Chitosan-based nanocomposites: preparation and characterization for food packing industry

In the present work, Cerium (IV)-Zirconium (IV) oxide nanoparticles (CeO4ZrNPs) was successfully dispersed into Chitosan/15Gelatin nanocomposites with different quantities. The obtained chitosan-based nanocomposites represented remarkable improvements in structural, morphological, mechanical, and thermal properties. Roughness increased from 74 nm to 6.4 nm, Young’s Modulus enhanced from 1.36 GPa to 2.99 GPa. The influence of dispersed CeO4ZrNPs contents on the phase transition temperature (T g) and the non-isothermal degradation processes of chitosan-based nanocomposites were examined using Differential Scanning Galorimetry (DSC) with different heating rates. Kinetic parameters of the thermal degradation for chitosan-based nanocomposites were evaluated using Kissinger-Akahira-Sunose (KAS) and Kissenger (KIS) procedures. Chitosan-based nanocomposites showed an increase in the thermal degradation temperature with higher activation energies, indicating improved thermal stability. Thermal analysis demonstrated that chitosan-based nanocomposites became more ordered by increasing CeO4ZrNPs as inferred from the negative entropy increase. Moreover, the degradation of chitosan-based nanocomposites has been described as a non-spontaneous process. The resulting information is particularly important in applications in which there is a need to obtain chitosan nanocomposites with improved mechanical and thermal properties such as food packing industry.


Introduction
Chitosan and its derivatives are a good choice as a polymeric matrix owing to their use in novel applications related to food science and packing technology [1]. The main requirements for polymeric composites for use in the packaging industry can be summarized as follows; They should have a good barrier to light, water vapor and oxygen, good mechanical performance and adequate thermal stability so that manufactured products made of polymeric composites will be worthy of participating in the packaging market. The necessity of using polymeric composites with adequate mechanical properties is related to maintaining food quality; And their ability to endure any kind of external force or stress that may occur at various stages of manufacturing, subsequent shipping, handling, and storage of prepackaged food [2]. The most common mechanical parameters governing mechanical characterizations of polymeric composites are related to modulus of elasticity, Young's modulus, tensile strength, and elongation of the fracture [3][4][5]. In addition, the thermal property of polymeric composites is a major factor in the packaging industry; Means how strong it is against potential thermal shocks to withstand [51][52][53][54][55][56][57][58][59][60][61][62][63][64][65][66]. For mechanical parameters enhancements of chitosan, various kinds of nanoparticles are incorporated, such as nanoclay [67,68], nanohydroxyapatite [69], nanometal oxides [70][71][72][73], and graphene oxide (GO) [74,75].
One of the trickiest challenges in the polymer industries and applications is its thermal stability. There are a considerable variety of factors with several effects make it difficult to study the mechanism and kinetics of reactions, such as specific area and porosity, layer thickness, formation and growth of new crystallization nuclei, reconstruction of solid state crystal lattice, amount and distribution of the active centers on solid state surface, diffusion of gaseous reagents or reaction products materials heat conductance and static or dynamic character of the environment, etc Nowadays, DTA (differential thermal analysis), TG (thermogravimetry) and DSC (differential scanning calorimetry) have been used successfully for characterizing the transformation processes of precipitation of solids during isothermal processes or non-isothermal heating [76][77][78][79][80][81]. The regular kinetic analysis is performed on the experimental data to provide mathematical description of the thermal processes with the characteristic kinetic triplet i.e., E a , A, and g(α). This information can be directly applied for fabrication various metals and alloys, ceramics glasses, cement, natural polymers and composite materials [82].
In this work, for first time, Cerium (IV)-Zirconium (IV) oxide (as a solid solution) nanoparticles (CeO 4 ZrNPs) was incorporated into Chitosan/15 wt.% Gelatin nanocomposite with different quantities. Structural, morphological, mechanical, and thermal properties of the obtained chitosan-based nanocomposites are presented. Moreover, phase transition temperature (T g ) and non-isothermal degradation processes were examined using Differential Scanning Galorimetry (DSC) with different heating rates. Kinetic parameters were evaluated using Kissinger-Akahira-Sunose (KAS) and Kissenger (KIS) procedures.

