Table Of Content

polymers Article Optical and Thermomechanical Properties of Doped Polyfunctional Acrylate Copolymers ThomasHanemann1,2,*andKirstenHonnef2 1 KarlsruheInstituteofTechnology,InstituteofAppliedMaterials,Hermann-vonHelmholtz-Platz1, D-76344Eggenstein-Leopoldshafen,Germany 2 LaboratoryforMaterialsProcessing,InstituteforMicrosystemsTechnology,Albert-Ludwigs-Universityof Freiburg,Georges-Koehler-Allee102,D-79110Freiburg,Germany;[email protected] * Correspondence:[email protected];Tel.:+49-721-608-22585 Received:9February2018;Accepted:16March2018;Published:19March2018 Abstract: Threedifferentpolyfunctionalacrylatemonomers—trimethylolpropantriacrylate(TMPTA), pentaerythritol triacrylate (PETA) and di(trimethylolpropane) tetraacrylate (DTTA)—have been usedascomonomersincombinationwithareactiveresinconsistingofpoly(methylmethacrylate), dissolvedinitsmonomermethylmethacrylate. Phenanthrenehasbeenaddedtoformaguest–host system. The level of phenanthrene present may be adjusted to tailor the refractive index in the system. Prior to curing, the shear rate and temperature-dependent viscosity as a function of the compositionweremeasured.Itcouldbedemonstratedthat,withrespecttodifferentshapingmethods, atailor-madeflowbehaviourcanbeadjusted. Afterthermally-inducedpolymerization,theresulting optical(refractiveindex, opticaltransmittance)andthermomechanical(glasstransitionbehavior, Vickers hardness) properties were characterized. A significant refractive index increase—up to a value close to 1.56 (@589 nm)—under the retention of good optical transmittance was able to be obtained. In addition, the photopolymerization behaviour was investigated to overcome the undesirableoxygeninhibitioneffectduringthelight-inducedradicalpolymerizationofacrylates. Thelevelofacrylateunitsinthecopolymercancompensatefortheplasticizingeffectofthedopant phenanthrene,enablinghigherconcentrationsofthedopantintheguest–hostsystemandtherefore largerrefractiveindexvaluessuitableforpolymerwaveguidefabrication. Keywords: refractiveindextailoring; opticaltransmittance; polymerwaveguides; photoinitiated curing;photopolymerization;UV-curing 1. Introduction Due to the rapid increase in data traffic both in the private as well as in the business sector, whichisbeingacceleratedbytheexpandinginternet,classicalcopper-cable-basedwidebandlines are reaching their limits. Therefore optical data transmission is increasingly important due to higher data transmission rate, lower damping and improved security. Beyond the application of the established fused silica cables in the fiber-optic network, polymer optical fibers are gaining more and more importance for short distance data transmission, e.g., in the automotive sector, due to their low weight and higher bending flexibility. In addition, the further development of customized wearables containing sensors for, e.g., health monitoring, requires the use of flexible polymerwaveguideswithtailoredcharacteristics[1–7].Infact,onlyveryfewpolymerswithacceptable opticalpropertiesareavailablecommercially. Themostimportantofthesefortechnicalapplications arepoly(methylmethacrylate)(PMMA),poly(methylmethacrylimide)(PMMI),poly(carbonate)(PC) andthecycloolefiniccopolymers(COP,COC).Eachofthesepossessafixedsetofphysicalattributes and can be replicated with established methods such as injection molding and related techniques Polymers2018,10,337;doi:10.3390/polym10030337 www.mdpi.com/journal/polymers Polymers2018,10,337 2of14 forthefabricationofopticalcomponents,e.g.,lensesordatastoragedevicessuchasCD/DVD/Blu Raydisks. With respect to the fabrication of customized devices, such as polymer waveguides with passive or active optical properties, new materials with tailored features including refractive index and optical transmittance have to be developed. In addition, these new materials have to be adapted to the surroundings, such as attached light sources and detectors, as well as to accommodate geometric device characteristics. Furthermore, thermomechanical properties such asglasstransitiontemperature(T ),whichisrelevantforthedevicemaximumoperationtemperature, g mustbeadapted. TheVickershardnesscanbeusedasameasureofsurfacestability. Allmaterial properties must also be adjusted to meet the requirements of the individual replication methods, such as the various melt processing techniques like hot embossing, injection molding or derived methods[8,9]. Ambienttemperatureprocessingmethods—theshapingofcurableresins,whichoften consistofpolymersdissolvedintheirmonomers—andthesubsequentthermalorUV-curing—light inducedpolymerizationorphoto-polymerization—aregainingmoreandmoreimportance. Examples that utilize photopolymerization are inkjet or flexo/offset printing, reaction molding, wafer-level microreplication[10],nanoimprintlithographyandstereolithography(SLA).Quiterecently,polymer opticalwaveguidesandopticalscatteringlayersforOLEDs(OrganicLightEmittingDiodes)from customizedresinsandinkjettingwithsubsequentUV-curinghavebeenreported[11–13]. In the last couple of years, different attempts at adjusting the optical and thermomechanical properties of polymers have been undertaken. Initially, highly refractive ceramic nanoparticles like alumina, zirconia, or titania were dispersed in reactive resins to increase the refractive index. Sincechangesintherefractiveindexcorrelatewiththenanofillerload,onlysmallchangescouldbe achievedbecauseofsignificantincreasesinresinviscosity,whichhinderedanyshaping. Thelossof opticaltransmittanceduetoparticleagglomerationandresultinglightscatteringalsorepresented limitations[3,14–16]. Sincetherefractiveindexcorrelateswiththenumberofpolarizableunits—e.g.,anaromaticsystem inanorganicmolecule,asdescribedbytheClausius–Mosotti(Lorentz–Lorenz)equation—copolymers withelectron-richmoietiesinthemainorsidechainhavebeensynthesized[1]. Polymerpurificationto achieveopticalqualityistimeconsumingandrepresentsafurtherlimitation. Rapidmaterialscreening isthereforeimpossible. Theformationofguest–hostsystemswithasuitableelectronrichmolecule dissolvedinapolymermatrixprovidesamoresuitableapproach[17–19]. Earlierworkinvestigated aseriesofdifferentelectron-richmoleculeswithrespecttotheirsolubilityindifferentphotocurable resins and their impact on optical and thermomechanical properties [19]. The largest effects due totheirenhancedsolubilityinMMA/PMMA-basedpolymermatricescouldbeobservedincaseof phenanthreneandbenzochinoline[19]. Someofthemoleculescausedayellowingofthemixtures, whichcannotbeacceptedforopticalapplications. Duetopricing(benzochinolineisfivetimesmore expensive),phenanthrenewasselectedforfurthercomprehensiveinvestigation. Therefore,theuse ofphenanthreneisthebestcompromisebetweenpricing,solubilityintheusedmonomermixtures, refractiveindexenhancementandlowestcoloring(yellowing)inthevisiblerange. Asadrawback,thepresenceofsmalldopantmoleculescausesapronouncedreductionofthe glass transition temperature range due to plasticizing. This is observed for different unsaturated polyester–styrene and methyl methacrylate–poly(methyl methacrylate) (MMA/PMMA)-based polymer matrixes [19]. The addition of a polyfunctional monomer as a comonomer to increase the crosslinking density has the potential to partially compensate the plasticizing effect. This has been shown for doped polymerized unsaturated polyester/styrene [17] and poly(methyl methacrylate-co-1,3-butyl dimethacrylate) [20]. In contrast, the use of ethyl glycol dimethyl methacrylate(EGDMA)asacomonomerdidnotreducethesofteningbehaviourofthecuredresin[20]. Polyfunctional(meth)acrylatesarewidelyusedinorthodonticsasonecomponentoftheapplied curableresinstoenablegoodmechanicalstability[21]. Morecurrentworkdescribescomplex,highly crosslinked,transparenttrimethylamineacrylatesandmethacrylateswithimprovedthermomechanical Polymers 2018, 10, x FOR PEER REVIEW 3 of 13 Polymers2018,10,337 3of14 ical stability. These were obtained from monomers with three (meth)acrylate moieties in one mole- sctualbei ltiatryg.eTtihnegs escwraetrceho-rbetsaiisntaendt,f rUoVm-cmuroanbolem