- Email: [email protected]
Time-dependent crosslinking of whey protein based films during storage
Materials Letters 215 (2018) 8–10
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Time-dependent crosslinking of whey protein based films during storage Markus Schmid a,b,⇑, Sarah Merzbacher b, Kerstin Müller b a b
Technical University of Munich, TUM School of Life Sciences Weihenstephan, Chair of Food Packaging Technology, Weihenstephaner Steig 22, 85354 Freising, Germany Fraunhofer Institute for Process Engineering and Packaging IVV, Giggenhauser Straße 35, 85354 Freising, Germany
a r t i c l e
i n f o
Article history: Received 10 June 2017 Received in revised form 26 November 2017 Accepted 10 December 2017 Available online 11 December 2017 Keywords: Whey protein isolate Whey protein films Post-crosslinking Storage time Degree of crosslinking Degree of swelling
a b s t r a c t Environmentally-friendly packaging based on whey protein offers the potential to replace petroleumbased oxygen barrier materials. Effective crosslinking of the whey protein is a prerequisite for this application. To quantify the effect of storage time on the post-crosslinking of whey protein isolate based films, swelling tests were conducted in order to determine the degree of swelling (DoS). In addition, the degree of crosslinking (DoC) and the crosslinking density (CLD) were calculated using existing models with new data from water vapour sorption isotherm measurements. The results showed that increasing storage time led to decreased swelling of whey protein isolate based films (p < .05) and increased DoC and CLD (p < .05). Comparison of the films on day 1 and day 21 showed that the crosslinking increased by a factor of 3 while the DoS reduced by almost half. The results of this study demonstrate that the storage time of whey protein isolated based films is a crucial factor for crosslinking. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Petroleum-based packaging materials are increasingly falling out of favour due to their unsustainable production and the environmental burden of plastic waste [1]. The packaging industry and R&D organisations are working on renewable and biodegradable alternatives to replace commercial plastics. Protein-based films offer substantial potential because they are more environmentally-friendly than fossil-based polymeric materials. A prerequisite for the use of whey protein films as a packaging material is heat-induced formation of a protein network, namely crosslinking [2–4]. Crosslinking leads to improved barrier and tensile properties. The crosslinking involves interactions such as ionic crosslinks between carboxyl and amino groups, hydrogen bonding and disulphide bridges [2,3]. Relatively little work has been carried out on the changes in the techno-functional properties of whey proteinbased films that occur during the storage time [5,6]. It is assumed that these changes are caused among other things by postdenaturation crosslinking of the proteins [7]. Thus, the crosslinking behaviour of whey protein based films during storage is a crucial factor for their mechanical and barrier performance, especially when used for packaging materials. The aim of this study was to quantify changes to the CLD and DoC of whey protein based films ⇑ Corresponding author at: Fraunhofer Institute for Process Engineering and Packaging IVV, Giggenhauser Straße 35, 85354 Freising, Germany. E-mail address: [email protected] (M. Schmid). https://doi.org/10.1016/j.matlet.2017.12.047 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
during their storage time. This study uses a direct approach for the quantification of crosslinking in whey protein based films and evaluation of the post crosslinking behaviour during storage. Accordingly, established crosslinking parameters were determined using existing models with data from swelling tests and water vapour sorption isotherms measurements. The findings will be invaluable for researchers and material developers, providing for the first time quantitative data about storage-dependent post-crosslinking in whey protein isolate based films. This information will enable future experiments and especially the characterisation procedures to be conducted more reliably.
2. Materials and methods 2.1. Materials Whey protein isolate (WPI, BiPro) was obtained from Davisco Foods International Inc. (Le Sueur, Minnesota, USA). Anhydrous glycerol was purchased from Merck KGaA (Darmstadt, Germany).
2.2. Film preparation A glycerol plasticised and denatured whey protein standard solution (WPSS) containing 10% (w/w) WPI was prepared according to the procedure developed by Schmid et al. [8]. Films were cast in petri dishes with a target thickness of 100 ± 10 mm and dried under ambient conditions in a climate chamber at 23 °C
M. Schmid et al. / Materials Letters 215 (2018) 8–10
and 50% relative humidity (RH). The films reached their equilibrium moisture content after approx. 24 h.
