In the polymer foaming process, the melt viscosity is an important parameter for the cell nucleation rate, the growth of nuclei, and the stabilization of the foamed cellular structure. However, due to their linear structure and high rigidity, PLA chains cannot entangle as rapidly as typical foaming materials can, and they show a low melt strength that decreases the foam stabilization. The effect of filler addition on the rheological properties was investigated on the basis of dynamic oscillatory shear measurements, and the resulting viscosity curves are shown in Figure 4. The complex viscosity of neat PLA exhibited typical Newtonian behavior in the low shear rate region and shear thinning commenced up to around 10 s-1. The complex viscosity of the CNF-incorporated nanocomposites was higher overall than that of neat PLA and increased with increasing CNF content. At a filler content of 1 and 2 wt%, similar Newtonian behavior with much higher complex viscosity was observed. A literature review suggested that the high viscosities of the polymer matrix with nanometric cellulose fillers are attributable to filler-filler interactions through hydrogen bonding and a good filler-matrix interfacial interaction. Consequently, the flow restrictions of the PLA chains result from the strong interaction between the modified CNFs and the PLA molecules, which enhances the melt viscosity of the PLA/CNF nanocomposites. However, the viscosity of the PLA/CNF (5 wt%) nanocomposite decreased rapidly even from the highest initial viscosity, which may have been cause by the tendency of the CNFs at high content to agglomerate into larger aggregates and induce a