Simplified extraction procedures (avoiding Soxhlet treatment and/or hemicellulose removal) were evaluated to valorize waste biomass from Posidonia oceanica leaves, obtaining cellulosic fractions and nanocrystals, which were subsequently used to produce films from their aqueous suspensions. Cellulose purification significantly improved mechanical and barrier properties of the films obtained from the fractions, while the extracted nanocrystals yielded
films with remarkably improved properties, outperforming most benchmark biopolymers. The lipids initially present in the fractions without Soxhlet treatment were not completely digested by the hydrolysis treatment, having a positive impact on the water vapor permeability of the films (up to 63% drop), although negatively impacting oxygen permeability (increased by 20–30-fold). On the contrary, some hemicelluloses present in the less purified fractions, strongly interacting with cellulose, remained in the extracted nanocrystals leading to enhanced mechanical properties (45% higher tensile strength and 2-fold increase in the elongation at break), but lower water barrier (up to 70% higher permeability than the pure cellulose nanocrystals) due to their hydrophilic character. Films produced from the less purified nanocrystals showed the best compromise between mechanical and barrier performance,
while offering a great advantage in terms of sustainability and reduced costs.