ceorastwinigths [t2h2r]e.e F(umrtehtehr)macorryel,a pteomlyfouientciteisoninalo ancerymlaotleesc uarlee tpaarrgte toifn gmsocdraetrcnh -UreVs-icstuarnatb,lUe Vin-ckusr aubseledc iona tfilnegxso-[ 2o2r] .inFkujretth eprrminotirne,gp tool yefnuhnacnticoen ea.lga.c, rtyhlea tceusrainregp sapreteodf m[23o]d. ernUV-curableinksusedinflexo-orinkjetprintingtoenhancee.g.,thecuringspeed[23]. AAiimmiinngg ttoo ccrreeaatteep poolylymmeerro opptitciaclalw wavaevgeugiudiedsebs ybyap applpyilnygindgi fdfeifrfeenretnsht asphianpginmge mtheotdhso,dths,e tphree spernet- wseonrtk wfoocruk sfeoscuosnetsh oend tehsec rdipetsicornipotifotnh eofc othmep coosmitipoons-idtieopne-dnedpeenntdpehnyts pichaylspicraolp perrtoipeserotifecso opfo clyompoelrys-, cmoenrssi,s tcionngsiosftindgif foefr edniftfeproelnytf upnoclytifounnacltimonoanlo mmoenros,mpeorlsy, (pmoelyth(mylemtheythlmacertyhlaactrey)laanted) athned ethleec terloenct-rroicnh- mricohle mcuolleecpuhleen pahnethnraenntehraesnteh aesd tohpea dnot.pant. 22.. MMaatteerriiaallss aanndd MMeetthhooddss AA ccoommmmeerrcciiaallllyy aavvaaiillaabbllee pprreemmiixxeedd ppoollyymmeerr rreessiinn ((PPlleexxiitt 5555,, CCaarrll RRootthh ccoommppaannyy,, KKaarrllssrruuhhee,, GGeerrmmaannyy)),, ccoonnssisitsitningg ofo pfoplyo(lmy(emtheythlmylemtheatchrayclraytela),t e(P),M(MPMAM, cAon,tecnotn 3te0n–t353 w0–t3 %5),w stol%ve)d, isno lmveedthyinl- mmeetthhyaclmryelathtea c(rMylMatAe,( M65M–7A0 ,w6t5 –%70) wwats% u)sewda sasu tsheed tahsertmhealtlhye ormr aplhlyotoorchpehmoitcoaclhlye mcuicraalbllye cmuraatrbilxe. mThartereix d.iTffherreeendt ipffoelrye-nfutnpcotliyon-fauln acctrioynlaatle amcroynlaotmeemrso,n torimmeertsh,ytlroimlpertohpyalnotlrpiraocpryalnattrei a(TcrMylPaTteA()T, MpePnTtaAe)-, pryetnhtraietoryl thtrriiatcorlytlraitaec r(yPlaEtTeA()P EaTnAd) dani(dtridmi(ettrhimyleotlhpyroloplapnreo)p atneter)aatectrryalaactery l(aDteTT(DAT, TaAls,oa ldsoendoetendo teads aDsiTDMiTPMTAP)T A(F)ig(uFrieg u1r)e, a1ll) ,obatlalinoebdta ifnroemd SfriogmmaS-Aiglmdraic-Ah l(dSrtiecihnh(eSitmei,n Gheerimm,anGye),r mwaenrey )u,sewde raes puosleyd- afusnpcotiloynfualn acctiroynlaatle accrorysslalitnekcerro rsessluinltkienrg rines cuolptionlgyminercso apfotelyr mcuerrisnga.f Itner a cfuirrsitn sge.rieIsn oaf ifinvrsetstsiegraiteisonosf, iinnvcreesatisgiantgio innsc,rienmcreenatsainl gaminocurenmtse nfrtoamla m0 ouupn ttos f2ro, m4, 08,u 1p6t oan2d,4 3,28 ,w16t a%n dof3 2thwe tp%oloyffuthnecptioolnyaful naccrtiyolnaatel amcroynloamteemrso nwoemree rasdwdeedre taod tdheed mtoaitnh ePlmexaiitn 5P5l erxesitin5.5 Irne sai ns.ecInonads esceorineds soefr iinesveosftiingvaetisotingsa, ttihoen sn,etwhe mneixw- mtuirxetsu wreesrew deroepeddo pweidthw pihthenpahnethnraennteh ruepn etou tphet osotlhuebisloitlyu bliimlitiyt alism ai treafsraactrivefer aincdtievxe binodosetxerb, oaoss dteer-, ascsrdibeesdcr iinbe pdreinviporuesv wioourskw [1o7r–k2[01]7. –20]. (a) (b) (c) Figure 1. Used polyfunctional acrylate comonomers: (a) trimethylolpropantriacrylate (TMPTA); Figure 1. Used polyfunctional acrylate comonomers: (a) trimethylolpropantriacrylate (TMPTA); (b) pentaerythritol triacrylate (PETA); (c) di(trimethylolpropane) tetraacrylate (DTTA). (b)pentaerythritoltriacrylate(PETA);(c)di(trimethylolpropane)tetraacrylate(DTTA). All components were mixed together using a high-speed stirrer (Ultraturrax T8, IKA, Staufe, All components were mixed together using a high-speed stirrer (Ultraturrax T8, IKA, Staufe, Germany) and polymerized thermally using 1 wt % dilauroylperoxide (DLP) (Sigma-Aldrich, Stein- Germany) and polymerized thermally using 1 wt % dilauroylperoxide (DLP) (Sigma-Aldrich, heim, Germany) at a curing temperature of 70 °C applied for 24 h. Table 1 lists the used sample series Steinheim,Germany)atacuringtemperatureof70◦Cappliedfor24h. Table1liststheusedsample denotation in this work and the related real composition of all investigated thermally-cured mixtures. seriesdenotationinthisworkandtherelatedrealcompositionofallinvestigatedthermally-cured Constant amounts of thermal initiator (DLP, 1 wt %) and release agent (INT54, 1 wt %) had to be mixtures. Constantamountsofthermalinitiator(DLP,1wt%)andreleaseagent(INT54,1wt%)had added to get 100%. Due to order of mixture fabrication (first monomer mixture, then dopant addition) tobeaddedtoget100%. Duetoorderofmixturefabrication(firstmonomermixture,thendopant the effective concentration of the polyfunctional acrylate monomers vary slightly from the denotation addition)theeffectiveconcentrationofthepolyfunctionalacrylatemonomersvaryslightlyfromthe value. denotationvalue. The photochemical curing behaviour was investigated for selected mixtures applying the pho- toinitiator D3358 (3 wt %, TCI, Eschborn, Germany) in combination with a powerful LED light source (405 nm, LED spot 100, Dr. Hoenle, Gräfelfing, Germany). To ensure complete polymerization, 1 wt % DLP as in the other systems was also added. To avoid adhesion to the glass substrate, a release agent (INT54, E. & P. Wuertz GmbH, Bingen, Germany) was added to the curable mixtures. The viscosity of all mixtures prior to curing was measured using a cone and plate rheometer (Malvern-Bohlin CVO50, Herrenberg, Germany, temperature range 20–60 °C, shear rate range 1–200 1/s, cone diameter 40 mm, cone inclination angle 4°, gap size 150 mm). The experimental uncertainty for viscosity values larger than 0.1 Pa s is around ±5% and ±10% for values below 0.1 Pa s. The refrac- tive indices of uncured liquid samples and cured solid samples (mean value of at least five measure- ments) were characterized by an Abbe refractometer AR2008 (589 nm, 20 °C, experimental uncer- tainty ±0.001, Kruess, Hamburg, Germany). The optical transmittance in the 400–800 nm range of uncured liquid samples was measured applying a Cary 50 UV/Vis spectrophotometer (Varian, now Polymers2018,10,337 4of14 Table1.Sampleseriesdenotationandrelatedmixturecomposition(inroundfiguresto100%without decimal place) All mixtures contain in addition 1 wt % thermal initiator DLP and release agent INT54each. Composition[wt%] SeriesDenotation Plexit55 PolyfunctionalAcrylate Phenanthrene 0wt%series 98 0 0 93 0 5 88 0 10 83 0 15 78 0 20 4wt%series 94 4 0 89 4 5 84 4 10 80 3 15 75 3 20 8wt%series 90 8 0 85 8 5 81 7 10 76 7 15 72 6 20 16wt%series 82 16 0 78 15 5 74 14 10 69 14 15 65 13 20 32wt%series 66 32 0 63 30 5 59 29 10 56 27 15 53 25 20 The photochemical curing behaviour was investigated for selected mixtures applying the photoinitiatorD3358(3wt%,TCI,Eschborn,Germany)incombinationwithapowerfulLEDlight source(405nm,LEDspot100,Dr. Hoenle,Gräfelfing,Germany). Toensurecompletepolymerization, 1wt%DLPasintheothersystemswasalsoadded. Toavoidadhesiontotheglasssubstrate,arelease agent(INT54,E.&P.WuertzGmbH,Bingen,Germany)wasaddedtothecurablemixtures. The viscosity of all mixtures prior to curing was measured using a cone and plate rheometer (Malvern-Bohlin CVO50, Herrenberg, Germany, temperature range 20–60 ◦C, shear rate range 1–2001/s, cone diameter 40 mm, cone inclination angle 4◦, gap size 150 mm). The experimental uncertaintyforviscosityvalueslargerthan0.1Pasisaround±5%and±10%forvaluesbelow0.1Pas. Therefractiveindicesofuncuredliquidsamplesandcuredsolidsamples(meanvalueofatleastfive measurements)werecharacterizedbyanAbberefractometerAR2008(589nm,20◦C,experimental uncertainty±0.001,Kruess,Hamburg,Germany). Theopticaltransmittanceinthe400–800nmrange ofuncuredliquidsampleswasmeasuredapplyingaCary50UV/Visspectrophotometer(Varian,now Agilent,Waldbronn,Germany)usingdisposableUVsemi-microcuvettes(12.5×12.5×45mm3outer dimensions). TheglasstransitiontemperaturesT ofthecuredsystemswereestimatedviadifferential g scanningcalorimetry(DSC)(Netzsch,DSC204F1Phoenix,heatingrate10◦C/min)withanuncertainty of±2◦C.TheVickershardness(Paar-PhysicaMHT10,load: 100p,indentionspeed: 10p/s,loading time: 1s)ofselectedsamples(meanvalueoffivemeasurements,standarddeviation±2.5units)were recordedafterpolymerizationforsuitabletestspecimens. Polymers2018,10,337 5of14 3. ResultsandDiscussion 3.1. ShearRateandTemperature-DependentViscosityofUncuredMixtures Theknowledgeoftheimpactofthepolyfunctionalacrylatetypeanddopantconcentrationas wellastheshearrateandtemperatureontheviscosityofthemixtureisessentialforfurtherprocessing, micro-structuringanddevicefabricationapplyingdifferentshapingmethods. Thedifferentvariants ofreactiveresinprocessingtechniquespossessdifferentacceptableresinviscositiesatprocessingor operationtemperatures. Figure2showsforbettercomparisontheshearrateandviscosity-dependent flowbehaviourforallinvestigatedpurecomponents.Duetothepolymeramountofaround30–35wt% in the commercial resin, Plexit 55 possesses a pseudoplastic flow at all investigated temperatures, aPsoldymeesrcsr 2i0b1e8d, 1e0,a xr lFiOerR [P1E7E]R. REVIEW 5 of 13 FFiigguurree 22.. SShheeaarr rraattee aanndd tteemmppeerraattuurree--ddeeppeennddeenntt vviissccoossiittyy ooff aallll iinnvveessttiiggaatteedd ppuurree ccoommppoonneennttss.. 