1. Field of the Invention
While cellulose-based films have achieved prominence as semipermeable membranes, starch-based films have not been available that would withstand prolonged exposure to water. Starch and cellulose are high polymers composed of D-glucose units. Their molecules differ only in weight and in the manner in which the glucose units are joined together. Cellulose is a linear polysaccharide consisting of 6,000 to 8,000 1,4-linked .beta.-D-glucose units. Because of this 1,4-.beta.-linkage, these chain molecules can align themselves alongside each other to form linear crystals or microfibrils. These structural properties contribute to strong hydrogen bonding, film-forming capabilities, and high resistance to gelatinization in water. In contrast, most common starches contain 17-27% linear polysaccharide (amylose) consisting of 400 to 1,000 1,4-linked .alpha.-D-glucose units and the remaining composition is a branched molecule (amylopectin) having 10,000 to 40,000 1,4- and 1,6-linked .alpha.-D-glucose linkage. Because of the 1,4-.alpha.-linkage, the amylose molecules assume a spiral or helical shape having six glucose units per spiral. Starch readily disperses in hot water to form starch-pastes possessing unique viscosity characteristics and film-forming behavior. However, such films are very brittle upon drying and are very sensitive to water. This invention relates to certain starch-based formulations which yield durable semipermeable films having potential applications as dialyzing membranes.
2. Description of the Prior Art
In U.S. Pat. No. 4,337,181, Otey et al. teach the preparation of flexible, starch-based films which are water resistant, yet biodegradable. The films are formed by shaping a composite of a water-dispersible ethylene acrylic acid (EAA) copolymer and a starchy material by the methods of extrusion blowing, simple extrusion, or molding. The resultant products have disclosed utilities as agricultural mulch, as well as disposable packaging and bagging materials. In this procedure, gelatinized starch and EAA are combined in a plasticized matrix, and then the acidic portion of the EAA is at least partially neutralized with ammonia or an amine. It is by virtue of the neutralization and adjustment of the moisture content that the matrix can be extrusion blown. Films prepared by this process have no observed permeability properties.