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Preparation of macroporous poly(acrylamide) hydrogels by radiation induced polymerization technique
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NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 265 (2007) 366–369 www.elsevier.com/locate/nimb
Preparation of macroporous poly(acrylamide) hydrogels by radiation induced polymerization technique Tuncer C ¸ aykara *, Melek Bulut, Serkan Demirci Gazi University, Faculty of Science, Department of Chemistry, 06500 Besevler, Ankara, Turkey Available online 8 September 2007
Abstract Macroporous poly(acrylamide) [poly(AAm)] hydrogels were prepared by using poly(ethylene glycol) (PEG) with three different molecular weight as the pore-forming agent during the radiation induced polymerization reaction. Scanning electron microscope graphs reveal that the macroporous network structure of the hydrogels can be adjusted by applying different molecular weights of PEG during the polymerization reaction. The swelling ratios of the PEG-modified hydrogels were much higher than those for the same type of hydrogel prepared via traditional method. However, the pulsatile swelling behavior of the PEG-modified hydrogels in water and in acetone was affected slightly by the change in the amount of the PEG. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Poly(acrylamide); Macroporous; Swelling
1. Introduction Poly(acrylamide) [poly(AAm)] has a broad applications as electrophoretic media for the separation or purification of biomolecules, as model drug delivery systems due to having random morphologies with a wide distribution of pore sizes [1,2], and as matrix for enzymes and living cells immobilization owing to its controllable pore size and no need for enzyme reactive groups for the attachment to an insoluble support, high residual activity, etc. [3]. In addition, high swelling capacity of poly(AAm) is very useful for water retention as grouting agents [4]. When a swollen poly(AAm) hydrogel in water is immersed in a non-solvent (e.g. acetone), deswelling immediately starts at the gel surface. Then the gel forms a dense polymer skin layer at the surface. This retards the permeability of water or a solute through the gel. To promote the applications of hydrogels in drug-release or fastresponse materials such as artificial muscles, reducing the *
Corresponding author. Tel.: +90 312 2126030x3116; fax: +90 312 2122279. E-mail address: [email protected] (T. C ¸ aykara). 0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.09.006
formation of a dense skin layer is important. Many researches have shown that incorporating a hydrophilic monomer such as acrylic acid or prepared a comb-type grafted hydrogel can reduce the dense skin layer [5]. In addition, using hydrogels with porous structures to increase the surface area can also achieve rapid swelling and deswelling [6,7]. In a previous report [8], the macroporous poly(acrylamide) [poly(AAm)] hydrogels were synthesized by using freeradical crosslinking polymerization method in presence of poly(ethylene glycol) (PEG) as pore-forming agent and the results showed that the swelling/deswelling ratios of these macroporous hydrogels in water and in dioxane are much higher than those of the same type of hydrogels prepared via traditional methods. In this report, we prepared poly(AAm) hydrogels with a macroporous network structure by using radiation induced polymerization method in the presence of PEG with three different molecular weight as the pore-forming agent. In this system, it is not necessary to add the crosslinker to monomer solution, because the polymers produced by irradiation can be crosslinked by exposure to the radiation. Moreover, radiation method is very useful for polymerization and
T. C¸aykara et al. / Nucl. Instr. and Meth. in Phys. Res. B 265 (2007) 366–369
crosslinking of acrylamide derivatives [9]. The PEG-modified poly(AAm) hydrogels were characterized by swelling ratio, swelling/deswelling kinetics, Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM).
