Formato Informes 1 336631

Carbohydrate Polymers 92 (2013) 157 166

Contents lists available at SciVerse ScienceDirect

Carbohydrate Polymers

jo u rn al hom epa ge: www.elsev ier .com

Prepar baof sodi

David Le Maa Department o de Chb Molecular De 4, 300c Imec, Polymer

a r t i c l

Article history:Received 16 MReceived in reAccepted 15 SAvailable onlin

Keywords:Sodium alginaHomopolymerPNIPAAmHydrogelsSEM

rmatte anpic neogelsd fromof m

capl types neaith p

2012 Elsevier Ltd. All rights reserved.

1. Introduction

Alginic aseaweeds; mannopyraThese two uand homop& Vsquez,Leal, Matsusalts are wshow medias delivery2011; Drag2008; Shap2008). The between thto the gelli& Thom, 1biodegradatoxic synthcan combin

CorresponSantiago, Chile

E-mail add

good biocompatibility (Dumitriu, Vidal, & Chornet, 1996; Hoare &

0144-8617/$ http://dx.doi.ocid is the major structural polysaccharide of brownit is a linear copolymer of 1 4 linked -d-nuronic acid (M) and -l-gulopyranuronic acid (G).ronic acids can be arranged in heteropolymeric (MG)olymeric (MM and GG) blocks (Chanda, Matsuhiro,

2001; Chanda, Matsuhiro, Mejias, & Moenne, 2004;hiro, Rossi, & Caruso, 2008; Painter, 1983). Alginateidely used in pharmaceutical and food industry; theycal applications in wound dressing, as scaffolds, and

matrices (Donati & Paoletti, 2009; Draget & Taylor,et et al., 2005; Han, Guenier, Salmieri, & Lacroix,iro & Cohen, 1997; Wang, Liu, Gao, Liu, & Tong,cooperative binding of divalent cations (usually Ca2+)e homopolymeric sequence of G residues gives riseng properties of alginates (Grant, Morris, Rees, Smith,973). Semi-interpenetrating hydrogels (semi-IPN) ofble polymers obtained from natural resources with non-etic polymers have potential applications because theye high responsiveness to an external stimulus with

ding author at: Facultad de Qumica y Biologa, Casilla 40, Correo 33,.ress: [email protected] (B. Matsuhiro).

Kohane, 2008). De Moura et al. (2005, 2006, 2009) prepared calciumalginate and PNIPAAm interpenetrated hydrogels with potentialbiomedical applications. Poly(N-isopropylacrylamide) (PNIPAAm)is a temperature-sensitive polymer with many applications inmedicine and pharmacology (Guo & Gao, 2007; Shi, Alves, & Mano,2007; Yoo, Seok, & Sung, 2004; Zhang & Peppas, 2002). In thelast two decades, also copolymerization of polysaccharides withvinyl polymers (graft hydrogels) has gained much attention sincestimuli-responsive hydrogels with novel properties can be pre-pared in this way (Jenkins & Hudson, 2001; Krishnamoorthi, Mal, &Singh, 2007; Li, Wu, & Liu, 2008; Prabaharan & Mano, 2006; Sanl,Ay, & Isklan, 2007).

Synthesis and characterization of graft and semi-IPN hydrogelsof commercial sodium alginate with PNIPAAm have been pub-lished (Ju, Kim, Kim, & Lee, 2002; Kim, Lee, Kim, & Lee, 2002;Zhang, Zha, Zhou, Ma, & Liang, 2005a, 2005b). In these publica-tions, the authors used commercial alginates with a M/G ratio of1.56, which indicated an alginate enriched in mannuronic acid;however the composition in terms of hetero- and homopoly-meric blocks was unknown. It is important to know the extentof the homopolyguluronic acid block fraction in the polymersince this fraction is responsible for gel formation in the pres-ence of calcium ions. Thus, the use of homo-polymeric blocksobtained from sodium alginate would allow the preparation ofhydrogels with characteristic and dened properties. In previous

see front matter 2012 Elsevier Ltd. All rights reserved.rg/10.1016/j.carbpol.2012.09.031ation and characterization of hydrogelsum alginate and PNIPAAm

ala, Wim De Borggraeveb, Maria V. Encinasa, Bettyf Environmental Chemistry, Faculty of Chemistry and Biology, Universidad de Santiagosign and Synthesis, Department of Chemistry, KU Leuven, Celestijnenlaan 200F, bus 240

