pH-responsive physical gels from poly (meth) acrylic acid-containing crosslinked particles: the ...

14 Pages·2013·1.58 MB·English

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Materials Chemistry B
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pH-responsive physical gels from poly(meth)acrylic
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n acid-containing crosslinked particles: the relationship
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AM. ed Li C40it6e5this:J.Mater.Chem.B,2013,1, between structure and mechanical properties†
52:31 Unport Silvia S. Halacheva,*a Tony J. Freemontb and Brian R. Saunders*a
023 12:on 3.0 Gelsthatfeaturehighinternalporosityandhavebothhighelasticityandductilityhavepotentialtoprovide
n 2/18/2Attributi iwmitmhecdeiallste. Hloeardeinsu,pwpeorrtepanodrterneasublltessfurobmseqauerenctetnistsuineverestgigenateiroantioonf onfovdealmpaoglye(dmseotfhtytlismsueethifaccroymlabtein-ceod-
d ons methacrylicacid),(PMMA-MAA)andpoly(ethylacrylate-co-methacrylicacid),(PEA-MAA)biodegradable,
eo
oadmm pH-sensitive particle gels which are with high porosity, elasticity and ductility. These gels formed at
wnlCo physiological pH range and are potentially injectable. The particles were prepared using solvent
Dove evaporation. They were functionalized by crosslinking the MAA groups of the particles via bis-amide
013. Creati formation with either cystamine (CYS) or 3,30-dithiodipropionic acid dihydrazide (DTP) which
ublished on 21 June 2e is licensed under a sTipclniioghhmcmheyruestpliclayotorsaolisocnnsirtgesgoiliooicsnnusatkllshientleyhdpkeeiHnppdfcHaoorrp.arrtmapnicCroglatoeiertcsnai.lotecweneAsd.neortFrkerfueaevrctyoteeohbrdnfiesssiernbimdrddivlieiioesntrpdryageebd,rblsuoycyioeofmdnmttysoohpnriataesfhormeeersdmitlcpauesrtdlteodiygischetewhnatlnecaasedscsPtaoiMdcttfthueMdaacrtitinAsnilude-gfMlopgtdrhAoeuiAldccssr/etwoCiwlsbeeYsaollSlsinnpaaodkhpnbesypddsswiercerPiaptvcEhlieaaAidrnbgt-iMlecftyrlhlseAoiesnmAwc/osriCtiotfzhhYseesiSsnliiummngpkteioolhealssnretr.,
Article. PThis articl RAeccceeipvteedd2291tshtMJuanye22001133 tisnhtrtoaesirnecogfnorneremacteetidrvittfyhr.oaFnmor1D0thT0eP%-PcMrwoeMssrleAin-okMebAdseArp/vaDerdTtPicflogereslashaagndelehlcaiogsnthitecarimneionladgsutlioucnistlyyv,a5tluhewickta%esrhoipgfohpreaasrwt1ica0lle0lss.kaPTnahdeaniimmdpparrooyivveeelddd
ess DOI:10.1039/c3tb20758g mechanical properties of these new gel-forming dispersions imply that they now have good potential
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A www.rsc.org/MaterialsB forfutureapplicationasinjectablegelsforregenerativemedicine.
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1 Introduction complications.1,2 Recently, dispersions of star-shaped poly((S)-
lactic acid)-based particles, prepared by emulsication of the
The use of injectable polymer/cell dispersions to repair tissue polymer in glycerol, were successfully utilized as an injectable
defectscouldconstituteasuperiorminimally-invasivealternative delivery system for hyaline cartilage regeneration in rabbits.3
tocurrentapproacheswhichinvolvesurgery.Aswellassigni- However,thedispersionsdidnotformphysicalgelsandtherefore
cantlyimprovingpatientcomfortandrecoverytime,theinject- could not provide immediate load support. The key essential
able therapy would be simple and eliminate post-treatment properties of materials intended for so tissue repair are
biocompatibility, biodegradability, interconnected porosity,
mechanical strength, durability and ease of preparation.
aUniversityofManchester,MaterialsScienceCentre,GrosvenorStreet,Manchester,M1
Furthermore, for potential load-bearing applications they are
7HS,UK.E-mail:[email protected];Brian.Saunders@manchester.
requiredtotransformtoahighelasticitygelimmediatelyupon
ac.uk;Tel:[phone]
bDivision of Regenerative Medicine, School of Medicine, Stopford Building, Oxford applicationofappropriateconditions.4Arangeofbiocompatible
Road,Manchester,M139PT,UK hydrogelshavebeenextensivelyexploredforbiomaterialappli-
†Electronic supplementary information (ESI) available:1H NMR and 13C NMR cations.5–12 In particular, stimuli-responsive degradable hydro-
spectra of DTP in DMSO-d6; 1H NMR spectra (DMSO-d6) and GPC traces for gels have recently attracted a great deal of attention in drug
some of the PMMA-MAA and PEA-MAA copolymers; potentiometric titration delivery, lm coating, tissue engineering and in fabrication of
data for non-crosslinked PMMA-MAA and PEA-MAA particles; a table showing
stimuli-responsive sensors due to their ability to undergo
the variation of the hydrodynamic diameter (Dh, in nm) with pH for
non-crosslinked and crosslinked particles; size distribution functions for reversible sol–gel transition in response to external chemical,
non-crosslinked and crosslinked PMMA-MAA-containing particles taken at physicalorbiochemicaltriggerssuchastemperatureandpH.13–15
different pH values; SEM images of freeze-dried PMMA-25MAA/4DTP gels The porosity of a hydrogel can be easily tuned by varying
formed at pH 7.0; strain sweep data for all PMMA-MAA and PEA-MAA
the structureofthe polymer, the crosslinker andthe extent of
crosslinkedparticlegelsstudied.SeeDOI:10.1039/c3tb20758g
ThisjournalisªTheRoyalSocietyofChemistry2013 J.Mater.Chem.B,2013,1,4065–4078 | 4065
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inter-chain crosslinks in the hydrogel's matrix. Elucidation of molecule surfactants or highly hazardous materials.40–46 The
the structure–properties relationship can potentially enable particles were based on amphiphilic PMMA-MAA or PEA-MAA
controlover theloadingand releasebehaviour.16,17Inorderto copolymers and were formed via copolymer migration to the
facilitate their removal from the body on demand a range of dichloromethane–waterinterface.Anadvantageofthissystem
crosslinkers with biodegradable groups, such as disulphides, for potential application is that MMA is used in bone cement
have been investigated.7,18–20 The disulphide linkage can be and MAA is a hydrolysis product of MMA. In that work up to
readilycleavedtothecorrespondingthiol(s)inthepresenceof 37% of the available carboxylic acid groups in MAA were
reducingagentssuchasglutathione,whichoccursnaturallyin crosslinked via bis-amide formation with CYS, which simulta-
e. the body.21 Disulphide-functionalized hydrogels have been neouslyimpartedelasticityandreversibilitytothecrosslinking
c
n widelyexplored for the preparationof stimuli-responsive gela- procedure due to the disulphide bonds, which can be readily
e
AM. ed Lic tToorsa,crhevieevresiblolnygcr-loassstliinngk,eidnjpeocltyapblleexehsyadnrodgpeolslytmhaetrccaapnsiunlecos.r2p2–o24- bhroollkoewnupparotnictlreesatsmweelnletdwiitnhathreedpuhcyinsigoaloggeincta.lTphHecrraonssgleinaknedd
2:31 nport ratecellsintotheir3DstructureLeeetal.25developedahyalur- formedphysicalgelsfromtheirconcentrateddispersions.The
23 12:5n 3.0 U opnroicperatcieids.,Sh(HuAe)t–aPll.u26ropnreicpacreodmtphoiosiltme owdiithedtHisAsuuesinagdhcaersbivoe- greealsdihlyaddismaiscsreommbelteedr-sucpaoleniandtedrictioonnneocftegdluptaotrhoisointye.aUndnfowreture-
