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Polymer Hydrogels: A Review

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Polymer Hydrogels: A ReviewWaham Ashaier Laftah a , Shahrir Hashim a & Akos N. Ibrahim aa Faculty of Chemical and Natural Resources Engineering, Department of PolymerEngineering, Universiti Technologi, Malaysia

Available online: 30 Sep 2011

To cite this article: Waham Ashaier Laftah, Shahrir Hashim & Akos N. Ibrahim (2011): Polymer Hydrogels: A Review, Polymer-Plastics Technology and Engineering, 50:14, 1475-1486

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Polymer Hydrogels: A Review

Waham Ashaier Laftah, Shahrir Hashim, and Akos N. IbrahimFaculty of Chemical and Natural Resources Engineering, Department of Polymer Engineering,Universiti Technologi, Malaysia

This review encompasses definitions, classification, main proper-ties, and application of polymer hydrogels. Raw materials and prep-aration techniques of polymer hydrogels were described. The factorsthat affect absorption capacity and swelling properties of polymerhydrogels were reviewed. PHG materials are defined as a viscoelas-tic network structure, swellable and not soluble in water with highabsorbent capacity, which may reach 1000 g/g of their dried weightto be termed ‘‘superabsorbent polymer hydrogels’’. PHGs have theability to release absorbed fluids under certain circumstances.

Keywords Absorption capacity; Polymer hydrogels; Superabsor-bent polymer hydrogels; Swelling

INTRODUCTION

Definitions of polymer hydrogels (PHGs) are madeaccording to the main properties and raw materials. PHGsare materials with viscoelastic properties and networkstructure caused by cross-linker and the solvent, respect-ively. Recently, PHG materials have been pervaded in dailylife in different forms depending on their needed appli-cation such as soap, shampoo, toothpaste, hair gel andcontact lenses[1]. There are some advance industrial appli-cations of PHGs such as oil recovery, pharmaceutical, agri-culture, textile, and water treatment. Therefore, gelmaterials have become one of the most popular materialsin our daily life. PHGs are different according to neededapplication such as the gels for body care is not same asthat for agriculture or oil recovery application.

A new class of PHGs were discovered and reported firstby the U.S. Department of Agriculture, which they calledsuperabsorbent polymer (SAP) materials. They arereported it as loosely cross-linked hydrophilic polymersthat can swell, absorb and retain a large volume of wateror other biological fluids[2–5], and also the absorbed fluidis hardly removed under some pressure[6]. In other wordsthis class of materials can be defined as an advance formof PHGs or can be said to be PHGs with high potential

to absorb water or other fluid and swell 1000 times thanof their dried weight. This kind of material can be appliedin areas where high absorption is needed. To enhance theabsorption capacity (AC), biodegradability, reduce priceof PHGs and SAP and make it more suitable for someapplication. PHGs and SAP have been filled with naturaland synthesized filers in the last decade; the product is thencalled a PHG or a SAP composite. Many scientists definedPHG materials based on different aspects.

PHGs are a three-dimensional network, which can beformed by cross-linking polymer chains. The cross-linkingof these polymers are a result of covalent, hydrogen bond-ing, van der Waals forces or physical entanglements[7].Another scientist said that PHGs are three-dimensionalmatrices, viscoelastic solid-like materials which are com-prised of a elastic cross-linked network and solvent as itsmain components. The solid-like gels are a result of entrap-ment and adhesion of the liquid in the large solid 3D sur-face area matrix. The formation of solid matrix is a resultof cross-linking polymer strands (macro) of moleculesdue to physical or chemical forces[1,8–12].

PHGs are also defined as a network of hydrophilic poly-mers[13–15] that can swell in water or biological fluids andhold a large amount of them[7,10,16–18] more than 400 timesits original weight[19], more than 20% of their dry weight[20],up to thousands of times their dry weight[21]. The 3D net-works are insoluble in water because of the presence ofchemical cross-link[8,22,23]. Other researchers defined PHGsare polymeric materials that have three-dimensional net-work structure and can swell considerably in aqueousmedium without dissolution[24–26]. Other workers definedPHGs as insoluble cross-linked polymer network structurescomposed of hydrophilic[27] homo-hetero-co-polymers,which have the ability to absorb significant amounts ofwater[28–31].

