Synthesis, Characterization and Ionic Conductive Properties of ...

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Macromol. Chem. Phys. 2003, 204, 850–858

Full Paper: Phosphorylated chitosan membranes were prepared from the reaction of orthophosphoric acid and urea on the surface of chitosan membranes in N,N-dimethylformamide. Their ionic conductivity in the wet state was investigated. Chemical modifications contributed to improved ionic conductivities of the chitosan membranes. Compared to the unmodified chitosan membranes, it was found that hydrated phosphorylated chitosan membranes with an appropriate phosphorus content showed an increasing ionic conductivity of about one order of magnitude. The phosphorylation reaction mechanism was explained based on 13C and 31 P NMR measurements. It was also observed that the crystallinity of the phosphorylated chitosan membranes and the corresponding swelling indices were changed pronouncedly, but these membranes did not lose either their tensile strength or thermal stability to a significant degree in comparison with the unmodified chitosan membranes. Possible reaction mechanism for preparation of phosphorylated chitosan membranes.

Synthesis, Characterization and Ionic Conductive Properties of Phosphorylated Chitosan Membranes Ying Wan, Katherine A. M. Creber,* Brant Peppley, V. Tam Bui Department of Chemistry and Chemical Engineering, Royal Military College of Canada, P.O. Box 17000, Station Forces, Kingston, Ontario, Canada, K7K 7B4 Fax: 613-542-9489; E-mail: [email protected]

Keywords: chitosan; fuel cell; ionic conductivity; membranes, modification

Introduction Chitosan, a copolymer of glucosamine and N-acetyglucosamine units linked by 1–4 glucosidic bonds, can be obtained by N-deacetylation of chitin, which is the second most abundant natural polymer. In recent years, chitosan has attracted much attention due to its specific properties such as biodegradability, biocompatibility, and bioactivity. Chitosan is being used extensively in industrial and biomedical areas such as pharmaceutical and biomedical engineering, paper production, textile finishes, photographic products, cements, heavy metal chelation, waste water treatment, and fiber and film formations.[1] Chitosan is also readily converted to fibers, films, coatings, and beads as well as powders and solutions, further enhancing its usefulness. Chitosan has both free hydroxyl groups and amine groups that can be modified readily to prepare chitosan derivatives,[2] which gives some sophisticated Macromol. Chem. Phys. 2003, 204, No. 5/6

functional polymers with properties quite different from those of synthetic polymers. Chitosan is also a cationic polyeletrolyte[3] and chitosan membranes have been used for active transport of chloride ions in aqueous solution.[4] It is a reasonable deduction that if these membranes are set in a closed circuit this kind of active transport can show, more or less, a corresponding ionic conduction. In its natural state (dry state), a chitosan film has a very low electrical conductivity. However, when a chitosan membrane is swollen in water, its amino groups may be protonated and thus contribute to ionic conduction in the membrane. Due to the crystalline nature of chitosan, highly crystalline portions in the chitosan membranes obviously render resistance to water uptake and in turn hinder hydroxide ion transport in the membranes. In order to increase the ionic conductivity of membrane in swollen state, some chemical modifications can be employed to increase the hydrophilicity of the membrane and decrease

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Synthesis, Characterization and Ionic Conductive Properties . . .

crystallinity of the membrane. Nishi and co-workers reported that phosphorylated chitosan with low degree of substitution had shown increased solubility because of a strong interaction between the phosphoryl groups and water.[5] In a similar way, it is expected that the hydrophilicity of phosphorylated chitosan membrane will be increased, and at the same time, its crystallinity will be also decreased, which is expected to leave hydrated chitosan membrane with an increased ionic conductivity. In the present study, phosphorylated chitosan membranes were prepared from the reaction of orthophosphoric acid and urea on the surface of chitosan membranes in N,Ndimethylformamide, and the ionic conductivity of phosphorylated chitosan in the swollen state was investigated. Some results of membrane characterization were also reported.