n 2/18/20Attributio dtthhiieimonlifgdorero-mmupeesddtiuoantdedidesurhlpypdhhryiasdzieoidsl.oeGgcihhcoaelsmhciosenttrdayil.t.i2To7hncersoHbssyAlaihniyrkdeordxoigdtheaeltsitowhnieoorlef- nnmaoottdeleeyrx,actteheeedbeiloa4mstkiecPcahm.aonTdihcuaolulseslovagadelu.lseswoefrtehotsoeogewlseawkerteoloswupapnodrdtida
ed oons modied HA with poly(ethylene glycol) diacrylate, (PEGDA) to MMA-containing biomaterials are frequently exceptional in
dm
oam form HA-based hydrogels. Their mechanical properties were mechanical strength and show various biological benets in
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wC evaluatedasafunctionoftheconcentrationandthemolecular clinicalapplications.47Wehavecontinuedourinvestigationof
oe
013. DCreativ wineciogrhptoroaftethbeioccroomsspliantkiberle. Shyyndtrhoeptihcilipchypsoiclya(lethhyydlernoegelosxitdhea)t, apnodly(mreeptohr)tacrhyelareteinbaosned-tbhieomsaytnetrhiaelssisf,orcuhsaeraacstteirsissuaetioscnaffaonldd
ublished on 21 June 2e is licensed under a psaepsutxcooipicmlldyyhlo(s)ugaraloleciysr-dccrh.eipb2dsa8oi–poro3lilyo2cd)(onCleamssh-bcieivptagtieoascmsetiadaebncnhlbietydaisdon)p,mrdoaopllagaoyotel(nelgplyrgir(incoaaalpwsstpcyeiiraltneohhffnltaoahevbcleedtigoossely.ndym3nc3ee,toeg3)h4rlr)ageanesdhiddUasabavnoplesffeoonpblryardte(otuegoumnnlmryaaicsataloeiil3nlnlsiDyogcs, mesupPulhMsaeieystcMdsathiibcoAahliln-eteoMyirgfceoAPiacrAMltafoluMpalytrnduAoder-pcmMeoePurmoAEtsnAiApees-asatMirtnniaoAbdtmfleAePinntEphesiAamwetr-rMtaaigtctlAeeyllynepAg--eyeibngrsaaevfsllaooisestrf,dyivwhepoghireftgieilchssophsuauwprareaaroteasrripeoopeppsnomiarttoiyeproa,n.lfcoEthhiyAaite.ghldilhAeys,
Article. PThis articl choanvevernestitorincateldhtyhderiorgueslesipnotsissseusseesnegveinraelerdinisgaadnvadndtrauggesdewlihviecrhy uprtielvizioinugslyDfToPrthorepCrYeSparaastiocrnososflirnekveerrssi.blDyTcProshsalisnkbeedenhyualsuerd-
s applications. Some of the disadvantages of these materials onanhydrogels,whichdemonstratedgoodpotentialforinsitu
s
e
cc include low solubility at physiological pH, higher swelling encapsulation.26,27,48,49 The strategy for the preparation of
A
n capacitywhichleadstorapiddissolutionandprematurerelease PMMA-MAA/DTP- and PEA-MAA/DTP-based gels involves
e
Op ofdrugs,potentialvariabilityofthenaturalmaterials,depending incorporation of the DTP crosslinker into the particle shells
onthesource,aswellasthenecessitytoensurepuritypriorto using carbodiimide-mediated amide formation, followed by
implantationtoavoidactivatinganimmuneresponse.Further- formation of the physical gels under physiological conditions.
more,thesescaffoldsoenhavealimitedrangeofmechanical AsthedisulphidelinkageoftheDTPcanbereadilycleavedto
propertiesandmayneedtobeoptimizedforstemcellculture.35 thecorrespondingthiolinthepresenceofreducingagents,the
Increasing the elasticity and ductility remains a challenge for formation of PMMA-MAA/DTP- and PEA-MAA/DTP-based gels
load-bearingapplications.6,36 offers an alternative redox route (cf. CYS) for biodegradation.
To generate durable microgels with interconnected micro- Herein, the mechanical properties and porosity ofthe CYS- or
metre-scale porosity and increased elasticity, Liu et al.37 DTP-containing particle gels are explored and compared in
prepared permanently crosslinked poly(MMA-co-MAA/EGD) termsoftheparticles'compositions,structuresandpK values.
a
(where EGD is ethylene glycol dimethacrylate) microgels by Avarietyofcomplementarycharacterisationmethodshavebeen
emulsion polymerization. Bird et al.38 prepared vinyl-function- employed, including dynamic light scattering (DLS), potentio-
alizedpH-responsivehollowpoly(MMA-co-MAA)particleswhich metrictitration,scanningelectronmicroscopy,opticalmicros-
were covalently interlinked to give injectable pH-responsive copyanddynamicrheology.
gels.However,thepoly(MMA-co-MAA/EGD)andvinyl-function-
alized poly(MMA-co-MAA) particle-based gels did not contain 2 Experimental section
any labile groups, such as disulphides, and therefore were
Materials
stableunderphysiologicalconditions.
Recently, a simple technique for preparing hollow, biode- Tetrahydrofuran (Aldrich, anhydrous, inhibitor-free, $99.9%),
gradable polymer-based particles that are both pH-responsive dichloromethane (Aldrich, HPLC grade, $99.8%), methanol
andredox-sensitivehasbeendeveloped.21,39Themethodusedis (Aldrich, HPLC grade, $99.9%), 2,20-azobis(2-methylpropioni-
basedonasolventevaporationapproachanddoesnotrequire trile)(AIBN,Aldrich,98%),methylmethacrylate(MMA,Aldrich,
the use of colloid templates, layer-by-layer assembly, small- $98.5%), methacrylic acid (MAA, Aldrich, 99%), ethyl acrylate
4066 | J.Mater.Chem.B,2013,1,4065–4078 ThisjournalisªTheRoyalSocietyofChemistry2013
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Paper JournalofMaterialsChemistryB
(EA,Fluka,$99.5%),cystaminedihydrochloride(CYS,Aldrich, washedsequentiallywithmethanol(50ml),water(10ml)and
96%), N-(3-dimethylaminopropyl)-N0-ethylcarbodiimide hydro- methanol(25ml),beforebeingair-driedtoconstantweight.The
chloride (EDC, Aldrich, $99%), N-hydroxysuccinimide (NHS, desired product, DTP, was obtained as a white solid (3.10 g,
Aldrich, 98%), 3,30-dithiodipropionic acid (Aldrich, 99%), 62%).51 The 1H and 13C NMR spectra of DTP in DMSO-d are
6
hydrazine monohydrate (Fluka, purum, $98%), concentrated presentedinFig.S1,ESI.†
sulphuric acid (Aldrich, ACS reagent, 95–98%), poly(vinyl pyr-
rolidone) (PVP, Aldrich, average molecular weight 40000 g Particlepreparation
mol(cid:2)1),wereusedasreceivedfromthechemicalsupplier.Milli-
A solventevaporation methodwasusedforthe preparation of
e. Q water was used throughout, unless otherwise stated. Reac- the PMMA-MAA and PEA-MAA particles. The following proce-
c
n tionsrequiringanhydrousconditionswereperformedundera
e dure is representative for the preparation of the non-cross-
c
2:31 AM. nported Li pSyonsitthiveesisproefsscuorpeoloyfmneitrrsogen. lP(i8Mn4kM:e1dA6-,MPvMA/vAM)swAo-aMlsveAdnAits.saTonhlvdiesdPwEianAs-4Mth4AemnAlapodafdraetidcm,leaisxt.eadFuiCrnsHitfl2oyC,rml22–.0MrategeOooHff
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023 12:on 3.0 ATlHlFfr,euen-rdaedricaanliptorolygmeneraitsmatoiosnpshewreer,euscianrgrieAdIBoNutasinananihnyitdiarotours. 1sh0emareldmaitn1(cid:2)01,00in0torpm120(wmhillsotfcowoaletedr,toco0n(cid:3)tCa)i,nuinsigng1awSti%lvePrsVoPn,
n 2/18/2Attributi TMhAeAcocopnoltyemnte.rFaobrberexvaimatpiolen,sPuMseMdAh-2e9reMiAdAenctiofnyttahienspe2r9cemntoalg%e LseRc4onhdigshaspeeredadmdiitxieorn.Tohfetheme upolsliymcaetriosnolwutaisonco.nTthineueemdufolsrio3n0
d ons MAA and 71 mol% MMA units. The following synthesis of was then allowed to stir gently overnight toenable removal of
nloadeommo PthMeMotAh-e2r9cMopAoAlyismreerpsr.eAsethnrteaeti-vneecokfthaeskp,rotcteedduwriethemacpolonydeednsfoerr tthioenCaHnd2Crle2.diTshpeerpsiroonduinctwwaatserpiunrioredderbtyorreepmeaotveedecxecnestrsifPuVgPa-.
