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Comparing homogeneous and heterogeneous routes for ionic crosslinking of chitosan membranes
Reactive and Functional Polymers 103 (2016) 156–161
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Comparing homogeneous and heterogeneous routes for ionic crosslinking of chitosan membranes J.S. Marques, J.A.O.D. Chagas, J.L.C. Fonseca, M.R. Pereira ⁎ Chemistry Institute, Universidade Federal do Rio Grande do Norte, Campus Universitário, Lagoa Nova, Natal, RN 59072-970, Brazil
a r t i c l e
i n f o
Article history: Received 29 July 2015 Received in revised form 16 March 2016 Accepted 17 April 2016 Available online 20 April 2016 Keywords: Chitosan Crosslinking Homogeneous Heterogeneous H2SO4
a b s t r a c t H2SO4 — ionically crosslinked chitosan membranes were prepared via homogeneous and heterogeneous routes. + The control variable in homogeneous crosslinking was the SO2− 4 /NH3 molar ratio (1:4 e 1:6) while for heterogeneous crosslinking it was the immersion time of pure chitosan membrane in H2SO4 0.5 M aqueous solution (5 and 30 min). FTIR-ATR suggested lower crosslinking degree for homogeneous crosslinking, corroborated by XRD analysis that indicated the maintenance of the crystalline structure for such membranes. Thermal analysis showed very similar degradation processes for homogeneous and pure chitosan but quite different for heterogeneous: not only in terms of degradation temperature but also in amount and signal of heat involved. Swelling index results were very dependent on pH of medium. Particularly in acidic medium, homogeneous crosslinked membranes, presented a higher swelling capacity than the heterogeneous ones. Mechanical properties revealed that both methodologies render membranes with lower tensile strength and elongation but with Young modulus + about four times higher, due to the interactions of SO2− 4 groups of H2SO4 with NH3 of chitosan. Finally, AFM images showed dramatic changes on surface topology, with reduction of roughness for heterogeneous and an increase for the homogeneous one. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Chitosan is a polyaminosaccharide, produced by partial Ndeacetylation of chitin. It is a biodegradable, non-toxic, semi-synthetic polymer with excellent biocompatibility [1,2]. It can be defined as a copolymer of 2-amino-2-deoxy-D-glucopyranose and 2-acetoamido-2deoxy-D-glucopyranose whose units are linked by β(1 → 4) bonds [3, 4]. Chitosan exhibits unique physicochemical properties and it is highly suitable to innumerable applications in a wide range of fields such as food and nutrition, biotechnology, material science, drugs and pharmaceuticals, agriculture, environmental protection, and gene therapy, among others [5]. As a copolymer soluble in acidic medium, chitosan can be obtained under the form of fibers, films, membranes, powder, particles, beads and solution, which enhances its usefulness. Chitosan has free amino and hydroxyl groups on its backbone, which can undergo chemical modifications in order to turn chitosan more bacteriostatic, and to improve its chemical and mechanical resistance [6]. Among this, crosslinking is one of the most effective approaches for improving membrane stability [7]. There are two main ways to crosslink these membranes: by covalent bonds, using agents such as glutaraldehyde or ethylene glycol, or through ionic bonds, using, e.g., sulfuric acid or tripolyphosphate [8]. The use of sulfuric acid as crosslinking agent has been proved to increase the separation factor in ⁎ Corresponding author. E-mail address: [email protected] (M.R. Pereira).
http://dx.doi.org/10.1016/j.reactfunctpolym.2016.04.014 1381-5148/© 2016 Elsevier B.V. All rights reserved.
pervaporation experiments of ethanol/water and methanol/methyl tbutyl ether [9–11]. Most of the work done on ionic crosslinking of chitosan membranes using sulfuric acid used the heterogeneous route, where the membrane is immersed in a sulfuric acid solution for a period of time and then removed and dried [1,12,13]. The homogeneous route is very little described in literature in comparison with the heterogeneous one. The main difficulty is to achieve a low degree of crosslinking without induce the solution gelification (which could hinder the casting process). As far as we know, most of the homogeneous route described in literature use glutaraldehyde as crosslinking agent [14] but no studies were found using sulfuric acid. In this work, we investigated the influence of the crosslinking routes (homogeneous and heterogeneous) on the final properties of sulfuric acid-crosslinked chitosan membranes. The purpose was to obtain membranes with low degrees of crosslinking, in order to confer chemical resistance in acid medium but still maintain a relatively high number of NH2 groups available for interactions related to sorption applications. 2. Experimental 2.1. Materials Chitosan used in this work was purchased from Polymar LTD (Brazil). It had a deacetylation degree of 88%, determined by CHN elemental analysis and conductometric titration as described elsewhere [15], intrinsic viscosity [η] = 0.360 g L−1, and average viscometric
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molar mass, Mv = 1.6 × 105 g·mol−1 (determined using Mark– Houwink–Sakurada equation) [16,17]. Acetic acid (P. A., Cromato Produtos Químicos LTD, Brazil), Sodium hydroxide (P. A. Vetec LTD, Brazil) and sulfuric acid (P. A. Vetec LTD, Brazil) were all analytical grade reagents and used as received.
submitted to a heating rate of 10 °C/min in nitrogen atmosphere, using alumina crucible and range of temperature from 25 °C to 600 °C. DSC analysis were obtained using a Shimadzu DSC 50, temperature range from 25 °C up to 450 °C, in nitrogen atmosphere (flow of 20 mL/min) and heating rate of 10 °C/min.
2.2. General procedure for membrane preparation
2.5. Mechanical characterization
2.2.1. Homogeneous route Chitosan was dissolved in a 2.0% v/v aqueous acetic acid solution for 24 h under mechanical stirring in order to form a 2.0% w/v solution. The resulting solution was filtered twice, first using a nylon filter and then using a Millipore® Millex filter, with 0.40 μm of pore diameter. A 0.5 M sulfuric acid solution was added to 25 mL of chitosan solution + so that SO2− 4 /NH3 molar ratios of 1:6 and 1:4 were obtained. The mixtures were kept under mechanical stirring for 1 h. Afterwards the solutions were cast onto a glass plate and left at room temperature for 2 days for solvent evaporation. The membranes were then neutralized trough immersion in a 5.0% (w/v) sodium hydroxide solution for 2 h. Following, they were repeatedly washed with distilled water and fixed on a support to dry at room temperature. These membranes were denominated CS16 and CS14.
Ultimate tensile strength, modulus and elongation percentage of the membranes were measured in a universal test dynamometer, Dyna View model AME-5kN (Oswaldo Filizola LTDA, Brazil), according to ASTM D638. The test samples were cut into strips (100 mm × 10 mm). The crosshead speed was 10 mm/min. All samples were tested at room temperature and the data were instantaneously recorded. At least 10 samples of each membrane were measured and the presented results are the average values.
