Structure–Property Relationships of Hydroxypropyl Methylcellulose Films Reinforced with Cellulose Nanocrystals Enriched in Cellulose I and Cellulose II from Different Agricultural Residues

Abstract Converting agricultural waste into cellulose nanocrystals provides a sustainable route for producing biodegradable packaging materials. This study investigated how the crystalline polymorphism of CNCs from different agricultural residues influences the structure–property relationships of hydroxypropyl methylcellulose (HPMC) nanocomposite films. Cellulose I-rich CNCs from durian rind (CNC I) and cellulose II-rich CNCs from sugarcane bagasse (CNC II) were directly compared under identical HPMC film-forming conditions at a fixed CNC loading to clarify their effects on film structure and performance. CNC I and CNC II were characterized by extraction yields of 41.8% and 32.7%, crystallinity indices of 37.4% and 53.1%, and aspect ratios of 18.3 and 10.5, respectively. Dynamic light scattering revealed bimodal apparent hydrodynamic diameters for CNC I (30.8 and 139.8 nm), whereas CNC II showed a single apparent diameter of 102.4 nm. These differences in crystalline structure, morphology, size distribution, aspect ratio, crystallinity, and aggregation behavior contributed to distinct mechanical, barrier, and water-stability properties of the resulting HPMC–CNC films. Compared with neat HPMC, HPMC–CNC I exhibited improved mechanical properties, with tensile strength and Young’s modulus increasing from 34.6 to 53.2 MPa and from 1097.9 to 3302.5 MPa, respectively. In contrast, HPMC–CNC II showed lower oxygen transmission rates of 76.0 and 142.4 cm3·m–2·day–1·bar–1 at 50% and 80% RH, respectively, together with reduced water solubility to 10.8–30.4%. These findings demonstrate that CNCs from agricultural residues can be strategically selected based on their source-dependent structural characteristics to tailor the mechanical, barrier, and water-stability properties of biodegradable HPMC films while supporting the value-added utilization of agricultural residues.