Diels–Alder-Crosslinked Polymers Derived from Jatropha Oil

17 Pages·2017·4.9 MB·English

polymers
Article
Diels–Alder-Crosslinked Polymers Derived from
Jatropha Oil
MuhammadIqbal1,2 ,RemcoArjenKnigge2,HeroJanHeeres2,AntoniusA.Broekhuis2
andFrancescoPicchioni2,*
1 DepartmentofChemistry,InstitutTeknologiBandung,JalanGaneshaNo.10,40132Bandung,Indonesia;
[email protected]
2 ENgineeringandTEchnologyInstituteGroningen(ENTEG),ChemicalProductEngineering,Universityof
Groningen,Nijenborgh4,9747AGGroningen,TheNetherlands;[email protected](R.A.K.);
[email protected](H.J.H.);[email protected](A.A.B.)
* Correspondence:[email protected];Tel.:[phone]
(cid:1)(cid:2)(cid:3)(cid:1)(cid:4)(cid:5)(cid:6)(cid:7)(cid:8)(cid:1)
(cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:6)(cid:7)
Received:23September2018;Accepted:15October2018;Published:22October2018
Abstract: Methyl oleate, methyl linoleate, and jatropha oil were fully epoxidized using in
situ-generatedperformicacid.Theepoxidizedcompoundswerefurtherreactedwithfurfurylaminein
asolvent-freereactiontoobtainfuran-functionalizedfattyesterswhich,then,functionedasoligomers
foranetworkpreparation. Thermoreversiblecrosslinkingwasobtainedthrougha(retro)Diels–Alder
reactionwithbismaleimide,resultingintheformationofabrittlenetworkforfuran-functionalized
methyllinoleateandjatrophaoil. Thefuran-functionalizedfattyestersweremixedwithalternating
(1,4)-polyketonereactedwithfurfurylamine(PK-Furan)fortestingthemechanicalandself-healing
propertieswithDMTAandDSC,respectively. Fullself-healingpropertieswerefound,andfaster
thermoreversibilitykineticswereobserved,comparedtoPK-Furan.
Keywords: fattyacidmethylester;jatrophaoil;epoxidation;polyketone;thermoreversible
1. Introduction
Thermosettingpolymersarewidelyusedinindustry,duetotheirtunablemechanicalproperties
(modulus, strength, and durability) as well as their chemical and thermal resistance. This is due
totherelativelyhighcrosslinkingdensitydisplayedbythesematerials. However,thesamefactor,
i.e.,highcrosslinkingdensity,hampersthepossibilityforthesematerialstobereshapedorrecycled.
This is related to the factual impossibility of selectively cleaving the crosslinking bonds without
affectingthemainchainones. Asaconsequence,end-of-lifethermosetscanberecycledonlyaccording
to a “cradle-to-grave” approach (e.g., by incineration), thus adding pollution to the environment.
Anotherdrawbackofthermosetsisthedependenceonoil,themainsourcesforrawmaterialsofthis
classofchemicalproducts. Inthelongterm,thesefactorsrenderthermosetpolymersunfit,inageneral
context,assustainable(chemical)products[1,2].
Recycling of thermosets has been an important field in sustainability research, since it may
decreasetheneedforrawmaterialsandsignificantlylowertheecologicalimpact. Themaindifficulties
arethedevelopmentofefficientprocessesfortherecoveryofthestartingmaterialsandtheworse
mechanicalpropertiesuponrecycling[3]. Theuseofreversiblecrosslinkingbondsmightrepresent
asolutiontothisproblem. Bydisassemblingthepolymernetwork,thematerialcouldberecovered
andusedinotherproductsofsimilarvalueandapplications(i.e.,accordingtoa“cradle-to-cradle”
approach).
Different approaches of obtaining reworkable thermosets properties have already been
investigated. Oneofmostpopularapproachesistheuseofthermallyreversiblecrosslinkedpolymers.
Polymers2018,10,1177;doi:10.3390/polym10101177 www.mdpi.com/journal/polymers
Polymers2018,10,1177 2of17
This concept involves a crosslinking reaction at relatively low temperatures (40–50 ◦C), while the
reversereaction(i.e.,thedisassemblingofthenetwork)occursathighertemperatures(110–140◦C)[4].