Dowve C and nitrogen inlet, was charged with AIBN (0.25 g) and anhy- The resultant suspension was then ltered through a 50 mm
ublished on 21 June 2013. e is licensed under a Creati r(dotd52oo5vrr.oooe2p(cid:3)mruu4CrssaogcfTTpt,oeeHHer0emrdF.Fi33op0((ade81etrm5q7oa7ufmtmi0niu9vlrl.0.(cid:3)))e).CA,maTanmnohitdnhvdeie.exasrAAtdtnuIirdriBregereeaNehrdcdottt(fthi0so(oe1Mo.n0t8lfh2Mue5hetemAidosgaion(x)tp6lhtupwuw.e0rtraa6ioersosexgna.ndw),c.iiaat0tstAriss.oo7aonglceeuveowrqennnduacipcfsoiovieuona.rnr)lml,gitlanoreMnagwrdthaAeetatyAddoet- PT9m(8CrhlltewYeeporStffa%oaritslanh,ltoder1iewocspmniotnronmeogrscfeoeepdclnnrrCotatoarctOsatesi2tOdvle–ieuHdn4rokP(cid:3)egfeCMrdfto.ohMuprepaAtghsr-et)2ein9wcpelMaerrsaseAlpdApailprruoaattcreietodidncuwleorieftshtPeomM1c8kpM.4ldoAimys-e2pld0eoM:rfs4Ai.0o2A.n51/
Article. PThis articl uapnondldyemtrheerrenwdpuaosceurderempdoreivnsestdoubr1ey0t0ol0tramatvilooonluf,mcrioneldsoefdwaatphteporrr.ooTxuihgmehalpytrewelycitih1p0iwt0aattmeedrl tMwhte%pH.pTa6rh.0teicpplheHso.sop(fhTtahhteeebbrueulffffaeteirrotnuostaihlffiizpoerddbedatewpneeeannldceotdhnecuepnpotHnratthaiotenpwoKhfai1co.h5f
s
es andair-dried.Residualwaterwasremovedbyfreeze-drying.The crosslinking was performed and the particle pK is discussed
c a
c
A molarratioofMMAtoMAAintheco-polymerwasdetermined later.) To the resulting stirred suspension, NHS (0.140 g, 1.22
pen by 1H NMR spectroscopic analysis. The other co-polymer equiv.)wasaddedfollowedbyEDChydrochloride(0.153g,0.80
O
compositions were obtained in an analogous manner from equiv.). Stirring was continued for a further een minutes
appropriatemixturesofeitherMMAorEA,withMAA. before CYS (0.095 g, 0.42 equiv.) was added. The reaction
mixturewasthenstirredatroomtemperaturefor24hoursand
Synthesisof3,30-dithiodipropionicaciddihydrazide the crosslinked particles were isolated by centrifugation, fol-
lowed by three cycles of redispersion in water and centrifuga-
DTP was synthesized according to a previously reported
method.50Intoa250mlaskwereplaced3,30-dithiodipropionic tion. The DTP crosslinked particles were prepared in an
analogous manner by substituting DTP for CYS. The cross-
acid(5.00g,23.8mmol)andmethanol(50ml).Threedropsof
linked particle abbreviation used here identies the mol%
concentrated sulphuric acid were added to the resulting solu-
tion and the mixture was reuxed for two hours. The reaction non-functionalized MAA groups and the mol% CYS or DTP
incorporated into the particles. For example PMMA-20MAA/
mixturewasthencooledtoroomtemperatureandthemethanol
9CYScontained20mol%MAAand9mol%CYS.
wasevaporatedunderreducedpressure.Theresiduewasthen
diluted with diethyl ether (60 ml), transferred to a separating
Analysis
funnel, washed sequentially with saturated NaHCO solution
3
(30ml)andwater(30ml).Theorganicphasewasdried(MgSO ) Protonmagneticresonancespectra(1HNMR)wererecordedon
4
and evaporated under reduced pressure to afford crude BrukerAvance400and500MHzspectrometers.Chemicalshis
dimethyl 3,30-dithiodipropionate as a transparent, pale amber (dH)are quoted inparts permillionand are referenced tothe
oil(5.21g,92%),whichwasusedwithoutfurtherpurication.50 residualsolventpeak.GPCanalyseswereperformedonaSho-
Dimethyl 3,30-dithiodipropionate (5.00 g) was dissolved in dex R101 Refractive index detector. Samples were dissolved in
methanol (100 ml) at room temperature. Hydrazine mono- anhydrous, inhibitor-free THF (1 mg ml(cid:2)1) at room tempera-
hydrate (6.30 g, 6.0 equiv.) was then added and the reaction ture. The columns contained Phenomenex Phenogel 5 mm
mixture stirred overnight (18 h approx.) at room temperature. beadswith500,104and106A˚poresizes.Theowratewas1ml
The resulting suspension was ltered and the white solid min(cid:2)1.PotentiometrictitrationwasconductedusingaMettler
ThisjournalisªTheRoyalSocietyofChemistry2013 J.Mater.Chem.B,2013,1,4065–4078 | 4067
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JournalofMaterialsChemistryB Paper
ToledoDL15titrator.Measurementswereperformedon40ml Table1 CharacterisationdataofPMMA-MAAandPEA-MAA
of a 1 wt% non-crosslinked dispersion using standardized
Mol%MAA
NaOH solutions. All pK values reported here are apparent
a
values. Optical microscopy was conducted with an Olympus Composition M a(gmol(cid:2)1) PDIa Theor. Exper.b Exper.c pKc
w a
BX41microscopeandwhitetransmittedlight.Dispersionswere
deposited on SEM stubs by evaporation at room temperature. PMMA-29MAA 40390 1.89 30 29 34.3 7.3
PMMA-38MAA 31130 1.89 40 38 38.5 6.9
Thinsectionsofgelwererapidlyfrozeninliquidnitrogenand
PMMA-65MAA 23410 1.96 70 65 68.3 6.3
residual moisture removed by freeze-drying. Rheology
PEA-30MAA 20370 1.75 30 30 26.1 6.2
e. measurements were performed at 25 (cid:3)C using a TA AR-G2 PEA-65MAA 18120 1.85 70 65 66.9 5.6
nc rheometeranda250mmgapwitha20mmdiametersteelplate
AM. ed Lice afrteq25ue(cid:3)nCc.yAollfs1traHinz-.swAlelepfremqeuaesnucrye-msweenetps wmeeraesucorenmduecnttesdwaetrae paoDteetnetrimomineetdricbtyitrGatPioCn..bDetermined by 1H NMR. cDetermined by
2:31 nport conducted at a strain of 0.1%. A particle concentration of 5.0
5U
23 12:n 3.0 wfotr%mewdasonusaeldigfhotrstchaettseerisntugdpiheso.toDmLSetemrecaosnusriesmtinengtosfwae5r0empeWr- experimental values obtained by 1H NMR were in good agree-
n 2/18/20Attributio MHpheao/lNvtoeedrnliaosdZeeert,a(oAspiPzeDerr)ataNinnagdnao9t0Z6(cid:3)S393d0entmeacu,ttiowocniothrorepaltaiscttosarn.cdoTanrhndeeacmvteaedlaasntuocrheae- mentwiththetheoreticalvalues(Table1).
