Synthesis of a Novel Carrageenan-Based Superabsorbent Hydrogel with Temperature Salt, and pH-Responsiveness Properties.
A novel thermo-sensitive superabsorbent hydrogel with salt- and pH-responsiveness properties was obtained by grafting of mixtures of acrylic acid (AA) and N-isopropylacrylamide (NIPAM) monomers onto kappa-carrageenan, kC, using ammonium persulfate (APS) as a free radical initiator in the presence of methylene bisacrylamide (MBA) as a crosslinker. Infrared spectroscopy was carried out to confirm the chemical structure of the hydrogel. Moreover, morphology of the samples was examined by scanning electron microscopy (SEM) and thermogravimetric analysis TGA. Results from SEM observation also showed a porous structure with smooth surface morphology of the hydrogel. The effect of MBA concentration and AA/NIPAM weight ratio on the water absorbency capacity has been investigated. The swelling variations of hydrogels were explained according to swelling theory based on the hydrogel chemical structure. The hydrogels exhibited salt-sensitivity and cation exchange properties.
The temperature-and pH-reversibility properties of the hydrogels make the intelligent polymers as good candidates for considering as potential carriers for bioactive agents, e.g. drugs.
Key words: superabsorbent, carrageenan, acrylic acid, N-isopropylacrylamide, hydrogel, swelling.
Superabsorbent polymers (SAPs) are defined as hydrophilic, three-dimensional networks with the ability of absorbing large values of water, saline solutions, or physiological fluids [1-2]. They are widely used in various applications such as hygienic, foods, cosmetics, and agriculture [3-5]. Thermo-sensitive hydrogels are known to exhibit phase separation properties in aqueous solution when the temperature is increased above a certain level. This phase separation temperature is referred to as the lower critical solution temperature (LCST) . This kind of material could be highly useful in many fields such as an on-off switch function by the change of temperature for the pulsatile controlled release of drugs . In general, the properties of the swelling medium (e.g. pH, temperature, and ionic strength) affect the swelling characteristics. SAPs responding to external stimuli such as heat, pH, electric field, chemical environments, etc, are often referred to as "intelligent" or "smart" polymers.
Vinyl graft copolymerization onto polysaccharides and proteins is a well-known method for synthesis of natural-based superabsorbent hydrogels [8-10]. The first industrial superabsorbent hydrogel, hydrolyzed starch-graft-polyacrylonitrile, was synthesized using this method .
Carrageenans are relatively new polysaccharides in the synthesis of natural-based superabsorbent polymers. These biopolymers are linear sulfated polysaccharides that are obtained from certain species of red seaweeds . Schematic diagram of the idealized structure of the repeat units for the most well-known and important type of carrageenan family, kappa-carrageenan (kC), is shown in Scheme 1. The presence of hydrophilic sulphate groups with high ionization tendency and less sensitivity to salt solution was the main goal for synthesis of carrageenan-based superabsorbent hyd- rogels. The presence of the natural parts, in addition, guarantees biocompatibility, biodegrade-bility, and non-toxicity of the superabsorbing materials. Foll- owing a continuous research on modi-fication of kappa-carrageenan (kC) [13-16] in this work, therefore, we attempted to synthesize and investigate the swelling behavior of a novel superabsorbing hydrogel from kC-g-poly (acrylic acid-co-N-iso- propylacrylamide).
The reaction variables affecting the water absorbency of the hydrogel as well as the salt-, pH-, and temperature sensitivity of the hydrogels were investigated in detail.
Results and Discussion
Preparation of Hydrogel
Scheme 1 shows a simple structural proposal of the graft copolymerization of AA and NIPAM monomers onto the kC backbones and crosslinking of the graft copolymer. The thermally dissociating initiator, i.e. APS, is first decomposed under heating (80 oC) to produce sulphate anion-radicals. Then, the anion-radicals abstract hydrogen from the kC backbones is used to form corresponding macroinitiators. These macroradicals initiate grafting of AA and NIPAM onto kC backbones leading to a graft copolymer. Crosslinking reaction also occurred in the presence of the crosslinker, i.e. MBA.
