acrylic acid hydrogels with superabsorbent properties by radiation-initiated crosslinking

Author’s Accepted Manuscript Synthesis of carboxymethylcellulose/acrylic acid hydrogels with superabsorbent properties by radiation-initiated crosslinking Tamás Fekete, Judit Borsa, Erzsébet Takács, László Wojnárovits www.elsevier.com/locate/radphyschem

PII: DOI: Reference:

S0969-806X(15)30060-8 http://dx.doi.org/10.1016/j.radphyschem.2015.09.018 RPC6919

To appear in: Radiation Physics and Chemistry Received date: 27 July 2015 Revised date: 28 September 2015 Accepted date: 28 September 2015 Cite this article as: Tamás Fekete, Judit Borsa, Erzsébet Takács and László Wojnárovits, Synthesis of carboxymethylcellulose/acrylic acid hydrogels with superabsorbent properties by radiation-initiated crosslinking, Radiation Physics and Chemistry, http://dx.doi.org/10.1016/j.radphyschem.2015.09.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis of carboxymethylcellulose/acrylic acid hydrogels with superabsorbent properties by radiation-initiated crosslinking Tamás Feketea,b, Judit Borsab,c*, Erzsébet Takácsa,c, László Wojnárovitsa a

Institute for Energy Security and Environmental Safety, Centre for Energy Research,

Hungarian Academy of Sciences, Budapest, Hungary b

Faculty of Chemical Technology and Biotechnology, Budapest University of Technology

and Economics c

Faculty of Light Industry and Environmental Engineering, Obuda-University

Abstract Superabsorbent hydrogels were prepared by gamma irradiation from aqueous solutions of carboxymethylcellulose (CMC) and acrylic acid (AAc) with varying CMC:AAc ratio. By partially replacing the CMC with AAc the gelation increased and led to a higher gel fraction and lower water uptake. Moreover, the gelation required significantly milder synthesis conditions. Decreasing both the dose and the solute concentration in the presence of AAc led to gels with higher gel fraction and higher degree of swelling compared to pure CMC gels. Increasing the AAc content up to 10% proved to be very effective, while very high AAc content (over 50%) hindered the gelation process. Keywords carboxymethylcellulose, acrylic acid, superabsorbent, hydrogel, irradiation, crosslinking 1. Introduction Carboxymethylcellulose (CMC) is one of the major low-cost, commercially available derivatives of cellulose with a wide array of industrial applications (Heinze and Koschella, 2005). An interesting potential application is the preparation of hydrogels with superabsorbent properties (Chang and Zhang, 2011). CMC has several advantageous properties for gel synthesis, such as good water solubility and the presence of reactive hydroxyl and carboxymethyl groups. The non-toxic nature and biocompatibility of such gels is advantageous for biomedical applications (Caló and Khutoryanskiy, 2015). Moreover, its good biodegradability in soil is advantageous for its use in agriculture and forestry (Nie et al., 2004). Cellulose derivative gels are prepared from aqueous solutions with several crosslinking *

Corresponding author, Tel:+36-20-596-5401, Fax:+36-1-666-5876, E-mail: [email protected]

obuda.hu (J. Borsa) 1

methods. While common crosslinking agents such as divinylsulfone (Sannino et al., 2004) and epichlorohydrin (Chang et al., 2010) are effective for the chemical crosslinking of CMC, they are mostly used for the synthesis of various CMC copolymer gels. However, pure CMC gels were also prepared with several non-toxic crosslinkers like citric acid (Demitri et al., 2008) and fumaric acid (Akar et al., 2012). The gelation can also be achieved without crosslinkers by free-radical crosslinking. Moreover, with high-energy irradiation no initiator is required for the synthesis (Fei et al., 2000; Liu et al., 2002). Besides pure CMC gels copolymers with other monomers such as acrylamide (Ibrahim et al., 2007;

Heimvichian et

al.,

2014) or acrylic acid

(AAc) were

also

prepared.

