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 (<i>T</i><sub>c</sub>). 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 <i>T</i><sub>c</sub> ≥ 160 °C gave the β-form. A mixture of α- and β-forms was obtained at <i>T</i><sub>c</sub> < 160 °C, and the β-form fraction increased with increasing <i>T</i><sub>c</sub>. The unit cell of the α-form was determined by the fiber diffraction method: <i>a</i> = 0.523 nm, <i>b</i> = 0.913 nm, <i>c</i> (fiber axis) = 2.082 nm, and α = β = γ = 90° (monoclinic <i>P</i>2<sub>1</sub>), 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 <i>T</i><sub>c</sub>, in which the minimum was ca. 200 s at <i>T</i><sub>c</sub> = 140–150 °C.