A study of crystallinity changes in oxidised celluloses

Abstract

Cellulose was oxidised to dialdehyde cellulose (DAC), sodium dicarboxy cellulose (NaDCC) and dicarboxy cellulose (DCC). Products having oxidation levels of 12, 30, 60, 80 and 98% based on glucose monomer units, were obtained by adjusting the quantity of oxidising agents. The wide-angle X-ray diffraction peak at 2θ = 22·7° for the various oxidised cellulose samples, which indicates the crystallinity of the cellulose, was found to decrease almost proportionately to the degree of oxidation of the starting cellulose. The differences in crystallinity between DAC, NaDCC and DCC of the same degree of oxidation are expected to result from differences in their hydrogen bonding ability with adjacent molecules. Their ability to hydrogen bond with water is reflected in their water absorption capacity. NaDCC has an exceptionally high water absorption capacity owing to its polyelectrolyte nature. DCC absorbs more water than the corresponding DAC since the carboxy group forms stronger hydrogen bonds than the aldehyde group. Thermogravimetric analysis of the three oxidised celluloses showed that DCC decomposes at a somewhat higher temperature than DAC in the initial stages up to around 400°C, reflecting greater stability, possibly due to better intermolecular hydrogen bonding. However, as the temperature is increased further, DCC degrades somewhat faster than DAC, NaDCC, as expected, being a polyelectrolyte shows much less weight loss, and the weight loss occurs over a very wide range. The changes in the structure and crystallinity are also seen in the CP-MAS C-13 NMR studies of these molecules.