European Polymer Journal 54 (2014) 172–180

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European Polymer Journal

journal homepage: www.elsevier .com/locate /europol j

Synthesis and properties of canola protein-basedsuperabsorbent hydrogels

http://dx.doi.org/10.1016/j.eurpolymj.2014.03.0070014-3057/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +1 514 398 7776; fax: +1 514 398 8387.E-mail address: marie-josee.dumont@mcgill.ca (M.-J. Dumont).

Weida Shi a, Marie-Josée Dumont a,⇑, Elhadji Babacar Ly b

a Bioresource Engineering Department, McGill University, 21111 Lakeshore Rd., Ste-Anne-de-Bellevue, QC H9X 3V9, Canadab Université Gaston Berger, BP. 234 St-Louis, Senegal

a r t i c l e i n f o

Article history:Received 23 December 2013Received in revised form 4 March 2014Accepted 5 March 2014Available online 16 March 2014

Keywords:HydrogelsCanola proteinsStructural propertiesSynthesis

a b s t r a c t

The present work reports, for the first time, the synthesis and characterization of canola pro-tein-based hydrogels. These hydrogels were synthesized by solution based graft copolymeri-zation of acrylic acid monomers on the canola protein backbones in the presence of acrosslinker (N,N0-methylenebis (acrylymide)) and initiators (sodium bisulfite and potassiumpersulfate). The grafting was confirmed by means of Fourier transform infrared spectroscopy.The contributions of the crosslinker, initiator and neutralization degree to the hydrogel wereinvestigated by applying differential scanning calorimetry, thermogravimetric analysis, swell-ing test, scanning electron microscope. The macromolecules exhibited extraordinary waterabsorbency capacity in distilled water. The highest equilibrium swelling of hydrogel in dis-tilled water reached 448 g/g of hydrogel in 48 h. The swelling properties of the optimizedhydrogel were also studied at various pH and saline concentrations. The hydrogels respondedspontaneously to these changes, which may confer them the title of smart materials.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Hydrogels are highly cross-linked macromoleculeswhich can undergo a change in volume (swelling/shrinkage)based on changes in environmental conditions, such as tem-perature, pH, ionic strength and the nature of the solvent.These spontaneous physical changes in response to changesin the environment conferred them the title of smart mate-rials [1,2]. Superabsorbent polymer (SAP) hydrogel is onetype of hydrogel that can absorb and retain a large amountof water or biological fluids in their polymeric structures[3]. Due to this characteristic, SAP materials are widely usedin various areas, for example in hospital products, for waterretention in agricultural and horticultural soils, in inconti-nence products, disposable diapers, and feminine hygieneproducts, for liquid radioactive waste treatment, in foodpackaging, biomedical products, and so forth [3–6].

Generally, SAPs are divided into synthetic and natural-based polymers. Synthetic hydrogels are usually made frompetroleum-based hydrocarbons such as poly(hydroxyalkylmethacrylates), polyacrylate, polyacrylamide, and poly-methacrylamide and its derivatives poly(N-vinyl-2-pyroli-done) and polyvinyl alcohol [7]. Even though synthetichydrogels exhibit several advantages such as large waterabsorption capacities, and reasonable gel strength and cost;the broader use of synthetic hydrogel is limited by theirtoxic character and poor biodegradability. Therefore, natu-ral-based SAPs have gained significant attention becauseof their nontoxicity, biocompatibility, and biodegradability[8]. This resulted in an increased number of studies report-ing the synthesis of bio-based hydrogels synthesized fromcellulose, starch, gelatin, chitosan, carrageenan, pectin, andproteins among others [9–12].

Of these natural polymers, proteins may be the mostunder-rated and underutilized feedstocks with respect totheir industrial applications. Although proteins have beenstudied as starting material for the manufacture of films

W. Shi et al. / European Polymer Journal 54 (2014) 172–180 173

and composites [13–16], their potential as structural ele-ments is still not well recognized. Proteins are character-ized by numerous reactive groups, which can be used assites for chemical modifications and cross-linking to devel-op polymeric structures [7]. An efficient way to obtain pro-tein-based SAP hydrogels is through graft polymerizationof vinylic monomers onto their backbones in the presenceof crosslinkers within different initiator systems [17].Addition of vinylic monomers enhances the hydrophiliccharacter of proteins, which consequently improves thewater absorption capacity of the resulting protein-basedSAP hydrogels. To the best of the knowledge of the authors,collagen and cottonseed proteins are the only proteinswhich have been investigated for the synthesis of SAPhydrogels by the graft polymerization technique [17–22].

