Unraveling the Effect of Citric Acid on Microstructure, Rheology, and Structural Recovery of Thermoplastic Potato Starch

摘要

The main objective of the present work is to explore the role of citric acid (CA) on microstructural changes and their impact on the melt rheological behavior, mechanical performance, and shape recovery of thermoplastic starch (TPS). The results of frequency sweep test show a pronounced nonterminal storage modulus (G') at lower frequencies (high melt elasticity) for TPS, which is found to be greatly reduced in favor of improving the processability through the addition of CA. This could be explained in terms of reduced disentanglements of amylopectin molecules, as a crucial parameter responsible for the 3D-type interconnected microstructure of TPS, through catalyzing effect of CA on hydrolysis. The results of the temperature sweep test (cooling mode) show a pronounced liquid-to-solid transition at similar to 136celcius for CA-modified TPS. This transition could be attributed to the temperature-driven significant strengthening of the formation of single helical amylose crystalline structure and/or the hydrogen bonds between hydrolyzed amylopectin chains at temperatures below 136celcius. The results are found to be consistent with the results obtained from the tensile and DMTA experiments. As expected, the shape recovery of the CA-modified TPS sample is lower than that of unmodified TPS because of the lower elastic stored energy (stiffness) in the presence of CA, which can be largely compensated by incorporating appropriate nanoparticles.