PREPARATION OF PHGs

To produce anything in this word as long as the humanbeing has the ability to think and organize. Two mainissues have to be considered: the first is the raw materialof that product and the second is the suitable techniqueto get unique and desirable products. Therefore, the raw

Address correspondence to Waham Ashaier Laftah, Faculty ofChemical and Natural Resources Engineering, Department ofPolymer Engineering, Universiti Technologi, Malaysia. E-mail:waham2007@yahoo.com

Polymer-Plastics Technology and Engineering, 50: 1475–1486, 2011

Copyright # Taylor & Francis Group, LLC

ISSN: 0360-2559 print=1525-6111 online

DOI: 10.1080/03602559.2011.593082

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materials and the techniques those have been used to pre-pare PHGs are briefly discussed next.

Raw Materials of PHGs

The main materials used for synthesis and preparationof PHG are monomers or polymers which can be synthe-sised or natural polymers such as Hydroxyethylmethacrylate (HEMA), Hydroxyethoxyethyl metha-crylate (HEEMA), Hydroxydiethoxyethylmethacrylate(HDEEMA), Methoxyethyl methacrylate (MEMA),Methoxyethoxyethyl methacrylate (MEEMA), Methoxy-diethoxyethyl methacrylate (MDEEMA), Ethylene glycoldimethacrylate (EGDMA), N-vinyl-2-pyrrolidone (NVP),N-isopropyl AAm (NIPAAm), Vinyl acetate (VAc),Acrylic acid (AA), N-(2-hydroxypropyl) methacrylamide(HPMA), Ethylene glycol (EG), PEG acrylate (PEGA),PEG methacrylate (PEGMA), PEG diacrylate (PEGDA),PEG dimethacrylate (PEGDMA)[18], and cross-linkingagent such as epichlorohydrin (ECH)[32], N,N0-Methylene--bis-acrylamide (N,N0-MBAAm)(BIS)[14,33,34] and divinylsulfone (DVS)[35]. In some methods require initiators suchas 2,20-azobis (isobutyronitrile)[36], ammonium persulfate(APS)[6], and potassium peroxidisulfate[37].

Polymerization Techniques of PHGs

Several polymerization techniques have been usedto synthesize and prepare PHGs such as solution poly-merization or aqueous polymer solution[38], radiationpolymerization[39] or photopolymerizatio[23,40], suspensionpolymerization[41], reversible addition-fragmentation chaintransfer (RAFT) polymerization[42] and free radical poly-merization[23,43,44]. According to literature most of PHGswere synthesized using solution polymerization, suspensionpolymerization and radiation polymerization[30,45–48]. Inthese techniques the products of PHG can be formed orreformed into small particles, powder, fibers, membranes,micro beads and even liquids by using some other substan-tial machines. Therefore, these three techniques are themain focus next.

Solution Polymerization. In solution polymerization,the ionic or neutral monomers are mixed with the multi-functional cross-linking agent. The polymerization isinitiated thermally by UV-irradiation or by a redoxinitiator system. The presence of solvent is the majoradvantage of the solution polymerization over the bulkpolymerization method to serve as a heat sink. The pre-pared PHGs have to be washed with distilled water toremove unreacted monomers, oligomers, cross-linkingagent, initiator, soluble and extractable polymers and otherimpurities. Phase separation occurs and the heterogeneousPHGs are formed when the amount of water during poly-merization is more than the water content corresponding to

the equilibrium swelling. This method has been used to pre-pare varieties of PHGs in the last decades[49,50].

Suspension Polymerization. Suspension polymerizationis one of the successful methods used to prepare sphericalPHGs or micro-particles with a size range of 1 mm to1mm. In suspension polymerization, the monomer sol-ution is dispersed in the non-solvent, forming fine mono-mer droplets, which are stabilized by the addition ofstabilizer. The polymerization is initiated by radicals fromthermal decomposition of an initiator.

The newly formed micro-particles are then washed toremove unreacted monomers, cross-linking agent, andinitiator. The shape of particles produced can be affectedby the viscosity of monomer phase, while the size of parti-cles can be controlled by the hydrophilic-lipophilic balance(HLB) of each type of suspending agent[51]. Some PHGsmicro-particles of poly (hydroxyethylmethacrylate) havebeen prepared by this method. Recently, the inverse sus-pension technique has been widely used for preparationof PHGs of polyacrylamide because of its easy removaland management of the hazardous residual acrylamidemonomer from the polymer[52].

