Effect water of content on viscoelastic properties of amorphous potato starch: DMA measurement

Due to its mechanical properties and the absence of endogenous lipids, amorphous potato starch is offering promising prospects as shape memory material despite its sensitivity to water [1]. Heating at a temperature higher than the glass transition temperature Tg is a stimuli of shape recovery of prestressed amorphous starches. Also, the absorption of water as a plasticizer of the starch, decreases the Tg, thereby promoting shape recovery behaviour. The linear viscoelastic behaviour of amorphous potato starch obtained by extrusion isinvestigated by dynamic thermomechanical analysis (DMA) in function of the temperature and moisture content with the aim to improve the knowledge of its rheological properties [2]. The extrudates were equilibrated for 3 weeks under different relative humidity to achieve levels of water content between 9.3 and 17.2% w.b. The samples were then covered with a thin layer of silicone grease to minimize dehydration during the DMA analysis which were performed using traction-compression mode, under a strain amplitude of 0.05% and simultaneous sweep of frequency (0.3 - 40 Hz) and temperature (20 - 135 ° C), with a heating rate of 3 ° C / min. Water loss was evaluated by performing a thermogravimetric analysis and via Tg measured by DSC. The storage and loss modulus (E '(ω) and E'' (ω)) were obtained as a function of the current water content in the range of studied temperatures. The isotherms of E’ and E’’ were obtained by curve fitting and interpolation of the data. The principle of time-temperature superposition [3], using the translation factor aT(T) allowed us to obtain master curves of E '(ω) and E'' (ω). These results were adjusted by generalized Maxwell model to calculate relaxation modulus E(t) at the reference temperature of 50 ° C for discrete values of water content. Relaxation times τi of 0.1, 1, 10, 102, 103, 104 and 105 s make possible a numerical simulation in the range 0.01 and 105 s. The obtained translation factor aT(T) between (10-9 - 108) allows us to compute E(t) at any temperature in the range studied and, by interpolation, its variation as a function of water content. The determination of visco-elastic parameters of the amorphous starch will contribute to the modelling of its thermo-mechanical behaviour and promote the shape memory programming and applications.