Materials and methods
Chitosan powder of average molecular weight (100, 000-300, 000) and molecular formula (C 6 H 11 NO 4 ) n . Gelatin type A were obtained from ACROS Organics. Cerium (IV)-zirconium (IV) oxide of molecular formula (CeO 4 Zr) and particle size of less than 50 nm was obtained from Sigma-Aldrich. A mixture of 50 ml distilled water, 2 % acetic acid and the desired amount of chitosan were magnetically stirred at 25°C for 3 h. The prepared chitosan solution was added to a previously prepared gelatin solution. Different amounts of x (wt.) % of CeO 4 Zr NPs (x=0, 1, 3, 5, 10, 20 and 30) were sonicated for 2 h in 10 ml distilled water, then later on slowly added into the obtained solution and stirred for 1 h. The resulted solutions were distributed into leveled hydrophobic polystyrene Petri dishes (10 cm diameter). To get the desired films, the solution was left to dry for 24 h at 40°C. Films were finally peeled off from the trays and placed in sealed containers to avoid moisture exchange. The morphology of CeO 4 ZrNPs was investigated using HRTEM (JEM 21OO HRT made in Japan, with accelerating voltage = 200 KV and resolution of 0.2 nm). The chitosan based nanocomposites (CS, and x-CG, x=0, 3, and 10) were investigated using spectrophotometer in the spectral range 400-4000 cm −1 , with a resolution of 4 cm −1 (JASCO, FTIR-300 E.), optical microscopy equipped with a camera, Atomic Force Microscopy AFM (Dimension ® Edge TM, Bruker) under tapping mode operation, Scanning Electron Microscope SEM (JEOL-JSM-T330) operated at 25 kV. Young's modulus and strain % were estimated using calibrated bench top tensile test setup attached with direct controlled drive linear motor XY stage and tension force sensor (type K-100 of measuring range of 1-100 KN, Lorenz Messtechnik GmbH) with strain rate of 10 mm min −1 . DSC measurements were carried out by NETZCH DSC 204 f1 phoenix. Samples (CS, and x-CG, x=0, 3, and 10) of about 4±0.1 mg mass were used for the experiments varied out at heating rates of 10°C, 20°C, 30°C and 40°C min −1 up to 500°C under N 2 at a flow rate of 50 ml min −1 . The samples were loaded without pressing into an open of 6 mm diameter and 3 mm high purity platinum crucible using an empty platinum crucible as a reference. The DSC curves were recorded simultaneously with 0.1 mg sensitivity. Due to the potential effect of moisture content in determining the T g of biopolymers, DSC data of each film were collected during the second heating run from 30 to 200°C at a rate of 20°C min −1 , after the first run of heating up to 110°C and cooling to 30°C at the same rate of 10°C min −1 . Dry nitrogen was used as the purge gas at a flow rate of 50 ml min −1 .

Characterizations
The Particles size range of incorporated CeO 4 Zr NPs was in nm scale, as deduced by broadening x-ray diffraction pattern and TEM micrograph, showing that the particle size are in range of 50 nm (figure 1).    formation of intermolecular hydrogen bonds between chitosan and gelatin. Additionally, the observed shift of the amide-II absorbed bands of the chitosan film from 1536 to 1530 cm −1 indicated to the presence of electrostatic interactions between the amino groups of chitosan and the carboxyl groups of gelatins. This also indicates to the miscibility between chitosan and gelatin in polymer blends which assigned to a specific interaction between the different components. Referring to the obtained results of x=3 and 10 CG nanocomposites, shown in figure 3 and table 1, a significant shift of the CG absorption bands (O-H, N-H, 3213 cm −1 ) to higher wavenumbers (3268 cm −1 ) was observed. However, in case of CG-10, this absorption band was returned back to lower value (3198 cm −1 ) close to what was observed for CS sample. This suggests that, (OH) and (NH) groups of x=3 CG nanocomposite are well involved to the coordination of the of CeO 4 Zr ions. However, in case of x=10 CG nanocomposite, the huge amount of the CeO 4 ZrNPs was easy to be accumulated, losing the chance of interaction with the functional groups of CG nanocomposite. The last curve shown in figure 3 represents the FTIR spectra of the CeO 4 ZrNPs and shows the absorbed bands centered at 3426, 1632, 1552 and 1061 cm −1 , which can be attributed to the stretchy vibration of the hydroxyl groups [84], the vibration of the connections Zr-O [85], Ce-O [86] and Zr-O respectively. Figure 4 represents AFM images of x=0, 10, 20 and 30 CG nanocomposites. The root mean square roughness (RMS) and particle size of the matrix (nm) were estimated and collected as shown in table 2. It is clear that, by increasing the incorporated CeO 4 ZrNPs contents in the CG nanocomposites, the roughness decreased from 74.8 nm for CG to 6.1 nm for x=30 CG nanocomposite, owing to the accumulated CeO 4 Zr NPs on the