Canola is the third most widely grown commercialgenetically modified crop after soybean and maize [23].Canola proteins are extracted from canola meal, one ofthe by-products of the vegetable oil refining industry.These proteins are mostly utilized for low-value animalfeed. Our research group saw an interesting potential in ca-nola proteins since their amino acid composition is similarto soy proteins, which are utilized for many applicationswhere polymers are involved. Therefore, in our previousstudy, we synthesized canola protein-based films [24].These films showed water absorption capacities of up to1150 wt.%. It should be noted that the films had not beendesigned for water retention applications. The high hydro-philicity of these canola protein-based films suggested thatcanola proteins could be excellent candidates for the syn-thesis of SAP hydrogels.

In this paper, we report the synthesis and characteriza-tion of canola protein-based SAP hydrogels by the graftpolymerization technique with acrylate monomers, in or-der to broaden the non-food applications of canola protein.The swelling behavior of these hydrogels in different med-ia shows their potential application as smart materials forcontrolled delivery applications, such as slow-release fer-tilizers for the agricultural sector.

2. Materials and methods

2.1. Materials

Hydrolyzed canola protein (HCP) (Vitalexx�) with a pro-tein content of 77% (dry basis) was provided by BioExxSpecialty Proteins Ltd. (Toronto, ON, Canada). Acrylic acid(AA), sodium bisulfite (SBS), potassium persulfate (KPS),N,N0-methylenebis (acrylymide) (NMBA) and anhydrousethanol were of analytical grades and purchased from Sig-ma Aldrich (St. Louis, MO, USA). Sodium chloride andhydrochloric acid (ACS reagent grade) were purchasedfrom Fisher Scientific (Fair Lawn, NJ, USA). Sodium hydrox-ide was purchased from EMD (Damstadt, Germany).

2.2. Preparation of hydrogels

In a typical experiment, hydrolyzed canola protein-polyacrylic acid (HCP-PAA) hydrogel were prepared as follows:0.06 g of NMBA in 5 ml H2O was added to 10 g of partially

neutralized (70 mol%) AA. Hydrolyzed canola proteins (3 g)were dissolved in 25 ml of distilled water at 70 �C in a ther-mostat water bath with agitation using a magnetic stirrerfor 5 min. Thereafter, initiators (2 g KPS and 1 g SBS) wereadded to the protein solution. After stirring for 10 min, theprotein solution was mixed with the prepared AA andNMBA solution. The mixture was incubated in a water bathat 70 �C for 60 min for completion of the reaction. Theresulting gel was immersed in an excess of non-solventethanol (200 ml) to dewater it. After 3 h, the ethanol wasdecanted. The gel was cut into small pieces and re-im-mersed with 100 ml fresh ethanol for 24 h. Afterwards,the gel was filtered and dried in an oven at 70 �C for 24 hafter which, the dried gel pieces were treated with liquidnitrogen before being ground into powder. Finally, thepowdered hydrogel was stored away from moisture, heatand light. To study the effects of the crosslinker, initiatorsand the neutralization degree on the swelling properties,hydrogels with different compositions were synthesizedas described in Table 1.

2.3. Infrared analysis (FT-IR)

The FT-IR spectra of the HCP, hydrogels and additiveswere conducted in triplicate on a Nicolet iS5 FT-IR spec-trometer (Thermo, Madison, WI, USA). The spectra were re-corded at 32 scans and 4 cm�1 resolution in the 4000–400 cm�1 range. The spectra were analyzed using the OM-NIC software package (version 8.2, Thermo Nicolet Corp).

2.4. Differential scanning calorimetry (DSC)

Around 10 mg of HCP and hydrogels were compressedin hermetic aluminum pans and scanned in duplicate usinga DSC (Q100, TA Instruments, Inc., New Castle, DE, USA)under a steam of nitrogen (50 ml/min). Samples wereheated from 0 �C to 200 �C at a rate of 10 �C/min to removethe thermal history. Samples were then cooled to 0 �C at arate of 10 �C/min. The samples were then reheated from0 �C to 350 �C at a rate of 10 �C/min.

