Tensile study of melt-spun poly(3-hydroxybutyrate) P3HB fibers: Reversible transformation of a highly oriented phase

Abstract

Poly-3-hydroxybutyrate (P3HB) typically crystallizes into the orthorhombic α-form. The additional intense and rather broad equatorial reflection in wide-angle X-ray diffraction (WAXD) patterns seen in our melt-spun P3HB fibers cannot be attributed to the α-form crystals. We propose that a non-crystalline mesophase, which consists of disordered but highly oriented and stretched molecules located between α-crystals, leads to the observed broad equatorial reflection. We show that the transformation of this mesophase from and into the α-form phase is partially reversible under cyclic tensile loading. Structure factor calculations, which are based on atomic positions from molecular dynamics simulations of a set of stretched P3HB molecules, support this model. The WAXD patterns were analyzed with azimuthal, radial, equatorial and meridional scans and the changes in the crystal orientation, changes in the percentages of individual phases and changes in the α-crystal lattice spacings were analyzed as a function of the applied tension. Changes in long spacings, crystal sizes and coherence lengths under cyclic loading were determined with small-angle X-ray scattering (SAXS) measurements. The long spacing between α-crystals increases when tensile stress is applied and it snaps back to the original spacing when the tensile stress is released.