reactive & functional polymers - anasayfa

Reactive & Functional Polymers 68 (2008) 1467–1475

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Reactive & Functional Polymers

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Formation of macroporous poly(acrylamide) hydrogels inDMSO/water mixture: Transition from cryogelation to phaseseparation copolymerization

M. Murat Ozmen, Oguz Okay *

Istanbul Technical University, Department of Chemistry, 34469 Maslak, Istanbul, Turkey

a r t i c l e i n f o a b s t r a c t

Article history:Received 19 May 2008Received in revised form 30 June 2008Accepted 12 July 2008Available online 25 July 2008

Keywords:Poly(acrylamide)MacroporousHydrogelsCryogelationElasticity

1381-5148/$ - see front matter � 2008 Elsevier Ltddoi:10.1016/j.reactfunctpolym.2008.07.005

* Corresponding author. Tel.: +90 212 2853156; faE-mail address: [email protected] (O. Okay).

Poly(acrylamide) hydrogels were prepared by free-radical crosslinking copolymerization ofacrylamide and N,N0-methylenebis(acrylamide) at �18 oC in aqueous DMSO solutions ofvarious composition. The hydrogels formed in the solvent mixture with less than 25%DMSO by volume have irregular large pores of about 101 lm in diameter, typical for mac-roporous networks created by the cryogelation technique. Non-porous hydrogels wereobtained in solutions containing 25% DMSO, while at larger DMSO contents, the structureof the hydrogel networks consists of aggregates of microspheres, which looks as cauliflow-ers, typical for a macroporous network formed by reaction-induced phase separationmechanism. Swelling measurements show that fast responsive PAAm hydrogels can beobtained as the DMSO content in the mixed solvent is decreased or increased starting from25 v/v%. The results were interpreted as the transition from cryogelation to the phase sep-aration copolymerization due to the marked freezing point depression of the solvent mix-ture as well as due to the action of the mixed solvent as a poor solvating diluent at �18 oC.It was also shown that the initial temperature of the cryogelation reactions in waterstrongly affects the hydrogel properties. Hydrogels formed at an initial temperature Tini

of 0 oC were very tough and can be compressed up to about 100% strain without any crackdevelopment while those formed at Tini = 21 oC were fragile.

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1. Introduction

Hydrophilic gels called hydrogels are hydrophilic poly-mer networks swollen in water. They exhibit drastic vol-ume changes in response to specific external stimuli,such as the temperature, solvent quality, pH, and electricfield [1,2]. These properties of hydrogels received consider-able interest in the last three decades. They are usefulmaterials for drug delivery systems, artificial organs, sepa-ration operations in biotechnology, processing of agricul-tural products, on–off switches, sensors, and actuators.Despite this fact and considerable research in this field,the design and control of hydrogel-based devices still pres-

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ent some problems since a number of network propertiesare inversely coupled. For example, decreasing the degreeof crosslinking of hydrogels in order to increase their meshsize (‘‘molecular porosity”) results in their acceleratedchemical degradation. Further, loosely crosslinked hydro-gels are soft and fragile materials when handled in theswollen state; typically, they exhibit moduli of elasticityin the order of kilopascals and fracture energy in the rangeof 10�1–100 J/m2, much smaller than the fracture energy ofrubbers [3]. The poor mechanical performance of highlyswollen hydrogels limits their technological applications.A fast response of hydrogels to the external stimuli is alsoa requirement in many application areas of these materi-als. However, the kinetics of hydrogel volume change in-volves absorbing or desorbing solvent by the polymernetwork, which is a diffusive process. This process is slow

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Fig. 1. Freezing temperature of aqueous DMSO solutions shown as afunction of their DMSO content by volume percent. Data were taken from[16,17].

1468 M. Murat Ozmen, O. Okay / Reactive & Functional Polymers 68 (2008) 1467–1475

and even slower near the critical point. Increasing the re-sponse rate of hydrogels has been one of the challengingproblems in the last 25 years.