2.5. Thermogravimetric analyzer (TGA)

A thermogravimetric analyzer (TGA) (Q500, TA Instru-ment, Inc., New Castle, DE, USA) was used for analyzingthe thermal properties of the HCP powder and HCP basedhydrogel. The thermogravimetric analyses were carriedout under a stream of nitrogen at a flow rate of 60 ml/min. Samples weighing between 5 and 10 mg was heatedfrom room temperature to 1000 �C at a constant rate of20 �C/ min. The HCP powder and selected hydrogel sam-ples were tested in duplicate.

2.6. Swelling measurement

The water uptake of hydrogel was determined as fol-lows: In the first step, No. 2 coffee filter cone (Loblaws, Tor-onto, ON, Canada) was weighed and recorded as theoriginal bag weight (M1). The bag was then immersed indistilled water for 2 h and hung in the air for 20 min to re-move the excess water. The bag was weighed again and the

Table 1Composition of HCP hydrogel.

Sample HCP AA Neutralization degree% NMBA KPS SBS

wt. (g) wt.% wt. (g) wt.% wt. (g) wt.% wt. (g) wt.% wt. (g) wt.%

HCP-PAA-1 3 18.73 10 62.44 70 0.015 0.093 2.00 12.49 1.00 8.01HCP-PAA-2 3 18.71 10 62.38 70 0.03 0.187 2.00 12.48 1.00 8.02HCP-PAA-3 3 18.68 10 62.27 70 0.06 0.374 2.00 12.45 1.00 8.03HCP-PAA-4 3 18.61 10 62.03 70 0.12 0.744 2.00 12.41 1.00 8.06HCP-PAA-5 3 16.61 10 55.37 70 0.06 0.332 3.33 18.46 1.67 9.03HCP-PAA-6 3 14.96 10 49.85 70 0.06 0.299 4.67 23.26 2.33 10.03HCP-PAA-7 3 18.68 10 62.27 65 0.06 0.374 2.00 12.45 1.00 8.03HCP-PAA-8 3 18.68 10 62.27 75 0.06 0.374 2.00 12.45 1.00 8.03HCP-PAA-9 3 18.68 10 62.27 80 0.06 0.374 2.00 12.45 1.00 8.03

174 W. Shi et al. / European Polymer Journal 54 (2014) 172–180

weight was recorded as the weight of bag after immersion(M2). After drying the bag in an oven at 60 �C for 24 h, thedried bag was weighed again and recorded as weight ofdried bag (M3). Around 0.2–0.5 g of hydrogel sample wasloaded into the dried bag and the weight of the samplewas recorded (M4). The loaded bag was then immersed in500 ml of distilled water. After swelling at room tempera-ture for pre-determined time intervals (2 h, 4 h, 6 h, 8 h,12 h, 24 h, 48 h), the bags were hung in the air for20 min to remove the excess solution. The total weight ofthe bags and samples after swelling was recorded (M5).The weight of water (WW) absorbed by the hydrogel sam-ple was calculated by the following equation (Eq. (1)):

WW ¼ M5 �M4 �M3 � ðM2 �M1Þ ð1Þ

The equilibrium swelling (ES) of hydrogel was calcu-lated using the following equation (Eq. (2)):

ESðg=gÞ ¼WW=M4 ð2Þ

Each hydrogel samples were tested using five replicates.The standard deviation was calculated and recorded.

The swelling property of the hydrogel samples was alsoevaluated for different concentrations of NaCl in triplicateaccording to the method described above. Finally, theswelling properties of the hydrogel samples were studiedin various pH solutions. Individual solutions with pH of1, 3, 5, 7, 9, and 11 were prepared by using HCl (10 M)and NaOH (1 M) solutions. The pH value of each solutionwas precisely adjusted using a Symphony SB70P pH meter(VWR, Wayne, NJ, US).

