Electron Beam Synthesis and Characterization of Acrylamide/Acrylic Acid Hydrogels Using ...

15 Pages·2016·7.56 MB·English

Hindawi Publishing Corporation
Journal of Chemistry
Volume 2016, Article ID 1470965, 14 pages
http://dx.doi.org/10.1155/2016/1470965
Research Article
Electron Beam Synthesis and Characterization of
Acrylamide/Acrylic Acid Hydrogels Using Trimethylolpropane
Trimethacrylate as Cross-Linker
GabrielaCraciun,1ElenaManaila,1andMariaDanielaStelescu2
1NationalInstituteforLaser,PlasmaandRadiationPhysics,ElectronAcceleratorsLaboratory,409AtomistilorStreet,
077125Magurele,Romania
2LeatherandFootwearResearchInstitute,NationalR&DInstituteforTextileandLeather,93IonMinulescuStreet,
031215Bucharest,Romania
CorrespondenceshouldbeaddressedtoMariaDanielaStelescu;[email protected]
Received15November2015;Accepted16February2016
AcademicEditor:BarbaraGawdzik
Copyright©2016GabrielaCraciunetal. This is an open access article distributed under the Creative Commons Attribution
License,whichpermitsunrestricteduse,distribution,andreproductioninanymedium,providedtheoriginalworkisproperly
cited.
Thepurposeofthepaperistopresentthesynthesisandcharacterizationofhydrogelspreparedbyfree-radicalcopolymerization
ofacrylamideandacrylicacidinaqueoussolutionsusingpotassiumpersulfateasinitiatorandtrimethylolpropanetrimethacrylate
as cross-linker, via the radiation technique. The samples were subjected to electron beam treatment in the dose range of 2 to
4kGyandtheinfluenceoftheabsorbeddoseandamountofcross-linkerontheswellingproperties,diffusioncoefficient,and
networkparametersofhydrogelswasinvestigated.Apossiblereactionmechanismforacrylamide/acrylicacid/trimethylolpropane
trimethacrylatehydrogelswasalsosuggested.ThestructureandmorphologyofhydrogelswerecharacterizedbyFourierTransform
InfraredSpectroscopyandScanningElectronMicroscopy.
1.Introduction dependsonthepolymer-waterinteractionparameterandthe
cross-link density [5, 7]. In the reversible or physical gel,
Hydrogels are three-dimensionally cross-linked hydrophilic
thenetworksareheldtogetherbymolecularentanglements
polymers capable of swelling and retaining huge volume in
and/orsecondaryforcesincludingionic,hydrogenbonding
theswollenstate,evenunderpressure.Thesemacromolecule
or hydrophobic interactions. In the physically cross-linked
networks can absorb water, many hundreds of times more
gels,dissolutionispreventedbyphysicalinteractionswhich
than their dried weight. Due to their unique characteristics
existbetweendifferentpolymerchains[5,7].
like hydrophilicity, swelling in aqueous media, nonsoluble
Thevarioustechniquesadoptedforhydrogelspreparation
natureinaqueousfluids,andionicaspects,theyareappliedin
are the physical and chemical cross-linking, the grafting
biomedicine,bioengineering,pharmaceutical,foodindustry,
polymerization, and the radiation cross-linking [5, 8–10].
or agriculture [1–3]. Because of their excellent response to
The initiation of chemical reactions by using radiation is
changingoftheenvironmentconditionssuchastemperature,
increasingly used for novel hydrogels obtaining. The radi-
pH,andsolventcomposition,hydrogelparticlesdistributed
ation technique is more preferable than the chemical one,
in soil are capable of absorbing water and then slowly
releasing it from the polymeric matrix [1, 4]. Hydrogels because of the advantage offered by the gentle control of
are broadly classified into two categories: permanent or cross-linking level by variation of the absorbed dose. It is
chemical gel and reversible or physical gel. Permanent or a simple additive-free process which is happening at any
chemicalgelisacovalentlycross-linkednetworkobtainedby temperature; the reactions such as polymerization, cross-
replacinghydrogenbondwithastrongerandstablecovalent linking, and grafting can be easily controlled and the treat-
bond[5,6].Theyreachanequilibriumswellingstatewhich mentcanbelimitedtoaspecificarea[11].Theprocessingof
2 JournalofChemistry
Table1:Thematerialsusedforthepreparationofthehydrogels.
