Novel high-performance materials from starch. 2. Orientation and mechanical properties of lightly cross-linked starch-ether films

Three types of reactants were used to introduce cross-links into the starch-ether polymers, specifically hexamethylene diisocyanate, a hydridosilane, and a silanol-terminated oligosiloxane. The resulting cross-linked starch-ether films in the dry state were extremely brittle (as they are in their un-cross-linked states), with glass transition temperatures between 30 and 40 degrees C. The desired chain orientations were therefore carried out in the swollen state, with an externally applied mechanical force, focusing on the diisocyanate-cured triethyl-amylose films. The solvents were removed from these materials while they were in the stretched state, either uniaxial or biaxial, and the improvements in the mechanical properties thus achieved were determined as a function of this stretch ratio. It was found that the tensile strengths, tensile moduli, and elongations at break increased monotonically and frequently dramatically with increase in orientation, as gauged by this ratio. In addition, two solvent-removal processes were investigated, drying in air and coagulation in a suitable nonsolvent. Films obtained from the coagulation processes showed smaller improvements in mechanical properties and rougher surfaces than those obtained by air-drying.