2. Experimental Acrylamide (AAm), poly(ethylene glycol)s with molecular weights of 4000, 6000 and 10 000 g/mol (defined as PEG4000, PEG-6000 and PEG-10000, respectively) and acetone were obtained from Aldrich Chemical Co. The AAm (1.0 g) and PEG (4.8–20.0 wt%) were dissolved in distilled water (5.0 mL). The solutions thus prepared were placed in poly(vinyl chloride) straws of 4 mm diameters and about 20 cm long and irradiated to 10 kGy in air at ambient temperature in PX-k-30 Isslodovateji irradiator at a fixed dose rate of 2.05 kGyh1. Here, PEGs with three different molecular weights are employed as pore-forming agent. Upon completion of the reaction, the hydrogels were cut into specimens of approximately 10 mm in length and immersed in large excess of water at room temperature for at least 72 h. The water was changed every several hours to wash out the pore-forming agent and unreacted materials. The hydrogels were then dried at 50 °C under vacuum to constant weight. Fourier transform infrared spectroscopy (FT-IR) of the hydrogels was performed with a Nicolet 6700 FT-IR spec-
367
trometer attached to an attenuated total reflection (ATR) apparatus, using diamond prism with an incident angle 45°. Scanning electron microscopy (JEOL JSM-6360 LV SEM instrument) was used to study the cross-section or interior morphology of the hydrogels. To prepare samples for SEM analysis, the swollen hydrogels at 22 °C were freeze-dried and then sputter coated with gold. For the swelling dynamic studies, the hydrogels were immersed in deionized water at 22 °C. At a prescribed time interval, the hydrogels were taken out from water and weighed after wiping the excess water at the surface of
Fig. 1. FT-IR spectra of the traditional and PEG-modified poly(AAm) hydrogels: (a) 20.0 wt% PEG-10000 modified poly(AAm), (b) PEG, (c) poly(AAm) and (d) difference spectrum (a b, (a b) c).
Fig. 2. SEM micrographs of the traditional and 20.0 wt% PEG-modified poly(AAm) hydrogels: (a) poly(AAm), (b) PEG-4000 modified poly(AAm), (c) PEG-6000 modified poly(AAm) and (c) PEG-10000 modified poly(AAm).
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T. C¸aykara et al. / Nucl. Instr. and Meth. in Phys. Res. B 265 (2007) 366–369
hydrogels. The swelling ratio (SR) of the hydrogels was defined as follows: mt md ; ð1Þ SR ¼ md where md and mt are the masses of the dry and swollen hydrogels at time t, respectively. The pulsatile swelling behavior was observed in water and in acetone. The hydrogels were first immersed in distilled water and kept at 22 °C for ten days to reach equilibrium. The equilibrated hydrogels were then quickly transferred into acetone. During the pulsatile swelling process, the weight change of hydrogels was measured at 5 min intervals and the solvent was switched every 85 min. Normalized deswelling ratio (NSR) was calculated from the following equation: mt md NDR ¼ ; ð2Þ me md
PEG molecules in aqueous solution increases with an increase in PEG molecular weight, the higher the molecular weight of PEG used during the polymerization, the larger the pores within the resultant hydrogel [10]. Fig. 3 shows the swelling ratio–time curves for the traditional and PEG-modified hydrogels depending on the content and molecular weight of PEG. As shown in Fig. 3, all the PEG-modified hydrogels absorb water more quickly than the traditional hydrogels because their macroporous structures make transfer of water molecules easier between the hydrogel matrix and the external aqueous phase. Another point shown in Fig. 3, the swelling ratio of the PEG-modified hydrogels slightly increased with increasing PEG content, as from 4.8 to 20.0 wt%. The swelling ratios
where me is the mass of the swollen hydrogels at the equilibrium. 3. Results and discussion The ATR-FT-IR spectra of the poly(AAm) and PEGmodified poly(AAm) hydrogels, which have been freezedried, are illustrated in Fig. 1(a) and (c). The ATR-FTIR spectra of all the hydrogels are similar. There exists a typical amide I band (1653 cm1), consisting of the C@O stretch of poly(AAm) and amide II band (1543 cm1), including NAH vibration in each spectrum. In addition, the ATR-FT-IR spectrum of the PEG was also given in Fig. 1(b). The characteristic band of the PEG was observed at 1100 cm1 due to the CAO asymmetric stretching. On the other hand, if there exists PEG in the PEGmodified hydrogels, a typical and strong band positioned at around 1100 cm1, which belongs to the CAO stretch of PEG, would appear. For this purpose, the spectrum subtraction of the PEG and poly(AAm) from the PEG-modified hydrogel was made to compensate for the CAO stretching band of the PEG at 1100 cm1 (Fig. 1(d)). However, from Fig. 1(d) there is no obvious band appearing around 1100 cm1 in the difference spectrum. These findings suggest that the PEG-modified poly(AAm) hydrogels have the same chemical composition as the traditional poly(AAm) hydrogel and PEG does not exist in the PEG-modified hydrogels after they are extensively washed. PEG acts as a pore-forming agent and does not participate with the polymerization. The cross-sectional SEM micropictures of the freezedried traditional and PEG-modified hydrogels are exhibited in Fig. 2. The internal morphology of the traditional hydrogels is a dense smooth surface and there are no apparent pores, while the PEG-modified hydrogels exhibits porous microstructure and discontinuous surface morphologies. This may be explainable by various porous structures that PEGs create. That is, the average diameter of
Fig. 3. Swelling kinetics of the traditional and PEG-modified poly(AAm) hydrogels. The content and molecular weight of PEG are indicated as the inset.