Chemical
Materials Chemicalstructure
characteristics
(i)Molecularweight:71.08g/mol O
3
(ii)Density:1.13g/cm
Acrylamide(AMD) ∘ H2C CH C
(iii)Meltingpoint:84.5C
∘ NH2
(iv)Solubilityinwater:2.04kg/Lat25C
(i)Molecularweight:72.06g/mol
(ii)Density:1.051g/cm3 O
Acrylicacid(AA) (iii)Meltingpoint:14∘C H2C CH C
(iv)Boilingpoint:141∘C NH2
(v)Solubilityinwater:miscible
(i)Molecularweight:270.322g/mol
3
(ii)Density:2.477g/cm
Potassiumpersulfate ∘
(PP) (iii)Meltingpoint:14∘C K2S2O8
(iv)Boilingpoint:141C
∘
(v)Solubilityinwater:1.75g/100mLat0C
O CH3
O C C CH2
(i)Molecularweight:338.4g/mol CH2
Ttrriimmeetthhayclroylplartoepane (ii)Boilingpoint:>2003∘C H2C C C O CH2 C CH2 CH3
(iii)Density:1.06g/cm
(TMPT) (iv)75±3%activeingredient CH3O CH2
O C C CH2
O CH3
materials by irradiation with accelerated electrons removes acid (AA) as comonomers, potassium persulfate (PP) as
many drawbacks of the conventional technologies, because initiator,andtrimethylolpropanetrimethacrylate(TMPT)as
ionizing radiation initiates polymerization without thermal cross-linker. All reagents were obtained from LACHEMA,
inputfromtheoutside,duetofreeradicalsthatareformed Germany,andwereuseddirectly,withoutpurification.
at the interaction with the monomers and especially with Other chemicals used in this study for physical and
the solvent (water in this case). So, a good solution to chemicalcharacterizationofthehydrogelswereofanalytical
producepolymericmaterialsistouseionizingradiationand grade.
especially electron beams, which direct their energy in the
entirevolumeofthemonomericsolutionstobeirradiated.
2.2. Experimental Installation and Sample Preparation. The
The purpose of this study is to present the synthesis
experimentswerecarriedoutusingtheelectronlinearaccel-
and characterization of some hydrogels prepared by free-
erator called ALIN 10 from the NationalInstitute for Laser,
radical copolymerization of acrylamide and acrylic acid in
PlasmaandRadiationPhysics.TheALIN10linearaccelerator
aqueous solutions using potassium persulfate as initiator
isatravelling-wavetype,operatingatawavelengthof10cm
andtrimethylolpropanetrimethacrylateascross-linker.The
andhaving164Wmaximumoutputpower.Theaccelerating
free-radicalcopolymerizationwasrealizedbyelectronbeam
structure is a disk-loaded tube, which operates in the 𝜋/2
irradiation in the dose range of 2 to 4kGy in atmospheric
mode. The optimum values of the electron beam, peak
conditions and at room temperature. The cross-linker was
usedinconjunctionwiththeradicalcuresystemsinorderto current𝐼EBandEBenergy𝐸EBtoproducemaximumoutput
improvephysicalpropertiesofthefinalproduct.Apossible power 𝑃EB for a fixed pulse duration 𝜏EB and repetition
reaction mechanism is proposed. The obtained hydrogels frequency𝑓EB,areasfollows:𝐸EB = 6.23MeV,𝐼EB = 75mA,
wereinvestigatedthroughswellinganddiffusionexperiments and 𝑃EB = 164W (𝑓EB = 100Hz, 𝜏EB = 3.5𝜇s). The EB
effects are related to the absorbed dose (𝐷) and absorbed
andalsousingtheinfraredspectroscopyinordertoevaluate doserate(𝐷∗).Theabsorbeddoseisthemajorparameterin
thechemicalstructureoftheobtainednetworks.Thesurface
theelectronbeam(EB)irradiation[12].Theperformancesof
appearance was also observed using the technique of Scan-
polymerizationandcopolymerizationprocessesareprovided
ningElectronMicroscopy.
bythestrictcontrolofthisparameter[12].Inourexperiments
theelectronbeamdoseratewasfixedat2kGy/min,inorder
2.Experimental toaccumulatedosesbetween2and4kGy.Theabsorbeddose
wasdeterminedusingtheconventionalFrickedosimeter.The
2.1.Materials. Thematerialsusedfortheobtainingofhydro- thicknessof2.5cmoftheirradiatedsampleswasestablished
−3
gels are shown in Table 1: acrylamide (AMD) and acrylic in accordance with the samples densities (of about 1gcm
JournalofChemistry 3
Table2:Thehydrogelssynthesisdetails.
Amountofchemicals(mol/L)
Samplescodes Irradiationdose(kGy)
AMD AA PP TMPT
H ,H ,H 5 0.5 3.70×10−3 2.95×10−3 2,3,4
1-1 1-2 1-3
H ,H ,H 5 0.5 3.70×10−3 5.90×10−3 2,3,4
2-1 2-2 2-3
ianquoeuoruscassoel)utainodnsEwBereeneprrgeyp.aFroedr i(rTraadblieati2o)n.,Thtweostoylpuetisonosf H2O EB irradiation H∙(e−aq), HO∙,H2,H2O2,H2O∗
were placed in polyvinylchloride (PVC) containers of 3cm S2O82− EB irradiation 2SO4∙−
diameter and irradiated in atmospheric conditions and at O O
roomtemperatureof25∘Cwith2kGy,3kGy,and4kGy.After O ∗ ∙
irradiation, the obtained hydrogels were cut into pieces of EB irradiation NH2 NH2
3±4mmlength,driedinairfor3daysandinalaboratory NH2 ∗ O ∙ O
∘ Acrylamide
ovenat50 Cfor12hourstoconstantweight,andthenstored
NH2 NH2
in desiccators. The dried hydrogels were used to determine
theparametersofswelling,diffusion,andnetwork. Excited state Free radicals
of AMD
O O
2.3.SampleCharacterization
O ∗ ∙
EB irradiation OH OH
2.3.1. Fourier Transform Infrared (FTIR) Spectroscopy Anal-
ysis. In order to obtain spectral information regarding the OH ∗ O ∙ O
Acrylic acid
chemicalstructureofhydrogels,aFTIRspectrophotometer,
OH OH
JASCOFT/IR4200,wasusedbyATRmeasurementmethod.