and Molecular Electronics, Kapeldreef 75, B-3001 Leuven, Belgium

e i n f o

ay 2012vised form 9 August 2012eptember 2012e 24 September 2012

teic fractions

a b s t r a c t

Graft copolymers were prepared by fo24,000), isolated from sodium alginarevealed the formation of a macroscodiameter of 1020 m. Semi-IPN hydrd-mannuronate (MW 21,000), isolateSEM micrographs showed porosities water containers, the water retentionof semi-IPN hydrogels. Both hydrogeand 45 C: the swelling ratio decreasecapacity of graft hydrogels increases w/ locate /carbpol

sed on homopolymeric fractions

tsuhiroa,, Robert Mllerc

ile, Av. B. OHiggins 3363, Santiago, Chile1 Heverlee, Belgium

ion of an amide bond between poly--l-guluronic acid (MWd the free amino group of PNIPAAm-NH2. SEM micrographstwork on the surface of the grafted hydrogels with a porosity

were prepared using different proportions of sodium poly-- sodium alginate and cross-linked PNIPAAm-NH2 polymers.

inor size (5 m). Though both types of hydrogels are goodacity of graft copolymers is more than 50% higher than thats showed signicant changes in swelling ratios between 20r the LCST of PNIPAAm. The water absorption and retentionH, reaching a maximum value at pH 7.0.

158 D. Leal et al. / Carbohydrate Polymers 92 (2013) 157 166

work, we performed the isolation of homopolymeric blocksof sodium alginate from the hybrid brown seaweed Lessonia-Macrocystis. Full characterisation of these homopolymers was doneby vibrational spectroscopy methods along with computational cal-culations (CThe aim ofgraft and seblock of soan amine gGraft hydrogroup to poence of Ca2

by the asso-d-manno

2. Materia

2.1. Materi

Poly--dacid (GG) bof sodium Macrocystis68250W)IsopropylacMO, USA) toluene (M(AESH), hydrochlorN,N-methySigmaAldr(CaCl2) wa2,2-azobisiMO, USA) wGermany). stead EasypTetrahydroDarmstadt,used witho

2.2. Synthecross-linked

Free radcarried out AIBN as phThe sample(Hamden, CIrradiation isopropylacAIBN concethe low moin ethyl eth

Along wsynthesized2 wt% MBA,merization.against dist(Spectrum Lprecipitatedwashed sevprecipitate

Averagewere deter25 C, using

chromatograph, equipped with three series of Styragel HR 1-2-4(Waters Co., Framingham, MA, USA) columns and THF as mobilephase with a ow rate of 1 mL min1. Differential refractometrywas used to detect the polymers, and monodisperse poly(methyl

crylaA, U

epar

sodPNIP

in wto lin

ratice tosly sted wueouilled

CitymenA).

epar

aqroniixed). Thtant

and

IR sp400 cclud

was nto t400.1C in ic exrime

of 0 sprobe0.90 ed usces. scop

a sp to gsionsnce 28

a nHz poment

M

surfannient

5 kV.ed innitroardenas-Jiron, Leal, Matsuhiro, & Osorio-Roman, 2011). the present work is to synthesize and characterizemi-IPN cross-linked hydrogels bearing a homopolymerdium alginate and poly(N-isopropylacrylamide) withroup at the end of the polymer chain (PNIPAAm-NH2).gels were synthesized by covalently linking this aminely--l-gulopyranuronic acid blocks (GG) in the pres-+ ions, meanwhile semi-IPN hydrogels were obtainedciation of the cross-linked PNIPAAm chains with poly-pyranuronic acid blocks (MM).

ls and methods

als

-mannuronic acid (MM) and poly--l-guluroniclocks were previously obtained by partial hydrolysisalginate from the hybrid brown seaweed Lessonia-

(LMH) collected in Tekenika Bay (551960S,, southern Chile (Cardenas-Jiron et al., 2011). N-rylamide (NIPAAm) (SigmaAldrich Co., St. Louis,was puried by recrystallization from n-hexane/erck, Darmstadt, Germany). 2-Aminoethanethiol1-ethyl-(3-3-dimethylaminopropyl) carbodiimide

ide (EDC), N-hydroxysuccinimide (NHS) andlenebisacrylamide (MBA) were purchased fromich Co. (St. Louis, MO, USA). Calcium chlorides purchased from Merck (Darmstadt, Germany).sobutyronitrile (AIBN) (SigmaAldrich Co., St. Louis,as recrystallized from methanol (Merck, Darmstadt,

Distilled water was puried using a Milli-Q Barn-ure II system (Thermo Scientic, Dubuque, IA, USA).