Polymerization by Irradiation. Ionizing high energyradiation, such as gamma rays and electron beams, hasbeen used to initiate the polymerization for preparing thePHGs of unsaturated compounds. The irradiation of aque-ous polymer solution results in the formation of radicals onthe polymer chains. Also, radiolysis of water moleculesresults in the formation of hydroxyl radicals, which alsoattack the polymer chains resulting in the formation ofmacro-radicals. Recombination of the macro-radicals ondifferent chains leads to forming of covalent bonds andfinally a cross-linked structure. Examples of polymerscross-linked by this method are poly(vinyl alcohol), poly(ethylene glycol) and poly (acrylic acid). The major advan-tage of the radiation initiation over the chemical initiationis that the production relatively pure and initiator-free[39,53–55].

CLASSIFICATION OF PHG MATERIALS

PHG materials are classified based on different points ofview. From a morphological point of view they are classi-fied into particle, powder, spherical, fibre, membrane,and emulsion. From a material resources point of view,PHG can also be divided into natural macromolecules,semi-synthesized polymer, and synthesized polymers. Fromthe preparation method point of view, they are classified asgraft polymerization, cross-linking polymerization, net-works formation of water-soluble polymer and radiationcross-linking. Also, PHG can be classified according to ori-gin constitution, type of cross-linking and environmentresponse[34].

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Environmental response PHG materials are called‘‘smart materials’’ as a result of their response to environ-mental change. Smart materials of PHGs are classifiedaccording to their sensitivity to temperature, electric, light,sound field, magnetic fields, and pH. This means the net-work structures of PHG are physiologically responsive tothe mentioned factors[18,22,56]. In view of the importanceof thermo-sensitive, pH response and electric smart materi-als of PHGs, they are discussed in detail. Classification ofPHGs depending on a different point of view is illustratedin Figure 1.

Thermo-Sensitive PHGs

This kind of PHGs is defined by its ability to swell andshrink when the temperature changes in the surroundingfluid which means the swelling and deswelling behaviourmostly depend on the surrounding temperature[57]. Thiskind of PHG can be classified into three categories, asfollows.

Negative Temperature-PHGs. This kind of PHGs hascritical parameter called low critical solution temperature(LCST), which means that the PHG will shrink when thetemperature increases above LCST and will showswelling behavior at lower LCST. The LCST is the most

important parameter for negative temperature-sensitivePHG and changes using different ways such as mixing asmall amount of ionic copolymer in the gels or by changingthe solvent composition. In general, the LCST of polymerwith more hydrophobic constituent shift to lower tempera-ture[57,58]. By changing the percentage ratio of hydrophobicto hydrophilic contents of the structure of PHGs, LCSTwill be changed[58]. The structures of some of these poly-mers are shown in Figure 2.

From the structure of negative thermo-sensitive PHGs,it can be seen that the polymers have two parts; the firstis the hydrophilic part -CONH-, and the second is hydro-phobic part -R-[31]. At temperatures lower than LCSTwater or fluid interact with the hydrophilic part by forminghydrogen bonds. As a result of these hydrogen bonds thedissolution and swelling will be improved. As the tempera-ture increases to greater than the LCST, the hydrophobicinteraction among the hydrophobic part will be stronger,while at same time the hydrogen bonds will becomeweaker. Therefore shrinking of sample will occur due tointer- polymer chain association[7], and the absorbed fluidwill go out by de-swelling process.

Positive Temperature PHGs. Positive temperaturePHGs are known by the upper critical solution tempera-ture (UCST)[22]. This means when the temperature is belowUCST the PHG contract and release solvents or fluidsfrom the matrix (de-hydration). At temperatures higherthan UCST swelling takes place. In view of the above itcan be concluded that, these types of PHGs are retrogress-ive at negative temperature. Positive temperature PHGs areshrinking at low temperature because of formation of com-plex structure by hydrogen bonds. The structure dissociatesat high temperature due to breaking of hydrogen bonds,and the gel will swell to the maximum possible extent rap-idly above the UCST. There are a lot of polymers andcopolymers that are positively temperature dependent,such as poly (AAm-co-BMA), and the random copolymergel, poly (AA-co-AAm-co-BMA)[57].

