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In this study, nanocrystalline celluloses were used to enhance physical, mechanical and water vapor barrier properties of chitosan films for potential food packaging applications. Two different mineral acids (sulfuric and phosphoric) were used to extract nanocrystalline cellulose from date palm sheath fibers. The influence of cellulose I and cellulose II on the properties of the isolated nanocrystalline celluloses (e.g., yield, energy and length of intra-and intermolecular hydrogen bonds, and degree of substitution) were studied too. The characteristics of chitosan biocomposite film with phosphorylated nanocrystalline cellulose were compared to those with sulfated nanocrystalline cellulose. Results showed that besides cellulose polymorphism, the ionic ester groups on the surface of nanocrystalline cellulose is one of the factors influencing the physical, chemical, mechanical, and water vapor barrier properties in chitosan/nanocrystalline cellulose biocomposites. Abbreviations:CI: Cellulose I CII: Cellulose II CS: Chitosan DPS: Date palm sheath NCC: Nanocrystalline cellulose PNCC: Phosphorylated nanocrystalline cellulose SNCC: Sulfated nanocrystalline cellulose Preparation of Sulfated and Phosphorylated Nanocrystalline Cellulose (SNCC & PNCC)Before NCCs extraction, DPS fibers were subjected to successive purification steps of pulping, bleaching and mercerizing treatments to extract CI and CII fibers [21]. For SNCCs preparation, 1 g of CI and CII fibers were added to a preheated 6.5 M sulfuric acid solution (fiber to liquor ratio of 1:20) for 1.45 h at 50°C under strong agitation [22]. PNCCs were prepared by soaking 1 g of CI and CII fibers in distilled water for 15 min, and then vigorously blended with an electrically powered hand blender, until a pulp like slurry was obtained. Phosphoric acid (85% v/v) was slowly added (keeping the temperature below 25°C) until a phosphoric acid concentration of 10.7 M was reached. Then, the reaction vessel was placed in a preheated oil bath at 90°C, and the mixture stirred for 30 min [8]. The produced SNCC and PNCC fractions were continuously washed by the addition of distilled water and centrifuged until the solution conductivity reached ≈ 5 μ S cm -1 (pH 4). The SNCC and PNCC colloidal suspension were dialyzed against distilled water for 3-5 days, till reaching pH ≈ 7. The final SNCC and PNCC suspensions were subsequently undergone ultrasonic treatment for 30 min (Falc Instruments, LSB2, 59 kHz) in an ice bath.All NCC suspensions prepared were filtered through sintered glass funnel No 1º before being lyophilized. The yields of the prepared NCCs were calculated as a percentage of the weight of the final products divided by the initial weights of the starting material (i.e., CI or CII DPS fibers). Tab. 1 shows yield % of NCCs obtained by sulfuric acid and phosphoric acid hydrolyses. Preparation of CS Biocomposite FilmsThe biocomposite films were prepared by the solution casting method. CS solution was prepared by mechanically stirring 2% CS (wt/v) in 1% acetic acid solution (v/v...