)]. The basic information about chain structures of polyesters is summarized in Table 1. Differential Scanning Calorimetry. Thermal properties of copolyesters were measured on a differential scanning calorimeter (DSC, NETZSCH 204 F1) equipped with an intercooler as the cooling system under a nitrogen atmosphere. The instrument was calibrated with indium and tin standards before use, and samples of ∼5 mg were used. To obtain the glass transition temperature (Tg), the polyester was quenched from molten state at 120 °C to −80 °C and kept for 10 min; the sample was heated to 100 °C at a rate of 10 °C/min. In a non-isothermal scan, the sample was melted at 120 °C for 5 min to eliminate any previous thermal history and then cooled to 0 °C at a constant rate of usually 10 °C/min. Subsequently, the sample was reheated to 120 °C at a rate of 10 °C/min. Wide-Angle X-ray Diffraction. Crystal structures of polyesters were examined by a Bruker AXS D8 Advance wide-angle X-ray powder diffractometer using Cu Kα radiation at 25 °C (λCu = 0.154 nm). Diffractograms were collected in the 2θ range interval of 5−40° with a scanning step of 0.01°. Two-dimensional X-ray diffraction patterns of oriented polyester fibers were recorded on a Rigaku R-Axis instrument with Mo radiation at 25 °C. Samples for one-dimensional diffractogram collection were prepared by isothermally crystallizing the polyester melt at the desired temperatures. The thickness of the sample was ∼300 μm. Oriented fibers were obtained using a two-step procedure. (1) The initial fiber was prepared using a Rosand 2000 capillary rheometer at a shear rate of 60 s−1 at 120 °C, and the diameter of the capillary was 1 mm. (2) The initial fiber was stretched to a drawn ratio of 6 using a homemade drawing mill at room temperature. Temperature-dependent WAXD was performed on a Rigaku Smartlab instrument using Cu Kα radiation operating at 40 kV and 30 mA. The testing temperature range was 30−120 °C with intervals of 5 °C. The diffractograms were collected in the 2θ range interval of 8−32° at a scanning rate of 3°/min and step of 0.01°.
phases can form at the same temperature, especially during the non-isothermal melt−crystallization process. Second, both comonomers can be hosted in two types of crystals, and the concentration of the minor comonomer in a majority of literature is lower than its concentration in the polymer chain.5 The pseudoeutectic composition range is determined by several factors, including comonomer structure, crystallization temperature, etc.4,5,12,15−17 Pseudoeutectic copolyesters provide typical models for studying the assembly of the complex structure (an amorphous phase and two crystalline phases), competition crystallization behavior, etc., thereby helping in the design of materials with novel properties. Nevertheless, the previously reported pseudoeutectic composition spans that are rather narrow (≤10 mol %) are opposed to further study,5 and effectively adjusting the span is also a challenge. With respect to the degree of inclusion, isomorphism is an extreme example of isodimorphism when the degree of inclusion of minor comonomeric units in a single lattice changes from partial to total.5 Previously, we had studied the crystallization behavior of poly(alkylene succinate-ran-alkylene fumarate)s.6,15,18 When the alkylene unit was varied from butylene to hexamethylene, the copolyester altered the crystallization behavior from isomorphism to isodimorphism with a pseudoeutectic span of ∼6 mol %. Increasing the carbon chain length of alkylene would decrease the miscibility in lattices between succinate and fumarate and, subsequently, lead to the case of the degree of inclusion being partially induced, i.e., isodimorphism. Therefore, a further increase in alkylene length could plausibly help in the fabrication of isomorphism with a larger pseudoeutectic region, with which more precise and adjustable properties and functions, e.g., mechanical performance and degradation behavior, can be designed, realized, and applied. In this work, random copolyesters of poly(decamethylene succinate-co-decamethylene fumarate) (PDSF) have been synthesized. The thermal properties, structure, and crystallization behavior of these PDSFs are studied. We have shown that PDSFs display the largest pseudoeutectic span (≥20 mol %) of all reported copolymers and reveal the particular crystallization and melting behavior, different from those of common isodimorphism, by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), small-angle Xray scattering (SAXS), and Fourier transform infrared spectroscopy (FTIR).
