Relation between the structure of matrices and their mechanical relaxation mechanisms during the glass transition of biomaterials: A review

It has been demonstrated that industrial polysaccharides (agarose, deacylated gellan and κ-carrageenan) form networks of reduced enthalpic content in the presence of high levels of non-crystallizing co-solute (e.g., glucose syrup) that exhibit time–temperature dependent behaviour of a typical rubberlike polymer. In contrast, amylose holds its structural characteristics unaltered and does not reach a state of molecular mixing with glucose syrup, with morphological features being those of a micro phase-separated mixture. Variation in phase morphology and density of intermolecular associations leads to entropic or enthalpic viscoelasticity in systems, and it was utilised to define distinct classes of food related biomaterials exhibiting an extensive glass transition region or absence of vitrification phenomena. The approach was extended to encompass the experimental parameters of a porous matrix and the application of hydrostatic pressure. In the former, work discusses discrepancies in the Tg – porosity relationship attributable to the different extent to which the two techniques of calorimetry and mechanical spectroscopy respond to degrees of molecular mobility. In the latter, it was shown that the time–temperature–pressure equivalence of synthetic amorphous polymers is not operational in the glass-like behaviour of high sugar systems in the presence of gelatin or gelling polysaccharides. The existing body of evidence allowed quantitative treatment of results based on the asymmetric distribution theory of molecular relaxation time that identifies the chemical fingerprint of the local motions operating at the vicinity of Tg. Furthermore, the diffusional mobility of a bioactive compound within a glassy matrix could be followed in relation to temperature induced changes in free volume using the time–temperature superposition principle.