3.1.1. Trimethylolpropantriacrylate (TMPTA)-Containing Systems ThepuremonomersTMPTA,PETAandDTTAshowinallcasesapureNewtonianflow,inwhich the twThoen aodnd-iptioolna romf ionncroemaseirnsg, TamMoPuTnAts aonfd TMDTPTTAA, teuxrhniebdit thsieg npisfieucadnotplloawstiecr fvloiwsc oinsittoy av Naleuwestotnhiaann tfhloewm, oasr eexppoelacrt,edon ferohmy dFrigouxyre-g 2r.o Fuigpu-croen 3t asuinminmgaPrEizTeAs t.hIne iandflduietinocne, othf etecmopmebraintuarteio annodf cahceommicmale crocimal- eppoosxityioanc orynl athtee wreistuhltEinGgD mMiAxt,utrhee vpisucroesietpyo fxoyr aa csreylelactteedi tssheelfa,ra rnadtea ollf d1e0r0i v1e/sd. Tmhiex tmuraeins, raelssuoltssh aorwe:e da Newtonianflow[24]. • A temperature increase causes a viscosity drop: this is a common behaviour for all organic liq- uids and can be described by the Andrade–Eyring relation [25]. 3.1.1. Trimethylolpropantriacrylate(TMPTA)-ContainingSystems • Increasing TMPTA amounts cause a viscosity drop due to simple mixing rules. • TInhcereaadsdinitgio pnhoefniannctrhearesnineg aammoouunntsts coafuTseM aP vTAisctousrintye ddrthope:p Tsheue dgoivpelans tpiclaflstoiwciziinntgo eafNfecetw otfo npihaen- flow, as expected from Figure 2. Figure 3 summarizes the influence of temperature and chemical nanthrene, solved in different reactive resins, is in agreement with other results [17–20]. c•o mpAo stiotitoanl oofn 20th wetr e%s uplhtienngamntihxrteunree evxiscceoesdisty thfoe rsoalsuebleilcittyed limshiet aart raa tTeMoPfT10A0 c1o/nst.eTnht eofm 4a wint r%es. ults are: • Atemperatureincreasecausesaviscositydrop: thisisacommonbehaviourforallorganicliquids andcanbedescribedbytheAndrade–Eyringrelation[25]. • IncreasingTMPTAamountscauseaviscositydropduetosimplemixingrules. • Increasing phenanthrene amounts cause a viscosity drop: The given plasticizing effect of phenanthrene,solvedindifferentreactiveresins,isinagreementwithotherresults[17–20]. • Atotalof20wt%phenanthreneexceedsthesolubilitylimitataTMPTAcontentof4wt%. Figure 3. Influence of temperature, TMPTA, and phenanthrene content on the viscosity of the mixture at a fixed shear rate of 100 1/s. Polymers 2018, 10, x FOR PEER REVIEW 5 of 13 Figure 2. Shear rate and temperature-dependent viscosity of all investigated pure components. 3.1.1. Trimethylolpropantriacrylate (TMPTA)-Containing Systems The addition of increasing amounts of TMPTA turned the pseudoplastic flow into a Newtonian flow, as expected from Figure 2. Figure 3 summarizes the influence of temperature and chemical com- position on the resulting mixture viscosity for a selected shear rate of 100 1/s. The main results are: • A temperature increase causes a viscosity drop: this is a common behaviour for all organic liq- uids and can be described by the Andrade–Eyring relation [25]. • Increasing TMPTA amounts cause a viscosity drop due to simple mixing rules. • Increasing phenanthrene amounts cause a viscosity drop: The given plasticizing effect of phe- nanthrene, solved in different reactive resins, is in agreement with other results [17–20]. Polymers2018,10,337 6of14 • A total of 20 wt % phenanthrene exceeds the solubility limit at a TMPTA content of 4 wt %. Figure 3. Influence of temperature, TMPTA, and phenanthrene content on the viscosity of the mixture Figure3.Influenceoftemperature,TMPTA,andphenanthrenecontentontheviscosityofthemixture at a fixed shear rate of 100 1/s. atafixedshearrateof1001/s. Polymers 2018, 10, x FOR PEER REVIEW 6 of 13 3.1.2. PentaerythritolTriacrylate(PETA)-ContainingSystems 3.1.2. Pentaerythritol Triacrylate (PETA)-Containing Systems TheadditionofincreasingamountsofPETAandphenanthrenetoPlexit55causesacomparable influeTnhcee aodndtihtieoflno owf ibnechreaavsiionugr a(Fmigouurnets4 )o.fA PEclToAse ranindv peshteignaatniothnroenfteh teo vPilsecxoisti t5y5 dcaautaseresv ae caolsmapcaerratabilne sincafltuteernicneg oonf tthhee vflioswco sbietyhadvaitoau:r (Figure 4). A closer investigation of the viscosity data reveals a cer- tain scattering of the viscosity data: • AtaconstantPETAcontent,atemperatureincreasecausesaviscositydrop. •• IAntc rae caosninsgtapnht ePnEaTnAth creonneteanmt, oau tnetmspcaeurasteuarep irnincrceipasael vcaisucsoessit ay vdirsocposdituye dtrooppl.a sticizing. •• WInictrheiansitnhge pinhveensatingtahtreednceo anmceonutnratst icoanursaen ag ep,rpinhceipnaanl vthisrceonseitiys cdormopp ldeuteel ytos polluasbtlieciuzpintgo. 20wt%. • Within the investigated concentration range, phenanthrene is completely soluble up to 20 wt %. FFiigguurree 44.. IInnflfluueennccee ooff tteemmppeerraattuurree,,P PEETTAA,,a annddp phhenenananththrerneenec ocnotnetnetnot nonth tehve ivscisocsoitsyitoyf othf ethme imxtiuxrteuraet aatc ao mcommomnosnh esahrearar treaotef o10f 01010/ s1./s. 3.1.3. Di(trimethylolpropane) Tetraacrylate (DTTA)-Containing Systems As with the other investigated systems, the addition of increasing amounts of DTTA and phe- nanthrene to Plexit 55 causes an almost analogous influence on the resulting mixture’s flow behav- iour (Figure 5). A closer investigation of the viscosity data reveals a slight scattering of the viscosity data: • At constant DTTA content, a temperature increase causes a viscosity drop. • Increasing phenanthrene amounts cause a principal viscosity drop due to plasticizing. • A total of 20 wt % phenanthrene exceeds the solubility limit at a DTTA content of 16 wt %. In general, all three polyfunctional acrylates lower the viscosity of the Plexit 55 matrix with in- creasing monomer content. The addition of phenanthrene also causes a viscosity drop. In a few cases, the solubility limit of phenanthrene in the monomer mixture is lowered. The presence of the hy- droxyl-functionality in PETA may prohibit a clear relationship between the composition and result- ing viscosity values. The information obtained in this section enables viscosity tailoring with respect to the aspired viscosity range attributed to the selected further processing. As an example, polymer waveguide inkjet printing requires UV-curable mixtures with a viscosity below 16 mPa s at printing temperatures (40–70 °C) [11,12]. Unfortunately, none of the investigated mixtures fulfills this viscos- ity requirement for inkjet printing. In contrast, flexoprinting and SLA accept viscosities up to 500 mPa s, which can be achieved here. Polymers2018,10,337 7of14 3.1.3. Di(trimethylolpropane)Tetraacrylate(DTTA)-ContainingSystems As with the other investigated systems, the addition of increasing amounts of DTTA and phenanthrene to Plexit 55 causes an almost analogous influence on the resulting mixture’s flow behaviour (Figure 5). A closer investigation of the viscosity data reveals a slight scattering of the viscositydata: • AtconstantDTTAcontent,atemperatureincreasecausesaviscositydrop. • Increasingphenanthreneamountscauseaprincipalviscositydropduetoplasticizing. • Atotalof20wt%phenanthreneexceedsthesolubilitylimitataDTTAcontentof16wt%. Polymers 2018, 10, x FOR PEER REVIEW 7 of 13 Figure 5. Influence of temperature, DTTA, and phenanthrene content on the viscosity of the mixture