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Swelling of radiation crosslinked acrylamide-based microgels and their potential applications
ARTICLE IN PRESS
Radiation Physics and Chemistry 74 (2005) 111–117 www.elsevier.com/locate/radphyschem
Swelling of radiation crosslinked acrylamide-based microgels and their potential applications H.A. Abd El-Rehim National Center for Radiation Research and Technology, Atomic Energy Authority, P.O. Box 29 Nasr City, Cairo, Egypt Received 8 June 2004; accepted 17 January 2005
Abstract Crosslinked polyacrylamide PAAm and acrylamide–Na–acrylate P(AAm–Na–AAc) microgels were prepared by electron beam irradiation. It was found that the dose required for crosslinking depends on the polymer moisture content, so that the dose to obtain PAAm of maximum gel fraction was over 40 and 20 kGy for dry and moist PAAm, respectively. The structural changes in irradiated PAAm were investigated using FTIR and SEM. The swelling property of such microgels in distilled water and real urine solution was determined and crosslinked polymers reached their equilibrium swelling state in a few minutes. As the gel content and crosslinking density decrease, the swelling of the microgels increases. The ability of the microgels to absorb and retain large amount of solutions suggested their possible uses in horticulture and in hygienic products such as disposable diapers. r 2005 Elsevier Ltd. All rights reserved. Keywords: Crosslinking; Irradiation; Polyacrylamide; Microgel; Superabsorbent; Diaper; Agriculture
1. Introduction A superabsorbent polymer is a network of flexible crosslinked molecules that exhibits an intriguing combination of the properties of both liquids and solids. Superabsorbent polymer gels can swell up to hundreds of times their own weight in aqueous media. The term encompasses a number of crosslinked polymers such as poly(vinyl alcohol), polyvinylpyrrolidone, poly(ethylene oxide), carboxymethylcellulose and polyacrylamide, all having the basic ability to swell in water. There has been increasing interest in the synthesis and applications of superabsorbent polymer or hydrogels (Buchholz, 1998). They have been studied intensively in recent decades for their promising applications in chemical engineering as sensors (Anderson et al., 2000), in the biomedical field as materials in medicine E-mail address: [email protected].
(Wichterle and Lim, 1960), in pharmacy as drug delivery systems (Peppas, 1986), in agriculture and industry as adsorbents and separation membranes (Abd El-Rehim et al., 1999, 2004), in solving some ecological problems (Abdel-Aal et al., 2003) and in other modern technologies (Hirasa, 1993). Acrylamide hydrogels have found extensive commercial applications as sorbents in personal care and for agriculture purposes (Etienne et al., 2002; Wingard, 1983; Sojka et al., 1998). The swelling behavior of the hydrogel is very sensitive to the network microstructure and its interaction with water. Further, the properties of the crosslinked polyacrylamide depend on the methods of preparation (Burillo and Ogawa, 1983, 1986; Rosiak et al., 1983). Ionizing radiation has long been recognized as a very suitable tool for the formation of hydrogels with unique properties. Moreover, easy process control, no initiators, crosslinkers, etc., no waste, and relatively low operating costs make irradiation the method of
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choice for the synthesis of hydrogels (Charlesby, 1960; Chapiro, 1962; Rosiak, 1994; Rosiak and Ulanski, 1999). Radiation-induced crosslinking of PAAm has been investigated and carried out under various conditions. The molecular weight of the polymer, dose as well as the external applied pressure are the most important factors for controlling the PAAm radiation crosslinking process (Burillo and Ogawa, 1981, 1983, 1986). Although the crosslinked PAAm polymer can be directly obtained by irradiating its monomers or polymer in solution (Burillo and Ogawa, 1986; Wada et al., 1976; Ye et al., 2002), it is better from the economical point of view if its raw materials are processed by irradiation in the solid state to form PAAm microgel. In the present work, crosslinking of PAAm and P(AAm–Na–AAc) powders using electron beam irradiation was investigated. The factors affecting the degree of crosslinking and swelling behavior of the prepared microgels were studied. The possible uses of such materials in agriculture and as hygienic products were discussed.
2. Materials and methods 2.1. Materials Commercial polyacrylamide PAAm supplied from ElNasser Co. Egypt—molecular weight over 5 000 000 and viscosity (0.5% aqueous solution at 50 1C) 282 cP—was used. Powder of 0.3 mm particle size was covered with stainless steel net and dried in a vacuum oven at 45 1C for 24 h before its use. 2.2. Sample preparation and irradiation Appropriate weights of dry and wetted polymer powders were put in polyethylene bags (the thickness of the bags filled at powder was 3 mm) and irradiated at different doses at an electron accelerator. The parameters of the accelerator were electron energy, 1.45 MeV; electron beam current, 4 mA; scanner width, 90 cm; and conveyor speed at 20 kGy, 3.6 m min1. After irradiation, powders were dehydrated for 2 h in absolute ethanol, dried at 37 1C for 24 h and stored in vacuum oven at 45 1C for 12 h before use. The wetted PAAm was obtained by mixing an appropriate amount of water with the dry powder before electron beam irradiation.
