Relating the properties of cellulose fibers to structure

Abstract

Polymer fibers are ubiquitous. Applications of polymer fibers are wide ranging, spanning diverse fields such as textiles, biomedical materials, etc. These fibers are manufactured using very different processing techniques. For example, silk is spun by silkworms, PET fibers are spun from the melt state, polyethylene fibers are spun from gels. This thesis is focused on the structural investigation of semicrystalline regenerated cellulose fibers and on structure-property relations in these materials. To convert cellulose into continuous filaments, it is dissolved and regenerated using two dominant industrially practiced processes – Viscose and Lyocell. Differences in processing techniques induce structural differences in regenerated cellulose fibers. Fibers manufactured using Viscose technique are characterized by skin core morphology whereas, Lyocell fibers show radial uniformity throughout the fiber diameter. Also, the shape of the fiber cross-section is different for Viscose and Lyocell fibers. Such structural differences manifest at length scales varying from O(Å) to O(micron). Implications of the structural differences are observed as differences in the fiber properties. For example, Lyocell fibers have higher modulus than Viscose fibers, Viscose fiber show better fibrillation resistance when soaked in water as compared to Lyocell fibers. Due to the hierarchy of structural differences, ascertaining the microstructural origin of the variations in these properties is a challenging task. Therefore, we present a systematic investigation of the structural features of regenerated cellulose fibers using tools that allow us to probe different length scales e.g. wide angle X-ray diffraction for Å-scale structure, small angle X-ray scattering for structure at tens of nm, ultra-small angle neutron scattering for structure at hundreds of nm, etc. We study the mechanical response of these fibers using a rheometer and relate the mechanical properties to the structure of the fibers. We demonstrate that polymer fibers, including natural silk and synthetic fibers, exhibit universal viscoelastic response. On stretching below yield, they show logarithmic stress decay. On unloading fibers with a glassy amorphous phase, the stress recovers. A simple phenomenological model accurately describes data from independent mechanical experiments and provides insights into the microstructural origins of the fiber response. Counter to intuition, the model indicates that it is the crystalline regions, rather than the amorphous glass, that deform predominantly on stretching fibers at high strain rates. On holding a stretched fiber, stress decays as a consequence of relaxations in amorphous regions. Finally, unloading the fiber transfers stress from the amorphous to crystalline regions resulting in stress recovery. Model parameters correlate well with the fiber microstructure. Crystal and amorphous moduli from the model match those from X-ray diffraction. Activation energies for the temperature dependence of the peak relaxation time are similar to those reported in the literature. Thus, a simple model that invokes only crystal-amorphous coexistence can successfully model the mechanical response of a wide variety of polymer fibers. We employ this model to compare commercially available regenerated cellulose fibers manufactured using Viscose and Lyocell processes. Single fibers are subjected to a variety of mechanical deformations to obtain stress-strain, stress relaxation and stress recovery data. Lyocell fibers are characterized by higher values of crystalline modulus relative to Viscose. Lyocell fibers also have a higher amorphous phase modulus and a wider relaxation spectrum than Viscose, suggesting that amorphous and crystalline phases are dispersed in close connectivity in Lyocell. Viscose and Lyocell fibers exhibit qualitative similarities in their mechanical response. On stretching, there is a transition in the stress-strain curve from a low strain elastic response at a critical value of strain. This critical strain has been incorrectly attributed to yielding of the fiber. We establish that this critical value corresponds to an apparent yield. When subjected to strains higher than this apparent yield point, the fibers develop a memory of the mechanical deformation. This memory decays slowly, logarithmically with time and is lost over about a day as the fiber structure transitions back to the original as spun fiber. We also demonstrate that on wetting the fibers with water, there is an increase in the apparent yield strain for Viscose fibers, but not for Lyocell. We interpret these results in terms of the semicrystalline microstructure of the fibers. We study the effect of stretching and stress relaxation on the orientation of crystal and amorphous phases of Lyocell and Viscose fibers. Our results show that on stretching, orientation in both crystal and amorphous phases increases linearly with strain, correlating with the increase in stress and with the stretching of the crystalline unit cell along the c-axis. On holding after stretching to a particular strain, the stress relaxes logarithmically in time, correlating with a decrease in the strain along the c-axis of the crystal unit cell. The stress relaxation is also correlated with a logarithmic increase in amorphous orientation, while crystalline orientation stays constant. We attribute the stress development during stretching to deformation of the crystal unit cell, while crystal reorientation in the fiber direction results in increase in the crystalline orientation parameter. On holding the fiber at a fixed total strain, the stress relaxes as strain is transferred from crystal to amorphous regions. Thus, the strain on the unit cell c-axis decreases and amorphous orientation increases. There are quantitative differences between the rate of increase in amorphous phase orientation during stress relaxation for Lyocell and Viscose fibers. For dry fibers, Lyocell shows a slower increase in orientation during stress relaxation. On wetting the fibers, their structural response changes qualitatively. We combine wide angle X-ray diffraction and birefringence experiments with our model to infer that that on stretching the wet fiber, the crystalline phase is neither strained nor oriented. However, orientation develops in the amorphous phase. During stress relaxation in wet condition, Lyocell fibers shows a faster increase in amorphous orientation than Viscose fibers, in line with the comparison of relaxation time spectra for wet Viscose and Lyocell fibers. We use small angle scattering to characterize the microvoids in regenerated cellulose fibers that might govern the onset of mechanical failure in these. In regenerated cellulose fibers, scattering of X-rays or neutrons at small angles is largely dominated by scattering from microvoids. We demonstrate that small angle X-ray scattering (SAXS) over the q-range that is typical for most commercial instruments arises from Porod scattering from the microvoid surfaces, viz. the scattered intensity scales as q^-4. Therefore, it is not possible to extrapolate this data to lower q to obtain microvoid dimensions and volume fraction. We combine SAXS with medium resolution small angle neutron scattering (MSANS) to characterize the microvoids in regenerated cellulose fibers. Specifically, we compare fibers produced using the Viscose process with those from the Lyocell process. For both Viscose and Lyocell fibers, microvoids have a high aspect ratio and are elongated in the fiber direction. Also, the volume fraction occupied by the microvoids is comparable for Viscose and Lyocell fibers (0.04% - 0.05%). However, there are differences in the microvoid size: Microvoids are more highly oriented for Lyocell fibers and have a larger average length and diameter, compared with Viscose fibers. This result might have important implications for understanding failure of these fibers. In summary, this thesis reports a detailed study of the structural differences between Lyocell and Viscose fibers at multiple length scales. We have developed a set of sophisticated methodologies for structural characterization of regenerated cellulose fibers. We report the non-intuitive structural response of Lyocell and Viscose fibers subjected to mechanical deformations and develop structure-property relations for these fibers. Our work uncovers the microstructural origins of the differences between Viscose and Lyocell fibers, and presents results that have implications for relations between semicrystalline microstructure and properties in general.