Natural Cellulose-Chitosan Crosslinked Superabsorbent Hydrogels

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Natural Cellulose-Chitosan Crosslinked Superabsorbent Hydrogels with Superior Swelling Properties Md Nur Alam, and Lew Christopher ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01062 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 23, 2018

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ACS Sustainable Chemistry & Engineering

Natural Cellulose-Chitosan Crosslinked Superabsorbent Hydrogels with Superior Swelling Properties Md Nur Alam, Lew P Christopher*

Biorefining Research Institute, Lakehead University, 1294 Balmoral Street, Thunder Bay, Ontario, P7B 5Z5, Canada

ABSTRACT: We have developed a new, aqueous-based process for production of superabsorbent materials that is catalyst-free and eco-friendly as the superabsorbent was derived from two completely biodegradable polymers with water as the only by-product. The new hydrogels were obtained by crosslinking partially oxidized bleached kraft pulp fibers with carboxymethylated chitosan. In distilled water, the maximum water retention value (WRV) of the crosslinked hydrogels reached 610 (g/g gel), which is several times higher than any neat cellulose-based superabsorbent material reported in literature. In saline water, the WRV of the new hydrogels (85 g/g) doubled that of commercial gels (40-50 g/g). In presence of potassium or ammonium cations, the WRV increased further to reach 91 and 96 g/g, respectively. Gels only lost 5-10% of their re-swelling capacity when reused four consecutive times. The hydrogels had high porous architecture and specific surface area that facilitates rapid mass penetration in superabsorbent applications. Due to their superior swelling properties, reusability, and biodegradable nature, the new hydrogels could serve as strong candidates to replace synthetic petroleum-derived polymers, and find uses in high-value hygiene, food, agricultural and pharmaceutical products.

KEYWORDS: Superabsorbent gels, Cellulose, Chitosan, Crosslinking; Water Retention Value, Re-swelling

* Corresponding author email address: [email protected]

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INTRODUCTION Superabsorbent materials, also referred to as superabsorbents, are three-dimensional crosslinked hydrophilic, linear or branched polymers capable of absorbing and retaining liquids, bodily fluids and blood solutions.1,2 Because of their excellent hydrophilic properties, high swelling capacity, and user-friendliness, superabsorbent gels have been widely used in personal care products,3,4 agriculture,5 biomedical applications,6,7 removal of heavy metals8,9 and drug delivery.10,11 Most of the common superabsorbent materials in use today are based on crosslinked synthetic polymers, in particular acrylic acid and its co-polymers with acrylamide.12–16 Superabsorbent polymers (SAPs), also referred to as “slush powder”, are produced by either solution polymerization of partially neutralized acrylic acid, or by suspension polymerization to form a gel that can absorb water up to 1,000 times its weight. The crosslinking reaction immobilizes the SAPs within their molecular structure to prevent dissolution in water. The crosslinking density and charge groups have a direct impact on the absorption capacity of SAPs.17 The higher the charge on the polymer, the better the water absorbancy, especially in presence of salts. Acrylic acids as well as acrylamide are currently derived from petroleum products that are not renewable or biologically degradable. In contrast, natural polymers such as natural cellulose-based superabsorbents have better biocompatibility and lower latent toxicity than most synthetic polymers.18-21 The superabsorbent cellulose-derived materials that currently exist on the market contain polymeric particles, which are either based on synthetic polymers, such as polyacrylates, sulfonated polystyrene, polyvinyl alcohol, etc., or on natural polymers, such as carboxyalkyl cellulose, gum, carboxyalkyl starch, cellulose sulphate, etc.22,23 The maximum absorbency of such materials ranges from 10 to 100 g water/g material, which is far less than that of commercial synthetic polymers (~1,000 g/g). More recently, cellulose-based superabsorbent gels were prepared using different crosslinking techniques.24-27 Yoshimura et al.28 described cellulose-based highly absorbent gels produced by crosslinking cellulose with succinic anhydride through etherification reaction. This gel was able to absorb water 400 times the gel’s dry weight.28 Jeanette et al.29 have reported a SAP prepared by crosslinking co-dissolved carboxymethyl cellulose and hydroxyethyl cellulose (HEC) with divinyl sulfone (DVS) with a water absorbent capacity of up to 425 g water/g material. However, in comparison to synthetic, polyacrylate-based SAP, cellulose-based superabsorbents have been often reported to lack complete biodegradability, be more expensive, and have inferior liquid absorption capacity. 2 ACS Paragon Plus Environment