furan (THF), ethyl ether and acetonitrile (ACN) (Merck, Germany) were of spectroscopic grade and they wereut any further purication.

sis and molecular weight determination of linear and poly(N-isopropyl-acrylamide)

ical polymerizations of N-isopropylacrylamide werein ACN solution under N2 gas atmosphere at 25 C, usingotoinitiator. AESH was used as chain transfer agent.s were irradiated in a Rayonet photochemical reactorT, USA) equipped with a 360 nm irradiation source.times were xed to obtain conversions

D. Leal et al. / Carbohydrate Polymers 92 (2013) 157 166 159

Table 1Polymerization conditions, and average molecular weight Mn (g/mol) of linearpoly(N-isopropylacrylamide)-NH2.

Polymera AESH (mM) Mn (g/mol)b

P-1 P-2P-3 P-4 P-5

a Poly(N-isob The avera

poly(methylm

York, NY, Ubrass holdeto avoid cha

2.7. Swellin

To measimmersed iAfter removpaper, the wtimes. The serable chanswelling ratperature rafrom 2.0 toabove and tdition, hydr24 h and weples with equation:

Swelling rat

where Ws isular conditiof the hydro

3. Results

3.1. Synthepoly(N-isop

The mousing differfound that tion were a(AESH) contration. Undgroup at thsame orderfrom sodiuet al., 2011)thesized linpointed oudecreases mas expectedtransfer agPNIPAAm-Nand NMR spP-3 polymeand 2929.8 and alkane 1654.0 cm

0

a)

b)

c)

T-IR sm po

II (showsynthxhib

H Cn (

a qu & Ze1 pp

tively mete sig

N-ials ber i

hows (C 35.1rbonthinNo unsaturated carbon signals were observed indicating thee of monomer molecules.ss-linked PNIPAAm polymers were synthesized using N-hylenebisacrylamide (MBA) at 2% (w/w) as cross-linkerer in the free radical polymerization. These polymers were

terized by 1H NMR spectroscopy. Fig. 2c presents the 1Hpectrum of a cross-linked polymer of PNIPAAm, it showslar prole to that obtained for linear chains. Furthermore,w intensity peaks between 2.85 and 2.50 ppm could beed; these signals are assigned to protons of the ethylene( CH2 CH2 ) from the chain transfer agent (AESH) (Kim002) as shown in the insert. Along with this, Fig. 2d shows0.0 35,7009.1 31,000

18.1 930020.0 900020.0 8900

propylacrylamide-NH2).ge molecular weight was determined by GPC using monodisperseethacrylate) with Mn from 1580 to 52,500 g/mol.

SA) for 48 h. The freeze-dried hydrogels were xed onrs and coated by sputtering with a thin Au layer in orderrging effects.

g properties and stimuli response of hydrogels

ure the swelling ratio, pre-weighed dry samples weren a CaCl2 solution (0.1 M) at room temperature (25 C).ing the water excess on the sample surface using a ltereight of the swollen samples was measured at differentwelling ratio was monitored until there was no consid-ge in weight of the hydrogel samples. In addition, theios of the hydrogel samples were measured in the tem-nge from 20 to 45 C (at pH 5.4) and in the pH range

7.0 (at 25 C) using the gravimetric method describedhe same medium conditions. Under each particular con-ogel samples were incubated in the medium for at leastighed after removing the water excess from the sam-

lter paper. Each swelling ratio was calculated using the

io = Ws WdWd

(1)

the weight of hydrogel in the swollen state at a partic-on (time, pH or temperature), and Wd is the dry weightgel.

and discussions

sis of linear and cross-linkedropylacrylamide)

lecular weight of PNIPAAm polymers was regulatedent concentrations of chain transfer agent, AESH. It wasthe optimal conditions for the polymerization reac-

non-protic polar solvent like acetonitrile, the thiolcentration around 20 mM, and a low monomer concen-er these conditions, polymers with an amine terminale end of polymer chain and a molecular weight in the

as the homopolymeric fractions (MM and GG) isolatedm alginate of LMH could be obtained (Cardenas-Jiron. Table 1 shows the molecular weights of various syn-ear PNIPAAm polymers obtained by GPC. It can be

t that the average molecular weight of the polymers

400

Arb

itra

ry u

nit

s(a

.u.)

