Bacterial cellulose nanocrystals with a great difference in aspect ratios: A comparison study of their reinforcing effects on properties of the sodium alginate film

Cellulose nanocrystal (CNC) incorporation is a practical way to improve the properties of polysaccharide films, and the aspect ratio (L/D) of CNC is a crucial morphological factor affecting its reinforcing effects. In this study, the changes in properties of sodium alginate (SA) films with bacterial CNCs (BCNCs) of great L/D differences were tested and the Young's modulus (E) results of films were compared with predictions from the Halpin–Kardos and Ouali models. At BCNC concentrations ≤0.5 wt%, BCNCs with L/D of 91.45 (BCNC91) and 130.31 (BCNC130) were more effective in reinforcing E than BCNC with L/D of 41.04 (BCNC41). Because AFM and FTIR results confirmed that at this stage, all three BCNCs were well-dispersed, but BCNC91 and BCNC130 formed more new hydrogen bonds with SA than BCNC41. Further increasing BCNC concentrations to 1 wt%–3 wt% continued to enhance E but all BCNCs' reinforcing efficiencies decreased in sync with the formation of BCNC percolation networks and aggregations until efficiency disparities of three BCNCs diminished with all SA-BCNC films' E at around 1800 MPa. The fact that the Halpin–Kardos model described E better than the filler-network-based Ouali model, but still seriously underestimated E at BCNC concentrations ≤3 wt%, was attributed to the greater interfacial area effect produced by nanosized BCNCs than that of the macro-fibers used to develop the Halpin–Kardos model and emphasized the importance of SA-BCNC interactions. Moreover, BCNCs with higher L/D required lower concentrations to significantly reduce films' water vapor permeabilities, and nanocomposites with BCNC130 were more transparent but less UV-light resistant than others.