T. C¸aykara et al. / Nucl. Instr. and Meth. in Phys. Res. B 265 (2007) 366–369
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about 8.9 and 11.8, and 12.1 and 15.9 within the same time frames and at the same molecular weight of the PEGs. In this case, the modified hydrogels by a high amount of PEG, such as 16.7 and 20.0 wt%, might have a more pore density, which would lead to a slight increase in the amount of water uptake. On the other hand, the increase in porosity affects their swelling ratios. Therefore, the PEG-10000 modified hydrogels lead to the highest swelling ratio among the PEG-modified hydrogels because of their highly porous structure. The pulsatile swelling ratio of the traditional and PEGmodified hydrogels at 22 °C in water and in acetone was studied to confirm swelling process reproducibility (Fig. 4). When the traditional hydrogel is immersed into the acetone, hydrogel may start the volume phase transition and shrink in the utmost surface region, resulting in a thick and dense skin layer at the beginning of the shrinking process. The resultant dense skin layer acts as a barrier for further solvent permeation and prevents the free solvent diffusion out from the hydrogel matrix. In contrast to the traditional hydrogels, the PEG-modified hydrogels have more solvent transfer because of their macroporous structures. This is, in case of the porous hydrogels, makes it possible for solvent molecules to rapidly transfer through the macropores into the innermost matrix, even though phase separation had occurred on the surface in non-solvent, and this resulted in a rapid deswelling. Acknowledgements This work was supported by the state Planning Organization (DPT), Contact Grant No. 2003K 120470–31. References
Fig. 4. Pulsatile swelling behavior of the traditional and PEG-modified poly(AAm) hydrogels in response to solvent changes between water and acetone. The content and molecular weight of PEG are indicated as the inset.
of the modified hydrogels with 4.8 wt% PEG were about 7.5 and 8.0 within 200 min, or 9.7 and 10.4 within 400 min for PEG-4000 and PEG-10000, respectively, whereas the modified hydrogels with 20.0 wt% PEG were
[1] H.R. Lin, Eur. Polym. J. 37 (2001) 1507. [2] D.G. Liu, Anal. Biochem. 339 (2005) 351. [3] C.L. Lester, S.M. Smith, A. Guymon, Macromolecules 34 (2001) 8587. [4] Y. Cohen, O. Ramon, I.J. Koperman, S.J. Mizrahi, J. Polym. Sci. 30 (1992) 1055. [5] Y. Kaneko, S. Nakamura, K. Sakai, T. Aoyagi, A. Kikuchi, Y. Sakuhisa, T. Okano, Macromolecules 31 (1998) 6099. [6] T. Gotog, Y. Nakatani, S. Sakohara, J. Appl. Polym. Sci. 69 (1998) 895. [7] J. Chen, K. Park, J. Control. Release 65 (2000) 73. [8] T. Caykara, M. Bulut, N. Dilsiz, Y. Akyu¨z, J. Macromol. Sci. Pure Appl. Chem. 43 (2006) 889. [9] A. Chapiro, Radiation Chemistry of Polymeric Systems, John Wiley, New York, 1962 (Chapters VI, IX). [10] J.K. Lin, M.R. Ladisch, J.A. Patterson, C.H. Noller, Biotechnol. Bioeng. 29 (1987) 976.
