Effect of molecular structure and water content on the dielectric relaxation behaviour of amorphous low molecular weight carbohydrates above and below their glass transition.
The dielectric relaxation behaviour of several amorphous low molecular weight carbohydrates and their 10% w/w water mixtures has been studied in the supercooled liquid and glassy regions in the frequency range 100 Hz to 100 kHz. The dry carbohydrates show a primary alpha-relaxation (activation energy 250-405 kJ mol(-1)) at temperatures above the calorimetric glass transition temperature, Tg, and, in most cases, a secondary beta-relaxation (activation energy 42-55 kJ mol(-1)) at sub-Tg temperatures. Whilst D-mannose showed a beta-relaxation similar in strength to D-glucose, its deoxy sugar, L-rhamnose showed a relatively weak beta-relaxation. This indicates that the hydroxymethyl group influences relaxation in carbohydrate glasses. Addition of water shifted the alpha-relaxations to lower temperatures and increased the strength of the beta-relaxations. In glucitol this resulted in a merging of the alpha- and beta-relaxations. The beta-relaxation increased in strength and decreased in temperature for the series of water mixtures: D-glucose, maltose, and maltotriose.
Noel
T R
TR
Institute of Food Research, Food Materials Science Division, Colney, Norwich, UK.
Parker
R
R
Ring
S G
SG
D002152:Calorimetry, Differential Scanning; D002241:Carbohydrates / Q000737:chemistry*; D013237:Stereoisomerism; D013696:Temperature; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
D002241:Carbohydrates; D014867:Water