2.3. Swelling tests Swelling tests were used to determine the DoS, CLD and DoC of the whey protein isolate-based cast films as described by Schmid et al. [9,10]. The DoS [%] was calculated using a gravimetric method. Fivefold determination was carried out using square samples of 50 mm 50 mm size. Further information about the crosslinking of whey protein based films was obtained by calculating the DoC [%] and CLD [mol g-1]. The DoC is defined as the quotient of the molecular weight of the mean amino acid (123.3 g mol 1 for WPI according to [10]) and the number average molecular weight of the polymer between the crosslinks [11]. The latter was determined using the Flory-Rehner equation according to the implementation by Schmid et al. [9,10]. The CLD is the reciprocal number average molecular weight of the polymer between the crosslinks [12,13].
2.4. Water vapour sorption isotherm measurements For calculation of the average molecular weight of the polymer between the crosslinks, the Flory-Huggins interaction parameter v is required. This was determined by measuring the water sorption isotherm. These measurements and the data evaluation were carried out following the approach of Schmid et al. [9,10] with a deviating sample diameter of 25 mm and a maximum measuring time of 3000 min. Triple determination was performed on each cast film.
1000
DoS [%]
900 800
700
ab
600 500
2.5. Statistical analysis The results were analysed with the statistical software Visual XSel (CRGRAPH GbR, Starnberg, Germany). All data were statistically evaluated for normal distribution according to the AndersonDarling (sample size 5) or Kolmogorov-Smirnov test (sample size 4). Multiple t-tests were performed in order to determine whether the mean values differed significantly from each other (p < .05). 3. Results and discussion 3.1. Degree of swelling It was assumed that the crosslinking increases due to the fact that the whey protein films post-crosslink during the storage time [5]. A consequence of the increased network formation is a decrease in the swelling capacity as less water can accumulate [14,15]. Fig. 1 shows the DoS as a function of the storage time of the cast films. A clear decline in DoS with storage time was observed with significant differences (p < .05) between day 1 and days 7, 10, 14 and 21. The hypothesis that DoS decreases with storage time due to increasing crosslinking is confirmed not only by the results of this work but also by other results in the literature. The DoS is strongly dependent on the DoC. The greater the crosslinking of the polymer matrix, the lower the DoS [14] and the lower the amount of bounded water in the cast films [16]. The following figure illustrates the relationship between the crosslinking and the swelling capacity (Fig 2.). The polymer chains are slightly crosslinked at the start of the storage period. Increasing crosslinking takes place during the storage period, leading to a decrease in water absorption. Crosslinked proteins can absorb less water and lower the swelling capacity [17]. In addition, the mobility of the chain segments decreases [18]. 3.2. Degree of crosslinking and crosslinking density
ab
a
9
b
b b
c
400 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 storage time [d] Fig. 1. Degree of swelling as a function of the storage time. Significant differences (p < .05) are indicated by different letters.
Water vapour sorption measurements were used to demonstrate the decrease in swelling capacity due to the increase in crosslinking. There is a significant difference (p < .05) between day 1 and days 10, 14 and 21, demonstrating that the DoC and CLD significantly increase with storage time (see Fig. 3). The results clearly show that the crosslinking increases with storage time. DoC and CLD on day 21 are almost a factor of 3 higher than on day 1. This is in compliance with former studies that also indicated a correlation between the increase in DoC and storage time [5]. Their investigations demonstrated that the oxygen permeability decreases with storage period. The reduction in oxygen permeability can be explained by the increase in crosslinking. Sol-
Fig. 2. Schematic representation of reduced swelling as a result of increased crosslinking [17].
10
M. Schmid et al. / Materials Letters 215 (2018) 8–10 3.5
4 c
3
DoC [%]
2.5
2.5
2
b
1.5 a
1
a
ab
b
2 1.5 1
a
0.5
CLD *10^4 [mol/cm^3]
3
References
3.5
0.5
0
0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 storage me [d]
Fig. 3. Degree of crosslinking and crosslinking density as a function of the storage time. Significant differences (p < .05) are indicated by different letters.