ed oons mentswerecarriedoutatpHvaluesrangingfrom5.4–8.0andat PreparationandcharacterisationofPMMA-MAAandPEA-
oadmm aconstantsolutionconcentrationof0.1wt%.Thedispersions MAAnon-crosslinkedparticles
nlo
wC were treated with 1.0 M NaOH solution until the desired pH DispersionsofPMMA-29MAA-,PMMA-38MAA-,PMMA-65MAA-,
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ss Article. Published on 21 June 2013. D This article is licensed under a Creativ vhm1vsf(dpADuwaa0Kilylaenllhdeauucam/clrDDvoeesltoiae,huirhndworl(t(ircucegeynneaoonrleldaslaslrolsaalata)riproapmizetdsosaftieeaeneoin)t)d9cc)hr,d03hvfwet,uQa(cid:3)ediowiplnnd,utiuoaahhc,steormbthietsfetinhtroieieacegttewntichhlneneDseaoeersrlharwaMleaPlanCa(eltpMDaomnrYaalswehdvSMveeg,eeaedaoiDrdAnrvsinranhe-nuaMtD(gnoolcrZyeoenTAepnzledl-mPtAeaHmqsap,dwusaesw)a(eitaoiwpz)nlaslieeoaetdlsbuhrerlrfrrleneeNsPaesbtamttEtsahelehmitoAnlmeaeeowf-optnoMwaeheltrptZe,s.yAeta,S1rtdhTdAre95arethps0tuoipmpepHc.dseleaaiAveayicnrrnonsvcttttgahl.iiialucvcleQTmwuellmpaeehlaeHs¼iysseecsst.,' ctctfstmPwrahowaiooEeonrtuippoArismntcoeoo-msh3ndllsayyp0tti(olmmusirFpoMnoecdirweenpgotAoyrrhe.vatArseoorssa1-erooftcnaayhdoall)huu.anlioebonttnTddpiirmyoohtetttinnhhPstehomhcaEisgegwsreetiAeiirzraewbn-eesd6smpeaeeodi5dtrosqerpMlepkusuyalppteosdeAawaarr,yndAbesffraectiaiv-ledstiobioitaiscoinolatplhotluhssneorneeesadeidmr.lddtvryeAtaaitmu(honth(l1lleSue0osayeecsiinrdohlnrmsy-aftietcecs5hatlma3rdptebeothmaeielsoscnoeersitrfsnd1naloiCpiaa-bo(cid:2)nsrsadH)nles1o.kerd)2ls2lte,i1svuiiCnfdtcwteAolilkaltno2rnehebip–tnndiaiaMdcilgetrmiohiivttenssflyiaOpepctw.ttpealoehhHIaeposrrneesst/---,
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cc tested.Thedegreeofneutralization,a,oftheavailableRCOOH highly elastic gels were formed when the pH was raised by
A
n groups was calculated using the Henderson–Hasselbalch treatment with sodium hydroxide solution. Optical micro-
Ope equation:a¼1/(10pKa(cid:2)pH+1). graphs of PMMA-38MAA dispersions taken aer CH2Cl2 evap-
orationarepresentedinFig.1a.Someofthecollapsedparticles
3 Results and discussion arewithdoughnut-likemorphology,whichisoenreportedfor
hollow particles.54 A representative SEM image of non-cross-
Synthesisandcharacterisationofthecopolymers
linkedPEA-30MAAparticlesisshowninFig.1b.Boththeoptical
PMMA-MAA and PEA-MAA were synthesized by free-radical (Fig. 1a) and SEM images of all non-crosslinked dispersions
polymerisationoftheappropriatemixtureofmonomersinthe studiedindicatedcollapsedobjectsofvarioussizesandshapes.
presence of AIBN using a previously reported method.52 The Theparticles'diametersrangefromlessthan1mmuptoaround
copolymerswerecharacterizedbyGPCand1HNMR.Theweight 5mm.Thepolydispersityofthesampleswasduetotheturbu-
averagemolecularweightandpolydispersityofthecopolymers lentshearowduringthehighshearmixing.21,39Ifrequired,the
were determined by GPC (Fig. S2, ESI†). The copolymers had polydispersitycouldbereducedbyfractionation.
molecular weights ranging from 18120 to 40390 g mol(cid:2)1 TheformationofthePMMA-MAAandPEA-MAAparticlesis
(Table 1). GPC analyses gave monomodal distributions with mainlycontrolledbytheamphiphilicnatureofthecopolymers.
polydispersities ranging from 1.75 to 1.96. The copolymers' The formation of spherical core–shell micelles has been
structural compositions were determined from their 1H NMR reported for both random and crosslinked PMMA-MAA and
spectra in DMSO-d (Fig. S3, ESI†). The compositions of the PEA-MAA, poly(styrene-b-methacrylic acid), poly(S-b-MAA),
6
PMMA-MAAsampleswerecalculatedbyexaminingtherelative poly(isobutylene-b-methacrylic acid), poly(IB-b-MAA), poly-
ratios of signal intensities of the methoxyl protons of MMA (styrene-b-acrylicacid)andpoly(S-b-AA)copolymersinaqueous
groups and the methyl and methylene protons of both MMA solution.55–57 In the present study, PMMA-MAA and PEA-MAA
andMAAgroups.ThecompositionofthePEA-30MAApolymer particleswerepreparedusingthesolventevaporationapproach.
wasdeterminedfromtheratiosofthesignalintensitiesofthe This type of technique has been previously used to manu-
methylene protons of the ethyl ester units to the methylene facture solid polymeric microparticles or core–shell micelles
protons of both EA and MAA groups (see Fig. S3, ESI†). The from amphiphilic block copolymers (such as poly(n-butyl
4068 | J.Mater.Chem.B,2013,1,4065–4078 ThisjournalisªTheRoyalSocietyofChemistry2013
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Scheme1 MethodforpreparationofpH-responsive,crosslinkeddoughnut-likeparticles:(a)depictsthepreparationofthenon-crosslinkedparticles;(b)and(c)depict
thepreparationofcrosslinkedparticlesusingCYSandDTP,respectively.ForclaritythePVPstabilizerisnotshown.
methacrylate)-b-poly(2-methacryloyloxyethyl phosphorylcho- ofthenewPMMA-MMAandPEA-MAA.Thisissupportedbythe
line)).58–60Thesolventevaporationapproachhasalsobeenused doughnut-likeparticleshighlightedinFig.1aandc.However,
to generate hollow particles.61,62 For the earlier investigated the gels formed from the crosslinked particles, under physio-
PMMA-MAAandPEA-MAAcopolymeranalogues,theformation logical conditions, are shown to have high elasticities and
ofhollowparticlesshortlyaeremulsicationwasattributedto ductilities,whichwastheultimateaimofthisresearch.
the polymers' precipitation at the CH Cl droplet/water inter- The molar percentages (mol%) of MAA in the PMMA-MAA
2 2
face due to transfer of methanol to the aqueous phase.21 and PEA-MAA non-crosslinked particles were measured by
Unfortunately, based on the optical and SEM images of the potentiometric titration. The values calculated by potentio-
present PMMA-MAA and PEA-MAA non-crosslinked particles metrictitrationandthoseestimatedfromthe1HNMRspectra
(Fig. 1) it is difficult to unambiguously determine the particle of the copolymers were in reasonable agreement with each
morphologies present. However, acoexistence ofhollow parti- other and with the theoretical values (Table 1). The pH vs.
