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two important points are extracted from these experimental data. One is about the crystallization of the different crystal forms: in the low temperature region (70–110°C), the δ form is produced, while the α form is crystallized in the high temperature region (>110°C), as already mentioned above. Another point is about the crystallization rate. As seen in Figure 6.12c, the cold‐crystallization gave the crystallization rate higher than the melt‐crystallization when compared at the same crystallization temperature. The higher crystallization rate in the cold crystallization phenomenon was ascribed to the higher content of the crystalline nuclei produced in the cooling process from the amorphous phase compared with that from the molten state [58, 59].

Schematic illustration of temperature dependence of 2D X-ray diffraction patterns of the oriented PLLA mesophase measured in the heating process.

      Source: Reproduced from Wasanasuk et al., Macromolecules 2011, 44, 9650–9660.

Schematic illustration of the temperature dependence of the crystallite size estimated for the equatorial and meridional directions of the oriented PLLA sample during the cold crystallization process starting from the meso form.

      Source: Reproduced from Wasanasuk et al., Macromolecules 2011, 44, 9650–9660.

      6.3.3 Mechanically Induced Phase Transition

Image described by caption.

      Source: Reproduced from Wasanasuk et al., Macromolecules 2011, 44, 9650–9660.

      In this way, two different types of the formation process of the α to β form seem to exist. One is a solid‐state transition via the αd and δ forms at a relatively low temperature below the melting point (100–170°C; refer to Figure 6.13). Another one is observed near the melting point. The starting α form changes to the αd form and to the δ form, then the melt of the δ form occurs in a highly superheated state, followed by the recrystallization to an extremely highly oriented β form (refer to Figure 6.14).

Schematic illustration of the 2D WAXD patterns measured at 25°C for the uniaxially oriented PLLA samples prepared by drawing the oriented alpha form at 165°C.

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