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EXPERIMENTAL SECTION
Materials. The synthesis of polyesters was performed by a twostage melt−polycondensation reaction, via combination of the esterification process at atmospheric pressure and the polycondensation process at reduced pressure, as reported previously.6 Succinic acid (SA) and fumaric acid (FA) were fed at different molar ratios in the presence of 1,10-decanediol (DDO), and the (SA + FA)/DDO B
DOI: 10.1021/acs.macromol.8b01848 Macromolecules XXXX, XXX, XXX−XXX
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temperature (Tc) of PDF (62.2 °C) than of PDS (50.4 °C) should be ascribed to the existence of fumarate units with the fixed trans conformation, which is favorable during crystallization for lowering the entropic barrier.6,21 However, Tc declines when comonomeric units are incorporated into the homopolyester chain of either PDS or PDF. Thus, as opposed to the isomorphic phenomenon,6,7,15 PDSF copolyesters are plausible in isodimorphism.4,5 The succinate and fumarate structures are not equivalently included in the crystal lattices of PDSF, even though fumarate and succinate have very similar chemical structures. Corresponding X-ray diffractograms in the following section further confirm the isodimorphism in PDSFs. In the subsequent heating DSC curves (Figure 2B), the melting point (Tm) of either PDS or PDF is depressed by incorporation of fumarate or succinate structures, respectively, while the melting enthalpy of copolyesters, or PDSF, remains considerably high irrespective of the copolymerized composition (see Table 1). Once DF units are completely excluded from the crystal structure and confined to the amorphous region and exert no influence on the crystallization DS fraction in PDSF20, the melting enthalpy (ΔHm) of PDSF20 should not be higher than 72.9 J/g (=91.1 J/g × 0.8), which is apparently lower than the experimental value (85.5 J/g). Therefore, the DF fraction does participate in the crystal structure. The emerging FTIR band at 3080 cm−1 proves the inclusion of CC−H groups, or fumarate units, in the crystalline region of all PDSFs, and the plot of the normalized intensity of PDSF20 that is lower than the “y = x2” curve demonstrates the fumarate concentration in the PDS lattice is lower than that in the polyester chain.15,18 A similar deduction about the participation of the DS fraction in the PDF lattice could be reached for PDSF76. The minor comonomeric units are not strongly desired as the major monomeric units in crystal lattices, displaying a slight decrease in ΔHm. Therefore, the dependence of ΔHm on DF content in Figure 2C directly demonstrates the pseudoeutectic behavior of PDSF. The Tc and Tm values of polyesters are plotted as a function of DF content in panels A and B, respectively, of Figure 3. It is clear that PDSFs do not exhibit a commonly V-shaped Tm− composition relationship like common eutectic systems that usually display only a pseudoeutectic point or rather narrow pseudoeutectic region with a minimum Tm value over the entire composition range.5 A discrete transition in the crystal structure has been examined at some intermediate composition and correlated with the pseudoeutectic point/region.5 PDSFs, however, display irregular changes in Tm at DF contents of 37 and 57 mol %. Combined with parallel X-ray diffractograms shown below, PDSFs are believed to emerge in an intriguingly and attractively broad pseudoeutectic range of >20 mol %, from 37 to 57 mol % at least. The X-ray diffractograms of PDSF30 and PDSF70 in Figure S4 reveal that the pseudoeutectic range is ν2] sector reveals the change of the 1724 cm−1 band is faster than that of the 1740 cm−1 band. The blue-shift behavior of the 1724 cm−1 band in Figures S9 and
Figure 9. In situ-measured WAXD data of PDSF57 during the cooling process.
