Essay: Investigation Into Chitosan-Based Coatings Technology

 
1. Introduction
Recently, there is a lot of considerable research works has been proceeded and implemented to meet the scientific challenges for protecting environment and ecosystems. The main concern of industry is to improve and employ a variety of bio-based polymers to eliminate or prevent contributions to air, water and land contamination. The role of these bio-materials in the development of modern technology needs to be emphasized. As the industrial interests in the development of eco-friendly materials increased rapidly, the materials used based on agricultural resources as feed stocks need more evolution, to be more efficient. Today, the bio-polymers are most importantly to be specialty substances based on abundant renewable resources called natural biodegradable polymers which classified as illustrated scheme in Fig.1 [1-3].
Fig. 1. Classification of the main biodegradable polymers.
Novel composites and bio-polymeric materials that have both environmental and economic concerns are being considered for many applications such as chitosan, cellulose derivatives, starch and protein [1]. The growing interest for clean and safe environment plays an important role for using materials offer extra advantages. The traditional non-degradable substances are not able to achieve all these needs. So, the enormous progress in materials technology focuses on the biodegradable substances. The biodegradable materials are bio-renewable and recyclable raw substances consist of family of natural polymers or fibers as matrix materials and considered an environmental waste management option. These family of bio-polymers able to degrade through action of microorganisms and they provides positive environmental benefits [1-4]. Several demands for biodegradables are forecast to grow due to the natural polymers are available in large quantities from renewable sources, while synthetic polymers are produced from non-renewable petroleum resources. According to the great efforts to utilize bio-renewable and biodegradable resources, the strategy for chemical synthesis is meanly dependent on the thermodynamic and kinetic barriers in the reaction [5]. The current review article described the investigation of chitosan as an important eco-friendly biodegradable polymer and its significant role in the coatings technology.
2. Coatings technology
Historically, the topic ‘coating’ is refers to an extensive covering that is applied for surface modification. The world of coatings is constantly changing by adding classes of new materials to the market. There are many implementations of coatings technology, from engine friction reduction to decorative coatings and food preservatives [6]. The enormous growth of coatings production (Fig. 2) led to raises in machinery operation, improvement of manufacture and reduction of waste materials.
Fig. 2. Global market of coatings technology.
However, there are many continual research challenges to improve and develop the current methods. Some of these techniques have been used for many decades such as physical and chemical vapor deposition processes for thin films [7], magnetron sputtering [8], ion beam [9] and laser deposition [10] because they avoid the complexities and costs of vacuum processing. A lot of modern research articles have been published deals with metals and alloys electrodeposition using different generations of ionic liquids [11-13]. The usage of coatings includes mechanical applications such as hardness, wear resistance and protective coatings with very wide industrial applications [14]. For decorative purposes, coatings technique is employed using significant advanced substances which used to enhance decoration performance [15]. The large-scale uses of coatings in preservation of fresh food [16] and catalysis [17] were also reported. On the other hand, new categories of materials are being developed and modified, based on natural biodegradable polymers, provide novel characterizations and applications. Continuously, several bio-based polymers have received the great attention for various applications such as chemical, biomedical and food industries. These bio-materials offer the availability of obtaining coatings to cover fresh fruits and foods to extend their life time [1-3]. Biodegradable coatings offer more advantages such as edibility, non-toxicity, non-pollution, biocompatibility, good performance, gas barrier and low cost [3,4]. For food preservation, seafood group is more perishable than animal meat as they contain relatively large quantities of crude protein and volatile nitrogenous bases [18]. The rising demand for preserving seafood using new methods and techniques is dependent on modified edible coatings [19]. Another field of increasing concern is the formation of antimicrobial edible chitosan-based coatings [20]. Also, edible chitin and chitosan composites are used currently in medicine as surgical suture and artificial skin [21].
