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

Abstract

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.

Analysis of mercerization process based on the intensity change of deconvoluted resonances of 13C CP/MAS NMR: Cellulose mercerized under cooling and non-cooling conditions

“…The concentration at which the spectrum changes is in good agreement with the concentration required for ramie fiber shrinking [13]. Furthermore, the spectrum at high concentrations corresponds to the spectrum of cellulose II, as reported by Yokota and Sei [37] and Vanderhart and Atalla [40].…”

“…The intensity change of their signals starts at [NaOH] = 0.07 (non-cooling) and [NaOH] = 0.05 (cooling), becomes significant with increasing [NaOH], and stops at [NaOH] = 0.10 (non-cooling) and [NaOH] = 0.07 (cooling). Interestingly, the above regions of [NaOH] closely correspond to the morphological change of ramie fibers treated with various concentrations of aqueous NaOH [13], although samples used in the present study, which is microcrystalline cellulose, are different from ramie fiber samples. This strongly suggests that the morphological change induced by mercerization is essentially due to the change in the intramolecular hydrogen bonds related to these carbon atoms.…”

“…The change in the intensity of C6-down reduces with the decrease in the C4-down intensity in the region, whereas almost no change was observed in the C1-up intensity; in contrast, the intensity of C1-up starts to decrease with an increase in the C4-down intensity immediately after the change of C6-down is complete. The regions of the intensity change of C1-up and C6-down correspond closely to the morphological change of ramie fibers treated with various concentrations aqueous NaOH solutions [13]; the former region represents the decrystallization of cellulose I and the latter the crystallization of cellulose II. Considering the previous results of morphological changes [13], it can be suggested that the change in the intensity of C1-up, C4-down, and C6-down relates cooperatively to the change in flexibility of cellulose chains caused by decrystallization (contraction along the longitudinal axis of ramie fibers) and the subsequent crystallization of cellulose II (recovery along the longitudinal axis).…”

“…Following up on the aforementioned studies, Roy et al [12] revealed the structure of cellulose by calorimetry, small X-ray scattering, and viscometry after treating it with a NaOH solution at low temperatures. We have also previously reported the effect of cooling on the morphological change of ramie under alkali treatment [13].…”

“…Since the treatment with NaOH reduces the crystallinity of microfibrils and induces longitudinal contraction, we believe that these observed changes are attributable mainly to the entropic elastic force [13,30,32]. These changes in the entropic force influence the mechanical relaxation behavior of wood upon NaOH treatment, and in particular, reflect the dynamics of the methylol group [33,34].…”