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In an attempt to address above challenges and overcome some of the shortcomings of existing technologies, we have developed a new, aqueous-based and environmentally-friendly process for production of superabsorbent biodegradable materials from wood-derived cellulosic fibers. The process is based on a chemically modified method for periodate oxidation of cellulose to generate aldehyde groups that can be further oxidized to carboxyl groups and subsequently crosslinked with chitosan. Next to cellulose and hemicellulose, chitosan is the third most abundant polysaccharide in nature. Chitosan is a linear cationic polysaccharide composed of randomly distributed β-(1→4)linked D-glucosamine (deacetylated unit) and N-acetyl-D-glucosamine (acetylated unit), produced commercially by deacetylation of chitin shells of shrimp and other crustaceans. Chitosan is nowadays recognized as a versatile biomaterial because of its non-toxicity, low allergenicity, antibacterial properties, biocompatibility and biodegradability, which justifies its wide biomedical and pharmaceutical applications. The cationic nature of chitosan allows it to form electrostatic complexes or multilayer structures with other negatively charged synthetic or natural polymers. Both the amino (-NH2) and hydroxyl (-OH) groups of chitosan can be engaged in specific grafting and crosslinking reactions with hydrophilic molecules, including natural polymers such as cellulose and starch.30-32 Synthetic SAPs have also made use of chitosan as a crosslinking agent for graft copolymerization with acrylamide, acrylic acid and acrylonitrile.33-34 In this work, we have employed chitosan as a green crosslinker by taking advantage of the functionality of its amine groups to assist in forming covalent bonds. Here we describe the new process and its advantages, and report on the properties of the new superabsorbent materials and their potential applications.

EXPERIMENTAL SECTION Materials. Bleached softwood kraft (BSWK) pulp was provided by a kraft pulp mill in Canada. Sodium (meta) periodate, sodium chloride, ethylene glycol, hydroxylamine hydrochloride, chitosan (high molecular weight 310-375 KDa, viscosity 800-2000 cP), sodium polyacrylate (cross-linked, particle size 90-850 µm, cat. no CAS Number 9003-04-7), and hydrochloric acid were purchased from Sigma-Aldrich (Mississauga, Ontario, Canada). Sodium hydroxide and ethanol were supplied by Thermo Fisher Scientific (Whitby, Ontario, Canada). All chemicals were used as received. Preparation of Reactive Cellulose. Cellulose fibers (softwood kraft pulp) were subjected 3 ACS Paragon Plus Environment


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to two successive chemical treatments that were carried out to achieve various degrees of reactive cellulose. Initially, periodate oxidation (well-known stereospecific reaction in cellulose chemistry) was performed to convert 27.5% of all hydroxyl groups present in cellulose to aldehyde groups.35 In the second step, chlorite oxidation was applied to convert 70% of all aldehyde groups, formed in the first step, to carboxyl groups.9 The periodate oxidation was carried out in aqueous media using a glass beaker with overhead stirrer, according to the following reaction conditions: bleached softwood kraft pulp (10.0 g), sodium meta-periodate (6.6 g; 50 mole % based on moles of anhydroglucose units) and sodium chloride (14.5 g; 0.5 N in the overall solution) were added in 500 mL deionised water. The reaction mixture was gently stirred at room temperature in the dark for 72 h. The modified cellulose with an aldehyde content of 5.1 mmol/g cellulose was filtered out and washed with diionized water repeatedly. Periodate-oxidized cellulose fibers were partially chemically modified via chlorite oxidation36 of the aldehyde groups to obtain fibers with 2-4 mmol carboxyl groups/g fiber. The chlorite oxidation was carried out on 3 g of periodate-oxidized pulp using 1.62 g of sodium chlorite (80% pure), 5.85 g of sodium chloride and 1.62 g of hydrogen peroxide (30 wt% solution), dissolved in 200 ml water. The reaction mixture was stirred at room temperature for 6 h and kept at pH 5 by drop-wise addition of NaOH solution (necessary during first 3 h). After reaction was complete, cellulose fibers were recovered by adding two volumes of ethanol to one volume of reaction mixture, which facilitated fiber coagulation and separation by filtration. The product was washed with acetone twice and dried at room temperature. Preparation of Carboxymethylated Chitosan. Carboxymethylation of chitosan was carried out according to a previously described method.9 Sodium hydroxide (1.35 g) was first dissolved in a propanol/water mixture with a volume ratio of 8:2. Thereafter, 1 g of high molecular weight chitosan was added, stirred, and allowed to swell at room temperature for 1 h. Subsequently, 1.5 g of chloroacetic acid was dissolved in 2 mL of propanol and added to the chitosan slurry. The mixture was let to react for another 4 h at room temperature. The reaction was then stopped by adding 50 mL of 70% ethanol and filtered through a nylon cloth. The white solid retained on the cloth was washed with 80% ethanol 4 times, and then washed with anhydrous ethanol. Finally, the powder was dried in an oven at 50°C to obtain carboxymethylated chitosan. 4 ACS Paragon Plus Environment

