The role of phase separation in determining the glass transition behaviour of thermally aggregated protein-based thermoplastics

The glass transition behaviour of a highly aggregated protein-based material (Novatein) was studied using dynamic mechanical analysis. Novatein was plasticized with up to 40 parts of tri(ethylene glycol) and glycerol and equilibrated at four different relative humidities. Phase separation was found to play a significant role in determining material properties due to the low saturation point of Novatein. The Couchman-Karasz model was used to determine the composition of each phase and tri(ethylene glycol) led to a greater proportion of an intermediate T-g phase compared to glycerol. The role of the intermediate phase was crucial in terms of total plasticization where water is needed for good compatibility between the plasticizer and protein. The constraint theory provided a unifying model for understanding phase behaviour, assuming the different phases are ideally mixed. Using the point of equivalence approach differentiated between primary and secondary plasticization, representing plasticization based on plasticizer content alone.