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GREENSUSTAINABLE PROCESSFOR CHEMICALAND ENVIRONMENTAL ENGINEERINGAND SCIENCE GREENSUSTAINABLE PROCESSFOR CHEMICALAND ENVIRONMENTAL ENGINEERINGAND SCIENCE SupercriticalCarbonDioxide asGreenSolvent Editedby
INAMUDDIN
DepartmentofChemistry,KingAbdulazizUniversity,SaudiArabia
ABDULLAHM.ASIRI
DepartmentofChemistry,KingAbdulazizUniversity,SaudiArabia
ARUNM.ISLOOR
DepartmentofChemistry,NationalInstituteofTechnology
Karnataka,India
Elsevier
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Contributors PoonamAggarwal
DepartmentofFoodScienceandTechnology,PunjabAgriculturalUniversity,Ludhiana, India
MuktaAgrawal
RungtaCollegeofPharmaceuticalSciencesandResearch,Bhilai,Chhattisgarh,India
MudasirAhmad
DepartmentofFoodScienceandTechnology,UniversityofKashmir,Srinagar,India
Ajazuddin
RungtaCollegeofPharmaceuticalSciencesandResearch,Bhilai,Chhattisgarh,India
SumiaAkram UniversityofEducation,BankRoadCampus,Lahore,Pakistan
AmitAlexander RungtaCollegeofPharmaceuticalSciencesandResearch,Bhilai,Chhattisgarh,India
FrancoRicoAmado
MaterialsandEnvironmentLaboratory(LAMMA),StateUniversityofSantaCruz—UESC, Ilheus,Brazil
AndrewN.Amenaghawon
DepartmentofChemicalEngineering,FacultyofEngineering,UniversityofBenin,BeninCity, Nigeria
NanjangudVAnilKumar
DepartmentofChemistry,ManipalInstituteofTechnology,ManipalAcademyofHigher Education,Manipal,India
ChineduL.Anyalewechi
DepartmentofChemicalEngineering,FacultyofEngineering,UniversityofBenin,BeninCity, Nigeria
RaoufAslam
DepartmentofProcessingandFoodEngineering,PunjabAgriculturalUniversity,Ludhiana, India
MojhdehBaghbanbashi
DepartmentofChemicalEngineering,AmirkabirUniversityofTechnology(Tehran Polytechnic),Tehran,Iran
MingBao
StateKeyLaboratoryofFineChemicals,DalianUniversityofTechnology,GanjingziDistrict, DalianChina;SchoolofPetroleumandChemicalEngineering,DalianUniversityof Technology,LiaodongwanNewDistrict,Panjin,China
DanielAssumpc ¸ a˜oBertuol
EnvironmentalProcessesLaboratory(LAPAM),ChemicalEngineeringDepartment,Federal UniversityofSantaMaria—UFSM,SantaMaria,Brazil
F.W.F.Bezerra
TechnologyInstitute,ProgramofPost-GraduationinFoodScience,Technologyand Engineering,FederalUniversityofPara ´ ,Belem,Brazil
P.N.Bezerra
TechnologyInstitute,ProgramofPost-GraduationinFoodScience,Technologyand Engineering,FederalUniversityofPara ´ ,Belem,Brazil
R.N.CarvalhoJunior
TechnologyInstitute,ProgramofPost-GraduationinFoodScience,Technologyand Engineering;TechnologyInstitute,ProgramofPost-GraduationinAmazonNaturalResources Engineering,FederalUniversityofPara ´ ,Belem,Brazil
RajeshChandra
BioenergyResearchLaboratory,DepartmentofPolymer&ProcessEngineering,IndianInstitute ofTechnologyRoorkee(SaharanpurCampus),Saharanpur,India
A.M.J.ChavesNeto
TechnologyInstitute,ProgramofPost-GraduationinAmazonNaturalResourcesEngineering; LaboratoryofPreparationandComputationofNanomaterials,FederalUniversityofPara ´ , Belem,Brazil
AhmadCheikhyoussef UniversityofNamibia,Windhoek,Namibia
NataschaCheikhyoussef MinistryofHigherEducation,TrainingandInnovation,Windhoek,Namibia
Gun-HeanChong
FacultyofFoodScienceandTechnology,UniversitiPutraMalaysia,Serdang,Selangor, Malaysia
R.M.Cordeiro
TechnologyInstitute,ProgramofPost-GraduationinAmazonNaturalResourcesEngineering, FederalUniversityofPara ´ ,Belem,Brazil
EstevanDornelesCruz
EnvironmentalProcessesLaboratory(LAPAM),ChemicalEngineeringDepartment,Federal UniversityofSantaMaria—UFSM,SantaMaria,Brazil
V.M.B.Cunha
TechnologyInstitute,ProgramofPost-GraduationinFoodScience,Technologyand Engineering,FederalUniversityofPara ´ ,Belem,Brazil
W.A.daCosta
TechnologyInstitute,ProgramofPost-GraduationinAmazonNaturalResourcesEngineering, FederalUniversityofPara ´ ,Belem,Brazil
J.N.daCruz