particles of the CG matrix and filling the cavities on the surface (figure 5). The average particles size was estimated as 44 nm as shown in table 2. This reflects the influence of the CeO 4 Zr NPs on the decreasing of the roughness of the CG nanocomposite and improving the surface morphology by making it much smooth. Figure 6 represents tensile stress against strain for x% CeO 4 ZrNPs/CG, x=0, 1, 5, 10, 20 and 30 nanocomposites thin films. The extracted mechanical parameters are estimated and collected in table 3. It is obvious that, nanocomposites thin films exhibited an increasing of bearing resistance force against the tensile force as incorporated CeO 4 Zr NPs contents increased; strength (MPa) and young's modulus (GPa) increased     4.1 for x=20 and 30 CG nanocomposites. This improvement of mechanical properties has been observed even for high concentration of CeO 4 ZrNPs (30%), which is referring to the well dispersion of the metal-oxide nanoparticles through the host material. The mechanical properties for the obtained chitosan-based nanocomposites were greatly increased as shown in figure 6 and table 3, which is attributed to the beneficial effects of the dispersed CeO 4 ZrNPs to produce a strong interfacial bonding with high ability to transfer the mechanical loads from the matrix to CeO 4 ZrNPs network [69][70][71]. A similarly large increase in mechanical properties for chitosan based nanocomposite has been reported previously, and has been connected to the enhanced stiffness of polymer chains, owing to the reinforce effect of the nano-filler [87]. Figure 7(a) shows the DSC data of CS at heating rate of 30°C min −1 . The glass transition temperature of CS was estimated as 121°C. Figure7(b) represents DSC curves of CS at different heating rates (b=10°C, 20°C, 30°C, 40°C min −1 ) in non-isothermal conditions. These curves showed an endothermic peak (peak 1) corresponding to the dehydration of water molecules followed by a crystalline melting endothermic peak (peak 2). The third one was assigned as an exothermic peak (peak 3) ascribed to the thermal degradation (first stage of the chitosan degradation), including dehydration of the saccharide rings, de-polymerization, and decomposition of the acetylated and de-acetylated units of chitosan [88,89]. Increasing of heating rates leads to an increasing of amplitude of the thermal peaks with peak position shift to higher temperature. It is clear that the characteristic temperature of the thermal degradation is a heating rate dependent and increase linearly. Following Kissinger-Akahira-Sunose (KAS) [90], the apparent activation energy (E a ) and the other kinetic parameters of the thermal   E a of CS was estimated as 112.5 KJ mol −1 . Moreover, the partial integrations of the first thermal degradation peak as a function of temperature, at different heating rates, are estimated as an evaluation of the thermal degradation fraction (a%) change with increasing of temperature ( figure 8(a)). These enabled us to estimate E a  as a function of a % ( figure 8(b)). At certain a% (from 10% to 90% step 10%), the estimated activation energies of the degradation process show a decrease from 172 KJ mol

Thermal analysis of pure chitosan
The linear regression of the least square's method is used for different algebraic expressions of g (a) function, looking for the straight line with a slope equal to −1.000, for which the linear correlation coefficient R 2 was close to unity. Consequently, the most probable mechanism function g (a) was determined as [-ln (1-a) 2/3 ], which is related to the random nucleation mechanism with subsequent growth, n=1.5. The pre-exponential factor A is then evaluated from equation (1) and listed in table 4. It is obvious that, A decreases by increasing of a% with average value 2.36 * 10 13 s −1 . These values of activation energy and pre-exponential factor A are in good agreement with earlier reported ones [97,98]. Knowing of E and A values following KAS procedures, the change of entropy ΔS # , enthalpy ΔH # , and the Gibbs free energy ΔG # for the formation of the activated complex from the reagent are evaluated and listed in table 4. Figure 9(a) represent DSC signals of x=0, 3 and 10 CG nanocomposites prior degradation stage. It is observed that, peaks (1 & 2) are shifted to higher temperatures for x=3 CG nanocomposite, while they shifted to lower temperature in x=10 CG nanocomposite. Regarding to peak (3), which is corresponding to the first thermal degradation process, its centered temperature is shifted to higher values for x=3 and 10 CG nanocomposites as shown in figure 9(b). Figure9(c) represents T g of x=0, 3 and 10 CG nanocomposites. T g is enhanced from 121°C for CS (as shown previously) to 141°C for CG(x = 0), referring to the effective role of 15 wt.