Fig. 1. F(b) sodiu

amide and b of the trum eto thefunctiodue toMeier,and 1.5respecthe sixof thesbond inof signmonomtrum scarbon42.20, lene cathe metively. absenc

CroN-metmonomcharacNMR sa simitwo loobservgroup et al., 2arkedly with the increase in the AESH concentration from the high efciencies of aliphatic thiols as chainents (Valdebenito & Encinas, 2010). The synthesizedH2 polymers were characterized by infrared (FT-IR)ectroscopies. Fig. 1a shows the FT-IR spectrum of the

r, it presents the characteristic bands at 3436.8, 3314.6cm1 assigned to primary amino (N H), amide (N H)(C H) groups, respectively, along with two bands at1 and 1544.00 cm1 assigned to amide I (C O) and

the 1H NMwithout cha2.50 ppm cofer agent in

3.2. Synthe

A comb-bond betw3500 3000 2500 2000 1500 1000 500

Wavenumber (cm-1 )

pectra in the 4000400 cm1 region of: (a) P-3 PNIPAAm-NH2 (lm),ly--l-guluronate (in KBr), (c) HGG-2 graft hydrogel (lm).

N H), vibrations, respectively (Conley, 1966). Fig. 2as, respectively, the 1H and 13C NMR spectra in D2Oesized PNIPAAm-NH2 polymer P-3.The 1H NMR spec-ited a broad signal at 3.82 ppm which was assigned

proton of the isopropyl group linked to the amideCH NH ); the broadening of the signal is probablyadrupolar effect of the neighbor nitrogen atom (Hesse,eh, 1997). The broad and low intensity signals at 1.94m correspond to the methine and methylene protons,, and the intense signal at 1.07 ppm was assigned to

hyl protons of the isopropyl group. The chemical shiftsnals are indicative for the transformation of the vinylsopropylacrylamide residues; furthermore, the absenceetween 5.0 and 6.0 ppm indicates the absence of vinyln the dialyzed samples. In addition, the 13C NMR spec-

a signal at 175.66 ppm that corresponds to the carbonylO) of the amide groups along with three more signals at6 and 22.07 ppm assigned to the methine and methy-s ( CH2 CH ) of the regular polymer chain, and toe carbon ( CH NH ) of the isopropyl group, respec-R spectrum of the PNIPAAm polymer P-1 synthesizedin transfer agent. No proton signals between 2.85 anduld be observed, indicating the absence of chain trans-side the polymer structure.

sis of graft copolymer, GG-g-PNIPAAm

type hydrogel was prepared by formation of an amideeen the carboxyl group of poly-l-guluronic acid (MW


Fig. 2. NMR sp 1 (400 M 13

of P-3 of linear ing MBchain transfer

24,000) andNH2 using Eas shown ithe synthethe reactivethe carboxy

lar w limiectra of synthesized polymers of PNIPAAm in D2O and 25 C. (a) H NMR spectrum PNIPAAm-NH2. (c) 1H NMR spectrum (400 MHz) of cross-linked PNIPAAm-NH2 us

agent.

the free amino group of samples P-3 to P-5 of PNIPAAm-DC to activate the carboxyl group in the polysaccharide

molecuas then Scheme 1. The reactive amounts employed duringsis are listed in Table 2. It can be pointed out that

amounts used in Table 2 were calculated on basis oflate groups present in the polysaccharide using the

percentageIn additionthe actual ron the nit

Scheme 1. Formation of graft copolymer of GG and linHz) of P-3 of linear PNIPAAm-NH2. (b) C NMR spectrum (100 MHz)A. (d) 1H NMR spectrum (400 MHz) of linear PNIPAAm (P-1) without

eight of it and the initial mass. Thus, NHS was usedting reagent in order to obtain a certain conjugation of polymer chains into the polysaccharide (Table 2)., elemental analysis of nitrogen was used to determineeaction composition as shown in Table 2. An increaserogen content can be observed according with the

ear PNIPAAm-NH2.


Table 2Reactive amounts of poly--l-guluronic acid fraction and linear semi-telechelic PNIPAAm-NH2 in the formation of graft copolymers.

