Surface‐Engineered Nanofibrillated Bacterial Cellulose for Enhanced Interfacial Compatibility and Thermomechanical Performance in Poly(3‐Hydroxybutyrate‐ <i>co</i> ‐3‐Hydroxyhexanoate) Nanocomposites

ABSTRACT Nanofibrillated bacterial cellulose (NFBC), characterized by an ultrahigh aspect ratio (fiber length: 17 μm; fiber width: 10–15 nm; aspect ratio: &gt; 1000), is a promising green nanofiller for biodegradable polymers. This study employed click chemistry to surface‐modify hydroxypropyl‐cellulose‐functionalized NFBC (HP‐NFBC) via grafting with poly(3‐hydroxybutyrate‐ co ‐3‐hydroxyhexanoate) (PHBH), yielding HPNFBC‐ g ‐PHBH as a compatibilized nanofiller. The modified nanofiller was incorporated into a PHBH matrix via solvent casting, followed by systematic evaluation of its mechanical and thermomechanical properties. At loading of 10 wt% HPNFBC‐ g ‐PHBH (corresponding to 3.9 wt% cellulose), tensile strength increased by 12.4% (26.3 MPa) and Young's modulus improved by 17.5% (866 MPa) relative to those of neat PHBH. Furthermore, the coefficient of thermal expansion substantially decreased from 20.6 to 7.07 ppm K −1 , indicating enhanced dimensional stability at elevated temperatures. These results demonstrate that covalent grafting of PHBH onto NFBC effectively improves interfacial compatibility, enabling efficient stress transfer and thermal stability enhancement. This study provides a viable strategy for designing high‐performance, fully bio‐based polymer nanocomposites.