Effect of water on glass transition in starch/sucrose matrices investigated through positron annihilation lifetime spectroscopy: a new approach

Abstract

Glass transition is studied through positron annihilation lifetime spectroscopy (PALS) in maize starch matrices containing 10 (batch STS10) and 20 (STS20) w/w% sucrose, as a function of temperature (T) and water content (c_w). To circumvent important losses of water upon heating while recording the PALS spectra, a new method is developed: instead of a series of measurements of τ₃, the triplet positronium lifetime, at different T, the latter is kept constant and the series relates to c_w, which is left to decrease at a constant rate. Similarly to the changes in τ₃ with T, the τ₃vs. c_w plots obtained show a smooth linear increase until a break, denoting the occurrence of glass transition, followed by a sharper increase. The gradients appear to be independent of T. The variation of the glass transition temperature, T_g, with c_w shows a broad sigmoid with a large linear central part; as expected from the plasticising effect of sucrose, the plot for STS20 lies some 10 K below that for STS10. Results from differential scanning calorimetry for STS20 yield T_g values some 15 K higher than from PALS. On the basis of the general shape of the τ₃vs. T variations, a general equation is set for τ₃(T, c_w), leading one to expect a similar shape for τ₃vs. c_w, as experimentally observed.