ubility tests with different buffer systems, which split specific interactions, give evidence that during the first days of storage disulphide bonds and hydrogen bonds are formed. As a result of those interactions, the oxygen permeability decreases. As the storage time increases, the formation of hydrogen bonds accelerates, which also leads to a reduction in oxygen permeability [5]. These results are also in accordance with the determined DoS, since DoS and CLD are always inversely proportional to each other [17]. 4. Conclusion It was demonstrated that the crosslinking, as quantified by the DoC and CLD, increases with storage time. The increase in DoC and CLD clearly indicates post-crosslinking, in agreement with the previously observed decrease in oxygen permeability reported by Schmid et al. [5]. Based on these findings, a minimum storage time of two weeks should be heeded prior to material characterisation. No substantial changes of material properties were detected thereafter. However, due to ongoing post crosslinking minor changes can still be expected subsequently and should be taken into account for future experiments and characterisation procedures. Conflicts of interest None.
[1] J.V. Schulze, Öffentliches Marketing zur Vermeidung von Kunststoffverpackungen im Lebensmittelbereich, Diplomica Verlag, 2015. [2] M. Pérez-Gago, P. Nadaud, J. Krochta, Water vapor permeability, solubility, and tensile properties of heat-denatured versus native whey protein films, J. Food Sci. 64 (6) (1999) 1034–1037. [3] M. Perez-gago, J. Krochta, Denaturation time and temperature effects on solubility, tensile properties, and oxygen permeability of whey protein edible films, J. Food Sci. 66 (5) (2001) 705–710. [4] U. Poth, Automotive Coatings Formulation: Chemistry, Physics and Practices, Vincentz Network, 2008. [5] M. Schmid, K. Reichert, F. Hammann, A. Stäbler, Storage time-dependent alteration of molecular interaction–property relationships of whey protein isolate-based films and coatings, J. Mater. Sci. 50 (12) (2015) 4396–4404. [6] J. Oses, I. Fernandez-Pan, M. Mendoza, J.I. Mate, Stability of the mechanical properties of edible films based on whey protein isolate during storage at different relative humidity, Food Hydrocolloids 23 (1) (2009) 125–131. [7] M. Anker, M. Stading, A.-M. Hermansson, Relationship between the microstructure and the mechanical and barrier properties of whey protein films, J. Agric. Food Chem. 48 (9) (2000) 3806–3816. [8] M. Schmid, K. Dallmann, E. Bugnicourt, D. Cordoni, F. Wild, A. Lazzeri, K. Noller, Properties of whey-protein-coated films and laminates as novel recyclable food packaging materials with excellent barrier properties, Int. J. Polym. Sci. 2012 (2012). [9] M. Schmid, S. Pröls, D. Kainz, F. Hammann, U. Grupa, Effect of thermally induced denaturation on molecular interaction-response relationships of whey protein isolate based films and coatings, Prog. Org. Coat. 104 (3) (2017) 161– 172. [10] M. Schmid, S. Pröls, D.M. Kainz, F. Hammann, A. Stäbler, Impact of hydrolyzed whey protein on the molecular interactions and cross-linking density in whey protein isolate-based films, Int. J. Polym. Sci. 2016 (2016). [11] Z. Ding, J. Aklonis, R. Salovey, Model filled polymers: VI. Determination of the crosslink density of polymeric beads by swelling, J. Polym. Sci. B: Polym. Phys. 29 (8) (1991) 1035–1038. [12] K. Ahmed, S.S. Nizami, N.Z. Raza, K. Mahmood, Characterization of mechanical properties of marble sludge/natural rubber composites, J. Chem. Soc. Pak. 34 (6) (2012). [13] A. Marzocca, Evaluation of the polymer–solvent interaction parameter v for the system cured styrene butadiene rubber and toluene, Eur. Polym. J. 43 (6) (2007) 2682–2689. [14] L.K. Wang, Y.-T. Hung, N.K. Shammas, Advanced Physicochemical Treatment Technologies, Springer, 2007. [15] K. Kabiri, H. Omidian, S. Hashemi, M. Zohuriaan-Mehr, Synthesis of fastswelling superabsorbent hydrogels: effect of crosslinker type and concentration on porosity and absorption rate, Eur. Polym. J. 39 (7) (2003) 1341–1348. [16] K. Kajiwara, Y. Osada, Gels Handbook, Four-Volume Set, Elsevier Science, 2000. [17] M. Jenkins, Biomedical Polymers, Elsevier, 2007. [18] O. Güven, Crosslinking and Scission in Polymers, Springer, The Netherlands, 2012.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Time-dependent crosslinking of whey protein based films during storage Markus Schmid a,b,⇑, Sarah Merzbacher b, Kerstin Müller b a b
