Crystal structure identification of poly(trimethylene 2,6-naphthalate) β-form crystal by X-ray diffraction and molecular modeling

X-ray powder diffraction and molecular modeling are used to identify the crystal structure and chain conformation of poly(trimethylene 2,6-naphthalate) (PTN) β-form crystal. The unit cell of PTN β-form crystal was determined to be a triclinic with dimensions of a=0.4665 nm, b=0.7014 nm, c=2.2177 nm, α=100.85°, β=88.78° and γ=120.63°, and the space group of the crystal is identified as P1̄. The observed crystal density of 1.37 g cm−3 and the determined dimensions of unit cell indicate that the unit cell contains one polymer chain with two repeating units. In the unit cell, each trimethylene unit in PTN backbone is in gauche/gauche conformation and neighboring naphthalene units are in face-to-face type arrangement, forming π-stacks that lead to the lowest energy of the unit cell.