Stearate-Driven Lamellar Ordering and Divalent-Ion-Triggered Disassembly in Heat-Reprocessable Cationic Starch Ionic Complex Films

Starch is a sustainable and low-cost polymer, yet its poor mechanical properties and limited processability restrict broader engineering applications. This study investigates how stearate-based ionic complexation regulates supramolecular ordering, mechanical properties, water resistance, and ion-responsive disassembly in cationic starch materials. Cationic starch (CS, DS ≈ 0.8) was complexed with stearate to form the stearate-based ionic complex CS-S and blended with the corresponding laurate-based complex CS-L. CS-S exhibited low-angle XRD reflections at 2.57° and 4.88°, consistent with periodic lamellar ordering supported by DSC and SEM observations of ordered stearate-rich domains. Increasing CS-S content increased the Young’s modulus from 46.0 to 143.1 MPa while decreasing elongation at break from 99.8% to 4.35%, indicating a transition from ductile to brittle behavior. The films were also reprocessable by repeated compression molding, with CS-S-60 retaining 78.6% of its original tensile strength. CS-S showed lower swelling in deionized water (∼45%) than CS-L (∼80%). In contrast to monovalent salts, MgCl2 and CaCl2 induced pronounced disassembly. EDX detected ∼2 wt % retained Mg in washed CS-S after MgCl2 immersion, supporting Mg2+ association with stearate-containing domains. Despite increased ordering and water resistance, CS-S reached ∼65% BOD/ThOD after 28 days in seawater. These results demonstrate that stearate incorporation and blend composition effectively regulate structure, mechanical performance, water resistance, reprocessability, and ion-responsive disassembly in starch-based ionic complex films.