Molecular and crystal deformation of cellulose: uniform strain or uniform stress?

The molecular and crystal deformations of a range of lyocell cellulose fibres, produced using different drawing conditions, are reported. The fibres are spun using increasing draw ratios to both increase the molecular and crystal orientation and, consequently, mechanical stiffness. Raman spectroscopy and X-ray diffraction are used to follow molecular and crystal deformation, respectively. It is shown that these techniques are complementary, and that both must be used for semicrystalline cellulose fibres if a full picture of their micromechanics is to be obtained. By following the shift in the 1095 cm(-1) Raman band with respect to external tensile deformation of the fibres we show that we can build up a picture of the microstructure. Using theoretical predictions of the relationship between the Raman band shift rates with respect to strain and stress and the modulus of the fibres we show that the fibres have properties that suggest a hybrid series-series aggregate structure. By using X-ray diffraction we show that the crystal modulus of the fibres appears to change with increasing draw ratio. Furthermore the crystal modulus of the fibres appears to vary systematically with the crystallinity of the sample. Other relationships between the predicted fibre modulus and the experimental values and between the Raman band shift rates and modulus suggest that the assumption of a uniform stress microstructure prior to the measurement of crystal modulus may be an incorrect one. A more realistic structure is proposed for semicrystalline regenerated cellulose fibres, wherein crystals and amorphous regions are both in series and in parallel with each other.

Influence of domain orientation on the mechanical properties of regenerated cellulose fibers.

The determination of the crystal orientation of regenerated cellulose fibers produced under different drawing regimes is presented. Orientation is determined by using wide-angle X-ray diffraction from a synchrotron source and by measuring the azimuthal width of equatorial reflections. The orientation parameter theta is then determined to compare fiber samples. By using a 500 nm beam size, clear differences between the crystal orientations of the skin and the core of the fibers are reported for a range of differently processed fibers for the first time. These results are shown to have implications for the mechanical properties of regenerated cellulose fibers. By applying tensile deformation to fiber bundles it is shown that the most misoriented samples undergo rapid decreases in the orientation parameter, which is an indication of crystal reorientation. However, the more highly oriented fibers undergo little reorientation. An average shear modulus for these fibers is determined by placing the data on a master curve and fitting with a model equation. By using another model for the fibers of low orientation and the shear modulus from the master curve analysis, it is shown that the deformation of less oriented fibers is dominated by shear between crystals, whereas the more oriented filaments are likely to undergo more significant chain deformation. By using a new model for fibers of low orientation, a parameter ksigma is introduced that gives the proportion of the fiber stress that is due to crystal shear. Systematic differences between this parameter for fibers of increasing initial orientation are reported. Moreover it is shown that the fibers of initially lower average orientation are governed by uniform strain, in agreement with the new model, whereas more highly oriented fibers deform under uniform stress. Furthermore, the model that we propose for misoriented domains and the use of a new factor dictating the proportion of shear stress may have general applications in materials engineering.