Article Cite This: Macromolecules 2019, 52, 4624−4633
pubs.acs.org/Macromolecules
Crystal Polymorphism of Biobased Polyester Composed of Isomannide and Succinic Acid Hironori Marubayashi,*,†,‡ Takaaki Ushio,† and Shuichi Nojima†,‡ †
Department of Chemical Science and Engineering, School of Materials and Chemical Technology, and ‡Research Institute of Polymer Science and Technology (RIPST), Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan
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S Supporting Information *
ABSTRACT: The crystallization and solid-state structure of the biobased polyester composed of isomannide and succinic acid (M4) were systematically investigated by changing the crystallization conditions such as casting solvent and crystallization temperature (Tc). It was found from wide-angle X-ray diffraction that M4 had the crystal polymorphism: casting from the chloroform solution gave the α-form, whereas thermal annealing at Tc ≥ 160 °C gave the β-form. A mixture of α- and β-forms was obtained at Tc < 160 °C, and the β-form fraction increased with increasing Tc. The unit cell of the α-form was determined by the fiber diffraction method: a = 0.523 nm, b = 0.913 nm, c (fiber axis) = 2.082 nm, and α = β = γ = 90° (monoclinic P21), where two 2/1 helices were packed in the unit cell in a parallel manner. M4 chains in the β-form took the same chain conformation but a different chain packing. The crystallization half-time of M4 drew an inverted bell-shaped curve against Tc, in which the minimum was ca. 200 s at Tc = 140−150 °C.
1. INTRODUCTION
Isohexides are aliphatic heterocyclic diols composed of two cis-fused hydroxytetrahydrofuran units.6 “Isohexide” is a general term for three stereoisomers, which are different in terms of orientations of two hydroxyl groups (endo or exo) connected to the second and fifth carbon atoms1,4:3,6dianhydro-D-mannitol (isomannide), 1,4:3,6-dianhydro-D-glucitol (isosorbide), and 1,4:3,6-dianhydro-L-iditol (isoidide) (Figure S1).6 The starch-derived sugar alcoholsD-sorbitol and D-mannitolcan be dehydrated to give isosorbide and isomannide, respectively.6 On the contrary, the precursor to isoidide (L-idose) is rarely found in nature, but recently, several trials have been done toward industrial production.21 “Isohexide polyesters”, biobased polyesters composed of isohexides and dicarboxylic acids (diacids), have been synthesized, and their basic properties have been investigated.3−7 The glass-transition temperature (Tg) of isohexide polyesters ranges from −10 to 80 °C depending dominantly on the diacid chain length.7 In addition, by using isohexides [isomannide (M) and isosorbide (S)] and diacids with k = 4, 5, 6, 8, 10, and 12 (k is the carbon number of the diacid unit), we have synthesized a series of polyesters (Mk and Sk) to examine their crystallizability and found a crystalline nature of M4, M10, M12, S10, and S12.7 Furthermore, we have conducted detailed studies on the crystallization of the polyesters with long diacid units (Mk and Sk with k = 10, 12) and clarified the effects of isohexide stereoisomerism (i.e.,
In the last few decades, “biobased polymers”, which can be synthesized from renewable natural resources (i.e., biomass), have attracted more and more attention because of urgent global concern about fossil fuel depletion, global warming, and ecosystem destruction, resulting from mass production and consumption of petroleum-based polymeric materials.1,2 However, it is very difficult to meet a wide variety of demands from the market of polymeric materials only by conventional biopolyesters such as poly(lactic acid) and polyhydroxyalkanoates. Therefore, it is essential to utilize various biobased value-added chemicals as building blocks of biobased polymers to tune their material properties and satisfy the demands from the market. In general, introducing a rigid cyclic structure such as aromatic rings into the backbone of polymers can significantly improve their material properties (e.g., thermal and mechanical properties). For instance, arylate-type polyesters such as poly(ethylene terephthalate) have excellent properties and are widely used as packaging (e.g., bottles and containers), clothing, and so forth. However, these polyesters are made from petroleum, so that their production must be reduced and the alternative biobased polymers should be produced in an upcoming sustainable society. Recently, biobased polymers having a rigid cyclic structure in the backbone have attracted increasing attention owing to their possibility to be highperformance biobased plastics. For example, 1,4:3,6-dianhydrohexitols (isohexides),3−11 2,5-furandicarboxylic acid,11−16 and vanillin17−20 have been utilized as such biobased cyclic building blocks. © 2019 American Chemical Society
Received: December 5, 2018 Revised: March 28, 2019 Published: June 13, 2019 4624
DOI: 10.1021/acs.macromol.8b02594 Macromolecules 2019, 52, 4624−4633
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Macromolecules