2.2.2. Heterogeneous route Heterogeneous crosslinked membranes were prepared using pure chitosan membranes obtained as described above, without the addition of sulfuric acid solution. After the neutralization and washing processes, the membranes were immersed in sulfuric acid solution 0.5 M for 5 and 30 min. Then the membranes were exhaustively washed with distilled water and fixed on a support to dry at room temperature. These membranes were denominated CS5 and CS30. 2.3. Physico-chemical characterization Infrared spectra were obtained using a Spectrum65 FTIR coupled to a universal ATR sampling accessory from Perkin Elmer, operating in the range of 400–4000 cm−1, 32 scans and resolution of 4 cm−1. Morphologies of chitosan membranes were studied with an X-ray diffractometer Shimadzu model XRD-7000, radiation of CuK α (λ = 1.5406 Å), angle variation 2θ from 5° to 40°, with scanning speed of 2°min−1 and step of 0.02°. The surface roughness and topology at nanometer scale of the membranes were measured, at room temperature, using a Shimadzu SPM9700 atomic force microscope (Kyoto, Japan) in dynamic mode with a resonance frequency of 320 kHz and scan rate of 1.0 Hz. The scan areas were 5.0 μm × 5.0 μm and 10.0 μm × 10.0 μm. Swelling index was gravimetrically determined. The dry membrane (Wd) was initially weighted and promptly immersed in distilled water at room temperature. The membranes were removed from water, wiped off, weighted, and returned to water at different time intervals: five minutes for the first 30 min, 1 h until complete 12 h, and them 24 h and 48 h. The experiments were done in triplicate. The water sorption capacity or swelling index, W(%) was calculated according to Eq. 1: Wð%Þ ¼ ½ðWw–W d Þ=W d 100
3. Results and discussion 3.1. FTIR spectroscopy The first difference observed when comparing all membranes is related to their physical aspects. While pure and heterogeneous crosslinked membranes were transparent, the homogeneous ones were opaque. However, despite their different appearances, all crosslinked membranes behaved similarly when immersed in acetic acid solution for over 24 h, maintaining their integrity and therefore confirming the crosslinking process. Fig. 1 shows the FTIR-ATR spectra of pure and crosslinked membranes while Table 1 summarizes the main absorption peaks for pure chitosan. Regarding the heterogeneous crosslinked membranes, the first spectral changes observed are in the 3600–2500 cm− 1 region, where the O\\H and N\\H2 absorption bands became less distinct, and a new absorption band appeared at 3220 cm−1 and broadened as the reaction time increased. Besides, C\\H stretching vibration peak shifted to lower wavenumbers (from 2875 to 2963 cm−1). The new band at 3220 cm−1 is assigned to the stretching vibration of N+–H [1] indicating that the NH2 groups in chitosan were protonated by the presence of sulfuric acid forming NH+ 3 groups. Crosslinking process occurs when an 2 anion interacts with two NH+ SO− 4 3 groups by ionic attraction. The shift at the C\\H stretching vibration peak is an indication that there was an additional interaction between sulfuric acid and chitosan membrane. At lower wavenumbers, between 1750 and 1500 cm− 1, it is
ð1Þ
where Ww is the membrane wet weight and Wd is the membrane dry weight. The swelling index was also determined in acidic medium using a H2SO4 0.01 mol/L aqueous solution. 2.4. Thermo characterization A Shimadzu TGA-50 thermogravimetric analyzer was used to study the thermal stability of the membranes. Samples with 6 mg were
Fig. 1. FTIR-ATR spectra of chitosan (CS), heterogeneous (CS5 and CS30) and homogenous (CS14 and CS16) crosslinked chitosan membranes.
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Table 1 Infrared band assignments for CS membranes. Peak position (cm−1)
Assignment
3420 3360 3300 2920 2875 1645 1585 1380 1150
O\H \ stretching N\\H2 sym. stretching N\\H2 asym. stretching C\H \ sym. stretching C\H \ asym. stretching C=O stretching (amide I) N\\H2 deformation N\\H2–bending (amide II) C\O \\\C asym. stretching
possible to observe the disappearance of the amide I band (at 1645 cm−1) and of amide II band (at 1580 cm−1) and the appearance of new bands at 1630 cm−1 and 1530 cm−1. In addition, the band at 1150 cm−1 is overlapped by the shifted band at 1055 cm−1. According to the literature [1,18] the new bands represent the bending vibrations of NH+ 3 (antisymmetric and symmetric, respectively), confirming the protonation process. Bands were more intense for the shortest crosslinking time. If one bears in mind that ATR is a technique decisively influenced by the sample-crystal contact, it is reasonable to assume that the membrane crosslinked at short times may be softer than the one crosslinked at longer times, leading to a better sample-crystal contact. Regarding the spectra of homogeneous crosslinked membranes, it was not possible to notice any significant changes when comparing them with uncrosslinked chitosan. These results confirm the low degree of crosslinking obtained using the homogeneous route. 3.2. XRD The XRD patterns of pure and crosslinked chitosan membranes are shown in Fig. 2. Chitosan presented two characteristic peaks around 2θ = 10.6° and 20.1° indicating its particular crystalline structure [14]. The XRD pattern of homogeneous crosslinked membranes, as occurs in the FTIR-ATR technique, was almost the same as that of pure chitosan, which indicated that the crystallinity of chitosan was not modified. Regarding the heterogeneous route, it was found that the peak at 2θ = 10.6° moved toward higher diffraction angle and the peak at 2θ = 20.1° stayed at the same position although its area decreased after crosslinking. Besides, a set of new peaks can be found above 20°. These results indicate that: a) the low degree of crosslinking achieved using homogeneous methodology was not enough to hinder the crystallization process, justifying the maintenance of crystalline structure and b) using the heterogeneous route, the membranes were crosslinked in the solid state and, therefore, the crystalline structure before
Fig. 2. X-ray diffractograms of chitosan (CS), heterogeneous (CS5 and CS30) and homogenous (CS14 and CS16) crosslinked chitosan membranes.
