Tensile yield properties of starch-filled poly(ester amide) materials

Composite materials were prepared with granular corn starch (CS) or potato starch (PS) and poly(ester amide) resin (PEA), with starch volume fractions (ϕ) up to 0.40. Tensile yield properties were evaluated at strain rates of 0.0017-0.05 s−1. Yield stress of the CS-PEA materials increased with strain rate and starch content. The strain rate effect became more pronounced as the starch content increased. A crossover effect was observed with PS-PEA materials: at low strain rates, the yield stress decreased with increasing ϕ, and increased with ϕ at higher strain rates. This crossover suggests that the time scale of debonding in the PS-PEA materials is comparable to the time scale of the tension test. The addition of either CS or PS to PEA induced a distinct maximum in the stress-strain curve at yield compared to the neat PEA. Debonding of starch granules from the PEA matrix occurred at lower stresses in the PS-PEA materials than the CS-PEA. In PS-PEA, debonding occurred in bands similar in appearance to shear bands throughout the tensile specimen. After yielding, the cross-section area decreased as the debonded zones coalesced. In the CS-PEA materials, debonding zones were more diffuse, and a distinct neck formed at yield. Yield stress data for the CS-PEA materials could be shifted with respect to strain rate to construct a master curve, indicating that yield properties at these strain rates were determined by the matrix response rather than debonding as observed in other starch-filled materials.