A widely used approach to obtain fast-responsivehydrogels is to create voids (pores) inside the hydrogel ma-trix, so that the response rate becomes a function of themicrostructure rather than the size or the shape of thegel samples. For a polymer network having an intercon-nected pore structure, absorption or desorption of wateroccurs through the pores by convection, which is much fas-ter than the diffusion process that dominates the non-por-ous hydrogels. There are two basic techniques to obtaincrosslinked polymers with a macroporous structure. Thefirst technique, called reaction-induced phase separation,involves the free-radical crosslinking copolymerization ofthe monomer-crosslinker mixture in the presence of an in-ert diluent, which is soluble in the monomer mixture[4–7]. In order to obtain macroporous structures, a phaseseparation must occur during the course of the gelationprocess so that the two-phase structure formed is fixedby the formation of additional crosslinks. After the poly-merization, the diluent is removed from the network, leav-ing a porous structure within the highly crosslinkedpolymer network. Another technique to create a macropo-rous network structure is the use of inert templates in thepreparation of hydrogels. By this technique, the polymerformation reactions are carried out in the presence of tem-plates; a macroporous structure in the final hydrogel ma-trix appears after extraction of template materials. Forexample, by the cryogelation technique, the polymer for-mation reactions are carried out below the bulk freezingtemperature of the reaction system [8]. Thus, the essentialfeature of such reaction systems is that the monomers andthe initiator are concentrated in the unfrozen microzonesof the apparently frozen system. The polymerization andcrosslinking reactions proceed in the unfrozen microzonesof the reaction system. A macroporous structure in the fi-nal material appears due to the existence of ice crystalsacting as a template for the formation of the pores [9–15].

Here, we describe, for the first time based on our knowl-edge, the change of the formation mechanism of the por-ous structures in hydrogels from cryogelation to phaseseparation polymerization by simply varying an experi-mental parameter. By conducting the crosslinking poly-merization of acrylamide in aqueous DMSO solutions, wewere able to manipulate the porosity formation mecha-nism through the amount of DMSO present in the reactionsystem. DMSO is a polyfunctional molecule with a highlypolar S@O group and two hydrophobic CH3 groups. Its po-lar site can interact with water forming strong hydrogenbonds, and its non-polar sites can cause effects of hydro-phobic hydration and hydrophobic association of DMSOmolecules. Therefore, aqueous solutions of DMSO displayinteresting intermediate behavior not seen in either neatsolvent and depart strongly from ideal behavior [16–18].For example, Fig. 1 shows the freezing point of aqueousDMSO solutions plotted against the DMSO content. It isseen that DMSO–water mixtures exhibit a marked freezingpoint depression. In the present work, the gelation reac-tions were carried out at �18 oC. As seen in Fig. 1, the reac-tion system will freeze at �18 oC if the DMSO content is

less than about 30 v/v%. Indeed, we observed formationof macroporous structures below 25 v/v% DMSO due tothe cryogelation mechanism. However, at higher DMSOcontents, materials with cauliflower morphology were ob-tained indicating that the porosity is induced by phase sep-aration mechanism. This paper describes the transitioncondition from one to another mechanism by simply vary-ing the mixed solvent composition. Most hydrogels, partic-ularly macroporous hydrogels, are known to suffer fromthe lack of mechanical toughness. Here, we also report for-mation conditions of macroporous hydrogels with a highdegree of toughness by adjusting the initial temperatureof the crosslinking polymerization at �18 oC.

2. Experimental

2.1. Materials

Acrylamide (AAm), N,N0-methylenebis(acrylamide)(BAAm), ammonium persulfate (APS), N,N,N’,N’-tetrameth-ylethylenediamine (TEMED), and dimethyl sulfoxide(DMSO), all from Merck, Germany, were used as received.Stock solutions of APS and TEMED were prepared by dis-solving 0.24 g APS and 0.50 mL TEMED each in 20 mL ofdistilled water. Poly(acrylamide) (PAAm) hydrogels wereprepared by free-radical crosslinking copolymerization ofAAm and BAAm in DMSO–water mixtures of various com-position. To obtain macroporous structures, the gel prepa-ration temperature Tprep was set to �18 oC. Controlexperiments were carried out similarly, except that Tprep

was 22 oC. The initial concentration of the monomers(AAm + BAAm) as well as the crosslinker ratio X, that is,the mole ratio of the crosslinker BAAm to the monomerAAm, were fixed at 0.15 g/mL and 1/80, respectively. Thereaction time was also set to 24 h. APS (5.26 mM) and