Carboxymethylcellulose/acrylic acid hydrogels were previously synthetized by using crosslinking agent/initiator system (Bajpai and Mishra, 2004) and electron beam irradiation (Said et al., 2004; El-Naggar et al., 2006). In the latter method gels with high gel fraction and moderate swelling properties were prepared successfully. However, it should be noted that relatively low carboxymethylcellulose concentration was used (4.2 w/v% as opposed to the acrylic acid concentration of 10-50 w/v%) and high absorbed doses (50-80 kGy) were required. As pure concentrated solutions of CMC can also form gels by high-energy irradiation, the gelation of CMC/AAc solutions with low AAc content should be also examined. In the present work we prepared gels using high concentration aqueous solutions of carboxymethylcellulose and acrylic acid with different blend ratios. The aim of this work is to form gels at milder synthesis conditions and to achieve better gel properties compared to the pure CMC gels reported in our previous study (Fekete et al., 2014) by substituting a part of the cellulose derivative content with acrylic acid while keeping the AAc concentration relatively low. 2. Experimental 2.1. Materials Carboxymethylcellulose Na-salt (Mw = 700 000 g mol-1, DS = 0.9, properties provided by the manufacturer) and acrylic acid (anhydrous, contains 180-200 ppm monomethyl ether of hydroquinone (MEHQ) as inhibitor) of analytical grade were purchased from Sigma-Aldrich. No purification was used. 2.2. Synthesis Aqueous solutions of CMC and AAc were prepared from 5 to 40 w/w% solute concentrations. The CMC:AAc ratio varied from 100:0 to 20:80. First, the acrylic acid was mixed with the water, and then the CMC powder was added to the solution. After stirring the solutions were 2

stored at room temperature for 24 hours to improve homogeneity. Spherical samples formed from the paste-like solution were put into polyethylene bags. The solutions were irradiated with

60

Co γ-source at a dose rate of 9 kGy h-1 with an absorbed dose of 1 to 80 kGy. Three

samples were used during the measurements to determine the standard deviation. 2.3. Gel fraction The sol fraction was removed by immersing the samples in deionized water for 48 hours (1000:1 liquid ratio). Swollen gels were removed by a metal sieve and dried to constant weight at 60 °C. The gel fraction (GF) was calculated from the dry gel weight before (w0; calculated from the solution concentration) and after (w1) the process: ( )

(1)

2.4. Degree of swelling Dried, washed (see 2.3) samples were immersed in deionized water. After 24 hours, gels were removed from the water and weighted. The degree of swelling (Q) was calculated from the weight of the swollen (ws) and the dry gel (wd): (

)

(2)

2.5. Composition of the hydrogel ATR-FTIR spectra were recorded using ATI Mattson Research Series FTIR spectrometer. The accessory contained a ZnSe flat plate with a nominal incident angle of 45°. Gel samples were prepared by freeze-drying after immersion in water for 48 hours. The spectra were recorded from 4000 to 500 cm-1 at a resolution of 8 cm-1, averaged from 128 scans. 3. Results and discussion Three synthesis parameters (absorbed dose, solute concentration and CMC:AAc blend ratio) were varied to determine their effect on the gel properties. ATR-FTIR spectroscopy was used to characterize the composition of the crosslinked samples. 3.1. Absorbed dose The effect of the dose on gel properties was determined at three different CMC:AAc ratios (Fig. 1). Gel formation occurred only over a critical absorbed dose. Acrylic acid-free solutions gelled at 7.5 kGy, but the gel fraction was very low until 15 kGy (Fig. 1a). The GF increased with the absorbed dose up to 60 kGy. At 10% AAc content the required dose for gelation decreased significantly, more than 30% GF was achieved even at 2.5 kGy. It should also be noted that the MEHQ additive acts as inhibitor (in presence of oxygen) and retarder for the 3