3. Chitosan
Recently, chitosan (CHI) is one of the most abundant basic biopolymer used in our life. Chitosan is a cationic biodegradable polymer obtained by N-deacetylation of chitin in basic medium as illustrated scheme in Fig.3. Chitin is an naturally occurring biopolymer derived from exoskeleton of crustaceans and fungi cell walls [21-23]. Chitosan (CHI) with average molecular weight of 110,000 and deacetylation degree of 84.7% was prepared locally from brown shrimp (metapenaeus monoceros) shell wastes [24-27]. Chitosan has a large scale range of applications due to its biodegradability, biocompatibility, nontoxicity, toughly, gas permeability and simple production method [28-32].
Fig. 3. Schematic diagram of chitosan processing method.
Due to its polycationic nature, CHI was used in some biomedical applications (Fig.4) such as anti-oxidant [33], anti-microbial [33,34], anti-tumor [34,35], hydrogels [35], anti-bacterial [36], anti-fungal [36], analgesic effects [37], nanoparticles [38] and food preservatives [39].
Fig. 4. Some biomedical applications of chitosan.
Owing to its high hydrophilicity and good chemical resistance [40] it has gained special attention in membrane separation technology including gas separation [41,42], reverse osmosis [43] and pervaporative dehydration [24, 43-47]. Moreover, a lot of separation methods were used for azeotropic mixtures throughout pervaporation techniques [48]. Separation of alcohols from water is vital in organic synthesis. Among the several methods, pervaporation gives high separation result and low-cost [49,50]. For dehydration technique, preparation of specific membranes with high hydrophilicity is very important to achieve high swelling degree and good permeation flux. For improving the CHI membranes, several methods were applied such as polymeric blends [51-56], hybrid polymer [57-59] and crosslinking [60-62].
4. Chitosan-based coatings
In the last few years, the chitosan-based polymers and their derivatives find great concerns in coatings technology due to their unique characterizations. The following sections will discussed the investigation of chitosan crosslinking with some selective polymers such as polyester (PE), polyurethane (PU), polyvinyl acetate (PVA) and carboxy methyl cellulose (CMC), respectively.
4.1. Chitosan-based polyester (CHI-PE)
Polyesters (PEs) are commonly refers to high molecular weight compounds containing ester links in the main structure of the molecules. These compounds incorporate acidic and alcoholic entities bonded alternately to one other. The polyester molecules may have a linear structure (from bifunctional alcohols and acids) or a branched structure (if the constituents greater than two) .The polyester resin molecules may be terminated by other groupings besides hydroxyl and carboxyl groups, depending on the polycondensation reaction conditions and the starting compounds as illustrated in Fig.5. However, even for an alcohol to acid ratio of 1:1 the numbers of terminal groups of each kind may not be equal owing to side reactions that occur during synthesis [63].
Fig. 5. Schematic reaction of dioic and diol for preparing polyester.
Owing to the large spectrum of characteristics and applications of polyester compounds, thermoplastic or thermosetting, it mostly used in moulding, films, coatings, composites, fibres, rubbers and plasticisers [64]. Polyesters (PEs) are produced by a variety of manufacturing techniques such as direct polycondensation esterification, melt transesterification, acylation, interfacial polycondensation and ring-opening polymerization [65]. Currently, essential bio-based polyesters have been widely used in biological industry [66]. Additionally, many thermoplastic polyesters are produced from bio-based diols with dioic acids like polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polytrimethyleneterephthalate (PTT) [67]. Mainly, the bio-based polyesters are prepared, completely or partially, from renewable bio-materials for the potential growth of polyester industry [68].
4.1.1. Synthesis
This part concerns with the investigations of new chitosan-based polyester (CHI-PE) films such as chitosan-polyethylene glycol maleate (CHI-PEGM) and chitosan-polyethylene glycol fumarate (CHI-PEGF), respectively. PEGM or PEGF macromers (as determined by GPC) were prepared by direct esterification reaction of maleic anhydride (MA) or fumaryl chloride (FCl) with diol such as polyethylene glycol (PEG) in the presence of benzoyl peroxide or propylene oxide as a catalysts, respectively, as illustrated in schematic diagram (Fig. 6). For modification, CHI-PEGM and CHI-PEGF films were prepared by polymer solution casting process of PEGM or PEGF solutions with CHI solution (dissolved in 1% v/v acetic acid) at definite ratios. The mixtures were cast into a suitable petri dishes, then dried at room temperature and carefully stored until used [69-71].
