Fracture behavior and optical properties of melt compounded semi-transparent polycarbonate (PC)/alumina nanocomposites

Two different molecular weight (MW) poly(styrenemaleic anhydride) (SMA) copolymers were employed to pretreat and coat spherical alumina (Al ₂O₃) nanoparticles to facilitate dispersion in a polycarbonate (PC) matrix. Melt compounding was done using a high intensity thermokinetic mixer (K-mixer). The low MW SMA polymer coating produced better dispersion of nanoparticles in the PC nanocomposites, resulting in fairly high light transmittance even through 2 mm thick specimens. The addition of 1 wt% well-dispersed nanoparticles improved the impact strength during brittle fracture of the PC/alumina nanocomposites through the formation of multi-level microcrazes induced by the nanoparticles. However, further increasing the alumina nanoparticle content altered the energy dissipation behavior, resulting in less effective reinforcement. Various fracture mechanisms affected by the content and processing of two alumina nanoparticles are presented together with the effect of thermal treatment on the PC/alumina nanocomposites. This paper also discusses why the preconditioned nanocomposites, which are typically ductile like neat PC, could occasionally exhibit a brittle fracture behavior.