Hydrogel GGa (mg) EDCb (mg) NHSc (mg) Polymer (mg) Recovery massd (mg) N content (wt%)

HGG-1 100 97 15 100 110 1.34HGG-2 100 97 29 100 HGG-3 100 97 44 100

a Homopolymeric blocks of the sodium salt of poly--l-guluronic acid.b 1-Ethyl-(3-3-dimethylaminopropyl) carbodiimide hydrochloride.c N-Hydroxysuccinimide.d Of the hydrogel.

reaction conditions, indicating the increase on the conjugation leveland the introduction of PNIPPAAm-NH2 on the gel structure. Theseresults are in agreement with those reported by Ju, Kim, and Lee(2001) for alginic acid hydrogels conjugated to PNIPAAm polymers.Three different grafted copolymers were obtained and character-ized by spectroscopic methods. Fig. 1b shows the IR spectrum ofthe sodium salt of polyguluronic acid, obtained by partial depoly-merization of sodium alginate, which presents characteristic bandsat 3427.5 cm1 (O H), 2930.7 cm1 (C H), 1614 cm1 (C O ofcarboxylate group), and at 816.67 cm1 assigned to C O H ringand C H of -anomeric linkage vibrations (Chanda et al., 2001;Leal et al., 2008). The IR spectrum (Fig. 1c) of the graft polymershows signals at 3276.6 cm1 (N H, amide), 2923.6 cm1 (C H),1623.4 cm1 (C O, amide I and carboxylate), 1537.2 cm1 (N H,amide II), and at 809.7 cm1 assigned to C O H ring and C H of-anomeric

Fig. 3 shIn the carboat least threcarbon atomand of the cthe third otion, at 174102 and 65Torres, & Gand 22 ppmdimensionastructure oof the 13C in Fig. 3.

Fig. 3. 13C NM

3.3. Preparwith cross-l

The semtions of hom(MW 21,00medium wof polymerpoly--d-mcondition aNH2 with tThree semi-of 20/80 pe(90 wt% yie

orphydro

hydnd thn mperl strm-Nond

on in GG sre o

with g. 4c--d, 200olymf PN

ty onrces re aM mthe fith aed in5c an

of eaophilylic t in linkage vibrations.ows the 13C NMR spectrum of HGG-2 graft copolymer.nyl region (170180 ppm) it is possible to distinguishe carbon peaks, two of them assigned to the carbonyls of the amide group of PNIPAAm-NH2 (174.25 ppm)arboxylic group of poly--l-guluronate (175.65 ppm),ne due to the amide group formed in the conjuga-.92 ppm. Moreover, the characteristic signals between

ppm assigned to poly--l-guluronic acid (Matsuhiro,uerrero, 2007; Leal et al., 2008) and between 43

assigned to PNIPAAm-NH2 chains are found. Two-l NMR spectra were registered to conrm the nef the grafted copolymer hydrogels, and assignmentsand 1H NMR spectra were accomplished as shown

3.4. MHMM h

TheGG), aelectroand suinternaPNIPAAcorresptribution thestructuAlong ple (Fiof polyWelshthetic pof P-3 oporosition fostructuthe SEreveal face wobservin Fig. graphsa hydrcarboxpresenR spectrum (150 MHz) at 25 C of the graft copolymer HGG-2 in D2O.

aqueous so(Prabaharangraphs of Hregularity apolysacchastructure (Gformation otion of the p116 2.50152 3.35

ation of semi-interpenetrating mixture of MM blocksinked PNIPAAm

i-IPN hydrogels were prepared using different propor-opolymeric blocks of sodium poly--d-mannuronate

0) and cross-linked PNIPAAm polymers. The pH of theas regulated at 4.0 where the amine groups at the end

chain are protonated and the carboxylic groups of theannuronate (pKa, 3.39) are deprotonated as well. Thisllows the electrostatic interactions of the PNIPAAm-he poly-mannuronate chains as shown in Scheme 2.IPN hydrogels were obtained with MM/PNIPAAm ratiosrcent HMM-1 (95 wt% yield), HMM-2 with a 50/50%ld), and HMM-3 with 80/20% ratio (96 wt% yield).

ologic characterization of graft HGG and semi-IPNgels by scanning electron microscopy

rogel samples, the homopolymeric blocks (MM ande PNIPAAm-NH2 polymer were analyzed by scanningicroscopy (SEM) in order to obtain the morphologycial proles. Fig. 4 shows the SEM micrographs of theucture of the homopolymeric blocks along with theH2 polymer. The images at different magnications

to the homopolymeric blocks showing the laminar dis- both structures. Small folds can be observed at 2000ample (Fig. 4 b) which may be assigned to the tertiaryf the poly--l-guluronic acid chains as pleated sheets.this, the laminar distribution observed in the MM sam-

and d) is probably attributed to the tertiary structure-mannuronic acid chains as extended sheets (Rees &3). A completely different pattern was found for the syn-er, PNIPAAm-NH2. The SEM micrographs (Fig. 4e and f)

