Pullulan Fatty Ester Nanocoatings with Tunable Surface Wettability and Bio-Inert Surface Properties

Polysaccharide derivatives with tunable hydrophobicity offer a versatile platform for tailoring material properties through controlled chemical modification. In this study, pullulan was functionalized with saturated fatty acids (C14-C18) via N,N-carbonyldiimidazole-mediated esterification, yielding degrees of substitution between 0.24 and 0.77. The effect of alkyl chain length and substitution levels on the thermal, structural, and surface properties of the resulting pullulan esters was systematically investigated. Increasing chain length enhanced thermal stability and promoted structural ordering, with stearate-modified pullulan exhibiting melting transitions at 40-42 °C and higher degradation temperatures (≥312 °C). Spin-coated nanoscale coatings (110-150 nm thick) exhibited extremely smooth surfaces (S_a < 1 nm), although differences in solubility influenced solvent evaporation and induced nanostructural variations. Surface wettability increased systematically with alkyl chain length and degree of substitution, resulting in water contact angles between 85° and 105°. All coatings were non-cytotoxic and inhibited bacterial growth at the interface without leaching, suggesting a contact-dependent antibacterial effect. These results establish clear structure-property relationships in pullulan esters, demonstrating that surface wettability, structural ordering, and surface functionality can be systematically adjusted via alkyl chain length and degree of substitution.