Thermo-Reversible PHGs. These kinds of PHGs havethe same structure and contents as that of negative and

FIG. 1. Classification of PHG materials depended on different point of

view.

FIG. 2. Negatively temperature-sensitive PHGs.

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positive temperature PHGs. The difference with the pre-vious two types of thermo-sensitive PHGs are in their typesof bonds. The polymer chains in this class are not cova-lently cross-linked, and the gel will undergo sol-gel phasetransitions instead of swelling-shrinking transition[57].Sol-gel phase transformation depends on the glucose con-centration in the surrounding medium. Sol-gel reversiblePHGs require glucose response cross-linking. The mostcommonly used thermo-reversible gels are Pluronics, andTetronics compounds as illustrated in Figure 3[7].

pH-Sensitivity

pH-sensitive hydrogels are materials that respond to pHvalues of the surrounding medium and they exhibit swell-ing and de-swelling according to pH of the environment.The swelling behavior occurs due to changes in thehydrophobic-hydrophilic nature of chains or due to hydro-gen bonds and the complexation of inter- and intramolecu-lar or electrostatic repulsion. pH-sensitive PHGs can beclassified into either anionic or cationic, depending on thenature of pendant group in the network[57].

Anionic. This class of PHGs often has carpoxylic orsulfonic acid groups[7]. The more important parameter inanionic hydrogels is the relation between pKa of the poly-mer and pH of surrounding medium. when the pKa ishigher than the pH of surrounding medium, the ionizedstructure will provide increased electrostatic repulsion ofthe net work and enhance the swelling properties[57]. Exam-ples of anionic pH sensitive PHGs are in Figure 4.

Cationic. Cationic PHGs usually have pendant groupsuch as amine[7]. The more important parameter in thesePHGs is the relation between pKb of the polymer andpH of surrounding medium. When the pH of surroundingmedium is lower than pKb, the amine group will changefrom NH2 to NHþ

3 , which subsequently increase the hydro-philicity, electrostatic repulsion and swelling rate[22].

Electric Signal-Sensitivity

This class of PHGs is similar to pH response becausethey are made of polyelectrolytes. Electric sensitive PHGundergo swelling and de-swelling depending on the applied

FIG. 3. Polymer structures of pluronic and tetronic.

FIG. 4. Anionic pH-senstive PHGs.

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electric signal. Sometimes electric sensitive PHG showsswelling on one side and shrinking on the other side. Thisphenomenon occurs when the surface of PHGs in contactwith the electrode resulting in bending of the PHGs. Elec-tric sensitive PHGs has three kinds of transition phases;swelling, shrinking or de-swelling, and bending. Thesephases depend on a number of conditions such as thesystem that has been used to apply electric field (contactsystem or spread system)[57,59].

SWELLING RATIO AND ABSORPTION CAPACITY

Swelling behavior and absorption capacity (AC) ofPHGs are the most important properties that give thePHG materials wide applications. Swelling and absorptionproperties are attributed to the presence of hydrophilicgroups such as -OH-, CONH-, -CONH2-, and -SO3H inthe network[18]. The ratio of the weight of sample at swell-ing and de-swelling behaviours is called swelling ratio(SR)[6,59,60]. There are many factors affecting the SR andAC of PHGs such as chemical structure of the repeatingunit or chemical compositions[4,8], network structure[9], sol-vent concentration, quality of solvent, cross-linking ratio,and the specific stimuli or the surrounding medium.

SR and AC are affected by the ratio between the molesof cross linking agent to moles of polymer repeating units.The higher the cross-linking ratio is the more cross-linkingagent are incorporated in the PHGs structure[22]. Also, SRand AC of PHGs are affected by polymers molecularweight. The higher average molecular weight with lowercross-linking densities exhibited higher swelling rate[10].The chemical structure of polymer repeating unit affectthe SR and AC by affecting the ratio of hydrophilic tohydrophobic groups[61].

