Summary: Polyhydroxybutyrate (PHB) is an ideal bioplastic, however, this polymer undergoes a severe embrittlement process because of its spherulitic structure, rendering the material brittle. Using a series of in-situ rheo techniques, we have previously observed only the rapid initial stage of shish formation, we term a partial shish, which existed at high shears in medium-molecular-weight PHB, = 360 000. The shish kebab morphology is anticipated to remove or severely lessen this embrittlement process whilst providing new properties and applications. For medium and ultra high-molecular-weight (MMWT, = 360 000/UHMWT, = 5 × 106) PHB 99/1 and 99.5/0.5 blends only a partial shish is identified. However, the initial shish formation stage and subsequent stages were observed at 98/2 and 97/3 blend ratios resulting in a complete shish, we term the full shish, and fiber formation was evident. We believe this fiber morphology achieved by high molecular weights is crucial to sustaining the shish kebab structure for an excessive period.
Left: In-situ rheo-light scattering micrograph; 97/3 MMWT/UHMWT PHB at 100 s−1 for 1 s shear shish held at 75 s. Right: In situ rheo-optical micrograph; PHB fiber morphology observed at 50 s−1 for 2 s shear 98/2 MMWT/UHMWT PHB after 1 min.