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Preparation of Cellulose-Chitosan Hydrogels. The chemically modified (reactive) cellulose (1 wt % solution) was heated with 1 wt% chitosan (both unmodified and carboxymethylated chitosan) solution at three different ratios (Table 1) while stirring at 60°C for 1 h. Then, the two sets of samples were mixed by magnetic stirring at 500 rpm for 1 min and set for 4-6 h at room temperature to form a cellulose-chitsan cross-linked gels. Characterization of Crosslinked Hydrogels. The crosslinked gel was dried in an oven at 60°C and then ground in a lab-scale, high-speed grinder to a particle size of 400 - 600 µm. The particle size was measured with an optical microscope (Olympus CKX41, Olympus Corp., Tokyo, Japan).The dried samples were analyzed by Fourier Transform Infrared Spectroscopy (FT-IR) spectrometer “Bruker Tensor 37” (Bruker, Ettlingen, Germany) with PIKE MIRacle diamond Attenuated Total Reflectance (ATR) accessory. Solid samples were placed directly on the ATR crystal. Maximum pressure was applied by lowering the tip of the pressure clamp using a rachet-type clutch mechanism. The sample spectra of all 32 scans were averaged (from 550 to 4,000 cm-1) with a resolution of 4 cm-1. The gel morphology was examined using Hitachi Su-70 (Hitachi, Chiyoda, Tokyo, Japan) Field Emission Scanning Electron Microscopy (SEM). The gels swollen to equilibrium in distilled or saline water at 23oC and neutral pH for 24 h, then frozen and freeze-dried using LABCONCO Free-zone 2.5 (LABCONCO, Kansas City, USA). The freeze-dried samples were placed on double-sided carbon adhesive discs attached to aluminium specimen stubs and then sputter-coated with gold to improve specimen conductivity. The images were taken at an accelerating voltage of 5 kV. Determination of Aldehyde Groups. The aldehyde content of periodate-oxidized cellulosic fibers was determined as described by Alam et al. 2012.37 The hydroxylaminehydrochloride (NH2OH·HCl) standard titration method was used, according to which the HCl released from the reaction of aldehydes with NH2OH·HCl is determined by titration with NaOH solution of known normality. The aldehyde content of cellulose was calculated using Eq. 1: Ac = (VNaOH x NNaOH)/DWc

(1)

where Ac is aldehyde content of cellulose (mmol/g cellulose), VNaOH is volume of NaOH (mL) required for titration, NNaOH is normality of NaOH (eq/L), DWc is weight of dry cellulose initially dissolved (g). Determination of Carboxyl Groups. The carboxyl content of the crosslinked gels was determined using a conductometric titration method38 with a METER pH/conductivity S470-KIT (Mettler-Toledo GmbH, Greifensee, Switzerland) titrator. A certain amount of cellulose-chitosan 5 ACS Paragon Plus Environment


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cross-linked gels (with a solid content of ∼20 mg dry basis) and 2.5 mL of a 0.02 M sodium chloride solution was added, and the mixture was sufficiently stirred to prepare a well-dispersed solution. Then 0.1 M HCl was slowly added to the mixture to set the pH to 3. Then, the suspension was titrated with a 0.01 M NaOH solution at a rate of 0.1 mL/min until the mixture reached pH 11. The carboxyl content of the cross-linked gels was determined from the conductivity curves by means of the following equation (Eq. 2): [COOH]g = (VNaOH x MNaOH)/DWg