Fig.6. A schematic pathway for preparing CHI-PEGM and CHI-PEGF.
4.1.2. Fourier transform infrared spectroscopy (FTIR)
FT-IR (Fig.7) shows absorption spectrum of polyethylene glycol maleate (PEGM), chitosan (CHI) and chitosan-based polyethylene glycol maleate (CHI-PEGM), respectively. The FT-IR measurements were mainly employed to check the interaction between CHI and PEGM. Moreover, the absorption spectra of the obtained CHI-PEGM film samples were compared with the unmodified polyester. The vital peaks of PEGM present at 2910 cm-1 for CH stretching, at 1724 cm-1 for C=O stretching (ester) and at 1100 cm-1 for ether group. The major absorption peaks range appeared at 3500’3200 cm-1 due to OH groups (for PEGM) and stretching NH groups (for CHI and CHI-PEGM), at 1720 cm-1 for stretching C=O group (PEGM) and at 810 cm-1 for the pending CH (PEGM). Intensity of ‘CH2 group bands increases sharply with increasing hydrocarbon chain length in diol portion. This observation revealed that chitosan and PEGM successfully reacted to form blended film. Meanwhile, shifting of absorption peaks of C=O from 1720 cm-1 (COO groups of PEGM due to conjugation with C=C before reaction) to an overlapping normal (saturated) COO group 1735 cm-1 in the film samples due to coupling of the two macromers [27,68,70].
Fig.7. FT-IR of PEGM, CHI and CHI-PEGM, respectively.
4.1.3. X-Ray Diffraction (XRD)
It can be observed that by XRD analysis in Fig. 8 both PE and CHI were amorphous [4,27]. Significant strong peak of PEGM at 2h = 20” meanwhile for CHI there are two sharp peaks at 2h =12.2” and 2h =19.2”, respectively. This behaviour decreased at 2h = 19.7” as a result of the CHI-PE formation [70].
Fig.8. XRD of PEGM, CHI and CHI-PEGM, respectively.
4.1.4. Scanning Electron Microscope (SEM)
Surface morphology of CHI, PEGM and CHI-PEGM films was analyzed using SEM technique as illustrated in Fig.9. The micrographs exposed that CHI particles were relatively well impeded in the PEGM matrix. In general, the PEGM film composition is homogeneous implying full miscibility of the PE components. By adding CHI to PEGM matrix, the surface homogeneity of the CHI-PEGM film was reduced with phase separation to some extent. However, phase separation between CHI and PEGM in the blend films may be detected by SEM. The CHI film morphology shows uniform film matrix with homogeneous surface. Surface of CHI-PEGM were rougher than CHI film [72].
Fig.9. SEM images of CHI, PEGM and CHI-PEGM, respectively.
4.1.5. Swelling performance
The swelling performance of CHI-PEGF films (Fig. 10) were investigated based on equilibrium water uptake (EWu%) using Eq. (1). The values of EWu showed that all films had water retention capability. Moreover, the blend films with 60% and 80% CHI content ratio yielded 258% and 305% of EWu, respectively, than the others. This may correlated to the increasing of hydrophilic OH groups in the films. This trend reduced in case of 40% CHI content ratio may due to the interactive effects between the functional groups of CHI and PEGF [69,73].
Fig.10. Swelling based on water uptake (EWu) of CHI-PEGF films.
