Temperature Dependence of Lamellar Thickness in Isothermally Crystallized Poly(butylene terephthalate)

In order to clarify the crystallization from the melt in poly(butylene terephthalate) (PBT), the heating processes of isothermally crystallized PBT have been investigated by differential scanning calorimetry (DSC), wide-angle X-ray diffraction, and small-angle X-ray scattering. Both the melting temperature and the crystallization temperature dependences of the lamellar thickness show two distinct regions separated by 208 degrees C, which indicate two equilibrium transition temperatures, 270 and 335 degrees C. On the basis of the proposed crystallization model, it is interpreted that the crystallization proceeds through the mesophase below 208 degrees C and directly from the melt above 208 degrees C. The experimental results also show the constant excess length of lamella over the experimental temperature range.