Experimental Part Materials Chitosan (shrimp-based product) was received in the form of flakes from Sigma-Aldrich Canada Ltd. with a claimed viscosity of 800–2 000 cps for 1 wt.-% chitosan solution in 1% (v/v) aqueous acetic acid. Its molecular weight (M.W.) was reported by the supplier as high M.W. The following chemicals were all obtained and used as reagent grade from either SigmaAldrich Canada Ltd. or Caledon Laboratories Ltd.: acetic acid (99.7%), sodium acetate (99.3%), N-acetyl-D-glucosamine (NAG, 99%, M.W. 221.21), sodium hydroxide pellets (98%), orthophosphoric acid (85%), urea (98%), and N,N-dimethylformamide (DMF, 99%). Deuterium oxide (99.9 atom-% D), sodium 3-(trimethylsilyl)propionate-2,2,3,3-d4 (TSP, 98 atom% D), and acetic acid-d4 (99.6 atom-% D) were provided by CDN ISOTOPES INC. (Canada). Deionized water (resistivity > 1.8 108 O cm) was used for all samples.

Degree of Deacetylation and Molecular Weight of Chitosan The commercial chitosan sample was treated with 50% NaOH aqueous solution under air with constant stirring, for 1 h, at 100 8C to increase its degree of deacetylation. The reaction product was washed subsequently in deionized water until it showed a neutral pH and dried in a convection oven at 50 8C for 2 d. The process was repeated one more time using a sample from the reaction product of the first deacetylation step with the same reaction conditions for 2 h under air. This product was also washed repeatedly in deionized water until neutral pH was reached. The samples were dried first in a convection oven at 50 8C for 2 d and then in a vacuum oven at 50 8C for 24 h. The commercial chitosan sample and deacetylated chitosan samples are designated as CH and CH2, respectively. The degree of deacetylation (DDA) of chitosan was determined using first derivative UV spectroscopy recording on a CARY 5E UV-VIS-NIR spectrometer.[6] A calibration curve from Nacetyl-D-glucosamine (NAG) was generated according to our previous method.[7] The DDA values for CH and CH2 were

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75.4% and 97.6%, respectively. The viscosity-average molecular weight of chitosan was examined using 0.25 M CH3COOH/0.25 M CH3COONa solvent system,[8] and the viscosity-average molecular weight of CH and CH2 were 2.4 106 and 9.27 105, respectively. Preparation of Phosphorylated Chitosan Membrane The chitosan membrane preparation consisted of dissolving chitosan (CH or CH2) in a 1 vol.-% aqueous acetic acid solution with a 0.5–1.0 wt.-% chitosan concentration. The solution was filtered in order to remove undissolved chitosan and debris, and cast on a polystyrene plate. After drying at room temperature for 2 d, the membranes were mounted on a stainless steel holding device and immersed in 2 wt.-% NaOH aqueous solution for 1 h for neutralization. They were then washed intensively with deionized water and air-dried for 1 d. Orthophosphoric acid (2 g, 85%) and 50 g of urea were dissolved in 100 mL of DMF, and added to the chitosan membrane in a reactor. The reaction was conducted at 70 8C for variable time between 10 and 50 min with stirring. After the reaction, the membranes were washed repeatedly with deionized water and dried at room temperature again. CH and CH2 were phosphorylated under the same conditions but various reaction times. These two series of membranes were designated as PCH-10, PCH-20, PCH-30, PCH-40 and PCH2-20, PCH230, PCH2-40 PCH2-50, respectively. Characterization of Membranes A Nicolet 510P Forrier-Transform-IR (FTIR, USA) spectrometer was used to record the infrared spectra of membranes with a resolution of 2 cm1, 64 scans, in transmission mode. Each membrane was dried until a constant weight was reached before the spectrum was made and the sample chamber was purged with dry nitrogen gas. 13 C NMR and 31P NMR measurements were performed on a Bruker AVANCE-500(dmx-500) NMR spectrometer (USA) under a static magnetic field of 125.77 and 202.47 MHz at 60 8C, respectively. For 13C NMR measurements, ca. 40 mg sample (PCH-20 or PCH2-30) was cut into very fine pieces and introduced into a 5 mm f NMR test tube, and dried in a vacuum oven at 50 8C for 2 d. Subsequently, 1.0 mL of a 2 wt.-% CD3COOD/D2O solution was added and the test tube was kept at 60 8C to dissolve the polymer in solution for 24 h. For 31P NMR measurements, PCH-20 or PCH2-30 was dissolved in 2 wt.-% CD3COOD/D2O at a 20 mg/mL concentration. Thermogravimetric analysis (Texas Instruments 2050 Thermogravimetric Analyzer, USA) was used to investigate the thermal stability of the chitosan membranes. Each sample (5–10 mg) was run from 25 to 400 8C at a scanning rate of 10 K/ min under a nitrogen atmosphere. The wide-angle X-ray diffractograms were recorded at room temperature using a SCINTAG X1 X-ray diffractrometer (USA). The X-ray source was Ni-filtered Cu-Ka radiation (45 kV and 40 mA). The dried membranes were scanned from 5 to 458 (2y) at a speed of 28 (2y)/min. To measure crystallinity of membranes, the amorphous areas and the areas of crystalline peaks were measured and the