followed by vacuum drying at room temperature. Chloroform, dichloromethane, and 2,2,2-trifluoroethanol were used as casting solvents. Film samples of M4h were prepared by the following procedures. The as-polymerized sample of M4h was sandwiched between aluminum plates (0.1 mm in thickness) with an aluminum spacer (0.1 mm in thickness), preheated at Ta for 1 min, and hot-pressed under 20 MPa at Ta for ca. 1 min. The resultant samples were quenched into ice-water at ca. 5 °C and dried in vacuum, and then the melt-quenched (i.e., amorphous) films of M4h with a thickness of ca. 0.2 mm were obtained. The uniaxially-oriented films for the fiber diffraction were prepared by drawing the melt-quenched films at 95 °C (>Tg = 78 °C) by six times the original length, followed by annealing at higher temperatures in the fixed state. Detailed conditions are shown in Section 3.1.2. 2.4. Differential Scanning Calorimetry. Crystallization and melting behaviors were examined for a ca. 5 mg sample sealed in an aluminum pan using a Diamond differential scanning calorimetry (DSC; PerkinElmer Inc.) equipped with a Cryofill (PerkinElmer Inc.) under a helium gas atmosphere. The first heating run [25 °C → Ta (=200 °C for M4) at 10 °C/min] was conducted for the aspolymerized sample. After quenching from Ta to −30 °C at −500 °C/ min, the second heating run (−30 °C → Ta at 10 °C/min) was carried out. A heating scan from Tc to Ta at 10 °C/min was done after the samples were isothermally annealed at Tc or solution-cast at room temperature (=Tc). 2.5. Polarized Optical Microscopy. The crystalline morphology was observed by a digital microscope VHX-600 (KEYENCE Corp.) equipped with an LTS350 (Linkam Scientific Instruments Ltd.). A fragment of a sample was sandwiched between glass plates and heated at Ta (=200 °C for M4). Then, a gentle pressure was applied to the cover glass to squeeze the melt into a thin film, and the temperature was quickly lowered to Tc. 2.6. Wide-Angle X-ray Diffraction. Crystallinity (Χc) and crystal structure were evaluated by a NANO-Viewer (Rigaku Corp.) operating at 45 kV and 60 mA with Cu Kα radiation (λ = 0.1542 nm), which was monochromatized by a confocal mirror. An imaging plate BAS-IP SR 127 (Fujifilm Corp.) was used as a detector and read by an R-AXIS DS3C (Rigaku Corp.). The total pixel number, pixel size, and dynamic range of the imaging plate were 2300 × 2300, 50 × 50 μm2, and 2.6 × 105, respectively. The d-spacing of (111) of silicon powder was used as a standard. X-ray radiation to a sample was conducted at room temperature under atmospheric pressure. 2D-to-1D conversion, intensity correction, crystallinity determination (see the Supporting Information), and peak separation were done by using a handmade GUI software.22,23 2D wide-angle X-ray diffraction (WAXD) patterns of the oriented samples (i.e., fiber diagrams) were analyzed by the same software. 2.7. Small-Angle X-ray Scattering. The higher-order structure (i.e., alternating structure of crystalline lamella and amorphous layer) was analyzed by using small-angle X-ray scattering (SAXS) with synchrotron radiation at the beam lines BL-10C and 6A in Photon Factory of High Energy Accelerator Research Organization, KEK (Ibaraki, Japan). The X-ray wavelength was set to 0.1488 and 0.1500 nm at BL-10C and 6A, respectively. PILATUS3 2M and 1M (DECTRIS Ltd.) were used as detectors at BL-10C and 6A, respectively. Silver behenate (d001 = 5.8380 nm)24 was used as a standard sample. Vacuum paths were set up between a sample holder and the detector. 2D-to-1D conversion, intensity correction, and correlation function analysis (see the Supporting Information) were done by using a handmade GUI software.25,26 2.8. Fourier Transform Infrared Spectroscopy. Conformation and relative crystallinity of M4 were evaluated by using an FT/IR6200 (JASCO Corp.) with a spectral resolution of 2 cm−1 (4 cm−1 for time-resolved measurements) and a cumulative number of 32 in air at room temperature (Tc for time-resolved measurements). KBr pellets including 1−4 wt % M4 were used as specimens. 2.9. Conformational Analysis. Conformations of M4 in the crystallized state were evaluated by a handmade GUI software.27
M or S) and diacid chain length (k) on their hierarchical crystalline structure and crystallization behavior.7 M4 (Figure 1), the polyester composed of isomannide and short diacid units (k = 4), also shows a crystalline nature. The
Figure 1. Chemical structure of polyester composed of isomannide and succinic acid (M4).
melting temperature of M4 (Tm = 185 °C7) is much higher than those of polyesters with long diacid units (T°m ≈ 100 °C for Mk and Sk with k = 10, 127), so that M4 is expected to be used as high-performance biobased plastics. However, little is known about the crystallization and structure−property relationship of M4, which are absolutely important for the material design. In this study, the solid-state structure, melting behavior, and crystallization rate of M4 were systematically investigated by changing the crystallization conditions. We demonstrate herein the crystal polymorphism of M4 (α- and βforms) mainly by one-dimensional (1D) and two-dimensional (2D) X-ray diffraction data.
