INTRODUCTION:

Since it is inexpensive, non-toxic and possesses potentially reactive amino functional groups, Chitosan has been widely used in the fields of medicine, food, cosmetics, agriculture, and wastewater treatment and so on (1-5). Because Chitosan has many functional groups, its modification by blending with other polymeric materials is expected to be useful for many applications. Blending of Chitosan with Carrageenan (AG) or with Hydroxyethyl cellulose (HEC) has been already reported in the literature. However, the study of CS/AG/HEC ternary blended system gel is still rare.

The combination of these three most widespread natural biopolymers (Chitosan, Carrageenan, and Cellulose), is of great interest from the standpoint of production of composite filaments, fibers, films, membranes, and other polymeric materials with a unique set of properties. In particular the combination of the sorption activity and hydrophobicity of the three polymers. The high mechanical resistance of Cellulose; gelling, thickening and stabilizing properties of the Carrageenan and the antibacterial properties of Chitosan makes it possible to substantially extend their fields of application.

The resemblance of chemical structures of macromolecules provides conditions for the kinetic compatibility of the polymers in the same solvent, thus preventing the negative consequence of phase separation on the physical and mechanical properties of composite materials. Lots of Chitosan blends and composites with other polymers have been proposed in the literature, for example, Chitosan/Cellulose membrane (6), Chitosan/ Starch, Chitosan/Collagen (7) and Chitosan/Gelatine (8). The compatibility of Chitosan with these polymers may be obtained since Chitosan is considered as a strongly interacting polymer. The most important property of Agar is its ability to form reversible gels simply by cooling hot aqueous solutions due to the formation of hydrogen bonds (9). Some of its hydrogel blends, such as PVP-Agar Hydrogel, have previously been prepared (10). Agar has also a wide variety of uses in industry. For example, it has been used in the food area (processed cheese, ice cream, bread, and soft candy) due to its ability to form hard gels at very low concentrations (11).

In this study, we report on the preparation of blended CS gels obtained by the physical solution mixing of Chitosan, Carrageenan, and Cellulose at different proportions. The rheological properties i.e. shearing viscosity and shear stress as a function of shear rate of the blend gels are investigated. And an optimum mixture ratio for a better gel is obtained by Experimental Design using a model of Response Surface Methodology (RSM). Characterization of the optimized gel is performed using Fourier Transform Infrared spectroscopy (FTIR), Thermal Gravimetric Analysis TGA and Diﬀerential Scanning Calorimetry DSC. The optimized blended CS gel viscoelastic properties are also investigated through rheological measurements.

Therefore, the present work is aimed to study of Chitosan/Carrageenan/Cellulose ternary gels with the view to further explore their properties and applications.

MATERIAL AND METHODS:

Materials:

Chitosan (CS) is prepared from shrimp Parapenaeus longirostris shells, as described in a previous study (12). Chitosan (CS) with a higher Degree of Deacetylation (DD) is obtained by alkaline treatment of Chitin. Carrageenan (AG) and Hydroxyethyl Cellulose (HEC) are purchased from the market. Homogenization of particle size of the three components (CS, AG and HEC) is performed at a diameter of 0.05mm.

Preparation of blended CS gels:

The preparation of blended gels of Chitosan, Carrageenan and Cellulose is carried out at various proportions. The ternary mixture aqueous solutions of Chitosan / Carrageenan / Hydroxyethyl Cellulose (CS/AG/HEC) at different weight ratios (considering Chitosan as the main component, i.e. ≥ 50%) are prepared by dissolving (0.1g) of the three pure component in (20 ml) of Acetic Acid (0.1M) followed by stirring for 15 min at 1400 r/min. The blended CS gel is prepared at room temperature with a pH in the range of 4. Air bubbles formed in the solution are eliminated by keeping the solutions at room temperature for 2 h. The aqueous Carrageenan and Cellulose solutions are added drop by drop to the Chitosan solution, under continuous stirring in various proportions. The range of the added Carrageenan/Cellulose to Chitosan solution is from 0-50% in weight. Stirring at 1400 r/min is allowed to continue for 30 minutes until a gel solution is obtained.