soluble polymer was precipitated by methanol, washed several times with dry methanol, dried at 45 1C in vacuum oven, and grained (sieves of 0.2 and 0.3 mesh sizes were used to obtain powder of 0.3 mm particle size). The nitrogen content in the prepared P(AAm– Na–AAc) was determined by the Microanalysis Unit, Cairo University, Cairo, Egypt. The conversion of CONH2 groups to COONa by the NaOH treatment was calculated to be 19.8%. 2.4. Swelling measurement Dried raw irradiated microgels of known weights were put in stainless steel net bags and immersed in distilled water or urine solutions at 30 1C for 15 min. Equilibrium swelling was reached in 3–10 min depending on the polymer gel content. The bags were removed from the solution, blotted quickly with absorbent paper and then weighed. The experiment was repeated three times for each sample and the average weight of swelled polymer was taken. The following equation was used to determine the swelling ratio of the microgels: swelling ratio ðg=gÞ ¼ ðW s W g Þ=W g , where Ws and Wg represent the weights of wet and dry gel, respectively. 2.5. Gel determination Dried powders of known weight were put in stainless steel net bags and immersed in distilled water for 48 h at 75 1C. The gelled part was washed several times with hot water to remove the soluble fraction, then dried and weighed. The experiment was repeated three times for each sample and the average weight of insoluble fraction was taken. The gel percent in the microgels was determined from the following equation: gel ð%Þ ¼ ðW g =W i Þ 100, where Wg and Wi are dry hydrogel weights after and before extraction, respectively. 2.6. SEM examination The microgels were immersed in distilled water for 20 min, freeze dried and examined using a JEOLJSM5400 (Japan) scanning electron microscope (SEM) after gold deposition in vacuum for 3 min.
2.3. Preparation of P(AAm–Na–AAc) powders 2.7. Soil preparation and plantation An appropriate weight of dry PAAm polymer (14 g) in 200 ml distilled water was stirred at 40 1C until it completely dissolved. In all, 1.5 g NaOH in 20 ml distilled water was added to the PAAm solution. The mixture was stirred for 5 h at 70 1C. After cooling, the
A plot of 1 0.5 m2 was constructed in sandy soil and divided into two ridges 1 m long at equal distances of 25 cm. Eight holes per ridge (diameter 8 cm) were dug to 8 cm depth into which 0.02 g dry P(AAm–Na–AAc)
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was added. Radish plant seeds were sown 3 cm apart from the gel–soil holes. Similarly, another plot was prepared for sandy soil reference (control) without any treatment. Water of irrigation remained constant during the experiment. Fifty days after planting, one ridge from each plot was exposed to drought and water stress to determine the wilting time of the plants. The details are described in Abd El-Rehim et al. (2004).
3.1. Crosslinking of PAAm Dry PAAm was electron irradiated at 30 kGy in the presence of air and in vacuum. The gel content of PAAm was 52% in air and 56% in vacuum. This result agrees with that obtained by Burillo and Ogawa (1983), who found that the gelation yield of irradiated PAAm did not increase appreciably when the irradiation was carried out in the absence of air. The reason is that in the presence of air, some of the initially generated carboncentered macroradicals in the powder surfaces react with oxygen to form peroxyl radicals, which form unstable bonds (crosslinks) upon recombination (Rosiak and Ulanski, 1999). PAAm in dry or moist form was irradiated at different doses in air and the gel fraction as a function of irradiation dose is shown in Fig. 1(A). The PAAm gel content increases as the irradiation dose increases to reach its maximum stable level at 40 and 20 kGy for dry and moist PAAm, respectively. As the moisture content in the PAAm increases, the dose required to obtain maximum gel fraction decreases. In the case of irradiation of polymer in dry form, the crosslinking was achieved from a direct effect of irradiation. The radiation creates radicals localized mostly on the main chains. The random recombination of these radicals creates covalent chemical bonds between the chains of polymer. It causes an increase in the branching and increase of average molecular weight of the polymer, which reflects on the polymer properties as insolubility (Chapiro, 1962). Doses higher than the gelation dose cause the PAAm chains to form a threedimensional network. In this case of irradiation of polymer in wet form, in addition to the direct effect of irradiation on the PAAm chains, there are two ways for the water to contribute in the crosslinkng process of PAAm. First, it enhances mobility and elasticity of the rigid PAAm chains. As a consequence, molecular movement of the molecules takes place, allowing the macroradicals to recombine and to enhance intermolecular and intramolecular recombination reactions. Second, the products of water radiolysis, namely,
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Fig. 1. (A) Gel fraction of crosslinked PAAm as a function of irradiation dose. Sample irradiated: (&) in dry form, (’) with 10% (wt/wt) water, () with 20% (wt/wt) water. (B) Gel fraction of crosslinked P(AAm–Na–AAc) as a function of irradiation dose. Sample irradiated: () in dry form, (&) with 5% (wt/wt) water, (’) with 10% (wt/wt) water.
hydrogen and hydroxyl radicals, create macroradicals by abstracting H from the polymer chains. Thus, the crosslinking process of PAAm occurs faster and easier in the presence of water (Rosiak and Ulanski, 1999). To enhance the crosslinking reactions, water should be homogeneously dispersed in the powder before irradiation. P(AAm–Na–AAc) copolymers were also irradiated and their gel contents were determined (Fig. 1(B)). The effect of radiation and polymer moisture content on the crosslinking process of P(AAm–Na–AAc) is similar to that observed for PAAm crosslinking; however, the introduction of sodium acrylate to PAAm reduces the extent of crosslinking and raises the dose required. This is due to the change in PAAm structure and properties when moving from neutral polymer to polyelectrolyte copolymer. The electrostatic repulsion between the polarized charged groups on the network chains plays a role in hindering the recombination of radicals and, consequently, reduces the extent of the copolymer crosslinking.