Figure5.Influenceoftemperature,DTTA,andphenanthrenecontentontheviscosityofthemixtureat at a common shear rate of 100 1/s. acommonshearrateof1001/s. 3.2. Optical Properties of Uncured and Cured Systems In general, all three polyfunctional acrylates lower the viscosity of the Plexit 55 matrix with increWasiitnhg rmesopneoctm teor tchoen tfeanbrt.icTathieona dodf iptioolnymofepr hweanvaengthuridenese aulssiongc aduisffeesreanvt isschoaspitinygd rmoept.hIondsa, ftehwe acadsjuess,tmtheents oolfu tbhiel irteyfrlaimctiitveo finpdhiceensa noft hthreen ceorine athned mclaodndoimnge rfomr iwxtauvreegiusidloinwge irse dcr.ucTihael. Ipnr easdednicteioonf, kthneowhyleddrogxey olf- ftuhnec otipotnicaalli ttyrainnsPmEiTttAanmcea iys persosehnibtiiatla focrle laorwr elloastsi omnashteirpiable stewleecetniotnh. ecompositionand resultingviscosityvalues. Theinformationobtainedinthissectionenablesviscositytailoringwith 3.2.1. Refractive Index respecttotheaspiredviscosityrangeattributedtotheselectedfurtherprocessing. Asanexample, polymTheer wreaavl epgaurti doef tinhke jceotmprpinletxin rgefrreaqcutiivrees inUdVe-xc,u tryapbilceamllyi xdteunreostewdi aths arevfrisaccotisviittyyb oerl orewac1t6ivme Pinadseaxt, ipsr ian mtinegastuemrep oefr tahtue rienste(4ra0c–t7i0on◦ Co)f [t1h1e, 1e2l]e.cUtronmfoartgunneattiec lfyi,enldo noef othfeth iencinomveisntgig wataedvem wixitthu rtehsef uelleficltlrsotnhiics sviitsucaotsiiotny irne qmuairtetemr,e enstpfeocriainllkyj ethtep rpinoltainrigz.aIbnilcitoyn itnra tshte, flaeffxeocpterdin mtinogleacunldesS L[2A6]a. cAcne pintcvriesacsoes iotife tshue ppoto- l5a0r0izmabPialitsy, winh oicrhgacnainc bmeoalcehcuielvees dcahne rbee. realized by an increase of the number of movable electrons, e.g., by the extension of delocalized π-electrons as given in the dopant phenanthrene. In addition, a 3.2. OpticalPropertiesofUncuredandCuredSystems further interaction—e.g., by forming charge–transfer complexes—can lead to a light absorption (yel- lowinWg)it ihn rthesep veicstibtloe etqhueivfaalbernict attoi otnhe oimf pagoilnyamrye rpawrat voef gthueid ceosmupslienxg redfrifafcetrievnet insdheaxp.i nWgitmh reethspoedcst, ttoh eteacdhjunsictmal eanptpolfictahteiornefsr aocf ttihvee innedwic mesaotferthiaelsc—oree.agn.,d asc lwadadvienggufiodrews—avaeng aubidsoinrgptiisocnr uinc itahl.e Ivnisaidbdleit hioans, tkon boew slterdicgtelyo afvthoeidoepdt.i cFaiglutrraen 6s mshitotwansc feoirs aelsl stehnet iinalvfeosrtilgoawtedlo ssyssmteamtesr iaa lchsealnegctei oinn .the refractive index with mixture composition and dopant concentration. The most important correlations found were: 3.2.1. RefractiveIndex • Due to the lower density of the liquid monomer mixtures, the refractive index is lower in general Therealpartofthecomplexrefractiveindex,typicallydenotedasrefractivityorreactiveindex, than the related ones in the solidified state. •is amAelal smuereasoufrtehde vinaltuereasc atiroen cloofsteh teoegleetchteror,m thaeg nriestei cinfi ethlde orefftrhaectiinvceo imndinegx wcaanv beew aittthribthueteedle mctraoinnliyc to the increasing phenanthrene moiety. • As found in other systems, an almost linear correlation of the refractive index with the phenan- threne concentration can be derived [18,19]. • In the case of DTTA-based systems, the use of larger DTTA amounts are favorable in the liquid state. • A clear correlation of the polyfunctional acrylate monomer structure with the refractive index values cannot be found. Polymers2018,10,337 8of14 situation in matter, especially the polarizability in the affected molecules [26]. An increase of the polarizabilityinorganicmoleculescanberealizedbyanincreaseofthenumberofmovableelectrons, e.g., bytheextensionofdelocalizedπ-electronsasgiveninthedopantphenanthrene. Inaddition, a further interaction—e.g., by forming charge–transfer complexes—can lead to a light absorption (yellowing)inthevisibleequivalenttotheimaginarypartofthecomplexrefractiveindex. Withrespect totechnicalapplicationsofthenewmaterials—e.g.,aswaveguides—anabsorptioninthevisiblehas tobestrictlyavoided. Figure6showsforalltheinvestigatedsystemsachangeintherefractiveindex withmixturecompositionanddopantconcentration. Themostimportantcorrelationsfoundwere: • Duetothelowerdensityoftheliquidmonomermixtures,therefractiveindexisloweringeneral thantherelatedonesinthesolidifiedstate. • Allmeasuredvaluesareclosetogether,theriseintherefractiveindexcanbeattributedmainlyto theincreasingphenanthrenemoiety. • As found in other systems, an almost linear correlation of the refractive index with the phenanthreneconcentrationcanbederived[18,19]. • In the case of DTTA-based systems, the use of larger DTTA amounts are favorable in the liquidstate. • Aclearcorrelationofthepolyfunctionalacrylatemonomerstructurewiththerefractiveindex valuescannotbefound. Polymers 2018, 10, x FOR PEER REVIEW 8 of 13 FiFgiugruer6e. 6R. eRfreafrcaticvtievien idnidceicseosf oafl lailnl ivnevsetisgtaigteadtemd mixtiuxtruesrepsr piorrio(rli q(luiqidu)ida)n adnadf taefrtecru cruinrgin(gs o(lsiodl)i.d). 3.2.2. Optical Transmittance in the Visible Range 3.2.2. OpticalTransmittanceintheVisibleRange Exemplarily, the influence of the copolymer composition on the optical transmittance properties Exemplarily,theinfluenceofthecopolymercompositionontheopticaltransmittanceproperties were investigated applying the DTTA-based mixtures. Without dopant, the influence of the increas- wereinvestigatedapplyingtheDTTA-basedmixtures. Withoutdopant,theinfluenceoftheincreasing ing DTTA content was negligible; an influence was found for low amounts of DTTA in combination DTTAcontentwasnegligible;aninfluencewasfoundforlowamountsofDTTAincombinationwitha with a moderate phenanthrene load of 10%. Higher DTTA moieties in combination with 10% dopant moderatephenanthreneloadof10%. HigherDTTAmoietiesincombinationwith10%dopantresulted resulted in some higher absorbance (Figure 7a). The combination of low amounts of DTTA with in- insomehigherabsorbance(Figure7a). ThecombinationoflowamountsofDTTAwithincreasing creasing phenanthrene contents up to 20% produced only a small drop in the transmittance values phenanthrenecontentsupto20%producedonlyasmalldropinthetransmittancevalues(Figure7b). (Figure 7b). (a) (b) Figure 7. Optical transmittance of DTTA-containing mixtures: (a) Influence of DTTA content without and with 10% phenanthrene; (b) Influence of phenanthrene content at constant DTTA amount. Mixtures consisting of TMPTA or PETA in combination with phenanthrene showed comparable transmittance behaviour due their similar chemical structure without functional groups absorbing in the visible range. The main result of this subsection can be summarized as follows: large refractive indices, gener- ated with huge dopant concentrations, have the drawback of the reduced optical transmittance of the used doped copolymer. The amount of polyfunctional acrylate monomer has a certain influence on Polymers 2018, 10, x FOR PEER REVIEW 8 of 13 Figure 6. Refractive indices of all investigated mixtures prior (liquid) and after curing (solid). 