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 265 (2007) 366–369 www.elsevier.com/locate/nimb
Preparation of macroporous poly(acrylamide) hydrogels by radiation induced polymerization technique Tuncer C ¸ aykara *, Melek Bulut, Serkan Demirci Gazi University, Faculty of Science, Department of Chemistry, 06500 Besevler, Ankara, Turkey Available online 8 September 2007
Abstract Macroporous poly(acrylamide) [poly(AAm)] hydrogels were prepared by using poly(ethylene glycol) (PEG) with three different molecular weight as the pore-forming agent during the radiation induced polymerization reaction. Scanning electron microscope graphs reveal that the macroporous network structure of the hydrogels can be adjusted by applying different molecular weights of PEG during the polymerization reaction. The swelling ratios of the PEG-modified hydrogels were much higher than those for the same type of hydrogel prepared via traditional method. However, the pulsatile swelling behavior of the PEG-modified hydrogels in water and in acetone was affected slightly by the change in the amount of the PEG. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Poly(acrylamide); Macroporous; Swelling
1. Introduction Poly(acrylamide) [poly(AAm)] has a broad applications as electrophoretic media for the separation or purification of biomolecules, as model drug delivery systems due to having random morphologies with a wide distribution of pore sizes [1,2], and as matrix for enzymes and living cells immobilization owing to its controllable pore size and no need for enzyme reactive groups for the attachment to an insoluble support, high residual activity, etc. [3]. In addition, high swelling capacity of poly(AAm) is very useful for water retention as grouting agents [4]. When a swollen poly(AAm) hydrogel in water is immersed in a non-solvent (e.g. acetone), deswelling immediately starts at the gel surface. Then the gel forms a dense polymer skin layer at the surface. This retards the permeability of water or a solute through the gel. To promote the applications of hydrogels in drug-release or fastresponse materials such as artificial muscles, reducing the *
Corresponding author. Tel.: +90 312 2126030x3116; fax: +90 312 2122279. E-mail address: [email protected] (T. C ¸ aykara). 0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.09.006
formation of a dense skin layer is important. Many researches have shown that incorporating a hydrophilic monomer such as acrylic acid or prepared a comb-type grafted hydrogel can reduce the dense skin layer [5]. In addition, using hydrogels with porous structures to increase the surface area can also achieve rapid swelling and deswelling [6,7]. In a previous report [8], the macroporous poly(acrylamide) [poly(AAm)] hydrogels were synthesized by using freeradical crosslinking polymerization method in presence of poly(ethylene glycol) (PEG) as pore-forming agent and the results showed that the swelling/deswelling ratios of these macroporous hydrogels in water and in dioxane are much higher than those of the same type of hydrogels prepared via traditional methods. In this report, we prepared poly(AAm) hydrogels with a macroporous network structure by using radiation induced polymerization method in the presence of PEG with three different molecular weight as the pore-forming agent. In this system, it is not necessary to add the crosslinker to monomer solution, because the polymers produced by irradiation can be crosslinked by exposure to the radiation. Moreover, radiation method is very useful for polymerization and
T. C¸aykara et al. / Nucl. Instr. and Meth. in Phys. Res. B 265 (2007) 366–369
crosslinking of acrylamide derivatives [9]. The PEG-modified poly(AAm) hydrogels were characterized by swelling ratio, swelling/deswelling kinetics, Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM).
