Thermal transitions and non-isothermal crystallization kinetics of thermoplastic starch: amylose-driven ordering...

The thermal and crystallization behavior of thermoplastic starch (TPS) is critical for determining processability and final material properties. However, the influence of compositional variability and processing conditions remains insufficiently understood. In particular, variability in amylose-amylopectin ratio and the presence of non-starch components influence molecular ordering and crystallization. This study investigated how amylose content and matrix complexity affect thermal transitions and non-isothermal crystallization behavior of TPS systems. TPS materials were prepared from purified rice starch, rice dust, and colored rice, covering increasing amylose content (15.8%-29.5%) and non-starch components. Heating and cooling rates (2.5 °C/min-10.0 °C/min) were evaluated using differential scanning calorimetry. During heating, higher amylose content was associated with increased cold crystallization and melting enthalpies, while these effects appeared to be attenuated by non-starch components. Two distinct cold crystallization events were consistently observed across all TPS formulations, suggesting multi-stage crystallization. During cooling, crystallization enthalpy increase was correlated with amylose content and negatively correlated with matrix complexity, while crystallization temperature decreased with increasing cooling rate, inversely to crystallization enthalpy. Kinetic modeling (Avrami, Tobin, Kissinger) pointed to heterogeneous crystallization and three-dimensional crystal growth, with activation energy increasing with matrix chemical complexity. VH-type crystallinity and spherulitic morphologies were further confirmed by X-ray diffraction and microscopy. Overall, amylose content appears to be strongly associated with crystallization behavior, while non-starch components appear to modulate crystallization kinetics, providing insight for TPS processing and formulation design.