Structure Evolution during Phase Separation and Gelation of Biopolymer Mixtures

Abstract: This thesis focuses on the kinetics of phase separation and gelation in aqueous mixtures of gelatin and maltodextrin. The structure evolution of these mixtures at different length scales was recorded by confocal laser scanning microscopy (CLSM) and transmission electron microscopy (TEM). The microstructure was quantified by image analysis. All mixtures phase separated during cooling into the incompatible region of the phase diagram. The main conclusion of this work is that it is possible to control the final microstructure of mixed biopolymer systems by the kinetics of phase separation and gelation, which can lead to new materials with unique properties in the future.

The work shows that the phase separation was kinetically trapped by the gelation in a non-equilibrium situation. The final morphology was determined by the relative rates of phase separation and gelation, the onset of phase separation and gelation, and the gelation context in which the phase separation proceeded. The phase separation processes involve self-similar growth, percolation-to-cluster transition, coalescence, secondary phase separation and sedimentation. Start, stop, rate and which of these processes are involved in the time evolution of the microstructure in relation to the rate and progress of the gelation determine the final morphology. The phase separation mechanisms and the energetically most favourable growing wavelength in the early stage of spinodal decomposition strongly affected the final morphology. Another interesting result was that it was possible to determine the phase separation mechanism by comparing the time evolution of the microstructure with existing theories about the kinetics of phase separation and coarsening.

Three kinetic zones were established according to the size of the maltodextrin inclusions and the temperature of phase separation. The kinetic zones were an effect of the relative rates of phase separation and gelation. It was generally found that the size of the maltodextrin inclusions increases with an increasing phase separation temperature, a decreasing cooling rate, an increasing maltodextrin concentration and an increasing end temperature.

The conformational ordering of gelatin gives an extra incentive for phase separation, which results in increased incompatibility of the mixture with time. It was found that the gelation can trap the coalescence in intermediate stages, resulting in partial coalescence and contracted flocculation. In the end state, after the microstructure has been trapped by the gelation, small maltodextrin inclusions can migrate towards the larger inclusions and the gelatin network becomes more uniform. Turbidity and rheological measurements showed that several hundred Pascals are required to trap the microstructure, and the gel point does not necessarily reflect this trapping.

A crossover from diffusive growth to hydrodynamic growth was found in bicontinuous microstructures. The growth rate after the crossover, and the increase of the peak intensity with time were found to increase with increasing quench depth at constant composition. The dimensionality of the growth was three and the time evolution of the microstructure obeyed dynamical scaling in both the intermediate and late stages of spinodal decomposition.