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Y. Wan, K. A. M. Creber, B. Peppley, V. T. Bui

percent relative crystallinities (Xc) were calculated from the following relationship:[9] Xc ¼ ½Ac =ðAc þ Aa Þ 100%

ð1Þ

where Ac and Aa are the areas of crystalline and amorphous peaks, respectively. Three specimens were measured for each sample. The phosphorus content of chitosan membranes was measured using the molybdenum blue method.[10]

Swelling Index The membrane (dry mass ¼ Wd) was immersed in an excess amount of deionized water at ambient temperature until swelling equilibrium was attained. The mass of the wet membrane (Ww) was obtained after gently removing the surface water with blotting paper. Five specimens were measured for each sample. Swelling index (SI) was then calculated on the basis of the masses of wet membrane and dry membrane using the formula: SI ¼ ½ðWw Wd Þ=Wd 100%

ð2Þ

Ionic Conductivity The conductance measurements were made by following a reported method.[11] A Hewlett Packard Impedance/Gainphase Analyzer, model 4194A (Japan), was used for the impedance spectroscopic analysis of the membranes. Complex impedance measurements were carried out in AC mode, in the frequency range from 0.1 to 104 kHz, and 1 V amplitude of the applied AC signal. The dry membranes were sandwiched between two brass blocking electrodes in the measurement cell. For the impedance analysis in the swollen state, the membranes were immersed in deionized water at room temperature for the required time. Before starting measurements, the surface water was removed, and the swollen membrane was placed quickly between electrodes in the measurement cell. The water content of the membrane was assumed to remain constant during the short period of time required for making the measurement. Five specimens were measured for each sample.

Results and Discussion FTIR Analysis Infrared spectra of the chitosan membranes and the phosphorylated chitosan membranes are shown in Figure 1 and Figure 2. The infrared spectrum of CH in Figure 1 shows a clear amide I band at 1 655 cm1 and an amide II band at 1 586 cm1. In the case of CH2 in Figure 2, however, the amide I band at 1 655 cm1 almost disappears and only a shoulder is observed, and at the same time, bands at the 1 415, 1 375, 1 318 and 1 260 cm1 are decreased in intensity respectively due to the high DDA of CH2. These results are in agreement with published reports.[12] Compared to CH, in the spectrum of PCH-30, the peaks at 1 655 and 1 586 cm1 from amide absorption still remain while new peaks for the stretching vibrations of P O and P–O near 1 258 and 1 000 cm1 have appeared. Compared to CH2, the spectrum of PCH2-40 has only a slight increase in the intensity of the band 1 586 cm1 and new peaks at 1 258 and 1 000 cm1 are also easily noted. These infrared spectra suggest that some hydroxyl groups in chitosan are phosphorylated.