IPAAm-NH2 polymer show a dense surface with a lack of it. This is indicative for the contribution of weak attrac-on the interaction between chains leading to a compactcross the entire analyzed sample. Fig. 5a and b showsicrographs of the HGG-1 graft hydrogel. The imagesormation of a macroscopic network on the sample sur-

porosity diameter of 1020 m. This porous pattern is the rest of the synthesized grafted hydrogels as shownd d from HGG-2 and is not observed in the SEM micro-ch hydrogel component (Fig. 4). According to literature,ic dipole is generated between the non-functionalizedgroups of the polysaccharide with the amide groupsthe synthetic polymer which allow the interaction of

lutions and the inside of the hydrogel polymer network

& Mano, 2006). Fig. 5e and f shows the SEM micro-GG-2 cross-linked hydrogel with Ca2+ ions. The porouslong the structure is due to the arrangement of the

ride chains around the Ca2+ ions, forming the eggs boxrant et al., 1973). It can be pointed out that the poren the graft hydrogel surface depends on the interac-olysaccharide chains with the PNIPAAm-NH2 polymers


Scheme 2. Formation of semi-IPN hydrogel of MM and cross-linked PNIPAAm-NH2.

Fig. 4. SEM micrographs of the homopolymeric blocks (MM and GG) and semi-telechelic PNIPAAm-NH2 polymer: (a) GG at 500, (b) GG at 2000, (c) MM at 500, (d) MMat 1000, (e) PNIPAAm-NH2 at 500, and (f) PNIPAAm-NH2 at 2000.


Fig. 5. SEM mHGG-2 at 1000

as shown ithe polysacregularity aanalysis of

Fig. 6. SEMicrographs of graft hydrogels HGG-1, and HGG-2 along with the HGG-2 cross-linked wit, (d) HGG-2 at 2000, (e) HGG-2-Ca at 500, and (f) HGG-2-Ca at 2000.

n Fig. 5d. Calcium ions allow a better distribution ofcharide chains resulting in an improvement in the porecross the sample. In agreement with the morphologicthe grafted hydrogels, the most regular arrangement

observed coSEM microand HMM-gels varies

micrographs of semi-IPN hydrogels HMM-2, and HMM-1: (a) HMM-2 at 1000, (b) HMh Ca2+ ions (HGG-2-Ca): (a) HGG-1 at 500, (b) HGG-1 at 2000, (c)

rresponds to HGG-2 hydrogel (Fig. 5d). Fig. 6 shows thegraphs of semi-IPM hydrogels HMM-2 (Fig. 6a and b)1 (Fig. 6c and d). The pore distribution in these hydro-with respect to the graft hydrogels. The electrostatic

M-2 at 5000, (c) HMM-1 at 2000, and (d) HMM-1 at 10,000.


Fig. 7. Swellin0.1 M CaCl2 so

interaction NH2 and thepores of rein graft hydrounding ethe interactpolysaccharthe semi-IP

3.5. SwellinHGG and se

The mechecked by solution ovecated in Eq.is an index ment of theIn case of gris attributedparison, algalginate froJiron et al.,ratio of HGGnate microg

with the increase of polymer chains bound to the HGG polysac-charide. HGG-3 showed the highest and calcium alginate had thelowest swelling ratio. In addition, these synthesized graft hydrogelsshowed a remarkable increment in the water retention capacity

comtics bellinal co

grao theand,els ii-IPN

in F thelar forthyty coe tocks ily de; thisng) inM hyinor els iate wtionpolyvaluesacterisThe swexternfor themers tother hhydrogin semshowntainingparticunotewcapaciably duGG blotion onchains(graftiin HMThe mhydrogdomininteracof the g kinetics of (a) graft HGG, and (b) semi-IPN HMM hydrogels in alution at pH = 5.4 and 23 C.

between the protonated amino groups of PNIPAAm- carboxylate groups from poly-mannuronate generates

duced size (5 m) in comparison to those observedrogels. Moreover, satellite shapes can be seen sur-very pore (Fig. 6b and d) which is probably due toion of the polymeric chains of PNIPAAm-NH2 with theide surface. Interestingly, this pattern is observed in allN hydrogels.

g properties and external stimuli response of graftmi-IPN HMM hydrogels

chanical behavior of the synthesized hydrogels wasanalyzing their absorption capacity of an aqueous CaCl2r a controlled period of time. The swelling ratio as indi-

(1) is indicative for this absorption capacity. This ratioof the water absorption capacity along with the incre-

hydrogels volume and the hydrogel resistance in time.aft hydrogels, the formation of the cross-linked network

to the presence of Ca2+ ions in the medium. For com-inate microgels were obtained by treatment of nativem the hybrid seaweed Lessonia-Macrocystis (Cardenas-

