RESUMEN
A shear-thickening polysaccharide from the New Zealand Black tree fern (Cyathea medullaris, commonly known as mamaku) extracted from different age fronds (stage 1: young, stage 2: fully grown and stage 3: old) was characterised in terms of structure and rheological properties. Constituent sugar analysis and 1H and 13C NMR revealed a repeating backbone of -4)-ß-D-GlcpA-(1 â 2)-α-D-Manp-(1â, for all mamaku polysaccharide (MP) samples from different age fronds without any alterations in molecular structure. However, the molecular weight (Mw) was reduced with increasing age, from ~4.1 × 106 to ~2.1 × 106 Da from stage 1 to stage 3, respectively. This decrease in Mw (and size) consequently reduced the shear viscosity (ηs-Stage 1 > ηs-Stage 2 > ηs-Stage 3). However, the extent of shear-thickening and uniaxial extensional viscosity of MP stage 2 was greater than MP stage 1, which was attributed to a greater intermolecular interaction occurring in the former. Shear-thickening behaviour was not observed in MP stage 3.
RESUMEN
A water-soluble polymer was extracted from the fronds of the black tree fern (Cyathea medullaris or "Mamaku" in Maori) and characterized under various physico-chemical conditions. The rheological properties (in the semi-dilute region) and particle size and charge (in the dilute region) were investigated under various salt (0-1M NaCl), pH (1-12) and temperature conditions (5-80°C), using rheometry and dynamic light scattering techniques. A 7% (w/w) Mamaku crude extract showed strong shear-thickening properties at high salinity levels (1M NaCl) and over the whole range of pH (1-12). However, the thickening properties disappeared above 50°C. Apparent viscosity remained constant over the pH range 3-9, although the particle size systematically decreased with increasing pH. Overall, Mamaku solutions showed very good salt and pH resistance and exhibited strong temperature dependency. Hydrophobic and electrostatic interactions are not likely to be the cause of the shear-thickening phenomenon observed. Other forces such as hydrogen bonding may play a dominant role on the formation of shear-induced associations.