Synthesis and Characterization of PH-Responsive Starch Nicotinate via High-Pressure Solid-State Esterification

The development of bio-based polymers exhibiting pH-responsive behavior represents a promising strategy for designing functional materials with potential applications in controlled drug delivery. In this work, starch nicotinate was synthesized from native corn starch through a high-pressure solid-state esterification process using nicotinoyl chloride as the esterifying agent, without organic solvents or catalysts during the esterification step. The reaction was performed at 353 K and 80 kg cm-2 for 96 h, yielding a degree of substitution (DS) of 0.086 and a reaction efficiency (RE) of 8.6%, as determined by elemental nitrogen analysis. The resulting material was comprehensively characterized by Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), polarized light microscopy (PLM), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG). FT-IR confirmed the introduction of ester functionalities, while XRD revealed an almost complete disruption of the characteristic A-type crystalline structure of native corn starch. Consistently, PLM showed the disappearance of granular birefringence, whereas SEM demonstrated that the overall granular morphology remained largely preserved despite the pronounced loss of internal structural order. Thermal analysis indicated a reduction in thermal stability and an increase in residual mass following esterification. The water uptake behavior was investigated gravimetrically at 310 K in acidic (pH 1.2) and near-neutral (pH 6.8) media, using native starch as a reference. The modified starch exhibited enhanced water uptake (WU) under acidic conditions, suggesting that protonation of the introduced pyridine groups contributes to its pH-dependent hydration behavior. Overall, these findings demonstrate the feasibility of high-pressure solid-state nicotinoylation as a strategy for obtaining functional starch derivatives and highlight the potential of starch nicotinate as a bio-based pH-responsive material for future controlled drug delivery applications.