2011) with calcium ions. Fig. 7a shows the swelling graft hydrogels at 23 C and pH 5.4, and calcium algi-els. The swelling ratio of HGG graft hydrogels increases

groups of tha better intthe swellingels, chainscovalent linof free hydring aqueousolutions inpared with retention cato semi-IPNlinked polycross-linkingels, the cohydrogels rits the watecan be poinhydrogels iand a remagels it is noionic interathus increaa consideraels of conjuby these hyrigidity of HHGG-3 andexpansion)gels spherbe inferredto the stiffnble explanacan be pointers observthe observehydrogel sha higher inpared with reported data for hydrogels of similar char-ut using commercial sodium alginate (Ju et al., 2001).g ratios of semi-IPN hydrogels (HMM) under the samenditions are shown in Fig. 7b. Similarly to that describedft hydrogels, the incorporation of the synthetic poly-

network increases the water retention capacity. On the the water equilibrium inside of the network of the grafts reached after 2 h of exposition while the equilibrium

hydrogels is reached after 3045 min of exposition asig. 7b. These results indicate that both microgels, con-

homopolymer blocks, are good water containers witheatures compared with the native polysaccharide. It is

that calcium alginate showed lower water absorptionmpared with graft and semi-IPN hydrogels. This is prob-

the polysaccharide arrangement, where the amount ofs lower than 50% of the composition and the gel forma-pends on the calcium coordination among the alginate

leads to a cross-linked structure but the conjugation HGG hydrogels or the semi interpenetrating networkdrogels improved on the polymers network swelling.period of time to reach the equilibrium in semi-IPNs probably due to the nature of the interactions that pre-ithin the polymeric network. Since polymeric chains

s are electrostatic type, between carboxylate groups--d-mannuronate chains and the protonated aminee PNIPPAAm-NH2 chains, the hydrophilic setting allowseraction with aqueous solutions which leads to reachg equilibrium in less time. In contrast, on graft hydro-

from different nature interact between them due toks after the conjugation which reduces the availabilityophilic carboxylate groups to interact with the incom-s solutions. This leads to slower retentions of aqueousside the polymeric network for graft hydrogels com-

semi-IPN hydrogels. On the other hand, the higher waterpacity observed in graft hydrogels (Fig. 7a) with respect

hydrogels is probably due to the rigidity of the cross-mer chains of PNIPAAm in these hydrogels. Unlike theg with calcium ions on the GG chains in graft hydro-valent cross-linking of PNIPAAm chains in semi-IPNestricts the polymeric network expansibility and lim-r retention capacity of the three semi-IPN hydrogels. Itted out that the water retention capacity of semi-IPNs more than 50% less compared with graft hydrogelsrkable difference among the set of semi-IPN hydro-t observable. In the case of graft hydrogels HGG, thections with calcium ions generate greater exibility andse the retention capacity. Moreover, it can be observedble increment on the retention capacity at higher lev-gation despite the higher equilibrium times reacheddrogels compared with semi-IPN types. However, theGG hydrogels is affected at higher levels of conjugation;

HGG-2 samples presented a higher exibility (network and therefore a more complex handling of the hydro-es compared with the most rigid sample, HGG-1. It can

that the conjugation degree is inversely proportionaless of the synthesized hydrogels. In addition, a possi-tion of the major retention capacity of graft hydrogelsted from the analyzed SEM images. The porous diame-ed in the graft hydrogels (Fig. 5d and f) are bigger thand to the semi-IPN samples (Fig. 6b and d). The HGG-2ows a porous diameter close to 10 m which involvesux of water inside the polymeric network compared to


the semi-IPN HMM-1 hydrogel, whose porous diameter is around5 m.

The effect of the temperature and pH on the water retentioncapacity of graft (HGG) and semi-IPN (HMM) hydrogels was evalu-ated in ordestimuli. Thefunction of both guresincrementssignicant cature rangesolution temA more drason grafted hvalues lowepresent expwater retendue to the pnetwork in ity of thesePNIPAAm, tdecreases. PNIPAAm c2006; Guo in Fig. 8a awith highermore prono4045 C thlower compwith PNIPAbe explaineHGG-3 andpolymer nedecrease inbic forces tnetwork.