M. Murat Ozmen, O. Okay / Reactive & Functional Polymers 68 (2008) 1467–1475 1469

TEMED (0.25 mL/100 mL reaction solution) were used asthe redox initiator system. The following example illus-trates the synthetic procedure for hydrogels prepared in50 v/v% DMSO/water mixture: 1.46 g AAm, 0.0396 g BAAm,water (3 mL), DMSO (5 mL), and TEMED stock solution(1 mL) were first mixed in a graduated flask of 10 mL involume. The solution was purged with nitrogen gas for10 min at 21 oC and then APS stock solution (1 mL) wasadded. The solution was transferred to a plastic syringeof 4.4 mm in diameter, immersed in a thermostated bathat Tprep, and the polymerization was conducted for oneday. After polymerization, the syringe was taken out ofthe bath and the gel rod was removed from the syringe.The gel was cut into specimens of approximately 10 mmin length and immersed in a large excess of water at21 oC to wash out any soluble polymers, unreacted mono-mers, and the initiator. In order to obtain PAAm networks,the equilibrium swollen gel samples were taken out ofwater and immersed in acetone followed by drying undervacuum at 40 oC.

2.2. Gel fraction measurements

For the gel fraction measurements, the hydrogel sam-ples were extracted with water at least for one month.For this purpose, each hydrogel sample was placed in anexcess of water at 21 oC and water was replaced severaltimes. The gel fraction Wg was calculated as

Wg ¼mdry

mocMð1Þ

where mdry and mo are the weights of the gel samples afterdrying and just after preparation, respectively, and cM isthe initial monomer concentration (0.15 g/mL).

2.3. Swelling measurements

For the swelling measurements, the hydrogel samplesafter preparation in the form of rods of 4.4 mm in diameterand about 10 mm length were placed in an excess of waterat 21 ± 0.5 oC. In order to reach swelling equilibrium, thehydrogels were immersed in water for at least two weeksreplacing the water every other day. The swelling equilib-rium was tested by measuring the diameter of the gel sam-ples by using an image analyzing system consisting of amicroscope (XSZ single Zoom microscope), a CDD digitalcamera (TK 1381 EG), and a PC with the data analyzing sys-tem Image-Pro Plus. The swelling equilibrium was alsotested by weighing the gel samples. Thereafter, the hydro-gel samples equilibrium swollen in water were dried as de-scribed above to constant weight. The equilibrium volumeand the equilibrium weight swelling ratios of the hydro-gels, qv and qw, respectively, were calculated as

qv ¼ ðDw=DdryÞ3 ð2Þ

qw ¼ mw=mdry ð3Þ

where Dw and Ddry are the diameters of the equilibriumswollen and dry gels, respectively, mw and mdry are theweights of gels after equilibrium swelling in water andafter drying, respectively.

For the measurement of the deswelling times of gels,the equilibrium swollen hydrogel samples in water wereimmersed in acetone at 21 oC. The volume changes of gelswere measured in situ by following the diameter of thesamples under microscope using the image analyzing sys-tem. For the measurement of the swelling times of gels, thecollapsed gel samples in acetone were transferred intowater at 21 oC. The diameter changes of gels were alsodetermined as described above. The normalized swellingratio Vrel at a given swelling time t was calculated as

V rel ¼ ðD=DwÞ3 ð4Þ

where D is the diameter of the gel sample at time t.

2.4. Elasticity tests

Uniaxial compression measurements were performedon equilibrium swollen gels in water. All the mechanicalmeasurements were conducted in a thermostated roomof 21 ± 0.5 oC. The stress–strain isotherms were measuredby using an apparatus previously described [19,20]. Briefly,a cylindrical gel sample of 4–8 mm in diameter and 7–15 mm in length was placed on a digital balance (SartoriusBP221S, readability and reproducibility: 0.1 mg). A loadwas transmitted vertically to the gel through a rod fittedwith a PTFE end-plate. The compressional force acting onthe gel was calculated from the reading of the balance.The resulting deformation was measured after 20 s ofrelaxation by using a digital comparator (IDC type Digi-matic Indicator 543-262, Mitutoyo Co.), which was sensi-tive to displacements of 10�3 mm. The measurementswere conducted up to about 15% compression. From the re-peated measurements, the standard deviations in the mod-ulus value were less than 3%. The elastic modulus G wasdetermined from the slope of linear dependence [21],

f ¼ Gða� a�2Þ ð5Þ

where f is the force acting per unit cross-sectional area ofthe undeformed gel specimen, and a is the deformation ra-tio (deformed length/initial length).

