Homogeneous covalent internal plasticization of high-amylose starch via post-gelatinization hydroxypropylation: Critical substitution threshold for ductility and anti-retrogradation.

High-amylose starch (HAS) films exhibit high mechanical strength but suffer from intrinsic brittleness and retrogradation-induced instability due to strong intermolecular hydrogen bonding. Conventional hydroxypropylation, performed in the granule state, yields low and non-uniform substitution that inadequately plasticizes the dense amylose network. Here, we report a homogeneous covalent internal plasticization strategy achieved by hydroxypropylating fully gelatinized (molecularly dispersed) HAS. This approach enables efficient and uniform introduction of hydroxypropyl groups with 0.18-2.11 M substitution (MS) and ≥ 75% reaction efficiency. A critical transition from gel-like to liquid-like rheological behavior was identified at MS ≈ 0.67. At this substitution level, the elongation at break increased dramatically from 6.12% (unmodified) to 29.6%, demonstrating effective covalent internal plasticization that weakens intermolecular association. DMTA, XRD, and LF-NMR analyses revealed enhanced chain mobility, suppressed retrogradation, and a shift from semi-bound-water-dominated to bound-water-rich networks. Remarkably, after 180 days of storage, modified films (MS ≥ 0.67) retained their mechanical properties and amorphous structure, whereas unmodified films showed severe embrittlement and increased crystallinity. These results establish homogeneous hydroxypropylation as a powerful covalent internal plasticization method that simultaneously improves flexibility, suppresses long-term retrogradation, and enhances structural stability of HAS films, overcoming key limitations of both external plasticizers and heterogeneous chemical modification.