Anomalous Mechanical Response of Stretched Film of Poly(3-Hydroxybutyrate-co-3-Hydroxyhexanoate)

The mechanical responses during loading, unloading, and reloading cyclic tensile tests of a tubular blown film of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) are studied. Although the stress–strain curve recorded during the initial stretching process is typical for a crystalline polymer, the stretched film behaves like a rubber during the reloading process; that is, low modulus with a small residual strain after unloading. Furthermore, the stress–strain curves during the reloading process are an inverted “S” shape. During the first stretching process of the polymer film, small crystals are destroyed without reorganization into a crystalline structure, leading to the observed decrease of crystallinity. In contrast, well-developed crystals that orient to the machine direction of the film do not disappear during the first stretching and act as crosslink points during reloading. As a result, a rubber-like response is detected. This mechanical response during reloading is considerably different from those of conventional crystalline plastics such as polyethylene and polypropylene.