Converse piezoelectric effect in cellulose I revealed by wide-angle X-ray diffraction.

The converse piezoelectric effect in cellulose I was studied by exposing thin pine wood slices to an electric field. Macroscopically, a strong extension of wood was observed in its transverse anatomical direction (grain angle 90 degrees), perpendicular to the direction of the electric field. The same effect, albeit to a lesser extent, was observed for specimens with a 45 degree grain angle, whereas no measurable dimensional change was observed for specimens with grain oriented parallel to the testing direction (0 degree grain angle). The measured extension in the transverse direction was proportional to the intensity of the applied electric field and amounted to 0.0278% on average at a field intensity of 1 MV m(-1), which results in a piezoelectric charge constant of 278 pm V(-1). At the nanoscale, changes in the cellulose crystallites due to the applied electric field were studied by means of wide-angle X-ray diffraction using the same specimens as in macroscopic experiments. Significant radial shifts of the scattering intensity peak attributed to the cellulose 200 crystallographic plane toward smaller scattering angles were observed, while the electric field was applied. These peak shifts were attributed to an increase in the spacing of the 200 crystallographic planes of cellulose I. At an electric field intensity of 1 MV m(-1), a crystallite strain epsilon(perpendicular 200) normal to the 200 reflection plane of 0.2% was estimated from Bragg's law.

Cellulose in never-dried gel oriented by an AC electric field.

Never-dried cellulose gel obtained by slow coagulation from LiCl/N,N-dimethylacetamide (DMAc) solution was exposed to an alternating current electric field. Making use of the birefringence of oriented cellulose and by means of wide-angle X-ray scattering, it was demonstrated that preferred orientation of cellulose molecules parallel to the electric field lines is induced in the cellulose gel. The preferred orientation remained unchanged for several days after storage in water and persisted after drying of the cellulose gel.

Actual versus apparent within cell wall variability of nanoindentation results from wood cell walls related to cellulose microfibril angle.

Hardness and elastic modulus of spruce wood cell walls parallel to their axial direction were investigated by means of nanoindentation. In the secondary cell wall layer S2 of individual earlywood and compression wood tracheids, a systematic pattern variability was found. Several factors potentially affecting nanoindentation results were investigated, i.e. specimen orientation related to the indenter tip, cutting direction during specimen preparation, tip geometry, specimen and fibre inclination, respectively, and finally micro fibril orientation. Mechanical property measurements were correlated with structural features measured by confocal Raman spectroscopy. It was demonstrated that very high variability in the measurement of micromechanical cell wall properties can be caused by unintentional small fibre misalignment by few degrees with regard to the indentation direction caused by sub-optimal specimen preparation.

Changes in the molecular orientation and tensile properties of uniaxially drawn cellulose films.

Cellulose films were prepared by dissolving lyocell fibers in LiCl/N,N-dimethylacetamide solvent and subsequently coagulating and drying them under ambient conditions. To introduce preferred orientation, the films were uniaxially drawn under air-dry and rewetted conditions, respectively. Preferred orientation was determined by birefringence measurements and by wide-angle X-ray scattering. Mechanical properties were characterized by means of tensile tests with films conditioned to standard temperatures and humidity. Drawing resulted in the substantial reorientation of cellulose, whereby the molecular chains in the amorphous regions exhibited clearly stronger reorientation than the crystalline fraction. The average degree of orientation was comparable to orientation achieved in spun cellulose fibers. Wet-drawing resulted in improved tensile strength and modulus of elasticity but reduced elongation at break. The mechanical properties of wet-drawn films are competitive with regard to cellophane and melt-blown cellulose films, particularly considering their high modulus of elasticity of up to 26 GPa, which is also comparable to values obtained for industrially produced cellulose fibers.