Samples spectra are the average of 30 scans realized in Excited state Free radicals
absorption,intherangeof4000–600cm−1,witharesolution of AA
−1
of4cm . Scheme1:Theschemeofpossiblereactionmechanismforobtaining
free radicals by electron beam irradiation from a system formed
of water, potassium persulphate, and monomers (acrylamide and
2.3.2. Experiments of Swelling and Diffusion. The swelling
acrylicacid).
of dried hydrogels was carried out by immersion in doubly
distilled water at 25 ± 0.1∘C in a water bath, in order to
determine the parameters of swelling and diffusion. The
amount of absorbed water was determined by weighing
the samples, after wiping with filter paper, at various time similar to classic polymerization through addition of free
intervals. The swollen hydrogels were weighed using an radicals.Theinitialphaseofpolymerizationinvolves,inboth
electronicbalance(HR200,0.1mgresolution). types of polymerizations, classic and induced by ionizing
radiations, the introducing of a certain amount of energy
in the system, the initiating energy. In the case of radio-
2.3.3.ScanningElectronMicroscopy(SEM). Themorphology
induced polymerization, the initiating energy is introduced
ofthelyophilizedhydrogelswasdeterminedusingascanning
byionizingradiationandinvolvestheradicalformation.The
electron microscope (FEI/Phillips, USA). Small pieces of
advantageofradio-inducedcopolymerizationisthattheradi-
swelledgelswerefreeze-driedinordertoavoidthecollapse
calscanbeformedbydecompositionofallcomponentsofthe
ofporousstructure.Freeze-dryingwascarriedoutinvacuum
at−80∘Cfor40husingtheChristAlfa2–4(MartinChrist, system:solvent(inthiscase,water),initiator,andmonomers
[12–14]. A possible reaction mechanism for obtaining free
Germany)lyophilizer.Forscanning,thelyophilizedsamples
radicalsbyelectronbeamirradiationfromasystemformed
havebeencutinordertoexposetheinnersurface.Samples
ofwater,potassiumpersulphate,andmonomers(acrylamide
were placed on an aluminum mount, sputtered with gold
andacrylicacid)isgiveninScheme1.
palladium,andthenscannedatanacceleratingvoltageupto
30kV. Themainfunctionoftheionizingradiationistoachieve
the first step of the polymerization process: the initiation
phaseleadingtofreeradicalformationandtosomespecific
3.ResultsandDiscussion
side effects. The next polymerization phases, propagation,
3.1.SynthesisandReactionMechanismAspects. Inourstudy, completion, and the chain transfer process, occur almost
the EB radiation technique was used in order to prepare identically as in classic polymerization processes. Based on
AMD/AA hydrogels in the presence of TMPT as cross- our results and literature studies [2, 15], a possible reaction
linker. The initiating of chemical reactions by irradiation is mechanism for the copolymerization of acrylamide and
usuallydoneusingionizingradiationswithenergiesranging acrylic acid by electron beam irradiation is proposed in
from 0.5 to 10MeV. Radio-induced polymerization is quite Scheme2.
4 JournalofChemistry
R∙ + H2C CH EB R CH2 C∙H + H2C CH EB R H2C CH CH2 C∙H
CONH2 CONH2 COOH CONH2 COOH
AMD/AA copolymer
EB
∙ ∙
CH COOH CH CONH2 AMD or AA + R H2C C∙ CH2 C∙H
CH2 CH2
CONH2 COOH
∙
R H2C C CH2 CH CH2 CH CH2 CH Macroradical
CONH2 COOH CONH2 COOH
Cross-linking
Scheme2:Theschemeofapossiblereactionmechanismforobtaininghydrogelsbasedonacrylamideandacrylicacidinthepresenceoffree
radicalsformedbywaterradiolysisorinitiatordecomposition.
−1
The cross-linkers are usually used in conjunction with 3200–3190cm arecharacteristicfortheabsorptionsof–OH
the radical cure systems in order to promote cross-linking groups of AA. The methylene group vibrations are used to
reactions and to improve the physical properties. They are monitor the extent of polymerization. The absorbance in
typicallypolyfunctionalmonomersandareeffectiveinmod- therangeof2937–2933cm−1isassignedtotheasymmetrical
ification of materials not only by cross-linking but also by stretching vibrations of –CH2 from AMD or AA [18, 19].