crosslinking process remains unchanged. However, the appearance of new peaks indicates the formation of another crystalline arrangement. One possibility is the formation of an inorganic salt (Na2SO4) resulting from the reaction between H2SO4 and some residual NaOH, used at the neutralization step [19]. Xiang J. and co-workers [20] found similar results preparing absorbents using chitosan, iodine and sulfuric acid. In this case the authors attribute the new patterns to the formation of K2SO4 crystalline phase. 3.3. TG/DTG and DSC Fig. 3 presents the results of thermogravimetric analysis for CS, CS16, CS14, CS5 and CS30 membranes. The first event occurs between 35 and 100 °C and is accompanied by a weight loss ranging from 12% to 16%. This stage can be assigned to residual water loss present on chitosan [21–23]. Although the amount of residual water did not change drastically, only from 12% on CS to 16% on heterogeneous membranes, it is possible to notice that the peak shapes are not similar and that especially for CS30 the peak is shifted to higher temperatures. Neto et al., 2005 [16] have carried out a detailed analysis of such peak position and shape using DTG while Rueda et al., 1999 [24] made similar investigation with respect to the water at low water content in chitosan using infrared spectroscopy and differential scanning calorimetry (DSC). According to the authors, water interacts with two different polar groups of chitosan, being the interactions with amine groups weaker than those with hydroxyl ones. With that consideration in mind, one can explain the peak shapes observed in Fig. 3. Starting from homogeneous membranes it is possible to observe that peak shapes and areas are very similar to pure chitosan, confirming the low degree of crosslinking, insufficient to promote a drastic change on membrane hydrophilicity. On the other hand, for heterogeneous membranes one observes an increase in peak area and a shift to higher temperatures. In this case it is possible to conclude that, for CS5 and CS30, water preferentially interacts with OH groups, since some of the NH2 groups are involved in crosslinking and, therefore, are not more available to form hydrogen bonds with water molecules. The second significant weight loss for CS is in the range of 240–390 °C with a weight loss of 42%, usually related to the decomposition of the macromolecule. Considering homogeneous membranes, here, once again, their behavior was very similar to CS due to the low degree of crosslinking, as already mentioned. However, for heterogeneous membranes, a completely different pattern of decomposition is observed. In this case, one observe not one, but two peaks, one at 230 °C and the other, smaller, at 290 °C, indicating that the degradation process occurs in two steps. The total weight loss is also a little higher
Fig. 3. DTG curves obtained for chitosan (CS), heterogeneous (CS5 and CS30) and homogenous (CS14 and CS16) crosslinked chitosan membranes.
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(about 48%) that the one observed for CS. In order to get more insight about this process, DSC analysis was carried out and the results are presented in Fig. 4. According to the results, it possible to deduce that the first weight loss is related to an endothermic process and that the enthalpy involved on dehydration of heterogeneous membranes is higher, confirming the preferential interactions of water molecules with OH groups instead of NH2 groups. Regarding the macromolecule degradation itself, DSC also confirms the behavior observed on thermograms for CS and the homogenous membranes i.e. the process occurs in one single step. Besides, it is possible to see that the enthalpy for CS is higher indicating a small decrease in thermal stability due to crosslinking, also observed on DTG through a higher weight loss. Differently, for heterogeneous membranes, DSC confirms the two steps observed on DTG and, in addition, shows that they are endothermic events and not exothermic as the one observed on CS and on homogenous membranes. Similar results were obtained by Mukoma et al. [25], emphasizing the similarity between event temperatures registered by both techniques (TG and DSC) as an indication of their reliability. In order to try to understand such differences it is necessary to take a look on the proposed mechanisms of degradation reported in literature for chitosan. A very comprehensive study was carried out by Mucha et al. [26], suggesting that the process usually begins with random splitting of β-1,4-glycosidic bonds causing formation of free radicals that initiate oxidation. As a result, a carbonyl group appears. Simultaneously, cleavage and/or destruction of its functional groups (amino, amide, carbonyl and hydroxyl) also occurs, generating free radicals that can form strong intermolecular interactions between fragments of chitosan. Both processes justify the exothermic nature of the peak observed on DSC for chitosan and homogeneous membranes. Unlikely, the two endothermic peaks observed for heterogeneous membranes, suggest that, in this case, a different mechanism of degradation is taking place. Reminding that the methodology for heterogeneous membrane involves its immersion in acid solution, followed only by washing, it is reasonable to suppose that the membrane still has an acid residue which would be responsible for the different degradation pattern observed. Several authors have studied the degradation of chitosan in solution [27–29], as much as on solid state [30] in acid medium, using H2SO4 or HCl. According to them, in this case, the above mentioned oxidative-reductive degradation mechanism does not dominate, being replaced by the acid catalyzed hydrolysis, where hydrolysis of the N-acetyl linkages occurs in addition to the cleavage of the glycosidic linkages. Besides the cleavage of glycosidic linkages will form OH terminal groups instead of carbonyl, being therefore an endothermic process.
Fig. 4. DSC analyses of chitosan (CS), heterogeneous (CS5 and CS30) and homogenous (CS14 and CS16) crosslinked chitosan membranes.
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3.4. Swelling index Fig. 5 (a) and (b) show the results for swelling index for all membranes under investigation in neutral and acidic medium, respectively. Starting from Fig. 5 (a), one notices that, in neutral medium, the swelling process occurs very rapidly, reaching equilibrium values almost instantaneously, for all membranes. Besides, the crosslinked membranes present lower water sorption capacity when compared to pure chitosan. The water sorption in chitosan membranes is due to the formation of hydrogen bonds between water molecules and the OH and NH2 groups present on chitosan [7,31,32]. The reduction on swelling index can therefore be associated to the crosslinking process, as it uses some NH2 groups, reducing its availability and also decreases chain mobility, hindering the access of water molecules to the polar groups of chitosan (OH and NH2). This effect is clearly observed on homogeneous membranes, where a reduction from 75% to 60% on swelling index is observed for an increase from 1:6 to 1:4 on SO24 −/NH+ 3 molar ratio. For heterogeneous membranes, it seems that the crosslinking reaction is very fast, reaching its maximum in less than 30 min [1]. Swelling in acidic media (Fig. 5 (b)) for heterogeneous membranes results in a very similar behavior observed in neutral medium. Pure chitosan and the homogeneous ones behaved quite differently, taking more than 1 h to establish equilibrium and reaching much higher values of swelling indexes — from 90% to 160% for CS; 75% to 140% for CS16 and 60% to 125% for CS14. Sorption process takes place from the surface to the inner part of the membrane. In acidic medium NH2 groups become protonated generating NH+ 3 groups. The electrostatic repulsion between them causes chain expansion. This expansion, then, facilitates
Fig. 5. Swelling index of chitosan membranes in neutral (a) and acid medium (b).
