This paper presents dilatometric and calorimetric experiments on potato starch extruded into a transparent amorphous glass with a density of 1.5 g/cm3 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 °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.

This paper presents a theoretical and experimental study of the sigmoidal water sorption isotherms of amorphous starch. Sorption isotherms have been measured by gas chromatography at infinite dilution of water in starch and by isothermal and isosteric sorption experiments in an extended concentration and temperature range in which the biopolymer behaves either as a brittle glass or as a rubbery melt. A maximum value of the isothermal activity coefficient of water is observed at a composition corresponding to the glass transitions measured by calorimetry. Therefore, the partial derivatives of the activity coefficient of water with respect to concentration and temperature are positive in glasses and negative in melts. A transition from sigmoidal to Flory type sorption is estimated to occur at 175 °C, which is lower than the glass transition of dry starch. The distribution of water molecules in glasses and melts is analyzed with the Kirkwood theory of solutions. In glasses, water shows large negative excluded volumes typical of an antiplasticizer, as reflected also in the density increase observed at low water concentrations. In melts, water shows positive excluded volumes typical of a plasticizer having recovered its motional freedom restricted in the glassy state. Up to 80 °C, the self-clustering functions of water in melts diverge at higher water contents. These functions only take finite values above this temperature once a full melting of the starch−starch hydrogen bond interaction has occurred. The sigmoidal water sorption isotherms are analyzed with a new explicit relationship combining the generalized Freundlich adsorption model and the Flory model of polymer solutions. A restricted translational and rotational freedom is predicted for the adsorption water, and a clustering tendency is predicted for the solution water. The Freundlich−Flory sorption model provides a consistent description of the solvation, the swelling, and the dissolution of hydrophilic polymer glasses in a solvent like water whereas the Brunauer−Emmet−Teller model is only physically meaningful for the adsorption of nonsolvents such as oxygen or nitrogen gases.

The structural basis for polymorphism in amyloids is unraveled with a model system. The hydrogen‐bonding pattern within the β sheets of fibrils is strongly influenced by the pH of the solution from which the fibrils are formed. Solid‐state NMR spectroscopy experiments allow quantification of the relative amounts of two different β‐sheet structures over the pH range 2.0–7.3.

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