Читать книгу Poly(lactic acid) - Группа авторов - Страница 134

The crystallization phenomenon caused by heating the amorphous sample is known as cold‐crystallization in contrast to the melt‐crystallization that occurs during the cooling process from the molten state. The temperature‐dependent 2D‐WAXD patterns were measured for the uniaxially oriented mesophase sample in the cold‐crystallization process, as shown in Figure 6.10 [5]. The mesophase started to transform to the δ form at around T _g. The δ form grew gradually during heating to 120°C. Once the sample was heated above 120°C, the δ form transformed to the α form. By further heating, the α form did not directly transform to the molten state, but it first changed to the mesophase once again and then to the melt. The growth of the crystallite size was estimated by analyzing the change of the half‐width of the X‐ray diffraction spots using Scherrer’s equation [52]. Figure 6.11 shows the results obtained for the equatorial (200/110 peak) and meridional (0010 peak) directions. The starting mesophase has a quite small crystal size of ca. 30 Å (c‐axis) × 20 Å (ab‐plane). It changes gradually to the δ form with a larger size of ca. 75 Å (c‐axis) × 100 Å (ab‐plane). The α form developed from δ form has a larger crystal size of ca. 175 Å (c‐axis) × 300 Å (ab‐plane).