LaboratoryofPreparationandComputationofNanomaterials,FederalUniversityofPara ´ ,Belem, Brazil
M.S.deOliveira
TechnologyInstitute,ProgramofPost-GraduationinFoodScience,Technologyand Engineering,FederalUniversityofPara ´ ,Belem,Brazil
AnaLu ´ ciaBarbosadeSouza
UniversityCenterSENAICIMATEC,HealthInstituteofTechnologies(CIMATECITS), NationalServiceofIndustrialLearning—SENAI,Salvador,Bahia,Brazil
NoorUDinReshi
DepartmentofChemistry,UniversityofKashmir,Srinagar,India
JaniceIzabelDruzian
FederalUniversityofBahia(UFBA),FacultyofPharmacy,Salvador,Bahia,Brazil
SunilKumarDubey DepartmentofPharmacy,BirlaInstituteofTechnologyandScience,Pilani,Rajasthan, India
AdilGani
DepartmentofFoodScienceandTechnology,UniversityofKashmir,Srinagar,India
SandraGonc ¸ alves FacultyofSciencesandTechnology,MeditBio,UniversityofAlgarve,Faro,Portugal
NaghmehHadidi
DepartmentofClinicalResearchandElectronicMicroscope,PasteurInstituteofIran,Tehran, Iran
NoorHadzuinNikHadzir
FacultyofFoodScienceandTechnology,UniversitiPutraMalaysia,Serdang,Selangor, Malaysia
ShahryarJafarinejad
DepartmentofChemicalEngineering,CollegeofEngineering,TuskegeeUniversity,Tuskegee, AL,UnitedStates
JunaidKhan
UniversityTeachingDepartment(Pharmacy),SantGahiraGuruUniversity,Sarguja,Ambikapur, Chhattisgarh,India
HeriSeptyaKusuma
DepartmentofChemicalEngineering,FacultyofIndustrialTechnology,InstitutTeknologi SepuluhNopember,Surabaya,Indonesia
Wan-JunLee
FacultyofFoodScienceandTechnology,UniversitiPutraMalaysia,Serdang,Selangor, Malaysia
XinLiu
StateKeyLaboratoryofFineChemicals,DalianUniversityofTechnology,GanjingziDistrict, DalianChina;SchoolofPetroleumandChemicalEngineering,DalianUniversityof Technology,LiaodongwanNewDistrict,Panjin,China
BrunaAparecidaSouzaMachado
UniversityCenterSENAICIMATEC,HealthInstituteofTechnologies(CIMATECITS), NationalServiceofIndustrialLearning—SENAI,Salvador,Bahia,Brazil
MahfudMahfud
DepartmentofChemicalEngineering,FacultyofIndustrialTechnology,InstitutTeknologi SepuluhNopember,Surabaya,Indonesia
LeonidasMatsakas
BiochemicalProcessEngineering,DivisionofChemicalEngineering,DepartmentofCivil, EnvironmentalandNaturalResourcesEngineering,Lulea ˚ UniversityofTechnology,Lulea ˚ , Sweden
SyedKazimMoosvi
SchoolEducationDepartment,J&KGovernment,Srinagar,India
MuhammadMushtaq
DepartmentofChemistry,GCUniversity,Lahore,Pakistan
RavishPatel
RamanbhaiPatelCollegeofPharmacy,CharotarUniversityofScienceandTechnology, CHARUSATCampus,Changa,Ta-Petlad,Anand,Gujarat,India
AlokPatel
BiochemicalProcessEngineering,DivisionofChemicalEngineering,DepartmentofCivil, EnvironmentalandNaturalResourcesEngineering,Lulea ˚ UniversityofTechnology,Lulea ˚ , Sweden
GholamrezaPazuki
DepartmentofChemicalEngineering,AmirkabirUniversityofTechnology(Tehran Polytechnic),Tehran,Iran
FernandoLuizPellegriniPessoa
UniversityCenterSENAICIMATEC,HealthInstituteofTechnologies(CIMATECITS), NationalServiceofIndustrialLearning—SENAI,Salvador,Bahia,Brazil
R.H.H.Pinto
TechnologyInstitute,ProgramofPost-GraduationinFoodScience,Technologyand Engineering,FederalUniversityofPara ´ ,Belem,Brazil
JagbirRehal DepartmentofFoodScienceandTechnology,PunjabAgriculturalUniversity,Ludhiana,India
Joa˜oHenriquedeOliveiraReis
FederalUniversityofBahia(UFBA),FacultyofPharmacy,Salvador,Bahia,Brazil
MasoodAhmadRizvi DepartmentofChemistry,UniversityofKashmir,Srinagar,India
AnabelaRomano FacultyofSciencesandTechnology,MeditBio,UniversityofAlgarve,Faro,Portugal
ShailendraSaraf
UniversityInstituteofPharmacy,Pt.RavishankarShuklaUniversity,Raipur,Chhattisgarh,India
SwarnlataSaraf
UniversityInstituteofPharmacy,Pt.RavishankarShuklaUniversity,Raipur,Chhattisgarh,India
KmSartaj
MolecularMicrobiologyLaboratory,BiotechnologyDepartment,IndianInstituteofTechnology (IIT-R),Roorkee,India
