Regenerated cellulose fibers: Relating mechanical response to semicrystalline microstructure

We compare regenerated cellulose fibers manufactured using Viscose and Lyocell processes. These fibers show qualitatively similar mechanical response in different experiments e.g. stretching, stress relaxation and stress recovery. We show that the linear viscoelastic response of these fibers is accurately captured using a phenomenological model. The model parameters correlate well to the structural features and thus provide a robust structure-property relation for the linear region response. When stretched beyond the linear region, regenerated cellulose fibers show a transition in the slope of stress strain curve at a critical strain. This transition has been attributed in the literature to yielding of fibers. We demonstrate that this is incorrect and that the critical strain corresponds to an apparent yield. When subjected to strains higher than this apparent yield point, the fibers retain a memory of the mechanical deformation, that decays logarithmically with time and is lost over about a day as the fiber structure transitions back to the original as spun fiber.