PHGs with hydrophilic groups swell to a higher degreethan PHGs with hydrophobic groups. Hydrophobicgroups may collapse in the presence of water. The col-lapsed chains minimize their exposure to the water mol-ecule and lower SR. Also swelling behavior and AC areaffected by surrounding medium, if the PHGs are environ-mentally sensitive. Therefore, the SR can change withchange in temperature of the swelling medium forthermo-sensitive PHGs. The change in ionic strength andpH of surrounding medium result in SR and AC changefor ionic and pH sensitive PHGs[22,57]. The swelling processof PHGs can be explained as follows:

The solvent tries to penetrate the polymer networks toproduce 3D-molecular network at the same time expand-ing the molecule chain between the cross-linked points,thus decreasing the configuration enthalpy value. Themolecule network has an elastic contractive force that triesto make the networks contract. When these opposingforces reach equilibrium, the expansion and contractionalso reach a balance. In this process, the osmotic pressureis the driving force for the expansion of swelling, and the

network elastic force is the driving force of the contractionof the gel.

There are many ways to measure AC; a standard is yetto be established. Usually, AC is measured using a volu-metric method, a gravimetric method, a spectroscopicmethod, and a microwave method. The volumetric methodmeasures the volume changes of PHG sample or absorbedfluid before and after the absorption. The gravimetricmethod is dependent on measuring the weight changes ofPHGs sample. The spectrometric method measures thechanges of the UV-spectrum of the PHG sample, and themicrowave method measures the microwave absorptionby energy changes. Absorbed water in the network ofPHGs can exist in three states: bound, half-bound and freewater. Free water shows a freezing point when the environ-mental temperature is around 0�C. However, this freezingpoint cannot be seen with bound water. The half-boundwater shows property between them. Bounded water inPHG usually is 0.39–1.18 g=g. The principle of waterabsorption by PHGs can be illustrated by the Flory theoryof ionic networks.

Q53 ¼ 1

2� i

Vu�

1S1

2

� �þ

12 � X1

V1� V0

v

� �� �ð1Þ

where Q is maximum swelling ratio of SAP, i is electroniccharge on the polymer structure per polymer unit, Vu ispolymer repeating unit volume, S is ionic strength of sol-ution, X1 is interaction parameter of polymer with solvent,V1 is molar volume of solvent in a real network, V0 isun-swollen polymer volume, and n is the effective numberof chains.

The equation shows that the water absorption powermainly depend on the osmotic pressure, the affinity ofwater and polymer, and the cross-linking density of the net-work. By using Flory equation the main factors that affectSR and AC of PHGs can be determined in the followingmethods.

Cross-Linker and Cross-Linking Density. A cross-linker plays a major role in modifying the properties ofPHGs in terms of absorption and mechanical properties.Cross-linking density of PHGs is controlled by the fractionof cross-linking agent present in the polymerization and thedouble bond conversion. Therefore, a smaller amount ofcross-linking agent leads to different cross-linking degreesand changes in the AC of PHGs. The chemical structureof a cross-linker can affect absorption properties. Cross-linking agents with hydrophilic properties such as MPAenhances the AC as a result of the presence of amidegroups[62]. Water solubility, short chains and high activityof MPA give this kind of cross-linker property wide usein free radical polymerization[63]. At low percentage ofcross-linking agents, the three-dimensional network of

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polymers does not form effectively, and the water mole-cules cannot be held, which means a decrease in SR andAC of PHGs. At high concentrations of cross-linkers, alarge number of growing polymer chains are involved toproduce an additional network structure. This high con-centration of network doesn’t allow the water to enterthe network and decrease the AC[63].

Other researchers found the same results indicate theaffect of crossliker content on SR as shown in Figure 5[64].Some researchers had reported that the number of func-tional groups in composition of cross-linkers will affectthe AC. They reported that the AC of PHG of polyacryla-mide (PAAm) and polyacrylamide sodium acrylate (SA)(PAAm=SA) when different type of cross-linker is used(e.g., 1,4-butanediol dimetacrylate, BDMA, ethylene glycoldimethaacrylate; EGDMA, N,N0-methylenebisacrylamide;MPA, trimethylolpropane triacrylate; TMPTA in the fol-lowing order.