SabahuddinSiddique PatelCollegeofPharmacy,MadhyanchalProfessionalUniversity,Bhopal,MadhyaPradesh,India
M.P.Silva
TechnologyInstitute,ProgramofPost-GraduationinFoodScience,Technologyand Engineering,FederalUniversityofPara ´ ,Belem,Brazil
EmmaSuali
FacultyofEngineering,JalanUMS,UniversitiMalaysiaSabah,KotaKinabalu,Sabah,Malaysia
NorhidayahSuleiman
FacultyofFoodScienceandTechnology,UniversitiPutraMalaysia,Serdang,Selangor,Malaysia
DhanyaSunil DepartmentofChemistry,ManipalInstituteofTechnology,ManipalAcademyofHigher Education,Manipal,India
EduardoHiromitsuTanabe
EnvironmentalProcessesLaboratory(LAPAM),ChemicalEngineeringDepartment,Federal UniversityofSantaMaria—UFSM,SantaMaria,Brazil
NaderVahdat
DepartmentofChemicalEngineering,CollegeofEngineering,TuskegeeUniversity,Tuskegee, AL,UnitedStates
Wan-HuiWang
StateKeyLaboratoryofFineChemicals,DalianUniversityofTechnology,GanjingziDistrict, DalianChina;SchoolofPetroleumandChemicalEngineering,DalianUniversityof Technology,LiaodongwanNewDistrict,Panjin,China
PoojaYadav
RungtaCollegeofPharmaceuticalSciencesandResearch,Bhilai,Chhattisgarh,India
MudasirYaqoob DepartmentofFoodScienceandTechnology,PunjabAgriculturalUniversity,Ludhiana,India
GiovaniL.Zabot
LaboratoryofAgroindustrialProcessesEngineering(LAPE),FederalUniversityofSantaMaria (UFSM),CachoeiradoSul,Brazil
PoojaYadava,MuktaAgrawala,AmitAlexandera,RavishPatelb, SabahuddinSiddiquec,ShailendraSaraf d,Ajazuddina
aRungtaCollegeofPharmaceuticalSciencesandResearch,Bhilai,Chhattisgarh,India bRamanbhaiPatelCollegeofPharmacy,CharotarUniversityofScienceandTechnology,CHARUSATCampus,Changa, Ta-Petlad,Anand,Gujarat,India
cPatelCollegeofPharmacy,MadhyanchalProfessionalUniversity,Bhopal,MadhyaPradesh,India dUniversityInstituteofPharmacy,Pt.RavishankarShuklaUniversity,Raipur,Chhattisgarh,India
Inthepastfewdecades,polymershavebecomeanimportantpartofdailylife.Their feasiblesynthesisandprocessingareneededforvariousapplications.Insteadofusingconventionalsolvents,nowtheresearchhasbeenshiftedtosupercriticalfluids.Thesupercriticalfluidsaredescribedasamaterialmaintainedataconditionoftemperatureand pressuremorethanitscriticalvalues.Itholdstheuniquecombinationofviscosity(like gas)anddensity(likeliquid)whichmakesasupercriticalfluidanexcellentsolvent [1,2] Supercriticalcarbondioxide(SCO2)isanon-toxicsolvent,whichisthebestalternative
Fig.1 Schematicrepresentationoftemperature–pressurephasediagramofthepurecomponent, surroundedbytriplepoint(T)andcriticalpoint(C). (AdaptedfromS.P.Nalawade,F.Picchioni,L.P.B.M. Janssen,Supercriticalcarbondioxideasagreensolventforprocessingpolymermelts:processingaspects andapplications,Prog.Polym.Sci.31(1)(2006)19–43.)
fornoxiousorganicsolventsandchlorofluorocarbons.Owingtotheirimmensephysical propertiessuchasnon-flammability,chemicallyinertness,andcheapness,thesefindapplications,notonlyinpolymersynthesis,butalsoinpolymerproduction.Thesupercritical conditionsofcarbon-dioxide(CO2)caneasilybeachieved(Tc ¼ 304Kand Pc ¼ 7.38 MPa)(Fig.1)andextractedoutfromthereactionbysimplydepressurizingthereactor. ThehighyieldandincreasedproductqualityattracttheusageofSCO2 [4]
Moreover,CO2 isagasinambientconditions,whichmakesiteasytoremovefrom thepolymericproductinsteadofusingcostlymethodsofsolventevaporationordrying. Mixturesofsolventsareusedtoenhancethestrengthofchemicalreactionduringthe synthesisandprocessingofpolymers [5].However,byincorporatingSCO2 intothe responsegivesadvantagesoverconventionalsolvents,suchas:
•itpossessesenvironmentallybenigncharacteristicssuchasbeingnon-toxic,nonflammable,andlowcost;
•easyremovalofthesolventafterthereaction;
•fasterrateofreactionatmildconditions;and
•higherselectivityandhigheryieldinchemicalreactions(obtainedduetotemperature andpressureabovecriticalvalues).