2. EXPERIMENTAL SECTION 2.1. Materials. Isomannide (Sigma-Aldrich Co. LLC., 95%) was recrystallized from acetone before use. Succinyl chloride (Tokyo Chemical Industry Co., Ltd., >95%) was used as received. All other reagents were used as received from commercial suppliers. 2.2. Polymerization of Isomannide Polyester. As shown in Scheme S1, isomannide was polycondensed with succinyl chloride. The polyester (M4) with Mn = 0.94 × 104 and Mw/Mn = 2.1 (Mn: number-average molecular weight, Mw: weight-average molecular weight) had been synthesized in our previous study.7 The polyester with higher molecular weight (Mn = 2.9 × 104, Mw/Mn = 2.1; denoted M4h) was newly prepared by raising the polymerization temperature and used for the fiber diffraction analysis. The polymerization method of M4h is detailed below. Isomannide was weighed to be an equimolar amount to 1 mL (8.84 mmol) of succinyl chloride and put in a 100 mL three-necked flask equipped with a mechanical stirrer. The flask was vacuum-dried, and subsequently, the atmosphere was replaced with argon. The flask was heated at 95 °C with stirring to melt crystals of isomannide. Under a nitrogen gas flow, bulk polycondensation was started by adding 1 mL of succinyl chloride to the system. After 6 h, the temperature was raised to 100 °C and held for 20 min. The temperature was increased by 10 °C every 20 min (100 → 150 °C) and held at 150 °C for 1 h. During these steps, the product became solidified but able to be stirred by the mechanical stirrer. The temperature was further raised to 200 °C and held for 1 h. Afterward, polycondensation was carried out at the same temperature under vacuum for 1 h. After the system was cooled to room temperature, the product was dissolved in chloroform and the polymer was precipitated into methanol, followed by suction filtration and vacuum drying. Finally, the polymer was obtained in a yield of 57%. 2.3. Sample Preparation. The annealing temperature (Ta) at which thermal history of each sample would be removed (Ta > Tm) was set to 200 and 210 °C for M4 and M4h, respectively. Just after being melted at Ta for enough time (≥1 min), samples were annealed at the crystallization temperature (Tc) for the selected period of time (tc = 1 h for Tc < 170 °C and 3 h for Tc ≥ 170 °C for sufficient crystallization), followed by quenching with liquid nitrogen (meltcrystallized samples). In order to obtain the solution-crystallized samples, M4 solution (10 mg/mL) was cast on a glass Petri dish at room temperature, 4625
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Macromolecules MMFF9428 was used as the force field, and the definition of the intramolecular interaction energy (Eintra) was the same as our previous study.7 An M4 chain with 6 residues was constructed in the Z-matrix coordinate system, and 28 atoms in the third residue were targeted in energy calculations, where the cutoff distance was set to 1.5 nm. 2.10. Packing Analysis. In calculations of the intermolecular interaction energy (Einter), MMFF9428 was also used as the force field, where van der Waals and electrostatic interactions were calculated and summed up to be Einter. The standard M4 chain with 6 residues was constructed in the Z-matrix coordinate system. Other M4 chains were generated as needed. As in the conformational analysis, 28 atoms in the third residue were targeted and the cutoff distance was set to 1.5 nm. The reliability factor (R) is given by the following equation R=
∑ ||Fo| − K −1|Fc|| ∑ |Fo|
Thus, peak intensities of dβ1 and dβ2 increase and that of dα1 decreases with increasing Tc. These results clearly indicate the crystal polymorphism of M4. Only one crystal modification, which is named the α-form, is generated by solution-casting using chloroform. A mixture of the α-form and another polymorph named the β-form is developed by the melt-crystallization at Tc = 120−150 °C, where the fraction of the β-form increases with increasing Tc. M4 crystallizes into only the β-form in the melt-crystallization at Tc ≥ 160 °C. Note that the melt-crystallization gives a mixture of α- and β-forms, even if Tc is further lowered (e.g., 110 °C). In addition, the pure α-form can be obtained by casting from the chloroform solution, whereas pure β- and βrich crystals are formed by casting from dichloromethane and 2,2,2-trifluoroethanol solutions, respectively (Figure S3). Namely, the crystal structure of M4 strongly depends on casting solvents. Although we tried to use other solvents such as acetone, ethyl acetate, tetrahydrofuran, and toluene, M4 was not dissolved in these solvents. Here, Χc of the solution-crystallized sample (49%) is obviously higher than that of the melt-crystallized samples (27−34%). The effect of residual solvent (chloroform) in the sample was suspected, but Χc was unchanged (47%) even by reannealing at 120 °C for 1 h after solution-casting (Figure S4). Such an increase in Χc would be ascribed to a significant increase in chain mobility of M4 by plasticizing effects of solvent molecules. 3.1.2. Fiber Diagram Analysis of Oriented Samples. In order to understand the crystal polymorphism of M4 in more detail, we prepared the uniaxially-oriented crystallized films of M4h for X-ray fiber diffraction (i.e., 2D WAXD patterns of oriented samples). Figure 3 depicts the fiber diagrams of α-
(1)
where Fo is the observed structure factor, K is the scaling factor, and Fc is the calculated structure factor. The isotropic atomic displacement parameter was assumed to be 0.120 nm2 for all constituent atoms. More detailed information is shown in the Supporting Information.
