Cocrystallization behavior of poly(butylene terephthalate-co-butylene 2,6-naphthalate) random copolymers

A series of poly(butylene terephthalate-co-butylene 2,6-naphthalate) (P(BT- co-BN)) random copolymers were synthesized, and their cocrystallization beh avior was investigated by differential scanning calorimetry (DSC) and wide- angle X-ray diffraction (WAXD). The P(BT-co-BN) copolymers, which are found to be statistical random copolymers, show a single melting and a single cr ystallization peak over the entire range of copolymer composition and have a eutectic melting point in the plot of melting point versus composition. W AXD patterns of copolymers are divided into two groups, i.e., the PET type crystal and the PEN type crystal. These facts indicate that the P(BT-co-BN) copolymers show isodimorphic cocrystallization. The eutectic composition a t which the crystal transition between PET and PEN type crystal occurs was estimated ca. 35 mol % BN content. When the defect free energy was calculat ed by using the equilibrium inclusion model, the defect free energy (6.10 k J/mol) in the case of incorporation of BN units in the PET type crystal was higher than the opposite case (3.63 kJ/mol). It means that the incorporati on of bulkier BN units in the PET type crystal is more sterically hindered than the opposite case. The changes of heat of fusion and d spacings with c omposition also support isodimorphic crystallization.