AAc gelation (Cutié et al., 1997), thus MEHQ-free solutions would require even lower doses. The increase of the dose had little effect on the gel fraction over 10-15 kGy compared to lower doses. Increasing the AAc ratio to 30% resulted in a significant increase in the gel fraction only over 10 kGy. At 40 kGy a GF over 80% was achieved. At 80 kGy a slight decrease was observed in the gel fraction, which can be explained by the degradation which becomes more dominant than the crosslink formation at high doses. The improved crosslinking in the presence of acrylic acid can be attributed to several effects. The mobility of the acrylic acid monomer molecules is much higher than that of the CMC macromolecules. Moreover, the viscosity of the solution is lower, which improves the mobility of the CMC chains, as well. This is not only related to the molecular weight differences between the two components, but also the change in the pH of the solution. Pure CMC solutions are slightly alkaline (pH of ~7.4 for 20 w/wpolymer%), but solution is shifted towards acidic character with the acrylic acid content. This leads to the decrease of the electrostatic repulsion between carboxymethyl and carboxyl groups due to the protonation, which also promotes the crosslink formation. The water uptake decreased with the absorbed dose for all samples (Fig. 1b). This can be explained with the higher crosslink density of the polymer networks. While the degree of swelling decreased rapidly at lower doses, higher doses resulted in only a small change of the water uptake. At 10% AAc content the degree of swelling decreased significantly compared to the AAc-free samples, but at low doses (2.5-5 kGy) good swelling properties were observed. Interestingly, while increasing the AAc content to 30% resulted in even lower water uptake at low doses, at 10-40 kGy they had higher water uptake than gels with 10% AAc content. Over 40 kGy pure CMC gels showed only a slight decrease in Q, while for CMC/AAc gels no significant change was observed. 3.2. Solute concentration The effect of solute (CMC+AAc) concentration was examined with different CMC/AAc blend ratios at two different doses. At 20 kGy AAc-free solutions gelled in the 10 to 40 w/w% concentration range, with the highest gel fraction measured for 15-30 w/w% solutions (Fig. 2a). In dilute solutions the relatively large distances between the individual polymer chains hindered the crosslinking process, while at high concentrations the low chain mobility due to the high viscosity had a negative effect on the gelation. The addition of AAc significantly increased the crosslink formation and the gelation occurred even at 5 w/w% solute concentration. The gel fraction increased with the solute content up to 15 w/w%, which was followed by a slight decrease. Increasing the AAc ratio to 30% further improved the GF; 4

moreover, its decrease at high concentrations was smaller. This can be explained by the smaller increase in the viscosity with the solute concentration compared to pure CMC gels due to the lower CMC macromolecule content and lower pH. At 5 kGy, in AAc-free solutions there was no gel formation. In contrast, CMC/AAc solutions reached a relatively high gel fraction over a wide solute concentration range. While the gelled fraction was smaller than at 20 kGy, it was still significantly higher than for pure CMC gels. Moreover, while solutions with 30% AAc content had higher gel fraction in higher solute concentrations, the maximum gel fraction achieved at 10 w/w% was roughly the same for both CMC/AAc ratios. The water uptake decreased with the solute concentration for all gels (Fig. 2b). The decrease is due to the increase in the crosslink density with increasing solute concentration. CMC/AAc gels showed lower degree of swelling than CMC gels in the same solute concentrations due to their higher crosslink density. However, copolymer gels synthesized from dilute solutions (510 w/w%) had better swelling properties than the pure CMC gels with the highest water uptake. Decreasing the absorbed dose to 5 kGy further improved the swelling properties and 700-1000 gwater/ggel was reached at low concentrations. The swelling of the two CMC/AAc gels was very similar at low concentrations, while over 15 w/w% solutions with 10% AAc had higher water uptake. In summary, by decreasing the dose and solute concentration both gel characteristics were improved in the presence of 10% AAc. Increasing the acid content to 30% was mostly beneficial in higher solute concentrations. 3.3. Acrylic acid ratio The change of the gel properties with the CMC:AAc ratio was similar at different doses. The gel fraction increased with the acrylic acid concentration (Fig. 3a) due to the effects of the acrylic acid on solution properties explained in 3.1. The substitution proved to be very effective up to 10%, but further increase of the AAc content lead to a smaller improvement in the gelation. Taking into consideration the molecular weight of the monomer unit of CMC (MCMC,DS=0.9 = 234) and that of AAc (MAAc = 72), 10 m% AAc means that is the molar ratio of CMC and AAc is about 3:1. The gel fraction increased with the dose for all CMC/AAc gels. Interestingly, the gel fraction decreased at very high AAc content. Moreover, its maximum shifted to higher AAc concentrations and the decrease became lower with the dose. This is related to the decrease of the average molecular weight of the solute with the replacement of the CMC of high molecular weight: significantly more reactions were required for the formation of the gel network, which was only partly compensated by the improved mobility. However, with the 5

