Water-soluble hydroxypropyl curdlan derivatives with thermosensitive properties

Curdlan is a relatively little investigated bacterial polysaccharide because it is insoluble in water and many organic solvents. Therefore, hydroxypropyl curdlan (HPCD) derivatives with different degrees of substitution were obtained by homogeneous etherification reaction with propylene oxide. The unwinding of the triple helix structure of curdlan ensures the water solubility of the samples when the substitution degree was situated above 1, while the hydrophilic-hydrophobic balance of the hydroxypropyl groups provides the thermo-responsive characteristics. HPCD samples with different degrees of substitution were characterized using ¹H, ¹³C, DEPT135, and 2D HSQC NMR spectroscopy. The thermal behaviour of HPCD, studied by rheology and turbidimetry, was found to be nearly similar to that of cellulose derivatives containing hydroxypropyl substituents: a minimum in the viscoelastic moduli was observed and attributed to the phase separation of a polymer-rich phase in a polymer-depleted matrix. The thermal behaviour in solution was also studied by ATR-FTIR spectroscopy, that can brought information about the selective dehydration of the side chains in comparison with the main chain. The hydrophobic interactions between the hydroxypropyl side groups are the first to manifest during heating, and the dehydration of the main chain takes place much faster, but with a delay of 10-12 °C.