MPA > BDMA > EGDMA > TMPTA

The changes in SR and AC are as a result of functionalgroups of each type of cross-linker. MPA, BDMA andEGDMA are tetrafunctional cross-linkers, and TMPTAis a hexafunctional cross-linker Figure 4. The NH groupof MPA may increase the SR and AC by causing newhydrophilic interaction. In TMPTA cross-linkers, thereare many cross-linking sites; therefore, the cross-linkingdensity will be higher than other cross-linkers at same con-centration, as such result in decreased AC[65].

Initiator Content. AC of PHGs is affected by initiatorcontent as a result of change in molecular weight of poly-mer repeating unit. At low molecular weight the relative

amount of polymer chain ends increase (polymer chainends do not contribute to AC) therefore, AC will decreaseat high content of initiator[66]. Moreover, when the contentof initiator is low the polymerization reaction takes placeslowly, which means low polymerization rate. This maycause large space volume of network production. At higherinitiator content, the polymerization rate is high and theproduct will has smaller space size and that will preventfluid molecules from entering the network of PHGs[67].Figure 6 shows the effect of APS on SR of carboxy-methylchitosan-g-poly (acrylic acid) (CMCT-g-PAA) anddepicts the same preceding determinations[64].

Degree of Neutralization (DON). DON is normallyused between 0–80% for most polymerization pro-cesses[68,69]. In neutralization of acrylic acid (AA) asexample using sodium hydroxide (NaOH), the negativelycharged carboxyl groups attached to polymer chains pro-duce an electrostatic repulsion (ESR), which leads to net-work expansion. Electrostatic repulsion is the key to highor low AC. This means high AC is associated with highESR. At certain range of neutralization ESR increase leadsto increased AC. Moreover, after a certain degree of neu-tralization, the AC decreases, and that might be as a resultof increase in chain stiffness and counterion condensationon the polyion ‘‘(screening effect)’’[70].

The review indicates that the PHGs of AA reaches themaximum AC at 30% of DON; and at low DON, the i

Vuin Equation (1) is low, meaning decreased ESR anddecrease in AC as well. When DON starts to increase,the content of -CO2-Na groups increases as well; this leadsto enhancement in osmotic pressure between the inside andoutside of the network and also enhanced AC of PHG.However that will stop at certain values of DON, and the

FIG. 6. Effect of mAPS=mAA on the swelling ratio of the superabsorbent

polymer. Reaction condition, mMBAM=mAA, 0.012; mAA=mCMCTS, 6.0;

time, 5 h; temperature, 60�C; neutralization degree 50%; water volume,

150mL.

FIG. 5. Effect of mMBAN=mAA on the swelling ratio of the superabsor-

bent polymer. Reaction condition, mAPS=mAA, 0.04; mAA=mCMCTS, 6.5;

time, 5 h; temperature, 60�C; neutralization degree 50%; water volume,

150mL.

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AC starts to decline. The decrease in AC is as a result ofthe presence of Naþ ion, which leads to shielding of thecarboxylate anions on PAA chains and inhibits the anion-anion repulsion forces. Furthermore, increased DONleads to increases in S as shown in Equation (1), and thatis the main reason for the reduced osmotic pressure ofthe network and decreased SR and AC[71]. The outcomeof another study by other scientists in Table 1 declaringthe effect of DON of WAC and support that abovementioned[51].

Solvent Volume and Concentration. PHG propertiesare affected by solvent concentration as a result ofenhanced primary cyclization rate of multi-vinyl monomerduring the polymerization process. Decreasing the contentreaction of cross-linking agent, initiator and monomer isaccompanied with increase in solvent volume in the reac-tion container. Decreasing the content reaction leads todecrease in the polymerization rate and cross-linking den-sity. In contrast to low solvent volume, the viscosity ofthe reaction content is high, which leads to hindered move-ment of free radicals and monomers in the reaction. Sol-vent concentration can be used to control free radicalpolymerization of PHG in both microscopic and molecularlevels[71,72].

Solvent concentration affects the PHG network proper-ties by affecting the dynamics of radical propagation aswell. At low solvent concentration the double bond con-centration surrounding the free radical is relatively high.This leads to a faster propagation step and less opportunityfor the free radicals to cycle by reacting with its own pen-dant double bond. Solvent type and quality also has beenreported to affect the PHGs network properties. The resultof using a poor solvent is loose network structure and firmnetworks result from using good solvents[73]. Previous stu-dies indicated that high solubility monomer and homopo-lymer in the solvent lead to difficulty in graftingpolymerization. In the same study, the effects of solventcontent in the reaction were determined by fixing the otherreaction content. The result showed that the maximum per-centage of grafting was obtained at 50ml total reactionmixture[74].