3. RESULTS AND DISCUSSION 3.1. Crystal Structure. 3.1.1. WAXD Analysis of Unoriented Samples. Figure 2 shows WAXD profiles of
Figure 3. Fiber diagrams of (a) α- and (b) β-forms of M4h. (b) Film contains the β- and α-crystals as major and minor components, respectively. Drawing direction is vertical. The oriented α-crystal film (a) was prepared by drawing at 95 °C and stepwise annealing from 100 to 160 °C in the fixed state, whereas the oriented β-crystal film (b) by drawing at 95 °C and annealing at 176 °C in the fixed state.
Figure 2. WAXD profiles of solution-crystallized or melt-crystallized M4 samples. λ = 0.1542 nm (Cu Kα).
solution-crystallized or melt-crystallized samples. A strong diffraction peak is seen at 2θ = 19.0° (dα1 = 0.467 nm) for the solution-crystallized sample, where Χc is 49%. Although Χc of the melt-crystallized samples is almost constant (27−34%) irrespective of Tc, the profile shape shows a strong dependence on Tc. The samples melt-crystallized at 120−130 °C show strong diffraction peaks at 2θ = 17.8° (dβ1 = 0.498 nm) as well as 19.0° (dα1 = 0.467 nm), where the latter peak position is the same as the dα1 peak position of the solution-crystallized sample. At Tc = 140 °C, the peak intensity of dβ1 increases and that of dα1 decreases. In addition, the diffraction peak at 2θ = 20.9° (dβ2 = 0.425 nm) becomes observable. At Tc = 150 °C, the peak intensity of dβ1 further increases, that of dα1 decreases to be seen just as a shoulder, and that of dβ2 increases. With further increasing Tc (160−170 °C), the dα1 peak is no longer observed, so that representative peaks arise from dβ1 and dβ2.
and β-forms of M4h, in which the drawing direction is vertical. Well-separated diffraction spots are seen in the fiber diagram of the α-form (Figure 3a), although the number of reflections is not sufficient. Unfortunately, the fiber diagram of the β-form (Figure 3b) is poor because both degrees of crystallinity and orientation of the β-form sample are obviously lower than those of the α-form, which would be due to the orientation relaxation competing against the oriented-crystallization of M4h chains during annealing at high Tc (176 °C). As shown in Figure S5, the very strong (vs) equatorial peak in the fiber diagram of the α-form corresponds to dα1 (Figure 2) and vs and strong (s) equatorial peaks in that of the β-form correspond to dβ1 and dβ2 (Figure 2), respectively. It is found from these equatorial peaks that the α-form sample contains only the α-crystals, whereas the β-form sample 4626
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Macromolecules consists of the β- and α-crystals as major and minor components, respectively. For both fiber diagrams, it was difficult to draw the layer lines on which all of the reflections lay by the plausible values of fiber periods (dp = 1−2 nm), which were judged by taking a small lamella thickness (4−6 nm) and the Laue condition into account. Such a deviation of reflections from the layer line can be explained by the so-called “tilting phenomenon”.29,30 As a result, dp of the α-form was determined to be ca. 2.1 nm, where the strong meridional reflection (d = 1.03 nm) was indexed as (002), and hence, the twofold helix was proposed as the conformation of the α-form. In our previous work on the isomannide polyester with a longer diacid unit (M12), the right-handed 2/1 helix with planar zig-zag methylene sequences was proposed on the basis of the fiber diagram, infrared progression bands, and energy calculations.7 Note that dp ≈ 1 nm (i.e., a half value of 2.1 nm) was confirmed to be invalid because it did not give the energetically stable conformation, as described later in the conformational analysis. Diffraction peaks with the same ξ and different ζ values, where ξ is the distance and ζ the height in the reciprocal cylindrical coordinate,31 were seen for the α-form, clearly indicating that the angles α and β in the unit cell were 90°. A total of 13 unique reflections were extracted from the fiber diagram of the α-form by peak separation (Figure S6) and indexed by the orthogonal cell with a ≈ 0.52 nm, b ≈ 0.91 nm, and c (fiber axis) ≈ 2.1 nm, where b/a ≈ 31/2 meant the hexagonal packing. The c-axis length had uncertainty to some extent owing to the “tilting phenomenon”, so that a final decision of lattice constants was not conducted at this stage. Note that the 2/1 helix of M4 has no symmetries required for the hexagonal system such as 6/m, so that the orthorhombic cell is appropriate despite b/a ≈ 31/2. Here, dα1 that gives a vs diffraction peak (Figure 2) is indexed as the (020)/(110) reflection. Unfortunately, it was very difficult or impossible to identify diffraction peaks derived from the β-form because (ξ, ζ) values of the β-rich film were similar to those of the α-crystal film. To make matters worse, the aforementioned low diffraction intensity and orientation degree of the β-rich film hindered the identification of the β-form-derived peaks. However, at least limited but useful information on the β-form has been obtained: (i) vs dβ1 and s dβ2 peaks are indexed as (hk0) reflections (Figure S5) and (ii) dp of the β-form is comparable to that of the α-form (ca. 2.1 nm). These results clearly show that M4 chains in α- and β-forms take the same chain conformation (2/1 helix) but a different chain packing (i.e., hexagonal and nonhexagonal packing of α- and β-forms, respectively). 3.1.3. Conformational Analysis. Figure 4 shows atomic numbering and definition of torsional angles for M4. Molecular building is mainly based on isosorbide32 and dimethyl succinate,33 and details are shown in Tables S1−S3. Unfortunately, as far as we know, the crystal structure of isomannide has not been reported yet, so that we utilized that of isosorbide as a reference. The torsional angles around ester linkages (ω1, ω2) were fixed to 180°. Plausible conformations of M4 in the crystallized state were explored by changing the selected torsional angles (φ, ψ, θ1, θ2, and θ3) considering dp (≈2.1 nm), helical sense (2/1 helix), and intramolecular interaction energy (Eintra). As a result, it was found that some twofold helical conformations with θ2 = 60° (gauche) or 180° (trans) met well such requirements. Note that valid
Figure 4. Atomic numbering and definition of torsional angles (φ, ψ, ω1, θ1, θ2, θ3, and ω2) for M4.