increase of the absorbed dose the crosslink formation increased, thus even solutions with very high AAc content could gel properly. The increased gelation with increasing monomer content also had a major effect on the water uptake (Fig. 3b). The degree of swelling decreased significantly with the AAc content up to 10% due to the increasing crosslink density. Over 10% the CMC:AAc ratio had no major impact on the water uptake, though a slight increase was observed above 50% AAc content. This is also related to the weaker crosslink formation due to the low CMC content. The water uptake decreased with the dose; however, over 10 kGy the dose had very little effect on it for all samples. 3.4. Composition of the hydrogel FTIR spectra of CMC/AAc hydrogels with different blend ratios from 2000 to 700 cm-1 are presented in Fig. 4. Pure CMC gels showed a wide absorption band from 1100 to 1000 cm-1 due to the ether bonds in the polymer. The peak at 1321 cm-1 is due to the O-H stretching, while the minor peak at 1268 cm-1 is attributed to the C-O stretching. The COO- groups of the carboxymethyl substituent appear at 1580 and 1410 cm-1 (Biswal and Singh, 2004). The dual peak is due to the symmetric and antisymmetric stretching of the functional group. The C=O stretch of free protonated COOH group only shows a very small peak since Na-salt of CMC was used for gelation. The replacement of 10% CMC with AAc resulted in the peaks attributed to the ether bonds and the COO- groups becoming significantly smaller, while the intensity of the COOH peak at 1730 cm-1 increased due to the protonated COOH group in the acrylic acid (Kirwan et al, 2003). The changes in the peak intensities became even more prominent at 30% AAc content. Thus it confirms that both CMC and AAc are present in the samples and most possibly participate in the gelation process. 4. Conclusions The gelation of carboxymethylcellulose/acrylic acid solutions required much milder synthesis conditions than pure CMC solutions. Absorbed dose required for adequate gelation decreased with the increase in AAc ratio. Moreover, significantly higher gel fraction was achieved at the expense of lower water uptake. Unlike CMC solutions, CMC/AAc systems showed good gelation even below 10 w/w% solute concentration. Moreover, irradiation of 5-7.5 w/w% CMC/AAc solutions with small doses resulted in gels with excellent swelling properties and gel fraction. The replacement of CMC was very effective up to 10%. Very high AAc content (more than 50-60%) resulted in lower gel fraction unless higher absorbed doses were used. In summary, by substituting a small part of CMC with AAc under mild synthesis conditions both 6

gel properties could be significantly improved simultaneously compared to pure carboxymethylcellulose gels. Acknowledgements The authors thank the Hungarian Science Foundation (NK 105802) for partial support, Eva Horvathne Koczog and Zoltan Papp for technical assistance. References Akar, E., Altınışık, A., Seki, Y., 2012. Preparation of pH- and ionic-strength responsive biodegradable fumaric acid crosslinked carboxymethyl cellulose. Carbohydr. Polym. 90, 1634-1641. Bajpai, A.K., Mishra, A., 2004. Ionizable interpenetrating polymer networks of carboxymethyl cellulose and polyacric acid: evaluation of water uptake. J. Appl. Polym. Sci. 93, 2054–2065. Biswal, D.R., Singh, R.P., 2004. Characterization of carboxymethyl cellulose and polyacrylamide graft copolymer. Carbohydr. Polym. 57, 379-387. Caló, E., Khutoryanskiy, V., 2015. Biomedical applications of hydrogels: A review of patents and commercial products. Eur. Polym. J. 65, 252-267. Chang, C., Duan, B., Cai, J., Zhang, L., 2010. Superabsorbent hydrogels based on cellulose for smart swelling and controllable delivery. Eur. Polym. J. 46, 92-100. Chang, C., Zhang, L., 2011. Cellulose-based hydrogels: Present status and application prospects. Carbohydr. Polym. 84, 40-53. Cutié, S.S., Henton, D.E., Powell, C., Reim, R.E., Smith, P.B., Staples, T.L., 1997. The effects of MEHQ on the polymerization of acrylic acid in the preparation of superabsorbent gels. J. Appl. Polym. Sci. 64, 577-589. Demitri, C., Sole, R.D., Scalera, F., Sannino, A., Vasapollo, G., Maffezzoli, A., Ambrosio, L., Nicolais, L., 2008. Novel superabsorbent cellulose-based hydrogels crosslinked with citric acid. J. Appl. Polym. Sci. 110, 2453-2460. El-Naggar, A.W.M., Alla, S.G.A., Said, H.M., 2006. Temperature and pH responsive behaviours of CMC/AAc hydrogels prepared by electron beam irradiation. Mater. Chem. Phys. 95, 158-163.

