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Most significant amount of regenerated cellulose fibers are industrially manufactured using either Viscose or Lyocell processes. Differences in processing conditions induce different structural features of Lyocell and Viscose fibers. The surface morphological differences in Lyocell and Viscose fibers are widely studied in literature. It is known that the Lyocell fibers exhibit relatively circular cross section whereas, the cross section of Viscose fibers is nonuniform. However, there is not much information available on the microstructural features present at the length scales on the order of nanometers e.g., microvoids, fibrillar structure, lamellar structure etc. We employ scanning electron microscopy on acid etched Viscose and Lyocell fibers to show that both fibers exhibit similar fibrillar structure. However, small angle X-ray and medium resolution small angle neutron scattering studies reveal that regenerated cellulose fibers possess elongated microvoids, oriented in the fiber direction. We analyze two dimensional SAXS data using Ruland's equatorial streak method to obtain the average length and average misorientation of microvoids in Viscose and Lyocell fibers. Porod's law and invariant calculations from the combined SAXS and MSANS are employed to evaluate the radius. We show that Lyocell fibers have bigger, better orientated voids than Viscose fibers. Whereas, Viscose fibers exhibit a wider distribution of radii than Lyocell fibers. Interestingly, our SAXS data shows signatures of stacked crystal/amorphous lamellar structure in case of Viscose fibers. On the other hand, such structure is not observed for Lyocell fibers. We analyze the stacked lamellar structure in Viscose fibers using 1-D correlation function analysis of the Lorentz corrected data. We show that the lamellar structure in Viscose fibers swell heterogeneously, when soaked in water. This shows that the process differences in Lyocell and Viscose strongly affect the microstructure of fibers at length scales of few nanometers. The methodologies presented by us could be used to explore these differences in detail. [ABSTRACT FROM AUTHOR] more...

Regenerated cellulose fibers are bio-derived materials, well known for their applications in clothing materials. There are two industrially dominating processes for manufacturing regenerated cellulose fibers: Viscose and Lyocell process. Viscose is a conventional process that utilizes harmful chemicals like carbon disulfide whereas, Lyocell is relatively recent and environmentally more benign as compared to the Viscose process. Due to differences in the manufacturing process, the properties of Lyocell fibers are substantially different from those of the Viscose fibers. These properties are anticipated to be governed by the hierarchical microstructure of semicrystalline fibers, which is ordered over multiple length scales. The understanding of structure-property relations in regenerated cellulose fibers is crucial for manufacturing fibers with desirable properties. However, the complexity of the microstructure makes it difficult to relate the properties of cellulose fibers to the relevant structural details. We examine the microstructure of Viscose as well as Lyocell fibers at different length scales from a few Å to hundreds of nanometers. Dynamics of fiber microstructure is captured during mechanical deformation by employing in-situ experiments. A simple model relates the viscoelastic response of fibers to the microstructural features. Our model and in-situ measurements establish that the mechanical response of regenerated cellulose fibers is governed by the interplay between crystalline and amorphous domains in the semicrystalline microstructure. This whole framework allows us to eliminate the complexities of hierarchical microstructure and pins down only the features that are relevant to the mechanical properties of fibers. We show that our model is universally applicable for various semicrystalline fibers with glassy amorphous phase and could be used to study a variety of other similar systems. [ABSTRACT FROM AUTHOR] more...