2.7. Scanning electron microscopy

The morphological properties of HCP based hydrogelswere examined using a JEOL JSM-7600 TFE scanning elec-tronic microscope (SEM) (JEOL Ltd.; Tokyo, Japan). Theexamination was carried out at magnifications of 5000Xand 10,000X at an accelerated voltage of 2 kV. The surfaceof the particle was coated with gold using a Polaron SC-502sputter coater (Fison, Ashford, UK) under an argon atmo-sphere. The coating process was conducted at a plasmacurrent of 5 mA for 30 s so as to have a coating thicknessof less than 10 nm.

3. Results and discussion

3.1. Synthesis and mechanism

The SAP hydrogels were synthesized by graft copoly-merization of acrylic acid onto the hydrolyzed canola pro-tein backbone in the presence of NMBA, a crosslinker. Theproposed mechanism for the graft copolymerization isshown in Scheme 1 [19]. SBS and KPS were used as initia-tors, which decomposed under heating to generate sulfateanion-radicals. The hydrogen from the functional groups ofthe protein backbone was attracted to the anion radical toform macroradicals. Thereafter, the macroradicals initiatedpolymerization of the acrylic acid, creating the graftcopolymer [17,18]. Due to the presence of crosslinker, thecopolymer formed a three dimensional cross-linked struc-ture, which rendered the copolymer stable in solution andable to hold large quantities of water. This will bediscussed in section 3.4.

3.2. FT-IR

Grafting was confirmed by comparing the FT-IR spec-trum of the hydrolyzed canola proteins with that of thehydrogel, which are shown in Fig. 1. From the HCP spec-tra, the bands observed at 1582 cm�1, 1393 cm�1 and1112 cm�1 were related to the C@O stretching (amide I),NAH bending (amide II) and CAN stretching (amide III)of the protein backbone respectively. The bands at3269 cm�1, 3077 cm�1 and 2960 cm�1 were assigned tothe stretching of ANH, AOH and ACH groups of canolaproteins respectively. The hydrogels comprised of proteinbackbones linked by amide bonds, which was evidencedby a new characteristic absorption band at 3367 cm�1.Meanwhile, the dissipation of ANH and AOH stretchingbands was also observed when comparing the spectraof the hydrogels to the canola proteins spectra. The in-crease in the CAN stretching at 1105 cm�1 suggests thatthe carboxylic group of canola proteins had been re-placed with amide bonds linked to the acrylic acidmonomers. A comparison of the FT-IR spectra of HCPand the HCP-PAA hydrogel indicated the grafting copoly-merization of acrylic acid monomers onto the canolaprotein backbone.

Scheme 1. Proposed mechanism of the grafting copolymerization.

Fig. 1. FT-IR spectra of HCP and HCP-PAA hydrogel.

W. Shi et al. / European Polymer Journal 54 (2014) 172–180 175

3.3. Thermal properties

Typical DSC thermograms of HCP and HCP-PAA hydro-gel are presented in Fig. 2A. The dried HCP powder exhib-

ited two denaturation transitions at 109 �C and 130 �C,which corresponded to the napin and cruciferin proteinfractions of canola [24]. Meanwhile, two endothermic tran-sitions were also observed in certain hydrogel samples

Fig. 2. Thermal properties of HCP and HCP-PAA hydrogel (A) DSC thermogram and (B) TGA curve of HCP (left) and HCP-PAA hydrogel (right).

Table 2The transition temperatures of hydrogels.

Wt.% orneutralizationdegree

Maximumdenaturationtemperature tofirstdenaturationtransition (�C)

Maximumdenaturationtemperatureto seconddenaturation(�C)

NMBA to AAratio (wt.%)

0.15 97 2950.30 101 3010.60 118 3251.19 148 333

Initiators to AAratio (wt.%)

23 118 32533.3 156 31441.2 159 290

Neutralizationdegree (%)

65 N/A 29570 118 32775 N/A 33280 N/A 241

176 W. Shi et al. / European Polymer Journal 54 (2014) 172–180

(Table 2). From Table 2, an increase in NMBA concentrationwas associated with an increase in the denaturation tem-perature of both protein fractions. However, no concentra-tion dependence was observed for hydrogels formulatedwith different concentrations of initiators. For hydrogels

formulated with various neutralized AA, no denaturationwas observed in the spectrum except for the one formu-lated with 70 mol% acrylic acid, which only showed onedenaturation transition.

