Starch bio-nanocomposites based on phosphorylated and sulphated cellulose nanocrystals extracted from pepper plant residue: effect of surface functionality on property improvements

This work identifies Pepper (Capsicum annuum L) stems residue as a new potential source for producing high-quality cellulose nanocrystals (CNCs), making a significant contribution to the field of natural waste material utilization and sustainability. It also compares the properties of CNCs produced using two different acids, phosphoric acid and sulfuric acid (CNCPs and CNCSs), providing new insights into their properties and polymer reinforcing ability. Hence, the as-isolated CNCPs (D = 43 ± 15.11 nm, CrI = 84.78%) and CNCSs (D = 31 ± 8.80 nm, CrI = 85.41%) were separately dispersed in starch at different weight loadings, and their reinforcing effects on the chemical, morphological, thermal, transparency and mechanical properties of the resulting starch-based biocomposite films were discussed. Globally, the incorporation of CNCs into starch biopolymer improved all these properties due to their rather similar polysaccharide structure and thus good interfacial interaction. However, the capacity of phosphoric acid to produce char made CNCPs effective flame retardants, thus providing bionanocomposite films with better thermal stability than CNCSs ones. In counterpart, the negative charge provided by the residual sulfate group (SO 4 2− ) in CNCSs, coupled with their high aspect ratio, high surface areas and crystallinity, significantly improves the dispersibility of CNCSs in the ST matrix compared to the phosphoric acid-derived CNCs. Hence, adding up to 5 wt% CNCSs improved the film’s transparency by 25%, stiffness by 196%, and ductility by 169%, compared to 11%, 164% and 114%, respectively, for CNCPs. Here, this work offers valuable insights into the utilization of pepper stems residue and the type of acid for CNCs production with outstanding characteristics.