graftingandradicaladdition.Theadditionofanappropriate The bands in the range of 1774–1785cm−1 correspond to
polyfunctional monomer in the polymer matrix may lead the carbonyl stretching vibration of C=O connected to the
to the obtaining of a desired cross-linking density even at −1
carboxyl group but in the range of 1647–1657cm can be
lower absorbed doses. By increasing the cross-link density,
attributedtotheC=Ogroupconnectedtotheamidegroup
thenetworkperformancecanbeimprovedalso.Therearetwo
[18]. The characteristic band which appeared in the range
importantclassesinwhichthepolyfunctionalmonomerscan −1
of 1600–1620cm is due to the amide II bands. Amide II
begrouped,accordingtotheirinfluenceoncurekineticsor
results from the N–H bending vibration and from the C–
totheirphysicalproperties.Thepolyfunctionalmonomersof −
N stretching vibration. Symmetric stretching of COO is
typeIarehighlyreactiveandveryeffectivefortheincreasing −1
foundat1450–1410cm inpoly(acrylamide-co-acrylicacid)
of both rate and state of cure (acrylate, methacrylate, or −1
spectra [18]. On the other hand, the bands at 1443cm
maleimide functionality). The polyfunctional monomers of −1
typeIIarebasedonallylreactivesitesandincreasethestate and 1353cm are attributed to the CH2 bending and –
CHbendingvibrations,respectively.Forthepolyacrylamide,
of cure only. In order to obtain hydrogels with improved
−1
the –CN stretching appears at 1350–1345cm and –C–
networkcharacteristicsandgoodphysicochemicalproperties
−1
O stretching at 1125–1116cm . The C–O–C asymmetric
we used TMPT (a polyfunctional monomer of Type I) as
stretchingandC–OstretchingfromAAareconfirmedwith
cross-linker.
−1 −1
It is suggested that the polymerization reaction of theabsorptionsaround1190–1180cm and1065–1025cm ,
AMD/AAinpresenceofTMPTcanbedepictedasatwo-step respectively[20].
process:inthefirststep,arapidinitialpolymerizationofthe
polyfunctionalmonomeroccursandinthesecondstepthe 3.3.SwellingandDiffusionExperiments
reactionbetweenthepolymerizedpolyfunctionalmonomer
3.3.1.TheMeasurementofGelFraction. Normally,thehydro-
andtheAMD/AAchainstakesplaceinordertoformacross-
gelcontentofagivenmaterialisestimatedbymeasuringits
linkedAMD/AA-TMPTnetwork[16,17].Scheme3presents
insoluble part in dried sample after immersion in water. In
apossiblereactionmechanismbetweenAMD/AAchainsin
ordertodeterminethegelfractionandthesolublefraction
thepresenceofTMPTwhichoccursinEBcross-linking.
in the obtained hydrogels, dried samples of 0.05g were
immersed in double distilled water in order to swell for
3.2.SpectralCharacterization. Theinfraredspectroscopywas 72h[4].Afterfiltration,theextractedgelwasdewateredby
carriedoutinordertoinvestigatethechemicalstructureof nonsolventethanol,driedinairandthentoconstantweight
∘
the obtained hydrogels and the results presented in Figures inalaboratoryovenfor5hat50 C,andfinallyreweighed.The
1 and 2 were obtained in absorbance in the range of 4000– gelfractionandsolublefractionweredeterminedasfollows
−1 [21]:
650cm .
𝑊 −𝑊
The broad bands in the range of 3345 to 3330cm−1 are Sol fraction(%)= 0 1 ×100,
𝑊
attributedtothesymmetricandasymmetric–NHstretching 0 (1)
vibrations of AMD and the broad bands in the region of Gel fraction(%)=100−Sol fraction,
JournalofChemistry 5
∙
∗
O
O O
O EB O O + H+
O
O O
TMPT Excited state of TMPT
Radical of TMPT
+nTMPT
(homopolymerization)
O O
O O
[
O O
O
] O
p
O
+ AMD/AA macroradical addition
+ AMD/AA (H-transfer)
ONH2 OH
C O
C C
C ON
H
C 2
C
O
N
H
O O 2
C C
O
O O OH
[
O O
O CO
O
H
] O
n
COOH O
C
CONH2 COOH
C
CONH2
Scheme3:TheschemeofapossiblereactionmechanismbetweenAMD/AAandTMPTinEBcross-linking.
where 𝑊0 is the initial weight of the dried sample and 𝑊1 and solvent. The swelling 𝑆, expressed in percentages, was
is the weight of sample after extraction from the water and calculatedfromthefollowingrelation[15,22]:
drying.
𝑀 −𝑀
Figure 3 shows the effect of the absorbed dose and 𝑆(%)= 𝑡 0 ×100, (2)
𝑀
amount of TMPT on the gel fraction and soluble fraction 0
of hydrogels. It was found that the gel fraction depends on where𝑀𝑡 isthemassoftheswollengelattime𝑡and𝑀0 is
the concentration of TMPT, increases with radiation dose theinitialmassofthedriedgel(attime𝑡=0).
increasing, and attains a maximum at the radiation dose of
Thewaterintakeofinitiallydriedhydrogelswaspursued
4kGy. foralongperiodoftime.Thehydrogelsswellingisotherms,
asafunctionoftheabsorbeddoseandtheamountofcross-
linker,areshowninFigure4.