2.PropertiesofsupercriticalCO2 SCO2 haswidelygainedimportanceinrecentyearsbecauseofitsassureddiffusivityand density.Thesecanchangetheglasstransitiontemperature(Tg)ofthepolymersleadingto theirreducedviscosity [6].Undeniably,eventhoughCO2,duetoitsdistinctsymmetrical
structure,hasnodipolemoment,itcanshowinviolablequadruplemoment(actingata smallerdistancethandipolarinteraction),henceincreasingthesolubility [7,8].TheCO2 itselfshowsexcellentsolubilizingpowerandisthereforeusedasasolventandantisolvent. CO2 ispreferredasapolymerizationmediumbecause:
•thesearecost-effective,non-flammable,non-toxic,andarereadilyobtainablein pureform;
•solventrecoverybecomeseasyduetotheuseofCO2;
•thehighsolubilityofvariouspolymerswithinCO2 findsitsapplicationfortheir synthesis;
•forporouspolymerstobesynthesized,thesupercriticalfluidhavingCO2 facilitatesas non-solvatingpowerandactsasanorganicdiluent;and
•carbondioxideusuallydoesn’tinteractwiththestrongnucleophiles(e.g.,alkoxides, primaryamines,etc.),andhenceitcanbesuggestedpolymerizationinCO2 is generallythroughtheanionicmechanism.
Thenonpolarmolecules,havinglowmolecularweight,canbereadilysolubilizedin supercriticalCO2 ratherthanwaterandioniccompoundsbecausesupercriticalCO2 hasarelativelysmalldielectricconstant(ε ¼ 2).SCO2,owingtoitstremendousphysiochemicalproperties,isalsocheap,non-toxic,andnon-flammable,hencefindingapplicationsintheproductionandrefinementofpolymers.Owingtothehighmasstransfer, lowviscosity,andlowtoxicityprofile,thesupercriticalcarbondioxidecanbepotentially usedinpolymerprocessing,whichalsoovercomestheTrommsdorffeffect.Also,with supercriticalCO2,drypolymerscanbeeasilymadebymerelydepressurizingthereactor afterpolymerization [9,10]
3.ApplicationsofSCO2 inpolymerproductionandprocessing 3.1Purificationofpolymers Apriorquantitativeanalysisoftheextractedcompoundfromthepolymericmatrixbydifferentconventionalmethods,likeduringsynthesisorprocessingofpolymers,isessential becausethereisachancetoobtainby-productsorresidualrawmaterialsthathavetobe extracted.Theconventionalmethodsforpurificationofpolymersincludevariousmethods suchassolvent-intensive,Soxhletextraction,orpolymerdissolutionmethod [11].SCO2 possessestheabilitytoextractouttheseby-productsbyfirstlyswellingthepolymerand thenpermeatingwithinthematrixwheretheseimpuritiesgetsolubilizedintothe SCO2 andagainuponreducingthepressure,SCO2 iseasilydiffusedoutofthepolymer. Interestingly,thesolvatingpowerofSCO2 isregulatedbyslightmodificationinpressure andtemperature.Hence,itisusedinpurificationofpolymers [12,13].Moreover,thesedo notleaveanyharmfulresidue,representingthenon-toxicbehaviorofSCO2.Inaddition, thesecanbeeasilyrecycledduetotheirvolatilenature.Comparedtotheconventional extractionprocesses,ifweuseadrymembranethatispre-treatedwithsupercriticalcarbon dioxide,thiscanresultinlessshrinkageandimprovedwaterpermeability [14,15].
