The results of the TMOE measurements were in accordance with the rheological data on MFI. Because of the intrinsic rigidity of CNF, the TMOE were higher than those of neat PLA and PLA + PHB composites, especially for the highest CNF content (10 wt.%), indicating stress transfer from the matrix to the CNF. Increasing TMOE of the composites compared with the matrix polymer can also be associated with the restrictions of macromolecular mobility and deformability imposed by the presence of CNF. The presence of CNF perturbed normal polymer flow and restricted the mobility of chain segments. Thus, higher amounts of CNF result in lower MFIs or higher viscosity values of the filled polymer