4.1.6. Antibacterial activity
The results of antibacterial activity tests of CHI and CHI-PEGF blend films as surviving bacteria and population reduction are depicted in Fig. 11(a) and (b), respectively. All samples of CHI and CHI-PEGF reduced number of P. aeruginosa as a Gram (‘ve) and S. aureus as a Gram-(+ve) bactera. The results showed that all CHI composites showed significantly antibacterial activity against P. aeruginosa except for 80% CHI content films. By increasing PEGF, PEGF may decrease the possibility of CHI interaction with bacteria due to CHI is reduced the bacterial growth as well as hydrophilicity of the film [74,75].
Fig.11. Antibacterial activity of CHI-PEGF films for different blend ratios.
4.2. Chitosan-based polyurethane (CHI-PU)
Polyurethanes (PUs) are one of the most versatile synthetic polymers. They have significant characterizations such as good mechanical and physical properties, flexibility and abrasion resistance that find large scale of applications for various purposes. Currently, polyurethanes are used in various industries and biomedical implementations such as synthetic resin, coatings, vehicles, fibers, foams, paints, adhesives, elastomers, artificial skin [4,76].
4.2.1. Synthesis
Polyurethane elastomers are mainly prepared from hard isocyanate portion (-NCO) with soft diol portion like polyethylene glycol (PEG). Many research works studied the influence of chemical structures of diisocyanate compounds on crystallinity degree, surface morphology and thermal stability of polyurethane films. Soft polyurethanes accept plasticizers with almost predictable changes in physical characterizations, meanwhile hard polyurethanes which large increase in elasticity and decreasing of Tg would be undesirable. Small additions of plasticizers or other liquids can produce foam of equal density with higher compressive strength, finer cell structure, and tougher cell wall. Polyurethane prepared from polyols and diisocyanates by a prepolymer method gives good mechanical properties [77,78]. In the present section, polyurethane (PU) elastomers were prepared from TDI and PEG. For modification, addition of chitosan (CHI) to the polyurethane (PU) chain matrix was investigated. Firstly, polyurethane (PU) (with molar ratio NCO/OH=4) was prepared by addition of toluene diisocyanate (TDI) to a solution of polyethylene glycol (PEG) and carbon tetrachloride (CCl4) under continuous stirring and heat. Chitosan-based polyurethane (CHI-PU) was prepared with the addition of chitosan solution (5mg/mL chitosan in 1% glacial acetic acid) giving amorphous layer [4].
The obtained products of PU and CHI-PU were characterized for the purpose of comparison. Thus, chemical modification has led to opportunities for new compositions with significant properties for many end-use applications [78]. Preparation of chitosan-polyurethane (CHI-PU) composition; a preparation pathway is schematically illustrated in Fig 12.
Fig.12. A schematic pathway for preparing CHI-PU elastomers.
4.2.2. Fourier transform infrared spectroscopy (FTIR)
The structure of CHI-PU composition was confirmed by FT-IR spectra as given in Fig. 13 and compared with the unmodified PU sample (in absence of CHI). The observed peaks in the FT-IR spectra of CHI-PU were appeared at range 3300 cm’1-3500 cm’1 for both OH and NH stretching vibrations, at 2910 cm’1 and at 2880 cm’1 for CH2 stretching, at 1720 cm’1 for C=O stretching, at 1440 cm’1 for CH2 bending, at 1260 cm’1 for C-O stretching, at 1173 cm’1 for C-O-C stretching, repectively. The intensity of ‘CH2 group bands increases sharply with increasing the hydrocarbon chain length in the glycol portion as shown for CHI-PU as compared with PU.The FT-IR spectra of PU exposed broad peak at 3223 cm’1 due to the OH stretching peak at 2277 cm’1 attributed to the N-C=O group attached to TDI. The spectra also show sharp peaks at range 1500 cm’1-1600 cm’1 attributed to the C-C stretching of aromatic ring. There are other sharp peaks appeared at 1445 cm’1 due to CH2 bending. The peak intensity of OH group increased and NCO group decreased slightly. These are the strong evidence for the formation of NCO-terminated PU [77,79].