Phosphorus Content and Possible Reaction Mechanism It is found that PCH-10, PCH-20, PCH2-20 and PCH2-30 membrane are soluble in 2 vol.-% aqueous acetic acid, while PCH-30 can be swollen into a gel, and PCH-40, PCH2-40 and PCH2-50 are insoluble in the same solution. The variance of phosphorus content with the reaction time is listed in Table 1. It was observed that phosphorylation occurred most rapidly in the first 30 min. In order to find out the possible reaction mechanism, the 13C NMR and 31P NMR measurements were made to examine the substitution on the chitosan unit. 13C NMR spectra of PCH2-20 and

Tensile Testing Ultimate tensile strength and percent breaking elongation of the membranes were evaluated on an INSTRON tensile testing machine, Model 4206 (USA), according to the standard method (ASTM D882). The test specimens were cut into strips with 70 mm length and 25 mm width. The average thickness of membrane was about 0.2 mm. The relative humidity, gauge length and the crosshead speed were selected as 50%, 30 mm and 5 mm/min, respectively. All specimens were drawn at ambient temperature and all data were instantaneously recorded using a computer. At least five specimens were measured for each sample and the results quoted are the average values.

Figure 1.

Infrared spectra of (a) CH and (b) PCH-30.


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and even polyphosphates may be formed. The polyphosphates are formed not only from inter- and intramolecular condensation of phosphates in chitosan polymer chains, but also from condensation of chitosan phosphate and incoming orthophosphoric acid from the reaction media. The condensation of phosphates can be regarded as a crosslinking reaction as in the case of cellulose[16] because chitosan has a similar chemical structure in the C-6 position of the glucosamine units with cellulose.

Thermal Properties

Figure 2.

Infrared spectra of (a) CH2 and (b) PCH2-40.

PCH2-30 are presented in Figure 3. The spectra show that all the signals from each carbon are well separated from each other. The signal of ca. 63.1 ppm may be assigned to the substituted C-6 which is indicated by C-60 in Figure 3, according to the 13C NMR studies for phosphorylated chitin.[5] The position of phosphorylation can also be distinguished from the signals of proton-decoupled 31P NMR spectra in Figure 4. According to the space hindrance and migration situation of 13C NMR,[13] the peak around 235.9 ppm means that phosphorylation occurs on C-6 hydroxyl groups of chitosan. The appearance of a small peak at ca. 231.7 ppm indicates that one other kind of group in chitosan is phosphorylated. It is believed that some amino groups in chitosan could undergo the reaction to form phosphorylamide groups according to Wang’s report.[14] Based on the 13C NMR and 31P NMR results, formation of the phosphate is thought to involve mainly the primary hydroxyl groups on the chitosan units because they are more reactive than the secondary hydroxyl groups. A possible reaction mechanism of chitosan phosphate in the presence of urea is illustrated in Figure 5, which is similar to the phosphate formation from the reaction of orthophosphoric acid and cellulose.[15] At the beginning of the reaction, the phosphate groups are thought to replace the hydroxyl groups at the C-6 position of glucosamine units, and as the reaction proceeds, pyrophosphates, triphosphates Table 1. Sample

CH PCH-10 PCH-20 PCH-30 PCH-40

Figure 6 and Figure 7 exhibit the thermogravimetric analysis of chitosan membranes and phosphorylated chitosan membranes. The thermal degradation of CH takes place at a maximum rate at about 275 8C, meanwhile the corresponding value for CH2 is shifted to around 279 8C. Even though this temperature shifting seems to be small, it is considered beyond the experimental uncertainty which is estimated as 1.0 8C. In general, it is known that hydrogen bonds between polymer chains contribute to raising the degradation temperature. CH2 has a higher DDA which will facilitate the inter- and intramolecular hydrogen-bonding formation due to the fact that chains of chitosan with higher DDA are more flexible[17] and thus CH2 shows a little higher degradation temperature than that of CH. The degradation temperature for all phosphorylated chitosan membranes drops off a little compared with the unmodified membranes. From the reaction mechanism described above (Figure 5) some hydroxyl groups are replaced by bulky phosphate groups as well as crosslinking the chitosan membrane on the surface or epidermal areas by polyphosphates, resulting in a loss of hydrogen bonding, and hence a lower degradation temperature for corresponding phosphorylated chitosan membranes. However, compared with unmodified membranes, all phosphorylated chitosan membranes exhibit a minimal degree of change in their thermal stability.

Wide-Angle X-Ray Analysis The X-ray patterns are illustrated in Figure 8 and Figure 9, the corresponding crystallinity and swelling index of phosphorylated chitosan membranes are listed Table 2.

