Mechanical properties and highly ordered structure of ultra-high-molecular-weight poly[( R)-3-hydroxybutyrate] films: Effects of annealing and two-step drawing

Abstract

Uniaxially oriented films with high mechanical properties were processed from ultra-high-molecular-weight poly[( R)-3-hydroxybutyrate] (P(3HB)) by two-step drawing procedure. When the annealing procedure was applied to one-step hot-drawn films at a temperature (100 °C) showing the maximum growth rate of spherulites, the tensile strength reached up to 277 MPa with 84% elongation to break. The increase of mechanical properties is due to the orientation of molecular chains and the finger-joint structure of lamellar crystals in shishkebab structure. To further increase the mechanical properties, two-step drawing was performed at room temperature against hot-drawn films before annealing. Tensile strength of two-step drawn films increased with an increase in the two-step draw ratio at room temperature. X-ray fibre diagrams showed a new reflection on the equatorial line which indicates the planar zigzag conformation of P(3HB). The tensile strength of the two-step drawn film reached nearly 400 MPa, indicating that two-step drawing procedure is quite useful to obtain high strength films of P(3HB) homopolymer. These mechanical properties remained unchanged for 2 months, suggesting that the development of planar zigzag conformation and the highly ordered structure with finger-joint structure of lamellar crystals avoid the deterioration by secondary crystallization.