title: 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 τ(3), 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 τ(3) with T, the τ(3)vs. 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 τ(3)vs. T variations, a general equation is set for τ(3)(T, c(w)), leading one to expect a similar shape for τ(3)vs. c(w), as experimentally observed.
authors.affiliation: Chimie Nucléaire, Institut Pluridisciplinaire Hubert Curien, UDS/IN2P3/CNRS, 23 rue du Loess, B.P. 28 670737 Strasbourg Cedex 2, France. skumars@barc.gov.in
authors.forename: Sandeep Kumar
authors.initials: SK
authors.lastname: Sharma
authors.forename: Abdelhadi
authors.initials: A
authors.lastname: Zaydouri
authors.forename: Gaëlle
authors.initials: G
authors.lastname: Roudaut
authors.forename: Gilles
authors.initials: G
authors.lastname: Duplâtre
mesh_terms: D004583:Electrons; D005898:Glass / Q000737:chemistry*; D013057:Spectrum Analysis / Q000379:methods; D013213:Starch / Q000737:chemistry*; D013395:Sucrose / Q000737:chemistry*; D044366:Transition Temperature; D014867:Water / Q000737:chemistry*
chemical_list: D014867:Water; D013395:Sucrose; D013213:Starch