epitaxial crystallization surroundings caused by DF-rich crystals formed in advance. Epitaxy enhances crystallization, while confinement slows crystallization; the single Tc of PDSF57 (in Figure 2A) supports the possibility that epitaxial crystallization might play a more prominent role. The similar chemical structures of succinate and fumarate and the close lattice parameters on some axes between PDS and PDF ensure the epitaxial mechanism. The c axis (1.968 nm) of the PDS lattice
Figure 10. (A) Synchronous and (B) asynchronous 2D IR correlation spectra of PDSF57 being cooled from 140 to 70 °C in the wavenumber range of 1750−1710 cm−1. The vertical and horizontal axes are υ1 and υ2, respectively. Red and green colors represent the positive and negative signals, respectively. G
DOI: 10.1021/acs.macromol.8b01848 Macromolecules XXXX, XXX, XXX−XXX
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Figure 11. (A) DSC thermograms of PDSF37 being heated at different rates after non-isothermal melt crystallization. (B) In situ-measured WAXD data of PDSF37 during the heating process.
Figure 12. (A) X-ray diffractograms and (B) one-dimensional scattering intensity distribution profiles of PDSF37 isothermally crystallized at different temperatures. (C) In situ-measured WAXD data of PDSF37 during the cooling process. The arrows in panels A and C indicate the (040) diffraction shoulder.
Figure 13. (A) In situ-measured one-dimensional scattering intensity distribution profiles of PDSF37 during the heating process. (B) d* values plotted a function of temperature. “Step vert”-type lines have been drawn joining the data points to show the change.
increases. However, still only the (220)DS‑rich diffraction peak appears in all PDSF57 samples, ensuring DF-rich crystals form before DS-rich crystals. Melting and Crystallization Behavior of PDSF37. When PDSF37 is heated at different rates, only a single melting peak can be observed during each process (Figure 11A), indicating DS-rich and DF-rich crystals melt simultaneously. In situ WAXD measurements during heating were performed, and the results are shown in Figure 11B. The diffraction signal of the minor crystalline phase, DF-rich crystal, is maintained at a temperature as high as 75 °C and decreases with the same tendency and disappears at the same
S10 suggests some orderly assembly of DF units, which is in accord with the enhancement of the conjugated “CC− CO” structure upon formation of a hydrogen bonding interaction.15,18 Hence, the fast formation of the preordered structure of the DF structure in the melt helps induce crystallization of DF-rich crystals in PDSF57 first. When the melt crystallized at low temperatures, the contribution of the preordered structure to the crystallization competition would weaken; meanwhile, the accommodation ability of the DF unit in the PDS lattice is better than that of the DS unit in the PDF lattice (inferred from the change in interplanar distances in Figure S11), so the relative content of the DS-rich crystal H
DOI: 10.1021/acs.macromol.8b01848 Macromolecules XXXX, XXX, XXX−XXX
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Figure 14. (A−C) Linear fitting lines of “Tm versus lc−1” for PDS, PDSFs, and PDF. (D) Dependence of calculating Tm° and σe on DF content. The linear fitting data in panels B and C are from DS-rich crystals and DF-rich crystals, respectively. The data of DS-rich crystals in PDSF57 (★) are plotted in panel B, and the data of DF-rich crystals in PDSF37 (◆) are plotted in panel C for comparison. The calculated densities of PDS and PDF crystals are both close to 1.2 g/cm3, so this value was adopted for all polyesters to determine Δh from ΔHm°.