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3.2.1. Swelling behavior of the hydrogels Equilibrium swelling of the irradiated polymer takes place in a few minutes and results in a drastic increase of its volume, while the unirradiated PAAm dissolved in water (Fig. 2). The time needed for all crosslinked microgels under investigation to reach their equilibrium swelling ranged from 3 to 10 min, depending on the polymer crosslinking density. The swelling ratio for irradiated PAAm in distilled water is shown in Fig. 3. The swelling ratio of irradiated PAAm follows the order dry PAAm4PAAm containing 10%water4PAAm containing 20% water. The swelling properties of the hydrogel are mainly related to its degree of crosslinking. The reduction in the swelling property of irradiated PAAm containing 20% water is attributed to the high degree of crosslinking, which restricts the mobility of the polymer chains and decreases its swelling ratio. The lack of ionic character in the PAAm polymers structure greatly reduces their ability to absorb water. Therefore, in order to raise their swelling efficiency in aqueous media, partial hydrolysis in polyacrylamide chains was carried out in order to obtain PAAmbased microgels containing both amide and Na–AAc groups. From Fig. 3 it is clear that, at the same absorbed doses, the crosslinked P(AAm–Na–AAc) copolymer swelling ratio is much higher than that of PAAm. A major difference between PAAm and P(AAm– Na–AAc) is in the presence of ionizable hydrophilic Na–AAc groups of polar negative charges in P(AAm–Na–AAc). Thus, the high swelling ability of P(AAm–Na–AAc) can be attributed to the electrostatic repulsion between the polarized charged groups on the network chains and the concentration difference of Na+ mobile ions inside the hydrogel and outside (in the swelling solution) (Abd El-Rehim et al., 2004).
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Fig. 2. Change in volume in the unirradiated and irradiated PAAm containing 10% (wt/wt) water: (A) dry polymer, (B) unirradiated swelled polymer, (C) irradiated swelled polymer.
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3.2. Properties of the microgels
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Irradiation Dose (kGy.) Fig. 3. Swelling ratio of PAAm and P(AAm–Na–AAc) in distilled water prepared at different conditions. PAAm samples: () irradiated in dry form, (J) irradiated in wet form, 10% (wt/ wt) water, (.) irradiated in wet form, 20% (wt/wt) water, (’) P(AAm–Na–AAc) irradiated in wet form, 10% (wt/wt) water.
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Wavenumbers (cm-1) Fig. 4. FTIR spectra of irradiated PAAm prepared in dry form at different irradiation doses: (A) unirradiated PAAm, (B) irradiated at 20 kGy, (C) irradiated at 40 kGy.
3.2.2. FTIR study FTIR was performed for the irradiation of dry PAAm powders (Fig. 4). The FTIR spectra show a new characteristic band at around 1600 cm1, which might be due to the unsaturated double bond ( C C ). The intensity of this band increases by increasing dose. This result suggests that besides the formation of crosslinking bonds between the PAAm chains, another reaction takes place resulting in radical disproportionation and unsaturated bond formation. The pale yellow color of irradiated PAAm confirmed the formation of unsaturated double bonds.
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Fig. 6. Swelling ratio of PAAm and P(AAm–Na–AAc) in urine: (’) PAAm sample irradiated in dry form, (J) PAAm sample irradiated in wet form containing 10% (wt/wt) water, (.) PAAm sample irradiated in wet form containing 20% (wt/ wt) water, () P(AAm–Na–AAc) sample irradiated in wet form containing 10% (w/wt) water.
Fig. 5. SEM photos of (A) unirradiated PAAm, (B) PAAm particles irradiated in a dry form at dose 30 kGy.