3.3.2. Swelling Measurements. On the cross-linked AMD/ Itcanbeseenthattheswellingincreaseswiththeincrease
AA/TMPT hydrogels, the dynamic swelling experiments of the TMPT amount at the same absorbed dose, until a
wereperformedindistilledwater,atroomtemperature(25± certain point when it becomes constant. It can be noticed
0.1∘C), and the mass increasing was pursued as a function that, for the sample which contains a higher amount of
of time. A fundamental relationship exists between the TMPT (5.90 × 10−3mol/L) even at the highest irradiation
swellingofapolymerinasolventandthenatureofpolymer dose(4kGy),theswellingisaround3900%,comparedwith
6 JournalofChemistry
0.5
1.6
1.4
0.4
1.2
nce 1.0 nce 0.3
a a
b b
or 0.8 or
Abs Abs 0.2
0.6
0.4
0.1
0.2
0.0 0.0
750 1000 1250 1500 1750 2000 2000 2250 2500 2750 3000 3250 3500 3750 4000
Wavenumber (cm−1) Wavenumber (cm−1)
2.95×10−3mol/L TMPT 2.95×10−3mol/L TMPT
2kGy 2kGy
3kGy 3kGy
4kGy 4kGy
(a) (b)
Figure1:FTIRspectraforhydrogelshaving2.95×10−3mol/LTMPT,intherangeof(a)750–2000cm−1and(b)2000–4000cm−1.
0.5
1.6
1.4
0.4
1.2
ce 1.0 ce 0.3
n n
a a
b b
or 0.8 or
Abs Abs 0.2
0.6
0.4
0.1
0.2
0.0 0.0
750 1000 1250 1500 1750 2000 2000 2250 2500 2750 3000 3250 3500 3750 4000
−1 −1
Wavenumber (cm ) Wavenumber (cm )
5.90×10−3mol/L TMPT 5.90×10−3mol/L TMPT
2kGy 2kGy
3kGy 3kGy
4kGy 4kGy
(a) (b)
Figure2:FTIRspectraforhydrogelshaving5.90×10−3mol/LTMPT,intherangeof(a)750–2000cm−1and(b)2000–4000cm−1.
the sample irradiated at the same dose, but with a lower 3.3.3. Equilibrium Water Content. Another parameter used
contentofTMPT(2.95×10−3mol/L),forwhichtheswelling fortheassessmentofhydrogelsswellingisthepercentageof
isalmost3500%.Also,theswellingisstrictlydependenton equilibriumwatercontent(EWC%)anditcanbecalculated
theabsorbeddose.Theswellingdecreaseswiththeincreasing fromthefollowingequation[22,23]:
ofabsorbeddose,sincethedegreeofcross-linkingincreases 𝑀 −𝑀
EWC= 𝑆 0 ×100, (3)
andthewaterenteringisrestricted. 𝑀
𝑆
JournalofChemistry 7
90 100
20
87
%)
Gel fraction (%) 77885814 111468 Soluble fraction (%) water content, EWC ( 999789
12 m
u
72 bri
10 uili 96
q
2.0 2.5 3.0 3.5 4.0 E
Absorbed dose,D(kGy)
95
H1-1,H1-2,H1-3(gel fraction) 2.0 2.5 3.0 3.5 4.0
H ,H ,H (gel fraction)
2-1 2-2 2-3 Absorbed dose (kGy)
H ,H ,H (sol. fraction)
1-1 1-2 1-3
H ,H ,H (sol. fraction) 2.95×10−3mol/L TMPT
2-1 2-2 2-3 5.90×10−3mol/L TMPT
Figure3:TheeffectoftheabsorbeddoseandamountofPPand
TMPTongelfractionandsolublefractionofhydrogels. Figure5:TheeffectoftheabsorbeddoseandamountofTMPTon
theequilibriumwatercontentofhydrogels.
10000
3.3.4.SwellingKinetics. Inordertoexaminethecontrolling
9000 mechanismoftheswellingprocesses,severalkineticmodels
areusedtotestexperimentaldata.Asimplekineticanalysis
8000
isthefirst-orderequationintheform[15]
7000
𝑑𝑆
ng (%) 56000000 𝑑𝑡 =𝑘1,𝑆(𝑆max.−𝑆), (4)
welli 4000 where 𝑘1,𝑆istherateconstantoffirst-orderswellingand𝑆max.
S isthedegreeofswellingatequilibrium.
3000 After the definite integration by applying the initial
2000 condition(𝑆=0at𝑡=0and𝑆=𝑆at𝑡=𝑡)equationbecomes
1000 ln𝑊=𝑘1,𝑆𝑡,
0 𝑆 (5)
0 500 1000 1500 2000 𝑊= max. .
𝑆 −𝑆
Time (min) max.
2.95×10−3mol/L TMPT 5.90×10−3mol/L TMPT Asecond-orderequationbasedonswellingequilibrium
2kGy 2kGy degreeisexpressedintheform[15]
3kGy 3kGy
4kGy 4kGy 𝑑𝑆 =𝑘 (𝑆 −𝑆)2, (6)
𝑑𝑡 2,𝑆 max.
Figure4:TheeffectoftheabsorbeddoseandamountofTMPTon
thehydrogelsswelling. where𝑘2,𝑆istherateconstantofsecond-orderswelling.