Fig.13. FT-IR of PU, CHI and CHI-PU.
4.2.3. X-Ray Diffraction (XRD)
It can be observed that by XRD analysis in Fig. 14 both PU and CHI were generally amorphous, as described in previous report [27,79]. For CHI-PU composition, it showed clearly strong peaks at 2h = 19.2” and 2h = 22.6”, respectively, meanwhile this performance decreased at 2h = 14.7”, 2h = 23.5” and 2h = 25.7”, respectively, as a result of the PU segments. The peaks associated with their crystalline structure were also observed. A similar behavior was observed for CHI-PU but yielded residue that was difficult to handle. The rigid segment exposure was also verified by XRD analysis [77,78].
Fig.14. XRD of PE, PU and EDA-CHI-PEU.
4.2.3. Scanning Electron Microscope (SEM)
SEM images in Fig. 15 show the surface morphology of PU, CHI and CHI-PU, respectively. A contrast appearance is developed by amorphous part of the soft PU matrix phase and crystalline of the hard CHI segments. It can be clearly seen that PU showed bad surface film. These formation are commonly due to the employed condition of O2 and heat, under high energy beam of electron, decompose the macromolecules into gaseous products like CO or CO2. CHI-based PU appears as small particles and micro holes which might be physical crosslinking occurred. This appearance may due to tri-functional CHI molecules were well impeded in the polymer matrix to form a 3D network structure, which primarily confirmed the formation of CH-based PU composites [27,77, 78].
Fig.15. SEM image of PU, CHI and CHI-PU, respectively.
4.2.4. Wettability
Surface hydrophilicity of CHI-PU films based on the measured water contact angles (”) was investigated (Table 1). PU values are decreased by increasing the polarity. It reduced significantly for CHI-PU films. Moreover, CHI contains hydrophilic OH and NH2 groups which increased the hydrophilicity. The dropping of angle values is depends on the functional groups of both CHI and PU. The results expose an enhanced hydrophilicity and reduce in contact angle about 57′ and 41’ for PU-CH-0.5 and PU-CH-2.0, respectively [78].
Table 1. Wettability measurements of PU and CHI-PU, respectively.
4.2.5. Antibacterial activity
The antibacterial activity of all CHI-PU films were investigated using Gram +ve (S. aureus) and Gram -ve (P. aeruginosa) bacteria (Fig.16). For P. aeruginosa control sample, bacteria contents reduced significantly in all PU films. With S. aureus bacteria, there is no change in control or PU films but the values were reduced in case of CHI- PU films. For both bacteria species, CHI films have strong antibacterial efficiency parallel to the CHI content [80].
Fig.16. Antibacterial activity of CHI-PEGF films for different blend ratios.
4.3. Chitosan-based polyvinyl acetate (CHI-PVA)
Polyvinyl acetate (PVA) is a rubbery synthetic polymer used mainly in coatings technology and biomedical preservation applications. There are many great interests about this substance in a large scale of modern techniques. PVA, as an emulsion in water, is applied as a good adhesive for several materials such as wood, paper, cloth and for porous building components [81]. On the other hand, many microorganisms like bacteria, algae and yeasts have the ability to degrade and damage PVA polymer [82]. The modification of PVA with biodegradable polymer like chitosan enhances the film-forming characterizations. CH-PVA has more hydrophobic, stability and lower Tg than pure CHI. Moreover, CH-PVA is synthetic’natural hybrid copolymers having beneficial mechanical properties and dual hydrophilic/hydrophobic nature [83].
4.3.1. Synthesis
CHI-PVA blends were prepared by adding ratios of CHI solution to PVA emulsion (Fig. 17). As more CHI solution was added, the diffusion rate increased and the dispersion solution appeared with more blending [51].
Fig. 17. A Schematic diagram for preparing CHI-PVA films.