conformations cannot be obtained when θ2 = −60° (gauche minus). Unfortunately, it was difficult to determine the best conformation from several energetically similar conformations. Therefore, the final conformation was determined in the process of the crystal structure analysis mentioned later. Figure 5 shows Fourier transform infrared spectroscopy (FTIR) spectra of M4 solution-crystallized by chloroform or melt-crystallized at Tc = 120−170 °C. In the fingerprint region of 1000−600 cm−1 (Figure 5a), complicated multiple peaks are observed for both solution-crystallized and melt-crystallized samples. As shown in Figure 5b, the carbonyl stretching bands are clearly seen around 1735 cm−1 for all of the samples. First, the spectrum of the solution-crystallized sample (αform) is compared with that of the sample melt-crystallized at 120 °C (α-rich). Peak positions and shapes are comparable for these two samples except for the peaks at 750 and 740 cm−1 for the solution-crystallized and melt-crystallized samples, respectively (Figure 5a). Such similarity of FTIR spectra would be reasonable because of similar compositions of α- and β-forms in these two samples (i.e., α- or α-rich). Because the casting solvent, chloroform is known to have strong absorbance around 760 cm−1, there is a possibility that the residual chloroform in the sample has some effect on FTIR spectra. However, such a possibility can be eliminated by the spectrum of the solution-cast and subsequently reannealed (“castannealed”) sample (red), in which the absorbance around 760 cm−1 is very similar to that of the solution-cast sample (blue). In addition, the reannealing temperature (120 °C) is much higher than the boiling point of chloroform (61 °C) and the reannealing period is sufficiently long (1 h). Therefore, we conclude that chloroform has been completely removed from the sample by vacuum-drying after solution-casting. As shown in Figure 5a, absorbances at 878 and 813 cm−1 increase and those at 860 and 852 cm−1 decrease by reannealing at 120 °C. These small spectral changes might be linked with the changes in WAXD curves at relatively high angles (2θ > 25° in Figure S4), which correspond to relatively small d-spacings. Next, the Tc dependence of FTIR spectra is discussed for the melt-crystallized samples. Almost the same spectra are seen regardless of Tc in both the fingerprint (Figure 5a) and carbonyl stretching (Figure 5b) regions. Thus, FTIR spectra of crystallized M4 samples are very similar to each other irrespective of the crystallization conditions (use/non-use of casting solvent and Tc), so that it is concluded that M4 chains in the α- and β-crystals have similar conformations (i.e., 2/1 helix). The peak around 634 cm−1 (Figure 5a) is seen only in the crystallized samples (i.e., crystalline band), so that this peak is used as a measure of crystallinity in Section 3.4. Here, the peak 4627
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Figure 5. FTIR spectra of solution-crystallized or melt-crystallized samples of M4: (a) 1000−600 cm−1 and (b) 2000−1500 cm−1. The spectrum of the solution-cast and reannealed (cast-annealed) sample (red) is shown for comparison with the solution-cast sample (blue). The spectrum of the melt-quenched (i.e., amorphous) sample is also shown for comparison.
Figure 6. Schematic illustration of crystal structure analysis processes for the α-form of M4. P and A represent parallel and antiparallel chains, respectively. P′ means the parallel chain with W2 ≠ W1 and Z2 ≠ Z1.
around 760 cm−1 is also observed only in the crystallized samples, but overlaps with the lower wavenumber peak. 3.1.4. Crystal Structure Analysis of the α-Form. Figure 6 shows the schematic illustration of the crystal structure analysis processes of the α-form. First, we assumed the antiparallel 2chain model (herringbone-type packing) with the space group of P212121 (Figure 6a). When the parallel chains are on the corners of the unit cell, the antiparallel one is on the center of the cell (vice versa). Here, we define the chain on the corner as the “parallel chain” and that on the center as the “antiparallel chain”. Two variables were considered in calculating R and Einter: W1 is the rotation angle of the parallel chain around the chain axis, and Z1 is the translation distance parallel to the chain axis. Note that the rotation angle (W2) and translation distance (Z2) of the antiparallel chain can be automatically determined by those of the parallel chain (W1 and Z1, respectively) because of the P212121 symmetry. Selected 26 types of energetically similar 2/1 helices with dp ≈ 2.1 nm were packed in the unit cell whose parameters had been optimized for dp (a, b: free, c: fixed to dp) with W1 (=W2) and Z1 (=Z2) changed in a stepwise manner. The plausible crystal structure (i.e., chain conformation and packing) was explored on the basis of R and Einter.
