Determination of polymorphic changes in cellulose from Eucalyptus spp. fibres after alkalization

Delignified fibres from E. benthamii, E. nitens and E. smithii were treated with different NaOH concentrations (up to 30%, w/v) at two different temperatures (25 °C and 80 °C) to investigate the polymorphic and morphological variations in cellulose. The results showed that de-crystallization and the polymorphic transformation of cellulose started at 10% (w/v) NaOH. At 10% (w/v) NaOH, the cellulose II content was similar among the samples from the different species, but changes were observed in the cellulose I content and amorphous fraction. Alkalization at 25 °C had a higher de-crystallization effect than that at 80 °C. E. benthamii showed the highest cristallinity index values and E. nitens the lowest. The crystallite size of cellulose I (L₍₂₀₀₎) increased as the NaOH concentration increased, while the temperature seemed to not have a significant influence on the L₍₂₀₀₎ variation. E. benthamii showed the highest L₍₂₀₀₎ (3.5–5.0 nm) while E. nitens showed the lowest (3.3–4.5 nm). The crystallite size of cellulose II (L₍₀₂₀₎) was higher (4.8–5.2 nm) than that of cellulose I L₍₂₀₀₎ (3.3–5.0 nm). Alkalization at a higher temperature resulted in a higher cellulose II L₍₀₂₀₎. Morphologically, at a 0.5% NaOH concentration, the fibres were embedded in a gel-like substance, and, with the NaOH concentration above 10%, the fibres were converted into a swollen and roughened state. Thus, different Eucalyptus species at given NaOH concentrations displayed different structural features, which could lead to procedures and products with different requirements towards the manufacturing of cellulose derivatives.