The variance of phosphorus content of membranes. Reaction time

Phosphorus content

Sample

2

min

mg/m (2.6%)

0 10 20 30 40

0 62.17 79.63 87.31 96.59

CH2 PCH2-20 PCH2-30 PCH2-40 PCH2-50

Reaction time

Phosphorus content

min

mg/m2 (2.9%)

0 20 30 40 50

0 69.42 83.51 94.28 101.73

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Figure 3. 13C NMR spectra of phosphorylated chitosan in CD3COOD/D2O. The chemical shift was measured from TSP in ppm. Temperature: 60 8C; concentration: 40 mg/mL; pulse angle: 308 (11 ms); 24 000 accumulations.

Figure 6. PCH-40.

TGA thermograms of (I) CH, (II) PCH-30, and (III)

Figure 4. 31P NMR spectra of phosphorylated chitosan in CD3COOD/D2O. Concentration: 20 mg/mL; scans: 1 280. The other conditions are as the same as Figure 3.

The diffractogram of the membranes in Figure 8 and Figure 9 consists of three major crystalline peaks around 108 (2y), 158 (2y) and 208 (2y), which are in agreement with Samuels’s report.[18] The X-ray patterns indicate that, compared to unmodified chitosan membrane, the crystalline structure of phosphorylated chitosan membranes has not been remarkably modified but the peak intensity has decreased markedly. It is also observed that, in Table 2, the crystallinity of phosphorylated chitosan membrane decreases gradually with increasing phosphorus content. The possible reasons for the decreasing crystallinity of membranes may arise from following facts. The chitosan membranes already have their own crystalline structures before they are phosphorylated. Introduction of some bulky phosphate groups and formation of polyphosphates in the membrane only break some inter- and intramolecular

Figure 5. Possible reaction mechanism for preparation of phosphorylated chitosan membranes.

Figure 7. TGA thermograms of (a) CH2, (b) PCH2-40, and (c) PCH2-50.


Figure 8.

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X-ray diffraction patterns of (a) CH and (b) PCH-30.

Figure 9. X-ray diffraction patterns of (A) CH2 and (B) PCH2-40.

hydrogen bonding on the surface or epidermal areas, and the crystalline domains inside the membrane will be not destroyed at all because the chitosan membranes only can be swollen slightly in reaction media (DMF) under the reaction conditions, which finally only decreases the crystallinity of membranes.

The variance of tensile strength and breaking elongation for dry samples of phosphorylated chitosan membranes are illustrated in Table 3. In general, it is known that the larger crystalline regions and higher crystallinity in membranes can enhance the mechanical strength of the membrane if only one factor, crystallinity of membrane, has been considered.[20] The results shown in Table 2 indicate that the crystallinity of phosphorylated chitosan membranes decreases with increasing phosphorus content, and thus, more or less, the tensile strength of membranes should be decreased. However, compared with unmodified membranes, the tensile strength and breaking elongation of phosphorylated chitosan membrane are almost unchanged within experimental uncertainties. The possible reason may come from the effects of polyphosphates. As described in Figure 5, the crystallinity of membranes can be decreased by the incorporation of phosphate, which may decrease the tensile strength of the membrane, but on the other hand, the membranes will be enhanced somewhat by the crosslinking effects of polyphosphates, resulting in an almost unchanged tensile strength and breaking elongation of membranes.

Swelling and Tensile Properties As can be seen in Table 2, the swelling index of phosphorylated chitosan membranes first increases to some point gradually with increasing phosphorus content and decreasing crystallinity of membranes and then decreases. Normally, the grown crystalline portion in a chitosan membrane prevents water from entering membrane and the larger the crystallinity of a chitosan membrane, the smaller the swelling index of the membrane.[19] On the other hand, a bulky phosphate group in phosphorylated chitosan is also more hydrophilic than a hydroxyl in chitosan. Both effects, decreasing crystallinity of membranes and increasing hydrophilicity of membrane, help to increase the swelling index of membranes. However, as the phosphorylated reaction is prolonged, some more polyphosphates will be formed and the crosslinking reaction may occur. This may be the reason why the swelling index of membranes increases for PCH-30 and PCH2-40 but drops off for PCH40 and PCH2-50. Table 2. Sample