It was found that the 2/1 helix with θ2 = 60° (gauche) was superior to that with θ2 = 180° (trans) in terms of R and Einter: the former had lower R and Einter compared to the latter. R was lowered (22−25%) but larger than 20% even by optimizing the packing parameters (W1 and Z1) and changing θ2 around 60°. In addition, Einter was still high (>100 kcal/mol). In order to decrease R and Einter, the space group was changed from the orthorhombic P212121 to the monoclinic P21 (P1121 to be more exact) by removing 21 screw axes perpendicular to the caxis (Figure 6b). Two chains (parallel and antiparallel) are independent in the space group of P21, so that 4 independent parameters (W1, W2, Z1, and Z2) have to be considered. As a result, the monoclinic P21 was found to be more appropriate as the space group of the α-form compared to the orthorhombic P212121 because the former gave lower R (19.2%) and Einter (30 kcal/mol) compared to the latter. There was no necessity that the chain at the center of the unit cell was antiparallel, so that we also tried the parallel packing model with the space group of P21, in which two independent parallel chains were packed (Figure 6c). After the packing parameters (W1, W2, Z1, and Z2) were optimized, the parallel P21 model gave lower R and Einter compared to the antiparallel P21 model. Finally, one of the parallel-packing P21 4628
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Macromolecules models was selected as the most plausible (final) model of the crystal structure of the M4 α-form. Figure 7 depicts the ab and
Figure 8. Lorentz-corrected SAXS curves (Iq2 vs q) of M4 solutioncrystallized (chloroform, room temperature) or melt-crystallized at Tc = 120−170 °C. The SAXS measurement of the solution-crystallized sample was done at 120 °C, and those of melt-crystallized samples at 120 °C for Tc = 120−150 °C and at 150 °C for Tc > 150 °C. The scattering intensity of each curve was normalized by the primary peak intensity, where the normalized intensity of the solution-crystallized sample was set to a half value of those of melt-crystallized samples.
phases. A scattering peak derived from the alternating structure composed of crystalline and amorphous layers is seen for both solution-crystallized and melt-crystallized samples, although the scattering peak intensity of the solution-crystallized sample is much lower than those of melt-crystallized samples. The peak position shifts to a lower q (i.e., an increase in the long period, Lp) with increasing Tc. Figure 9 shows the Tc dependence of Lp, lamella thickness (lc), and amorphous layer thickness (la) obtained from the 1D correlation functions [γ(r), Figure S7]. No clear Tc-dependence of lc is seen at Tc = 120−150 °C, whereas a small but distinct increase in lc is observed at Tc ≥ 150 °C.
Figure 7. Crystal structure of the α-form of M4. Unit cell parameters: a = 0.523 nm, b = 0.913 nm, c (fiber axis) = 2.082 nm, and α = β = γ = 90° (Z = 2, monoclinic P21). Torsional angles: φ = −70°, ψ = −120°, θ1 = 175°, θ2 = 60°, and θ3 = 170°. The calculated density is 1.525 g/cm3.
bc planes of the final model whose R is 19.1% 150 °C because of a broad SAXS pattern at room temperature, which is probably due to a small difference between electron densities of crystalline and amorphous
Figure 9. Long period (Lp), lamella thickness (lc), and amorphous layer thickness (la) of melt-crystallized M4 plotted against Tc. 4629
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Macromolecules Unfortunately, γ(r) with a valid shape was unable to be obtained for the solution-crystallized sample owing to a quite low peak intensity and a drastic increase in the intensity at q < 0.4 nm−1 with decreasing q. The former should be due to irregular stacking of lamellar crystals.34 The latter trend can be explained by scattering from loosely packed powder grains among which polydisperse microvoids exist.35 In fact, the M4 sample cast from the chloroform solution was fragile, so that fragments of the sample were hand-pressed at room temperature to obtain a disc-shaped specimen for WAXD and SAXS measurements. Alternatively, lc was approximately estimated to be 4.7 nm based on lc = Lp × (Χc/100), where Lp was obtained from the Lorentz-corrected scattering curve. This value (4.7 nm) is similar to lc of the sample melt-crystallized at 175 °C (4.9 nm by the same method). Such a good agreement is consistent with comparable Tm values of these two samples (≈185 °C), considering the Gibbs−Thomson equation.36,37 Figure S8 shows 2D SAXS patterns of the uniaxially-oriented crystallized films of M4h. The oriented film with pure αcrystals exhibits two-point scattering characteristic of the oriented alternating structure consisting of crystalline lamellae and amorphous layers.38 We obtained Lp = 10.7 nm, lc = 4.3 nm, and la = 6.4 nm by the correlation function analysis for the azimuthally-averaged profile around the peak (Figure S9). On the other hand, the β-rich film exhibits streaks parallel to the drawing direction (i.e., meridional streaks). It was difficult to obtain some structural parameters from such streaks because of no maxima. The equilibrium melting temperature (Tm ° ) of the β-form was estimated from SAXS (lc for Tc ≥ 150 °C) and DSC (Tm for Tc ≥ 150 °C, see Section 3.3) on the basis of the Gibbs− Thomson equation,36,37 as shown in Figure S10. The obtained ° (231 °C) was much higher than expected (i.e., overTm ° ) probably because a relatively small lc (4−6 estimation of Tm nm) and its small Tc-dependence (Δlc ≈ 1.4 nm for Tc = 150− 175 °C) caused a linear extrapolation with low accuracy. 