CrosslinkedpolymerscanbepreparedusingaDiels–Alder(DA)cycloadditionoffuranandmaleimide,
givingareversedreactionthrougharetroDiels–Alder(rDA).Forinstance,self-healingthermosets,
basedonpolymerizationthroughDiels–Aldercrosslinkingwithatetra-methoxyfuranmonomerand
1,8-bis(maleimide)-1-ethylpropane,havealreadybeenreported[5]. Likewise,thermallyreversible
reactions,basedonfuran-functionalizedpolyketoneandbismaleimidethroughDAandrDA,have
alreadybeenreported,withexcellentefficiencyforthermosetsself-healing[6,7].
Whiletheapproachesdescribedabovemightprovideasustainablesolutionfortheend-of-lifeof
thermosettingpolymers,theuseofrenewableresourcesfor(totalorpartial)substitutionofthermosets
raw material might also constitute another added value. Indeed, renewable resources are more
environmentally friendly and, also, interesting from an economic point of view (i.e., low cost and
abundance). Vegetable oils are considered as one of the potential sources for biopolymers, due to
theirunsaturatedfattyestersstructureswhichcontainsdoublebonds(forpossiblefunctionalgroup
transformation)[8]. Jatrophaoil(JO)isconsideredasapromisingsourceforpolymer,notonlybecause
ofitshighunsaturatedfattyacidcontent(3.2–3.8doublebondspertriglyceridemolecule)but,also,
due to its toxic nature of common variety (no direct competition with the food sector) [9]. Even
thoughJOismainlyinvestigatedforbiofuelproduction,asagreatpotentialreplacementtodiesel,
theoilispotentiallydevelopedforothertypesofapplications(e.g.,lubricant,pharmaceutical,polymer,
etc.)[10–14].
Regarding the potentiality of JO in polymer applications, several synthetic routes have been
reported in literature, in order to obtain a vegetable oil-based polymer [8]. Some popular routes
involvethemodificationofthereactivefunctionalgroupsonthefattychains(i.e.,doublebondsand
estergroup),inordertoyieldamoleculewithdesiredfunctionalities(e.g.,hydroxyl,oxiranering,etc.).
Ahigh-yieldepoxidationmethodofvegetableoilshasbeenreported,usinginsitu-generatedperformic
acid[15]. Inaddition,kineticstudiesonepoxidationofJO,usingperoxyaceticandperoxyformicacid,
havealsobeenreported[16]. Intermsofimprovingtheversatilityoftriglycerides,theepoxidation
processisconsideredtobeanimportantreactionroute,sincetheoxiraneringismuchmorereactive,
and opens the possibility for reaction with various nucleophiles [8]. As an example is the aniline
functionalizationofepoxidizedmethyloleate,catalyzedbyanionicliquid[17]. Similarly,additionof
aceticanhydridetomethyloleatehasalsobeenreported[18]. Apromisingsalt-catalyzedaminolysis
reactionhasalsobeenwidelystudied,forinstance,theuseofLiBr[19]and,also,ZnCl [20],forepoxide
2
ringopeningusingseveraldifferentaliphaticandaromaticamines. Aninterestingresultwasreported
regardingthereactionsofepoxidizedtriglyceridewithn-hexylamineand1,6-hexamethylenediamine.
Insteadofonlyaminolysis,thereactionalsoshowsamidationasaprimaryreaction,whichleadsto
crosslinkingproducts[21]. However,thecrosslinkedproductwasnotexploredanyfurther.
Basedonpreviouslyreportedresults,theresearchstrategyofthisworkisdividedintoseveral
steps. Thefirststepinvolvesthechemicalmodificationofthedoublebondsonunsaturatedfattyester
chainsintoepoxygroups. Thesecondoneisthereactionoffurfurylaminewithepoxidizedfattyesters,
yieldingafuran-modifiedmonomer.Thirdly,thepolymerizationofthefuran-functionalizedmonomers
withbismaleimideviaDAreaction,followedbycharacterizationasapotentiallythermoreversible
polymer.Lastbutnotleast,theuseoftheresultedpolymerasapartialsubstituteinthewell-established
thermoreversiblesystemofcrosslinkedfuran-functionalizedalternatingpolyketone. Tothebestofour
knowledge,thesecond,third,andfourthstepsoftheresearchhavenotbeenpreviouslyreportedin
theliterature.