Experimental Design:

A Response Surfaces Design Method (RSM) is performed, using "Design experiment 8" version 8.0.6.1 (Stat-Ease Corporation software), in order to find an optimum mixture ratio for a better blended CS gel. Mixture design technique which is a specialized form of Response Surface methods (RSM) is well used to discover the optimal formulations (13). Factors that influencing the characteristics of the blended CS gels are viscosity Mu, storage modulus G' (Elasticity behaviour), pH and sol/gel blend ratio (the latter is visually examined).

Characterization:

The three components (Cs, AG and HEC), as well as the ternary blended Cs/AG/HEC gel sample prepared under optimum conditions, are characterized by Fourier Transform Infrared spectroscopy (Perkin Elmer FTIR 1600, USA), of spectral frequency ranging from 4000 to 400cm^-1. The FTIR spectroscopy was used to identify the chemical structure and possible interaction between the components. The obtained FTIR spectra for the three components (Cs, AG and HEC) as well as for the blended gel are shown in Fig. 3.

Thermodynamic properties of the blended Cs gel are studied using the Thermal Gravimetric Analysis (TGA) and Diﬀerential Scanning Calorimetry (DSC) techniques. Measurements are performed using a TGA/DSC of type (3⁺Store System, Mettler-Toledo GmbH, and Switzerland). The thermal contours of blended gel are analyzed in a temperature range of 50-500°C, at a heating rate scan of 10°C/min and under a nitrogen stream rate of 50ml/min.

The Rheological measurements are performed using a Dynamic Bohlin CVO-100-901 Rheometer from Malvern Company (UK), equipped with CP4/40 cone of 40 mm diameter (with a cone angle of 4°), at various temperatures and frequencies in the linear viscoelastic regime.

RESULTS AND DISCUSSION:

Experimental Design results:

The significant ANOVA statistical analysis results in an optimum ratio, with a desirability value of 0.968, (Fig. 1). Thus, gels derived under these optimized conditions with a significantly higher viscoelastic parameter, are found for a given composition of weight ratio equals 51/42/07 of CS/AG/HEC respectively. With these conditions, a sample of blended Cs gel is prepared and characterized.

Fig. 1. Desirability ramp for optimization.

FTIR Analysis results:

The obtained FTIR spectra for the three components Chitosan (CS), Carrageenan (AG), Hydroxyethyl Cellulose (HEC) and for the blended gel are presented in Fig. 2.

Analyses of CS blended gel FTIR spectrum show a broader band at 3100–3600 cm^-1 with stronger absorbance intensities, this corresponds to –OH stretching vibrations of water and hydroxyls as well as –NH stretching vibrations of free amino groups. The band that appears at 1652 cm^-1 is an amide I resulting from hydrogen and hydroxyl group interactions, this is due to the removal of the acetyl group. Moreover, a disappearance of the bands at 1411cm-1, 1380 cm-1 and at 1033 cm-1; that is obviously observed in the pure chitosan and cellulose as well as bands in the 1210-1260
cm–1 region of the pure Carrageenan (corresponding to the S=O of Sulphate Esters); is observed. The FTIR spectrum of CS/AG/HEC ternary blended system gel also showed an absorption band at 1625 cm-1 (though with lower absorbance values) which corresponds to –NH₃⁺ group (14). Thus, the FTIR spectroscopy of the blended gel comparing to those of each component (i.e. Cs, AG and HEC) show from chemical structures that there is an interaction between the three components.

Fig. 2. IR of Chitosan, Carrageenan, Hydroxyethyl Cellulose and their blended gel.

Thermal properties:

The thermo-gravimetric analysis was performed to assess the thermal properties of the blended Chitosan formulation. Fig.3,4 shows the thermal degradation, in a nitrogen atmosphere, of the blended Chitosan gel. Fig. 4 presents the relative weight loss (TG curves), together with the weight loss first derivative (DTG curves), as functions of temperature.

