Estimation of the free volume of starch-water barriers

The permeation of volatiles through amorphous barrier materials is affected by their size and geometry as well as by their physical-chemical affinity for the barrier. The gas barrier property of an amorphous polymer is generally a function of density, cohesive energy density and stiffness as well as of the free volume available for the diffusion of volatiles in the microstructure of polymers. The barrier property of amorphous starch is investigated by measuring the density and the cohesive energy density as a function of water contents in an extended temperature range in which the material is either a brittle to rubbery solid or a viscoelastic liquid melt. Free volumes available for the diffusion of volatiles in starch barriers are deduced numerically from a mean field equation of state providing molecular scaling parameters for starch-water mixtures from a least squares fit of experimental data. The consistency of the mean field scaling parameters is reflected in their ability to predict accurate surface tension for these systems. The adsorption of small amounts of water is found to increase the density and to reduce the free volume of starch glasses by a phenomenon called antiplasticization as it results in an increase of Young's moduli of materials whereas the opposite is true for plasticization. At higher moisture contents, an overwhelming plasticization leads to a weakening of gas barrier properties as the free volume of starch is increased during swelling by water. (C) 1999 Elsevier Science Ltd. All rights reserved.