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To elucidate the detailed crystal structures and phase transition behaviors of PLLA, X‐ray diffraction method of highly oriented and highly crystalline samples is most useful. Figure 6.1 shows that the various crystal modifications are obtained depending on the sample preparation conditions. When the sample in the molten state is quenched into ice water, predominantly amorphous‐phase PLA is obtained [4, 5]. Casting PLA films using chloroform solution at room temperature also produces amorphous phase. The oriented PLLA sample of the mesophase is prepared by stretching 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].

Figure 6.2 shows the typical WAXD patterns of the uniaxially oriented PLA samples of the mesophase, δ, α, and β forms [5, 9, 14, 20], where an incident X‐ray beam is a graphite‐monochromatized Mo‐Kα line (wavelength λ = 0.711 Å). Compared with the X‐ray diffraction patterns observed for the general crystalline polymer samples, the α crystal form shows the anomalously beautiful X‐ray diffraction pattern with many sharp spots, reflecting the well‐developed crystal domains with highly regular chain packing mode. The diffraction pattern of the δ form is similar to that of the α form but diffuse as a whole, and the several characteristic diffraction peaks of the α form are lack, indicating that the δ form is not simply a disordered α form, but it is a crystalline form independent of the α form. The mesophase shows the further poor and diffuse diffraction pattern with the similar characteristic structural feature to those of the α and δ forms. The β form shows the remarkably different diffraction pattern from those of the abovementioned α and δ forms and meso phase. The streaks are more remarkable in the β form.

FIGURE 6.1 Sample preparations and phase transitions of the various crystal modifications of PLLA.

FIGURE 6.2 2D‐WAXD patterns of PLLA (a) mesophase, (b) δ form, (c) α form, and (d) β form.

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