Drying-Induced Surface Wrinkles Generated on Chitosan Films Having Polyion Complex Skin Layers: Effects of Physical Properties of Skin Layers and Substrates on Surface Wrinkling upon Drying

Abstract

Abstract The effects of the physical properties of skin layers and substrates on drying-induced surface wrinkling are investigated with a system using chitosan films having polyion complex (PIC) skin layers. The PIC layers are fabricated with diverse anionic polymers having similar molecular weights on a chitosan (CS) film surface, and the films are dried. In all cases, surface wrinkles form but their sizes vary widely depending on the molecular structure. In particular, wrinkles formed with ligninsulfonate are much larger than those formed with the others. The skin layers differ significantly in surface hardness but not in thickness, indicating that wrinkle sizes induced by PIC skin layers are predominately determined by the hardness of the layers or by physical properties affected by hardness, such as shrinkage volume. The effects of the elastic moduli and shrinkage volume of the substrates are evaluated with higher molecular weight CS and surface-deacetylated chitin nanofiber composite CS films that have different elastic moduli but show very similar swelling–drying behaviors. The wrinkle sizes and skin layer thicknesses also suggest that wrinkle size in this system depends on the hardness of the skin layers or on the physical properties affecting the hardness.