Afterintegrationandapplyingtheinitialcondition(𝑆=0
at𝑡=0and𝑆=𝑆at𝑡=𝑡),equationbecomes
𝑡
where𝑀𝑆 isthemassoftheswollengelatequilibriumand =𝐴+𝐵𝑡, (7)
𝑀0isthemassofthedriedgelattime𝑡=0. 𝑆
TheresultsarepresentedinFigure5anditcanbenoticed where𝐴isthereciprocaloftheinitialswellingrateand𝐵is
that all samples show values of EWC% over 95%, even for theinverseofthedegreeswellingatequilibrium:
highirradiationdoses.Also,theEWC%valuesofhydrogels
1
whichcontainhighamountsofTMPTarelargercompared 𝐴=𝑟 = ,
0 𝑘 ×𝑆 2
to those that contain small amounts at the same absorbed 2,𝑆 max.
dose.ThisresultcanbeexplainedbythefactthatTMPTisa (8)
1
hexafunctionalcross-linkeranditsuseinlargeamountsmay 𝐵= .
𝑆
formseveralbondsbetweenchains[22]. max.
8 JournalofChemistry
0.6
1.4
0.5
1.2
1.0 0.4
Wln 0.8 t(s) 0.3
0.6
0.2
0.4
0.1
0.2
0.0 0.0
0 100 200 300 400 500 0 500 1000 1500 2000
Time (min) Time (min)
2.95×10−3mol/L TMPT 5.90×10−3mol/L TMPT 2.95×10−3mol/L TMPT 5.90×10−3mol/L TMPT
2kGy 2kGy 2kGy 2kGy
3kGy 3kGy 3kGy 3kGy
4kGy 4kGy 4kGy 4kGy
(a) (b)
Figure6:Thefirst-order(a)andthesecond-order(b)swellingkineticsofhydrogels.
Table3:Thevariationoffirst-order𝑘1,𝑆/min−1 andsecond-order Table4:Thevariationofinitialswellingrate𝑟0/gwater(ggelmin−1)
𝑘2,𝑆/ggel(gwatermin)−1swellingrateconstantswiththeamountof andequilibriumswellingdegrees(theoretical)𝑆max./gwater(ggel)−1
TMPTandabsorbeddose. withtheamountofTMPTandabsorbeddose.
Absorbeddose(kGy) Absorbeddose(kGy)
TMPT(mol/L) TMPT(mol/L)
2 3 4 2 3 4
𝑘1,𝑆 ×103 𝑟0 ×102
2.95×10−3 3.14 2.59 2.65 2.95×10−3 3.42 6.29 9.21
5.90×10−3 2.81 2.08 2.61 5.90×10−3 3.63 6.44 7.76
𝑘2,𝑆 ×107 𝑆max.
2.95×10−3 6.28 4.65 5.83 2.95×10−3 6820 5843 4315
5.90×10−3 2.15 2.65 5.46 5.90×10−3 11321 7651 4859
comparedwiththesamplesthathavealowercontentofcross-
Also, by fitting the experimental data using the first-
linkeratthesameabsorbeddose(4kGy).TMPThasahigh
order equation and the second-order equation (Figure 6),
degree of functionality, fast undergoes addition reactions,
theswellingkineticparametersincludingtherateconstantof
andincreasesthecross-linkingdegreeofthehydrogels.The
first-andsecond-orderswelling(𝑘1,𝑆and𝑘2,𝑆),thetheoretical swellingpropertiesofhydrogelscontainingcross-linkersare
degreeswellingatequilibrium(𝑆 ),andtheinitialswelling
max. changed because of the structure of cross-linker. When the
rate(𝑟0)canbecalculatedfromtheslopeandinterceptoflines cross-linkersareaddedtothehydrogelsystems,itisknown
(Figure6).TheresultsarelistedinTables3and4.
thattheywillcausechangesintheswellingratioofhydrogels,
FromTable3itisobservedthattheswellingrateconstant
because the molecules of cross-linkers are placed between
decreaseswiththeincreaseintheamountofTMPT,indicat-
the chains of monomers [25]. The maximum equilibrium
ing that the diffusion rate decreases at higher TMPT con-
swelling ratios theoretically calculated are in a good agree-
centrations. This suggests that the water diffusion at higher
ment with the equilibrium swelling ratios experimentally
TMPT concentrations is hindered due to steric obstacles
obtained.AsitisshowninFigure6,thesecond-orderswelling
causedbyassociationwhichleadstodensergelstructure[24].
equationseemstofitbetterthanthefirst-orderequation.
Itcanbenoticedthattheinitialswellingrateconstant(𝑟0)
ofthehydrogelshasrapidlyincreasedwiththeincreasingof
absorbed dose for both TMPT concentrations. For samples 3.3.5.DeterminationofSwellingPower. Becauseofthehydro-
which contain a higher amount of cross-linker, the initial gels applications in biomedicine, pharmaceutical, environ-
swellingrateconstantissmallatthehighestabsorbeddose, mental, and agricultural engineering, the analysis of the
JournalofChemistry 9
mechanismsofwaterdiffusionhasgainedconsiderableatten- Table5:Thevariationof𝑛and𝑘withtheamountofTMPTand
tion lately. When a glassy hydrogel is brought in contact absorbeddose.
withwater,waterdiffusesintothehydrogelandthehydrogel
Absorbeddose(kGy)
swells.Diffusioninvolvesmigrationofwaterintopreexisting TMPT(mol/L)
2 3 4
ordynamicallyformedspacesbetweenthechainsofhydrogel.