(2)

where [COOH]g is the carboxyl content of cross-linked gels (mmol/g gels), VNaOH is the volume of NaOH (mL) required for deprotonation of carboxylic groups, MNaOH is the molarity of NaOH (eq/L), and DWg is the weight of dry gels initially dissolved (g). Water Retention Value (WRV). The superabsorbent gels were immersed in distilled water or 0.9 wt% NaCl aqueous (saline) solution. At each time interval, swelling gels were withdrawn from the solution and weighed out after removing the excess liquid off the gel surface. Excess water (or salt solution) was removed by filtration using a 20 µ mesh nylon cloth. The free swell water retention value (WRV), a measure of the dynamic water absorption properties of the gel, was calculated (Eq. 3) as follows: WRV = (Wt - Wd)/Wd × 100

(3)

where Wt is the weight of the wet gel at time t, and Wd is the weight of the dried gel. Re-swelling of Crosslinked Hydrogels. To measure the water re-swelling kinetics, the hydrogels (crosslinked with 25 wt% modified chitosan) were first bone-dried in an oven at 50oC for 12 h. Thereafter, 0.2 g of the dried, water-free gels were soaked in 200 mL of distilled water at room temperature for up to 180 min. The WRV of the swollen gels was then measured according to Eq. 3. The drying-soaking operation described above represented one re-swelling cycle. This re-swelling cycle was repeated four times using the same hydrogel. Crosslinking Density (CD). The CD of the hydrogel is provided by the average molar mass (Mc) that is entrapped between any two crosslinking points contained in the gel. The CD value is reversely proportional to Mc, therefore the higher the Mc the lower the CD. The Mc value of any hydrogel can be calculated using a previously developed method39 based on the Flory-Huggins theory40. Mc = Q5/3 (D2 V1/(0.5 – X1))

(4)


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where Q is the equilibrium water absorption of the hydrogels, D2 is the density of the hydrogels, V1 is the molar volume of the solvent used for swelling, and X1 is the Flory-Huggins interaction parameter between solvents and hydrogels. Salt Sensitivity Factor (f). The influence of ions (cations and anions) on the swelling capability of hydrogels was studied as described by Francisco et al.41 and a dimensionless salt sensitivity factor (f) was calculated (Eq. 5) as follows: f = 1 - (Ws / Ww)

(5)

where Ws and Ww is the equilibrium swelling capacity in saline solution, and in deionized water, respectively. All salts were used at a concentration of 0.15 mol/L.

RESULTS AND DISCUSSION Chemical Crosslinking of Hydrogels. The chemically modified cellulose (following periodate and chlorite oxidation) was crosslinked with chitosan using the Schiff base reaction. Crosslinking occurred between the amine groups of chitosan and the aldehyde groups of cellulose to create imine bonds, as shown in Figure 1.9,42 The crosslinking reaction required no catalyst, and no by-products were formed (except water), hence, no additional post-purification steps were needed. The cellulose and chitosan content of the crosslinked gels is listed in Table 1. Crosslinking plays an important role in the formation and properties of superabsorbent gels: 1) reduces gel hygroscopicity and prevents swelling;43 2) the degree of crosslinking impacts the integrity and strength of the crosslinked network; 3) prevents dissolution; 4) can increase crosslinking density/ decrease swelling capacity by adding more chitosan (Table 1).

Figure 1. Crosslinking reaction between cellulose aldehyde and chitosan.



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Increased crosslinking caused a denser macromolecular network with less capillary spaces in the crosslinked gel. The crosslinked network structures effectively screened the interior of the gels from the flux of incoming water, which limits the hydration of cellulose molecules. The higher the water absorbency, the better of the gel quality for absorbing applications. The water retention value of gels crosslinked with 25% carboxymethylated chitosan was around 600 g water/g gel (Table 1). Increasing the chitosan content to 35% led to a reduction in the WRV whereas gels prepared with 15% chitosan had unstable physical appearance and could not retain water due to unstable physical formation (Table 1). We chose to continue further work with crosslinked gels containing 25 wt% chitosan. Bonding between dialdehyde cellulose and chitosan was studied using FT-IR (Figure 2). The broad peak at 3,300-3,500 cm-1 was due to stretching of a large number of (–OH) groups. The peak at 1,300 cm-1 indicated –OH bending vibration44 whereas the peaks at 1,050, 1,430 and 2,900 cm-1 were assigned to CH2–O–CH2 stretching, –CH2 scissoring, and C–H vibration, respectively.45 The chitosan spectrum revealed: a band at 3,423 cm-1 that is characteristic of –OH

