Structural characteristics of biodegradable thermoplastic starch/poly(ethylene–vinyl alcohol) blends

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Abstract
To establish relationships among the blend composition, processing history, and the resultant properties of starch‐based thermoplastics, three varieties of corn starch: (1) Waxy Maize, (2) Native Corn, and (3) high‐amylose Hylon VII were extrusion‐blended with poly(ethylene‐vinyl alcohol) (EVOH) containing 56 mol % VOH. Wide‐angle X‐ray scattering (WAXS), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were used to examine the structural characteristics of the blends. All starches were destructurized upon compounding and a fine dispersion was achieved with EVOH. The Native Corn and Hylon VII blends were phase‐separated and exhibited some miscibility between the polymer components as evident in EVOH melting‐point depression, smaller domain sizes, lower contrast between phases in TEM, and increased resistance to moisture and enzyme‐etching treatments. Starches containing amylose exhibited complexation and crystallinity in the starch fraction, although most of the crystallinity in the blends was due to the EVOH component. Waxy Maize blends were well phase‐separated with larger domain sizes and underwent phase coarsening as a function of time in the melt. When subjecting the blends to capillary flow, orientation of both starch‐rich and EVOH‐rich domains was observed at various compositions, with the EVOH component undergoing significantly more orientation relative to starch as evident by the presence of EVOH‐rich fibrils. Finally, EVOH was found to coat the surfaces of filaments produced from the blends even at rather high levels of starch (70%), which is expected to improve moisture sensitivity and slow down the initial rate of biodegradation. © 1995 John Wiley & Sons, Inc.