3.2Impregnationandsupercriticaldyeing CO2 canbeusedasasolventtointroducenumerousdyesandmetalcomplexesintothe polymerichosts.Hence,itmaybeapplicableduringdyeing.Adopingsolutebehaving likeaguestisintroducedwithinthehostpolymermatrixduringpolymerimpregnation anddyeing.ThesupercriticalpropertiesofSCO2,includinglowsurfacetension,higher diffusivity,andeasyrecoveryofsolvent,assiststhepreparationofanewpolymer [16,17] Impregnationisthetechniquetoabsorbtheliquidswithinthepolymermatrix.During impregnationusingSCO2,thetherapeuticdrugisdissolvedintothesolventandthen impregnatedwithinthepolymermatrix,whichcanbepreparedfordrugdelivery.There aretwomechanismsforSCO2 infiltrationasadditivesinthepolymericmatrix.Thefirst mechanismdealswiththesolute,whichgetssolubilizedintotheSCO2 andthenispassed throughthepolymermatrix.Afterreducingthepressure,CO2 vacatesthepolymer matrix,andthesoluteparticlesbecomestrappedwithinthepolymer [4,18].Thesecond mechanismdealswiththepartitioningofsolutewithinthepolymermatrixbecauseof theirlowsolubilityinSCO2.Thehighaffinityofsolutetowardspolymeristheprimary factorforsuccessfulsupercriticaldyeing [19]
3.3Particleproduction Normalmillingandgrindingmaydeterioratethepolymerparticles;therefore,administeringSCO2 asasuitablesolvent,orantisolvent,couldofferamorepromisingapproachto reducetheparticlesizeandcontrolthemorphologicalpropertiesofthepolymer.Inadditiontothis,adjustingtheprocessparameterssuchaspressure,temperature,rateofdepressurization,andnozzlediametercanalsoresultindifferentparticlesize [20].Althoughahigh pressureisrequiredfortheparticleformationwhileusingSCO2 insteadofusingorganic solvents,whichtendtoproduceundesirablesolventresidues,itisbeneficialtouseSCO2 wherenosuchtoxicwastesaregenerated.Themicronsizedparticlescanbeproducedby low-meltingpolyestercoatings,acryliccoating,andpolyester-epoxysystems [21].Either ofthetwomethodscanformpolymericparticles,precipitationgenerallythrougharapid expansion,whichinvolvestheuseofSCO2 orbyusingSCO2 asanantisolvent.
3.4Polymermodification MonomersandinitiatorscangetdissolvedwithintheCO2 leadingtoenhanceddiffusion withinthepolymermatrixandcanthereforeeasilymodifythemorphologyofpolymer. Graftingofchemicalgroupswithinthepolymersubstratecanbedonethroughadditional reactions [22].
Theisopropyl-isocyanategraftingonethylene-vinylalcoholcopolymers(EVOH) baretheadvantageofSCO2 providingaselectivedispersionmethodinadditiontofrequentreactionsofthemonomerinamorphousformofEVOHmaintainingtheircrystallinenature [23].
4.Polymerproduction 4.1Step-growthpolymerization Inthismethod,thefunctionalgroupofdifunctionalmonomersiscondensed.Byusing SCO2,polycondensationofcomponentssuchaspolyesters,polyamides(nylon),polyurethanes,andpolyureasispossible(Fig.2).
Thenameitselfshowsthatthepolymerizationprocesswillbestepwisefromdimersto largemoleculesandthentopolymerchains.Themainproblemassociatedwiththisprocessistediousremovalofhighviscosityby-products,whichcanlargelybeovercomeby supercriticalCO2,showinganexcellentplasticizingeffectthatwilldecreasetheviscosity ofthawinfluencingfinerstirring.Predominantly,supercriticalCO2 permitsextractionof by-products [8,24,25]
PolycarbonatescanbesynthesizedbyincorporatingSCO2 inthetransesterification ofBisphenolAusingdiphenylcarbonate.Here,SCO2 hasatendencytoremovethe phenolby-productsandenhancethediffusivityofphenolduringsolidstatepolymerizationprocess [8,26].Similarly,polyurethanesandpolydimethylsiloxane(PDMS) weresynthesizedusingSCO2 asdispersantmediaat60°Candabove200barpressure whereethyleneglycolandtolylene-2,4-diisocyanateactasmonomersandhydroxyorisocyanate-terminatedpolydimethylsiloxaneserveasasurfmer(Fig.3) [8,27]
Fig.2 Schematicrepresentationofstep-growthpolymerizationusingSCO2. (AdaptedfromC.Boyère, C.Jer ^ ome,A.Debuigne,Inputofsupercriticalcarbondioxidetopolymersynthesis:anoverview,Eur.Polym. J.61(2014)45–63.)
Fig.3 Reactionbetweentolylene2,4-diisocyanate,andethyleneglycolinthepresenceofsurfmer. (AdaptedfromP.Chambon,E.Cloutet,H.Cramail,T.Tassaing,M.Besnard,Synthesisofcore-shell polyurethane–polydimethylsiloxaneparticlesincyclohexaneandinsupercriticalcarbondioxideusedas dispersantmedia:acomparativeinvestigation,Polymer46(4)(2005)1057–1066.)
4.2Chaingrowthpolymerization SupercriticalCO2 isbestsuitedforradicalpolymerizationwhereallthethree components,i.e.,monomer,initiator,andcontrolreagents,areeffortlesslysolubilized insupercriticalCO2 [28].Itactsasagreensolventforthesynthesisofvariousfluorinated andnon-fluorinatedpolymersbyatomtransferradicalpolymerization(ATRP).Moreover,itplaysabinaryrolebystabilizingthegrowingchainsofPMMAandcomplexionof thecoppersaltinthesynthesisofpoly(methylmethacrylate)(PMMA)usingATRP [29, 30].Aone-potprocessusingdispersionATRPandazide-alkyne1,3-dipolarHuisgen’s cycloadditionwasemployedforpreparingfunctionalizedpolymers(Fig.4) [31].