𝑛
The swelling of hydrogels involves a larger scale segmental
2.95×10−3 0.86 0.89 0.85
motion, resulting ultimately in an increased distance of
separationbetweenhydrogelchains.Bytheexploitingofthe 5.90×10−3 0.82 0.70 0.88
swellingexperiment,thediffusionofwaterintohydrogelcan 𝑘
bedetermined.Byapplyingthefollowingequationtothe60% 2.95×10−3 0.281 0.164 0.137
of swelling curves, the nature of the diffusion of water into 5.90×10−3 0.444 0.507 0.142
hydrogelscanbeevaluated[25]:
𝑀 −𝑀 Table 6: The variation of the diffusional coefficient (𝐷×
𝐹 = 𝑡 0 =𝑘𝑡𝑛, 10−4/cm2sec−1)withtheamountofTMPTandabsorbeddose.
swp 𝑀
0 (9)
Absorbeddose(kGy)
ln𝐹 =𝑛ln𝑡+ln𝑘, TMPT(mol/L)
swp 2 3 4
2.95×10−3 2.43 2.15 1.63
where𝑀𝑡and𝑀0arethemassoftheswollenanddrysample 5.90×10−3 16.17 5.49 4.08
at time 𝑡, 𝑘 is the swelling constant, and 𝑛 is the swelling
exponentwhichisindicativeofthetransportmechanism.
The abovementioned equation is valid for the first 60%
werecalculatedfromtheslopesofthelinesandtheresultsare
from the fractional uptake. For the hydrogels obtained in
this study, ln𝐹 versus ln𝑡 values are plotted and are shown listedinTable6.
FromTable6theincreasingtrendfordiffusionalcoeffi-
in Figure 7. The swelling constant and swelling exponent
cientswiththeincreaseofTMPTamountandtheirdecreas-
are calculated from the slopes and intercepts of the lines,
ing trend with the decrease of absorbed dose are observed.
respectively,andarelistedinTable5.Accordingtotherelative
ratesofdiffusion(𝑅diff)andrelaxation(𝑅relax),therearethree From Figure 5 it is observed that the equilibrium water
cases of diffusion [26–28]: Case I: 𝑛 = 0.45–0.5 indicates a contentshowsthesamevariationtrend.Thiscanbeexplained
asfollows:theincreasingofthenumberofcross-linksinside
Fickiandiffusionmechanism,inwhichtherateofdiffusion
is much smaller than the rate of relaxation (𝑅diff ≪ 𝑅relax) thenetworkpervolumeunitleadstothedecreasingofthefree
and the system is controlled by diffusion; Case II: 𝑛 = 1.0, spacesforwatermolecules,sothewateruptakedecreases.
wherethediffusionprocessismuchfasterthantherelaxation
process (𝑅diff ≫ 𝑅relax) and the system is controlled by 3.3.6. Network Studies. One of the most important struc-
relaxation; Case III: 0.5 < 𝑛 < 1.0 indicates non-Fickian turalparametersforthecharacterizationofthecross-linked
(anomalous) diffusion mechanism, which describes those polymersis𝑀𝑐,theaveragemolarmassbetweencross-links,
caseswherethediffusionandrelaxationratesarecomparable whichisdirectlyrelatedtothecross-linkdensity.Theswelling
(𝑅diff ≈ 𝑅relax). Occasionally, when 𝑛 > 1, the situation is equilibrium is widely used to determine 𝑀𝑐. According to
regardedasSuperCaseIIkinetics[26,29,30]. thetheoryofFloryandRehnerforaperfectnetwork,𝑀𝑐 is
From Table 5 it is observed that the values of swelling calculatedusingthefollowingrelation[15]:
coefficientvarybetween0.7and0.89.Duetotheseresults,we
cansaythatthediffusionofwaterintothehydrogelsobtained ]1/3−] /2
𝑀 =−𝑉𝑑 𝑆 𝑆 , (11)
inthisstudyshowsanon-Fickiancharacter. 𝑐 1 𝑝ln(1−]𝑆)+]𝑆+𝜒]2𝑆
The study of water diffusion phenomena in hydrogels
clarifies the polymer behavior. The swelling-time curves where𝑉1isthemolarvolumeofthesolvent(inthiscasewater:
of hydrogels in water are used to calculate the diffusion 18cm3mol−1),𝑑𝑝 isthepolymerdensity(1.106gcm−3),]𝑆is
coefficients.Oneofthemethodsforcalculatingthediffusion
3
coefficient(𝐷)istheshorttimeapproximationmethod.This thevolumefractionofthepolymerintheswollengel(cm )
and is equal to 1/𝑆, and 𝜒 is the Flory-Huggins interaction
methodisvalidonlyforthefirst60%oftheswelling[15].The
parameterbetweenthesolventandpolymer.
diffusioncoefficientshavebeencalculatedusingthefollowing
Thevalueof𝜒iscalculatedasfollows[31–33]:
relation:
𝐹=4[ 𝐷 ]1/2𝑡1/2, (10) 𝜒=0.431−0.311]𝑆−0.036]2𝑆. (12)
𝜋×𝑟2
Thecross-linkdensity,𝑞,isdefinedasbeingthemolefraction
where 𝐷 is in cm2sec−1, 𝑡 is in sec, and 𝑟 is the radius of ofthecross-linkedunits:
cylindricalpolymersample(cm).
