Structure-Dynamics-Property Relationships in Silk Fibroin Films Probed via Solid-State NMR

Silk fibroin films offer tunable mechanical properties, yet the structural origins of this tunability are incompletely understood. Here, we show how solvent choice and post-treatment govern the mechanics of silk fibroin films by combining solid-state NMR relaxation measurements (¹³C T ₁/T _1ρ; ¹H T ₁/T _1ρ), X-ray diffraction, and TG-DTA analyses. Formic acid-derived films exhibited a 6-fold higher Silk II content and greater backbone constraint (longer ¹³C T _1ρ) than water-derived films, resulting in higher initial tensile strength. Methanol treatment had opposite effects on mechanical strength in formic acid-derived and water-derived films. In the former, it relaxed backbone constraints (shorter ¹³C T _1ρ) without changing the Silk II content, reducing strength. In the latter, it created nanoscale Silk II domains (<5 nm by ¹H T _1ρ), increasing strength. These findings demonstrate that the mechanical response of silk fibroin films is governed not solely by Silk II content but also by the interplay among domain size and chain dynamics.