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access to the inner part of the membrane, increasing its sorption capacity. For homogeneous membranes, the Coulombic attractions between and NH+ SO2− 4 3 groups will restrain the expansion process, decreasing their swelling indexes, compared to pure chitosan. In case of heterogeneous membranes, it seems that the higher crosslinking degree does not allow chain expansion in acid medium and therefore membrane swelling index is kept practically unchanged. These results corroborate previous ones that indicate a higher degree of crosslinking for membranes obtained by the heterogeneous route. 3.5. Mechanical properties Mechanical properties of the membranes were studied by determining rupture tension, elongation at break and Young modulus. Table 2 summarizes the results for all membranes under investigation. Membrane thickness increases with crosslinking, indicating a shrinkage process that increases with crosslinking degree. Rupture tension decreases around 30%, deformation 85% but the Young modulus increases more than 300%. It is known that mechanical resistance of chitosan is mainly maintained by intra and intermolecular hydrogen bonds [23]. Hence it follows that a decrease on tensile strength and elongation with crosslinking may occur due to a decrease on the amount of hydrogen bonds. On the other hand, the replacement of such hydrogen bonds by and NH+ Coulombic attractions between SO2− 4 3 groups provides a rigid network that is beneficial since improves the Young modulus. Similar results were obtained by some authors, using different crosslinking agents [33–35]. 3.6. AFM Fig. 6 shows a dynamic mode AFM images for pure chitosan membranes as well as the crosslinked ones. Table 3 reports the quantitative data on roughness in terms of average roughness (Ra). For pure chitosan it was found an irregular peak and valley pattern with average roughness of 9.32 nm. Comparing the images and the results showed on Table 3 it can be noted that membranes' roughness and topology changes drastically according to the crosslinking route used. For the heterogeneous route, it can be observed that the contact of the H2SO4 solution with pure chitosan membrane even for short time (5 min) was enough to reduce roughness (from 9.32 to 2.31), turning the surface more regular, relatively flat and smoother. Similar results were already reported by some authors [36]. Homogeneous membranes, on the other hand, present much higher values of roughness (42.23 for CS14 and 45.33 for CS16). Such roughness increase could be a result of cluster formation and/or the mechanism involved on film formation [37,38]. On the first case, the lighter regions are taken as the hydrophilic phase that contains the ionic interactions between NH+ 3 and SO24 − groups while the darker regions contains the hydrophobic uncrosslinked regions. Considering the second possibility, the ionic interaction would increase the solution viscosity and consequently slower the surface flow velocity causing slower motion and, consequently, stronger surface deformations. It is worthwhile to remind that a surface with higher roughness will consequently presents a higher interaction area which would contribute to increase the sorption capacity and also affect the surface wettability and surface reactions. Table 2 Mechanical properties of chitosan membranes. Sample Thickness (mm) CS CS16 CS14 CS5 CS30
0.05 ± 0.01 0.05 ± 0.01 0.06 ± 0.01 0.07 ± 0.01 0.07 ± 0.01
Rupture tension (MPa)
Deformation Young modulus (%) (GPa)
61.8 ± 9.7 43.1 ± 6.2 43.5 ± 4.7 39.9 ± 4.6 39.7 ± 4.2
27.9 ± 8.0 4.02 ± 0.79 4.64 ± 0.89 4.89 ± 0.95 4.46 ± 1.20
1.1 ± 0.3 5.7 ± 0.9 4.4 ± 0.5 4.1 ± 0.5 4.6 ± 1.2
Fig. 6. Dynamic mode AFM images for a) pure chitosan sample measuring 5.0 μm × 5.0 μm; b) CS5 sample measuring 5.0 μm × 5.0 μm; c) CS30 sample measuring 5.0 μm × 5.0 μm; d) CS16 sample measuring10.0 μm × 10.0 μm and e) CS14 sample measuring 10.0 μm × 10.0 μm.
4. Conclusion On the basis of the results reported in the present work one can conclude that both methodologies (homogeneous and heterogeneous crosslinking) are effective to confer acid resistance to the membranes. However, different and significant characteristics were observed. Membranes obtained by heterogeneous route have a higher degree of crosslinking, especially near the surface, lower thermal stability and a different degradation process compared with pure chitosan, they also present lower water sorption capacity, particularly in acidic medium, and a decrease in rupture tension, elongation and increase in Young modulus (compared to CS). Homogeneous membranes, on the other hand, have a low degree of crosslinking, maintaining their crystalline structure, pretty much the thermal stability of pure chitosan and little reduction in water sorption capacity. The most significant changes observed were a decrease in rupture tension and elongation and an increase in Young modulus (compared to CS). The choice of the methodology to be used will depend of the final use of the membranes. Table 3 Ra Values obtained by AFM on 5.0 μm × 5.0 μm scanning area for all chitosan membranes under investigation. Sample
Roughness (Ra) (nm)
CS CS5 CS30 CS16 CS14
9.32 2.31 3.11 45.33 42.23
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For sorption studies, one can conclude that homogeneous membranes would be more recommended since they have higher roughness, that would contribute to a higher sorption capacity, and lower degree of crosslinking where most of the NH2 groups present on chitosan are still available to interact with other compounds. Acknowledgments The authors would like to thank the Materials Department of Federal University of Rio Grande do Norte for AFM analysis. The scholarship for J.S. Marques and J.A.O das Chagas from ANP (Agência Nacional de Petróleo) – PRH22 is also gratefully acknowledged. References [1] Z. Cui, et al., Ionic interactions between sulfuric acid and chitosan membranes, Carbohydr. Polym. 73 (1) (2008) 111–116. [2] C. Muzzarelli, R.A.A. Muzzarelli, Natural and artificial chitosan-inorganic composites, J. Inorg. Biochem. 92 (2) (2002) 89–94. [3] M. Dash, et al., Chitosan — a versatile semi-synthetic polymer in biomedical applications, Prog. Polym. Sci. 36 (8) (2011) 981–1014. [4] M.S.P. De Lima, et al., Chitosan membranes modified by contact with poly(acrylic acid), Carbohydr. Res. 344 (13) (2009) 1709–1715. [5] K.V. Harish Prashanth, R.N. Tharanathan, Chitin/chitosan: modifications and their unlimited application potential — an overview, Trends Food Sci. Technol. 18 (3) (2007) 117–131. [6] M.M. Beppu, et al., Crosslinking of chitosan membranes using glutaraldehyde: effect on ion permeability and water absorption, J. Membr. Sci. 301 (1–2) (2007) 126–130. [7] W. Won, X. Feng, D. Lawless, Separation of dimethyl carbonate/methanol/water mixtures by pervaporation using crosslinked chitosan membranes, Sep. Purif. Technol. 31 (2) (2003) 129–140. [8] A.D.A. Gonsalves, et al., Diferentes estratégias para a reticulação de quitosana, Química Nova On-Line 34 (7) (2011) 1215–1223. [9] Y.M. Lee, S.Y. Nam, D.J. Woo, Pervaporation of ionically surface crosslinked chitosan composite membranes for water-alcohol mixtures, J. Membr. Sci. 133 (1) (1997) 103–110. [10] S. Yong Nam, Y. Moo Lee, Pervaporation of ethylene glycol-water mixtures: I. Pervaporation performance of surface crosslinked chitosan membranes, J. Membr. Sci. 153 (2) (1999) 155–162. [11] S. Yong Nam, Y. Moo Lee, Pervaporation separation of methanol/methyl t-butyl ether through chitosan composite membrane modified with surfactants, J. Membr. Sci. 157 (1) (1999) 63–71. [12] A. Kamari, W.S.W. Ngah, Isotherm, kinetic and thermodynamic studies of lead and copper uptake by H2SO4 modified chitosan, Colloids Surf. B: Biointerfaces 73 (2) (2009) 257–266. [13] P.O. Osifo, A. Masala, Characterization of direct methanol fuel cell (DMFC) applications with H2SO4 modified chitosan membrane, J. Power Sources 195 (15) (2010) 4915–4922. [14] Y. Wan, et al., Ionic conductivity and related properties of crosslinked chitosan membranes, J. Appl. Polym. Sci. 89 (2) (2003) 306–317. [15] Z.M. Dos Santos, et al., Determination of deacetylation degree of chitosan: a comparison between conductometric titration and CHN elemental analysis, Carbohydr. Res. 344 (18) (2009) 2591–2595.