3.2.2. Optical Transmittance in the Visible Range Exemplarily, the influence of the copolymer composition on the optical transmittance properties were investigated applying the DTTA-based mixtures. Without dopant, the influence of the increas- ing DTTA content was negligible; an influence was found for low amounts of DTTA in combination with a moderate phenanthrene load of 10%. Higher DTTA moieties in combination with 10% dopant resulted in some higher absorbance (Figure 7a). The combination of low amounts of DTTA with in- creasing phenanthrene contents up to 20% produced only a small drop in the transmittance values (Figure 7b). Polymers2018,10,337 9of14 (a) (b) Figure 7. Optical transmittance of DTTA-containing mixtures: (a) Influence of DTTA content without Figure7.OpticaltransmittanceofDTTA-containingmixtures:(a)InfluenceofDTTAcontentwithout and with 10% phenanthrene; (b) Influence of phenanthrene content at constant DTTA amount. andwith10%phenanthrene;(b)InfluenceofphenanthrenecontentatconstantDTTAamount. Mixtures consisting of TMPTA or PETA in combination with phenanthrene showed comparable MixturesconsistingofTMPTAorPETAincombinationwithphenanthreneshowedcomparable transmittance behaviour due their similar chemical structure without functional groups absorbing in transmittancebehaviourduetheirsimilarchemicalstructurewithoutfunctionalgroupsabsorbingin the visible range. thevisiblerange. The main result of this subsection can be summarized as follows: large refractive indices, gener- The main result of this subsection can be summarized as follows: large refractive indices, ated with huge dopant concentrations, have the drawback of the reduced optical transmittance of the generatedwithhugedopantconcentrations,havethedrawbackofthereducedopticaltransmittance used doped copolymer. The amount of polyfunctional acrylate monomer has a certain influence on oftheuseddopedcopolymer. Theamountofpolyfunctionalacrylatemonomerhasacertaininfluence on the optical properties, enabling a certain flexibility in the viscosity value for use in different shapingmethods. 3.3. PhotoinitiatedCuringBehaviour Ingeneral,theradicalpolymerizationofacrylatessufferfromtheinhibitinginfluenceofoxygen ifthecuringprocessiscarriedoutunderambientconditionsinanairatmosphere. Severalnegative effectscanbeobservedduringphotopolymerization[27]: • Occurrenceofaninductionperiodpriortothechemicalreaction; • Reductionofthepolymerizationrateandfinalconversion; • Reductionofthechainlengthandconversionrate; • Formationofastickysurfaceappearanceduetooligomerformation. Differentapproachescanbeappliedtoovercometheoxygeninhibitionprocess[28–30]: • UV-curinginaninertatmosphere; • Preventingoxygendiffusionbyprotectingfilmsortransparentfoils; • Selectionofhighlyeffectivelightsourceslikevaporpressurearclamps; • Useofsuitableadditivestrappingoxygen-liketriphenylphosphine[31]; • Useofmonomerswithhugechemicalreactivity. Especiallyinthelattercaseitisrecommendedtoaddpolyfunctionalacrylatestoaphotocurable system to enhance the cure speed, reduce the oxygen inhibition effect and enhance the resulting physical properties [23,29]. Earlier investigations applying different acrylate mixtures exhibited sticky surface properties after UV-curing in air, hence a polymerization between glass plates was necessary[32]. Withrespecttotheverificationofthepositiveinfluenceofthepolyfunctionalacrylates onthecuringbehaviour[23],DTTA-containingmixtureswerecuredexemplarilybyUV-light. Two differentsetupsforthephotopolymerizationprocesswereinvestigated. First,theresincompositions Polymers2018,10,337 10of14 wereplacedbetweentwoglassplatesandUV-curedwiththeexclusionofoxygen. Second,theresins wereUV-curedunderexposuretoair. ItwasfoundthatevenatlowconcentrationsofDTTA(4wt%) bothsetupsdeliveredtack-free,hardandtransparentsurfaces,whileDTTA-freesamplesremained stickyatthesurface. Hence,asimplifiedcuringprocedureispossibleandshowsthepositiveinfluence ofthepolyfunctionalacrylatemonomer. AtlargeDTTAamounts(32wt%)andhigherphenanthrene concentrationsthesamplesbecameturbidsomeweeksafterpreparation. TheUV-curingofpureDTTA deliveredaverybrittlepolymerduetothermosetformation[23]. Theaboveresultsarealsovalidfor thetwootherpolyfunctionalacrylates,PETAandTMPTA. 3.4. ThermomechnicalProperties Onemotivationfortheuseofpolyfunctionalacrylatesascomonomersistheirabilitytoform rigidpolymernetworksaftercuring. Itcanbeexpectedthatwithahigherpolyfunctionalacrylate contentintheresultingpolymer,enhancedthermomechanicalpropertieswillbefoundincomparison topurethermoplastics[23]. 3.4.1. GlassTransitionTemperature Figure8showstheT valuesmeasuredbyDSCofallinvestigatedmixtures,thedatafordoped g Plexit without any other comonomer was addedfor reference. Inall cases, increasing amounts of phenanthrenelowersT relativetothereferencematerial(curedPlexit,i.e.,PMMA)duetoplasticizing, g as has been reported elsewhere [17–21]. This is also valid for the use of a polyfunctional acrylate comonomer irrespective of whether phenanthrene is present or not. A more careful data analysis shows a certain order of the comonomers: PETA caused a pronounced T drop, the influence of g DTTA was moderate, and TMPTA showed the smallest decrease. Higher amounts of TMPTA generated the aspired T elevation at higher dopant concentrations in comparison to the related g Plexit/phenanthrenemixtures. Polymers 2018, 10, x FOR PEER REVIEW 10 of 13 Figure 8. Glass transition temperatures of all investigated systems after curing. Figure8.Glasstransitiontemperaturesofallinvestigatedsystemsaftercuring. At present, inconsistent Tg values for the polymerized pure polyfunctional acrylates have been Atpresent,inconsistentT valuesforthepolymerizedpurepolyfunctionalacrylateshavebeen described. While [23] lists 98 °Cg for poly(TMPTA): 62 °C, poly(PETA): 103 °C and poly(DTTA), some described. While[23]lists98◦Cforpoly(TMPTA):62◦C,poly(PETA):103◦Candpoly(DTTA),some other references provide different information. One group measured a glass transition around 88 °C otherreferencesprovidedifferentinformation.Onegroupmeasuredaglasstransitionaround88◦Cfor for a doped PETA polymer [33], while in [34] no glass transition in the temperature range from −50– adopedPETApolymer[33],whilein[34]noglasstransitioninthetemperaturerangefrom−50–220◦C 220 °C was found. The aspired effect of an increase in glass transition temperature, within one series, wasfound. Theaspiredeffectofanincreaseinglasstransitiontemperature,withinoneseries,and and with an increasing polyfunctional acrylate comonomer content and a constant dopant amount, can only be observed at large TMPTA amounts (Figure 8). This may be attributed to the flexible chem- ical structure of the comonomers allowing a more elastomeric substructure. One can assume that huge amounts of TMPTA enables the formation of a more rigid thermoset-like network due to its more compact chemical structure. This can be also deduced from the low viscosity value of the TMPTA monomer, shown in Figure 2, as an indication of low inter-molecular friction in the liquid phase due to a more compact structure. Earlier work described that the use of rigid reactive molecules, like divinylbenzene, can increase Tg if they are used as comonomer in an unsaturated polyester–resin matrix after solidification [17]. Within an acrylate-based resin matrix, the use of 1,3-butandiol dimethacrylate (BDMA) as small di- functional molecule resulted in an observed drop of Tg with increasing amounts and a clear correla- tion between BDMA-content and Tg stabilization at high plasticizer content could be detected [18]. Without any crosslinker, a pronounced Tg decay with increasing phenanthrene concentration in PMMA (polymerized Plexit) and polymerized unsaturated polyester was found [19]. 3.4.2. Vickers Hardness Previous work investigating phenanthrene/unsaturated polyester/divinylbenzene mixtures, de- scribed a significant increase in the Vickers hardness of cured specimen with increasing divinylben- zene content [17]. In the case of a comparable system consisting of phenanthrene/Plexit/BDMA, no remarkable hardness increase could be measured [19]. Exemplarily, the DTTA-based mixtures were investigated with respect to the influence of the comonomer and the phenanthrene content (Figure 9). In all cases, increasing dopant concentrations show a decline in hardness due to plasticizing.

Description:
2 of 14 for the fabrication of optical components, e.g., lenses or data storage devices such as CD/DVD/Blu. Ray disks. With respect to the fabrication of customized devices, such as polymer waveguides with passive or active optical properties, new materials with tailored features including refracti
ebook img

Optical and Thermomechanical Properties of Doped Polyfunctional Acrylate Copolymers PDF

14 Pages·2017·9.28 MB·English
by  
Save to my drive
Quick download
Download
Upgrade Premium
Most books are stored in the elastic cloud where traffic is expensive. For this reason, we have a limit on daily download.