Technical University of Munich, TUM School of Life Sciences Weihenstephan, Chair of Food Packaging Technology, Weihenstephaner Steig 22, 85354 Freising, Germany Fraunhofer Institute for Process Engineering and Packaging IVV, Giggenhauser Straße 35, 85354 Freising, Germany
a r t i c l e
i n f o
Article history: Received 10 June 2017 Received in revised form 26 November 2017 Accepted 10 December 2017 Available online 11 December 2017 Keywords: Whey protein isolate Whey protein films Post-crosslinking Storage time Degree of crosslinking Degree of swelling
a b s t r a c t Environmentally-friendly packaging based on whey protein offers the potential to replace petroleumbased oxygen barrier materials. Effective crosslinking of the whey protein is a prerequisite for this application. To quantify the effect of storage time on the post-crosslinking of whey protein isolate based films, swelling tests were conducted in order to determine the degree of swelling (DoS). In addition, the degree of crosslinking (DoC) and the crosslinking density (CLD) were calculated using existing models with new data from water vapour sorption isotherm measurements. The results showed that increasing storage time led to decreased swelling of whey protein isolate based films (p < .05) and increased DoC and CLD (p < .05). Comparison of the films on day 1 and day 21 showed that the crosslinking increased by a factor of 3 while the DoS reduced by almost half. The results of this study demonstrate that the storage time of whey protein isolated based films is a crucial factor for crosslinking. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Petroleum-based packaging materials are increasingly falling out of favour due to their unsustainable production and the environmental burden of plastic waste [1]. The packaging industry and R&D organisations are working on renewable and biodegradable alternatives to replace commercial plastics. Protein-based films offer substantial potential because they are more environmentally-friendly than fossil-based polymeric materials. A prerequisite for the use of whey protein films as a packaging material is heat-induced formation of a protein network, namely crosslinking [2–4]. Crosslinking leads to improved barrier and tensile properties. The crosslinking involves interactions such as ionic crosslinks between carboxyl and amino groups, hydrogen bonding and disulphide bridges [2,3]. Relatively little work has been carried out on the changes in the techno-functional properties of whey proteinbased films that occur during the storage time [5,6]. It is assumed that these changes are caused among other things by postdenaturation crosslinking of the proteins [7]. Thus, the crosslinking behaviour of whey protein based films during storage is a crucial factor for their mechanical and barrier performance, especially when used for packaging materials. The aim of this study was to quantify changes to the CLD and DoC of whey protein based films ⇑ Corresponding author at: Fraunhofer Institute for Process Engineering and Packaging IVV, Giggenhauser Straße 35, 85354 Freising, Germany. E-mail address: [email protected] (M. Schmid). https://doi.org/10.1016/j.matlet.2017.12.047 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
during their storage time. This study uses a direct approach for the quantification of crosslinking in whey protein based films and evaluation of the post crosslinking behaviour during storage. Accordingly, established crosslinking parameters were determined using existing models with data from swelling tests and water vapour sorption isotherms measurements. The findings will be invaluable for researchers and material developers, providing for the first time quantitative data about storage-dependent post-crosslinking in whey protein isolate based films. This information will enable future experiments and especially the characterisation procedures to be conducted more reliably.
2. Materials and methods 2.1. Materials Whey protein isolate (WPI, BiPro) was obtained from Davisco Foods International Inc. (Le Sueur, Minnesota, USA). Anhydrous glycerol was purchased from Merck KGaA (Darmstadt, Germany).
2.2. Film preparation A glycerol plasticised and denatured whey protein standard solution (WPSS) containing 10% (w/w) WPI was prepared according to the procedure developed by Schmid et al. [8]. Films were cast in petri dishes with a target thickness of 100 ± 10 mm and dried under ambient conditions in a climate chamber at 23 °C
M. Schmid et al. / Materials Letters 215 (2018) 8–10
and 50% relative humidity (RH). The films reached their equilibrium moisture content after approx. 24 h.
