Reinforcement of Microbial Thermoplastics by Grafting to Polystyrene with Propargyl-Terminated Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)

Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) is one of the biomass plastics synthesized by intracellular fermentation of plant oils and saccharides, which has good flexibility and mechanical toughness. However, its industrial applications have been limited because of its poor elastic modulus, low mechanical strength, and high cost to produce. To expand the application of PHBH, this study reported the preparation of the graft copolymer consisting of PHBH and polystyrene (PSt) which is a rigid, transparent, and low-cost polymer. PSt-g-PHBH was synthesized from propargyl-terminated PHBH (Propargyl-PHBH) and azide-modified PSt via copper-catalyzed azide–alkyne cycloaddition reaction. PHBH/PSt blend films showed heterogeneous, opaque, and brittle. In contrast, PSt-g-PHBH films exhibited surprisingly good homogeneity, transparency, and high mechanical properties (Young's modulus ∼ 1.24 GPa and tensile strength ∼ 30 MPa). Furthermore, thermal analysis and theoretical discussion based on the Pukánszky model explained that the compatibility and stress transfer capacity of PHBH/PSt blend films were enhanced by grafting to PSt backbones with Propargyl-PHBH. Hence, the physical properties of PHBH such as optical and mechanical properties are improved by grafting to PSt. The present study can contribute to expanding the application of PHBH to various fields such as engineering materials, commodity plastics, and food packaging.