For identification of the hydrogel, infrared spectroscopy was used. Fig. 1 shows the IR spectroscopy of kC-g-poly (AA-co-NIPAM) hydrogel. The superabsorbent hydrogel product comprises a kC backbone with side chains that carry sodium carboxylate and carboxamide functional groups that are evidenced by peaks at 1573 and 1668 cm-1 , respectively. The intense characteristic band at 1573 cm-1 is due to C=O asymmetric stretching in carboxylate anion that is reconfirmed by another sharp peak at 1416 cm-1 which is related to the symmetric stretching mode of the carboxylate anion.
To obtain additional evidence of grafting, a similar polymerization was conducted in the absence of the crosslinker. After extracting the homopolymer and unreacted monomers using a cellophane membrane dialysis bag (D9402, Sigma-Aldrich), an appreciable amount of grafted kC (88%) was observed. The graft copolymer spectrum was very similar to Fig. 1(b). According to the preliminary measurements, also, the sol (soluble) content of the hydrogel networks was as little as 2.8 %. This fact, practically, proves that all monomers are almost involved in the polymer network. So, the monomers' percent in the network will be very similar to that of the initial feed of reaction.
Scanning Electron Microscopy
One of the most important properties that must be considered is hydrogel microstructure morphologies. The surface morphology of the samples was investigated by scanning electron microscopy. Fig. 2 shows an SEM micrograph of the polymeric hydrogels obtained from the fracture surface. The hydrogel has a porous structure. It is supposed that these pores are the regions of water permeation and interaction sites of external stimuli with the hydrophilic groups of the graft copolymers.
TGA of kappa-carrageenan (Fig. 3a) shows a weight loss in two distinct stages. The first stage ranges between 15 and 120 oC and shows about 17% loss in weight. This may correspond to the loss of adsorbed and bound water.17 No such inflexion was observed in the TGA curve of kC-g-poly(AA-co- NIPAM). This indicated that the grafted copolymers were resistant to moisture absorption. The second stage of weight loss starts at 330 oC and continues up to 440 oC during which there was 60% weight loss due to the degradation of kappa-carrageenan. Grafted samples, however, show almost different behavior of weight loss between 15 and 550 oC (Fig. 3b). The first stage of weight loss starts at 205 oC and continues up to 330 oC due to the degradation of kappa-carrageenan. The second stage from 370 to 480 oC may contribute to the decomposition of different structure of the graft copolymer.
The appearance of these stages indicates that the structure of kappa-carrageenan chains has been changed, which might be due to the grafting of PAA and PNIPAM chains. In general, the copolymer had lower weight loss than kappa-carrageenan. This means that the grafting of kappa-carrageenan increases the thermal stability of kappa-carrageenan in some extent.
Investigation of Effect Parameters onto Water Absorption
In this work, the main factors affecting on the swelling, i.e. concentration of MBA, AA/NIPAM ratio, salinity, pH and thermo-sensitivity of the hydrogel were systematically optimized to achieve superabsorbent with maximum water absorbency.
Effect of MBA Concentration on Swelling Capacity
The effect of crosslinker concentration on swelling capacity of kC-g-poly(AA-co-NIPAM) was investigated. As shown in Fig. 4, more values of absorbency are obtained by lower MBA concentration as reported by pioneering scientists [17-20]. In fact, higher crosslinker concentrations decrease the free space between the copolymer chains and consequently the resulted highly crosslinked rigid structure can not be expanded and hold a large quantity of water. The maximum absorbency (125 g/g) is achieved at 0.006 mol/L of MBA.