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Comparing homogeneous and heterogeneous routes for ionic crosslinking of chitosan membranes J.S. Marques, J.A.O.D. Chagas, J.L.C. Fonseca, M.R. Pereira ⁎ Chemistry Institute, Universidade Federal do Rio Grande do Norte, Campus Universitário, Lagoa Nova, Natal, RN 59072-970, Brazil
a r t i c l e
i n f o
Article history: Received 29 July 2015 Received in revised form 16 March 2016 Accepted 17 April 2016 Available online 20 April 2016 Keywords: Chitosan Crosslinking Homogeneous Heterogeneous H2SO4
a b s t r a c t H2SO4 — ionically crosslinked chitosan membranes were prepared via homogeneous and heterogeneous routes. + The control variable in homogeneous crosslinking was the SO2− 4 /NH3 molar ratio (1:4 e 1:6) while for heterogeneous crosslinking it was the immersion time of pure chitosan membrane in H2SO4 0.5 M aqueous solution (5 and 30 min). FTIR-ATR suggested lower crosslinking degree for homogeneous crosslinking, corroborated by XRD analysis that indicated the maintenance of the crystalline structure for such membranes. Thermal analysis showed very similar degradation processes for homogeneous and pure chitosan but quite different for heterogeneous: not only in terms of degradation temperature but also in amount and signal of heat involved. Swelling index results were very dependent on pH of medium. Particularly in acidic medium, homogeneous crosslinked membranes, presented a higher swelling capacity than the heterogeneous ones. Mechanical properties revealed that both methodologies render membranes with lower tensile strength and elongation but with Young modulus + about four times higher, due to the interactions of SO2− 4 groups of H2SO4 with NH3 of chitosan. Finally, AFM images showed dramatic changes on surface topology, with reduction of roughness for heterogeneous and an increase for the homogeneous one. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Chitosan is a polyaminosaccharide, produced by partial Ndeacetylation of chitin. It is a biodegradable, non-toxic, semi-synthetic polymer with excellent biocompatibility [1,2]. It can be defined as a copolymer of 2-amino-2-deoxy-D-glucopyranose and 2-acetoamido-2deoxy-D-glucopyranose whose units are linked by β(1 → 4) bonds [3, 4]. Chitosan exhibits unique physicochemical properties and it is highly suitable to innumerable applications in a wide range of fields such as food and nutrition, biotechnology, material science, drugs and pharmaceuticals, agriculture, environmental protection, and gene therapy, among others [5]. As a copolymer soluble in acidic medium, chitosan can be obtained under the form of fibers, films, membranes, powder, particles, beads and solution, which enhances its usefulness. Chitosan has free amino and hydroxyl groups on its backbone, which can undergo chemical modifications in order to turn chitosan more bacteriostatic, and to improve its chemical and mechanical resistance [6]. Among this, crosslinking is one of the most effective approaches for improving membrane stability [7]. There are two main ways to crosslink these membranes: by covalent bonds, using agents such as glutaraldehyde or ethylene glycol, or through ionic bonds, using, e.g., sulfuric acid or tripolyphosphate [8]. The use of sulfuric acid as crosslinking agent has been proved to increase the separation factor in ⁎ Corresponding author. E-mail address: [email protected] (M.R. Pereira).
http://dx.doi.org/10.1016/j.reactfunctpolym.2016.04.014 1381-5148/© 2016 Elsevier B.V. All rights reserved.
pervaporation experiments of ethanol/water and methanol/methyl tbutyl ether [9–11]. Most of the work done on ionic crosslinking of chitosan membranes using sulfuric acid used the heterogeneous route, where the membrane is immersed in a sulfuric acid solution for a period of time and then removed and dried [1,12,13]. The homogeneous route is very little described in literature in comparison with the heterogeneous one. The main difficulty is to achieve a low degree of crosslinking without induce the solution gelification (which could hinder the casting process). As far as we know, most of the homogeneous route described in literature use glutaraldehyde as crosslinking agent [14] but no studies were found using sulfuric acid. In this work, we investigated the influence of the crosslinking routes (homogeneous and heterogeneous) on the final properties of sulfuric acid-crosslinked chitosan membranes. The purpose was to obtain membranes with low degrees of crosslinking, in order to confer chemical resistance in acid medium but still maintain a relatively high number of NH2 groups available for interactions related to sorption applications. 2. Experimental 2.1. Materials Chitosan used in this work was purchased from Polymar LTD (Brazil). It had a deacetylation degree of 88%, determined by CHN elemental analysis and conductometric titration as described elsewhere [15], intrinsic viscosity [η] = 0.360 g L−1, and average viscometric
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molar mass, Mv = 1.6 × 105 g·mol−1 (determined using Mark– Houwink–Sakurada equation) [16,17]. Acetic acid (P. A., Cromato Produtos Químicos LTD, Brazil), Sodium hydroxide (P. A. Vetec LTD, Brazil) and sulfuric acid (P. A. Vetec LTD, Brazil) were all analytical grade reagents and used as received.
submitted to a heating rate of 10 °C/min in nitrogen atmosphere, using alumina crucible and range of temperature from 25 °C to 600 °C. DSC analysis were obtained using a Shimadzu DSC 50, temperature range from 25 °C up to 450 °C, in nitrogen atmosphere (flow of 20 mL/min) and heating rate of 10 °C/min.
2.2. General procedure for membrane preparation
2.5. Mechanical characterization
2.2.1. Homogeneous route Chitosan was dissolved in a 2.0% v/v aqueous acetic acid solution for 24 h under mechanical stirring in order to form a 2.0% w/v solution. The resulting solution was filtered twice, first using a nylon filter and then using a Millipore® Millex filter, with 0.40 μm of pore diameter. A 0.5 M sulfuric acid solution was added to 25 mL of chitosan solution + so that SO2− 4 /NH3 molar ratios of 1:6 and 1:4 were obtained. The mixtures were kept under mechanical stirring for 1 h. Afterwards the solutions were cast onto a glass plate and left at room temperature for 2 days for solvent evaporation. The membranes were then neutralized trough immersion in a 5.0% (w/v) sodium hydroxide solution for 2 h. Following, they were repeatedly washed with distilled water and fixed on a support to dry at room temperature. These membranes were denominated CS16 and CS14.
Ultimate tensile strength, modulus and elongation percentage of the membranes were measured in a universal test dynamometer, Dyna View model AME-5kN (Oswaldo Filizola LTDA, Brazil), according to ASTM D638. The test samples were cut into strips (100 mm × 10 mm). The crosshead speed was 10 mm/min. All samples were tested at room temperature and the data were instantaneously recorded. At least 10 samples of each membrane were measured and the presented results are the average values.