Preview Optical and Thermomechanical Properties of Doped Polyfunctional Acrylate Copolymers

polymers Article Optical and Thermomechanical Properties of Doped Polyfunctional Acrylate Copolymers ThomasHanemann1,2,*andKirstenHonnef2 1 KarlsruheInstituteofTechnology,InstituteofAppliedMaterials,Hermann-vonHelmholtz-Platz1, D-76344Eggenstein-Leopoldshafen,Germany 2 LaboratoryforMaterialsProcessing,InstituteforMicrosystemsTechnology,Albert-Ludwigs-Universityof Freiburg,Georges-Koehler-Allee102,D-79110Freiburg,Germany;[email protected] * Correspondence:[email protected];Tel.:+49-721-608-22585 Received:9February2018;Accepted:16March2018;Published:19March2018 Abstract: Threedifferentpolyfunctionalacrylatemonomers—trimethylolpropantriacrylate(TMPTA), pentaerythritol triacrylate (PETA) and di(trimethylolpropane) tetraacrylate (DTTA)—have been usedascomonomersincombinationwithareactiveresinconsistingofpoly(methylmethacrylate), dissolvedinitsmonomermethylmethacrylate. Phenanthrenehasbeenaddedtoformaguest–host system. The level of phenanthrene present may be adjusted to tailor the refractive index in the system. Prior to curing, the shear rate and temperature-dependent viscosity as a function of the compositionweremeasured.Itcouldbedemonstratedthat,withrespecttodifferentshapingmethods, atailor-madeflowbehaviourcanbeadjusted. Afterthermally-inducedpolymerization,theresulting optical(refractiveindex, opticaltransmittance)andthermomechanical(glasstransitionbehavior, Vickers hardness) properties were characterized. A significant refractive index increase—up to a value close to 1.56 (@589 nm)—under the retention of good optical transmittance was able to be obtained. In addition, the photopolymerization behaviour was investigated to overcome the undesirableoxygeninhibitioneffectduringthelight-inducedradicalpolymerizationofacrylates. Thelevelofacrylateunitsinthecopolymercancompensatefortheplasticizingeffectofthedopant phenanthrene,enablinghigherconcentrationsofthedopantintheguest–hostsystemandtherefore largerrefractiveindexvaluessuitableforpolymerwaveguidefabrication. Keywords: refractiveindextailoring; opticaltransmittance; polymerwaveguides; photoinitiated curing;photopolymerization;UV-curing 1. Introduction Due to the rapid increase in data traffic both in the private as well as in the business sector, whichisbeingacceleratedbytheexpandinginternet,classicalcopper-cable-basedwidebandlines are reaching their limits. Therefore optical data transmission is increasingly important due to higher data transmission rate, lower damping and improved security. Beyond the application of the established fused silica cables in the fiber-optic network, polymer optical fibers are gaining more and more importance for short distance data transmission, e.g., in the automotive sector, due to their low weight and higher bending flexibility. In addition, the further development of customized wearables containing sensors for, e.g., health monitoring, requires the use of flexible polymerwaveguideswithtailoredcharacteristics[1–7].Infact,onlyveryfewpolymerswithacceptable opticalpropertiesareavailablecommercially. Themostimportantofthesefortechnicalapplications arepoly(methylmethacrylate)(PMMA),poly(methylmethacrylimide)(PMMI),poly(carbonate)(PC) andthecycloolefiniccopolymers(COP,COC).Eachofthesepossessafixedsetofphysicalattributes and can be replicated with established methods such as injection molding and related techniques Polymers2018,10,337;doi:10.3390/polym10030337 www.mdpi.com/journal/polymers Polymers2018,10,337 2of14 forthefabricationofopticalcomponents,e.g.,lensesordatastoragedevicessuchasCD/DVD/Blu Raydisks. With respect to the fabrication of customized devices, such as polymer waveguides with passive or active optical properties, new materials with tailored features including refractive index and optical transmittance have to be developed. In addition, these new materials have to be adapted to the surroundings, such as attached light sources and detectors, as well as to accommodate geometric device characteristics. Furthermore, thermomechanical properties such asglasstransitiontemperature(T ),whichisrelevantforthedevicemaximumoperationtemperature, g mustbeadapted. TheVickershardnesscanbeusedasameasureofsurfacestability. Allmaterial properties must also be adjusted to meet the requirements of the individual replication methods, such as the various melt processing techniques like hot embossing, injection molding or derived methods[8,9]. Ambienttemperatureprocessingmethods—theshapingofcurableresins,whichoften consistofpolymersdissolvedintheirmonomers—andthesubsequentthermalorUV-curing—light inducedpolymerizationorphoto-polymerization—aregainingmoreandmoreimportance. Examples that utilize photopolymerization are inkjet or flexo/offset printing, reaction molding, wafer-level microreplication[10],nanoimprintlithographyandstereolithography(SLA).Quiterecently,polymer opticalwaveguidesandopticalscatteringlayersforOLEDs(OrganicLightEmittingDiodes)from customizedresinsandinkjettingwithsubsequentUV-curinghavebeenreported[11–13]. In the last couple of years, different attempts at adjusting the optical and thermomechanical properties of polymers have been undertaken. Initially, highly refractive ceramic nanoparticles like alumina, zirconia, or titania were dispersed in reactive resins to increase the refractive index. Sincechangesintherefractiveindexcorrelatewiththenanofillerload,onlysmallchangescouldbe achievedbecauseofsignificantincreasesinresinviscosity,whichhinderedanyshaping. Thelossof opticaltransmittanceduetoparticleagglomerationandresultinglightscatteringalsorepresented limitations[3,14–16]. Sincetherefractiveindexcorrelateswiththenumberofpolarizableunits—e.g.,anaromaticsystem inanorganicmolecule,asdescribedbytheClausius–Mosotti(Lorentz–Lorenz)equation—copolymers withelectron-richmoietiesinthemainorsidechainhavebeensynthesized[1]. Polymerpurificationto achieveopticalqualityistimeconsumingandrepresentsafurtherlimitation. Rapidmaterialscreening isthereforeimpossible. Theformationofguest–hostsystemswithasuitableelectronrichmolecule dissolvedinapolymermatrixprovidesamoresuitableapproach[17–19]. Earlierworkinvestigated aseriesofdifferentelectron-richmoleculeswithrespecttotheirsolubilityindifferentphotocurable resins and their impact on optical and thermomechanical properties [19]. The largest effects due totheirenhancedsolubilityinMMA/PMMA-basedpolymermatricescouldbeobservedincaseof phenanthreneandbenzochinoline[19]. Someofthemoleculescausedayellowingofthemixtures, whichcannotbeacceptedforopticalapplications. Duetopricing(benzochinolineisfivetimesmore expensive),phenanthrenewasselectedforfurthercomprehensiveinvestigation. Therefore,theuse ofphenanthreneisthebestcompromisebetweenpricing,solubilityintheusedmonomermixtures, refractiveindexenhancementandlowestcoloring(yellowing)inthevisiblerange. Asadrawback,thepresenceofsmalldopantmoleculescausesapronouncedreductionofthe glass transition temperature range due to plasticizing. This is observed for different unsaturated polyester–styrene and methyl methacrylate–poly(methyl methacrylate) (MMA/PMMA)-based polymer matrixes [19]. The addition of a polyfunctional monomer as a comonomer to increase the crosslinking density has the potential to partially compensate the plasticizing effect. This has been shown for doped polymerized unsaturated polyester/styrene [17] and poly(methyl methacrylate-co-1,3-butyl dimethacrylate) [20]. In contrast, the use of ethyl glycol dimethyl methacrylate(EGDMA)asacomonomerdidnotreducethesofteningbehaviourofthecuredresin[20]. Polyfunctional(meth)acrylatesarewidelyusedinorthodonticsasonecomponentoftheapplied curableresinstoenablegoodmechanicalstability[21]. Morecurrentworkdescribescomplex,highly crosslinked,transparenttrimethylamineacrylatesandmethacrylateswithimprovedthermomechanical Polymers 2018, 10, x FOR PEER REVIEW 3 of 13 Polymers2018,10,337 3of14 ical stability. These were obtained from monomers with three (meth)acrylate moieties in one mole- sctualbei ltiatryg.eTtihnegs escwraetrceho-rbetsaiisntaendt,f rUoVm-cmuroanbolem ceorastwinigths [t2h2r]e.e F(umrtehtehr)macorryel,a pteomlyfouientciteisoninalo ancerymlaotleesc uarlee tpaarrgte toifn gmsocdraetrcnh -UreVs-icstuarnatb,lUe Vin-ckusr aubseledc iona tfilnegxso-[ 2o2r] .inFkujretth eprrminotirne,gp tool yefnuhnacnticoen ea.lga.c, rtyhlea tceusrainregp sapreteodf m[23o]d. ernUV-curableinksusedinflexo-orinkjetprintingtoenhancee.g.,thecuringspeed[23]. AAiimmiinngg ttoo ccrreeaatteep poolylymmeerro opptitciaclalw wavaevgeugiudiedsebs ybyap