3.2.3. Structure and particle shape of irradiated PAAm Unirradiated and irradiated freeze dried PAAm hydrogels were examined by SEM (Fig. 5). The unirradiated PAAm lost its particle shape to form highly porous film (Fig. 5(A)), while the irradiated powders show distinct aggregated particles (Fig. 5(B)). It is clear that the morphology of PAAm was affected by radiation The network structure prevented the PAAm particles from dissolving in water, and kept their shape, although in distorted form. 3.2.4. Potential application of the microgels in the diaper industry The most important property in a commercial superabsorbent used in personal care applications is the extent of swelling. According to Buchholz (1998), an
acceptable swelling capacity is approximately 20–40 g of urine per gram of polymer. Therefore, the amount of urine absorbed by different types of dry PAAm-based hydrogels was investigated and the results are shown in Fig. 6. In general, the swelling of hydrogels in urine solution is much lower than the swelling obtained in distilled water (Figs. 3 and 6). The absorption of urine decreases as the crosslinking degree of the PAAm-based polymer increases. P(AAm–Na–AAc) microgels can swell 30–50 times their own weight in urine solution, while PAAm polymers swell 15–35 times, depending on the degree of crosslinking. Minute changes in external conditions such as pH, temperature, solvent composition, and ionic strength can induce drastic changes in the state of the swollen network (Osada and Gong, 1993; Osada and Khokhlov, 2002). Thus, the reduction in polymer swelling in urine can be attributed to the effect of urinal salts on the macromolecules network, which may cause a swelling reduction. These effects are usually explained by changes of the solution structure induced by ion dehydration (Zhang et al., 1992; Horky et al., 2000). The introduction of polar negative charges to PAAm leads to better salt resistance and solution absorption rate. The high swelling of P(AAm–Na–AAc) gels in urine is associated with the difference between the concentration of mobile ions in the gel and in urine. The presence of Na+ mobile ions inside the gel increases the electroneutrality conditions and creates an additional osmotic pressure that expands the gel (Abd El-Rehim et al., 2004). Swelling of the hydrogels was also tested in artificial urine prepared according to the method of Kark et al. (1964). The swelling behavior and results obtained by
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commercial superabsorbents. The promising results reveal that such prepared materials may be of great interest in horticulture and industrial applications.
References
(A)
(B)
Fig. 7. Radish plants planted in soil: (A) untreated (control), (B) treated with P(AAm–Na–AAc).
the use of artificial urine were approximately the same as that obtained by real urine. 3.2.5. Possible use of microgels in agriculture From an economic point of view, agricultural hydrogels should be evaluated through the plant growth and wilting time. Therefore, the effect of pure sandy soil (control) and that treated with P(AAm–Na–AAc) microgels on the vegetative growth and time to wilting of radish plants was investigated. It is clear that the presence of hydrogels in the soil enhances radish plant growth and performance (Fig. 7). Also, radish plants planted in sandy soil treated with P(AAm–Na–AAc) microgel take a long time to wilt (14 days) if compared with the control one (9 days). This means that the soil treated with P(AAm–Na–AAc) microgel possesses a high ability to absorb and retain water, and significantly delays plant losses due to drought and water stress conditions. Thus, plant survival is improved and its probability for fast growth becomes high.
4. Conclusion An effort was made to use radiation to develop a superabsorbent process technology and suggest an easy method for their production. Crosslinked AAm-based microgels, which were already used for the production of commercial superabsorbents, can be produced in powder form by electron beam irradiation. The crosslinking extent in irradiated AAm-based microgels was controlled by polymer moisture content and irradiation doses. The crosslinked polymers and copolymers possess high and fast swelling properties in aqueous media. The swelling ratios of AAm-based microgels in urine or saline solution are acceptable when compared with
Abd El-Rehim, H.A., Hegazy, E.A., Ali, A.M., 1999. Preparation of poly(vinyl alcohol) grafted with acrylic acid/styrene binary monomers for selective permeation of heavy metals. Appl. Polym. Sci. 74, 806–815. Abd El-Rehim, H.A., Hegazy, E.A., Abd El-Mohdy, H.L., 2004. Radiation synthesis of hydrogels to enhance sandy soils water retention and increase plant performance. Appl. Polym. Sci. 93, 1360–1371. Abdel-Aal, S.E., Hegazy, E.A., Abdel-Rehim, H.A., Khalifa, N.A., El-Hosseiny, E.M., 2003. Radiation copolymerization of (PVA/4-VP) hydrogel for treatment of industrial waste dyes. Int. J. Polym. Mater. 52, 901–916. Anderson, J.M., Allen, J., Dempsey, G.J., 2000. Sensor device. US Patent 6 055 448. Buchholz, F.L., 1998. In: Buchholz, F.L., Graham, A.T. (Eds.), Modern Superabsorbent Polymer Technology. Wiley-VCH, New York, pp. 190–193. Burillo, G., Ogawa, T., 1981. Obtention of hydrogels for use as water retainers in soil by gamma irradiation. Radiat. Phys. Chem. 18, 1143–1147. Burillo, G., Ogawa, T., 1983. The gamma ray-induced crosslinking of polyacrylamide in the solid state. J. Polym. Sci.: Polym. Lett. 21, 615–620. Burillo, G., Ogawa, T., 1986. The gamma-ray-induced crosslinking of polyacrylamide. J. Appl. Polym. Sci. 32, 3783–3789. Chapiro, A., 1962. Radiation Chemistry of Polymeric Systems. Interscience, New York. Charlesby, A., 1960. Atomic Radiation and Polymers. Pergamon Press, Oxford. Etienne, S., Bulcke, E.V., Bernard, D., Jean-Pierre, D., 2002. Medicaments based on polymers composed of methacrylamide-modified gelatin. US Patent 6 458 386. Hirasa, O., 1993. Research trends of stimuli-responsive polymer hydrogels in Japan. J. Intell. Mater. Systems Struct. 4, 538–544. Horky, F., Tasaki, I., Basser, P.J., 2000. Effect of monovalent–divalent cation exchange on the swelling of polyacrylate hydrogels in physiological salt solutions. Biomacromolecule 2, 195–199. Kark, R.M., Lawrence, J.R., Pollack, V.E., Pirani, C.L., Muehrcke, R.C., Silva, H., 1964. A Primer of Urinalysius, second ed. Hoeber Medical Division, Harper & Row Publishers, New York. Osada, Y., Gong, J., 1993. Stimuli—responsive polymer gels and their application to chemo mechanical systems. Prog. Polym. Sci. 18, 187–226. Osada, Y., Khokhlov, A.R., 2002. Polymer Gels and Networks. Marcel Dekker, New York. Peppas, N.A., 1986. Hydrogels in Medicine and Pharmacy. CRC Press, Boca Raton. Rosiak, J.M., 1994. Radiation formation of hydrogels. J. Control Release 31, 9–19.
