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the melt‐quenched sample by four to five times the original length near the glass transition temperature (T g ~ 60°C) [5]. The oriented δ form is obtained by annealing the as‐drawn mesophase in the temperature range of 70–120°C. Annealing of the δ form at a higher temperature of 120–170°C induces a phase transition to the α form [5, 7, 9, 14]. The oriented β form cannot be obtained easily by the usual heat treatment of other such crystal modifications as the δ and α forms. To prepare the highly oriented pure β form, a high shear or tensile stress has to be applied to the oriented α form at a temperature higher than 120°C [17–20,42–45]. The γ form is obtained as a single crystal by casting from hexamethylbenzene solution [21, 22]. A PLLA–CO2 complex is prepared by treating the PLLA sample with supercritical fluid CO2. The desorption of CO2 under vacuum at room temperature gives the empty α″ form. PLLA forms a crystalline complex with organic solvents like cyclopentanone (CPO) and N,N‐dimethylformamide (DMF), which is known as the ε form [25]. This complex is stable only at a low temperature; it transforms spontaneously to the α (or δ) form by leaving the sample at room temperature.

      PLLA and PDLA are enantiomers with the same chemical formula but with the opposite configuration around the asymmetric carbon atoms or the opposite optical activity. The blend sample of PLLA and PDLA at 1 : 1 molar ratio was found to form the so‐called stereocomplex (SC) [27]. However, the SC sample can be obtained in a wider range of PLLA/PDLA ratio of 7/3–3/7 [37, 46, 47].

Schematic illustration of sample preparations and phase transitions of the various crystal modifications of PLLA. Schematic illustration of 2D-WAXD patterns of PLLA (a) mesophase, (b) delta form, (c) alpha form, and (d) beta form.

      Source: (a)–(c): Reproduced from Wasanasuk et al., Macromolecules 2011, 44, 9650–9660; (d): Wang et al., Macromolecules 2017, 50, 3285–3300.

      6.2.2 Crystal Structure of the α Form

      The crystal structure of PLLA α form proposed at first was of the orthorhombic unit cell of a = 10.7 Å, b = 6.45 Å, c (chain axis) = 27.8 Å [15], which contains the two chains of 10/3 helical conformation [13, 48], where 10/3 indicates that the 10 monomeric units are contained and the 3 turns in the repeating period. The internal rotation angles of the skeletal chain are approximately expressed as the repetition of TTG where T and G are trans and gauche bonds, respectively. The crystal structure was refined by Sasaki et al. by assuming the space group symmetry P212121 [13]. However, several unsolved problems are still relevant in the abovementioned crystal structure analysis of the α form. For example, if the structure of the P212121 space group is assumed correct, the molecular chain must be deformed more or less from the regular and uniform 10/3 helical conformation, since the unit cell possesses only the 21 screw symmetries. If the 21 screw axis passes through the center of the molecular chain along the c axis, the five monomeric units should form one crystallographically asymmetric unit. The total number of the atomic coordinates to be determined is remarkably large, making the structure analysis greatly difficult. A more serious problem is the usage of the space group P212121 itself.

      The observed X‐ray diffraction pattern shows a series of 00l reflections containing the odd l values (003, 007 etc.) in addition to the even 00l peaks. This is not consistent with the extinction rule (00l with even l values) required for the space group P212121.

      In the X‐ray structure analysis of the α form, the total number of the adjustable parameters of one asymmetric unit (i.e., the coordinates and thermal factors of the atoms in one asymmetric unit) is 101 for C and O atoms of isotropic thermal factors, and 306 for C, H, and O atoms of anisotropic thermal factors. So, for the determination of the accurate crystal structure of the α form, we need to collect the X‐ray diffraction spots of about two to three times larger than the number of the parameters, i.e., 600–900 spots. The usage of an X‐ray beam of a shorter wavelength is useful for this purpose. For example, the X‐ray beam of 0.328 Å wavelength, generated from the synchrotron radiation facility, was incident to the ultra‐drawn PLLA α form sample [14]. From the collected 2D‐WAXD pattern, 700 independent diffraction spots were recognized, which are high enough when compared with the above‐mentioned number of the adjustable parameters. The quantitative analysis was performed manually by reading the positions and integrated intensities of all the observed diffraction peaks. By taking the above‐mentioned problems into consideration, the space group was reduced to the P1211 of the monoclinic system, which is lower than the space group P212121. The following unit cell parameters were obtained.

StartLayout 1st Row a equals 10.683 modifying above upper A with ring comma b equals 6.170 modifying above upper A with ring comma 2nd Row c left-parenthesis c h a i n a x i s right-parenthesis equals 28.860 modifying above upper A with ring and gamma equals 90 degree EndLayout Schematic illustration of crystal structure of PLLA alpha form.

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