clesandsolidparticlesisfavouredintheaqueousdispersions degreeofneutralisation(a)plotsforPMMA-MAAandPEA-MAA
ThisjournalisªTheRoyalSocietyofChemistry2013 J.Mater.Chem.B,2013,1,4065–4078 | 4069
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of available RCOOH groups which were successfully reacted
with either CYS or DTP were 5–31% for the MMA-containing
copolymersand9–67%forthePEA-MAA(Table2).Theseresults
werelowerthanthetheoreticallypredictedvalueswhichwould
correspond to a 100% chemical yield (see the Experimental
section). It was found that the extent of crosslinking achieved
washighlydependentuponthepHofthereactionmixtureand
this effect has been reported for related systems.64–66 For
e. example, a previously reported investigation into the reaction
c
n betweencarboxylicacidsandEDC,aswellassubsequentamide
e
AM. ed Lic afocrtmivaattiioonn,osfhcoawrebdoxtyhlagtroauppHs boyf 3E.D5–C4.5.59 Hwoawseovpetri,mtahlefoamr tinhee
2:31 nport groups of CYS (pKa of CYS ¼ 8.35)68 would be extensively
5U
23 12:n 3.0 preraocttoinvea,t6e5d,67ubnudterassutchhe apcHidiicnccroenadsietsiontoswaanrdd ntheeurterfaolirtey ltehses
n 2/18/20Attributio F3i8gM.A1A((paH) a¼nd6.5(b))anshdoPwEAa-3n0oMpAtiAca(lpmHi¼cro6g.0r)a,prhespaencdtivSeElMy.(icm)aangde(fdo)rshPMowMaAn- pshheaarvcreepnblyte.aeIgnnecfooounfntrdsaustcto,cethrsesetfauteilnrlymthianecaitrlivnnaitutercdolegoecnpahrabitlooicxmiytlsyoagftrhoauycdpidrsaiczfiadpleHlss
ded omons o10pMticAaAlm/2ic0rDoTgPra(pphHa¼nd6.S0E)M,reimspaegcetivoeflyP.MMA-20MAA/9CYS(pH¼6.5)andPEA- values which would render amines relatively unreactive.26,49,69
oam Thisallowedthehighestextentofcrosslinkerincorporationto
nlo
wC beachievedforDTP(Table2,entry7).However,weshowbelow
oe
ublished on 21 June 2013. De is licensed under a Creativ attwdPohsrihseettipphhcPeaheeMrrshaiMMistioigApAnohArn-sMoeacrpaAornogeAnsldastepeshdcanshorttatwtoriircnanablncceeistrnseetiwthraFiieoseisgernar.esteitSsh(oe4suimne,glteEhpooSeTeffrIrac.alt†brohtoluwTaesrhnese1lreit)onh.cpfhkoTKpeashadeioenvloayppf(laKMuPeraexEtMisivAcbadAl-ie3lle)uis0.ct5eMyr7s,e6dA3afuosAeer, P4prttinhhe)MaaearttCcttMehitsortceioAhmonlle-necus2ssrsist9odeioMw(osefShsnracAtliashihgAtnpheieookHmarfnlineneerdvscgeoeu1raPilfrbltscteMiihcaatonhaeMncfgldtedDiAgoict-Tempn3h)lPK,s8'epsaaMiiowrnsmrAvecttcearhAaoocelnreruhpprcpedaeaoeseinsrrsrtpaidpfioctocoueiaflnrorrelmsdnistpnihieo(rnwgeTdongeapstrinbhepnedolreKeitnnpsiaoecs2-Hcosrtv,ro.alebov6slneesu.s0sntdlerilesniipwen.khshkcFioniee1oasndg–rl-
s Article. P This articl TtohrheDeirTpcPaa,rrtubicsolixenysgliwEceDarCcei-dcmrgoersdosiulaiptneskdwecdiothbuypthlbieinssg-aymmmemitdheeotfrdoicoralmolgadytiia(oSmncihtnheemrsoeCugY1hbS p5(T.h6aa–bt6lee.3b,2u,PffMeenrMt,rwAie-hs6e55rMe–a8As)A,w,dPeurEeeAt-co3r0othMsseAliirAnrkaeelndadtiivPneElpyAH-l6o5w5M.4pAKbAaupvffaaelrrut.iecTslehosef,
s
cce and c).64–66 It is important to note that although the amide collapsed crosslinked PMMA-20MAA/9CYS and PEA-10MAA/
A
n bondsformedarestable,thecrosslinkingitselfisreversibledue 20DTPparticleswerevisualizedbyopticalmicroscopy(Fig.1c)
e
Op to the presence of labile disulphide bonds within the cross- andSEM(Fig.1d).Thecollapsedmorphologywasalsoapparent
linker molecules.26,27,67 In all crosslinking reactions, the same forthe respective parent non-crosslinked particles (see Fig. 1a
molarproportionsofreagents(EDC,NHS,CYSorDTP)relative andb).Someoftheparticlesfoldedinwardsuponthemselves,a
to the number of available COOH groups were employed. feature which has been previously observed for other related
Elementalanalysismeasurementsrevealedthattheproportion systems.21,39,54
Table2 Crosslinkedparticlecompositions,swellingratiosandgelproperties
Crosslinkedcopolymer (cid:1) (cid:3)
Entry Copolymer RCONHRa(%) particlescomposition Qmaxb pHGm0axc Gm0 axd(kPa) gc pHGm0ax e(%)
1 PMMA-29MAA 31 PMMA-20MAA/9CYS 5.9 7.0 8.7 —
2 14 PMMA-25MAA/4DTP 11.4 7.0 56.3 —
3 PMMA-38MAA 37 PMMA-24MAA/14CYS 3.4 7.0 5.4 17.7
4 5 PMMA-36MAA/2DTP 11.9 7.0 100.0 —
5 PMMA-65MAA 31 PMMA-45MAA/20DTP 4.7 6.5 0.2 43.7
6 PEA-30MAA 33 PEA-20MAA/10CYS 11.5 6.5 7.3 53.9
7 67 PEA-10MAA/20DTP 8.4 6.5 1.4 18.3
8 PEA-65MAA 9 PEA-59MAA/6DTP — 6.5 24.2 —
aPercentageofRCONHRformed.bSwellingratioatpHG0 ,IneachcasethisvalueofQcorrespondedtothemaximumswellingratio.cpHatwhich
gelswithGm0 axareformed.dThemaximumstoragemodumlauxsvalue.eYieldstrainatpHGm0ax.Thesw(cid:1)elling(cid:3)ratioforentry8wasnotdetermined,asthe
particleswerealreadyswollenatthelowestpHtested(pH5.4).Forentries1,2,4and8theg valueswerenotdeterminedbecausethey
c pHGm0ax
exceededthemaximumgvaluesusedintheexperiments.
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Fig. S5a, ESI† show that upon increasing the pH any large
particleswhichwerepresentgraduallydissociatedintosmaller
aggregatesandeventuallytheparticlesdissolved.