2. Experimental Acrylamide (AAm), poly(ethylene glycol)s with molecular weights of 4000, 6000 and 10 000 g/mol (defined as PEG4000, PEG-6000 and PEG-10000, respectively) and acetone were obtained from Aldrich Chemical Co. The AAm (1.0 g) and PEG (4.8–20.0 wt%) were dissolved in distilled water (5.0 mL). The solutions thus prepared were placed in poly(vinyl chloride) straws of 4 mm diameters and about 20 cm long and irradiated to 10 kGy in air at ambient temperature in PX-k-30 Isslodovateji irradiator at a fixed dose rate of 2.05 kGyh1. Here, PEGs with three different molecular weights are employed as pore-forming agent. Upon completion of the reaction, the hydrogels were cut into specimens of approximately 10 mm in length and immersed in large excess of water at room temperature for at least 72 h. The water was changed every several hours to wash out the pore-forming agent and unreacted materials. The hydrogels were then dried at 50 °C under vacuum to constant weight. Fourier transform infrared spectroscopy (FT-IR) of the hydrogels was performed with a Nicolet 6700 FT-IR spec-
367
trometer attached to an attenuated total reflection (ATR) apparatus, using diamond prism with an incident angle 45°. Scanning electron microscopy (JEOL JSM-6360 LV SEM instrument) was used to study the cross-section or interior morphology of the hydrogels. To prepare samples for SEM analysis, the swollen hydrogels at 22 °C were freeze-dried and then sputter coated with gold. For the swelling dynamic studies, the hydrogels were immersed in deionized water at 22 °C. At a prescribed time interval, the hydrogels were taken out from water and weighed after wiping the excess water at the surface of
Fig. 1. FT-IR spectra of the traditional and PEG-modified poly(AAm) hydrogels: (a) 20.0 wt% PEG-10000 modified poly(AAm), (b) PEG, (c) poly(AAm) and (d) difference spectrum (a b, (a b) c).
Fig. 2. SEM micrographs of the traditional and 20.0 wt% PEG-modified poly(AAm) hydrogels: (a) poly(AAm), (b) PEG-4000 modified poly(AAm), (c) PEG-6000 modified poly(AAm) and (c) PEG-10000 modified poly(AAm).
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T. C¸aykara et al. / Nucl. Instr. and Meth. in Phys. Res. B 265 (2007) 366–369
hydrogels. The swelling ratio (SR) of the hydrogels was defined as follows: mt md ; ð1Þ SR ¼ md where md and mt are the masses of the dry and swollen hydrogels at time t, respectively. The pulsatile swelling behavior was observed in water and in acetone. The hydrogels were first immersed in distilled water and kept at 22 °C for ten days to reach equilibrium. The equilibrated hydrogels were then quickly transferred into acetone. During the pulsatile swelling process, the weight change of hydrogels was measured at 5 min intervals and the solvent was switched every 85 min. Normalized deswelling ratio (NSR) was calculated from the following equation: mt md NDR ¼ ; ð2Þ me md
PEG molecules in aqueous solution increases with an increase in PEG molecular weight, the higher the molecular weight of PEG used during the polymerization, the larger the pores within the resultant hydrogel [10]. Fig. 3 shows the swelling ratio–time curves for the traditional and PEG-modified hydrogels depending on the content and molecular weight of PEG. As shown in Fig. 3, all the PEG-modified hydrogels absorb water more quickly than the traditional hydrogels because their macroporous structures make transfer of water molecules easier between the hydrogel matrix and the external aqueous phase. Another point shown in Fig. 3, the swelling ratio of the PEG-modified hydrogels slightly increased with increasing PEG content, as from 4.8 to 20.0 wt%. The swelling ratios
where me is the mass of the swollen hydrogels at the equilibrium. 3. Results and discussion The ATR-FT-IR spectra of the poly(AAm) and PEGmodified poly(AAm) hydrogels, which have been freezedried, are illustrated in Fig. 1(a) and (c). The ATR-FTIR spectra of all the hydrogels are similar. There exists a typical amide I band (1653 cm1), consisting of the C@O stretch of poly(AAm) and amide II band (1543 cm1), including NAH vibration in each spectrum. In addition, the ATR-FT-IR spectrum of the PEG was also given in Fig. 1(b). The characteristic band of the PEG was observed at 1100 cm1 due to the CAO asymmetric stretching. On the other hand, if there exists PEG in the PEGmodified hydrogels, a typical and strong band positioned at around 1100 cm1, which belongs to the CAO stretch of PEG, would appear. For this purpose, the spectrum subtraction of the PEG and poly(AAm) from the PEG-modified hydrogel was made to compensate for the CAO stretching band of the PEG at 1100 cm1 (Fig. 1(d)). However, from Fig. 1(d) there is no obvious band appearing around 1100 cm1 in the difference spectrum. These findings suggest that the PEG-modified poly(AAm) hydrogels have the same chemical composition as the traditional poly(AAm) hydrogel and PEG does not exist in the PEG-modified hydrogels after they are extensively washed. PEG acts as a pore-forming agent and does not participate with the polymerization. The cross-sectional SEM micropictures of the freezedried traditional and PEG-modified hydrogels are exhibited in Fig. 2. The internal morphology of the traditional hydrogels is a dense smooth surface and there are no apparent pores, while the PEG-modified hydrogels exhibits porous microstructure and discontinuous surface morphologies. This may be explainable by various porous structures that PEGs create. That is, the average diameter of
Fig. 3. Swelling kinetics of the traditional and PEG-modified poly(AAm) hydrogels. The content and molecular weight of PEG are indicated as the inset.
