Thermomechanical properties of small-carbohydrate–water glasses and ‘rubbers’. Kinetically metastable systems at sub-zero temperatures

Abstract

Aqueous solutions of 24 small carbohydrates have been analysed by low-temperature differential scanning calorimetry (d.s.c.). The method, based on analogue derivative thermograms, measured two thermomechanical properties characteristic of each non-crystallizing solute: T′_g, the sub-zero glass transition temperature of a maximally freeze-concentrated solution and W′_g, the amount of unfrozen water (UFW) kinetically immobilized in the glass at T′_g. For 84 low-molecular-weight polyhydroxy compounds (PHCs) analysed to date, T′_g ranged from –85 °C for ethylene glycol to –13.5 °C for maltoheptaose, and increased monotonically with increasing M_w. T′_g plotted vs.M^–1_w showed a linear correlation characteristic of an homologous family of glass-forming linear oligomers. W′_g ranged from 1.9 g UFW g^–1 for ethylene glycol to 0.2–0.3 g UFW g^–1 for several sugars and polyols, including maltoheptaose, and decreased with increasing T′_g, showing fair linear correlations for several series of homologous solutes. We describe here the use of T′_g and W′_g, as invariant physico-chemical properties of glass-forming solutions at sub-zero temperatures, to interpret thermomechanical behaviour of small-carbohydrate–water systems in non-equilibrium glassy and ‘rubbery’ states, define structure–activity relationships and explain and often predict functional behaviour of such PHCs in various applications.