4.3.2. Fourier transform infrared spectroscopy (FTIR)
FTIR analysis of CHI, PVA and CHI-PVA are shown in Fig. 18. For PVA diagram, there is no OH groups exist meanwhile significant absorption bands of C=O and C-O groups appeared at 1727cm’1, 1370 cm’1 and 1220 cm’1, respectively, as a result of COOCH3 groups. Compared with PVA, CHI diagram shows common broad bands of OH and NH2 groups appeared at 3460 cm’1 in absence of C=O group bands. From the spectra of CHI-PVA, strong band of NH2 groups at 3450 cm’1 reduced dramatically due to the blending between CHI and PVA. Moreover, the COO group appeared with absorption peaks at 1725cm’1, 1369 cm’1 and 1290 cm’1, respectively. Also there are two new absorption bands appeared clearly at 2011 cm’1 and 2165cm’1 may due to double or triple bond types between C and N during the blending process. This due to some NH2 groups on CHI structure were protonated to the NH groups by the blending with PVA. Based on this behaviour, the variation between the band intensities may refers to the crosslinking degree of CHI-PVA [51,84].
Fig.18. FTIR spectra of CHI, PVA and CHI-PVA, respectively.
4.3.3. X-Ray Diffraction (XRD)
XRD patterns of CHI, PVA and CHI-PVA are shown in Fig. 19. It can clearly see significant peaks at diffraction angles of 10” for CHI and of 20” for both CHI and PVA, revealing the partial crystalline structure of CHI and PVA blends. This CHI structure is explained the formation of several crystalline phases with two types of hydrogen bonding. These links occurred between NH2 and OH groups itselves. Furthermore, the band intensity at diffraction angles of 20” and 40” for CHI-PVA is decreased, correlating the ionic chelation between CHI and PVA, breaking of H bonds and reducing of crystallinity. It was observed that the signals of NH2 groups disappeared after the blending which limit the chain mobility of both CHI and PVA [51].
Fig. 19. XRD diagrams of CHI, PVA and CHI-PVA, respectively.
4.3.4. Morphology
Figs. 20 expose the optical microscopic images of PVA and CHI-PVA blends at room temperature. Microscopic investigations obtained from pure PVA do not have any fractions, whereas those of CHI with PVA show presence of small CHI spots. With the addition of CHI to PVA chains, the embedded CHI matrix has several branched sites as observed clearly in CHI-PVA image. Dispersal composites of CHI-PVA may due to the blend formation. The diffusion structure of CHI-PVA blend is mainly reveal to blend formation degree, CHI content and drying time. The common formation pathway which studies the elongation behaviour of the morphological structures of CHI is depending on the phenomena of molecular diffusion. Several techniques used to characterize CHI structures as function of concentration such as box counting method [85].
Fig. 20. Optical microscopic images of PVA and CHI-PVA, respectively.
4.3.5. Swelling performance
Fig. 21 shows the swelling degree (SD) of CHI-PVA blends in different feed ratios of methanol-water (MW) mixtures. SD of CHI-PVA blends in pure water observed higher than in pure alcohol due to increasing of CHI-PVA flexibility with increasing water ratio which enhances more interactive effects with blends than alcohol. Moreover, the outer layer constituents of CHI-PVA blend have the availability to undergo relaxation and diffusion of alcohol into the blend [12]. The polar groups of water are the major cause for the high SD of CHI-PVA blends in MW mixture. SD of MW was obtained in the range of 50-70% (20% CHI ratio) due to the influence of alcohol and water. To about 50% of water ratio in MW, the penetration of alcohol and water is shows proper rate. As the major water ratio exist, SD increased slightly. The swollen films decrease the resistivity to permeate owing to the high relaxation of the hydrophilic films. With increasing the blends swelling with high water ratio, more alcohol is permeating together with water due to the sorption coupling process [86].
Fig. 21. Swelling of CHI-PVA blends in methanol-water mixtures for 48h at RT.