Ionic Conductivity Ionic conductivity of the phosphorylated chitosan membranes was determined using the complex impedance method. All impedance measurements were done before

The crystallinity and swelling index (SI) of phosphorylated chitosan membranes. Crystallinity

SI

% (2.1%)

% (4.5%)

13.7 13.1 11.6 10.3 9.1

41 42 51 54 49

Sample


Crystallinity

SI

% (2.7%)

% (3.1%)

18.9 16.2 14.1 11.5 10.2

31 35 41 49 42

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Table 3. Tensile properties of phosphorylated chitosan membranes in dry state. Sample


Tensile strength

Breaking elongation

MPa (3.1%)

% (2.8%)

36.7 33.9 34.2 34.1 35.6

22.1 20.7 21.2 19.8 22.4

and after hydration of the membranes. Typical complexplane plots of imaginary impedance (Z 00 ) versus real impedance (Z 0 ) for the membranes after hydration for 1 h at room temperature are illustrated in Figure 10. These spectra comprise two well-defined regions in the complex-plane, a typical partial semi-circle arc in the high frequency zone that is related to the conduction process in the bulk of the membrane, and a linear region in the low frequency zone that is attributed to the solid electrolyte-electrode interface,[21] and they are also rather similar to the impedance spectrum for Nafion1 117.[22] The starting point of the arc at low frequency for most of the samples, which is normally frequency-dependent, is obtained around 100 kHz, also, at this point the phase angle is also found to be quite small and near zero. Since the complex impedance will be dominated by the ionic conductance when the phase angle is close to zero, all the conductivities reported in this paper were obtained using the real impedance values for which the frequency is near 100 kHz. The variances in conductivities of phosphorylated chitosan membranes are listed in Table 4. It is noted that in Table 4, membranes in the dry state exhibit ionic

Sample


Tensile strength

Breaking elongation

MPa (2.4%)

% (2.6%)

41.1 38.2 39.3 38.5 40.3

19.3 17.8 18.8 19.6 17.4

conductivities between 109 and 1010 S cm1 and the entire conduction process occurs after the water is incorporated in the membranes. Compared with the unmodified membranes, the ionic conductivity of PCH-30 and PCH240 show an increase of about one order of magnitude. From the fact that the ionic conductance will occur only after the membrane is hydrated, it can be concluded that the hydrated properties of membranes will critically affect the ion permeability through the membrane, and a membrane with a relatively high swelling index may allow ions to go through more easily in the swollen state of the membrane. The results given in Table 2 indicate that the swelling indexes of PCH-30 and PCH2-40 are higher than unmodified membranes and, as expected, their corresponding conductivities in Table 4 are also higher. A possible mechanism for ionic conductivity through the membrane may come from the function of free amino groups in the chitosan backbone. When water is incorporated into chitosan membranes, some free amino groups are partly protona ted (NH2 þ H2O $ NHþ 3 þ OH ) leading to the formation of hydroxide ions. Since the NHþ 3 groups are bonded on the backbone, only OH ions are free to move and give an AC ionic current under the action of an AC signal. As seen in Table 1, the content of phosphates is very low and incorporation of these phosphates into chitosan membranes

Table 4. Variances in ionic conductivities of phosphorylated chitosan membranes. Sample

Conductivity S cm1

Figure 10. Impedance spectra of chitosan membranes and phosphorylated chitosan membranes after hydration for 1 h at room temperature. AC mode; 1 V amplitude of applied signal; frequency range: 0.1–104 kHz; & -CH; * -PCH-30; ~ -CH2; ! -PCH2-40.