ARTICLE IN PRESS H.A. Abd El-Rehim / Radiation Physics and Chemistry 74 (2005) 111–117 Rosiak, J.M., Ulanski, P., 1999. Synthesis of hydrogels by irradiation of polymers in aqueous solution. Radiat. Phys. Chem. 55, 139–151. Rosiak, J.M., Burczak, K., Czolczynska, T., Pekala, W., 1983. Radiation crosslinked hydrogels from acrylamide water solutions. Radiat. Phys. Chem. 22, 917–928. Sojka, R.E., Westermann, D.T., Lentz, R.D., 1998. Water and erosion management with multiple applications of polyacrylamide in furrow irrigation. Soil Sci. Soc. Am. J. 62, 1672–1680. Wada, T., Sekiya, H., Machi, S., 1976. Synthesis of high molecular weight polyacrylamide flocculant by radiation polymerization. Appl. Polym. Sci. 20, 3233–3240.
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Wichterle, O., Lim, D., 1960. Hydrophilic gels for biological use. Nature 185, 117–118. Wingard, S.G., 1983. Shaped articles for rapid hair drying utilizing polymer blends products. US Patent 4 421 129. Ye, Q., He, W., Ge, X., Jia, H., Liu, H., Zhang, Z., 2002. Formation of monodisperse polyacrylamide particles by radiation-induced dispersion polymerization, Part I Synthesis and polymerization kinetics. Appl. Polym. Sci. 86, 2567–2573. Zhang, Y., Tanaka, T., Shibayama, M., 1992. Super-absorbency and phase transition of gels in physiological salt solutions. Nature 360, 142–144.
Radiation Physics and Chemistry 74 (2005) 111–117 www.elsevier.com/locate/radphyschem
Swelling of radiation crosslinked acrylamide-based microgels and their potential applications H.A. Abd El-Rehim National Center for Radiation Research and Technology, Atomic Energy Authority, P.O. Box 29 Nasr City, Cairo, Egypt Received 8 June 2004; accepted 17 January 2005
Abstract Crosslinked polyacrylamide PAAm and acrylamide–Na–acrylate P(AAm–Na–AAc) microgels were prepared by electron beam irradiation. It was found that the dose required for crosslinking depends on the polymer moisture content, so that the dose to obtain PAAm of maximum gel fraction was over 40 and 20 kGy for dry and moist PAAm, respectively. The structural changes in irradiated PAAm were investigated using FTIR and SEM. The swelling property of such microgels in distilled water and real urine solution was determined and crosslinked polymers reached their equilibrium swelling state in a few minutes. As the gel content and crosslinking density decrease, the swelling of the microgels increases. The ability of the microgels to absorb and retain large amount of solutions suggested their possible uses in horticulture and in hygienic products such as disposable diapers. r 2005 Elsevier Ltd. All rights reserved. Keywords: Crosslinking; Irradiation; Polyacrylamide; Microgel; Superabsorbent; Diaper; Agriculture
1. Introduction A superabsorbent polymer is a network of flexible crosslinked molecules that exhibits an intriguing combination of the properties of both liquids and solids. Superabsorbent polymer gels can swell up to hundreds of times their own weight in aqueous media. The term encompasses a number of crosslinked polymers such as poly(vinyl alcohol), polyvinylpyrrolidone, poly(ethylene oxide), carboxymethylcellulose and polyacrylamide, all having the basic ability to swell in water. There has been increasing interest in the synthesis and applications of superabsorbent polymer or hydrogels (Buchholz, 1998). They have been studied intensively in recent decades for their promising applications in chemical engineering as sensors (Anderson et al., 2000), in the biomedical field as materials in medicine E-mail address: [email protected].
