Melting and crystallization behavior of poly(trimethylene 2,6-naphthalate)

The melting and crystallization behavior of poly(trimethylene 2,6-naphthalate) (PTN) are investigated by using the conventional DSC, the temperature-modulated DSC (TMDSC), wide angle X-ray diffraction (WAXD) and polarized light microscopy. It is observed that PTN has two polymorphs (alpha- and beta-form) depending upon the crystallization temperature. The alpha-form crystals develop at the crystallization temperature below 140 degreesC while beta-form crystals develop above 160 degreesC. Both alpha- and beta-form crystals coexist in the samples crystallized isothermally at the temperature between 140 and 160 degreesC. When complex multiple melting peaks of PTN are analyzed using the conventional DSC, TMDSC and WAXD, it is found that those arise from the combined mechanism of the existence of different crystal structures, the dual lamellar population, and melting-recrystallization-remelting. The equilibrium melting temperatures of PTN alpha- and beta-form crystals determined by the Hoffman-Weeks method are 197 and 223 degreesC, respectively. When the spherulitic growth kinetics is analyzed using the Lauritzen-Hoffmann theory of secondary crystallization, the transition temperature of melt crystallization between regime II and III for the beta-form crystals is observed at 178 degreesC. Another transition is observed at 154 degreesC, where the crystal transformation from alpha- to beta-form occurs. (C) 2003 Elsevier Science Ltd. All rights reserved.