Fig. 2b shows the variation of the particle size (D ) with a.
h
The data in Fig. 2b show that for all systems, particle size
decreases as the value of a increases, resulting in complete
dissolution of the non-crosslinked particles at 100% neutrali-
sation (a ¼ 1). The increasing solubility of the particles with
e. increasingpHisduetotheionisationofRCOOHgroupsalong
c
en Fig.2 Sizedistributionfunctiondatafortheparentnon-crosslinkedparticlesare the polymer chain, which causes the polymer chains to repel
AM. ed Lic PshMoMwAn-i3n8M(a)A.AT,hoenDlyhtahseaDshfluowncatrioencoonfsaidearreeds.hownin(b).ForPEA-65MAAand ewaecahkeonththereirduheydtroopthhoebiricminutteuraacltinoengsa.t70iv,7e1 Ecvheanrtgueasllyan(adt hthiguhs
2:31 nport a)thepolymerchainsbecamehydrophilicenoughtoovercome
5U
23 12:n 3.0 Dynamiclightscatteringdata tahnedidnitsesromlvoel.ecTuhlaerpfaorrtciecslesbesthwreinenk aindjasiczeentduweatteor imndolievicduuleasl
n 2/18/20Attributio TimnheveeassstuiizrgeeasmteoedfntthbseywnDeorLneS-ccaraorsrsisealdinfoukunetdctaaitonpndHcorvfoasltuhsleeisnskroaenldugtpiinoagrntifcrploHems.w5Te.h4re–e csthhhaoaniwnnsdibsueinnlodtewegr,rgtaohtiiinosgnprtiohfcisienstpserrcocachneaslisenaodcfrtoodsispslsianorlktuisctlieaornse.w72pelrAleisnsegnwrtia.lltFhboeerr
ded omons 8.0. Examples of size distributions for PMMA-38MAA-, PEA- PMMA-38MAAandPEA-30MAAtheDhdecreasesgraduallywith
oam 30MAA-andPMMA-29MAA-basednon-crosslinkedparticlesare a over the entire course of neutralisation, reecting the
wnlCo showninFig.2aaswellasinFig.S5a,ESI.†TheD valuesare comparativelylowersolubilityofthesecopolymers.TheD vs.a
oe h h
013. DCreativ scuromssmlianrkiezeddPMinMTAa-b2l9eMSA1A, (EFSigI..†SF5ao,rEtShIe†),diPsMpeMrsAio-6n5sMoAfAnaonnd- cthuervselsoepxehbibeictoamgeesnstlteeesploepreaisnaa,inracnregaisnegsfuronmtil(cid:4)c0o.m1ptole0te.8naenud-
ublished on 21 June 2e is licensed under a pPtseiaEosrAtnte-is3dcl0.teMwFs)ooAwrAmet,hrtoeehdaePepsdMpipsMatrarAerintb-i3tuc8lateMitospAnsHAslow6(wF.e0irg–(eD.6m2.sh5laoo.)wnT)aohnamedndodPadEtfaaaAls-ia6ntt5(aTMDlalfhAbapsAlHte)dSv(i1sasmlpauneaerldsl- ptunranearudltiteisrcaralgtelioiossaniatzieoiscndo.ern5ec7f,ar7o3ecrahmsFeeaoddtr.iosTnhPhaMailrsMptlsryAua-an6gsg5sieMatsioAtisnnActrhdaeaunatrsdientdhgP,eEitnAphd-ae6irc5taciMcotliuAensrAgsemththaoaeytf
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Fig.3 EffectofpHontheDhofcrosslinkedandnon-crosslinkedparticles.Selectedsizedistributionfunctionsforcrosslinkedparticlesareshownin(a)and(b).TheDhas
afunctionofpHfor(c)PMMA-38MAA,PMMA-36MAA/2DTPandPMMA-24MAA/14CYSand(d)PEA-30MAA,PEA-10MAA/20DTPandPEA-20MAA/10CYS.For
PMMA-38MAAandPEA-20MAA/10CYS,DhrefersspecificallytoDshlowonly.
ThisjournalisªTheRoyalSocietyofChemistry2013 J.Mater.Chem.B,2013,1,4065–4078 | 4071
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dissociation of the carboxylic acid groups for these particles ratio(ofabout5.9atpH7.0)wasobservedfortheformer,which
leadstoacorrespondinglygreaterincreaseincopolymerchain featuresalowerlevelofcrosslinking(seeTable2,entries1and
solubility compared to their less hydrophilic, (meth)acrylate- 3). The same trend is present for the dispersions of PMMA-
richcounterparts.ComparedtothePMMA-65MAA,theparticles 36MAA/2DTP, PMMA-25MAA/4DTP and PMMA-45MAA/20DTP
formedfromPEA-65MAAarelarger,whichcouldbeduetothe wherethelargerswellingratio(ofabout11.9atpH7.0)isfound
greaterexibilityofPEAcomparedtoPMMAsegments. for the PMMA-36MAA/2DTP particles (see Table 2, entries 2, 4
Compared to the non-crosslinked particles, dispersions of and 5). This is due to the increased expansion of the polymer
particlescrosslinkedwithCYSorDTPshowedaninitialsharp chains as the extent of the crosslinking decreases.27 Particles
e. increase in the particles' diameter, i.e., pH-triggered swelling composed of EA-containing copolymers were found to swell
c
n (see Fig. 3). This was followed by a peak in D and then a moreatagivenpHthantheirstructurallyisomericMMA-based
e h
AM. ed Lic istusbmseaqxuimenutmdevcarleuaes.eTahtihsiwghaesrdpuHe tvoalpuHes-tarigegrertehde dDihssroelaucthioend cvaoluunetse,rpwahritcsh.TrheissuclatsnbineeaxpcloarirneesdpobnydthinegirlyreglaretiavteelryleoxwteenrtpKoaf
2:31 nport (see Fig. 3c and d). Representative crosslinked particle size carboxyl group ionisation at a particular pH. For example,
5U
23 12:n 3.0 dfoirstPriMbuMtiAo-n2s5MfoArAP/M4DMTAP-2a4rMe pArAe/s1e4nCtYedS,iPnEFAi-g2.03MaAaAn/1d0bCYaSndanind cthoemEpAar-ceopPoMlyMmAe-r20pMarAtAic/l9eCs,YwSitwhitahnPaEpAp-2ro0xMimAAat/e1l0yCeYqSu,ivwahleenret
n 2/18/20Attributio FMS1igM,.EASS,5Ib.t†,hEeFSoIdr.i†sttThrhiebeuccrtiooorsnrselsisnpwkoeenrddeipnmagrotDincholemardeoidssauplmerasmtioaanrlislzecpdoHnintavTianaluibnelges aptwlesirocceehntahtsaegaveocolounfmtfrueibn(usctetiieoonTnaafblriolzemed2,thCeenOtiOrniHecrseg1arsaoenuddpcs6h,)a.siTwnhemlilsodtboiiffliaetlyrmednoucseet
ed oons tested. For the PEA-20MAA/10CYS- (Fig. 3b), PEA-10MAA/ to more exible EA segments. The same trend occurs for the
dm
oam 20DTP- and PEA-59MAA/6DTP-based crosslinked particles the dispersions of PMMA-45MAA/20DTP and PEA-10MAA/20DTP,
nlo
wC distributions were typically bimodal or an intermediate mode whichfeatureswellingratiosof4.7and8.4,respectively(Table
oe
013. DCreativ wapapseaalrsotoaspwpealrletnot.aAleltshsoeurgehxtethnettfhasatnatnhdeisnlotewrmpaerdtiiaclteesp,asrimticilleasr 2sw,eenlltrtoiesa5laarngedr7e)x.tFeunrtththearnmtohroe,sepawrhtiicclhesacrreocsrsolisnskliendkwedithwiDthTPa
ss Article. Published on 21 June 2 This article is licensed under a dp2vt1mwha04HoeaerMCwriFxeaY7PnioAtmS.cMwriA0olue/aatMn1(ahrmnFs0drAedilCsgiy-sn2pY.twPo0oaSM3etbMrhpclatslM)ieHAneic,nirAdlArvwge6/-ePs9D.3hv5dCE6heac(AMYr(lrFvuseo-Sia1eAaesg,le0ssAsu.PMl/T3e(if2MQnodsaADkbrm)MAw.Tela/teTAdiP2xht)h-0hSe2pwaiD15srac,PiTMethcrMEhPtoapAiSrcMntrIAerlh†egeei/Atsi)4elssh.ae-sD4ntemQ5eTirtnmsMeaPCdaxw,AtxiYhemPiASinleMs/lu2wosTmMo0oraibDtlAbuhsiDTs-let2ePTtirs4hov,Ph2Mnee,.PidpAFEtpheKoAAaHderat/- cl14(lDseeoTwoff45TnwamCMeePgbeclY,tplAlrtiheSeaanAodsr2tg/ofah2,tfrb0heteerlhDtaenseshettTtwnirperoPieciorcseDorltt,lsoaippiTonsb3aohPnsglrralyattimstirinnhocoadklowintbegieleaos5ehorer)cfea.dnasurCttTtlrtlYephahuet.SHdivhecs,Feeteppul7oqisrar.orus0eerfpaastewmaictu(xceCrraamlloeteYsmyiscsaS3sp'upbcl.Helilov4laenyxem,,,rpkrdwlsafipytuunohhneeisrgeelnslt,riDuPsooewvasMTanattthhsPelrMuiaade)fcffeotAhiuinrovf-pwo2rercPeod4rraMod.enMesfaudMTsAttc4hhhebeAA.d7eeesy/-
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cc values of the corresponding (parent) non-crosslinked particles mechanicalpropertiesofthegelsisdiscussedbelow.