Polymers2018,10,1177 3of17
2. MaterialsandMethods
2.1. Materials
Methyl oleate (methyl cis-9-octadecenoate ≥99%, Sigma-Aldrich, Munich Germany), methyl
linoleate(methylcis,cis-9,12-octadecadienoate ≥99%, Sigma-Aldrich), andjatrophaoil(3.3double
bondspertriglycerideunits,locallyhot-pressedfromJatrophacurcasL.seeds,CapeVerde)areused
as sources of fatty acid moieties. For the epoxidation reaction, formic acid (ACS Reag PH Eur
≥99%,MerckChemicals,Darmstadt,Germany),hydrogenperoxide(stabilized,PhEur,BP,USP30%,
Merck Chemicals), and benzene (ReagentPlus®, thiophene free, ≥99%, Sigma-Aldrich) were used
as received. Trimethylsulfonium hydroxide (derivatization reaction for GC, 0.25 M in methanol,
Fluka) and tert-butyl ether (analytical standard, Sigma-Aldrich) are used for fatty esters analysis
usingGC-MS.LiBr(anhydrous,≥99%,Sigma-Aldrich)wasusedtoassistthefuranfunctionalization
reactionofepoxidizedcompoundusingfurfurylamine(2-aminomethylfuran≥99%,Sigma-Aldrich)
andbismaleimideforthecrosslinkingreaction(1,1(cid:48)-(methylenedi-4,1-phenylene)bismaleimide≥95%,
Sigma-Aldrich), and were used without further purification. In addition, THF (anhydrous, BHT
inhibitor,≥99.9%,Sigma-Aldrich)andchloroform(anhydrous,PCRreagent,amylenestabilizer,≥99%
Lab-Scan,Munich,Germany)wereusedassolvent. Chloroform-d(99.96atom%D,Sigma-Aldrich)
andDMSO-d (99.96atom%D,Sigma-Aldrich)wereusedforNMRanalysis.
6
2.2. EpoxidationReaction
Theepoxidationreactionwasadaptedfrompreviouslyreportedworks[15].Two-stepepoxidation
reactionswereperformedtoimproveconversiononalargerreactionscale. Fattyesterorjatropha
oil was diluted using benzene (molar ratio of C=C on fatty ester/benzene 1/5) in a three-neck
round bottom flask. H O and formic acid (molar ratio C=C/H O /FA = 1:10:1) as oxidant were
2 2 2 2
addeddropwisetotheflaskintwosteps(40◦Cat400rpm). Thefirststepwasallowedtoreactfor
12 h, followed by water layer removal. Subsequently, another batch of FA and H O (molar ratio
2 2
C=C/H O /FA=1:10:1)wasalsoaddeddropwisetothereactionmixture.Afteranother12h,thewater
2 2
layerwasremovedfollowedbywashingthesolutionusingdemineralizedwatervialiquid–liquid
extraction (5 times). Afterwards, benzene was removed using rotary evaporation. The resulted
productswereanalyzedwith1HNMRandGC-MS.
2.3. FuranFunctionalizationReaction
2.3.1. FunctionalizationofFattyEstersorJatrophaOil
Theepoxidizedfattyester(orepoxidizedjatrophaoil)andfurfurylamine(molarratiooxirane
ring/furfurylamine1/5)weremixedinaglassvessel. Toassisttheepoxidationreaction,LiBr(110mol
% for epoxidized fatty ester or 50 mol % for epoxidized jatropha oil, with respect to the mol of
doublebonds)wereaddedtothemixture. Afterwards,thevesselwasclosedandsealed,toprevent
evaporationoffurfurylamine. Themixturewasfurtherheatedat80◦C,andmechanicallystirredat
400rpmfor24h. Theproductwasdriedinvacuumovenat50◦Ctoremovetheexcessofunreacted
furfurylamine.ThedriedproductwasfurtherdissolvedinCHCl ,followedbyliquid–liquidextraction
3
usingdemineralizedwatertoremovetheLiBrsalt. Afterwards,thechloroformwasremovedusing
rotary evaporation followed by drying in vacuum oven at 50 ◦C until constant weight achieved.
Thefinalproductswereanalyzedwith1HNMR.
2.3.2. FuranFunctionalizationofAlternatingPolyketone
Polyketone-50(50%ethylenecontent)wasaddedtoasealed250mLglassreactor,equippedwith
anoilbathandamechanicalstirrerwithaU-typepropeller. Thepolyketonewaspreheatedtothe
liquidstateat100◦C.Furfurylamine(equimolaramountsof1,4-dicarbonylgroupsandaminegroups)
wasadded,dropwise,in10min. Thereactionproceededfor4h,withastirringspeedof400rpm.