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Fei, B., Wach, R.A., Mitomo, H., Yoshii, F., Kume, T., 2000. Hydrogel of biodegradable cellulose derivatives. I. Radiation-induced crosslinking of CMC. J. Appl. Polym. Sci. 78, 278283. Fekete, T., Borsa, J., Takács, E., Wojnárovits., L., 2014. Synthesis of cellulose derivative based superabsorbent hydrogels by radiation induced crosslinking. Cellulose 21, 4157-4165. Heinze, T., Koschella, A., 2005. Carboxymethyl ethers of cellulose and starch – A review. Macromol. Symp. 223, 13-40. Hemvichian, K., Chanthawong, A., Suwanmala, P., 2014. Synthesis and characterization of superabsorbent polymer prepared by radiation-induced graft copolymerization of acrylamide onto carboxymethyl cellulose for controlled release of agrochemicals. Radiat. Phys. Chem. 103, 167-171. Ibrahim, S.M., Salmawi, K.M.E., Zahran, A.H., 2007. Synthesis of crosslinked superabsorbent carboxymethyl cellulose/acrylamide hydrogels through electron-beam irradiation. J. Appl. Polym. Sci. 104, 2003-2008. Kirwan, L.J., Fawell, D.P., Bronswijk, W.V., 2003. In situ FTIR-ATR examination of poly(acrylic acid) adsorbed onto hematite at low pH. Langmuir 19, 5802-5807. Liu, P., Zhai, M., Li., J., Peng, J., Wu, J., 2002. Radiation preparation and swelling behavior of sodium carboxymethyl cellulose hydrogels. Radiat. Phys. Chem. 63, 525-528. Nie, H., Liu, M., Zhan, F., Guo, M., 2004. Factors on the preparation of carboxymethylcellulose hydrogel and its degradation behavior in soil. Carbohydr. Polym. 58, 185-189. Said, H.M., Alla, S.G.A., El-Naggar, A.W.M., 2004. Synthesis and characterization of novel gels based on carboxymethyl cellulose/acrylic acid prepared by electron beam irradiation. React. Funct. Polym. 61, 397-404. Sannino, A., Madaghiele, M., Conversano, F., Mele, G., Maffezzoli, P., Netti, P.A., Ambrosio, L., Nicolais, L., 2004. Cellulose derivative-hyaluronic acid-based microporous hydrogels cross-linked through divinyl sulfone (DVS) to modulate equilibrium sorption capacity and network stability. Biomacromolecules 5, 92-96.

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Figure captions Fig. 1 Effect of the absorbed dose on gel fraction (a) and degree of swelling (b) for CMC/AAc gels with different AAc content (20 w/w% solution) Fig. 2 Effect of the solute concentration on gel fraction (a) and degree of swelling (b) for CMC/AAc gels with different AAc content (absorbed dose: 5 or 20 kGy) Fig. 3 Effect of acrylic acid ratio on gel fraction (a) and degree of swelling (b) for CMC/AAc gels at different doses (20 w/w% solution) Fig. 4 FTIR spectra of freeze-dried CMC/AAc gels with different AAc content (20 w/w% solution, 20 kGy)

HIGHLIGHTS     

CMC/AAc hydrogels were prepared by radiation-induced crosslinking. Gelation required lower dose and solute concentration in CMC/AAc solutions. Increased AAc concentration improved the gel fraction at the expense of water uptake. In mild synthesis conditions CMC/AAc gels had better properties than pure CMC gels. Substitution of CMC with AAc up to 10% proved to be the most effective.

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Figure 1

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