4.2.1Homogeneouspolymerization ThefluoropolymerscanbesynthesizedbyusingSCO2 viacationicpolymerizationor freeradicalpolymerization [32].Thehomogeneouspolymerizationof1,l-dihydroperfluorooctylacrylate(FOA)andazobisisobutyronitrile(AIBN)usingSCO2 yieldsvarious fluoropolymersassociatedwithFOA [8,33].Furthermore,FOAcanbe co-polymerized usingvinylhydrocarbonmonomersinthepresenceofSCO2,whichyieldspoly(1,ldihydroperfluorooctylacrylate)(PFOA)(Fig.5).
Anotherco-polymertetrafluoroethylene(TFE)canbeusedforthesynthesisofvariousfluoropolymers;althoughTFEasamonomercausesrapidexplosions,whencombinedwithCO2 itformsa“pseudo”azeotrope,whichmakesiteasiertohandle.Various TFE-basedfluoropolymershavebeensynthesizedusingCO2 suchasfluorinatedethylenepropylene(FEP),perfluoroalkoxyalkanes(PFA),ethylenetetrafluoroethylene (ETFE),TFE/vinylacetate,Nafiontypematerials,andTeflon-AF-typematerials (Fig.6) [35] .
Fig.4 ReactionbetweendispersionATRPandclickreactioncontainingfunctionalgroupforthe formationoffunctionalPMMAmicrospheres. (AdaptedfromB.Grignard,C.Calberg,C. Jerome,C.Detrembleur, “One-pot” dispersionATRPandalkyne-azideHuisgen’s1,3-dipolar cycloadditioninsupercriticalcarbondioxide:towardstheformationoffunctionalmicrospheres,J. Supercrit.Fluids53(1)(2010)151–155.)
Fig.5 HomogeneouspolymerizationPFOAfromFOAmonomerusingSCO2. (AdaptedfromC.Boyère, C.Jer ^ ome,A.Debuigne,Inputofsupercriticalcarbondioxidetopolymersynthesis:anoverview,Eur.Polym. J.61(2014)45–63.)
Fluorinated ethylene propylene resin
Perfluoroalkoxy resin (FEP)
tetrafluoroethylene resin
Fig.6 DifferentTetrafluoroethylene-basedfluoropolymerssynthesizedbyusingcarbondioxide (AdaptedfromC.D.Wood,A.I.Cooper,J.M.DeSimone,Greensynthesisofpolymersusingsupercritical carbondioxide,Curr.Opin.SolidStateMater.Sci.8(5)(2004)325–331.)
CF2
Ethylene
Tetrafluoroethylene-co-vinyl acetate (ETFE) (PFA)
However,TFEandSCO2 basedfluoropolymersproducehighmolecularweight polymersduetothepresenceofmuchlessacidicendgroups.Hence,itbecomesnecessarytoaddfurtherchaintransferagents,whichwilldecreasemolecularweightandoptimizemeltprocessabilityofthepolymer [36].SCO2 playsadualroleherebyfacilitating themonomerintothepolymerphase(becauseoftheplasticizingpropertyofCO2)and favoringcrosspropagationduetodecreasedtemperature [34,37].
4.2.2Precipitationpolymerization Assuch,vinylmonomersarereadilysolubleinCO2 buttheirrespectivepolymersafter synthesisprecipitateoutfromthesolution.TheuseofSCO2 heresignifiesthatuponsimpledepressurizationofthereactor,theprecipitatedpolymerswillcrystallizeoutindry formwithnosolventleftoverbehind [38]
Semi-crystallinefluoropolymerscanbetreatedusingthemethodofprecipitation polymerization,whichyieldshighmolecularweightpolymers.ByusingTFEandperfluoro(propylvinylether)copolymers,highmolecularweightpolymerswereproduced andconfirmedbyusingFT-IRanalysiswheretheroleofSCO2 wasalsotoeliminatethe unwantedendgroups [8,39].Polymersthatarethermoresponsivewerealsosynthesized byusingN-isopropylacrylamide(NIPAM),whichreactedwithacrylicacid(AA).These weresubjectedtohydration-dehydrationbehaviortoanalyzethetemperatureresponse [40].Similarworkwasdonebyusing2-hydroxyethylmethacrylate(HEMA) [41] and vinylidenefluoride(VDF) [42].Insolublealiphaticpolyesterswereproducedfromfeedstocksofnon-petrochemicalcompoundsbyringopeningpolymerization(ROP)of lactones [43].Thereactionresultedindeleteriousenvironmentalhazardsinthepresence ofLewisacidcatalyst.Therefore,theROPwasperformedusingCO2.Thealiphatic polyesterswereinsolubleinCO2.Hence,thelatterwaspolymerizedusingprecipitation polymerization(Fig.7) [8,44]
4.2.3Dispersionpolymerization Indispersionpolymerization,thepolymerisobtainedbypolymerizingthemonomerand initiatorintoCO2 byusingastabilizer,whichtendstostabilizethedispersedpolymer
Fig.7 ROPofe-caprolactoneactingasmonomeranditscorrespondingpolycaprolactone(PCL). (AdaptedfromC.Boyère,C.Jer ^ ome,A.Debuigne,Inputofsupercriticalcarbondioxidetopolymer synthesis:anoverview,Eur.Polym.J.61(2014)45–63.)
