Calorimetric evaluation of the glass transition in hydrated, linear and branched polyanhydroglucose compounds

By comparing glass transition temperatures, T-g, determined by differential scanning calorimetry (midpoint of heat capacity step at 3 degrees C.mn(-1)) on powders of varying water contents for different polysaccharides, the influence of molecular weight, degree of branching and (1-4) vs (1-6) glycosidic linkage ratio upon the depression of T-g is illustrated, thus extending results of former studies. Due to both of the doubts concerning the heat capacity change of water at its glass transition, when dispersed in such media, and limitations of second-order entropy-based derivations, the effect of water plasticization is described by the Couchman's correlation in its degenerated form, which is similar to the Gordon-Taylor formulation. Strong enthalpy relaxation effects ale observed following aging treatments at temperatures below, and even Far below T-g. This makes it necessary to erase the history of moisture-conditioned samples and, thus, only the second DSC scan results are presented. As expected, linear chains appear to favor chain-chain interactions and induce partial crystallinity; branched molecules display lower T-g values, due to chain end effects, as well as flexibility of branching points. The three dihedrals present in alpha(1-6) linkages seem to depress T-g in a similar fashion to internal plasticization. The case of a linear alpha(1-4) amylose chain bearing (1-6) grafted fructoses is examined as a first step towards tailored structures, designed to optimize mechanical properties and internal plasticization (as for chemically modified polysaccharides) and inhibit recrystallization. The extension to ciliated structures (sparse brushes) is proposed as a target for biosynthesis optimization. (C) 1997 Published by Elsevier Science Ltd.