As shown in Fig. 3, the TGA of the blended CS gel showed a two-stage weight loss. The first ranged up to 60°C showing approximately 4% losses in weight. This is attributed to the vaporization of volatile components (absorbed and bound water). The second stage of weight loss began at about 70°C and continued up to 200°C, during this stage a faster thermal degradation and a major weight loss (more than 95%) is demonstrated. This is attributed to the decomposition of the blended gel. Due to the respective compositions of the blended Cs gel components, therefore fewer residues remain at 200°C and the total weight loss of the sample at about 500 °C is of 95.8%. The 4.20 % remaining residue of the gel is owing to the formation of an inorganic complex containing C, N, and O.

The thermal stability of materials is determined by major mass loss, after which thermal degradation starts. Thus, the greatest rate point of the weight loss change revealed at the peak of the first derivative curve is observed at a temperature of 112.42°C (Fig. 4). This is responsible for the release of water bounded to the three polymers functional groups, which was not completely removed in the first dehydration step, together with degradation of carboxylic groups. These stages can be attributed to the degradation of Chitosan and the decomposition of different structures in the graft copolymer, respectively (15).

DSC run curve of the blended gel exhibited a broad endothermic peak centred at about 110.55°C corresponding to 315mW of heating. This endothermic peak often termed as dehydration temperature, is attributed to the evaporation of water associated with the hydrophilic groups of the polymers (14-16). The peak position situated at a higher temperature indicates a stronger interaction with water. Thus, these results suggest that the interaction between Chitosan, Carrageenan and Hydroxyethyl Cellulose may occur to form a gel with more stability.

Fig.3. TGA and DSC run curves of blended gel.

Fig.4. Thermal decomposition curves of the ternary blended gel (DTG curves).

Rheological measurement results and viscoelastic properties:

Critical strain and strain sweep:

Usually, the rheological properties of a viscoelastic material are independent of strain up to a critical strain level (Linear Viscoelastic Region, LVER). Beyond this critical strain level, the material’s behaviour is non-linear and the storage modulus declines. So, measuring the strain amplitude dependence of the storage and loss moduli (G' and G'') is a good first step taken in characterizing viscoelastic behaviour: A strain sweep will establish the extent of the material’s linearity.

Fig. 5. Critical strain and strain sweep (LVER region).

Thus, a strain sweep test at a frequency of 1 Hz is performed to determine the linear viscoelastic region, from which an appropriate strain is selected. The evaluation of shear strain effects on the measured G' and G'' shows that, below 0.02 deformation (strain), values of G' are independent of the applied strain i.e. LVE behaviour (Fig. 5).

Strain values used in the experiments are chosen to be in the linear viscoelastic (LVE) range, where G' and G'' are independent of the strain amplitude. Thus, the frequency sweeps are selected to ensure that the test is really carried out in the LVE Range (Fig. 5). As a result, the test frequencies are chosen between 0.1 Hz and 16 Hz.

Structure and frequency sweep:

After the linear viscoelastic region has been defined by strain sweeps, the CS blended gel structure can be further characterized using frequency sweep at strain below the critical strain (corresponding to shear stress equals to 1Pa in the present case study). Thus, a measurement is made from a constant strain frequency sweep under a frequency range of 0.1-16 Hz and at temperature equals 25°C. The measured results G' and G'' are virtually independent of lower range frequency values, and the material behaves predominately as elastic (G'>G''); which stands for structure in the material capable of storing energy, Fig. 6. Consequently, these results show that the blended gel has a solid-like behaviour.

The plot of Fig. 6, shows also that with increasing frequency both values of G' and G'' gradually increase. Since the storage modulus G' is higher than the loss modulus G'' (i.e. G'>G'') in the whole frequency range of Linear viscoelastic region (LVER), the sample elastic behaviour predominates over its viscous behaviour and the blended gel exhibits a mechanical rigidity. Afterward, the gel displays a weak dependence on frequency.

Fig. 6. Frequency sweep of the blend gel below the critical strain (at a shear stress of 1Pa and t=25°C).

In addition, the storage modulus is about an order of magnitude larger than the loss modulus with very weakly frequency dependence; this is dependable with the formation of a solid gel structure. The loss modulus is as well nearly independent of frequency exclusively at lower values.

