Analysis of side group motion in O-acetyl-starch using regioselective 2-O-acetyl-starches by means of dielectric spectroscopy

The complex dielectric constant of O-acetyl-starches with regioselective an d the statistical distribution of substituents at the anhydroglucose unit ( AGU) are measured in an extended frequency range from 10 mHz to 2 MHz and i n the low temperature range from -120 to 0 degrees C. The experimentally de termined relaxation spectra are evaluated with the well-known model functio n of Havriliak-Negami for relaxation processes. In the case of 2-O-acetyl-s tarch two significant relaxations are detected which are associated with th e orientational motion of the methylolgroup at the AGU and the acetylgroup in position C-2. The activation energies are 48 and 60 kJ/mol, respectively . Owing to that assignment of relaxations at the AGU with acetylation in po sition C-2 has proved that it is possible to describe further relaxation pr ocesses in acetylated amylomaize starch with a statistical substitution pat tern. It was found that an additional relaxation process is associated with the dynamics of the acetylgroup at the position C-6 at the AGU and charact erized by a lower activation energy of 28 kJ/mol. From the analytical point of view these results of dielectric spectroscopy analysis demonstrate that the mobility of the side groups depend significan tly on the position at the AGU and the type of substituent in their frequen cy and temperature dependence. (C) 2000 Elsevier Science Ltd. All rights re served.