Effect of temperature on the intrinsic viscosity and conformation of chitosans in dilute HCl solution

Abstract

The effects of temperature on the intrinsic viscosity and on the conformation of chitosans in dilute HCl solution were studied. Ten chitosans with the same degree of deacetylation but different molecular weights were produced by alkali deacetylation of chitin which was prepared from red shrimp wastes. The degree of deacetylation at 83% and weight average molecular weight of the chitosans ranging 78–914 kDa were determined by infrared spectroscopy and static light scattering, respectively. The intrinsic viscosities ([η]) of these 10 chitosans in 0.01 M hydrochloric acid were measured at 10, 20, 30, 40, and 50°C. Then, d ln [η]/d(1/T) and the Mark-Houwink exponents were calculated as the indices for chain flexibility and molecule conformation, respectively. These results showed: the intrinsic viscosities decreased linearly with increasing temperature, therefore, a temperature-induced conformational transition did not occur for all 10 different molecular weight chitosans in the temperature range studied. Values of d ln [η]/d(1/T) were between 633 and 1334 and increased with decreasing molecular weight, indicating that higher molecular weight chitosans are more flexible. Between 10° and 50°C, the Mark-Houwink exponents ranged 0.64–0.76 and increased with increasing temperature, indicating that the conformation of these chitosans were all in random coil, and a temperature-induced conformational transition did not occur. The a* and a** Mark-Houwink exponents represent those chitosans whose molecular weights are larger and smaller than 223 kDa, respectively, and were obtained by using 223 kDa as the break point in the double logarithmic plots of the intrinsic viscosities and weight average molecular weight. Values of a** were between 0.41 and 0.54, while the a* values were from 0.96 to 1.07. These values for a** and a* indicate that larger and smaller molecular weight chitosans were in random coil and rod shape, respectively.