Magnetic field-induced alignment of starch molecules enhances the mechanical strength of starch films

Starch-based films suffer from brittleness and poor tensile properties, limiting their industrial applicability. In this study, magnetically modified starch (MMS) was prepared by covalently grafting azide-Fe₃O₄@SiO₂ nanoparticles onto the reducing ends of soluble starch via reductive amination and Cu(I)-catalyzed azide-alkyne cycloaddition (click chemistry). During film formation, application of a 1.4 T NdFeB magnetic field promoted chain alignment and crystallization. X-ray diffraction revealed a 19 % increase in crystallinity (from 9.6 % to 11.4 %), while FTIR analysis showed a 27 % enhancement in the (994 + 1047)/1022 ratio, indicating greater short-range molecular order. As a result, the tensile strength of MMS films increased to 19.8 ± 1.9 MPa, representing an 81.7 % improvement over native starch films (10.9 ± 1.4 MPa, p < 0.01). Meanwhile, the elongation at break decreased from 3.9 ± 0.8 % to 2.3 ± 0.3 %, reflecting enhanced molecular packing and reduced chain mobility. Superparamagnetic behavior ensured chain mobility only under field exposure, minimizing agglomeration after field removal. This work demonstrates a scalable, field-guided strategy to tailor starch film structure and performance, offering promising applications in eco-friendly packaging and biomedical materials.