Polymers2018,10,1177 4of17
Afterthereaction,themixturewaswashedseveraltimeswithdemineralizedwatertoremovethe
unreactedfurfurylamine. Theproductwasfilteredandfreeze-dried,obtainingPK-Furanasalight
brownpowder. Thisprocedurerepresentsaslightlymodifiedformofapublishedone[6,7].
2.4. CrosslinkingReaction
2.4.1. CrosslinkingReactionofFuran-FunctionalizedFattyEsterorJatrophaOil
Thefuran-functionalizedcompoundandbismaleimideweredissolvedinaroundbottomflask
equippedwithmagneticstirrerandacondenser. Afterwards,thereactionwascarriedoutat50◦C,
300rpm,for24h. Afterchloroformremoval,theproductwasdriedinvacuumovenat50◦Cuntil
constantweightachieved.
2.4.2. CrosslinkingofFuran-FunctionalizedFattyEsterorJatrophaOilwithPK-Furan
PK-Furan and the furan-modified fatty ester or jatropha oil were reacted with an equimolar
amountofbismaleimide(ratioofmaleimide/furangroups—1:1)usingCHCl (10wt%)assolvent,
3
in a round bottom flask equipped with magnetic stirrer and condenser. The fatty acid derivative
contained10%or20%ofthetotalfurangroups. Thereactionwascarriedoutat50◦Cand300rpmfor
24h. Theproductwasdriedinavacuumovenat50◦CuntilCHCl wasremoved.
3
2.5. Characterization
2.5.1. 1HNMR
Characterizationoftheproductsby1HNMRwasperformedbyusingaVarianMercuryPlus
400 MHz (Mundelein, IL, USA) using CDCl or DMSO-d as solvents. MestRenova software
3 6
(SantiagodeCompostela,Spain) was used for NMR spectra processing. The different peaks were
assignedaccordingtopreviouslypublishedresults[10]. Thereactionconversionofepoxidationof
methyloleatewascalculatedfromtheratiobetweenpeakareaofepoxidepeakformedinepoxidized
methyloleate(–CHOCH–proton2.8ppm)towardstheinitialpeakofthedoublebondfrommethyl
oleate(–CH=CH–proton5.4ppm),whichisdefinedas
A
epoxide
Conversion(%) = ×100 (1)
AC=C
Thecalculationforepoxidationconversionofmethyllinoleatealsofollowstheformulapresented
inEquation(1). However,thepeakusedforthecalculationsrepresents4protons(twodoublebonds)
each,insteadof2formethyloleate. Thepeakareaofformedepoxidesinepoxidizedmethyllinoleate
(–CH<proton3.1and3.2ppm)werecomparedtowardstheinitialpeakofthedoublebondfrom
methyllinoleate(–CH<proton5.4ppm).
Jatrophaoilconsistsmainlyofoleicandlinoleicchains,attachedtoaglycerol. Peaksbelongingto
theglyceridearepresentat4.2and4.3ppm(–CH –),representing4protons,and5.3ppm(–CH<),
2
represent 1 proton which overlaps with the double bond signal. The signals of the overlapped
double bond on the triglyceride were calculated by difference, based on the integration of other
non-overlappedpeaks(–CH –). Theepoxidesignalsappearat2.8to3.2ppm,similartotheoleicand
2
linoleicproducts. Therefore,theconversionofthereactionisalsocalculatedfollowingEquation(1).
2.5.2. GC-MS
EpoxidizedfattyacidwasanalyzedusingTHFassolvent. Fattyacidcompositionofjatrophaoil
wasanalyzedasitstransmethylatedderivates[22]. Forsamplepreparation,100mgofjatrophaoilwas
dissolvedintert-butylether(5mL).Asampleof130µLwasmixedwith70µLoftrimethylsulfonium
hydroxide inside a 2 mL autosampler vial. The vial was capped and properly shaken to ensure
mixingbeforeitwasmeasuredusingagaschromatography-massspectrometry(GC-MS)inaHewlett
Polymers2018,10,1177 5of17
Packard(HP,PaloAlto,CA,USA)5890seriesIIPluswithHPChemstationG1701BA,B0100/NIST
librarysoftwareandaHewlettPackard(HP)5972seriesmassselectivedetector. Fused-silicacolumn
(Solgel-1MScolumn: 30m×0.25mm×0.25µm)wasusedasthegascarrieratacolumnpressureof
49kPa. Thecolumntemperaturewas80◦C(hold4min),80–150◦C(rate10◦C/min),150–240◦C(rate
3◦C/min),240◦C(hold10min). Afterthat,thetemperaturewasfurtherincreasedat3◦C/minto
240◦C,andheldfor10min. Asplitratioof1:100wasused,theinjectoranddetectortemperaturewere
setat280◦Cwhereasthegasflowratewassetto1mL/min.
