Mechanism of Elastic Properties of Biodegradable Poly[(R)-3-Hydroxybutyrate-co-4-hydroxybutyrate] Films Revealed by Synchrotron Radiation.

Reversible elastic films of biobased and biodegradable poly[(R)-3-hydroxybutyrate-co-4-hydroxybutyrate] [P(3HB-co-4HB)] were prepared by uniaxial drawing procedures. Mechanical properties and highly ordered film structures were investigated by tensile testing and both static-state and in situ wide-angle X-ray diffraction and small-angle X-ray scattering with synchrotron radiation during stretching and relaxing. Despite the crystalline nature of the polymers, the elongation at break of these films was greater than 1500%. Reversible elasticity was achieved after the first 10 times of uniaxial stretching. X-ray measurement results indicated that on stretching, β-form molecular chains with a planar zigzag conformation were introduced from molecular chains with random coils in the amorphous regions between α-form lamellar crystals. Notably, the orientation of the α-form lamellar crystals increased after relaxation of the molecular chains with a planar zigzag conformation (β-form) between the lamellar crystals (α-form). Reversible elastic properties were regenerated by a planar zigzag conformation between the lamellar crystals, the extension of molecular chains in lamellar crystals by the rotation of molecular conformation, and changes in the degree of orientation of the lamellar crystals.