A
n (see Table 1). The Q values for all crosslinked particle disper-
e
Op sionsatvariouspHvaluesarelistedinTableS1,ESI.†Swelling
Particlegelmorphologyanddynamicrheology
ratios of up to 11.5 and 11.9 for CYS- and DTP-functionalized
particles, respectively, were measured. Unfortunately, for PEA- ForthePMMA-29MAAandPMMA-38MAAparticlescrosslinked
59MAA/6DTPtheswellingratiowasnotmeasuredastheparti- with either CYS or DTP strong physical gels were formed at
cleswerealreadyswollenatpH5.4(TableS1,ESI,†entry13). particle concentrations of only 5 wt% and in the pH range of
The crosslinked particles initially expand to swollen 7.0–7.5, which is highly desirable for future potential applica-
hydratedparticlesasthepHincreasesandthisprocessisdriven tioninvivo.GelationoccurredatpH6.5–7.0forPMMA-45MAA/
by electrostatic repulsion between negatively charged carboxy- 20DTP, PEA-20MAA/10CYS, PEA-10MAA/20DTP and PEA-
late groups. Whereas the non-crosslinked particles gradually 59MAA/6DTP. Formation of physical gels under these condi-
disintegrate into unimers or small clusters when this electro- tionsoccurredwhentheeffectivevolumefractionoccupiedby
static repulsion exceeds the hydrophobic attraction between theswollenparticlesexceededthatwhichpermitsinter-particle
MMA or EA groups, the polymer chains of the swollen cross- movement.21,38,39 The particles swelled to the point of inter-
linkedparticlesareeffectivelyheldtogetherbytheircrosslinks particle contact and this physically locked them in their loca-
(seeFig.3candd).However,disulphidegroups,suchasthose tions. At this point the gels were able to distribute load
presentinCYSandDTP,areknowntobereadilycleavedunder throughoutthenetworkofphysicallyinterconnectedparticles.
alkaline conditions.74,75 As a consequence the crosslinked RepresentativeSEMimagesoffreeze-driedgelsformedfrom
particlesalsosuccumbtodisintegrationandforthisreasonthe swollenPMMA-24MAA/14CYS,PMMA-36MAA/2DTPandPMMA-
pK values of the crosslinked particles (necessary in order to 25MAA/4DTPgels,atpH7.0,areshowninFig.4andinFig.S6,
a
correlatesolutionpHwithavalues)werenotdetermined. ESI.† All gels studied feature a highly porous interconnected
CloseexaminationofthepH-dependentswellingdataresults morphology, similar to what has been previously reported for
(Fig. 3 and Table 2) reveals some important general trends. analogoussystems.21,38,39Theirporesizesrangefrom1to5mm
Firstly,betweenparticlesofsimilarcompositionsuchasPMMA- and are comparable to those observed in the corresponding
20MAA/9CYS and PMMA-24MAA/14CYS, the larger swelling crosslinkedparticleprecursors(Fig.1).Comparedtogelsderived
4072 | J.Mater.Chem.B,2013,1,4065–4078 ThisjournalisªTheRoyalSocietyofChemistry2013
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crosslinked copolymer particles (5 wt% concentration) were
subjected to frequency-sweep studies from 0.1 to 100 Hz at a
constantstrainof0.1%.ThevariationofG0 andtand(tand¼
G00/G0) with the oscillatory frequencies for PMMA-24MAA/
14CYS,PMMA-36MAA/2DTPatpH7.0andPEA-20MAA/10CYS,
PMMA-45MAA/20DTP physical gels at pH 6.5 are shown in
Fig. 5a and b. The data are characterized by G0 exhibiting a
plateau in the whole frequency range which is indicative of a
e. stablecrosslinkednetwork.Withinthefrequencyrange1–10Hz
nc all systems behaved as elastic gels with tand values less than
e
AM. ed Lic 1th.0e,sit.eru.,cGtu0>reGs0d0(iFdign.o5tbh)a.vAetthimigheetrofrreelqauxeunpcoiensa,pfrpolmica2t0io–n10o0fHthze,
2:31 nport deformationstresswhichresultedintandvaluesincreasingto
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23 12:n 3.0 gorbesaetrevredthdaniffe1r.0enacnesd ionccGur0reanricsee ofrfonmetwthoerkdbiffreearekndtownnu.mTbheer
n 2/18/20Attributio FPiMg.M4A-2S4EMMAAim/1a4gCeYsSogfeflrseefozrem-derdiedat(paH) a7n.0d. (b) PMMA-36MAA/2DTP and (c) s(dFpeiongns.idt5ycwoaefnlledlwadsitt)ih.caTtlhhlyeoseleffoweocebtsitvtaeiGnc0ehdvaailnfursoemsinwttehhreeegsoetbrlasseinwrv-hsewidcehefopcrotrterhseet-
ed oons heavilycrosslinkedPMMA-45MAA/20DTP.Forthisphysicalgel
dm
wnloaCom f2r0oDmTPP,MPMEAA--2200MMAAAA//190CCYYSS, PanEdA-1P0MMMAAA-/2240MDATAP,/1P4MCYMSA(-F4i5gM. 4AcA)/, othcecufrrreeqduwenacsyaabtouwth2i0chHtza,ncodm¼pa1r.e0d(taoc8r0it–i9ca0lHfrzefqouretnhcey,otfhcreitr)
oe
ublished on 21 June 2013. De is licensed under a Creativ cPoataEhotnuMSeonIdtdT†sMsnteh)beabAegcpn)y-,te2admil5vdrPistniMeyEitwcncgiAeAhlhaee-sAam5isl.cna/94hWisihMcDctaiaeaATcrdrlihAPestptey/mfr6rooevoaDulrasonpmTcslgeduhPyretehtdstimPaaeh(nfEtssirecdAeotaokeh-msf5ePeubr9tsMrPehMeeplMeMmoAonwMprAAeee)hnw-/.A26ytw-5Dss3P.MiaT6MclDMPalAsMliAAsga/AgpA4ene-e/D3l2ldrss6sTDMiwhPoTaeinAPglr(ssAFhe(o/Fio2geiifDv.gnga.tTatSleh4vuP6raee--,, Pt12a(idn<ir40Mym0ocnCM.ruMe0aYeAnm1a(SAAsds-cia/ecscr10nido)t0.s.dn0¼tCr1rtaPTYaa1t1MhiieS–/nnfi0Mahcsia.rnta0inAtgwds)1d-h3o3hgaoi6feisshcrMlasthfcibeog.oAorthrTnAhuehrs/erter2iiessa0iDflntcr.acer0Trtixonate5Par(nrttras-pigwepbofhesaioonpmtyrurhsotetiiPnttccni1hMladm0etelssM0enggtoHeoerAfelct-szawta4o.)ht5rocTaeerMrrnrheikPtsdmeAiMprcAdaeooaM/pablsn2tahir0dAalieoDis-rflt2rsaoyTt4xrutePoMoarnPftasAEditnocahAAernndier/-t
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Fig.5 Frequency-sweepdata(a)and(b)andstrain-sweepdata(c)and(d)forPMMA-36MAA/2DTP,PMMA-24MAA/14CYS,PEA-20MAA/10CYSandPMMA-45MAA/
20DTPatpH¼7.0.Thefrequency-sweepdataweremeasuredatastrainof0.1%.Thestrain-sweepmeasurementswereconductedatafrequencyof1Hz.