Fig. 8c shgels (HGG) were dissol(0.1 M), andwith incremthe water athan that ation capacitat pH 7.0. Tues, probabnetwork. Thhave large aare protonaation of hyrestrained the incremeerate a strehydrogen bpolysaccharincreased antions aboveFurthermorpears at acidand inducesgels with aand HGG-2HGG-3 hydrgroups presmers. At pHleading to

welling ratios as function of temperature and pH of graft and semi-IPNls. Temperature effect (2045 C) in a 0.1 M CaCl2 solution at pH = 5.4 on (a)G hydrogels, and (b) semi-IPN HMM hydrogels swelling ratios. (c) pH effect) in a 0.1 M CaCl2 solution at 25 C on graft HGG hydrogels swelling ratios.r to obtain the swelling ratios behaviors under external swelling equilibrium of HGG and HMM hydrogels as atemperature is shown in Fig. 8a and b, respectively. In, the temperature range used was from 20 to 45 C with

of 5 C per sample measurement. Both hydrogels showhanges in the swelling ratios in all the studied temper-. The swelling ratio decreases nearby the lower criticalperature (LCST) of the PNIPAAm polymers at 3233 C.

tic change of the capacity of water retention is observedydrogels than on semi-IPN hydrogels. At temperaturer than LCST (2032 C), the PNIPAAm polymer chainsandable and hydrophilic characteristics allowing thetion capacity. Along with this, hydrophilic interactionsresence of free carboxylate groups inside the polymerboth hydrogel types help on the water retention capac-

materials. At higher temperatures, close to the LCST ofhe water absorption capacity in both hydrogel typesThis effect has been attributed to the collapse of thehains in the polymer network (De Moura et al., 2005,& Gao, 2007), and is in agreement with data shownnd b where the observed changes for the hydrogels

content of PNIPAAm, HGG-3 and HMM-1, shows theunced phase transition. At temperatures in the rangee water absorption capacity of HGG-3 and HMM-1 isared to that reported for hydrogels based on alginateAm (Zhao, Cao, Li, Li, & Xu, 2010). This behavior couldd considering the polysaccharide/polymer ratio, since

HMM-1 present higher amounts of PNIPAAm on thetwork; in these hydrogels the hydrophilic interactionsside the polymer network and encourage hydropho-hat release water outside of the cross-linked polymer

ows the water absorption dependence of graft hydro-with pH changes from 2.0 to 7.0. Hydrogels samplesved in water and then immersed in a CaCl2 solution

the pH was regulated with a NaOH solution (0.1 M)ent of 1.0 unit in the pH scale. These results show that

bsorption capacity of graft hydrogels at pH 2.0 is lowert pH 5.4. In addition, the water absorption and reten-y increases at higher pH values with a maximum valuehe water absorption increases with increasing pH val-ly due to the ionic nature inside the hydrogel polymere poly--l-guluronate (GG) chains in graft hydrogelsmounts of carboxylate groups which at acidic pH valuested (guluronic acid pKa = 3.2). This allows the gener-drophobic points along the polymer network whichthe entry of water into the network in addition withnt of the interactions by hydrogen bonds which gen-ngthening and avoiding the network expansion. Thus,ond interactions between carboxylic acid groups fromide and amide groups from the synthetic polymer ared then, lead to higher polymerpolysaccharide interac-

polysaccharidewater or polymerwater interactions.e, charge repulsion among carboxylate groups disap-ic pH values which mean that polymer network shrinks

the low water absorption at pH < 4.0. Moreover, hydro- lower degree of PNIPAAm conjugated chains (HGG-1) present lower water absorption capacity than that ofogels. This could be explained by the lower carboxylateent in HGG-3respect to HGG-1 and HGG-2 graft copoly-

values >4.0, the amount of carboxylate groups increasescharge repulsion interactions among these functional

Fig. 8. Shydrogegraft HG(2.07.0


groups, instead of hydrogen bond interactions. In consequence,the predominance of electrostatic repulsions leads to the networkexpansion and then, the water absorption capacity increases. Themaximum value is reached at pH 7.0.

On the other hand, semi-IPN HMM hydrogels collapsed at pHvalues between 2.0 and 5.0, probably due to the low electrostaticforce exerted by carboxylate groups and protonated amine groupson the polypact hydrogunmanagea

4. Conclus

A carefuthe synthesweights.

Graft hysuperior wathe same eretention c(graft HGG)swelling beswelling raabrupt chancorporal tehydrogels sthe pH increIPN hydrogand are stab

From ththat copoly(GG) with Presponsive

Acknowled

Financiagratefully aship (D-210and a fellowthank Dr. MBuenos Aire

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