2.5. Texture determination

For the texture determination of dried hydrogels, scan-ning electron microscopy studies were carried out at vari-ous magnifications between 50 and 3000 times (Jeol JSM6335F Field Emission SEM). Prior to the measurements,network samples were sputter-coated with gold for3 min using Sputter-coater S150 B Edwards instrument.

3. Results and discussion

We discuss the results of our experiments in two sub-sections. In the first subsection, development of a macro-porous structure in poly(acrylamide) (PAAm) hydrogelsdepending on the mixed solvent composition is discussed.In the second subsection, the possibility of obtaining toughPAAm hydrogels is discussed and experimental observa-tions are interpreted.


3.1. Porosity formation

Crosslinking copolymerization of AAm/BAAm comono-mer system was carried out at �18 oC in aqueous DMSOsolutions. Control reactions were carried out at 22 oC. Ifthe DMSO content is more than 60 v/v% in the solvent mix-ture, no gel formation was observed at �18 oC. Fig. 2 showsthe weight fraction of gel Wg and the modulus of elasticityof swollen hydrogels G plotted as a function of DMSO% ofthe reaction solution. Filled and open symbols are the re-sults of measurements on gels prepared at �18 and22 oC, respectively. Independent of the gelation tempera-ture, the gel fraction Wg, that is the conversion of monomerto the crosslinked polymer, is almost complete up to about25 v/v% DMSO, whereas it slightly decreases as the DMSO%is further increased. Complete monomer conversion evenat �18 oC is due to the high monomer concentration inthe unfrozen reaction zones of the apparently frozen reac-tion system [13,14]. Thus, when the reaction solution iscooled down to �18 oC, solvent freezes while the solutes(AAm, BAAm, APS, and TEMED) expelled from the solventcrystals accumulate in the liquid phase, in which the reac-tions proceed. It seems that the decrease of the rate con-stants of polymerization and crosslinking reactions at�18 oC is compensated by the increased monomer concen-tration in the reaction zones so that the gel fraction is unaf-fected by Tprep. Similar to the gel fraction results, the elasticbehavior of gels is almost independent of temperature inthe range of 0 to 25 v/v% DMSO (Fig. 2B). In this range,the gels exhibit an elastic modulus of about 20 kPa. Asthe DMSO content is further increased, the elastic modulusof low temperature gels rapidly decreases and approachesto 102 Pa at 60 v/v% DMSO.

Fig. 3A and B shows the equilibrium weight (qw) andvolume swelling ratios (qv) of the hydrogels prepared at�18 and 22 oC, respectively, plotted against the DMSO con-tent of the reaction solution. For the room temperaturegels, both qw and qv are equal and slightly increasing func-

DMSO v/v %

0 20 40 60 80

Wg

0.00

0.20

0.40

0.60

0.80

1.00

0

A

Fig. 2. The weight fraction of gel Wg (A) and the modulus of elasticity of swollenFilled and open symbols are the results of measurements on hydrogels prepare

tion of DMSO%. However, low temperature gels exhibitnon-monotonic volume swelling behavior with a maxi-mum appearing at about 20 v/v% DMSO at which qv be-comes equal to qw. The relative values of qw and qv ofgels provide information about their internal structure inthe swollen state. This is due to the fact that the weightswelling ratio includes the solvent locating in both poresand in the polymer region of the gel, while, assuming iso-tropic swelling, the volume swelling only includes the sol-vent in the polymer region. Thus, the larger the differencebetween qw and qv, the larger the amount of solvent in thepores, i.e., the larger the total volume of pores. From theweight and volume swelling ratios of PAAm hydrogels,their swollen state porosities Ps can be estimated usingEq. (5):

Ps % ¼ ð1� qv

1þ ðqw � 1Þd2=d1Þ � 102 ð6Þ

where d1 and d2 are the densities of the swelling agent(water) and the polymer, respectively. Assuming thatd1 = 1.0 g/mL and d2 = 1.35 g/mL, the swollen state porosi-ties Ps were calculated and are shown in Fig. 3C and D plot-ted against DMSO% for gels prepared at �18 and 22 oC,respectively. The gels formed at room temperature exhibitnegligible porosities in the swollen state while thoseformed at �18 oC are porous outside of the range of about15 to 25 v/v% DMSO. The swollen state porosity increasesas the DMSO content is decreased or increased from 15to 25 v/v%.