2.2.2. Heterogeneous route Heterogeneous crosslinked membranes were prepared using pure chitosan membranes obtained as described above, without the addition of sulfuric acid solution. After the neutralization and washing processes, the membranes were immersed in sulfuric acid solution 0.5 M for 5 and 30 min. Then the membranes were exhaustively washed with distilled water and fixed on a support to dry at room temperature. These membranes were denominated CS5 and CS30. 2.3. Physico-chemical characterization Infrared spectra were obtained using a Spectrum65 FTIR coupled to a universal ATR sampling accessory from Perkin Elmer, operating in the range of 400–4000 cm−1, 32 scans and resolution of 4 cm−1. Morphologies of chitosan membranes were studied with an X-ray diffractometer Shimadzu model XRD-7000, radiation of CuK α (λ = 1.5406 Å), angle variation 2θ from 5° to 40°, with scanning speed of 2°min−1 and step of 0.02°. The surface roughness and topology at nanometer scale of the membranes were measured, at room temperature, using a Shimadzu SPM9700 atomic force microscope (Kyoto, Japan) in dynamic mode with a resonance frequency of 320 kHz and scan rate of 1.0 Hz. The scan areas were 5.0 μm × 5.0 μm and 10.0 μm × 10.0 μm. Swelling index was gravimetrically determined. The dry membrane (Wd) was initially weighted and promptly immersed in distilled water at room temperature. The membranes were removed from water, wiped off, weighted, and returned to water at different time intervals: five minutes for the first 30 min, 1 h until complete 12 h, and them 24 h and 48 h. The experiments were done in triplicate. The water sorption capacity or swelling index, W(%) was calculated according to Eq. 1: Wð%Þ ¼ ½ðWw–W d Þ=W d 100
3. Results and discussion 3.1. FTIR spectroscopy The first difference observed when comparing all membranes is related to their physical aspects. While pure and heterogeneous crosslinked membranes were transparent, the homogeneous ones were opaque. However, despite their different appearances, all crosslinked membranes behaved similarly when immersed in acetic acid solution for over 24 h, maintaining their integrity and therefore confirming the crosslinking process. Fig. 1 shows the FTIR-ATR spectra of pure and crosslinked membranes while Table 1 summarizes the main absorption peaks for pure chitosan. Regarding the heterogeneous crosslinked membranes, the first spectral changes observed are in the 3600–2500 cm− 1 region, where the O\\H and N\\H2 absorption bands became less distinct, and a new absorption band appeared at 3220 cm−1 and broadened as the reaction time increased. Besides, C\\H stretching vibration peak shifted to lower wavenumbers (from 2875 to 2963 cm−1). The new band at 3220 cm−1 is assigned to the stretching vibration of N+–H [1] indicating that the NH2 groups in chitosan were protonated by the presence of sulfuric acid forming NH+ 3 groups. Crosslinking process occurs when an 2 anion interacts with two NH+ SO− 4 3 groups by ionic attraction. The shift at the C\\H stretching vibration peak is an indication that there was an additional interaction between sulfuric acid and chitosan membrane. At lower wavenumbers, between 1750 and 1500 cm− 1, it is
ð1Þ
where Ww is the membrane wet weight and Wd is the membrane dry weight. The swelling index was also determined in acidic medium using a H2SO4 0.01 mol/L aqueous solution. 2.4. Thermo characterization A Shimadzu TGA-50 thermogravimetric analyzer was used to study the thermal stability of the membranes. Samples with 6 mg were
Fig. 1. FTIR-ATR spectra of chitosan (CS), heterogeneous (CS5 and CS30) and homogenous (CS14 and CS16) crosslinked chitosan membranes.
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Table 1 Infrared band assignments for CS membranes. Peak position (cm−1)
Assignment
3420 3360 3300 2920 2875 1645 1585 1380 1150
O\H \ stretching N\\H2 sym. stretching N\\H2 asym. stretching C\H \ sym. stretching C\H \ asym. stretching C=O stretching (amide I) N\\H2 deformation N\\H2–bending (amide II) C\O \\\C asym. stretching
possible to observe the disappearance of the amide I band (at 1645 cm−1) and of amide II band (at 1580 cm−1) and the appearance of new bands at 1630 cm−1 and 1530 cm−1. In addition, the band at 1150 cm−1 is overlapped by the shifted band at 1055 cm−1. According to the literature [1,18] the new bands represent the bending vibrations of NH+ 3 (antisymmetric and symmetric, respectively), confirming the protonation process. Bands were more intense for the shortest crosslinking time. If one bears in mind that ATR is a technique decisively influenced by the sample-crystal contact, it is reasonable to assume that the membrane crosslinked at short times may be softer than the one crosslinked at longer times, leading to a better sample-crystal contact. Regarding the spectra of homogeneous crosslinked membranes, it was not possible to notice any significant changes when comparing them with uncrosslinked chitosan. These results confirm the low degree of crosslinking obtained using the homogeneous route. 3.2. XRD The XRD patterns of pure and crosslinked chitosan membranes are shown in Fig. 2. Chitosan presented two characteristic peaks around 2θ = 10.6° and 20.1° indicating its particular crystalline structure [14]. The XRD pattern of homogeneous crosslinked membranes, as occurs in the FTIR-ATR technique, was almost the same as that of pure chitosan, which indicated that the crystallinity of chitosan was not modified. Regarding the heterogeneous route, it was found that the peak at 2θ = 10.6° moved toward higher diffraction angle and the peak at 2θ = 20.1° stayed at the same position although its area decreased after crosslinking. Besides, a set of new peaks can be found above 20°. These results indicate that: a) the low degree of crosslinking achieved using homogeneous methodology was not enough to hinder the crystallization process, justifying the maintenance of crystalline structure and b) using the heterogeneous route, the membranes were crosslinked in the solid state and, therefore, the crystalline structure before
Fig. 2. X-ray diffractograms of chitosan (CS), heterogeneous (CS5 and CS30) and homogenous (CS14 and CS16) crosslinked chitosan membranes.