% gelatin added to chitosan resulting in thermal stability improvement. T g showed further enhancing to 152.7°C and 159.2°C, for x=3 and 10 CG nanocomposites, respectively. Figures 9(d)-(f) represents the effect of heating rates on peak (3). It is obvious that, the peak position is shifted to higher temperature with higher amplitude, as heating rate increases. These data were used to follow the activation energy of nanocomposites as increasing of CeO 4 ZrNPs contents. Figure 10 represents Kissinger plot of the peak (3). The estimated E g were 202.86, 166.1 and 141.58 KJ mole −1 for x=0, 3 and 10 CG nanocomposites, respectively. Figure 11 represents the effect of different heating rates on the behavior of peaks (1&2) for x=0, 3, 10 CG nanocomposites. By increasing of the heating rate, the peaks amplitude increased and their characterized temperature are shifted to higher values. Moreover, the thermal phase transition at T g becomes more observable (especially for heating rate=30 and 40°C for x = 10 CG nanocomposite), indicating the effect of 10 wt% of CeO 4 Zr NPs contents on the glass-crystal phase transition process. Figure 12 shows the decomposition fraction percent (α%), the corresponding Avrami plots and the estimated Avrami numbers (growth exponent) with temperature for x=0, 3 and 10 CG nanocomposites. It was found that the growth exponent values as a function of temperature and its ranges are (α %) and x contents dependent. It decreases in different ranges of 1.45-0.6, 2.07-1.06, and 1.95-0.41 as temperature increases for x=0, 3 and 10 CG Table 4. Activation energy E a , pre-exponential factor A, change of entropy ΔS ≠ , enthalpy ΔH ≠ , and Gibbs free energy ΔG ≠ at different thermal degradation fraction (α %) for chitosan, as following KAS procedure. nanocomposites, respectively. Figure 13 represents the activation energy of the first thermal degradation of the nanocomposites as the line slopes of Ln (b/T α 2 ) versus (1000/T α ) at different (α). Figure 14 shows how the activation energy of the first thermal degradation process for the nanocomposites of different compositions varies with the conversion degree (α). It is clear that E a against α for x=0, 3 and 10 nanocomposites behaved with different ways. E a increases with increasing of α for x=0 and 10 nanocomposites, while it decreases for pure CS (as mentioned before) and as well for x=3 CG nanocomposite. Figure 14   In General, the thermal analysis confirmed the impact of CeO 4 ZrNPs contents on the thermal properties of CG nanocomposites. T g was shifted to higher temperature referring to the enhancement of thermal stability of the chitosan based nanocomposites. CG(x = 0) nanocomposite showed the highest value of E a . Moreover, E a and n exhibited (α) dependent for all studied samples. CG(x = 0) showed the highest value of A which connected  with the estimated positive value of ΔS # , while ΔS # of the rest of compositions were negative. The ΔS # negativity (as the average value) increased by the increase of the incorporated CeO 4 ZrNPs contents (table 5). This confirmed that the structure of the host material becomes more order with CeO 4 ZrNPs incorporations. It is obvious that, the ΔH # values are all positive indicating that the involved reaction related to the degradation process is a thermodynamically unfavorable endothermic process. In addition, ΔG is almost constant with increasing of (α) for compositions. Moreover, it is noticed that all of E a , ΔH # and ΔG # values are positive. Therefore, the first degradation of CG nanocomposites could be described as a non-spontaneous process.

Conclusions
It was concluded that incorporating of CeO 4 ZrNPs significantly improved the structural, morphological, mechanical, and thermal properties of the chitosan-based nanocomposites. Structure analysis represented a well incorporation of the CeO 4 ZrNPs as inferred by FTIR, SEM, and OM. AFM analysis identified a significant enhancement of surface roughness. Young's modulus and strain% were significantly improved (from 1.36 GPa and 1.8 to 2.99 GPa and 4.1 for x=0 and 30, respectively). In addition, the glass transition temperature was shifted to higher temperatures (from 121°C for CS to 159 (x = 10). The activation energy was enhanced from 125.55 KJ mol −1 (CS) to 202.86 KJ mol −1 (CG). The host material structure became more ordered with the increase of CeO 4 ZrNPs contents, as inferred from the increase in the negativity of ΔS # . The estimated kinetic parameters confirmed that the first degradation of CG nanocomposites can be characterized as a nonspontaneous process. The resulting information indicated the importance of CeO 4 Zr nanoparticles dispersion to enhance the physical properties of chitosan to be amenable to various applications such as food packaging industry.