Glass transition temperature of thermoplastic starches

International Agrophysics 01/2005;
Source: DOAJ

ABSTRACT Thermoplastic starch was produced by mixing potato starch and glycerol in a single screw extruder. The glass transition temperatures of the materials obtained were measured by differential scanning calorimetry (DSC). Both the influence of extruder parameters and material parameters, such as moisture and glycerol content and amyloses/amylopectine ratio were investigated. Repeated extrusion cycles affect the glass transition temperature only to a very small extent.

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    ABSTRACT: By comparing glass transition temperatures, Tg, determined by differential scanning calorimetry (midpoint of heat capacity step at 3 °−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 Tg 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 are observed following aging treatments at temperatures below, and even far below Tg. 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 Tg values, due to chain end effects, as well as flexibility of branching points. The three dihedrals present in α(1–6) linkages seem to depress Tg in a similar fashion to internal plasticization. The case of a linear α(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.
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    ABSTRACT: The effects of glycerol and water content on the thermal transitions of plasticized barley starch were examined using differential scanning calorimetry. The glycerol contents studied were 14, 20, 29 and 39% and the water content, obtained by conditioning in different relative humidities, varied in the range 1–28%. On the basis of the observed calorimetric glass transition temperatures and corresponding heat capacity increments it was inferred that a single phase system occurred at low water and glycerol contents, while in other cases phase separation occurred and the system was composed of starch-rich and starch-poor phases. Dynamic mechanical thermal analysis on a phase-separated sample showed mechanical loss peaks corresponding to the glass transitions of both phases. Amylopectin crystallization did not occur within 1 week of storage in mixtures having less than 20% water, indicating that glycerol interacted with starch, inhibiting crystallization of amylopectin.
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