Thermodynamics of amorphous starch-water systems. 1. Volume fluctuations

This paper presents dilatometric and calorimetric experiments on potato 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 glass transition is used to estimate the transition temperature of samples 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 calorimetry. In the entire experimental temperature range, a maximal excess volume of mixing is observed at a composition corresponding to three water molecules per anhydroglucose in the mixture. This suggests that the large size difference between the chemical components allows the water molecules to saturate only one hydroxyl group of the anhydroglucose at a time. Specific volumes of starch glasses and melts are superimposed onto a single master curve by a simple empirical relation. The neglect of polar interactions in mean field equations of state results in underestimated internal pressures and cohesive energy densities. Free volumes estimated with the lattice fluid equation of state reflect semiquantitatively the effects of temperature and concentration on the density of starch, which goes through a maximum value at low water concentration. Such a reduction of the free volume of polymer glasses by low plasticizer concentrations is called antiplasticization, as the 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 occurrence can be used to optimize the gas barrier properties of polymers.