To visualize the pores in PAAm hydrogels, the networksamples were investigated by scanning electron micros-copy (SEM). As expected, the entire room temperaturegel networks were non-porous. Fig. 4 shows SEM imagesof the network samples prepared at �18 oC in DMSO/watermixture of various compositions, as indicated in the figure.In accord with Fig. 3C, the gels prepared in 25 v/v% DMSOexhibit glass-like fracture surfaces at the dry state butwithout pronounced microstructure. However, the gels

DMSO v/v %

20 40 60 80

G / Pa

102

103

104

B

hydrogels G (B) plotted as a function of DMSO% of the reaction solution.d at �18 and 22 oC, respectively. The curves only show the trend of data.

DMSO v/v %DMSO v/v %

0 20 40 60 0 20 40 60

Ps %

0

20

40

60

80

qw, qv

5

10

15

20qv

qwA

DC

B

Fig. 3. The equilibrium weight (qw, triangles) and volume swelling ratios (qv, circles) of PAAm hydrogels plotted against the DMSO content of the reactionsolution. Tprep = �18 (A) and 22 oC (B). The swollen state porosities Ps calculated using Eq. (6) shown as a function of DMSO% for the hydrogels prepared atTprep = �18 (C) and 22 oC (D). The curves only show the trend of data.

Fig. 4. SEM images of PAAm network samples prepared at �18 oC in DMSO/water mixture of various compositions. DMSO v/v% in the mixed solvent isindicated in the pictures. Magnification = 300�. Scaling bars are 10 lm.


Fig. 5. SEM image of the network sample prepared at �18 oC in anaqueous DMSO solution containing 60 v/v% DMSO. Magnifica-tion = 7500�. Scaling bar is 1 lm.


formed below or above 25 v/v% DMSO have a porous struc-ture; those formed below 25 v/v% DMSO have irregularlarge pores of about 101 lm in diameter, typical for macro-porous networks created by the cryogelation technique[13]. However, those obtained above 25 v/v% DMSO haverelatively small pores. Fig. 5 shows the SEM image of�18 oC gel prepared in 60 v/v% DMSO at a larger magnifica-tion than in Fig. 4. The morphology consists of aggregatesof 3–5 lm dimensions of well-defined microspheres. Twogeneration of pores building the microstructure of the gelnetwork can be seen from the SEM image; one submicrom-eter-sized pores between the microspheres and the othermicrometer-sized pores between the aggregates. Themicrospheres are about 0.1–0.5 lm in diameter. Thus, thestructure of the networks formed above 25 v/v% DMSOlooks like cauliflowers, typical for a macroporous copoly-

Fig. 6. Deswelling and swelling kinetics of PAAm hydrogels in acetone and in watof deswelling or re-swelling. Tprep = 22 oC, DMSO v/v% = 0 (s), 25 (}), 40 (h), an

mer network formed by reaction-induced phase separationmechanism [5].

Fig. 6 compares the response rate of the hydrogels pre-pared at �18 oC (filled symbols) and at 22 oC (open sym-bols). Here, the normalized gel volume Vrel (volume of gelat time t/equilibrium swollen volume in water) is plottedagainst the time t of deswelling in acetone and re-swellingin water. Both the swelling and deswelling rates of �18 oCgels prepared in water or in 50 v/v% DMSO solution aremuch faster than those prepared at 22 oC. The low temper-ature gel attains its equilibrium collapsed and equilibriumswollen states within 5–10 min, while the conventional gelrequires 60 and 300 min to attain the equilibrium states inacetone and in water, respectively. The semilogarithmicplots in Fig. 7 show the relative volume Vrel of low temper-ature gels prepared in aqueous DMSO solution of variouscompositions plotted against the swelling time in water.It is seen that the time to attain the equilibrium swollengel state increases, that is, the response rate decreases asthe DMSO content is increased up to 25 v/v% (Fig. 7A); atthis DMSO concentration, the response rate of the low tem-perature gel is almost equal to that of the room tempera-ture gels. Further increase in the DMSO% again increasesthe response rate of the hydrogels (Fig. 8B). Similar find-ings were also observed during the deswelling processesof the hydrogels in acetone. Since creation of pores insidethe hydrogel matrix increases its response rate, these re-sults are in accord with the microscopic observation ofthe network structures reported above. Thus, fast respon-sive PAAm hydrogels form as the DMSO content in themixed solvent is decreased or increased starting from25 v/v%.