CH PCH-10 PCH-20 PCH-30 PCH-40 CH2 PCH2-20 PCH2-30 PCH2-40 PCH2-50

before hydration

after hydration

2.2 109 3.7 1010 5.4 109 6.3 109 8.5 1010 8.3 1010 6.7 109 7.2 1010 3.1 109 7.6 1010

8.3 105 9.1 105 3.7 104 1.2 103 4.2 104 6.1 105 6.7 105 3.8 104 9.8 104 7.1 105


just decreases the crystallinity of membranes and improves the hydrated properties of membranes. According to this tentative mechanism, PCH2-40 which contains more NH2 groups compared to PCH-30, should give a higher ionic conductivity. However, by using PCH240, more substantial crystalline regions will be formed more easily because of its higher DDA such that water is prevented from entering the crystalline portion and a large resistance is rendered to water incorporation. This finally decreases the concentration of OH groups in the swollen membrane and in turn the ionic conductivity of the PCH240 membrane. The ionic conductivity of the phosphorylated chitosan membranes may be useful for an alkaline polymer electrolyte fuel cell where a carrier type of membrane for hydroxide ion transport is needed. As long as the water and the hydroxide ions are supplied continuously, the chitosan membrane can act as a hydroxide ion carrier and the fuel cell can work in an uninterrupted way. Of the various polyelectrolyte membranes considered for polymer electrolyte fuel cells (PEFC), the Nafion1 membrane, namely, the perfluorinated membranes, has been almost the only advanced membrane among the most effective and available membranes that are used in practical systems.[23] The Nafion1 N117 membrane has an ionic conductivity in the swollen state of around 2.6 102 S cm1 and a tensile strength in the dry state ca. 43 MPa.[24,25] In spite of their superior characteristics for PEFCs, these membranes do have some weak points.[26] They are strongly limited to the range of temperature over which they can be reliably used. The upper operation limit is usually considered to be 100 8C because of their low water content above that temperature (and hence lower ionic conductivity), and because of accelerated oxidative degradation. In addition, their relatively high cost may also limit their use in mass production of fuel cells. For the most of the known polyelectrolyte membranes used in fuel cells, adequate hydration of such membranes is critical to fuel-cell operation.[27] If the membrane is too dry, its ionic conductivity falls and even loses its ionic conductive property. An excess of water in the fuel cell will also result in a poor cell performance. Although PCH-30 and PCH2-40 show ionic conductivity ca. 103 S cm1, which is just around one order of magnitude lower than the Nafion1 N117 membrane, they exhibit their swelling indexes around 50% and tensile strength in dry state ca. 35 MPa, which are proper values for fuel-cell operation. PCH-30 and PCH2-40, therefore, are attractive candidates for fuel cell applications. However, a fundamental difference for the ionic functionality between the Nafion1 membrane and phosphorylated chitosan membranes has to be pointed out, that is, the Nafion1 membrane is a proton conductor and it only can be used for an acidic polymer electrolyte fuel cell,[28] and the phosphorylated chitosanmembraneisahydroxideionconductoranditmaybe used for an alkaline polymer electrolyte fuel cell.

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Further studies on the ionic conductivity of phosphorylated chitosan membranes and their possible applications in alkaline polymer electrolyte fuel cell are currently underway, and relevant results will be reported separately.

Conclusions Phosphorylated chitosan membranes were prepared from the reaction of orthophosphoric acid and urea on the surface of chitosan membranes in N,N-dimethylformamide. With an appropriate phosphorus content, the crystallinity of phosphorylated chitosan membranes can be decreased and the swelling index will be increased. Phosphorylated chitosan membranes are almost no conductive in their dry states and hydrated membranes show ionic conductive properties. Comparing to the unmodified chitosan membranes, the ionic conductivity of phosphorylated chitosan membranes with appropriate phosphorus content can be improved and increased about one order of magnitude. Although crystallinity of phosphorylated chitosan membranes and the corresponding swelling index were changed pronouncedly, these membranes did not significantly lose their tensile strength and thermal stability in comparison with the unmodified chitosan membranes.

Acknowledgement: The financial support for this work was provided by the National Science and Engineering Council of Canada under the Strategic Grants Program No. 239090-01.