The increase in the denaturation temperature with anincrease in the concentration of NMBA suggested the for-mation of more thermally stable hydrogels. This increasein the network thermal stability can result from an in-creased molecular weight and rigidity. With an increasein the initiator concentration, a shift was observed forthe first denaturation transition to a higher temperature,indicating that napin was more prone to react with the ini-tiators. The higher accessibility of napin was attributed toits small molecular size [25]. Various degrees of neutraliza-tion of acrylic acid did not have significant effects on thetwo denaturation transitions. This can be explained bythe role of the neutralization step which is to change thecharge density of the polymeric chains. This had no influ-ence on the internal structure of the hydrogel.

The TGA thermograms of HCP and HCP-PAA hydrogelare shown in Fig. 2B. HCP presented three degradationstages has shown by the derivative weight curve. The firstdegradation stage (from 20 �C to 130 �C) is attributed tothe loss of free and bound water. The second stage (130–680 �C) showed significant weight loss (73.6%) which

Fig. 3. Comparison of the original hydrogel powder (left) and the formed

W. Shi et al. / European Polymer Journal 54 (2014) 172–180 177

resulted from the degradation of various canola proteinfractions (mainly napin and cruciferin). The degradationafter 680 �C is caused by the degradation of the proteins’backbone.

The TGA curve of HCP-PAA hydrogels showed that thewater loss stage of the degradation curve is less significantthan for HCP. For the second degradation stage (from156 �C to 550 �C), four degradation fractions were ob-served, which was consistent with the HCP degradationcurve. However, there is a shift towards higher tempera-tures for the degradation of the protein fractions. This isdue to an increase in the thermal stability due to graftingcopolymerization. A major degradation stage was observedat around 740 �C which can be explained by the degrada-tion of thermally stable crosslinked structures in thehydrogels.

gel after swelling (right).

3.4. Swelling properties

3.4.1. Effects of NMBA concentration on swellingNMBA was used as a crosslinker in the formation of

hydrogels. The effects of the concentration of NMBA onthe water absorbency of the resulting hydrogels wereinvestigated (Table 3). The sample HCP-PAA-1 formulatedwith 0.09 wt.% of NMBA did not absorb water, but dis-solved within the first 2 h. The dissolution of hydrogelswith low concentration of crosslinkers probably resultedfrom a weak crosslinked network. At low NMBA concentra-tions, the hydrogels were only partially formed and there-fore had a weak dimensional stability [22]. The hydrogelsamples started to absorb water as the concentration ofNMBA increased to 0.18 wt.%, and the extent of swellingfurther expanded as the NMBA concentration increasedto 0.37 wt.%. A comparison of the original hydrogel powderand the gel formed after swelling is shown in Fig. 3. The in-crease in the concentration of NMBA resulted in an in-crease in the crosslinking density and a more stablethree-dimensional structure. As the hydrogel structure in-creased in stability, the network had higher water holdingcapacity. When the concentration was further increasedfrom 0.37 wt.% to 0.74 wt.%, more crosslinking points werecreated on the polymer chain, which resulted in an in-creased crosslinking density and a decrease in the spacebetween chains of the copolymer network [22]. Thus, thewater absorbency of the hydrogel decreased drastically.

3.4.2. Effects of initiator content on swellingThe effects of the initiator content on water absorbency

of hydrogels are presented in Table 4. Generally, the 24 h.and the 48 h. swelling capacities of the hydrogel decreasedas the initiator content increased. With increasing concen-

Table 3Effects of NMBA content on the absorbency of hydrogel.

NMBA (wt.%) 2 h (g/g) 4 h (g/g) 6 h (g/g) 8 h

0.09 Dissolved0.18 51.53 ± 25.11 73.83 ± 18.95 91.2 ± 21.48 1080.37 36.49 ± 11.64 67.53 ± 11.39 96.43 ± 8.95 1250.74 63.6 ± 9.79 75.99 ± 12.31 83.57 ± 9.47 91

trations of initiator, the copolymerization process bene-fited from the formation of more free radicals, which ledto a high crosslinking density [21]. Moreover, the free rad-ical degradation of canola protein backbones by sulfate an-ions might have contributed to swelling loss [26]. Althoughthe water absorbency decreased, the swelling rate in-creased in the first 6 h with the increase in initiator con-centration as shown (Table 4). The hydrogel formulatedwith 27.69 wt.% of initiator had the highest swelling rateuntil swelling for 12 h.