Our experimental results are consistent with the fol-lowing scenario: As reported in the literature, macropo-rous structures by the cryogelation mechanism start toappear at polymerization temperatures about 8 oC belowthe freezing point of the reaction system [13–15]. Thus,to obtain macroporous PAAm gels at �18 oC, the freezing

er, respectively, shown as the variation of the gel volume Vrel with the timed 60 (4). Tprep = �18 oC, DMSO v/v% = 0 (d), 50 (N).

Fig. 7. Swelling kinetics of PAAm hydrogels in water shown as the variation of the gel volume Vrel with the time of swelling. Tprep = �18 oC. DMSO v/v% = 0(d), 5 (s), 10 (N), 25 (4), 40 (.), and (r). (A) and (B) show the DMSO ranges between 0 and 25, and 25 and 50 v/v%, respectively.

Fig. 8. Stress–strain data of PAAm hydrogels prepared in water atTprep = �18 oC as the dependence of f on fractional deformation 1 � a.The initial temperature of polymerization Tini is indicated. The hydrogelsamples subjected to the mechanical tests up to complete compressionwere 16 mm in diameter and about 10 mm in length.


point of the reaction solution should be �10 oC, which cor-responds to a DMSO concentration of 23 v/v% (Fig. 1).Accordingly, at DMSO contents below 23 v/v%, porousstructures in PAAm hydrogels should form due to thecryogelation where the solvent crystals in the reaction sys-tem act as a template. Indeed, SEM images of the gel net-works formed below 25 v/v% DMSO are typical for thoseof the cryogels.

At larger DMSO contents, one would expect formationof non-porous hydrogels due to the fact that the reactionsystem is unfrozen at �18 oC (Fig. 1). In contrast, however,only gels formed around 25 v/v% DMSO were found to be

non-porous while at larger DMSO contents, macroporoushydrogels with cauliflower morphology were obtained.Thus, formation of macroporous PAAm hydrogels in thisrange of DMSO should be a result of a phase separationprocess with the action of DMSO/water mixture as an inertdiluent. To explore the condition of phase separation dur-ing the reactions, we investigated the swelling capacityof the hydrogels in the mixed solvents at �18 oC. We foundthat the gels are in a collapsed state in aqueous DMSOsolutions containing more than 30 v/v% DMSO indicatingthat the mixed solvent is a poor solvating diluent for PAAmat �18 oC. Indeed, recent work also shows that DMSO/water mixture of 1:1 by volume is a poor solvent for PAAmat subzero temperatures [22]. Thus, the polymer chainsformed during the reaction at subzero temperatures un-dergo a deswelling transition, which is responsible forthe formation of macroporous hydrogels in the mixed sol-vent containing more than 25 v/v% DMSO.

3.2. Effect of the initial temperature of polymerization

In the previous section, macroporous PAAm hydrogelswere prepared by setting the gel preparation temperatureTprep to �18 oC. However, Tprep is the temperature of thethermostated bath in which the reactions were carriedout. Since the polymerization initiator APS was added intothe monomer solution before the freezing of the reactionsystem, i.e., at 21 oC, the polymerization and crosslinkingreactions proceed non-isothermally from the moment ofthe initiator addition to the moment when the tempera-ture of the reaction system reaches to Tprep. In this section,we investigated the effect of the initial temperature of thepolymerization (Tini) on the hydrogel properties. Experi-ments were carried out at Tprep = �18 oC and under thesame experimental conditions as described above, exceptthat the initiator APS was added to the reaction systemat Tini = 0 and 21 oC. The reactions were conducted bothin water (cryogelation) and in aqueous DMSO solutionwith 60 v/v% DMSO (phase separated polymerization).

Fig. 9. Photographs of PAAm hydrogels formed at �18 oC during the compression tests. Tini = 21 oC (upper panel) and 0 oC (bottom panel).