2.5.3. DSC
Analysisofthermoreversiblecrosslinkingwasperformedusingdifferentialscanningcalorimetry
(DSC).Samplesof10to15mgwereplacedinanaluminumpan. Aftersealingthepan,thetemperature
wasraisedat5◦C/min,fromastartingtemperaturetoamaximumtemperature. Thesampleswere
keptatthemaximumtemperaturefor1min. Aftercoolingdown,thecyclewasrepeatedforanoverall
of4cycles. Theoutputwasgivenastheheatflowinthesystemagainsttemperature. Allfirstcyclesof
theDSCwereremovedfromthethermogram,sinceitmostlyrepresentsthermalhistoryofthesample.
Therefore,secondcycleswerefurtherreferredtoasfirstcycles,andsoon.
2.5.4. DMTA
DynamicmechanicalthermalanalysiswasachievedusingaRheometricsScientificsolidanalyzer
(RSAII,Midland,ON,Canada). Analysiswasconductedinthedualcantilevermodewithastrainrate
of0.025%andanoscillationfrequencyof1Hz. Thedistancebetweentheoutersetpointswas35mm.
Thesamplewasheatedfromroomtemperatureto120◦C,withaheatingrateof3◦C/min. Samples
neededfortheDMTAmeasurementswerepowderedinacoffeegrinder,andpressedinrectangular
barsat120◦Cand4MPafor20min,withalengthof54mm,awidthof6mm,andathicknessof
1.5mm.
Fortheanalysis,thebarswereclippedbetweenthescrews. Theouterscrewswerekeptinplace
withadistanceof35mm,andtheinnerscrewwasmovedupanddownwithastrainrateof0.025%and
afrequencyof1Hzinthedualcantilevermode. Thetemperaturewasraisedfromroomtemperature
to120◦C,withaheatingrateof3◦C/min. Theoutputexistsoutofalossmodulus,storagemodulus,
andthetan(δ)againsttemperature. Thelossmodulusorelasticmodulus(E’)showstheabilityof
theproducttoreturntoitsinitialstate. Itisalsoreferredtoasthematerial’s“stiffness.”Thestorage
modulus(E”)representsinternalstrainandviscousforces,ameasureofdeformation. Theratioof
the loss modulus to the storage modulus gives the dampening property, tan(δ). It relates to all of
themolecularmovementandphasetransitions. Forexample,apeakoftan(δ)isfoundattheT for
g
apolymer. TheDMTAanalysiswasdividedintotwosteps. Twocyclesweredoneconsequentlyper
sample,onlyallowingthesystemtocoolbetweenthetworuns. Afterwards,thesamplewas“healed”
for 24 h at 50 ◦C, to ensure bond reconstitution, followed by another two cycles of measurements
(Figure1).
Polymers2018,10,1177 6of17
Figure1.SequenceofDMTAanalysis.
3. Results
3.1. Furan-FunctionalizedMethylOleateandMethylLinoleate-BasedPolymers
Thissectiondiscussestheuseofmethyloleate(MO)andmethyllinoleate(LO)asapreliminary
study, with respect to further study of jatropha oil (JO) as a crosslinked polymer precursor. Even
thoughthefattyacidcontentofJOmayvarydependingonspecies, climate, soilquality, etc., MO
and LO are chosen based on the fact that both components are the major fatty acid constituents
in JO [9,10]. As the first step of the polymer synthesis, MO and LO were epoxidized according to
publishedprocedures[15]. FormicacidwasreactedinsituwithH O toformperformicacid,which
2 2
actsasanoxygendonorforthealkene. ThedonatedoxygenisreplenishedbyH O ,generatingwater
2 2
(Scheme1). Formicacidcanalsoopentheoxiranering,andhydrolysiscanoccurinthepresenceof
water. Benzenehasbeenaddedasdiluentoftheorganicphase(i.e.,fattyesters)tominimizeoxirane
ringopening[23]. Sincethetwolayersareimmiscible,thedegreeofmixinghasalargeinfluenceon
thereaction. However,atacertainpoint,ahighdegreeofmixingmaypromotetheunwantedoxirane
ringdegradationprocess. Inaddition,higherreactiontemperaturesgenerallyfavorthedegradation
reaction[15,16].