particleswhilethereisachangeinthereactionmediumfromhomogeneoustoheterogeneous [45].Thestabilizeractsbyinducingstericrepulsionbetweenthegrowing particles.However,nowadaystheresearchhasshiftedtowardsnon-fluorinatedstericstabilizer,whichismorebeneficialinpolymerizationinvolvingdispersionusingSCO2 Recently,hydrocarbonblockcopolymerswereemployedasstabilizersbut,duetolack ofsolubilityinCO2,resultedinagglomeratedpolymerparticles [46].Interestingly,some polyvinylesterslikepolyvinylacetateandpolyvinylpivalatewereabetterreplacement fortheconventionalfluorinatedsurfactants [47–50]
4.2.4Suspensionpolymerization Inoppositiontodispersionpolymerizati on,heretheseformaheterogeneousmedia whereneithermonomernorpolymeraresolubleinthemedia.Themonomerforms dropswhentheinitiatorisaddedtoitwiththeaidofmechanicalstirring.Suspension polymerizationyieldedsubmi cron-sizedwater-solublepolymersbyN-ethylacrylamide.Thesizeofthepolymerparticlesvarieswidely,dependinguponmonomerconcentration,stabilizer,initiator,and,moreimportantly,uponvaryingthepressureand temperature [8,51] .Thepoly(3-O-methacryloyl- D -glucopyranose)-b-PFOMA a is usedasanemulsifierforthesuspensionpo lymerization,whichisanamphiphilic di-blockcopolymer [52] . Recently,variouspolymerssuchaspoly(acrylicacid)(PAA) [53],poly(lactide-codioxanone)(PLDO) [41],andcopolymersofN-isopropylacrylamide(NIPA) [54] were alsosynthesizedbysuspensionpolymerizationinSCO2.
4.2.5Emulsionpolymerization Emulsionpolymerizationdealswithbiphasicliquids/organicsolventmixturescontaining stabilizers,whichwillmodifythestructureofpolymersafterpolymerization.This methodofpolymerizationyieldshighmolecularweightpolymersbecausethesegenerally donotdependupontheviscosity;rather,theelongatingchainsremainwithintheparticlesatlowconcentrations.ThestableemulsioncanbeformedbyusingsurfactantshavingCO2-phillicandCO2-phobicpartsattached,whichwilldeterminethenatureof emulsionformed,i.e.,CO2-in-waterorwater-in-CO2 typeemulsion.Ifwehavehydrophilicmonomersundergoingwater-in-CO2 (W/C)polymerization,thenwater-soluble polymerswillformandviceversa.
InthecaseofaW/Csystem,surfactanttendstoformmicellewithahydrophilic monomer,whichuponpolymerizationbecomessolubilizedwithintheaqueousphase andresultsinsuspendedparticleswithinSCO2 (Fig.8).
a Poly(3-O-methacryloyl-D-glucopyranose)-b-1,1-dihydroperfluorooctylmethacrylate.
Fig.8 Graphicalillustrationofheterogeneousmicroemulsionpolymerizationtechniqueusing(A)W/C and(B)CO2-in-water(C/W)systems,whichresultsinformationofsolidparticlesandporousmaterial, respectively. (AdaptedandmodifiedfromC.Boyère,C.Jer ^ ome,A.Debuigne,Inputofsupercriticalcarbon dioxidetopolymersynthesis:anoverview,Eur.Polym.J.61(2014)45–63.)
5.Polymerprocessing
5.1Plasticizationofpolymers
SupercriticalCO2 hasthepotentialtolowertheglasstransitiontemperature(Tg)of glassypolymers.ThesepolymerstendtoabsorbsupercriticalCO2 andhenceforthswell andreformtheirphysicalandmechanicals trength.Theinteractionofafunctional groupofpolymersandsupercriticalSCO2 tendtodecreasethechain-chaininteraction andelevatethemobilityofpolymers [55,56] .PlasticizationofpolymersusingsupercriticalSCO2 canbemeasuredbydifferentmethodssuchasgassorption,permeability [57,58] ,andpolymerswelling [59,60]
5.2Viscosityreduction
Highmolecularweightpolymersaredifficulttoprocessduetothehighviscositysolvents usedduringtheprocessing.However,viscositycanbereducedifweapplyahightemperaturetothesystem,asweknowthatbyincreasingthetemperature,duetotheincrease
inkineticenergy,molecularvibrationsincreases,andtheviscositydecreases.Indeed,the increaseintemperaturecanresultinpolymerdegradation.Therefore,asuitablesolvent suchasSCO2 canbeusedtoreducetheviscosityatlowtemperature [61].SCO2 hasa goodplasticizationeffectonpolymers.CO2 isreadilysolubilizedinpolymers,decreasing theirTg,andthusthedecreaseinthemeltingpointofpolymersalsoreducestheviscosity.
