Results and discussion
3.1. CNC and Oxidized CNC. CNC isolated from rice straw were oxidized using TEMPO/NaOCl mixture; TEM images of the isolated nanocrystals are shown in Fig. 1. The width of the isolated nano crystals was in the range from 3–5 nm while the length was in the range from 80–120 nm;i.e., aspect ratio of 25. Presence of silica particles in the nano-size range was also noticed in the images. Acid-insoluble ash content of the dried cellulose nano crystals was 3.81%. Carboxylic content determination showed total carboxylic groups content of about 0.22 mmole/g. It should be pointed out that previous investigations showed that oxidation of cellulose nano crystals with TEMPO did not result in changing dimensions or cryrtallinity of the nanocrystals
3.2. HPC/OXCNC films Films from HPC and OXCNC were prepared and characterized to choose the optimum ratio of OXCNC regarding mechanical and physical properties of the films. The ratios of OXCNC ranged ranged Fig. 1. Cellulose nano crystals isolated from bleached rice straw pulp.from 2.5% to 20%. The properties of the films were evaluated in terms of tensile strength, water vapor permeability, dynamic mechanical thermal properties and crystallinity. In addition, the surface of OXCNC/HPC films was examined using scanning electron microscopy.Fig. 2 shows the scanning electron microscopy images of neat HPC (A) and 10% OXCNC filled HPC (B) at 16000 x magnification.The images showed no significant signs for agglomeration in the composites’ films; also no phase separation was observed. This indicates good distribution of OXCNC in the HPC matrix due to the hydrophilic character of OXCNC and HPC. From the figure, it is also clear that the presence of OXCNC ameliorated the smoothness of the surface and reduced the pores. Good dispersion of OXCNC in HPC and homogeneity of films reflect good adhesion between OXCNC and HPC. This in turn can improve mechanical properties of films.The effect of addition of OXCNC on the crystallinity of HPC was studied using X-ray diffraction since the change of crystallinity could result in change of other properties such as mechanical, thermomechanical, and water vapor permeability. Fig. 3 shows XRD patterns of OXCNC (A), HPC (B) and HPC filled with 10% (C) and 20% (D) OXCNC. XRD diffraction pattern of the OXCNC (Fig. 3A)showed two characteristic peaks at 2 values of about 16◦and 23◦Corresponding to the (101) and (002) crystallographic planes of the cellulose I crystal, respectively. On the other hand, neat HPC film showed two broad peaks at 2 values of about 9◦and 20◦(Fig. 3B); the broad diffraction peak at 2 = 20◦is attributed to the slightly ordered amorphous phase of HPC while the peak at 2 = 9◦is attributed to the backbone–backbone d- spacing of the main chains in the crystalline phase of HPC . It was noticed that increasing the amount of OXCNC resulted in increasing the intensity of the peak at about 2 = 9◦. Previous studies showed that the high crystallinity of CNC is based on the basic chemical structure of cellulose. Each monomer (glucose unit) bears three hydroxyl groups which have ability to initiate extensive intra- and intermolecularhydrogen bonding. Bonding among the cellulose chains allows the crystalline packing of cellulose chains into highly compact system(crystal structures) .This high crystallinity of CNC could impart significant hardening