Morphological change induced with NaOH–water solution for ramie fiber: change mechanism and effects of concentration and temperature

The morphology of ramie fiber treated with NaOH–water solutions at various concentrations was observed with an epi-illumination microscope (EIM) equipped with a charge-coupled device (CCD) camera. The crystallinity was measured by X-ray diffraction. The morphological changes in length and width were quantified using image analysis. Changes in morphology were noted for samples treated with NaOH–water solutions at room temperature in the narrow concentration range of 0.08 < [NaOH] ≦ 0.12. For samples cooled at −5 °C after treatment, the morphological changes started at a lower concentration, i.e., at [NaOH] = 0.05. The change was observed as contraction in length and swelling in width. The mechanism for this dimensional change related closely not to the conformation of the whole microfibril but to the crystallinity of cellulose chains that had been de-crystallized by the NaOH–water solution: the calculated bond angle was too small for a zigzag conformation of the whole microfibril.