Mechanical Behavior of Thermoplastic Starch: Rationale for the Temperature-Relative Humidity Equivalence

Abstract

This paper aimed at understanding and rationalizing the influence of both temperature and relative humidity on the mechanical behavior of thermoplastic starch (TPS). DMA experiments revealed that water molecules impact the crosslinking network by reducing the intermolecular hydrogen bond density, resulting in a less dense entanglement network. In addition, the in-situ X-ray characterization during hydration of starch revealed structural changes, which were ascribed to conformational changes in the starch chain, due to their interaction with the uptake water molecules. Finally, the study of TPS uniaxially stretched at different temperatures and humidity showed that the mechanical behavior of TPS could be rationalized by considering the ΔT parameter, which corresponds to the temperature difference between the drawing temperature and the glass transition temperature of TPS.

Keywords: humidity; mechanical behavior; thermoplastic starch.

Figure 1

DSC thermograms of TPS stored at 20 °C and relative humidity of 58 and 89% RH recorded during heating at 10 °C/min.

Figure 2

Evolution of the storage modulus E’ and loss factor tanδ as a function of temperature for TPS58 and TPS89.

Figure 3

Evolution of the integrated intensity profile during re-hydration of a dry TPS samples stored at 89% RH, T = 20 °C.

Figure 4

Engineering stress-strain curves of TPS samples stretched (a) at different T_d and RH = 70% (b) at different relative humidity and T_d = 80 °C.

Figure 5

Engineering stress-strain curves of TPS samples stretched at different ΔT, the gap between the drawing temperature T_d and the glass transition temperature T_g.

Figure 6

Engineering stress-strain curves of TPS samples stretched at T_d = 55 °C and relative air humidity varying from 70 to 90% and corresponding WAXS patterns recorded on the post-stretched samples (the draw axis is horizontal).