3.3. Melting Behavior. Figure 10 shows DSC heating curves of M4 solution-crystallized by chloroform or meltcrystallized at Tc = 120−175 °C for enough time. The solution-crystallized sample shows a single endothermic peak at 185 °C with the melting enthalpy, ΔHm = 39 J/g. On the other hand, the melt-crystallized samples exhibit multiple melting behaviors depending strongly on Tc. For Tc < 140 °C at which the α-form is preferentially formed (Figure 2), a sharp endothermic peak (Tm,α1) is seen around 170 °C. Tm,α1 slightly increases, and its peak area (i.e., ΔHm) decreases with increasing Tc. It seems that the peak area of Tm,α1 corresponds to the initial fraction of the α-form in each sample (Figure 2). On the contrary, Tm,α1 cannot be confirmed for samples meltcrystallized at Tc > 140 °C, which can be understood by a very small amount or absence of α-crystals, as seen in WAXD (Figure 2). On DSC curves of samples isothermally crystallized at Tc ≥ 140 °C, another endothermic peak appears (e.g., 168 °C for Tc = 140 °C; 185 °C for Tc = 175 °C) and the peak temperature (Tm,β1) increases with increasing Tc. Note that the β-form is preferentially generated in this Tc range, as seen in Figure 2. Therefore, this result would be explained by lamella thickening of the β-form with increasing Tc. In order to clarify the relationship between Tm,α1 and the initial fraction of the α-form, the sample melt-crystallized at 120 °C (α-crystal rich) was reannealed at 170 °C, which was just above Tm,α1. As a result, the sample showed no Tm,α1 peak in the DSC heating scan (Figure S13a) and the crystal
Figure 10. DSC heating curves of M4 solution-crystallized (chloroform, room temperature) or melt-crystallized at Tc = 120−175 °C for enough time.
structure changed to the β-form (Figure S13b). These results strongly indicate that the α-form crystals that initially existed in the melt-crystallized sample melt-recrystallize to or transform directly to the β-form at Tm,α1. The Hoffman−Weeks plot39 was applied to Tm,β1 (Tc ≥ 150 °C) in order to determine T°m of the β-form. As a result (Figure S11), we obtained Tm ° = 195 °C, which was somewhat lower than expected (i.e., underestimation of Tm ° ), considering that melting was still seen around 190 °C. In this study, M4 was practically melted at 200 °C for 1−2 min to erase thermal history. Of course, higher temperatures are preferred for this purpose, but thermal decomposition cannot be neglected at temperatures >200 °C. As a result, a clear melt memory effect such as instant crystallization at temperatures just below Tm was not observed (i.e., removal of thermal history). Accordingly, Tm ° of M4 is roughly estimated to be equal to or slightly higher than 200 °C, which is an intermediate value between T°m’s estimated by the Gibbs−Thomson equation (231 °C) and Hoffman−Weeks plot (195 °C). Thus, we failed to estimate correctly Tm ° of the β-form, so that out interest moved to the magnitude relationship between T°m’s of α- and β-forms. As discussed in Section 3.2, lc and Tm of the sample solution-crystallized by chloroform (α-form) are comparable to those of the sample melt-crystallized at 175 °C (β-form). This result implies that T°m’s of α- and β-forms are comparable, if Tc dependences of Tm’s of both forms are comparable. As mentioned above, the melt-crystallized M4 shows a complicated multiple melting behavior. In this study, we focus only on the lowest melting peak for each sample (i.e., Tm,α1 and Tm,β1) and other peaks are not discussed because of lack of information. The possible mechanisms of such multiple melting are the crystal transition between α- and β-forms, lamella thickening, and melt-recrystallization. This point is currently being investigated by using time-resolved synchrotron WAXD measurements and will be reported elsewhere. 4630
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forms. The pure α-crystal was selectively formed by casting from the chloroform solution, whereas the pure β-form by the melt-crystallization at Tc ≥ 160 °C. The α- and β-forms coexisted in the samples melt-crystallized at Tc < 160 °C, and the fraction of the β-form increased with increasing Tc. The unit cell parameters of the α-form were determined on the basis of its fiber diagram: a = 0.523 nm, b = 0.913 nm, c (fiber axis) = 2.082 nm, and α = β = γ = 90° (monoclinic P21). The crystal structure of the α-form was proposed as follows: two 2/ 1 helices were located on the center and corner of the unit cell in a parallel manner. It was found from fiber diagrams and infrared spectra that M4 chains in α- and β-forms took the same chain