THERMODYNAMICS OF AMORPHOUS STARCH-WATER SYSTEMS - 1 - VOLUME FLUCTUATIONS

This paper presents dilatometric and calorimetric experiments on potat o starch extruded into a transparent amorphous glass with a density of 1.5 g/cm(3) at room temperature. The specific heat increment at the g lass transition is used to estimate the transition temperature of samp les containing up to 25 wt % water. The specific volume of the samples is studied between 25 and 165 degrees C and between 0.1 and 100 MPa. Glass transitions estimated from the compressibility increment at the transition temperature are found in agreement with those detected by c alorimetry. In the entire experimental temperature range, a maximal ex cess volume of mixing is observed at a composition corresponding to th ree water molecules per anhydroglucose in the mixture. This suggests t hat the large size difference between the chemical components allows t he water molecules to saturate only one hydroxyl group of the anhydrog lucose at a time. Specific volumes of starch glasses and melts are sup erimposed onto a single master curve by a simple empirical relation. T he neglect of polar interactions in mean field equations of state resu lts in underestimated internal pressures and cohesive energy densities . Free volumes estimated with the lattice fluid equation of state refl ect semiquantitatively the effects of temperature and concentration on the density of starch, which goes through a maximum value at low wate r concentration. Such a reduction of the free volume of polymer glasse s by low plasticizer concentrations is called antiplasticization, as t he reduction of the glass transition temperature is then coupled to an increase rather than a decrease of elastic moduli. Antiplasticization reduces gas sorption and permeation rates and the knowledge of its oc currence can be used to optimize the gas barrier properties of polymer s.