𝑀
Forthehydrogelsobtainedinthisstudy,thegraphsof𝐹 𝑞= 0,
(13)
versus𝑡1/2 areshowedinFigure8.Thediffusioncoefficients 𝑀𝑐
10 JournalofChemistry
Table7:Thevariationofthenumber-averagemolarmassbetween
4 cross-links (𝑀𝑐/gmol−1), cross-link density (𝑞), and number of
elastically effective chains (𝜐𝑒) with the amount of TMPT and
absorbeddose.
3 Absorbeddose(kGy)
TMPT(mol/L)
2 3 4
wp 𝑀𝑐 ×10−3
Fs 2 2.95×10−3 589817 407327 239736
n
l 5.90×10−3 1187695 584394 296537
𝑞×107
1
2.95×10−3 1.218 1.764 2.997
5.90×10−3 0.610 1.241 2.445
0 𝜐𝑒 ×1014
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 2.95×10−3 1.875 2.715 4.613
lnt 5.90×10−3 0.931 1.892 3.729
2.95×10−3mol/L TMPT 5.90×10−3mol/L TMPT
2kGy 2kGy
3kGy 3kGy Another parameter used for cross-link density charac-
4kGy 4kGy terization is 𝜐𝑒 which represents the number of elastically
Figure7:Theplotsofln𝐹versusln𝑡forhydrogels. effectivechainstotallyincludedinanetworkpervolumeunit
anditiscalculatedwiththefollowingrelation:
𝑑 𝑁 𝑞
𝑝 𝐴
𝜐 = , (15)
60 𝑒 𝑀
0
50 where𝑁𝐴istheAvogadronumber.
Thevaluesofparameters𝑀𝑐,𝑞,and𝜐𝑒werecalculatedas
40 aboveandarelistedinTable7.
From Table 7 it is observed that the number-average
F
molar mass between cross-links of hydrogels has increased
30
with the increasing of the amount of cross-linker (TMPT)
buthasdecreasedwiththeincreasingofabsorbeddose.The
20
values of the cross-link density and number of elastically
effective chains are inversed due to the value of number-
10
averagemolarmassbetweencross-links.Theincreasingofthe
amount of cross-linker decreases the cross-linking density
0
andincreasestheswelling(Figure5),becausethehydrogels
4 6 8 10 12 14 16 18 20 22 24
t1/2 obtainedarenotstiff.Increasingthecross-linkerconcentra-
2.9523×kkGG1yy0−3mol/L TMPT 5.9023×kkGG1yy0−3mol/L TMPT tThviaolenueoinobcftra𝑀eina𝑐seiednsd𝑀riecs𝑐autbeltesstlwoshneeognwchthtahienattswbtohetemw𝑀eaein𝑐nbvthaaelcuktewbsoonabreaesc.akAffbeolcantreegsde.
4kGy 4kGy bytheabsorbeddose.Theincreasingofabsorbeddoseleads
to the decreasing of 𝑀𝑐, since the hydrogel becomes more
Figure8:Plotsof𝐹versus𝑡1/2forhydrogels. andmoredense.However,AMDincludesseveralhydrophilic
fragments and thus the hydrogels show a high degree of
swelling.
where𝑀0isthemolecularweightoftherepeatingunitsfrom netwOotrhkesrairmegpeolrptaonrtepsiazreaomremteersshussiezde(f𝜉o)ratnhdepaosrsoesssitmye(𝑃nt%o)f.
polymerandiscalculatedusingthefollowingrelation[31,33]:
Using the calculated values of number-average molecular
𝑀 massbetweencross-links,𝑀𝑐,themeshsizewasdetermined
0
withthefollowingequation[34]:
(𝑚 ×𝑀 )+(𝑚 ×𝑀 )+(𝑚 ×𝑀 ) (14)
= AMD AMD AA AA TMPT TMPT ,
𝑚AMD+𝑚AA+𝑚TMPT 𝜉=]−1/3√2𝐶𝑛𝑀𝑐, (16)
𝑆 𝑀
𝑟
where𝑚 , 𝑚 ,and𝑚 arethemassesofacrylamide,
AMD AA TMPT
acrylic acid, and cross-linker (TMPT) expressed in grams where]𝑆isthevolumefractionofthepolymerintheswollen
and 𝑀 , 𝑀 , and 𝑀 are the molar masses of gel, 𝑙 is the length of the C–C bond along the polymer
acrylamAiMdeD,acryAliAcacid,andTTMMPTPTexpressedingmol−1. backbone (0.154nm), 𝐶𝑛 is the Flory characteristic ratio of