All of the films displayed an asymmetric structure; namely the pore size on the free surface was different from that on the back surface. With an increase of the cellulose concentration, the surface morphology of the films became denser and the pore size (both free surface and back surface) decreased, respectively. The RC-4 film had a distinct asymmetric structure including the free surface with many macrovoids and back surface with larger pores (Fig. 3a, e). When the cellulose concentration was 4.5 wt%, the macroporous structure on the back surface and interpenetrated network structure on the free surface were formed, but the macrovoids became inconspicuous (Fig. 3b, f). As the cellulose concentration increased to 5.5 wt%, the structure of the RC-5.5 became further denser, but it still had network structure on the free surface