Table 1. Composition and Properties of Crosslinked Hydrogels sample code

cellulose content (wt %)

chitosan content (wt %)

WRV (g distilled water/g gel)

G-1

85

15

ND*

weak

G-2

75

25

610

moderate

G-3

65

35

420

strong

physical condition of gel

*ND, not determined due to weak hydrogel condition

group stretching vibration; two bands at 1,651 cm-1 and 1,559 cm-1 for the stretching and bending vibrations of amide I and –NH2, respectively; and a band at 1,384 cm-1 indicative of stretching vibrations of –CH3 groups (Figure 2). The non-crosslinked dialdehyde cellulose did no show any bands in the 1,730 cm-1 region which suggested that all aldehyde groups had been involved in the crosslinking reaction. Noteworthy, following periodate oxidation, the characteristic aldehyde peak at 1,730 cm-1 was 8 ACS Paragon Plus Environment

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not very sharp (Figure 2). The accurate spectroscopic detection of this peak is often hampered by the presence of hydrates and hemiacetals, which are formed depending on the moisture content of the sample material.46 Previous studies, however, have suggested that the imine band at 1,610 cm-1 may represent the stretching vibrations of C=N for Schiff’s base reaction between aldehyde and chitosan.47-49 Although the crosslinked gel had a new strong peak at 1,580 cm-1, crosslinking could not be verified from the FT-IR spectra (Figure 2) because of possible overlap with amide I and unreacted –NH2 stretching.

158 Absorption

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Gel

165 155

Modified Unmodified Chlorite reaction of cellulose Periodate reaction of cellulose Unmodified cellulose 4000

3500

3000

2500 2000 Wavenumber (cm-1)

1500

1000

500

Figure 2. FT-IR spectra of cellulose hydrogels crosslinked with chitosan.

Swelling Properties of Hydrogels. The free swell water absorbency capacity (WRV) provided indication of the swelling behaviour and capacity of gels to retain water. Overall, the WRV in distilled water is significantly higher than saline water. For instance, the WRV of gels crosslinked with unmodified and modified chitosan (25 wt%) was 360 and 610 g distilled water/g gel, respectively (Table 2). With saline water, the WRV of gels crosslinked with modified chitosan was only 85 g saline water/g gel (Table 2). Although this value is several-fold 9 ACS Paragon Plus Environment


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lower than the WRV of the same gels in distilled water, it is two-times higher than that of commercial gels in saline water (40-50 g/g).19 Modified chitosan (carboxymethylated chitosan) gels exhibit higher absorption capacity than gels containing unmodified chitosan due to the presence of more carboxyl groups. The higher the carboxyl charge on the gel, the greater the water absorbancy, especially in presence of metal salts.50-51 The charge screening effect caused by cations (Na+, K+, Mg2+ and Ca2+) in saline water could induce a decline of anion-anion electrostatic repulsions, leading to a reduced osmotic pressure between the hydrogel network and the external solution.50 As a result, the osmotic swelling pressure of the mobile cations inside the gel decreases which leads to a gel collapse.51 Lower swelling ability is attributed to higher CD of hydrogels. Therefore, at higher CD, the water permeation into the hydrogel matrix becomes more difficult, which affects the gel swelling capacity. As evident from Table 2, the Mc of the modified chitosan gels was nearly 1.5-fold higher than unmodified chitosan gel. This was due to the repulsion effect caused by the carboxyl groups present on the modified chitosan and cellulose macromolecules. The repulsion between the carboxyl groups of the two polymers prevented the same extent of crosslinking as in the case of unmodified chitosan gels, where carboxylic groups in chitosan were absent. The higher Mc of the modified chitosan gels was also associated with higher amount of carboxyl groups (30%), higher WRV in distilled water (69%) and higher WRV in saline water (78%) (Table 2). In addition to cellulose, absorbent gels prepared from conventional bleached kraft pulp fibers contain up to 20% hemicellulose. As hemicellulose is often branched and carboxylated, this could contribute to enhanced liquid absorbency of gels. In addition, the hydroxyl groups of both cellulose and hemicellulose are hydrophilic in nature, which makes gels more hydrophilic and helps increase the gel absorption capacity.