3.4.3. Effects of neutralization degree of acrylic acid onswelling

The role of neutralization of acrylic acid was to changethe crosslinking density of the hydrogel polymer network[22]. The effects of neutralization degree of acrylic acidon the water absorbency of hydrogels are shown in Table 5.The swelling ability of the hydrogel network was attrib-uted to the electrostatic repulsion derived from the nega-tively charged carboxyl groups attached to the polymerchains [21,22]. Once the acrylic acid was neutralized withsodium hydroxide, the carboxylic acid group changed intoa carboxylate group, resulting in an increase in the electro-static repulsion. Thus, the hydrogel network expanded andcould hold more water. Therefore, the swelling extent in-creased as the neutralization of acrylic acid increased from65% to 70%.

However, swelling losses were observed when the neu-tralization degree increased from 70% to 75% as shown inTable 5. High neutralization degree of acrylic acid resultedin generation of more sodium ions, which reduced electro-static repulsion and the osmotic pressure by screening thenegative charges of the carboxyl groups, causing the

(g/g) 12 h (g/g) 24 h (g/g) 48 h (g/g)

.45 ± 19.43 131.25 ± 17.25 256.45 ± 19.96 359.16 ± 25.66

.83 ± 10.21 172.04 ± 14.34 282.35 ± 14.74 448.23 ± 35.73

.62 ± 9.66 113 ± 12.55 143.55 ± 18.73 207.07 ± 14.84

Table 4Effects of initiator content on the absorbency of hydrogel.

Initiator (wt.%) 2 h (g/g) 4 h (g/g) 6 h (g/g) 8 h (g/g) 12 h (g/g) 24 h (g/g) 48 h (g/g)

18.68 36.49 ± 11.64 67.53 ± 11.39 96.43 ± 8.95 125.83 ± 10.21 172.04 ± 14.34 282.35 ± 14.74 448.23 ± 35.7327.69 74.79 ± 16.93 112.04 ± 27.26 135.59 ± 31.45 155.23 ± 30.36 180.88 ± 30.18 215.84 ± 23.12 243.35 ± 11.4934.9 59.5 ± 7.73 82.03 ± 19.44 104.88 ± 23.69 120.2 ± 27.17 143.36 ± 30.82 175.11 ± 25.91 200.92 ± 16.16

Table 5Effects of ND on the absorbency of hydrogel.

ND (%) 2 h (g/g) 4 h (g/g) 6 h (g/g) 8 h (g/g) 12 h (g/g) 24 h (g/g) 48 h (g/g)

65 46.25 ± 11.33 91.28 ± 9.09 123.12 ± 10.36 151.42 ± 17.24 181.45 ± 22.92 239.77 ± 32.34 354.06 ± 43.8670 36.49 ± 11.64 67.53 ± 11.39 96.43 ± 8.95 125.83 ± 10.21 172.04 ± 14.34 282.35 ± 14.74 448.23 ± 35.7375 43.24 ± 14.92 67.09 ± 18.06 87.38 ± 19.34 107.38 ± 22.73 131.93 ± 25.98 208.37 ± 28.61 347.71 ± 25.7180 Dissolved

Fig. 4. Swelling properties. (A) Effect of NaCl concentration on water absorbency and (B) effect of pH on water absorbency.

178 W. Shi et al. / European Polymer Journal 54 (2014) 172–180

reduction in water absorbency in the resulting hydrogel[22,27,28]. Further increase in neutralization degree(above 75%) caused the dissolution of the resultinghydrogels.