conformation (2/1 helix) but a different chain packing (hexagonal and nonhexagonal packing, respectively). The α-crystal formed by the melt-crystallization at 120 °C changed to the β-crystal by reannealing at 170 °C. The ° ) of the β-form was equilibrium melting temperature (Tm estimated by the Hoffman−Weeks plot (195 °C) and Gibbs− Thomson equation (231 °C). The crystallization half-time of M4 showed an inverted bell-shaped dependence on Tc, and the minimum was ca. 200 s at Tc = 140−150 °C, where M4 crystallized fastest. However, there are still some open questions on the crystal polymorphism of M4. The crystal structure analysis of the βform (nonhexagonal packing) will be done, if a highly crystallized and highly oriented β-crystal film without αcrystals can be prepared. The mechanism of the α-to-β transitionmelt-recrystallization or crystal-to-crystal transitionis currently being investigated by time-resolved synchrotron WAXD measurements during the heating process. In addition, the crystallization rate of each polymorph (α- and β-forms) is currently being examined by time-resolved synchrotron WAXD measurements in the isothermal conditions. Results of these time-resolved WAXD analyses will be reported elsewhere. M4 has higher Tg (80 °C) and Tm (185 °C) as compared to poly(L-lactic acid) (Tg = 50−6040,41 and Tm = 170 °C40) with comparable molecular weight, a representative biobased plastic, so that M4 is expected to be used as high-performance biobased plastics. In such an application, an insight on the effect of crystal polymorphism on structure and properties will be of great importance and use.
3.4. Crystallization Rate. In this study, the crystallization rate was evaluated from a temporal change in FTIR peak area of the crystalline band of M4 (634 cm−1), where DSC measurements were unable to be used for the evaluation of crystallization rates because of complicated melting curves of M4 (Figure 10). The normalized peak area of the crystalline band can be treated as a normalized crystallinity (Χc′). Figure 11a shows a temporal change of Χc′ of M4 during the
Figure 11. (a) Tc dependence of time evolution of normalized crystallinity (Χc′) obtained by time-resolved FTIR measurements. (b) Crystallization half-time (t1/2) of M4 as a function of Tc.
isothermal crystallization at Tc = 120−160 °C. A typical sigmoidal dependence of Χ′c on tc is seen irrespective of Tc. As shown in Figure 11b, the crystallization half-time (t1/2) of M4 obtained from Figure 11a exhibits a typical inverted bellshaped dependence on Tc. The minimum of t1/2 is ca. 200 s at Tc = 140−150 °C. At the latest, t1/2 of M4 is ca. 10 min (Tc = 120 and 160 °C), for which the crystallization of polyesters with long diacid units (Mk and Sk with k = 10, 12) is still in the induction stage.7 As mentioned before, the α- and β-forms cannot be distinguished by FTIR, so that t1/2 of M4 obtained contains both those of α- and β-forms. There is a possibility that the crystallization rate of the α-form is different from that of the βform.
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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.8b02594. Chemical structures of isohexides; synthesis of polyester; intensity correction for WAXD data; crystallinity determination by WAXD; effect of casting solvent species on the crystal structure; effect of reannealing on the crystal structure of the cast sample (α-form); fiber diagrams of oriented-crystallized films; peak extraction from fiber diagram; molecular building of M4; crystal structure of M4 α-form; SAXS correlation function analysis; 2D SAXS of oriented-crystallized films; estimation of equilibrium melting temperature; crystalline morphology; and effect of reannealing on the crystal structure of melt-crystallized M4 (α-rich) (PDF)
4. CONCLUSIONS The hierarchical crystalline structure and polymorphism of the biobased polyester composed of isomannide and succinic acid (M4) were systematically investigated by changing the crystallization conditions such as casting solvent and crystallization temperature (Tc). It was found from 1D and 2D WAXD that M4 had the crystal polymorphism of α- and β4631
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hironori Marubayashi: 0000-0001-5922-0778 Shuichi Nojima: 0000-0003-4268-9363 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS H.M. thanks Dr. H. Sogawa (Tokyo Institute of Technology), Dr. D. Aoki (Tokyo Institute of Technology), and Dr. D. Ishii (The University of Tokyo) for giving beneficial comments on polymer synthesis; Prof. S. Asai (Tokyo Institute of Technology) for letting us use a hot press; Prof. N. Shimizu, Prof. N. Igarashi, Dr. T. Mori, and Dr. H. Takagi (KEK IMSS PF) for strongly supporting synchrotron X-ray measurements. This work has been performed under the approval of the Photon Factory Program Advisory Committee [proposal no. 2015G076 (H.M.)]. This work was supported by Tokyo Tech Research Grant for New Assistant Professor (H.M., 2013) and JSPS KAKENHI Grant-in-Aid for Young Scientists (B) 26740040 (H.M., 2014−2016).
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