Nucleating effect of expanded graphite nanoplatelets on poly(hydroxybutyrate)

Poly(hydroxyalkanoates) (PHAs) are thermoplastic, biodegradable biopolyesters typically synthesized by various bacteria and microorganisms that use them as reserves of carbon and energy. 1-3 One of the most investigated PHAs is poly(hydroxybutyrate) (PHB) which, besides being biodegradable and biocompatible, is attractive due to its availability, processibility, and mechanical and barrier properties which are comparable to isotactic polypropylene and other synthetic polymers. PHB is a highly crystalline thermoplastic that can be extruded, injection molded, and spun without modifying traditional polymer processing equipment. It also has a low elongation at break (less than 10 %), an impact strength of 3 J/mm, a modulus of 1.7 GPa and a fracture stress of 35 MPa. Despite having these characteristics, PHB is not widely applied in the manufacturing industry because of its inherent brittleness and narrow processibility window. The brittleness of PHB is known to stem from three factors: a glass transition temperature close to room temperature, secondary crystallization occurring upon storage at room temperature, and an extremely low nucleation density, which all derive from the high purity of PHB and its stereochemical regularity. There is an ongoing research thrust to discover efficient nucleating agents for PHB and its copolymers. Efficient nucleating agents would increase the polymer’s crystallization temperature, and the crystallization rates, and generate smaller and more numerous spherulites, leading to materials with increased mechanical properties. In addition microand nanosized inorganic particles, like clay and carbon nanotubes, may have the added advantage of acting as reinforcements for the polymer.