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Composites based on natural polymers alone are extremely sensitive to moisture and their mechanical properties deteriorate upon the absorption of water, limiting their usefulness in practical applications. Ongoing research cooperation between USDA and the University of Pisa, Italy, has yielded several composites based on poly(vinyl alcohol) (PVA) and corn fibers (CF). In this study, variable amounts of CF and PVA were processed in the presence of both dry and liquid plasticizers, glycerol and pentaerythritol. Cornstarch was introduced in the formulation to reduce the cost and to further increase the composition of natural components in the composites. Composites made with as low as 30% PVA were injection molded into tensile bars and evaluated. The addition of starch moderately reduced the tensile properties of the composites, lowering the elongation (∼600% to 400%) and increasing Young's modulus (∼36 MPa to ∼100 MPa) while the ultimate tensile strength remained constant at about 8 MPa. Composites prepared from CF and PVA showed little change in their mechanical properties even after conditioning them at various relative humidities, or after soaking in water. Composites tested after storage for one year, at 50% relative humidity and 23°C, exhibited mechanical properties similar to those of freshly prepared composites. [ABSTRACT FROM AUTHOR]

The relationship between the structure, microtexture and mechanical properties of a high-performance polyborazine-based BN fibre has been studied in detail. Techniques such as Raman and IRTF spectroscopies, X-ray diffraction, scanning and transmission electron microscopies and microtraction tests were used. It has been shown that the longitudinal mechanical properties of the fibre directly depend on the microtextural and structural properties. In particular, the Young''s modulus in the fibre-axis (E=440 GPa) is directly controlled by the fibre structure: the fibre presents a high degree of alignment of the (002) planes compared with the fibre axis (τ=0.90) and the high value of the elastic modulus in the fibre-axis is attributed to the strong covalent bonds in the basal planes. On the other hand, the ultimate tensile strength (σR=2000 MPa) and the low density (d=1.85 g cm−3) of the fibre are limited by the external defects and the porosity in the volume of the fibre. [ABSTRACT FROM AUTHOR]