(Wichterle and Lim, 1960), in pharmacy as drug delivery systems (Peppas, 1986), in agriculture and industry as adsorbents and separation membranes (Abd El-Rehim et al., 1999, 2004), in solving some ecological problems (Abdel-Aal et al., 2003) and in other modern technologies (Hirasa, 1993). Acrylamide hydrogels have found extensive commercial applications as sorbents in personal care and for agriculture purposes (Etienne et al., 2002; Wingard, 1983; Sojka et al., 1998). The swelling behavior of the hydrogel is very sensitive to the network microstructure and its interaction with water. Further, the properties of the crosslinked polyacrylamide depend on the methods of preparation (Burillo and Ogawa, 1983, 1986; Rosiak et al., 1983). Ionizing radiation has long been recognized as a very suitable tool for the formation of hydrogels with unique properties. Moreover, easy process control, no initiators, crosslinkers, etc., no waste, and relatively low operating costs make irradiation the method of
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choice for the synthesis of hydrogels (Charlesby, 1960; Chapiro, 1962; Rosiak, 1994; Rosiak and Ulanski, 1999). Radiation-induced crosslinking of PAAm has been investigated and carried out under various conditions. The molecular weight of the polymer, dose as well as the external applied pressure are the most important factors for controlling the PAAm radiation crosslinking process (Burillo and Ogawa, 1981, 1983, 1986). Although the crosslinked PAAm polymer can be directly obtained by irradiating its monomers or polymer in solution (Burillo and Ogawa, 1986; Wada et al., 1976; Ye et al., 2002), it is better from the economical point of view if its raw materials are processed by irradiation in the solid state to form PAAm microgel. In the present work, crosslinking of PAAm and P(AAm–Na–AAc) powders using electron beam irradiation was investigated. The factors affecting the degree of crosslinking and swelling behavior of the prepared microgels were studied. The possible uses of such materials in agriculture and as hygienic products were discussed.
2. Materials and methods 2.1. Materials Commercial polyacrylamide PAAm supplied from ElNasser Co. Egypt—molecular weight over 5 000 000 and viscosity (0.5% aqueous solution at 50 1C) 282 cP—was used. Powder of 0.3 mm particle size was covered with stainless steel net and dried in a vacuum oven at 45 1C for 24 h before its use. 2.2. Sample preparation and irradiation Appropriate weights of dry and wetted polymer powders were put in polyethylene bags (the thickness of the bags filled at powder was 3 mm) and irradiated at different doses at an electron accelerator. The parameters of the accelerator were electron energy, 1.45 MeV; electron beam current, 4 mA; scanner width, 90 cm; and conveyor speed at 20 kGy, 3.6 m min1. After irradiation, powders were dehydrated for 2 h in absolute ethanol, dried at 37 1C for 24 h and stored in vacuum oven at 45 1C for 12 h before use. The wetted PAAm was obtained by mixing an appropriate amount of water with the dry powder before electron beam irradiation.
soluble polymer was precipitated by methanol, washed several times with dry methanol, dried at 45 1C in vacuum oven, and grained (sieves of 0.2 and 0.3 mesh sizes were used to obtain powder of 0.3 mm particle size). The nitrogen content in the prepared P(AAm– Na–AAc) was determined by the Microanalysis Unit, Cairo University, Cairo, Egypt. The conversion of CONH2 groups to COONa by the NaOH treatment was calculated to be 19.8%. 2.4. Swelling measurement Dried raw irradiated microgels of known weights were put in stainless steel net bags and immersed in distilled water or urine solutions at 30 1C for 15 min. Equilibrium swelling was reached in 3–10 min depending on the polymer gel content. The bags were removed from the solution, blotted quickly with absorbent paper and then weighed. The experiment was repeated three times for each sample and the average weight of swelled polymer was taken. The following equation was used to determine the swelling ratio of the microgels: swelling ratio ðg=gÞ ¼ ðW s W g Þ=W g , where Ws and Wg represent the weights of wet and dry gel, respectively. 2.5. Gel determination Dried powders of known weight were put in stainless steel net bags and immersed in distilled water for 48 h at 75 1C. The gelled part was washed several times with hot water to remove the soluble fraction, then dried and weighed. The experiment was repeated three times for each sample and the average weight of insoluble fraction was taken. The gel percent in the microgels was determined from the following equation: gel ð%Þ ¼ ðW g =W i Þ 100, where Wg and Wi are dry hydrogel weights after and before extraction, respectively. 2.6. SEM examination The microgels were immersed in distilled water for 20 min, freeze dried and examined using a JEOLJSM5400 (Japan) scanning electron microscope (SEM) after gold deposition in vacuum for 3 min.
2.3. Preparation of P(AAm–Na–AAc) powders 2.7. Soil preparation and plantation An appropriate weight of dry PAAm polymer (14 g) in 200 ml distilled water was stirred at 40 1C until it completely dissolved. In all, 1.5 g NaOH in 20 ml distilled water was added to the PAAm solution. The mixture was stirred for 5 h at 70 1C. After cooling, the
A plot of 1 0.5 m2 was constructed in sandy soil and divided into two ridges 1 m long at equal distances of 25 cm. Eight holes per ridge (diameter 8 cm) were dug to 8 cm depth into which 0.02 g dry P(AAm–Na–AAc)
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was added. Radish plant seeds were sown 3 cm apart from the gel–soil holes. Similarly, another plot was prepared for sandy soil reference (control) without any treatment. Water of irrigation remained constant during the experiment. Fifty days after planting, one ridge from each plot was exposed to drought and water stress to determine the wilting time of the plants. The details are described in Abd El-Rehim et al. (2004).