Received: August 26, 2002 Revised: January 7, 2003 Accepted: February 19, 2003

[1] T. Rathke, S. Hudson, J. Macromol. Sci., Rev. Macromol. Chem. Phys. 1994, C34, 375. [2] K. Kurita, Prog. Polym. Sci. 2001, 26, 1921. [3] B. Phillip, H. K. Dautzenberg, J. Linow, J. Kotz, Prog. Polym. Sci. 1989, 14, 91. [4] T. Uragami, F. Yoshida, M. Sugihara, Makromol. Chem., Rapid Commun. 1983, 4, 99. [5] N. Nishi, A. Ebina, S. Nishimura, A. Tsutsumi, O. Hasegawa, S. Tokura, Int. J. Biol. Macromol. 1986, 8, 311. [6] R. A. A. Muzzarelli, C. Jeuniaux, G. W. Gooday, ‘‘Chitin in Nature and Technology’’, Plenum, New York 1986, p. 385. [7] J. Z. Knaul, M. R. Kasaai, V. T. Bui, K. A. M. Creber, Can. J. Chem. 1998, 76, 1699. [8] M. R. Kasaai, G. Charlet, J. Arul, in: ‘‘Advances in Chitin Science, Vol. II, Proceeding of the 7th International Conference Chitin and Chitosan’’, A. Domard, K. Varum, R. Muzzarelli, Eds., Jaques Andre Publisher, Lyon 1998, p. 421. [9] J.K. Rabek, ‘‘Experimental Methods in Polymer Chemistry: Applications of Wide-angle X-ray diffraction (WAXD) to the Study of the Structure of polymers’’, Wiley-Inter Science, Chichester 1980, p. 505.

858


[10] H. H. Furman, ‘‘Standard Methods of Chemical Analysis’’, Vol. 1, 6th edition, Van Nostrand, New York 1975, p. 818. [11] A. Mokrini, L. J. Acosta, Polymer 2001, 42, 8817. [12] S. Mima, M. Miya, R. Iwamoto, S. Yoshikawa, J. Appl. Polym. Sci. 1983, 28, 1909. [13] S. Tokura, N. Nishi, A. Tsutsumi, O. Somorin, Polym. J. (Tokyo) 1983, 15, 485. [14] X. Wang, J. Ma, Y. Wang, B. He, Biomaterials 2001, 22, 2247. [15] S. Ueda, K. Oyama, K. Koma, J. Chem. Soc. Jpn., Ind. Eng. Chem. Sect. 1963, 66, 586. [16] K. Katsuura, N. Inagaki, J. Chem. Soc. Jpn., Ind. Eng. Chem. Sect. 1966, 69, 681. [17] W. M. Anthonsen, K. M. Varum, O. Smidsrod, Carbohydr. Polym. 1993, 22, 193. [18] R.J.J.Samuels,J.Polym.Sci.,Polym.Phys.Ed.1981,19,1081. [19] R. H. Chen, H.-D. Hwa, in: ‘‘Advances in Chitin Science’’, Vol. 1, A. Domard, C. Jeuniaux, R. Muzzarelli, G. Roberts, Eds., Jacquea Andre Publisher, Lyon 1996, p. 346.

[20] J. H. Kim, J. Y. Kim, Y. M. Lee, K. Y. Kim, J. Appl. Polym. Sci. 1992, 45, 1711. [21] P. D. Beattie, F. P. Orfino, V. I. Basura, K. Zychowska, J. Ding, C. Chuy, J. Schmeisser, S. Holdcroft, J. Electroanal. Chem. 2001, 503, 45. [22] C. L. Gardner, A. V. Anantaraman, J. Electroanal. Chem. 1995, 395, 67. [23] S. Gottesfeld, T. A. Zawodzinski, Adv. Electrochem. Sci. Eng. 1997, 5, 195. [24] J. J. Sumner, S. E. Creager, J. J. Ma, D. D. DesMarteau, J. Electrochem. Soc. 1998, 145, 107. [25] S. J. Sondheimer, N. J. Bunce, C. A. Fyfe, J. Macromol. Sci., Rev. Macromol. Chem. Phys. 1986, C26, 353. [26] O. J. Savadogo, J. New Mater. Electrochem. Syst. 1998, 1, 47. [27] T. A. Zawodzinski, J. C. Derouin, S. Radzinski, R. J. Sherman, V. T. Smith, T. E. Springer, S. Gottesfeld, J. Electrochem. Soc. 1993, 140, 1041. [28] L. J. M. J. Blomen, M. N. Mugerwa, ‘‘Fuel Cell System’’, Plenum Press, New York 1993.