Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) nanocomposites with optimal mechanical properties

With the ultimate goal to design renewable polymer nanocomposites with optimal mechanical properties, this study reports an investigation of structure-property relationships for a model system - silica/poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHx) nanocomposites. Two molecular weights of PHBHx (Mw = 903,000 g/mol and Mw = 633,000 g/mol) and two types of silica nanoparticles (nominally spheres and fibers according to the manufacturer) were used to prepare the nanocomposites. Small-angle X-ray scattering shows that the sphere and fiber nanoparticles had similar surface areas and primary particle size, but differed in degree of aggregation of the primary particles. The thermal stability of the PHBHx matrix was slightly improved by the addition of nanofillers. Simultaneous improvement of both stiffness and toughness was observed at 1-wt% loading for the higher molecular weight matrix. The more highly aggregated SiO2 fibers had a greater toughening effect than the SiO2 spheres. Compared to the unfilled polymer matrix, a 30% increase in Young's modulus and 34% increase in toughness were obtained for the 1-wt% SiO2 fiber/PHBHx072 nanocomposite. The addition of SiO2 spheres to PHBHx072 resulted in the same increase in Young's modulus (30%) but a smaller increase (11%) in toughness. The dramatic increases in modulus for PHBHx072 cannot be explained on the basis of two-component micromechanical models. Apparently the filler alters the character of the semicrystalline matrix. When the loading was 3 wt% and above, Young's modulus continued to increase, but the strain at break and toughness decreased. The ultimate strength did not change compared with the unfilled polymer. In order to understand the mechanical properties observed, the thermal behavior, spherulitic morphology and the deformation mechanisms of the nanocomposites and the dispersion state of the nanofillers were studied. We found that a high molecular weight of the polymer matrix, weak interfacial adhesion and a good dispersion of the nanofillers are necessary to improve toughness and stiffness simultaneously.