tightly confined by the DS-rich crystals. For comparison, in situ heating SAXS measurement of PDSF57 has also been performed (shown in Figure S12), and the result makes it clear that both d* values of DF-rich and DS-rich phases increase during heating. Thus, the two types of crystalline structures in PDSF57 do not significantly confine each other, like common pseudoeutectic systems, while PDSF37 is different from that case. By comparing Figures 8A and 12A, one can see that DS-rich crystals exhibit almost the same diffraction peaks, (220)DS‑rich and (240)DS‑rich, in PDSF37 and PDSF57 except for the appearance of a rather weak (040)DS‑rich diffraction shoulder in PDSF37. Thus, we propose that DS-rich crystals in PDSF37 melt-crystallize in a manner similar to that of PDSF57 but, plausibly, with a weaker epitaxial effect. Detailed information in Figure 12C showing that the diffraction peak of (240)DS‑rich at 2θ = 30° does not emerge together with other diffraction peaks during the early crystallization stage at 60 °C also indicates DSrich crystals do not form prior to DF-rich crystals, but the DFrich phase displays obviously fewer diffraction peaks in PDSF37 than in PDSF57, such as the strongest one at (210)DF‑rich and (040)DF‑rich. Therefore, the DF-rich phase in PDSF37 could not crystallize freely and remarkably earlier than the DS-rich phase like that in PDSF57. Inferring from the result described above that DF units form preordered structure first in the PDSF57 melt (similar 2D correlation patterns were not directly obtained due to the low DF content in PDSF37 and the influence of basic noise. but fortunately, Figure S10C reveals the wavenumber shift of CO in fumarate), we can deduce a description for the crystallization process of PDSF37. During the cooling process, while some preordered structures that would contribute to formation of the DF-rich crystal were
temperature as the DS-rich phase. Therefore, it is possible that the two types of crystals are “tightly tied” together. The X-ray diffractograms in Figure 12A demonstrate that the diffraction curves are independent of the crystallization temperature for PDSF37, which is strangely different from PDSF57 and other previously reported pseudoeutectic forms.15 The relative stability of two crystalline structures in the pseudoeutectic region is not steady, so their contents are dependent on crystallization temperature. Consequently, two types of lamellar stacks could be observed in samples isothermally melt-crystallized at different temperatures (Figure 12B). Although DS-rich crystals are the major crystalline structure in PDSF37, some of the diffraction peaks, (001)DS‑rich and (040)DS‑rich, are absent. The in situ melt-crystallization behavior of PDSF37 was observed by WAXD (Figure 12C). When the melt is cooled from 90 to 60 °C, some weak diffraction peaks appear, including the (220)DS‑rich, (002)DF‑rich, (202)DF‑rich, and (124)DF‑rich peaks. Then the diffraction intensities of all peaks increase upon further cooling. Thus, the two types of crystals should form almost at the same time. Figure 13A gives the in situ SAXS measurement of PDSF37 during heating, and the corresponding d* values are plotted in Figure 13B. The main peak that has been ascribed to the DSrich phase shifts in the low-q direction with an increase in temperature. This phenomenon reveals an increase in the d* value of the DS-rich phase, and there are two main reasons for it. The thinner lamellar fraction melts, and the volume expands when the material changes from the crystalline to amorphous state. However, the d* value of the DF-rich phase remains almost constant at around 9.7 nm until 74 °C and then vanishes together with the DS-rich phase. The temperatureindependent d* value discloses that DF-rich crystals should be I
DOI: 10.1021/acs.macromol.8b01848 Macromolecules XXXX, XXX, XXX−XXX
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thickness of DF-rich lamellae in PDSF37 is substituted into the fitting line of PDSF57, the resulting melting point at 61.4 °C could be obtained. Therefore, the DSC-measured Tm of the DF-rich crystal in PDSF37 is much higher than the expected theoretical value, and the crystal is under strong superheating conditions before melting. The surrounding and attaching DSrich crystals contribute to the guarantee of overheating of DFrich crystals. A somewhat similar case of overheating crystals being surrounded and stabilized by thicker lamellae can be made for poly(ethylene oxide) crystals during heating.27