3.1. Crosslinking of PAAm Dry PAAm was electron irradiated at 30 kGy in the presence of air and in vacuum. The gel content of PAAm was 52% in air and 56% in vacuum. This result agrees with that obtained by Burillo and Ogawa (1983), who found that the gelation yield of irradiated PAAm did not increase appreciably when the irradiation was carried out in the absence of air. The reason is that in the presence of air, some of the initially generated carboncentered macroradicals in the powder surfaces react with oxygen to form peroxyl radicals, which form unstable bonds (crosslinks) upon recombination (Rosiak and Ulanski, 1999). PAAm in dry or moist form was irradiated at different doses in air and the gel fraction as a function of irradiation dose is shown in Fig. 1(A). The PAAm gel content increases as the irradiation dose increases to reach its maximum stable level at 40 and 20 kGy for dry and moist PAAm, respectively. As the moisture content in the PAAm increases, the dose required to obtain maximum gel fraction decreases. In the case of irradiation of polymer in dry form, the crosslinking was achieved from a direct effect of irradiation. The radiation creates radicals localized mostly on the main chains. The random recombination of these radicals creates covalent chemical bonds between the chains of polymer. It causes an increase in the branching and increase of average molecular weight of the polymer, which reflects on the polymer properties as insolubility (Chapiro, 1962). Doses higher than the gelation dose cause the PAAm chains to form a threedimensional network. In this case of irradiation of polymer in wet form, in addition to the direct effect of irradiation on the PAAm chains, there are two ways for the water to contribute in the crosslinkng process of PAAm. First, it enhances mobility and elasticity of the rigid PAAm chains. As a consequence, molecular movement of the molecules takes place, allowing the macroradicals to recombine and to enhance intermolecular and intramolecular recombination reactions. Second, the products of water radiolysis, namely,
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Fig. 1. (A) Gel fraction of crosslinked PAAm as a function of irradiation dose. Sample irradiated: (&) in dry form, (’) with 10% (wt/wt) water, () with 20% (wt/wt) water. (B) Gel fraction of crosslinked P(AAm–Na–AAc) as a function of irradiation dose. Sample irradiated: () in dry form, (&) with 5% (wt/wt) water, (’) with 10% (wt/wt) water.
hydrogen and hydroxyl radicals, create macroradicals by abstracting H from the polymer chains. Thus, the crosslinking process of PAAm occurs faster and easier in the presence of water (Rosiak and Ulanski, 1999). To enhance the crosslinking reactions, water should be homogeneously dispersed in the powder before irradiation. P(AAm–Na–AAc) copolymers were also irradiated and their gel contents were determined (Fig. 1(B)). The effect of radiation and polymer moisture content on the crosslinking process of P(AAm–Na–AAc) is similar to that observed for PAAm crosslinking; however, the introduction of sodium acrylate to PAAm reduces the extent of crosslinking and raises the dose required. This is due to the change in PAAm structure and properties when moving from neutral polymer to polyelectrolyte copolymer. The electrostatic repulsion between the polarized charged groups on the network chains plays a role in hindering the recombination of radicals and, consequently, reduces the extent of the copolymer crosslinking.
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3.2.1. Swelling behavior of the hydrogels Equilibrium swelling of the irradiated polymer takes place in a few minutes and results in a drastic increase of its volume, while the unirradiated PAAm dissolved in water (Fig. 2). The time needed for all crosslinked microgels under investigation to reach their equilibrium swelling ranged from 3 to 10 min, depending on the polymer crosslinking density. The swelling ratio for irradiated PAAm in distilled water is shown in Fig. 3. The swelling ratio of irradiated PAAm follows the order dry PAAm4PAAm containing 10%water4PAAm containing 20% water. The swelling properties of the hydrogel are mainly related to its degree of crosslinking. The reduction in the swelling property of irradiated PAAm containing 20% water is attributed to the high degree of crosslinking, which restricts the mobility of the polymer chains and decreases its swelling ratio. The lack of ionic character in the PAAm polymers structure greatly reduces their ability to absorb water. Therefore, in order to raise their swelling efficiency in aqueous media, partial hydrolysis in polyacrylamide chains was carried out in order to obtain PAAmbased microgels containing both amide and Na–AAc groups. From Fig. 3 it is clear that, at the same absorbed doses, the crosslinked P(AAm–Na–AAc) copolymer swelling ratio is much higher than that of PAAm. A major difference between PAAm and P(AAm– Na–AAc) is in the presence of ionizable hydrophilic Na–AAc groups of polar negative charges in P(AAm–Na–AAc). Thus, the high swelling ability of P(AAm–Na–AAc) can be attributed to the electrostatic repulsion between the polarized charged groups on the network chains and the concentration difference of Na+ mobile ions inside the hydrogel and outside (in the swelling solution) (Abd El-Rehim et al., 2004).
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Fig. 2. Change in volume in the unirradiated and irradiated PAAm containing 10% (wt/wt) water: (A) dry polymer, (B) unirradiated swelled polymer, (C) irradiated swelled polymer.
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Irradiation Dose (kGy.) Fig. 3. Swelling ratio of PAAm and P(AAm–Na–AAc) in distilled water prepared at different conditions. PAAm samples: () irradiated in dry form, (J) irradiated in wet form, 10% (wt/ wt) water, (.) irradiated in wet form, 20% (wt/wt) water, (’) P(AAm–Na–AAc) irradiated in wet form, 10% (wt/wt) water.
