Glass-transition behaviour of plasticized starch biopolymer system – A modified Gordon–Taylor approach

Two plasticizers namely, glycerol and xylitol, based on their similar molecular size (˜6.3 Å) but different molecular weights (Glycerol-92; Xylitol-152) were selected for studying the glass-transition behaviour (rubber like behaviour) in multi-plasticized starch biopolymer with about 70% amylopectin structure. In the calorimetry measurements, glass-transition temperatures (onset temperature for bulk viscous flow) of plasticized samples were higher than non-plasticized samples at low water activities, thus showing typical antiplasticization behaviour. However, when plasticizer concentration was increased up to 15% and 20% wt, all plasticized samples showed significant reduction in glass-transition temperature. We used a modified Gordon–Taylor model to understand the competitive plasticization of glycerol and xylitol in presence of water, and suggest that competitive plasticization exists and occurs at a threshold amount of matrix free water content, due to strong three-way interactions: starch–plasticizer, plasticizer–plasticizer/water and starch–water. This competitive interaction is significant in determining the onset temperature for viscous flow behaviour; at higher matrix water content, the Gordon–Taylor constant was relatively unaffected by the plasticizer amount, and water was the dominant plasticizer. A new interaction parameter that separates the starch–plasticizer interaction in a starch–plasticizer–water system is also discussed.

For a hydrophilic ternary system of starch, plasticizer and water, it is suggested here that competitive plasticization exists due to the strong interactions between water and the low molecular weight plasticizers (glycerol or xylitol), which can be interpreted as 3-way interactions: starch–plasticizer, plasticizer–plasticizer/water and starch–water. A modified Gordon–Taylor model was used to calculate interaction parameters for plasticizers and water, and it is shown here that the competitive interaction between plasticizers and free water is achieved only after a ‘threshold’ moisture value within matrix. This competitive interaction is significant in determining the Tg value and the equilibrium moisture content of the starch biopolymer. If Tg is to be understood from a free volume concept, this is an interesting finding because it highlights that the strong interactions between starch–water increases while plasticizer–starch decreases such that the matrix free volume reaches an optimum point corresponding to about 11% equilibrium moisture content. It is shown here that further increases in free water or plasticizer had minimal effect of the interaction within this ternary system as corroborated by insignificant changes in Tg data in those ranges.Figure optionsDownload as PowerPoint slide