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

Abstract

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.