established, it is still unable to construct sufficiently thick lamellae to form the crystal structure due to an insufficient number of DF units. Nevertheless, the preordered DF-rich structures could show some similar effect and facilitate the DSrich crystallization behavior as preformed DF-rich crystals in PDSF57, tentatively described as “soft epitaxy”. Subsequently, the DS-rich phase crystallizes, and the thin lamellae of the DFrich phase form at the same time with the assistance of tightly attached and soft epitaxial DS-rich crystals. This special formation condition of confined DF-rich crystals in PDSF37 might result in the disappearance of the strongest (210)DF‑rich diffraction and make them display a false melting point as high as that of DS-rich crystals, and no increase in d* occurs before the melt during the heating process. Discussion of the Unusual Melting Point−Composition Relationship. Figure 3 shows that PDSFs exhibit a unique Tm−composition relationship different from those of common pseudoeutectic systems. To further confirm the phenomenon, Tm values of PDS, PDSF, and PDF with different lamellar thicknesses (lc) were measured and linear fitting lines of “Tm versus lc−1” were used to extrapolate the equilibrium melting point (Tm°) based on the Gibbs− Thomson equation (eq 1) in Figure 14.25,26 2σe zyz ji Tm = Tm°jjj1 − z j Δhlc zz{ k
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CONCLUSION In this study, PDSF random copolyesters have been successfully synthesized via an adapted two-step melt polycondensation reaction with a wide succinate/fumarate composition range. The measured DSC and WAXD data show that PDSFs possess rather high crystallizability irrespective of composition and display the widest isodimorphism region that has been reported to date. The similar structures of succinate and fumarate are responsible for the inclusion of the minor component in the crystal lattices of the majority. Two compositions of PDSF in the pseudoeutectic region, PDSF57 and PDSF37, have been studied in detail. DF-rich crystals form prior to DS-rich crystals due to the assembly of fumarate-related preordered structure in the PDSF57 melt. Then the remaining amorphous form surrounded by DF-rich crystals converts to DS-rich crystals via a confined and epitaxial crystallization process, resulting in the absence of many diffraction peaks in the WAXD-measured profile. Decreasing the crystallization temperature can benefit the formation of DS-rich crystals. During the melt crystallization of PDSF37 that is located near the singular transition point from the single phase of cocrystallization to double phases of competition crystallization (i.e., the edge of the pseudoeutectic region), DF-rich preordered structure forms first but cannot crystallize itself. The preordered structure plays the role of a “soft epitaxial” substrate for DS-rich crystal formation, and meanwhile, DFrich crystals form with the assistance of DS-rich crystals. This particular crystallization process induces tightly attached structure between two types of crystals and results in fantastic phenomena. (1) Attached DF-rich crystals significantly increase the end surface free energy of DS-rich crystals and make it display a large melting point jump. (2) The DF-rich crystals can be stabilized in the overheating state by surrounding DS-rich crystals and melt together with DS-rich crystals, showing a false high melting point.
(1)
where Δh and σe are the latent heat of fusion per unit volume (joules per cubic centimeter) and end surface free energy (joules per square centimeter), respectively. The calculated data are summarized in Figure 14D. It can be found that the Tm°−composition relationship possesses the same tendency to chang as the Tm−composition relationship, in which PDSF37 shows an abrupt increase in both Tm° and σe. In the composition range of the single crystal structure, when the second comonomers are introduced, both Tm° and σe values decrease due to the lack of co-crystallization of minor units in crystal lattices, such as PDSF20 (compared with PDS) and PDSF76 (compared with PDF). For PDSF37, the abrupt increase in the melting point of the DS-rich crystal is evidently ascribed to the increase in σe, which supports the specifically tightly attached structure as suggested above. The attached DF-rich crystals on the surface of DS-rich crystals change, and their end surface free energy increases. The detailed mechanism needs to be studied further. When the measured data of DS-rich crystals in PDSF57 (sample prepared by crystallization at 60 °C) were plotted in Figure 14B, we could see that the point is located just right in the “Tm versus lc−1” fitting curves of PDSF37. Thus, DS-rich crystals might adopt a similar, or the same, structural state in PDSF37 and PDSF57, which is in accordance with the (“soft”) epitaxial/confined crystallization of DS-rich crystals on DF-rich crystals and, further, indicates that DS-rich crystals achieve their highest DF content in lattices around PDSF37. PDSF37 is almost at, or very close to, the critical composition at which PDSF changes from a single co-crystal structure to a pseudoeutectic structure. This particular composition position endows PDSF37 with the novel crystal structure and thermal properties. Considering DS units as defects, the melting point of DFrich crystals in PDSF37 should be lower than the melting point of those in PDSF57 even when they accept the same lamellar thickness and are under the same circumstances. When the
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01848.