Theopeningoftheoxiraneringmayoccurinthepresenceofwaterandformicacidand,toalesser
extent,H O . Highchemicalactivityofformicacidmaycausedepletionofoxygeninthesystemif
2 2
theH O isnolongerpresent,resultinginlowerconversions[15,16]. Therefore,two-stepepoxidation
2 2
reactionswereformulatedtoensurethepresenceofH O . Asexpected,theepoxidationreactionof
2 2
MOandLOgivesfullconversion(NMRspectranotshownforbrevity)and,also,goodyield(i.e.,MO:
76%andLO:92%). Inaddition,fullconversiongivesoneoxiranegroupforepoxidizedmethyloleate
(ep-MO),andtwogroupsforepoxidizedmethyllinoleate(ep-LO).Thisnumberisratherimportantfor
furthersteps,sincetheoxiraneringactsasthebasisforthefuranfunctionalizationreaction.
Tothebestofourknowledge,thereactionbetweenfurfurylamineandepoxidizedfattyesterhas
notbeenreportedinliterature. Thereactiondoesnotspontaneouslytakeplaceatatmosphericpressure
androomtemperature. Sincetheprimaryamineoffurfurylaminemoleculeisratherweaktoundergo
ringopeningreactionwiththeoxiranegroup,thisaminolysisreactionrequiresassistanceintheform
ofacatalyst. Inaddition,thecatalystisalsoexpectedtoassistamidationreactionontheestergroups
oftheepoxidizedfattyesters(Figure2). Bothreactionsareratherimportant,especiallyforep-MO,
sinceitrequiresatleasttwofurangroupsinordertoassembleapolymericchain.
Polymers2018,10,1177 7of17
Figure2.1HNMR-spectrumofFAOwithfunctionalgroupsassigned.
Polymers2018,10,1177 8of17
Scheme1.Reactionschemeofjatrophaoil-basedDiels–Alder-crosslinkedpolymer.
Polymers2018,10,1177 9of17
Basedonasimilaraminolysisprocedurewithdifferentprimaryaminesdescribedinliterature[15,
18], oxirane contained ep-MO or ep-LO was reacted with furfurylamine. After 24 h, the reaction
givesaconversionof82%ontheestergroupand89%ontheepoxideforep-MO.Whileep-LOhas
aconversionof83%ontheestergroupand75%ontheepoxide(confirmedbyNMR).Thepresence
oftwooxiraneringclosetoeachotheronep-LOisbelievedtoberesponsibleforloweraminolysis
conversion(sterichindrance). Thesepartialconversionsareactuallydesiredforlaterreactionswith
JO(seebelow). Inaddition,thefunctionalizedoleic(FAO)andfuran-functionalizedlinoleic(FALO)
chainshaveapproximately1.7and2.4furangroups/moleculerespectively. Themixturesofpartially
convertedFAO(includingalsostereo-isomers)displayabroadrangeofprotonsignalsin1HNMR
spectra. Therefore,signalassignmentishighlightedonlytotheimportantpeaks(Figure2).
Aspreliminarystudy,FAOwasreactedwithbismaleimidetoobtainpolymerasaresultoftheDA
adductformation. WithamaximumoftwopossibleadductsiteperFAOmolecule,thereactionmay
onlygivealinearchain. However,poorsolubilityoftheresultedpolymercr-FAOinsomesolvents
(i.e.,acetone,THF,chloroform,andDMSO)atroomtemperatureindicatesarelativelyhighmolecular
weightproductwasobtained. TheDA-rDAthermoreversibilitysystemoncr-FAOwasinvestigated
usingDSC(Figure3). At110◦C,bondsruptureviarDAreactionroutestartstakingplaceindicatedby
heatuptake. Eachconsecutivecyclerevealsthedecreaseonthematerial“thermo-recoveringdegree”
specifiedbythelossofenergyuptake.