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samples

carboxyl groups (mmol/g)

Mc (g/mol)

WRV in distilled WRV in saline water (g/g) water (g/g)

gels with unmodified chitosana

2.6

1.72 x 105

360

45

gels with modified chitosana

3.4

2.54 x 105

610

85

unmodified kraft pulp fibers

0.035

-

2-10

1-5

commercial SAP

~ 13

3.13 x 105

~ 1,00012

40-5012

Table 2. Properties of Cellulose Hydrogels a

Cellulose hydrogel crosslinked with 25 wt% chitosan

The WRV of modified chitosan gels in distilled and saline water are shown in Figure 3. As expected, the equilibrium WRV of the cellulose gels in distilled water increased rapidly (300 times within 10 min). The WRV of the modified chitosan gel peaked at 610, which is higher than any other neat cellulose-based gel described in literature.24-29 One of the main drawbacks of cellulose-based absorbent materials is their inferior liquid absorption capacity to that of synthetic SAP.50 This study however reports on improved liquid absorption capabilities of polysaccharidebased gels in terms of WRV in both distilled and saline water that are comparable to those of commercial, non-biodegradable SAP.

Figure 3. WRV of cellulose hydrogels crosslinked with 25 wt% modified chitosan.



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The hydrogel re-wetting performance in distilled water following two re-swelling cycles is displayed in Figure 4. As shown, the water uptake capacity of the hydrogel during the soaking phase of cycle 2 decreased by 5-10% compared to cycle 1. This is most likely due to the formation of additional hydrogen bonds between the COOH and OH groups of cellulose that occurred during cycle 1. Following cycle 2, a similar re-swelling behavior without a further drop in the WRV of the hydrogels was observed for cycle 3 and cycle 4 (data not shown). The gel swelling behavior is also strongly dependent on the “type” of salt added to the swelling medium. The effect of the ion types on the swelling capability of hydrogels is shown in Table 3. As observed before, due to the presence of more carboxyl groups, the modified chitosan gels exhibited a higher absorption capacity than gels containing unmodified chitosan. Interestingly, using the same salt medium, a small change in the f value corresponded to a

Figure 4. WRV of cellulose hydrogels crosslinked with 25 wt% modified chitosan following two consecutive re-swelling cycles.

significant change in the Ws value. For example, in presence of NH4Cl, the difference between the f values for unmodified and modified chitosan gels was only 0.04 points whereas the corresponding difference in the Wc values was 41 points. When comparing different monovalent cations, the Wc with both gels decreased in the following descending order: NH4+ > K+ > Na+ (Table 3). This can be explained by the fact that the charge screening effect of these cations on 12 ACS Paragon Plus Environment

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the Wc decreased in the same order. As can also be seen from Table 3, in presence of different salts, the absorption capacity of the hydrogel declined in the order monovalent > divalent > trivalent cations. Apparently, the degree of crosslinking in the gel was influenced by the cation charge of the salt solutions. Additional crosslinking points were formed due to the physic0chemical interaction of the divalent (Ca2+) and trivalent (Al3+) cations with the carboxylate groups present in the hydrogel matrix.52 This caused a decrease in the gel swelling capacity.

Table 3. Impact of Salt Type on the Salt Sensitivity Factor (f) and Equilibrium Swelling Capacity (Ws) of Modified Chitosan Gels

salt

gels with unmodified chitosan

gels with modified chitosan

Ws

f

Ws

f

NaCl

45 ± 1

0.875

85 ± 1.5

0.861

KCl

49 ± 1

0.864

91 ± 2

0.851

NH4Cl

55 ± 1.5

0.847

96 ± 2

0.843

CaCl2

14.3 ± 0.6

0.960

31.5 ± 1

0.948

AlCl3

6.6 ± 0.5

0.981

15.9 ± 0.6

0.974

Morphological Properties of Hydrogels. The cross-sectional SEM images of cellulosechitosan crosslinked superabsorbent hydrogels are displayed in Figure 5. It can be seen that the gel is highly porous with a macroporous architecture. This suggests that the electrostatic repulsions caused by the ionic character of the carboxylate anions (COO-) in the gel have enlarged the space of the crosslinked gel network. The SEM image shows that the gel has an open porous geometry with a pore size in the range of 350-600 µm, which is separated by sheetlike walls and ultrathin structures, as revealed on Figure 5b. The high porosity of the hydrogels would facilitate rapid mass penetration which is useful in superabsorbent applications.