3.4.4. Swelling in various NaCl concentrationHydrogel sample HCP-PAA-3 had the highest absor-

bency in distilled water as previously discussed. This opti-

mized hydrogel was then tested in solutions with differentconcentrations of NaCl. The effects of NaCl concentrationon the water absorbency of the hydrogel at 24 h. and48 h. are presented in Fig. 4A. As shown in Fig. 4A, thewater uptake of hydrogels was sensitive to the ionicstrength since the water absorbency decreased signifi-cantly in the salt solution as compared to the absorbencyin distilled water. Also, the absorbency gradually decreased

Fig. 5. SEM images of HCP and HCP-PAA hydrogel: (A) HCP 5000X, (B)HCP-PAA 5000X, (C) HCP-PAA hydrogel after swelling 5000X, (D) HCP 10,000X,(E) HCP-PAA 10,000X and (F) HCP-PAA after swelling 10,000X.

W. Shi et al. / European Polymer Journal 54 (2014) 172–180 179

as the concentration of NaCl increased, which was attrib-uted to the decrease in the osmotic pressure difference be-tween the saline solution and the hydrogels. The sodiumion-induced electrostatic screening of the counter-ions inthe external solution on the anionic groups of the hydrogelnetwork caused a reduction in the electrostatic repulsionforce, which may have resulted in the contraction of thehydrogel network and a decrease of the porosity in thehydrogel [21,29].

3.4.5. Effects of pH on swellingTo investigate the effects of pH on the swelling of

hydrogels, stock solutions of NaOH (1 M) and HCl (10 M)were used to adjust pH of solutions. The effects of pH onthe swelling behavior of hydrogels are shown in Fig. 4B.In a very acidic solution (pH 1), the carboxylate anionson the hydrogel chains were highly protonated, resultingin a diminishment of the anion-anion repulsive forces.Therefore, the water absorbency of hydrogels was verylow. However, as the pH increased from 1 to 3, the waterabsorbency increased sharply due to the ionization of thecarboxylate group caused by the enhancement of therepulsive forces [17]. The equilibrium swelling of thehydrogels reached a maximum at pH 7, where the solutionhad the lowest ionic strength. The absorbency decreasedfurther as the solution became basic (pH P 9). The swell-ing loss at higher pH was attributed to the sodium ion-in-duced charge screening effect, which shielded thecarboxylate anions and thus interfered with the electro-static repulsions [22].

3.5. Microstructure

The surface morphology of HCP and HCP-PAA hydrogelsbefore and after swelling were observed by SEM (Fig. 5).The surface morphology of HCP before grafting showed agranular structure, which changed to a dispersed structureafter grafting. Some hollow spots were observed on the

hydrogel samples before swelling. The porous region onthe surface of the hydrogel was probably the site wherethe water permeation occurred and the external stimuliinteracted with the hydrophilic groups of the graft AAmonomers [30]. Besides the porous region, some rod-likestructures were observed in the central part. It is interest-ing to notice that the pores disappeared on the surfaceafter swelling. However, more rod-like structures werepresent on the surface and were evenly distributed, whichcould have resulted from the crystallization of the non-re-acted initiators (KPS and SBS).

4. Conclusion

HCP-PAA superabsorbent hydrogels were synthesizedby graft copolymerization of acrylic acid monomers ontohydrolyzed canola proteins. The thermal stability of theHCP improved in the grafted copolymer. The optimum con-ditions to achieve a maximum water absorbency werefound to be: 18.68 wt.% HCP, 62.27 wt.% 70 mol% neutral-ized AA, 0.37 wt.% NMBA, 12.45 wt.% KPS and 6.23 wt.%SBS respectively. Swelling measurements of the hydrogelswith optimized conditions were tested at various pH andin various NaCl solutions. The hydrogels were highly sensi-tive to the concentration of saline solution, resulting in asignificant reduction in water absorption as concentrationincreased. The maximum and minimum equilibrium swell-ing were achieved at pH 7 and 1, respectively, indicatingthat the HCP-PAA hydrogel has a swelling preference in aneutral or minor basic or acidic environment. HCP-PAAhydrogels show a promising application of hydrolyzed ca-nola protein as absorbent and smart materials.

Acknowledgements

This research was financially supported by theStart-Up Fund of McGill University. We gratefully acknowl-edge the permission of using the laboratory equipment of

180 W. Shi et al. / European Polymer Journal 54 (2014) 172–180

Dr. Valérie Orsat and Dr. Michael Ngadi. We are also grate-ful to Mr. Yvan Gariepy for his expertise and technicalsupport.

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