Effect of Monomer Ratio on Swelling Capacity
The swelling capacity of the hydrogels, prepared with various ratios of monomers, is shown in Fig. 5. The presence of the ionic groups in polymer chains results in increasing of swelling because the ions are more strongly solvated rather than non-ionic groups in the aqueous medium. Therefore, the swelling enhancement versus higher AA/NIPAM ratio can be attributed to the formation of high carboxylate groups.
Salt-Sensitivity of kC-g-Poly(AA-co-NIPAM) Hydrogel
Swelling capacity in salt solutions is of prime significance in many practical applications such as personal hygiene products and water release systems in agriculture. The swelling ability of "anionic" hydrogels in various salt solutions is appreciably decreased compared to the swelling values in distilled water. This well-known undesired swelling-loss is often attributed to a "charge screening effect" of the additional cations causing a non-perfect anion-anion electrostatic repulsion . In salt solution, also, the osmotic pressure resulting from the difference in the mobile ion concentration between gel and the aqueous phases is decreased and consequently the absorbency amounts are diminished. In addition, in the case of salt solutions with multivalent cations, "ionic crosslinking" at surface of hydrogel particles causing an appreciably decrease in swelling capacity.
Since the kC-based hydrogels are comprised poly(NaAA) chains with carboxylate groups that can interact with cations, they exhibit various swelling capacity in different salt solutions with same concentrations. In the presence of the bivalent calcium ions, the crosslinking density increases because of a double interaction of Ca2+ with carboxylate groups leading to "ionic crosslinking". The swelling-deswelling cycle of the hydrogel in sodium and calcium salts are shown in Fig. 6. In sodium solution, swelling of the hydrogel is increased with time. When this hydrogel is immersed in calcium chloride solution, it deswells to a collapsed form. When the shrinked hydrogel is immersed in sodium chloride solution again, the calcium ions are replaced by sodium ions. This ion exchange disrupts the ionic crosslinks leading to swelling enhancement. When hydrogel is treated alternatively with NaCl and CaCl2 solutions with equal morality the swelling reversibility of hydrogel is observed.
pH-Responsiveness Behavior of the Hydrogel
Ionic superabsorbent hydrogels exhibit swelling changes at a wide range of pHs. In this series of experiments, therefore, we investigated the reversible swelling-deswelling behavior of this hydrogel in solutions with pH 2.0 and 8.0 (Fig. 7). At pH 8.0, the hydrogel swells due to anion-anion repulsive electrostatic forces, while at pH 2.0, it shrinks within a few minutes due to protonation of the carboxylate anions. This swelling-deswelling behavior of the hydrogels makes them as suitable candidate for designing drug delivery systems.
The dependence of swelling degree on temperature and time is shown in Fig. 8. It indicates that the swelling and deswelling of the hydrogels were reversible. The response to temperature change is very quick. An abrupt decrease of swelling ratio is observed from 20 C to 40 C.
The polysaccharide, kappa-carrageenan (kC, MW=100,000, from Condinson Co., Denmark), N,N'-methylene bisacrylamide (MBA, from Merck), ammonium persulfate (APS, from Fluka), and N- isopropylacrylamide (NIPAM, from Merck) were of analytical grade and used without further purification. Acrylic acid (AA from Merck) as ionic monomer was used after vacuum distillation for removing inhibitor. The solvents (all from Merck) were used as received. Bidistilled water was used for the hydrogel preparation and swelling measurements.
Preparation of Hydrogel
Certain amounts of distilled water (30 mL) and kC (2.0 g), were added to a three-neck reactor equipped with a mechanical stirrer (Heidolph RZR 2021), while stirring (600 rpm). The reactor was placed in a thermostated water bath preset at 80oC for 20 min. After dissolving kC and homogenizing the mixture, the monomers, AA and NIPAM, and the crosslinker, MBA, were simultaneously added and the reaction mixture was stirred for 20 min. Then, the APS initiator was added and gelation was observed after 30 min. After 1h, the mixture was treated with 1 N sodium hydroxide for 70% neutralization of the carboxylic groups of the grafted poly (acrylic acid). Finally, the gel product was poured into 100 mL of ethanol for 2 h and then scissored to small pieces. The non-solvent ethanol was then decanted and 100 mL fresh ethanol was added. The particles were remained for 24 h to completely solidify.