Figure3.DSCthermogramofcr-FAO.(25◦C→180◦Cfor1min,10◦C/min,3cycles).
The DA-rDA system is an equilibrium system with respect to temperature and also
a time-dependent mechanism, especially at lower temperature region. The decrease on
thermo-recovering degree most likely caused by the partial failure on molecular reconstitution
via DA reaction at given time. The presence of more stable DA adduct isomer (i.e., endo and exo
adduct)alsocontributetothedropofthermo-recoveringdegree,aspreviouslyreportedforsimilar
systems[7]. Inaddition,lossoffunctionalitythroughpermanentcrosslinkingalsoarisesasanother
possibility. Thelatterpossibilityonlyimpliesifreductionofweightoccurs. Thepossibilitycanbe
eliminatedsincethethermogramprofileat40◦Cto80◦Cshowsnoapparentdecreaseofheatuptake.
ThecrosslinkingreactionofFALOandbismaleimide(I =1)givesmuchbrittleproductcomparedto
m/f
Polymers2018,10,1177 10of17
cr-FAO.ThedifferencecausedbythenumberofpossiblefuranfunctionalitytoundergoDAreaction
withbisamleimide. Acrosslinkedproductwasobtainedsincetheaveragefurangroupspermolecules
aremorethantwo.
3.2. Furan-FunctionalizedJatrophaOil-BasedPolymers
Throughsamemethodusedonepoxidationofthefattyesters(i.e.,MOandLO),JOwasfully
epoxidizedresultinghighyieldof96%(Figure4). Therefore,thefullconversiongivesanaverageof
3.3epoxidesgrouppertriglyceridemoleculesofep-JO.Asmentionedearlier,thissumofoxiranering
permoleculesisimportantsinceitwillundergoaminolysisreactionwithfurfurylamineasnucleophile.
Inaddition,occurringamidationreactionontheestergroupssplitsthefattyesterfromtheoriginal
triglyceridemolecule.
Reactionsofep-JOwithfurfurylaminewereperformedwithlowerLiBrintake(i.e.,50%). Partial
amidationonfuranfunctionalizationreactionofep-JOresultinabroadvarietyofFurfuryl-amine
functionalizedJatrophaOil(FAJO)products(i.e.,furan-functionalizedmono-,di-,triglyceride,andfree
fattyamide). Themultipletat3.7ppmgivesstrongindicationofglycerolormonoglyceride(Figure4).
However,somesignaloverlapoccurssinceaminolysisproductsgiveabroadsignalbetween3.7to
4.0ppm. Sterichindranceofthetriglyceridestructurecontributestopartialaminolysisreaction,with
remainingunreactedepoxidepeaksat2.9to3.2ppm. Atriglyceridecontainsthreeestergroupsclose
togethermakingthereactionsitebulkyforamidation. Thestereochemistryofthemoleculehindersits
accessibilitytonucleophile(i.e.,furfurylamine)aswellasLiBrtoassistthereaction.Improvingreaction
conversionbyusinganexcessofLiBratcertainpointmightleadtocontra-productiveresultssincethe
metalionmayalsointeractwithtwocarbonylgroupsthroughachelationeffect[20]. Theamidesignal
at4.4ppmisusedtocalculatetheamidationreactionconversion,whiletheremainingoxiranegroup
isusedfortheaminolysis. Atotalof4.2furangroupswerefoundforeverytriglycerideascombined
reactionofamidation(47%conversion)andaminolysis(74%conversion).
Figure4.1HNMRspectraoffuran-functionalizedjatrophaoil(400MHz;CDCl ).
3
Crosslinking reactions of FAJO using bismaleimide (equimolar ratio) were performed in
a reflux setup. The resulted product (cr-FAJO, I = 1) was analyzed using DSC to investigate
m/f
itsthermoreversibilityprofile(Figure5). Anincreaseofheatflowstartstooccuraround125◦Cforeach
cyclewhichrepresentstherDAreaction.Similartocr-FAOandcr-FALO,thethermo-recoveringdegree
of each cycles decrease gradually compared to its previous cycles. The decrease can be explained
using same possibilities as for crosslinked fatty esters (i.e., short time for bond reconstitution and
formationofmostthermodynamicallystableadductsisomer)[6,7]. Inaddition,relativelyhighdegree
ofcrosslinkoncr-FAJOcausedtheproducttohaveabrittlenature.