T. C¸aykara et al. / Nucl. Instr. and Meth. in Phys. Res. B 265 (2007) 366–369
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about 8.9 and 11.8, and 12.1 and 15.9 within the same time frames and at the same molecular weight of the PEGs. In this case, the modified hydrogels by a high amount of PEG, such as 16.7 and 20.0 wt%, might have a more pore density, which would lead to a slight increase in the amount of water uptake. On the other hand, the increase in porosity affects their swelling ratios. Therefore, the PEG-10000 modified hydrogels lead to the highest swelling ratio among the PEG-modified hydrogels because of their highly porous structure. The pulsatile swelling ratio of the traditional and PEGmodified hydrogels at 22 °C in water and in acetone was studied to confirm swelling process reproducibility (Fig. 4). When the traditional hydrogel is immersed into the acetone, hydrogel may start the volume phase transition and shrink in the utmost surface region, resulting in a thick and dense skin layer at the beginning of the shrinking process. The resultant dense skin layer acts as a barrier for further solvent permeation and prevents the free solvent diffusion out from the hydrogel matrix. In contrast to the traditional hydrogels, the PEG-modified hydrogels have more solvent transfer because of their macroporous structures. This is, in case of the porous hydrogels, makes it possible for solvent molecules to rapidly transfer through the macropores into the innermost matrix, even though phase separation had occurred on the surface in non-solvent, and this resulted in a rapid deswelling. Acknowledgements This work was supported by the state Planning Organization (DPT), Contact Grant No. 2003K 120470–31. References
Fig. 4. Pulsatile swelling behavior of the traditional and PEG-modified poly(AAm) hydrogels in response to solvent changes between water and acetone. The content and molecular weight of PEG are indicated as the inset.
of the modified hydrogels with 4.8 wt% PEG were about 7.5 and 8.0 within 200 min, or 9.7 and 10.4 within 400 min for PEG-4000 and PEG-10000, respectively, whereas the modified hydrogels with 20.0 wt% PEG were
[1] H.R. Lin, Eur. Polym. J. 37 (2001) 1507. [2] D.G. Liu, Anal. Biochem. 339 (2005) 351. [3] C.L. Lester, S.M. Smith, A. Guymon, Macromolecules 34 (2001) 8587. [4] Y. Cohen, O. Ramon, I.J. Koperman, S.J. Mizrahi, J. Polym. Sci. 30 (1992) 1055. [5] Y. Kaneko, S. Nakamura, K. Sakai, T. Aoyagi, A. Kikuchi, Y. Sakuhisa, T. Okano, Macromolecules 31 (1998) 6099. [6] T. Gotog, Y. Nakatani, S. Sakohara, J. Appl. Polym. Sci. 69 (1998) 895. [7] J. Chen, K. Park, J. Control. Release 65 (2000) 73. [8] T. Caykara, M. Bulut, N. Dilsiz, Y. Akyu¨z, J. Macromol. Sci. Pure Appl. Chem. 43 (2006) 889. [9] A. Chapiro, Radiation Chemistry of Polymeric Systems, John Wiley, New York, 1962 (Chapters VI, IX). [10] J.K. Lin, M.R. Ladisch, J.A. Patterson, C.H. Noller, Biotechnol. Bioeng. 29 (1987) 976.