4.3.6. Conductivity
From the conductivity diagram of CHI-PVA blends in Fig. 22, the conductivity values increased with elevating temperature, compatible with the common polyelectrolyte performance, which is mainly due to the ionization of NH2 groups in CHI structure. The high ” values of CHI-PVA are probably because the interaction between CHI and PVA is powerful and not cracked. The difference of conductivity values of blends at the same conditions are possibly owing to the change of the crosslinking method [87].
Fig. 22. Conductivity diagrams of different ratios of CHI-PVA copolymer.
4.4. Chitosan-based carboxymethyl cellulose (CHI-CMC)
Cellulose is the most abundant eco-friendly biodegradable polymer exists in nature [24]. Many hydrophilic cellulose derivatives such as hydroxy ethyl cellulose, hydroxy propyl cellulose or carboxy methyl cellulose (CMC) can be prepared throughout etherification reaction. Among all cellulose derivatives, CMC is an anionic polyelectrolyte which shows significant physicochemical characterizations in chemical reactions. CMC-based blend exposes high swelling in water due to existence of Na+ and also repulsion of its COO- groups [24,89]. For modification, CMC was chemically blended and grafted with other synthetic polymers [90,91]. CMC-based hydrogels are used in water treatment for removal of pollutants such as heavy metal ions [92] and dyes [92,93]. Composites of CHI and CMC fibers have been modified and investigated for biomaterial applications [94].
4.4.1. Synthesis
For preparing CHI-CMC blends, the ionic bonding carried out between the cationic NH2 groups of CHI and the anionic COO groups of CMC as shown in Fig. 23. Polyelectrolyte complexes (PECs) of CHI-CMC were prepared in aqueous HCl then the solid complex was dissolved in aqueous NaOH to prepare uniform polyelectrolyte complex membranes (UPECMs) [24].
Fig. 23. A Schematic diagram of preparing PECs from CHI and CMC.
UPECM films have ionic-rich form which enhances its dehydration behaviours over a series of binary contents of methanol-water (MW), ethanol-water (EW), isopropanol-water (IW) and butanol-water (BW) mixtures, respectively. From the optical images, by adding CHI solution (Fig. 24a) to CMC solution (Fig. 24b), the turbidity shown in CMC solution (Fig. 24c) with the first addition of CHI solution, correlating the production of insoluble PECs due to the ionic crosslinking between CHI and CMC. With the high CHI content, the turbidity increased (Fig. 24d) and two phases obtained (Fig. 24e) at the complexation endpoint. The PECs accumulated, separated and dried. PECs were obtained (Fig. 24f) and its casting solution (Fig. 24g) which produces uniform polyelectrolyte complex membranes UPECMs (Fig. 24h) [24].
Fig. 24. Photographs of (a) CHI solution; (b) CMC solution; (c & d) addition of CHI
solution to CMC solution; (e) complexation; (f) PECs; (g) casting and (h) UPECMs.
4.4.2. Fourier transform infrared spectroscopy (FTIR)
FTIR diagrams of CHI, CMC and PECs are given in Fig. 25, respectively. There are absorption bands of C=O groups at 1600cm’1 and at 1627 cm’1 for CMC and PECs, respectively, attributed to the free COOH groups. Broad bands of OH groups appeared at 3500 cm’1, 3422 cm’1 and 3384 cm’1 for CHI, CMC and PECs, respectively. For CMC, strong band observed in 1600 cm’1 for the COOH groups of CMC. In PECs, two bands appeared at 1736 cm’1 and 1627cm’1 for both COOH and COO groups. This due to some COO groups in CMC converted to COOH. So, the variation between the band intensities at 1736 cm’1 and 1627 cm’1 refers to the crosslinking degree of PECs [84].
Fig.25. FTIR spectra of CHI, CMC and PECs, respectively.
