Mechanical behavior and fracture toughness of poly(L-lactic acid)-natural fiber composites modified with hyperbranched polymers

The use of hyperbranched polymers (HBP) with hydroxy functionality as modifiers for poly(L-lactic acid) (PLLA)-flax fiber composites is presented. HBP concentrations were varied from 0 to 50% v/v and the static and dynamic tensile properties were investigated along with interlaminar fracture toughness. Upon addition of HBP, the tensile modulus and dynamic storage modulus (E') both diminished, although a greater decline was noticed in the static modulus. The elongation of the composites with HBP showed a pronounced increase as large as 314% at 50% v/v HBP. The loss factor (tan delta) indicated a lowering of the glass transition temperature (T-g) due to a change in crystal morphology from large, mixed perfection spherulites to finer, smaller spherulites. The change in T-g could have also resulted from some of the HBP being miscible in the amorphous phase, which caused a plasticizing effect of the PLLA. The interlaminar fracture toughness measured as the critical strain energy release rate (G(IC)) was significantly influenced by HBP. At 10% v/v HBP, G(IC) was at least double that of the unmodified composite and a rise as great as 250% was achieved with 50% v/v. The main factor contributing to high fracture toughness in this study was better wetting of the fibers by the matrix when the HBP was present. With improved ductility of the matrix, it caused ductile tearing along the fiber-matrix interface during crack propagation.