2.3. Swelling tests Swelling tests were used to determine the DoS, CLD and DoC of the whey protein isolate-based cast films as described by Schmid et al. [9,10]. The DoS [%] was calculated using a gravimetric method. Fivefold determination was carried out using square samples of 50 mm 50 mm size. Further information about the crosslinking of whey protein based films was obtained by calculating the DoC [%] and CLD [mol g-1]. The DoC is defined as the quotient of the molecular weight of the mean amino acid (123.3 g mol 1 for WPI according to [10]) and the number average molecular weight of the polymer between the crosslinks [11]. The latter was determined using the Flory-Rehner equation according to the implementation by Schmid et al. [9,10]. The CLD is the reciprocal number average molecular weight of the polymer between the crosslinks [12,13].
2.4. Water vapour sorption isotherm measurements For calculation of the average molecular weight of the polymer between the crosslinks, the Flory-Huggins interaction parameter v is required. This was determined by measuring the water sorption isotherm. These measurements and the data evaluation were carried out following the approach of Schmid et al. [9,10] with a deviating sample diameter of 25 mm and a maximum measuring time of 3000 min. Triple determination was performed on each cast film.
1000
DoS [%]
900 800
700
ab
600 500
2.5. Statistical analysis The results were analysed with the statistical software Visual XSel (CRGRAPH GbR, Starnberg, Germany). All data were statistically evaluated for normal distribution according to the AndersonDarling (sample size 5) or Kolmogorov-Smirnov test (sample size 4). Multiple t-tests were performed in order to determine whether the mean values differed significantly from each other (p < .05). 3. Results and discussion 3.1. Degree of swelling It was assumed that the crosslinking increases due to the fact that the whey protein films post-crosslink during the storage time [5]. A consequence of the increased network formation is a decrease in the swelling capacity as less water can accumulate [14,15]. Fig. 1 shows the DoS as a function of the storage time of the cast films. A clear decline in DoS with storage time was observed with significant differences (p < .05) between day 1 and days 7, 10, 14 and 21. The hypothesis that DoS decreases with storage time due to increasing crosslinking is confirmed not only by the results of this work but also by other results in the literature. The DoS is strongly dependent on the DoC. The greater the crosslinking of the polymer matrix, the lower the DoS [14] and the lower the amount of bounded water in the cast films [16]. The following figure illustrates the relationship between the crosslinking and the swelling capacity (Fig 2.). The polymer chains are slightly crosslinked at the start of the storage period. Increasing crosslinking takes place during the storage period, leading to a decrease in water absorption. Crosslinked proteins can absorb less water and lower the swelling capacity [17]. In addition, the mobility of the chain segments decreases [18]. 3.2. Degree of crosslinking and crosslinking density
ab
a
9
b
b b
c
400 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 storage time [d] Fig. 1. Degree of swelling as a function of the storage time. Significant differences (p < .05) are indicated by different letters.
Water vapour sorption measurements were used to demonstrate the decrease in swelling capacity due to the increase in crosslinking. There is a significant difference (p < .05) between day 1 and days 10, 14 and 21, demonstrating that the DoC and CLD significantly increase with storage time (see Fig. 3). The results clearly show that the crosslinking increases with storage time. DoC and CLD on day 21 are almost a factor of 3 higher than on day 1. This is in compliance with former studies that also indicated a correlation between the increase in DoC and storage time [5]. Their investigations demonstrated that the oxygen permeability decreases with storage period. The reduction in oxygen permeability can be explained by the increase in crosslinking. Sol-
Fig. 2. Schematic representation of reduced swelling as a result of increased crosslinking [17].
10
M. Schmid et al. / Materials Letters 215 (2018) 8–10 3.5
4 c
3
DoC [%]
2.5
2.5
2
b
1.5 a
1
a
ab
b
2 1.5 1
a
0.5
CLD *10^4 [mol/cm^3]
3
References
3.5
0.5
0
0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 storage me [d]
Fig. 3. Degree of crosslinking and crosslinking density as a function of the storage time. Significant differences (p < .05) are indicated by different letters.
ubility tests with different buffer systems, which split specific interactions, give evidence that during the first days of storage disulphide bonds and hydrogen bonds are formed. As a result of those interactions, the oxygen permeability decreases. As the storage time increases, the formation of hydrogen bonds accelerates, which also leads to a reduction in oxygen permeability [5]. These results are also in accordance with the determined DoS, since DoS and CLD are always inversely proportional to each other [17]. 4. Conclusion It was demonstrated that the crosslinking, as quantified by the DoC and CLD, increases with storage time. The increase in DoC and CLD clearly indicates post-crosslinking, in agreement with the previously observed decrease in oxygen permeability reported by Schmid et al. [5]. Based on these findings, a minimum storage time of two weeks should be heeded prior to material characterisation. No substantial changes of material properties were detected thereafter. However, due to ongoing post crosslinking minor changes can still be expected subsequently and should be taken into account for future experiments and characterisation procedures. Conflicts of interest None.