Monomer Type and Ratio. PHGs prepared of polymeror monomer with hydrophilic groups showed high AC andSR in comparison to those with hydrophobic groups in itsconstitution. Hydrophobic groups collapse in the presenceof water, which minimizes numbers of hydrophobic groupsto be exposed to water molecules, therefore decreasing theAC of the PHG. The high concentration of monomer in thepolymerization leads to tough and large particle sizePHG[57,75].

Fiber Type and Content. Chemical composition offibers is a factor that can highly affect AC of PHG. Con-centration of hydrophilic groups on the surface of fibercan contribute to enhancing the AC of the net work. Whenthe fiber contributes to increase the cross-linking density ofthe network, its effect will be like that of a cross-linkingagent. Therefore, high concentration of fibers leads to arigid network structure and less AC. The presence of fiberinside the network composition is illustrated as in Figure 7.Previous work on sodium alginate (Na-AG) grafted withcarboxymethycellulose (CMC) showed that the SR ofPHG increase gradually with increase in Na-Ag contentand the maximum AC is achieved at 0.5 weight ratio ofNa-Ag=CMC. After this ratio the AC decreases as a resultof increase in the viscosity of reaction mixture which hin-dered the reactants movement[64,76]. In another work whichused Kappa-carrageenan (kC) as a fiber, results showedthat the AC of water of network increased with increasein the kC content. The same result has been found by usingCMC grafted with polyacrylonitril (PAN), and starch-poly(sodium acrylate-co- acrylamide)[77–79].

Ionic Strength of Absorbed Fluid. PHGs are mainlysensitive to the ionic strength of the surrounding solutionas long as the origin of their raw materials is polyelectro-lyte. Therefore, AC and SR of PHG are significantly affec-ted by the saline consternation of absorbed fluid. The affect

TABLE 1Effect of the neutralization degree of acrylic acid atvarious crosslinking agent concentrations on water

absorbency of the synthesized beads

Water absorbency capacity g g�1

Neutralizationdegree ofacrylic acid %

0.025mol %N,N-MBA

0.05mol%N,N-MBA

0.075mol%N,N-MBA

100 775� 99 568� 27 476� 1576 581� 18 557� 51 471� 4259 570� 68 523� 39 428� 2846 507� 59 476� 17 447� 14

TABLE 2Comparison of absorbency with various hydrogels

Absorbency g=g

Hydrogel typePurewater

0.9%NaCl aq

Sodium polyacrylate hydrogel 300 60Cotton cellulose-based hydrogel 400 100Starch-based hydrogelprepared in DMSO (sequence 2)

120 30

Starch-based hydrogelprepared in water

70 20

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of ionic strength can be described using Donnan equilib-rium theory. This theory considers that the electrostaticforce is the result of osmotic pressure that depends onthe concentration of the mobile ions in and out the networkstricture of the gels. In addition high different concen-tration of mobile ions in and out the network gives highAC and SR by enhancing the electrostatic repulsion. How-ever, if the absorbed fluid has high saline concentration theabsorption will be stop when the equilibrium between thedifferent charges will be reached[80].

Table 1 shows a comparison between the absorbancy ofdifferent hydrogels in distilled water and water with 0.9%NaCl. The results show that all kinds of hydrogels havehigher AC in distilled water than those in NaCl solution[19].Another study obtained the same findings using variouskinds of electrolyte solutions such as NaCl and CaCl2 —the ionic strength has a significant effect on absorptioncapacity by decreasing the ability of hydrogel to uptakemore water[47].

APPLICATION OF PHGs

The unique properties of PHGs such as absorption,swelling and de-swelling behavior, hydrophilicity, and bio-compatibility are the main reason for their wide applica-tions[17,47]. Natural polymers have better biocompatibilityand less latent toxicity than other synthetic polymer[81,82].Therefore, PHGs of natural polymers have attracted con-siderable attention as excellent candidates for controlled

release device, bio-adhesive device, and targetable thera-peutic devices. In pharmaceuticals field, PHG materialshave special application including diagnostic, therapeutic,and implantable devices such as catheters, biosensors[83,84],artificial skin[85], and tissue engineering[86,87]. Also,pH-sensitive PHGs are able to convert chemical energyinto mechanical energy. These systems can serve as actua-tors or artificial muscles in many applications[27,45,75].

