Mechanical properties of uniaxially cold-drawn films of poly([ R]-3-hydroxybutyrate)

Abstract

Independently of molecular weight, uniaxially oriented films of poly([ R]-3-hydroxybutyrate) (P(3HB)), with sufficient strength and flexibility were prepared by cold-drawing from an amorphous preform at a temperature below, but near to, the glass transition temperature. Melt-crystallized and solvent-cast films of P(3HB) are usually quite brittle, and the orientation is critical and difficult to reproduce consistently. Melt-quenched films with rubber state were stretched easily and reproducibly to a draw ratio more than 1000%, and, when annealed under tension, acceptable mechanical properties were generated. The tensile strength, elongation to break, and Young's modulus were 237 MPa, 112%, and 1.5 GPa, respectively. When the two-step drawing procedure was applied, the mechanical properties were further improved (287 MPa, 53%, and 1.8 GPa). In X-ray fibre diagrams of highly oriented films, shape reflections assigned to the β-form (zigzag conformation) together with those derived from the normal orthorhombic crystal system ( α-form, 2 1 helix conformation) could be observed. The improvement of mechanical properties is due to not only the orientation of molecular chains but also the generation of zigzag conformation and network structure formed by fibril and lamellar crystals (shishkebab structure). The mechanical properties of uniaxially oriented films remained almost unchanged for 4 months at room temperature, suggesting that the high orientation and crystallinity avoid secondary crystallization.