ThisjournalisªTheRoyalSocietyofChemistry2013 J.Mater.Chem.B,2013,1,4065–4078 | 4073
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PEA chains.57,63 For the PMMA-containing gels the data are particle that contributed to stress distribution. Gels with an
consistent with a decrease in chain mobility as the extent of optimumelasticresponsewereobservedatpH6.5–7.5,whichis
crosslinkingincreases. theidealpHrangeforfuturephysiologicalapplications.Inthis
Strain-sweepdynamicrheologyexperimentswereperformed pH region maximum values of G0 (G0 ) and minimum tand
max
onthecrosslinkedparticlegelsoverabroadpHrange(frompH values were observed. Va(cid:1)lues for(cid:3)the Gm0 ax of each gel and the
6.0–11.0). Fig. 5c and d show data for PMMA-24MAA/14CYS, corresponding pH value pHG0max are shown in Table 2. Inter-
P20MDMTPA-m36eMasAuAr/e2dDaTtPp,HP¼EA7-2.00.MFoArAs/1tr0aCinYsS(ga)nldessPtMhaMnA1-405%M,AthAe/ eosctciunrgsl,y,dethmeopnHstGra0matxinvgalauesstrocnorgrecsoprroenladtitoontbheetwpeHenwmhearxeimQummax
e. gelshadtandvalueslessthan1.0.Thecriticalyieldstrain(gc)is particle swelling and gel elasticity. Further increases in pH
nc thegvaluecorrespondingtotand¼1.0.Whengisgreaterthang resulted inadecrease inG0 (Fig.6a andb) andan increasein
e c
AM. ed Lic mcoonrseideenreerdgtyoisbedisusiidp.atTehdro(ausghhoeuatt)ththeasntrastionrreadnagnedstuthdeiegdetlhies tsawneldlin(Fgiga.nSd7,paErStiIc†l)eaisntaerpcoennseetrqautieonnc.eTohfedpeacrrteiacsleesdppaarrttiiacllley
2:31 nport concentrated dispersions of PMMA-24MAA/14CYS, PMMA- deswelled due to the increased ionic strength. This decreased
5U
23 12:n 3.0 4150MMAAAA//2200DDTTPP,paPrEtiAc-l2e0sMatApAH/1¼0C6Y.5S(F(Figig..S95ecananddf,EdS)I†a)nsdhoPwEeAd- tdheenseitxyteonfteolafstiinctaelrly-peaffrteicclteivecocnhtaaicnts.aInndterheesnticnegltyh,ethneucmorbreer-
n 2/18/20Attributio abdtoistphheigrhsuieoirdn-sdliwekfeeorraemnmdatogiroeenl-eslilkatsehteibce,rhweasipvthioontuasrne.Adwt<als1o.w0cl(deGae0rfl>oyrGmn00oa),tniwo-ehnlaesrstethiaces. s2Pp4MoMMnAdAAi-n/31g64MCdYeAcSAr/ea2anDsdTesPP,iMnPMMGA0M-w4A5e-M2re0AMmAA/o2Ar0e/D9CpTrYPoSnthogauenlnscfe(oFdrigfto.hre6aPr)eM.laMTtheAde-
ed oons However, under the same conditions, the behaviour of the rst two systems were more highly crosslinked and had lower
dm
nloaom PMMA-36MAA/2DTP dispersion was distinctly different and respective Qmax values than the latter. This implies that inter-
wC showedentirelyelasticresponsesatallstrainpercentages.Whilst particle molecular entanglements (i.e., inter-particle contact
oe
013. DCreativ tvhaelureeswgarseaatmerotdheasntd0e.1c5re%as(eFiing.G50cf)oarnPdMaMcAor-3re6sMpAonAd/2iDngTPin,cwrietahsge amreoareahtigthhelypsawrotilclelenp(PerMipMhAer-3ie6sM) AwAe/r2eDmTPoraenedxPteMnsMivAe-2f0oMr AthAe/
ublished on 21 June 2e is licensed under a (itooPnshvbEeeetsAearee-nFr5ntvi9hdgteMi.de(rFSeAfi8eoAgscnr.t/,r6t5tiadDdrhi,en)Teg,Pt,PrrhaahMpenn,aMvggSraee9tAlicucs-ol2etaefu5sndgMfdsoeiteAlrdrsdaAt,aia/En(n4tuSDppIdp†THret)Pe.or¼mc,e1Pan07Mi0.tn0a%Megade)An.sw-d2Ee0tl6elaMl.ss5bttA,eeicAdrloe/b9swwepCh1aeY.sac0Stvioiaavovlensueldoyrr 9spgrrueueCasblsrYpctbSieoece)lpnlraetsspieitbgavilcaleeretilstsiptcyita.lacoeritTtdypighecHelctieshlr-stee.graioisesgIrlengysoe.tipearnSrenpiedocmdsrstoidiiiltnssaiessgralslityotnour,lkeunntttihdhiqdoeoeusnnsee.wstrifeFetgeyrnoaeerdaltsucottrpbhuwersoaeeeosldrrlefeyvicet(itdhanpelehcdsfsroyoeesrfairlpcsoteahHemsldes)-
Article. PThis articl forTPhMeMvaAr-i2a4tiMonAsAo/1f4GC0YanS,dPtManMdAw-2it0hMpAHA/m9CeaYsSu,rPeMdaMtA1-%36sMtrAaiAn/ mPEoAr-ecopnrtoaninoiunngcegdelsf.oArsPsEeAen-10inMFAiAg/.260bDTthPecdoemcrpeaarseedintoG0PwEaAs-
s 2DTP, PMMA-45MAA/20DTP, PEA-10MAA/20DTP and PEA- 20MAA/10CYS. The PEA-20MAA/10CYS gel was less extensively
s
Acce 20MAA/10CYSareshowninFig.6andS7,ESI.†Thefullsetof crosslinkedandalsohadalowerQmaxvalue.
en dataforallthegelsareshowninFig.S8andS9,ESI.†Similarto ThevaluesofGm0 axforthesePMMA-andPEA-containinggels
Op what has been previously reported for analogous systems,21,39 areuptofactorsof10and2greater,respectively,comparedto
theinitialincreaseinpHresultedinanincreaseinG0 (Fig.6a therelatedgelswhichwerereportedpreviously.21,39Alongwith
andb)andadecreaseintand(Fig.S7,ESI†).Thisisattributed theoptimizedparticlecompositions,whichresultedinforma-
to the swelling of the particles as the pH increased. As the tion of highly elastic MAA-containing physical gels, another
particlesswelled,andimpingedoneachother,agreaterinter- reason for these high G0 values is the improved dispersion
max
particle contact area resulted and this, in turn, increased the stability achieved by the slower feed employed during particle
number density of elastically effective chains within each preparation. A particle gel that forms from well dispersed,
Fig.6 VariationofG0withpHforPMMA-MAAandPEA-MAA-basedcrosslinkedparticles.
4074 | J.Mater.Chem.B,2013,1,4065–4078 ThisjournalisªTheRoyalSocietyofChemistry2013