Aliphatic polyester copolymer

A stretchable polyester, consisting of:

• an aliphatic copolymer polyester consisting of, in polymerized form, a 3-hydroxybutyrate unit and a 4-hydroxybutyrate unit,
• wherein the stretchable polyester is produced by crystallizing aliphatic copolymer polyester;
• stretching the crystallized aliphatic copolymer polyester by subjecting the crystallized aliphatic copolymer polyester to an orientation treatment;
• and removing a tensile load which was added by the orientation and stretching of the crystallized aliphatic copolymer polyester,
• wherein the stretchable polyester has an α-form and an amorphous structure,
• wherein the α-form has a degree of orientation of 50% or greater as determined by X-ray,
• wherein the stretchable polyester, in a stretched state, has a β-form detectable by wide-angle X-ray diffraction and small-angle X-ray scattering,
• wherein the stretchable polyester, in an unloaded state, has the β-form reduced compared to the stretched state, or not detectable, by wide-angle X-ray diffraction and small-angle X-ray scattering,
• wherein the stretchable polyester has an elasticity with a tensile elongation recovery rate in a range of from 20 to 100%, and
• wherein the 4-hydroxybutyrate unit is present in the aliphatic copolymer polyester in a range of from 10 to 30 mol %, with respect to all monomer units.