Temperature dependence of the blended Gel:

Fig. 7. Temperature dependence of the blended Gel (Shear stress Vs. Shear rate).

The Fig. 7 shows the effect of temperature on the ternary blended gel shear stress as a function of shear rate. Influence of the shear rate and temperature on rheological curves shows that shear stress increases with increasing shear rate especially for higher temperatures (40°C and 50°C), however, it is nearly constant at a temperature of 30°C and slightly reduces at 25°C. Furthermore, from this plot it can be observed that the ternary blended gel behaves as a viscoelastic fluid (or yielded stress fluid); and a notable shear stress/ shear rate dependence is only observed at higher shear rate values.

Effects of temperatures on the blended gel viscosity:

Fig. 8. Temperature dependence of the blended Gel (Viscosity Vs. Shear rate).

The study of the shear rate-dependent viscosity of the blended gel at a range of temperatures (from 25°C to 50°C) is performed and presented in Fig. 8. The flow curve measurements show that shear rates/ temperatures variations have an effect on the blended gel viscosity. Thus, higher viscosities are found for lower temperatures and non-Newtonian behavior is observed at all temperatures. In addition, the temperature has more effect at lower shear rate values, i.e. the viscosity increase associated with temperature is less important at a high shear rate. It is also apparent in these curves, that this type of fluid displays increases in the shear rate with decreasing viscosity, suggesting pseudo-plastic non-Newtonian behaviour. Negative slopes are also displayed indicating the destruction of internal structure by overcoming internal forces. Thus the blended gel has a shear thinning behaviour. Though the absence of a second Newtonian plateau indicates the absence of a complete loss of internal forces, thus it could be assumed that the gel still retains its internal structure in the applied shear rate regime.

Strain sweep:

Fig. 9. Strain sweep, storage (G') and loss (G") modulus as a function of strain.

In the strain sweep of Fig. 10, linear response for a strain below < 0.1 is observed, where both elastic modulus G' and viscous modulus G'' are independent of the strain and where G' is always greater than G'' (fluid-like behaviour). Further increase in strain amplitude results in a decrease in both G' and G'' up to ≃0.26, above which (i.e. above a yield strain) both modulus drop precipitously indicating a marked strain thinning feature. A strain of around 0.69, G'< G'' and viscous behaviour of the gel is shown; thus at larger strains, the gel is no longer recoverable, indicating an irreversible breakage of bonds between clusters. At sufficiently large strain amplitude range, however, a viscous behaviour is revealed because the storage modulus demonstrates a sharper decrease with increasing strain amplitude than does the loss modulus, Fig. 9.

CONCLUSIONS:

In this research work, a number of ternary mixture gels of Chitosan/Carrageenan/Hydroxyethyl Cellulose (CS/AG/HEC) at different ratios (considering Chitosan as the main component) are synthesized, chemically identified and characterized. An optimal mixture ratio for a better gel with significantly higher viscoelastic properties is obtained by an Experimental Design using a model of Response Surface Methodology (RSM). The significant ANOVA statistical analysis results in an optimum ratio, with a desirability value of 0.968. And the optimized conditions are found for a given composition of weight ratio equals 51/42/07 of CS/AG/HEC respectively. CS blended gel FTIR spectrum results show a broader band at 3100–3600 cm-1 with stronger absorbance intensity, corresponding to –(make – with OH) vibrations of water and hydroxyls as well as –NH stretching vibrations of free amino groups. The band that appears at 1652cm^-1 is an amide I resulting from hydrogen and hydroxyl group interactions. Results also show that bands at 1411cm^-1, 1380cm^-1 and at 1033 cm^-1 that is obviously observed in the Chitosan and Cellulose as well as bands found at 1210-1260cm^–1 region of the Carrageenan (corresponding to the S=O of sulfate Esters) have disappeared in CS blended gel FTIR spectrum. Moreover, the FTIR spectrum of CS/AG/HEC ternary blended system gel showed an absorption band at 1625 cm^-1 (though with lower absorbance value) which corresponds to –NH3⁺ group. Thus, the FTIR spectroscopy of the blended gel comparing to those of each component (i.e. CS, AG and HEC) shows from chemical structures that there is an interaction between the three components. Thermo-Gravimetric analysis of the blended CS gel showed a two-stage weight loss. The first ranged up to 60°C, attributed to the vaporization of volatile components, which is of approximately 4% loss in weight. And the second stage of weight loss began at about 70°C and continued up to 200°C, with faster thermal degradation and a major weight loss (more than 95%). The greatest rate point of the weight loss change (revealed on the peak of the first derivative curve), is observed at a temperature of 112.42°C. This is responsible for the release of water bounded to the three polymers' functional groups, which was not completely removed in the first dehydration step, together with degradation of carboxylic groups. This stage can be attributed to the degradation of Chitosan and the decomposition of different structures in the graft copolymer, respectively. DSC run curve of the blended gel exhibited a broad endothermic peak centred at about 110.55°C corresponding to 315 mW of heating. The peak position situated at a higher temperature indicates a stronger interaction with water. Thus, these results suggest that the interaction between Chitosan, Carrageenan, and Hydroxyethyl Cellulose may well occur to form a gel with more stability.

