Polymorphism and β-form structure of poly(octamethylene 2,6-naphthalate)

It was revealed that poly(octamethylene 2,6-naphthalate) (PON) existed in two different crystal structures, α- and β-form, depending on crystallization process: The α-form crystal was dominantly developed from the cold-crystallization, whereas the β-form was from the melt-crystallization. The apparent melting temperatures of α- and β-form crystals were characterized to be 175 and 183 °C, respectively. On the basis of X-ray diffraction and molecular modeling studies, the crystal structure of β-form, developed dominantly from the melt-crystallization, was identified to be triclinic with dimensions of a = 0.601 nm, b = 1.069 nm, c = 2.068 nm, α = 155.68°, β = 123.25°, γ = 52.85°, and with the space group of P1¯. The calculated crystal density was 1.243 g/cm3, supporting that one repeating unit of PON exists in a unit cell. The octamethylene units in the PON backbone take largely all-trans conformation in the β-form unit cell.