Effect of moisture content on tensile properties of starch/poly(hydroxyester ether) composite materials

The effect of moisture content on the mechanical properties of starch/poly(hydroxyester ether) (PHEE) composite materials has been characterized. Cornstarch with either 1 or 10% moisture content was extruded with PHEE. Total moisture content (TMC) during subsequent injection molding ranged from 1.0 to 10% (total solids basis). Starch granule structure, determined by X-ray diffraction and scanning electron microscopy, was progressively disrupted but not completely destroyed (as in thermoplastic starch) as TMC during molding increased. Tensile strength and modulus values were not significantly impacted when the TMC was 6% or less. As TMC increased above 6%, both properties decreased rapidly, while strain to break increased. Tensile strength increased when samples prepared at high TMC were equilibrated to a lower value. Changes in weight average molecular weight of the PHEE during processing were not significant as measured by gel permeation chromatography, even at the highest moisture contents. The effects of TMC on mechanical properties of starch/PHEE materials are shown to be due to changes in the viscoelastic response of the PHEE matrix, by comparison of starch/PHEE results to those of neat PHEE [Polymer 42 (2001) 5643].