Comparison of the mechanical properties of cellulose and starch films.

Some effects of water at levels up to 25% (dry solids basis db) on regenerated cellulose films with a thickness of 100 mum were investigated by dynamic vapor sorption (DVS), X-ray diffraction, tensile testing, and proton-nuclear magnetic resonance (NMR). The sorption isotherm fitted by the D'Arcy and Watt model and the increase in NMR T(2) with water content suggest that a mobile water fraction appears at water contents above 10%db. Water absorption increased the crystallinity of cellulose films from 31% (dry) to 38% (25%db) and altered the dimensions of crystallites. Mechanical measurements on planar and notched samples at all the water contents used here showed ductile fracture behavior. Although the properties of water in these cellulose films are comparable to previously reported data on starch, cellulose films at low water content are much less brittle than starch. The reasons for this difference are explored.