[1] J.V. Schulze, Öffentliches Marketing zur Vermeidung von Kunststoffverpackungen im Lebensmittelbereich, Diplomica Verlag, 2015. [2] M. Pérez-Gago, P. Nadaud, J. Krochta, Water vapor permeability, solubility, and tensile properties of heat-denatured versus native whey protein films, J. Food Sci. 64 (6) (1999) 1034–1037. [3] M. Perez-gago, J. Krochta, Denaturation time and temperature effects on solubility, tensile properties, and oxygen permeability of whey protein edible films, J. Food Sci. 66 (5) (2001) 705–710. [4] U. Poth, Automotive Coatings Formulation: Chemistry, Physics and Practices, Vincentz Network, 2008. [5] M. Schmid, K. Reichert, F. Hammann, A. Stäbler, Storage time-dependent alteration of molecular interaction–property relationships of whey protein isolate-based films and coatings, J. Mater. Sci. 50 (12) (2015) 4396–4404. [6] J. Oses, I. Fernandez-Pan, M. Mendoza, J.I. Mate, Stability of the mechanical properties of edible films based on whey protein isolate during storage at different relative humidity, Food Hydrocolloids 23 (1) (2009) 125–131. [7] M. Anker, M. Stading, A.-M. Hermansson, Relationship between the microstructure and the mechanical and barrier properties of whey protein films, J. Agric. Food Chem. 48 (9) (2000) 3806–3816. [8] M. Schmid, K. Dallmann, E. Bugnicourt, D. Cordoni, F. Wild, A. Lazzeri, K. Noller, Properties of whey-protein-coated films and laminates as novel recyclable food packaging materials with excellent barrier properties, Int. J. Polym. Sci. 2012 (2012). [9] M. Schmid, S. Pröls, D. Kainz, F. Hammann, U. Grupa, Effect of thermally induced denaturation on molecular interaction-response relationships of whey protein isolate based films and coatings, Prog. Org. Coat. 104 (3) (2017) 161– 172. [10] M. Schmid, S. Pröls, D.M. Kainz, F. Hammann, A. Stäbler, Impact of hydrolyzed whey protein on the molecular interactions and cross-linking density in whey protein isolate-based films, Int. J. Polym. Sci. 2016 (2016). [11] Z. Ding, J. Aklonis, R. Salovey, Model filled polymers: VI. Determination of the crosslink density of polymeric beads by swelling, J. Polym. Sci. B: Polym. Phys. 29 (8) (1991) 1035–1038. [12] K. Ahmed, S.S. Nizami, N.Z. Raza, K. Mahmood, Characterization of mechanical properties of marble sludge/natural rubber composites, J. Chem. Soc. Pak. 34 (6) (2012). [13] A. Marzocca, Evaluation of the polymer–solvent interaction parameter v for the system cured styrene butadiene rubber and toluene, Eur. Polym. J. 43 (6) (2007) 2682–2689. [14] L.K. Wang, Y.-T. Hung, N.K. Shammas, Advanced Physicochemical Treatment Technologies, Springer, 2007. [15] K. Kabiri, H. Omidian, S. Hashemi, M. Zohuriaan-Mehr, Synthesis of fastswelling superabsorbent hydrogels: effect of crosslinker type and concentration on porosity and absorption rate, Eur. Polym. J. 39 (7) (2003) 1341–1348. [16] K. Kajiwara, Y. Osada, Gels Handbook, Four-Volume Set, Elsevier Science, 2000. [17] M. Jenkins, Biomedical Polymers, Elsevier, 2007. [18] O. Güven, Crosslinking and Scission in Polymers, Springer, The Netherlands, 2012.