Melting point depression and cocrystallization behavior of poly(ethylene‐co‐butylene 2,6‐naphthalate) copolymers

The melting point depression and cocrystallization behavior of a series of random poly(ethylene‐ co ‐butylene 2,6‐naphthalate) P(EN‐ co ‐BN) copolymers were investigated using differential scanning calorimetry (DSC) and wide‐angle X‐ray diffraction (WAXD). These copolymers crystallize over the entire range of composition and they exhibit eutectic behavior. WAXD patterns showed that copolymers form exclusively poly(ethylene 2,6‐naphthalate) (PEN) type crystals or poly(butylene 2,6‐naphthalate) (PBN) type crystals. A transition from the PEN‐ to PBN‐type crystal was observed in the middle of the copolymer composition. This confirms that the P(EN‐ co ‐BN) copolymers show isodimorphic cocrystallization. The cocrystallization was thermodynamically analyzed using the Wendling–Suter model. The defect free energy was found to be about 12.17 kJ mol −1 in the case of incorporation of BN units in the PEN‐type crystals, and about 5.66 kJ mol −1 in the case of incorporation of EN units in the PBN‐type crystal. This means that the incorporation of bulkier BN units in the PEN crystals is more difficult. For low comonomer concentrations it was found that comonomer exclusion is probable.