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Figures S1−S12, Tables S1 and S2, and eqs S1 and S2 (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hai-Mu Ye: 0000-0001-7566-9559 J
DOI: 10.1021/acs.macromol.8b01848 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Notes
Random Copolymerization of Biodegradable Aliphatic Polyester. Macromolecules 2008, 41, 3162−3168. (15) Ye, H.-M.; Liu, P.; Wang, C. X.; Meng, X.; Zhou, Q. Polymorphism regulation in poly(hexamethylene succinate-co-hexamethylene fumarate): Altering the hydrogen bonds in crystalline lattice. Polymer 2017, 108, 272−280. (16) Yu, Y.; Wei, Z.; Zhou, C.; Zheng, L.; Leng, X.; Li, Y. Miscibility and competition of cocrystallization behavior of poly(hexamethylene dicarboxylate)s aliphatic copolyesters: Effect of chain length of aliphatic diacids. Eur. Polym. J. 2017, 92, 71−85. (17) Yu, Y.; Sang, L.; Wei, Z.; Leng, X.; Li, Y. Unique isodimorphism and isomorphism behaviors of even-odd poly(hexamethylene dicarboxylate) aliphatic copolyesters. Polymer 2017, 115, 106−117. (18) Ye, H.-M.; Tang, Y.-R.; Song, Y.-Y.; Xu, J.; Guo, B.-H.; Zhou, Q. Aliphatic copolyester with isomorphism in limited composition range. Polymer 2014, 55, 5811−5820. (19) Wu, Q.; Tian, G.; Sun, S.; Noda, I.; Chen, G.-Q. Study of microbial polyhydroxyalkanoates using two-dimensional Fouriertransform infrared correlation spectroscopy. J. Appl. Polym. Sci. 2001, 82, 934−940. (20) Fox, T. G. Glass transitions of mesophase macromolecules. Bull. Am. Phys. Soc. 1956, 1, 123−129. (21) Ye, H.-M.; Tang, Y.-R.; Xu, J.; Guo, B.-H. Role of Poly(butylene fumarate) on Crystallization Behavior of Poly(butylene succinate). Ind. Eng. Chem. Res. 2013, 52, 10682−10689. (22) Gestí, S.; Casas, M. T.; Puiggalí, J. Crystalline structure of poly(hexamethylene succinate) and single crystal degradation studies. Polymer 2007, 48, 5088−5097. (23) Sun, Y.; Li, H.; Huang, Y.; Chen, E.; Zhao, L.; Gan, Z.; Yan, S. Epitaxial Crystallization of Poly(butylene adipate) on Highly Oriented Polyethylene Thin Film. Macromolecules 2005, 38, 2739− 2743. (24) Wang, H.; Gao, Z.; Yang, X.; Liu, K.; Zhang, M.; Qiang, X.; Wang, X. Epitaxial Crystallization Behavior of Poly(butylene adipate) on Orientated Poly(butylene succinate) Substrate. Polymers 2018, 10, 110. (25) Hoffman, J. D.; Davis, G. T.; Lauritzen, J. I. The rate of crystallization of linear polymers with chain folding. In Treatise on Solid State Chemistry; Hannay, N. B., Ed.; Plenum Press: New York, 1976; Vol. 3, Chapter 7. (26) Wunderlich, B. Crystal melting. In Macromolecular Physics; Academic Press: New York, 1980; Vol. 3. (27) Zhu, D.-S.; Liu, Y.-X.; Shi, A.-C.; Chen, E.-Q. Morphology evolution in superheated crystal monolayer of low molecular weight poly(ethylene oxide) on mica surface. Polymer 2006, 47, 5239−5242.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21674128), the National Key Research and Development Plan (2016YFC0303708), and the China University of Petroleum, Beijing (2462018BJC005). The authors are grateful to Prof. Bijin Xiong from Huazhong University of Science and Technology for his kind help with SAXS analysis and Mr. Zhining Xie from Tsinghua University for his kind help with in situ WAXD measurements.
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REFERENCES
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DOI: 10.1021/acs.macromol.8b01848 Macromolecules XXXX, XXX, XXX−XXX