crosslinking process remains unchanged. However, the appearance of new peaks indicates the formation of another crystalline arrangement. One possibility is the formation of an inorganic salt (Na2SO4) resulting from the reaction between H2SO4 and some residual NaOH, used at the neutralization step [19]. Xiang J. and co-workers [20] found similar results preparing absorbents using chitosan, iodine and sulfuric acid. In this case the authors attribute the new patterns to the formation of K2SO4 crystalline phase. 3.3. TG/DTG and DSC Fig. 3 presents the results of thermogravimetric analysis for CS, CS16, CS14, CS5 and CS30 membranes. The first event occurs between 35 and 100 °C and is accompanied by a weight loss ranging from 12% to 16%. This stage can be assigned to residual water loss present on chitosan [21–23]. Although the amount of residual water did not change drastically, only from 12% on CS to 16% on heterogeneous membranes, it is possible to notice that the peak shapes are not similar and that especially for CS30 the peak is shifted to higher temperatures. Neto et al., 2005 [16] have carried out a detailed analysis of such peak position and shape using DTG while Rueda et al., 1999 [24] made similar investigation with respect to the water at low water content in chitosan using infrared spectroscopy and differential scanning calorimetry (DSC). According to the authors, water interacts with two different polar groups of chitosan, being the interactions with amine groups weaker than those with hydroxyl ones. With that consideration in mind, one can explain the peak shapes observed in Fig. 3. Starting from homogeneous membranes it is possible to observe that peak shapes and areas are very similar to pure chitosan, confirming the low degree of crosslinking, insufficient to promote a drastic change on membrane hydrophilicity. On the other hand, for heterogeneous membranes one observes an increase in peak area and a shift to higher temperatures. In this case it is possible to conclude that, for CS5 and CS30, water preferentially interacts with OH groups, since some of the NH2 groups are involved in crosslinking and, therefore, are not more available to form hydrogen bonds with water molecules. The second significant weight loss for CS is in the range of 240–390 °C with a weight loss of 42%, usually related to the decomposition of the macromolecule. Considering homogeneous membranes, here, once again, their behavior was very similar to CS due to the low degree of crosslinking, as already mentioned. However, for heterogeneous membranes, a completely different pattern of decomposition is observed. In this case, one observe not one, but two peaks, one at 230 °C and the other, smaller, at 290 °C, indicating that the degradation process occurs in two steps. The total weight loss is also a little higher
Fig. 3. DTG curves obtained for chitosan (CS), heterogeneous (CS5 and CS30) and homogenous (CS14 and CS16) crosslinked chitosan membranes.
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(about 48%) that the one observed for CS. In order to get more insight about this process, DSC analysis was carried out and the results are presented in Fig. 4. According to the results, it possible to deduce that the first weight loss is related to an endothermic process and that the enthalpy involved on dehydration of heterogeneous membranes is higher, confirming the preferential interactions of water molecules with OH groups instead of NH2 groups. Regarding the macromolecule degradation itself, DSC also confirms the behavior observed on thermograms for CS and the homogenous membranes i.e. the process occurs in one single step. Besides, it is possible to see that the enthalpy for CS is higher indicating a small decrease in thermal stability due to crosslinking, also observed on DTG through a higher weight loss. Differently, for heterogeneous membranes, DSC confirms the two steps observed on DTG and, in addition, shows that they are endothermic events and not exothermic as the one observed on CS and on homogenous membranes. Similar results were obtained by Mukoma et al. [25], emphasizing the similarity between event temperatures registered by both techniques (TG and DSC) as an indication of their reliability. In order to try to understand such differences it is necessary to take a look on the proposed mechanisms of degradation reported in literature for chitosan. A very comprehensive study was carried out by Mucha et al. [26], suggesting that the process usually begins with random splitting of β-1,4-glycosidic bonds causing formation of free radicals that initiate oxidation. As a result, a carbonyl group appears. Simultaneously, cleavage and/or destruction of its functional groups (amino, amide, carbonyl and hydroxyl) also occurs, generating free radicals that can form strong intermolecular interactions between fragments of chitosan. Both processes justify the exothermic nature of the peak observed on DSC for chitosan and homogeneous membranes. Unlikely, the two endothermic peaks observed for heterogeneous membranes, suggest that, in this case, a different mechanism of degradation is taking place. Reminding that the methodology for heterogeneous membrane involves its immersion in acid solution, followed only by washing, it is reasonable to suppose that the membrane still has an acid residue which would be responsible for the different degradation pattern observed. Several authors have studied the degradation of chitosan in solution [27–29], as much as on solid state [30] in acid medium, using H2SO4 or HCl. According to them, in this case, the above mentioned oxidative-reductive degradation mechanism does not dominate, being replaced by the acid catalyzed hydrolysis, where hydrolysis of the N-acetyl linkages occurs in addition to the cleavage of the glycosidic linkages. Besides the cleavage of glycosidic linkages will form OH terminal groups instead of carbonyl, being therefore an endothermic process.
Fig. 4. DSC analyses of chitosan (CS), heterogeneous (CS5 and CS30) and homogenous (CS14 and CS16) crosslinked chitosan membranes.
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3.4. Swelling index Fig. 5 (a) and (b) show the results for swelling index for all membranes under investigation in neutral and acidic medium, respectively. Starting from Fig. 5 (a), one notices that, in neutral medium, the swelling process occurs very rapidly, reaching equilibrium values almost instantaneously, for all membranes. Besides, the crosslinked membranes present lower water sorption capacity when compared to pure chitosan. The water sorption in chitosan membranes is due to the formation of hydrogen bonds between water molecules and the OH and NH2 groups present on chitosan [7,31,32]. The reduction on swelling index can therefore be associated to the crosslinking process, as it uses some NH2 groups, reducing its availability and also decreases chain mobility, hindering the access of water molecules to the polar groups of chitosan (OH and NH2). This effect is clearly observed on homogeneous membranes, where a reduction from 75% to 60% on swelling index is observed for an increase from 1:6 to 1:4 on SO24 −/NH+ 3 molar ratio. For heterogeneous membranes, it seems that the crosslinking reaction is very fast, reaching its maximum in less than 30 min [1]. Swelling in acidic media (Fig. 5 (b)) for heterogeneous membranes results in a very similar behavior observed in neutral medium. Pure chitosan and the homogeneous ones behaved quite differently, taking more than 1 h to establish equilibrium and reaching much higher values of swelling indexes — from 90% to 160% for CS; 75% to 140% for CS16 and 60% to 125% for CS14. Sorption process takes place from the surface to the inner part of the membrane. In acidic medium NH2 groups become protonated generating NH+ 3 groups. The electrostatic repulsion between them causes chain expansion. This expansion, then, facilitates
Fig. 5. Swelling index of chitosan membranes in neutral (a) and acid medium (b).