The dewatered gel particles were filtered and dried in oven at 45 oC for 6 h. After grinding, the powdered superabsorbent hydrogel was stored away from moisture, heat and light.
An accurately weighed sample (0.2 +- 0.001 g) of the powdered superabsorbent with average particle sizes between 40-60 mesh (250-350 um) was immersed in distilled water (200 mL) and allowed to soak for 3 h at room temperature. The equilibrium swelling (ES) capacity was measured twice at room temperature according to a conventional tea bag (i.e. a 100 mesh nylon screen) method using the following formula:
Weight of swollen gel [?] Weight of dried gel
ES (g/g) =
Weight of dried gel
Swelling in Various Salt Solutions
Absorbency of the kC-g-poly(AA-co- NIPAM) hydrogel sample was evaluated in 0.15 M solutions of NaCl and CaCl2 according to the above method described for swelling measurement in distilled water.
Fourier transform infrared (FTIR) spectroscopy absorption spectra of samples were taken in KBr pellets, using an ABB Bomem MB-100 FTIR spectrophotometer (Quebec, Canada), at room temperature. The surface morphology of the gel was examined using scanning electron microscopy (SEM). After Soxhlet extraction with methanol for 24 h and drying in an oven, superabsorbent powder was coated with a thin layer of gold and imaged in a SEM instrument (Leo, 1455 VP). Thermogravimetric analyses (TGA) were performed on a Universal V4.1D TA Instruments (SDT Q600) with 8-10 mg samples on a platinum pan under nitrogen atmosphere. Experiments were performed at a heating rate of 20 oC/min until 600 oC.
A novel biopolymer-based superabsorbent hydrogel,kC-g-poly(AA-co-NIPAM)was used Hydrogel was synthesized through simultaneous crosslinking and graft polymerization of acrylic acid/N-isopropylacrylamide mixtures onto kappa- carrageenan. In order to prove that AA and NIPAM molecules were grafted, FTIR and SEM spectroscopies, TGA analysis and gravimetric analysis were used.
Swelling capacity of the hydrogels is affected by the crosslinker (MBA) concentration and monomer ratio, so that the swelling is decreased by increasing the MBA concentration and NIPAM/AA ratio. The swelling capacity in CaCl2 is much lower than that in NaCl solution and distillated water. The swelling-deswelling process of the hydrogel alternatively carried out in CaCl2 and NaCl solutions results in a high capability of ion exchanging of the kC-based hydrogel.
The swelling of hydrogel exhibited high sensitivity to pH study of the effect of H+/OH- concentration which was carried out at various pHs shows that the swelling of hydrogel causes several large volume changes. So, we investigated the pH- sensitivity of the hydrogel. Ionic repulsion between charge groups incorporated in the gel matrix by an external pH modulation could be assumed as the main driving force responsible for such abrupt swelling changes. This superabsorbent network intelligently responding to pH may be considered as an excellent candidate to design novel drug delivery systems.
Finally, the swelling-deswelling process of the hydrogels alternatively carried out in solutions with various temperatures results in a high thermo- sensitivity of the kC-based polymer networks.
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1Chemistry Department, Science Faculty, Islamic Azad University, Arak Branch, Arak, Iran., 2Chemistry Department, Payame Noor University, 19395-4697, Tehran, Iran., email@example.com
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|Author:||Sadeghi, Mohammd; Hosseinzadeh, Hossein; Mohammadinasab, Esmat; Shafiei, Fatemeh|
|Publication:||Journal of the Chemical Society of Pakistan|
|Date:||Jun 30, 2012|
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