In this research study, detailed rheological profiling of Chitosan blended gel is carried out. After the linear viscoelastic region has been defined by strain sweeps, the CS blended gel structure is characterized using frequencies sweep below the critical strain. Since the storage modulus G' is higher than the loss modulus G'' in the whole frequency range of Linear Viscoelastic Region (LVER), the sample elastic behaviour predominates over its viscous behaviour and the blended gel exhibits a mechanical rigidity (solid-like behaviour). The storage modulus is about an order of magnitude larger than the loss modulus and has very weakly frequency dependence; this is dependable with the formation of a solid gel structure. The loss modulus is as well nearly independent of frequency exclusively at lower values. Thus, the data from frequency sweep experiments established the pseudo-plastic behaviour of Chitosan blended gel.

Rheological curves at different temperatures show that shear stress increases with increasing shear rate especially for higher temperatures (at 40°C and 50°C), though it is nearly constant at a temperature of 30°C and slightly reduces at 25°C. Therefore, the ternary blended CS gel behaves as a viscoplastic fluid (yielded stress fluid); and a notable shear stress/shear rate dependence is only observed at higher shear rate values.

Flow curve measurements also show that shear rates/temperatures variations have an effect on the blended CS gel viscosity. Thus, higher viscosities are found for lower temperatures, in addition, a non-Newtonian behaviour is observed at all temperatures. Furthermore, the viscosity increase associated with temperature is less important at high shear rates. It is also shown that this type of fluid displays increases in the shear rate with decreasing viscosity, suggesting pseudo-plastic non-Newtonian behaviour. Negative slopes indicate the destruction of internal structure by overcoming the internal forces. Thus a shear-thinning process of the blended gel is observed. Moreover, the absence of a second Newtonian plateau indicates the absence of a complete loss of internal forces, so it may well be assumed that the gel still retains its internal structure in the applied shear rate regime.

In the strain sweep tests, a linear response is observed for a lower strain (below < 0.1), where both elastic modulus G' and viscous modulus G'' are independent of the strain and where G' is always greater than G'' (solid-like behaviour). Further increase in strain amplitude results in a decrease in both G' and G'' (up to ≃0.26), above which (i.e. above a yield strain) both modulus drop precipitously indicating a marked strain thinning feature. At a sufficiently larger strain amplitude range, however, a viscous behaviour is revealed because the storage modulus demonstrates a sharper decrease with increasing strain amplitude than does the loss modulus.

The present study unveiling many of aspects of Chitosan blended gel. The application of various characterization methods also helped in identifying its properties. The study results obtained could be employed in developing Chitosan blended gel with better properties.

ACKNOWLEDGMENT:

The present work is supported by the Laboratory of Industrial Analysis and Material Engineering (LAIGM), University 8 Mai Guelma Algeria.

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Received on 21.03.2020 Modified on 15.04.2020

Asian J. Research Chem. 2020; 13(3):209-215.

DOI: 10.5958/0974-4150.2020.00040.1