The present invention involves the synthesis of a series of amylose (starch)
derivatives with various degrees of substitution and amylose/amylopectin ratios.
These chains are chemically crosslinked and then mechanically deformed (stretched)
to produce a biodegradable and mechanically superior material. Specifically, the
process consists of chemically modifying starch into starch derivatives such as
starch ethers, starch esters and starch carbamates. The polymers have a percentage
degree of substitution of from about 35% to about 95% (degree of substitution is
from about 1.05 to about 2.85) and preferably have a percentage degree of substitution
of from about 65% to about 90% (degree of substitution is from about 1.95 to about
2.70). The starch derivatives are crosslinked to obtain crosslinked chains and
processed into sheets, films, fibers, threads or other articles as known in the
art. After processing, the articles are swollen in a thermodynamically acceptable
solvent or solvent mixture to a desired volume and deformed in a uniaxial or biaxial
extension. The polymers materials are preferably stretched from about 1% to about
500% in the direction of stretching. Finally, the solvent is removed, yielding
a homogeneous, highly-ordered material. The present invention improves the properties
and the quality of sheets, films, fibers, threads or other articles with respect,
for example, to mechanical strength. The materials are developed from starch, a
natural renewable source which has low cost, high production levels and which replaces
petroleum-based, synthetic polymers; the materials acquire high-strength, high-modulus,
toughness and flexibility; and the materials exhibit structural and functional
stability during processing, storage and use, yet are susceptible to biodegradation
upon disposal.