Thermo-sensitive PHGs have been used as drug deliverysystems[25,38,88]. Electro-sensitive PHGs have been appliedin controlled drug delivery. Light-sensitive PHGs havebeen used to develop photo-response artificial muscles,switches and memory devices[89]. In addition, PHGs areinitially proposed to be utilized as ocular lenses. Recentlythe ocular lens made of PHGs was commercialized as softcontact lenses (SCL), which have been used widely as anadaptor for drug delivery system. The ability of these lensesto release and control the amount of drug over an extendedduration during treatment has attracted their wide accept-ance[90]. For most of these mentioned applications ofPHGs, the fast response to environment is one of the mostimportant factors for the application of environmentalsensitive PHGs, especially in the application of controlleddrug delivery systems[91].

Pulp and paper industry use large amount of water intheir production processes. For economic purposes andenergy efficiency water removal using temperature-sensitivePHGs method is a sufficient way to reduce the usedamount of water via recycling process. The principlebehind this method is that large amounts of water will beabsorbed at low temperatures without dissolving PHGsand then re-activated to release clean water when the tem-perature will be raised. In addition the PHGs could bere-used more than 1,000 times without losing the potency.In this process the required energy is less than 20 timescompared with the old triple-effect evaporator method.Other advantages of this technology are: lower capitalcosts, a simpler water-removal process, and the potentialfor essentially no energy costs, if a waste-heat source isavailable to heat the PHGs to the water-release tempera-ture[37,92].

PHGs used in textile industry by grafting or adsorbedonto the surface of polymer fibers[93]. Smart materials ofPHGs are in use for application of removal of heavy metalsfrom aqueous solution as well[45,94,95]. PHG materials havespecial application for control-release fertilizers[3,96–99], toreduce environmental pollution[24,33,96,100,101], reduce irri-gation water consumption, improve fertilizer retention insoil[3,5,43], eliminates the leaching of nutrients, increase soilaeration and diminishes soil density[16]. The function ofPHGs in the soil is to absorb water from rainfall or irri-gation and release it slowly to meet the need of plantgrowth and to enhance the production in terms of qualityand quantity. Figures 8 and 9 show the effects of PHG

FIG. 7. The present fiber inside the network composition.

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materials of the Radish and Faba bean plants. Clearly,there are significant affects on the plants growth and theirproduction as well[102,103]. There are many ways to usePHGs in agriculture such as mixing the PHGs with soil,seeds, fertilizers and any agricultural chemicals use inplanting

PHGs have been used in oil industry to enhanced oilrecovery application as a controller of mobility of fluid inthe formation. PHG will act as blocking agents in high per-meability regions of the formation leading the other sol-ution of the flooding process to flow in low permeabilityplaces to sweep as much oil as possible[35].

CONCLUSIONS

PHGs are viscoelestic materials with cross-linked struc-tures. They have the ability to absorb and release certainamounts of water or biological fluids. PHGs with high

AC can be called superabsorbent polymer hydrogels(SAPHGs). When fibers or fillers are used for preparationof PHGs, they can be called polymer hydrogel composites(PHGCs) or superabsorbent polymer hydrogel composites(SAPHGCs). Smart materials are PHGs that have the abil-ity to absorb and release the water or biological fluids inresponse to environmental change.

PHGs can be prepared using different raw materials.However, the main materials are monomer or polymer,initiator, cross linker, and solvent. Many preparationmethods have been used to prepare PHGs such as solution,suspension and radiation polymerization. PHGs are classi-fied by different workers using different points of view suchas morphology, preparation method, environmentalresponse and material resource. The most property thatgave the PHGs a wide range of application is that the highAC. PHGs have been applied in many areas: pharmaceuti-cal, drug delivery, bio sensors, artificial skin and mussels,oil industry, agriculture, water treatment, and textile indus-tries[2,37,104–113].

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