Not surprisingly, the hydrogels prepared in 60 v/v%DMSO exhibited similar properties independent on the ini-tial temperature Tini. This is due to the fact that the reac-tion system remains unfrozen during the crosslinkingreactions and the extent of phase separation does notchange much depending on the cooling rate. However, sig-nificant changes in the hydrogel properties were observedwhen the reactions were conducted in water. DecreasingTini from 21 to 0 oC decreased the modulus of elasticity ofthe swollen hydrogels from 30 to 13 kPa. Further, animportant improvement in the degree of toughness of thehydrogels was observed by decreasing the initial tempera-ture Tini. Hydrogels formed at Tini = 0 oC were very toughand can be compressed up to about 100% strain withoutany crack development while those formed at Tini = 21 oCwere fragile. This behavior is shown in Fig. 8 where thestress f is plotted against the fractional deformation (de-formed length/initial length, 1 � a) for gels formed usingan initial temperature Tini of 0 oC (filled symbols) and21 oC (open symbols). Although the gels prepared at Tini =21 oC broke at a stress of 11 kPa and a strain of about50%, those prepared at Tini = 0 oC did not break even ata strain of more than 98%. Photographs in Fig. 9 also

Fig. 10. SEM images of PAAm network samples prepared at �18 oC in water. The10 lm.

demonstrate how the hydrogel sample prepared at Tini =0 oC sustains a high compression. As shown in the upperpanel of Fig. 9, the swollen gel prepared using Tini = 21 oCfractured under relatively low deformation suggesting thatcracks develop easily in the gel. However, those obtainedat Tini = 0 oC remain mechanically stable up to completecompression (bottom panel of Fig. 9). Important point isthat, as the gel is squeezed under the piston, the gel releasesall its water so that it can completely be compressed. Afterthe release of the load and after the addition of water, thesample immediately recovers its original shape.

Fig. 10 shows SEM images of the gel networks preparedin water and at Tini = 0 oC (left) and 21 oC (right). It is seenthat decreasing the initial temperature of cryogelationdecreases the size of the pores in the network. At the sametime, the pore walls become thinner and a second genera-tion of pores starts to appear with decreasing the initialpolymerization temperature from 21 to 0 oC.

As the initial polymerization temperature is decreased,a longer period of the reactions takes place in the frozenstate, which slows down the crosslinking reactions andleads to a twofold decrease in the elastic modulus of thehydrogels. Thus, the flexibility of the polymer making the

initial temperature Tini is indicated. Magnification = 300�. Scaling bars are


pore walls increases by decreasing Tini so that the hydrogelbecomes mechanically stable even at very large deforma-tion ratios. Further, formation of relatively thin pore wallsand the existence of a second generation of pores withinthe pore walls may also contribute the stability of thematerials formed at a lower initial polymerizationtemperature.

4. Conclusions

Free-radical crosslinking copolymerization of AAm/BAAm comonomers was carried out at �18 oC in aqueousDMSO solution of various compositions. PAAm hydrogelsformed in the mixed solvent with less than 25 v/v% DMSOhave irregular large pores of about 101 lm in diameter,typical for macroporous networks created by the cryogela-tion technique. Non-porous hydrogels were obtained insolutions with 25 v/v% DMSO. At larger DMSO contents,the structure of the hydrogel networks consists of aggre-gates of microspheres, which look like cauliflowers, typicalfor a macroporous network formed by reaction-inducedphase separation mechanism. Swelling measurementsshow that fast responsive PAAm hydrogels can be obtainedas the DMSO content in the mixed solvent is decreased orincreased starting from 25 v/v%. The results were inter-preted as the transition from cryogelation to the phaseseparation polymerization due to the marked freezingpoint depression of the solvent mixture as well as due tothe action of the mixed solvent as a poor solvating diluentat �18 oC. It was also shown that the initial temperature ofthe cryogelation reactions in water strongly affects thehydrogel properties. Hydrogels formed at an initial tem-perature Tini of 0 oC were very tough and can be com-pressed up to about 100% strain without any crack

development while those formed at Tini = 21 oC were frag-ile and broke at a strain of about 50%. Experiments are inprogress to obtain fast responsive tough PAAm hydrogelbeads by inverse suspension polymerization technique.

Acknowledgement

Work was supported by the Scientific and Technical Re-search Council of Turkey (TUBITAK), TBAG 105T246.

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