Crystal Polymorphism of Biobased Polyester Composed of Isomannide and Succinic Acid

The crystallization and solid-state structure of the biobased polyester composed of isomannide and succinic acid (M4) were systematically investigated by changing the crystallization conditions such as casting solvent and crystallization temperature (Tc). It was found from wide-angle X-ray diffraction that M4 had the crystal polymorphism: casting from the chloroform solution gave the α-form, whereas thermal annealing at Tc ≥ 160 °C gave the β-form. A mixture of α- and β-forms was obtained at Tc < 160 °C, and the β-form fraction increased with increasing Tc. The unit cell of the α-form was determined by the fiber diffraction method: a = 0.523 nm, b = 0.913 nm, c (fiber axis) = 2.082 nm, and α = β = γ = 90° (monoclinic P21), where two 2/1 helices were packed in the unit cell in a parallel manner. M4 chains in the β-form took the same chain conformation but a different chain packing. The crystallization half-time of M4 drew an inverted bell-shaped curve against Tc, in which the minimum was ca. 200 s at Tc = 140–150 °C.