4.4.3. X-Ray Diffraction (XRD)
Fig. 26 exhibits the XRD of CHI, CMC and PECs, respectively. There are peaks at 10” for CHI and at 20” for both CHI and CMC, revealing the partial crystalline phases of CHI and CMC due to two types of hydrogen bonding [63]. However, the band intensity at diffraction angles of 20” and 40” for PECs was decreased, correlating the ionic interactions between CHI and CMC, breaking down of hydrogen bonding and reducing of the crystallinity. Consequently, it was exposed that the signals of NH2 groups disappeared after the ionic crosslinking reactions which limit the chain mobility of both CHI and CMC which in turns reduce the crystallinity of PECs [95].
Fig.26. XRD of CHI, CMC and PECs, respectively.
4.4.4. Swelling performance
Swelling of prepared UPECMs in alcohol-water mixtures, shown in Fig. 27, is generally based on hydrophilic behaviour of the films. SD of UPECMs in MW mixture rose with increasing water value. Flexibility of UPECMs in water is higher than in alcohol due to the outer particles of UPECMs undergo relaxation and diffusion of alcohol molecules into the bulk [92]. In general, the high SD in all alcohol-water mixtures, except BW, was observed in the range of 30-70%, especially in case of IW mixture. Up to 50% water content ratio in both MW and EW, the penetration of alcohol and water in UPECM films is the same. For IW binary feed mixture, high swelling was exposed at a range of 40-60% more than in both MW and EW systems. The high swelling of IW mixture at 50% water content is due to the more permeation of water than alcohol. The membrane degradation is beginning from 50% as correlating to the high relaxation. Besides, the low polarity of isopropanol is allows more water molecules to permeate through the membrane. In BW mixture, degradation of membranes was observed clearly from 10% water concentration in feed, indicating the poor stability of membranes in butanol [24].
Fig. 27. Swelling degree (SD) of UPECMs in (a) methanol-water, (b)ethanol-water,
(c) isopropanol-water and (d) butanol-water mixtures at room temperature for 48h.
4.4.5. Antimicrobial activity
Antimicrobial test is employed to check the antibacterial efficacy of films. The activities of CHI and CHI-CMC films against E. coli and S. aureus are shown in Table 2. The mechanism is the interaction of NH4+ groups with the -ve bacterial cell membranes, causing membrane leakage and decompose of intracellular components, which kill the bacteria. There is more inhibition activity of the films against S. aureus than against E. coli. The difference is due to S. aureus is more receptive to antibiotics than is E. coli because the latter has a relatively less permeable, lipid-based outer membrane [96,97].
Table 2. Antibacterial activity of CHI and CHI-CMC films.
5. Summary
Bio-based polymers have received much more interest in the last decades due their potential implementations in several fields correlated to environmental maintenance and protection of physical health. The present review reported that chitosan (CHI) could be incorporated into synthetic polymers such as polyester (PE), polyurethane (PU), polyvinyl acetate (PVA) and carboxymethyl cellulose (CMC) by simple blending techniques. These polymeric materials are completely miscible with CHI to produce unique chitosan-based polymers. The results explained that incorporation of CHI into the polymeric blends improved its strength, wettability, resistivity and antimicrobial activity ultimately. Based on the obtained investigations, polymeric composites containing CHI can be used in several technologies such as coating, painting, hydrogel, membrane separation, food preservation, vesicular drug delivery and tissue engineering.
6. Future perspectives
The most dynamic development of CHI-based polymer production is foreseen. Thus, the global market is expected to shift dramatically to new natural polymers derived from animal and plant origins. The future trend is to produce bio-based polymers using renewable resources as the following:
‘ Bio-based monomers: Using bacterial fermentation of lignocellulosic biomass (starch & cellulose), fatty acids and organic waste, polylactic acid (PLA), polybutylene succinate (PBS) and polyethylene (PE).
‘ Bio-based polymers: Directly by bacteria polyhydroxyalkanoates (PHA).
‘ Natural bio-based polymers: proteins, nucleic acids, collagen, starch, cellulose & chitosan.
Acknowledgement
The review article was supported by the Egyptian Petroleum Research Institute.
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