Competing roles of aggregation and interfacial interactions in sustainable protein/cellulose nanocrystal-reinforced soft composites

Renewable protein matrix nanocomposites reinforced with high-aspect-ratio cellulose nanocrystals (CNCs) offer promising alternatives to petroleum-based plastics. However, they exhibit mechanical properties far below theoretical predictions, often approaching the Hashin–Shtrikman lower bound despite filler geometries that should approach upper-bound behavior. This discrepancy suggests that microstructural features not captured in standard homogenization approaches dominate the mechanical response. We develop a hierarchical Mori–Tanaka framework that accounts for two competing effects: CNC agglomeration, which diminishes load transfer, and interphase stiffening at CNC–matrix interfaces, which enhances it. Applying this model to soy protein isolate (SPI) composites with unmodified and polydopamine-modified CNCs, we demonstrate attenuation of the high agglomeration inherent to SPI/CNC composites without diminishing favorable interfacial effects. Phase maps reveal conditions that could shift composite performance toward the upper bound, making SPI/CNC bio-nanocomposites a potential sustainable alternative to petroleum-based plastics.