applpyilnygindgi fdfeifrfeenretnsht asphianpginmge mtheotdhso,dths,e tphree spernet- wseonrtk wfoocruk sfeoscuosnetsh oend tehsec rdipetsicornipotifotnh eofc othmep coosmitipoons-idtieopne-dnedpeenntdpehnyts pichaylspicraolp perrtoipeserotifecso opfo clyompoelrys-, cmoenrssi,s tcionngsiosftindgif foefr edniftfeproelnytf upnoclytifounnacltimonoanlo mmoenros,mpeorlsy, (pmoelyth(mylemtheythlmacertyhlaactrey)laanted) athned ethleec terloenct-rroicnh- mricohle mcuolleecpuhleen pahnethnraenntehraesnteh aesd tohpea dnot.pant. 22.. MMaatteerriiaallss aanndd MMeetthhooddss AA ccoommmmeerrcciiaallllyy aavvaaiillaabbllee pprreemmiixxeedd ppoollyymmeerr rreessiinn ((PPlleexxiitt 5555,, CCaarrll RRootthh ccoommppaannyy,, KKaarrllssrruuhhee,, GGeerrmmaannyy)),, ccoonnssisitsitningg ofo pfoplyo(lmy(emtheythlmylemtheatchrayclraytela),t e(P),M(MPMAM, cAon,tecnotn 3te0n–t353 w0–t3 %5),w stol%ve)d, isno lmveedthyinl- mmeetthhyaclmryelathtea c(rMylMatAe,( M65M–7A0 ,w6t5 –%70) wwats% u)sewda sasu tsheed tahsertmhealtlhye ormr aplhlyotoorchpehmoitcoaclhlye mcuicraalbllye cmuraatrbilxe. mThartereix d.iTffherreeendt ipffoelrye-nfutnpcotliyon-fauln acctrioynlaatle amcroynlaotmeemrso,n torimmeertsh,ytlroimlpertohpyalnotlrpiraocpryalnattrei a(TcrMylPaTteA()T, MpePnTtaAe)-, pryetnhtraietoryl thtrriiatcorlytlraitaec r(yPlaEtTeA()P EaTnAd) dani(dtridmi(ettrhimyleotlhpyroloplapnreo)p atneter)aatectrryalaactery l(aDteTT(DAT, TaAls,oa ldsoendoetendo teads aDsiTDMiTPMTAP)T A(F)ig(uFrieg u1r)e, a1ll) ,obatlalinoebdta ifnroemd SfriogmmaS-Aiglmdraic-Ah l(dSrtiecihnh(eSitmei,n Gheerimm,anGye),r mwaenrey )u,sewde raes puosleyd- afusnpcotiloynfualn acctiroynlaatle accrorysslalitnekcerro rsessluinltkienrg rines cuolptionlgyminercso apfotelyr mcuerrisnga.f Itner a cfuirrsitn sge.rieIsn oaf ifinvrsetstsiegraiteisonosf, iinnvcreesatisgiantgio innsc,rienmcreenatsainl gaminocurenmtse nfrtoamla m0 ouupn ttos f2ro, m4, 08,u 1p6t oan2d,4 3,28 ,w16t a%n dof3 2thwe tp%oloyffuthnecptioolnyaful naccrtiyolnaatel amcroynloamteemrso nwoemree rasdwdeedre taod tdheed mtoaitnh ePlmexaiitn 5P5l erxesitin5.5 Irne sai ns.ecInonads esceorineds soefr iinesveosftiingvaetisotingsa, ttihoen sn,etwhe mneixw- mtuirxetsu wreesrew deroepeddo pweidthw pihthenpahnethnraennteh ruepn etou tphet osotlhuebisloitlyu bliimlitiyt alism ai treafsraactrivefer aincdtievxe binodosetxerb, oaoss dteer-, ascsrdibeesdcr iinbe pdreinviporuesv wioourskw [1o7r–k2[01]7. –20]. (a) (b) (c) Figure 1. Used polyfunctional acrylate comonomers: (a) trimethylolpropantriacrylate (TMPTA); Figure 1. Used polyfunctional acrylate comonomers: (a) trimethylolpropantriacrylate (TMPTA); (b) pentaerythritol triacrylate (PETA); (c) di(trimethylolpropane) tetraacrylate (DTTA). (b)pentaerythritoltriacrylate(PETA);(c)di(trimethylolpropane)tetraacrylate(DTTA). All components were mixed together using a high-speed stirrer (Ultraturrax T8, IKA, Staufe, All components were mixed together using a high-speed stirrer (Ultraturrax T8, IKA, Staufe, Germany) and polymerized thermally using 1 wt % dilauroylperoxide (DLP) (Sigma-Aldrich, Stein- Germany) and polymerized thermally using 1 wt % dilauroylperoxide (DLP) (Sigma-Aldrich, heim, Germany) at a curing temperature of 70 °C applied for 24 h. Table 1 lists the used sample series Steinheim,Germany)atacuringtemperatureof70◦Cappliedfor24h. Table1liststheusedsample denotation in this work and the related real composition of all investigated thermally-cured mixtures. seriesdenotationinthisworkandtherelatedrealcompositionofallinvestigatedthermally-cured Constant amounts of thermal initiator (DLP, 1 wt %) and release agent (INT54, 1 wt %) had to be mixtures. Constantamountsofthermalinitiator(DLP,1wt%)andreleaseagent(INT54,1wt%)had added to get 100%. Due to order of mixture fabrication (first monomer mixture, then dopant addition) tobeaddedtoget100%. Duetoorderofmixturefabrication(firstmonomermixture,thendopant the effective concentration of the polyfunctional acrylate monomers vary slightly from the denotation addition)theeffectiveconcentrationofthepolyfunctionalacrylatemonomersvaryslightlyfromthe value. denotationvalue. The photochemical curing behaviour was investigated for selected mixtures applying the pho- toinitiator D3358 (3 wt %, TCI, Eschborn, Germany) in combination with a powerful LED light source (405 nm, LED spot 100, Dr. Hoenle, Gräfelfing, Germany). To ensure complete polymerization, 1 wt % DLP as in the other systems was also added. To avoid adhesion to the glass substrate, a release agent (INT54, E. & P. Wuertz GmbH, Bingen, Germany) was added to the curable mixtures. The viscosity of all mixtures prior to curing was measured using a cone and plate rheometer (Malvern-Bohlin CVO50, Herrenberg, Germany, temperature range 20–60 °C, shear rate range 1–200 1/s, cone diameter 40 mm, cone inclination angle 4°, gap size 150 mm). The experimental uncertainty for viscosity values larger than 0.1 Pa s is around ±5% and ±10% for values below 0.1 Pa s. The refrac- tive indices of uncured liquid samples and cured solid samples (mean value of at least five measure- ments) were characterized by an Abbe refractometer AR2008 (589 nm, 20 °C, experimental uncer- tainty ±0.001, Kruess, Hamburg, Germany). The optical transmittance in the 400–800 nm range of uncured liquid samples was measured applying a Cary 50 UV/Vis spectrophotometer (Varian, now Polymers2018,10,337 4of14 Table1.Sampleseriesdenotationandrelatedmixturecomposition(inroundfiguresto100%without decimal place) All mixtures contain in addition 1 wt % thermal initiator DLP and release agent INT54each. Composition[wt%] SeriesDenotation Plexit55 PolyfunctionalAcrylate Phenanthrene 0wt%series 98 0 0 93 0 5 88 0 10 83 0 15 78 0 20 4wt%series 94 4 0 89 4 5 84 4 10 80 3 15 75 3 20 8wt%series 90 8 0 85 8 5 81 7 10 76 7 15 72 6 20 16wt%series 82 16 0 78 15 5 74 14 10 69 14 15 65 13 20 32wt%series 66 32 0 63 30 5 59 29 10 56 27 15 53 25 20 The photochemical curing behaviour was investigated for selected mixtures applying the photoinitiatorD3358(3wt%,TCI,Eschborn,Germany)incombinationwithapowerfulLEDlight source(405nm,LEDspot100,Dr. Hoenle,Gräfelfing,Germany). Toensurecompletepolymerization, 1wt%DLPasintheothersystemswasalsoadded. Toavoidadhesiontotheglasssubstrate,arelease agent(INT54,E.&P.WuertzGmbH,Bingen,Germany)wasaddedtothecurablemixtures. The viscosity of all mixtures prior to curing was measured using a cone and plate rheometer (Malvern-Bohlin CVO50, Herrenberg, Germany, temperature range 20–60 ◦C, shear rate range 1–2001/s, cone diameter 40 mm, cone inclination angle 4◦, gap size 150 mm). The experimental uncertaintyforviscosityvalueslargerthan0.1Pasisaround±5%and±10%forvaluesbelow0.1Pas. Therefractiveindicesofuncuredliquidsamplesandcuredsolidsamples(meanvalueofatleastfive measurements)werecharacterizedbyanAbberefractometerAR2008(589nm,20◦C,experimental uncertainty±0.001,Kruess,Hamburg,Germany). Theopticaltransmittanceinthe400–800nmrange ofuncuredliquidsampleswasmeasuredapplyingaCary50UV/Visspectrophotometer(Varian,now Agilent,Waldbronn,Germany)usingdisposableUVsemi-microcuvettes(12.5×12.5×45mm3outer dimensions). TheglasstransitiontemperaturesT ofthecuredsystemswereestimatedviadifferential g scanningcalorimetry(DSC)(Netzsch,DSC204F1Phoenix,heatingrate10◦C/min)withanuncertainty of±2◦C.TheVickershardness(Paar-PhysicaMHT10,load: 100p,indentionspeed: 10p/s,loading time: 1s)ofselectedsamples(meanvalueoffivemeasurements,standarddeviation±2.5units)were recordedafterpolymerizationforsuitabletestspecimens. Polymers2018,10,337 5of14 3. ResultsandDiscussion 3.1. ShearRateandTemperature-DependentViscosityofUncuredMixtures Theknowledgeoftheimpactofthepolyfunctionalacrylatetypeanddopantconcentrationas wellastheshearrateandtemperatureontheviscosityofthemixtureisessentialforfurtherprocessing, micro-structuringanddevicefabricationapplyingdifferentshapingmethods. Thedifferentvariants ofreactiveresinprocessingtechniquespossessdifferentacceptableresinviscositiesatprocessingor operationtemperatures. Figure2showsforbettercomparisontheshearrateandviscosity-dependent flowbehaviourforallinvestigatedpurecomponents.Duetothepolymeramountofaround30–35wt% in the commercial resin, Plexit 55 possesses a pseudoplastic flow at all investigated temperatures, aPsoldymeesrcsr 2i0b1e8d, 1e0,a xr lFiOerR [P1E7E]R. REVIEW 5 of 13 FFiigguurree 22.. SShheeaarr rraattee aanndd tteemmppeerraattuurree--ddeeppeennddeenntt vviissccoossiittyy ooff aallll iinnvveessttiiggaatteedd ppuurree ccoommppoonneennttss.. 