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Potential Applications of Hydrogels. Crosslinked cellulose-chitosan hydrogels appear as promising “green” superabsorbents, because of the biodegradable composition and the ecofriendly crosslinking process used for their preparation. It should be noted that sodium periodate (expensive chemical) that is used in the preparation of these hydrogels can be completely recovered from its reduced iodate form (product of the oxidation reaction) using sodium hypochlorite (inexpensive chemical).53 This certainly improves the economic viability of the hydrogel production process. The superabsorbent materials would be suitable for applications such as diapers, feminine hygiene products, wound dressings, meat soaking pads, wiping papers, and agricultural end-uses. They could also create new opportunities for production of advanced bioabsorbents with novel properties and uses.

Figure 5. Cross-sectional SEM image of cellulose hydrogel crosslinked with 25 wt% modified chitosan (a) 50-fold magnification (b) 500-fold magnification. CONCLUSIONS Cellulose-based absorbent materials already exist on the market; however, in most cases they suffer several major drawbacks in comparison to the incumbent petroleum-based superabsorbents, such as: 1) lack of complete biodegradability; 2) higher cost; 3) inferior liquid absorption capacity. In attempt to address these challenges, a novel polysaccharide-based superabsorbent hydrogel was prepared from chemically modified wood-derived cellulosic fibers crosslinked with carboxymethylated chitosan. The new process was described, and the composition, swelling and morphological properties of the superabsorbent gels were investigated and characterized. The main findings can be summarized as follows: 14 ACS Paragon Plus Environment

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•

The superabsorbent hydrogels were prepared by crosslinking modified cellulose and chitosan, a reaction that did not require use of expensive catalyst and post-reaction purification steps, as water was the only by-product. The process is therefore eco-friendly, and the product is derived from two completely biodegradable polymers. Biodegradability tests however are needed to verify that the crosslinked polymers is biodegradable.

•

The extent of crosslinking influenced the swelling properties of the hydrogels, with use of greater chitosan amounts causing a denser macromolecular network with less capillary spaces in the crosslinked gel. The absorption capacity improved in gels that had lower CD, higher Mc, and higher COOH group content.

•

The presence of salts also influenced the gel swelling capacity. Cations, depending on their charge strength, diminished the absorption capacity of gels to a different extent, depending on the charge screening effect and amount of additional crosslinking points created by these cations.

•

The hydrogels showed good re-swelling properties only losing 5-10% of their abilities to absorb distilled water when reused for four consecutive times. Their reusability makes them a good candidate for practical applications.

•

The superabsorbent hydrogels were prepared using conventional bleached kraft pulps containing up to 20% hemicelluloses that due to their branched, hydroxylated and carboxylated structures further increase the gel’s hydrophilicity and swelling potential. This could add additional cost and yield advantages in favor of the newly developed process that need further examination.

•

The cellulose-based hydrogels prepared with 25% carboxymethylated chitosan reached a maximum WRV of 610 in distilled water. Although this value is still lower than the WRV of commercial synthetic (polyacrylate) SAP (1,000 g distilled water/g polymer), it is several times higher than that of other neat cellulose-based superabsorbent materials, most of which have a reported maximum absorbency of 10-100 g/g.

•

In saline water, the WRV of the new cellulose-chitosan superabsorbent hydrogel was 85 g/g gel, which is approximately 2-fold higher than that of commercial gels (40-50 g/g).

•

The gel had a highly porous architecture, with a pore size in the range of 350-600 µm and open-pore geometry, separated by sheet-like walls and ultrathin structures that provide a



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high specific surface area and would facilitate rapid mass penetration in superabsorbent applications. •

The newly developed hydrogels appear as promising “green” superabsorbents, because of their biodegradable content, reusability, eco-friendly crosslinking and chemical recovery process used for their preparation, and superior liquid absorption properties. These superabsorbent materials may find suitable applications in a number of industry sectors, including hygiene, pharmaceutical, food and agricultural manufacturing.

AUTHOR INFORMATION Corresponding Author Email: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by Biorefining Research Institute (BRI) at Lakehead University. Special thanks to Mr. Michael Sorokopud at Lakehead University Instrumentation Laboratory for the FTIR and SEM facility.

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TOC

Biodegradable, reusable and eco-friendly superabsorbent hydrogels with excellent swelling properties were obtained by crosslinking partially-oxidized cellulose aldehyde with carboxymethylated chitosan.

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Natural Cellulose-Chitosan Crosslinked Superabsorbent Hydrogels

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