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access to the inner part of the membrane, increasing its sorption capacity. For homogeneous membranes, the Coulombic attractions between and NH+ SO2− 4 3 groups will restrain the expansion process, decreasing their swelling indexes, compared to pure chitosan. In case of heterogeneous membranes, it seems that the higher crosslinking degree does not allow chain expansion in acid medium and therefore membrane swelling index is kept practically unchanged. These results corroborate previous ones that indicate a higher degree of crosslinking for membranes obtained by the heterogeneous route. 3.5. Mechanical properties Mechanical properties of the membranes were studied by determining rupture tension, elongation at break and Young modulus. Table 2 summarizes the results for all membranes under investigation. Membrane thickness increases with crosslinking, indicating a shrinkage process that increases with crosslinking degree. Rupture tension decreases around 30%, deformation 85% but the Young modulus increases more than 300%. It is known that mechanical resistance of chitosan is mainly maintained by intra and intermolecular hydrogen bonds [23]. Hence it follows that a decrease on tensile strength and elongation with crosslinking may occur due to a decrease on the amount of hydrogen bonds. On the other hand, the replacement of such hydrogen bonds by and NH+ Coulombic attractions between SO2− 4 3 groups provides a rigid network that is beneficial since improves the Young modulus. Similar results were obtained by some authors, using different crosslinking agents [33–35]. 3.6. AFM Fig. 6 shows a dynamic mode AFM images for pure chitosan membranes as well as the crosslinked ones. Table 3 reports the quantitative data on roughness in terms of average roughness (Ra). For pure chitosan it was found an irregular peak and valley pattern with average roughness of 9.32 nm. Comparing the images and the results showed on Table 3 it can be noted that membranes' roughness and topology changes drastically according to the crosslinking route used. For the heterogeneous route, it can be observed that the contact of the H2SO4 solution with pure chitosan membrane even for short time (5 min) was enough to reduce roughness (from 9.32 to 2.31), turning the surface more regular, relatively flat and smoother. Similar results were already reported by some authors [36]. Homogeneous membranes, on the other hand, present much higher values of roughness (42.23 for CS14 and 45.33 for CS16). Such roughness increase could be a result of cluster formation and/or the mechanism involved on film formation [37,38]. On the first case, the lighter regions are taken as the hydrophilic phase that contains the ionic interactions between NH+ 3 and SO24 − groups while the darker regions contains the hydrophobic uncrosslinked regions. Considering the second possibility, the ionic interaction would increase the solution viscosity and consequently slower the surface flow velocity causing slower motion and, consequently, stronger surface deformations. It is worthwhile to remind that a surface with higher roughness will consequently presents a higher interaction area which would contribute to increase the sorption capacity and also affect the surface wettability and surface reactions. Table 2 Mechanical properties of chitosan membranes. Sample Thickness (mm) CS CS16 CS14 CS5 CS30
0.05 ± 0.01 0.05 ± 0.01 0.06 ± 0.01 0.07 ± 0.01 0.07 ± 0.01
Rupture tension (MPa)
Deformation Young modulus (%) (GPa)
61.8 ± 9.7 43.1 ± 6.2 43.5 ± 4.7 39.9 ± 4.6 39.7 ± 4.2
27.9 ± 8.0 4.02 ± 0.79 4.64 ± 0.89 4.89 ± 0.95 4.46 ± 1.20
1.1 ± 0.3 5.7 ± 0.9 4.4 ± 0.5 4.1 ± 0.5 4.6 ± 1.2
Fig. 6. Dynamic mode AFM images for a) pure chitosan sample measuring 5.0 μm × 5.0 μm; b) CS5 sample measuring 5.0 μm × 5.0 μm; c) CS30 sample measuring 5.0 μm × 5.0 μm; d) CS16 sample measuring10.0 μm × 10.0 μm and e) CS14 sample measuring 10.0 μm × 10.0 μm.
4. Conclusion On the basis of the results reported in the present work one can conclude that both methodologies (homogeneous and heterogeneous crosslinking) are effective to confer acid resistance to the membranes. However, different and significant characteristics were observed. Membranes obtained by heterogeneous route have a higher degree of crosslinking, especially near the surface, lower thermal stability and a different degradation process compared with pure chitosan, they also present lower water sorption capacity, particularly in acidic medium, and a decrease in rupture tension, elongation and increase in Young modulus (compared to CS). Homogeneous membranes, on the other hand, have a low degree of crosslinking, maintaining their crystalline structure, pretty much the thermal stability of pure chitosan and little reduction in water sorption capacity. The most significant changes observed were a decrease in rupture tension and elongation and an increase in Young modulus (compared to CS). The choice of the methodology to be used will depend of the final use of the membranes. Table 3 Ra Values obtained by AFM on 5.0 μm × 5.0 μm scanning area for all chitosan membranes under investigation. Sample
Roughness (Ra) (nm)
CS CS5 CS30 CS16 CS14
9.32 2.31 3.11 45.33 42.23
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For sorption studies, one can conclude that homogeneous membranes would be more recommended since they have higher roughness, that would contribute to a higher sorption capacity, and lower degree of crosslinking where most of the NH2 groups present on chitosan are still available to interact with other compounds. Acknowledgments The authors would like to thank the Materials Department of Federal University of Rio Grande do Norte for AFM analysis. The scholarship for J.S. Marques and J.A.O das Chagas from ANP (Agência Nacional de Petróleo) – PRH22 is also gratefully acknowledged. References [1] Z. Cui, et al., Ionic interactions between sulfuric acid and chitosan membranes, Carbohydr. Polym. 73 (1) (2008) 111–116. [2] C. Muzzarelli, R.A.A. Muzzarelli, Natural and artificial chitosan-inorganic composites, J. Inorg. Biochem. 92 (2) (2002) 89–94. [3] M. Dash, et al., Chitosan — a versatile semi-synthetic polymer in biomedical applications, Prog. Polym. Sci. 36 (8) (2011) 981–1014. [4] M.S.P. De Lima, et al., Chitosan membranes modified by contact with poly(acrylic acid), Carbohydr. Res. 344 (13) (2009) 1709–1715. [5] K.V. Harish Prashanth, R.N. Tharanathan, Chitin/chitosan: modifications and their unlimited application potential — an overview, Trends Food Sci. Technol. 18 (3) (2007) 117–131. [6] M.M. Beppu, et al., Crosslinking of chitosan membranes using glutaraldehyde: effect on ion permeability and water absorption, J. Membr. Sci. 301 (1–2) (2007) 126–130. [7] W. Won, X. Feng, D. Lawless, Separation of dimethyl carbonate/methanol/water mixtures by pervaporation using crosslinked chitosan membranes, Sep. Purif. Technol. 31 (2) (2003) 129–140. [8] A.D.A. Gonsalves, et al., Diferentes estratégias para a reticulação de quitosana, Química Nova On-Line 34 (7) (2011) 1215–1223. [9] Y.M. Lee, S.Y. Nam, D.J. Woo, Pervaporation of ionically surface crosslinked chitosan composite membranes for water-alcohol mixtures, J. Membr. Sci. 133 (1) (1997) 103–110. [10] S. Yong Nam, Y. Moo Lee, Pervaporation of ethylene glycol-water mixtures: I. Pervaporation performance of surface crosslinked chitosan membranes, J. Membr. Sci. 153 (2) (1999) 155–162. [11] S. Yong Nam, Y. Moo Lee, Pervaporation separation of methanol/methyl t-butyl ether through chitosan composite membrane modified with surfactants, J. Membr. Sci. 157 (1) (1999) 63–71. [12] A. Kamari, W.S.W. Ngah, Isotherm, kinetic and thermodynamic studies of lead and copper uptake by H2SO4 modified chitosan, Colloids Surf. B: Biointerfaces 73 (2) (2009) 257–266. [13] P.O. Osifo, A. Masala, Characterization of direct methanol fuel cell (DMFC) applications with H2SO4 modified chitosan membrane, J. Power Sources 195 (15) (2010) 4915–4922. [14] Y. Wan, et al., Ionic conductivity and related properties of crosslinked chitosan membranes, J. Appl. Polym. Sci. 89 (2) (2003) 306–317. [15] Z.M. Dos Santos, et al., Determination of deacetylation degree of chitosan: a comparison between conductometric titration and CHN elemental analysis, Carbohydr. Res. 344 (18) (2009) 2591–2595.
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