Duetotheelongationofpolymermoleculesduringsynthesis,CO2 couldnotescape fromthefibers;hence,aftersolidification,inadditiontoreducedviscosity,wegetalowdensitypolymer [3]
5.3Microcellularfoam Amicrocellularfoamedpolymerconsistsofacelldiameteroffewerthan10 μmandacell densityofmorethan100cells/cm3 [17].MicrocellularfoamcanbeformedbyasinglephasesolutionwhereafterdepressurizingtheSCO2,supersaturationoccurs,leadingto nucleationofcells.Drugentrapmentwithinthepolymerfoamusingsupercritical CO2 hasbecomeaninterestingtopicforresearch.Inarecentinvestigation,hydrophilic druggentamicinandhydrophobicdrugcurcuminhavebeenloadedasmicrocellular foamusingpoly(lactic-co-glycolicacid)(PLGA)polymerwiththeaidofSCO2.The entrapmentefficiencywasfoundtobe75%byusingthedrug:apolymerratioof 75:25 [62].Thefoamingprocessisgenerallycompletedintwobasicsteps: (a)dissolutionofCO2 inthepolymericmatrixunderanoptimumpressureforminga polymer/gassolution;and(b)nucleationandgrowthoftheparticlesuponadecrease inpressureoranincreaseintemperature [12]
5.4Polymerblending Forprocessingandblending,interactionsassociatedwiththesupercriticalmediaandthe functionalgroupofpolymershavetobekeptinmind.Theirratiooftheviscositieswill determinethesizeofthephases.Thesupercriticalmediabehavesdifferentlywithevery componentoftheblendbecauseoftheirvaryingmolecularstructures,whichwillhavea differenteffectontheTgofeachelement.Therefore,theplasticizingeffectofeachcomponentwillbedifferent.Duringextrusionusingsupercriticalfluid,thesolubilityofCO2 playsanimportantroleuponvaryingtemperature,pressure,andshearrate.Afteremploying SCO2,theshearthinningisdecreasedandafinedispersionisobtained.Therefore,onecan easilymanipulatethemorphologyofpolymerblendandtuneitssizeeasily [63].Byblending thepolymerwithinitiatorandSCO2,apolymercompositecanalsobeformed.
6.Futureprospects Duringthemanufactureoffluoropolymers,SCO2 showsincreasedsafetyaswellaspolymerproperties.Therefore,thesecanfindapplicationduringhandlingofexplosive
monomers.SCO2 canbefoundadvantageouswhenconsideringthedrawbacksassociatedwiththescaffoldfabricationmethod.Thesedrawbacksnotonlycanmanipulatescaffoldporositybutcanalsobeusedtomakebiocompositesandpreserveproteinactivity.As discussedabove,CO2 notonlyisabetteralternativeforpolymerproduction,butinmany casesisthemostneededone.AwiderangeofapplicationsassociatedwithSCO2 suggest ittobeapromisingsolvent,butfurtherinvestigationsrelatedtotheinteractionofthe polymerandSCO2 systemarerequired.
7.Challengesahead AlargevolumeofSCO2 isneededduringtheencapsulationofdrugsintopolymerichosts whichprovedtobelimited.Anotherchallengeisthehighsolubilityofmonomers requiredforpolymerprocessingandsynthesis.HighlySCO2 solublepolymersshowsignificantpolymerprocessingability.Overall,ithasbecomeclearthatSCO2 hasmoved aheadfromscientificcuriosity,butitstillneedstobemorecommercializedconcerning polymerprocessing.Akeychallengeistomakealargenumberofdrugstobeprocessedin SCO2.However,usingSCO2 asanantisolventwithotheradditionalsolventsovercomes thisproblem.Hence,SCO2 willplayanessentialroleinfuturedrugdeliveryapplications.
8.Conclusion SCO2 isanexcitingsubstituteforvariousorganic/non-polarsolventsinmultipleapplicationssuchasextraction,purification,impregnationofpolymers,andsupercriticaldyeing.Itcanalsobeusedindifferentpolymerizationtechniquesincludinghomogeneous polymerization,dispersion,suspension,precipitation,andemulsionpolymerizations. VariousprocessingstrategieshaveemergedthatdependonsupercriticalCO2 because ofitsplasticizingandantisolventproperties.Insummary,supercriticalfluidsofferatremendousopportunityinresearchassociatedwithpolymers,andprovidegreatscopefor developmentofmorepromisingandsustainabletechnologiestothepolymericindustry.
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