Differences in hierarchical structural changes between unoriented P(3HB) and P(3HB- <i>co</i> -3HH) under stretching

Poly[(<i>R</i>)-3-hy-droxy-butyrate] [P(3HB)] is a promising environmentally friendly polymer. However, P(3HB) exhibits brittle fracture, and the brittleness makes P(3HB) unsuitable for practical application. To improve the brittleness, 3-hy-droxy-hexano-ate (3HH) is introduced as a second monomer into P(3HB). The random copolymer P(3HB-<i>co</i>-3HH) shows a reduction in brittleness. In this paper, we investigate the changes in the hierarchical structures in unoriented P(3HB) and P(3HB-<i>co</i>-11mol%-3HH) during uniaxial stretching using <i>in situ</i> ultra-small-angle, small-angle and wide-angle X-ray scattering to clarify the effects of the introduction of 3HH. We reveal that the introduction of amorphous component units to P(3HB) not only suppresses crystallization and reduces crystal thickness on the nanometre scale but also destabilizes the crystal structure. We observed stretch-induced density fluctuations near the yield point by <i>in situ</i>X-ray scattering, showing heterogeneous deformation on scales from slightly larger than the lamellae to submicrometres in P(3HB) and on the submicrometre scale in its copolymer under uniaxial stretching. The enhancement of density fluctuations in the copolymer was smaller than that in P(3HB) because stress dispersion was caused by the hierarchical structure formed with the suppression of crystallization.