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Wavenumbers (cm-1) Fig. 4. FTIR spectra of irradiated PAAm prepared in dry form at different irradiation doses: (A) unirradiated PAAm, (B) irradiated at 20 kGy, (C) irradiated at 40 kGy.
3.2.2. FTIR study FTIR was performed for the irradiation of dry PAAm powders (Fig. 4). The FTIR spectra show a new characteristic band at around 1600 cm1, which might be due to the unsaturated double bond ( C C ). The intensity of this band increases by increasing dose. This result suggests that besides the formation of crosslinking bonds between the PAAm chains, another reaction takes place resulting in radical disproportionation and unsaturated bond formation. The pale yellow color of irradiated PAAm confirmed the formation of unsaturated double bonds.
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Fig. 6. Swelling ratio of PAAm and P(AAm–Na–AAc) in urine: (’) PAAm sample irradiated in dry form, (J) PAAm sample irradiated in wet form containing 10% (wt/wt) water, (.) PAAm sample irradiated in wet form containing 20% (wt/ wt) water, () P(AAm–Na–AAc) sample irradiated in wet form containing 10% (w/wt) water.
Fig. 5. SEM photos of (A) unirradiated PAAm, (B) PAAm particles irradiated in a dry form at dose 30 kGy.
3.2.3. Structure and particle shape of irradiated PAAm Unirradiated and irradiated freeze dried PAAm hydrogels were examined by SEM (Fig. 5). The unirradiated PAAm lost its particle shape to form highly porous film (Fig. 5(A)), while the irradiated powders show distinct aggregated particles (Fig. 5(B)). It is clear that the morphology of PAAm was affected by radiation The network structure prevented the PAAm particles from dissolving in water, and kept their shape, although in distorted form. 3.2.4. Potential application of the microgels in the diaper industry The most important property in a commercial superabsorbent used in personal care applications is the extent of swelling. According to Buchholz (1998), an
acceptable swelling capacity is approximately 20–40 g of urine per gram of polymer. Therefore, the amount of urine absorbed by different types of dry PAAm-based hydrogels was investigated and the results are shown in Fig. 6. In general, the swelling of hydrogels in urine solution is much lower than the swelling obtained in distilled water (Figs. 3 and 6). The absorption of urine decreases as the crosslinking degree of the PAAm-based polymer increases. P(AAm–Na–AAc) microgels can swell 30–50 times their own weight in urine solution, while PAAm polymers swell 15–35 times, depending on the degree of crosslinking. Minute changes in external conditions such as pH, temperature, solvent composition, and ionic strength can induce drastic changes in the state of the swollen network (Osada and Gong, 1993; Osada and Khokhlov, 2002). Thus, the reduction in polymer swelling in urine can be attributed to the effect of urinal salts on the macromolecules network, which may cause a swelling reduction. These effects are usually explained by changes of the solution structure induced by ion dehydration (Zhang et al., 1992; Horky et al., 2000). The introduction of polar negative charges to PAAm leads to better salt resistance and solution absorption rate. The high swelling of P(AAm–Na–AAc) gels in urine is associated with the difference between the concentration of mobile ions in the gel and in urine. The presence of Na+ mobile ions inside the gel increases the electroneutrality conditions and creates an additional osmotic pressure that expands the gel (Abd El-Rehim et al., 2004). Swelling of the hydrogels was also tested in artificial urine prepared according to the method of Kark et al. (1964). The swelling behavior and results obtained by
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commercial superabsorbents. The promising results reveal that such prepared materials may be of great interest in horticulture and industrial applications.
References
(A)
(B)
Fig. 7. Radish plants planted in soil: (A) untreated (control), (B) treated with P(AAm–Na–AAc).
the use of artificial urine were approximately the same as that obtained by real urine. 3.2.5. Possible use of microgels in agriculture From an economic point of view, agricultural hydrogels should be evaluated through the plant growth and wilting time. Therefore, the effect of pure sandy soil (control) and that treated with P(AAm–Na–AAc) microgels on the vegetative growth and time to wilting of radish plants was investigated. It is clear that the presence of hydrogels in the soil enhances radish plant growth and performance (Fig. 7). Also, radish plants planted in sandy soil treated with P(AAm–Na–AAc) microgel take a long time to wilt (14 days) if compared with the control one (9 days). This means that the soil treated with P(AAm–Na–AAc) microgel possesses a high ability to absorb and retain water, and significantly delays plant losses due to drought and water stress conditions. Thus, plant survival is improved and its probability for fast growth becomes high.
4. Conclusion An effort was made to use radiation to develop a superabsorbent process technology and suggest an easy method for their production. Crosslinked AAm-based microgels, which were already used for the production of commercial superabsorbents, can be produced in powder form by electron beam irradiation. The crosslinking extent in irradiated AAm-based microgels was controlled by polymer moisture content and irradiation doses. The crosslinked polymers and copolymers possess high and fast swelling properties in aqueous media. The swelling ratios of AAm-based microgels in urine or saline solution are acceptable when compared with
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