3.1.1. Trimethylolpropantriacrylate (TMPTA)-Containing Systems ThepuremonomersTMPTA,PETAandDTTAshowinallcasesapureNewtonianflow,inwhich the twThoen aodnd-iptioolna romf ionncroemaseirnsg, TamMoPuTnAts aonfd TMDTPTTAA, teuxrhniebdit thsieg npisfieucadnotplloawstiecr fvloiwsc oinsittoy av Naleuwestotnhiaann tfhloewm, oasr eexppoelacrt,edon ferohmy dFrigouxyre-g 2r.o Fuigpu-croen 3t asuinminmgaPrEizTeAs t.hIne iandflduietinocne, othf etecmopmebraintuarteio annodf cahceommicmale crocimal- eppoosxityioanc orynl athtee wreistuhltEinGgD mMiAxt,utrhee vpisucroesietpyo fxoyr aa csreylelactteedi tssheelfa,ra rnadtea ollf d1e0r0i v1e/sd. Tmhiex tmuraeins, raelssuoltssh aorwe:e da Newtonianflow[24]. • A temperature increase causes a viscosity drop: this is a common behaviour for all organic liq- uids and can be described by the Andrade–Eyring relation [25]. 3.1.1. Trimethylolpropantriacrylate(TMPTA)-ContainingSystems • Increasing TMPTA amounts cause a viscosity drop due to simple mixing rules. • TInhcereaadsdinitgio pnhoefniannctrhearesnineg aammoouunntsts coafuTseM aP vTAisctousrintye ddrthope:p Tsheue dgoivpelans tpiclaflstoiwciziinntgo eafNfecetw otfo npihaen- flow, as expected from Figure 2. Figure 3 summarizes the influence of temperature and chemical nanthrene, solved in different reactive resins, is in agreement with other results [17–20]. c•o mpAo stiotitoanl oofn 20th wetr e%s uplhtienngamntihxrteunree evxiscceoesdisty thfoe rsoalsuebleilcittyed limshiet aart raa tTeMoPfT10A0 c1o/nst.eTnht eofm 4a wint r%es. ults are: • Atemperatureincreasecausesaviscositydrop: thisisacommonbehaviourforallorganicliquids andcanbedescribedbytheAndrade–Eyringrelation[25]. • IncreasingTMPTAamountscauseaviscositydropduetosimplemixingrules. • Increasing phenanthrene amounts cause a viscosity drop: The given plasticizing effect of phenanthrene,solvedindifferentreactiveresins,isinagreementwithotherresults[17–20]. • Atotalof20wt%phenanthreneexceedsthesolubilitylimitataTMPTAcontentof4wt%. Figure 3. Influence of temperature, TMPTA, and phenanthrene content on the viscosity of the mixture at a fixed shear rate of 100 1/s. Polymers 2018, 10, x FOR PEER REVIEW 5 of 13 Figure 2. Shear rate and temperature-dependent viscosity of all investigated pure components. 3.1.1. Trimethylolpropantriacrylate (TMPTA)-Containing Systems The addition of increasing amounts of TMPTA turned the pseudoplastic flow into a Newtonian flow, as expected from Figure 2. Figure 3 summarizes the influence of temperature and chemical com- position on the resulting mixture viscosity for a selected shear rate of 100 1/s. The main results are: • A temperature increase causes a viscosity drop: this is a common behaviour for all organic liq- uids and can be described by the Andrade–Eyring relation [25]. • Increasing TMPTA amounts cause a viscosity drop due to simple mixing rules. • Increasing phenanthrene amounts cause a viscosity drop: The given plasticizing effect of phe- nanthrene, solved in different reactive resins, is in agreement with other results [17–20]. Polymers2018,10,337 6of14 • A total of 20 wt % phenanthrene exceeds the solubility limit at a TMPTA content of 4 wt %. Figure 3. Influence of temperature, TMPTA, and phenanthrene content on the viscosity of the mixture Figure3.Influenceoftemperature,TMPTA,andphenanthrenecontentontheviscosityofthemixture at a fixed shear rate of 100 1/s. atafixedshearrateof1001/s. Polymers 2018, 10, x FOR PEER REVIEW 6 of 13 3.1.2. PentaerythritolTriacrylate(PETA)-ContainingSystems 3.1.2. Pentaerythritol Triacrylate (PETA)-Containing Systems TheadditionofincreasingamountsofPETAandphenanthrenetoPlexit55causesacomparable influeTnhcee aodndtihtieoflno owf ibnechreaavsiionugr a(Fmigouurnets4 )o.fA PEclToAse ranindv peshteignaatniothnroenfteh teo vPilsecxoisti t5y5 dcaautaseresv ae caolsmapcaerratabilne sincafltuteernicneg oonf tthhee vflioswco sbietyhadvaitoau:r (Figure 4). A closer investigation of the viscosity data reveals a cer- tain scattering of the viscosity data: • AtaconstantPETAcontent,atemperatureincreasecausesaviscositydrop. •• IAntc rae caosninsgtapnht ePnEaTnAth creonneteanmt, oau tnetmspcaeurasteuarep irnincrceipasael vcaisucsoessit ay vdirsocposdituye dtrooppl.a sticizing. •• WInictrheiansitnhge pinhveensatingtahtreednceo anmceonutnratst icoanursaen ag ep,rpinhceipnaanl vthisrceonseitiys cdormopp ldeuteel ytos polluasbtlieciuzpintgo. 20wt%. • Within the investigated concentration range, phenanthrene is completely soluble up to 20 wt %. FFiigguurree 44.. IInnflfluueennccee ooff tteemmppeerraattuurree,,P PEETTAA,,a annddp phhenenananththrerneenec ocnotnetnetnot nonth tehve ivscisocsoitsyitoyf othf ethme imxtiuxrteuraet aatc ao mcommomnosnh esahrearar treaotef o10f 01010/ s1./s. 3.1.3. Di(trimethylolpropane) Tetraacrylate (DTTA)-Containing Systems As with the other investigated systems, the addition of increasing amounts of DTTA and phe- nanthrene to Plexit 55 causes an almost analogous influence on the resulting mixture’s flow behav- iour (Figure 5). A closer investigation of the viscosity data reveals a slight scattering of the viscosity data: • At constant DTTA content, a temperature increase causes a viscosity drop. • Increasing phenanthrene amounts cause a principal viscosity drop due to plasticizing. • A total of 20 wt % phenanthrene exceeds the solubility limit at a DTTA content of 16 wt %. In general, all three polyfunctional acrylates lower the viscosity of the Plexit 55 matrix with in- creasing monomer content. The addition of phenanthrene also causes a viscosity drop. In a few cases, the solubility limit of phenanthrene in the monomer mixture is lowered. The presence of the hy- droxyl-functionality in PETA may prohibit a clear relationship between the composition and result- ing viscosity values. The information obtained in this section enables viscosity tailoring with respect to the aspired viscosity range attributed to the selected further processing. As an example, polymer waveguide inkjet printing requires UV-curable mixtures with a viscosity below 16 mPa s at printing temperatures (40–70 °C) [11,12]. Unfortunately, none of the investigated mixtures fulfills this viscos- ity requirement for inkjet printing. In contrast, flexoprinting and SLA accept viscosities up to 500 mPa s, which can be achieved here. Polymers2018,10,337 7of14 3.1.3. Di(trimethylolpropane)Tetraacrylate(DTTA)-ContainingSystems As with the other investigated systems, the addition of increasing amounts of DTTA and phenanthrene to Plexit 55 causes an almost analogous influence on the resulting mixture’s flow behaviour (Figure 5). A closer investigation of the viscosity data reveals a slight scattering of the viscositydata: • AtconstantDTTAcontent,atemperatureincreasecausesaviscositydrop. • Increasingphenanthreneamountscauseaprincipalviscositydropduetoplasticizing. • Atotalof20wt%phenanthreneexceedsthesolubilitylimitataDTTAcontentof16wt%. Polymers 2018, 10, x FOR PEER REVIEW 7 of 13 Figure 5. Influence of temperature, DTTA, and phenanthrene content on the viscosity of the mixture Figure5.Influenceoftemperature,DTTA,andphenanthrenecontentontheviscosityofthemixtureat at a common shear rate of 100 1/s. acommonshearrateof1001/s. 3.2. Optical Properties of Uncured and Cured Systems In general, all three polyfunctional acrylates lower the viscosity of the Plexit 55 matrix with increWasiitnhg rmesopneoctm teor tchoen tfeanbrt.icTathieona dodf iptioolnymofepr hweanvaengthuridenese aulssiongc aduisffeesreanvt isschoaspitinygd rmoept.hIondsa, ftehwe acadsjuess,tmtheents oolfu tbhiel irteyfrlaimctiitveo finpdhiceensa noft hthreen ceorine athned mclaodndoimnge rfomr iwxtauvreegiusidloinwge irse dcr.ucTihael. Ipnr easdednicteioonf, kthneowhyleddrogxey olf- ftuhnec otipotnicaalli ttyrainnsPmEiTttAanmcea iys persosehnibtiiatla focrle laorwr elloastsi omnashteirpiable stewleecetniotnh. ecompositionand resultingviscosityvalues. Theinformationobtainedinthissectionenablesviscositytailoringwith 3.2.1. Refractive Index respecttotheaspiredviscosityrangeattributedtotheselectedfurtherprocessing. Asanexample, polymTheer wreaavl epgaurti doef tinhke jceotmprpinletxin rgefrreaqcutiivrees inUdVe-xc,u tryapbilceamllyi xdteunreostewdi aths arevfrisaccotisviittyyb oerl orewac1t6ivme Pinadseaxt, ipsr ian mtinegastuemrep oefr tahtue rienste(4ra0c–t7i0on◦ Co)f [t1h1e, 1e2l]e.cUtronmfoartgunneattiec lfyi,enldo noef othfeth iencinomveisntgig wataedvem wixitthu rtehsef uelleficltlrsotnhiics sviitsucaotsiiotny irne qmuairtetemr,e enstpfeocriainllkyj ethtep rpinoltainrigz.aIbnilcitoyn itnra tshte, flaeffxeocpterdin mtinogleacunldesS L[2A6]a. cAcne pintcvriesacsoes iotife tshue ppoto- l5a0r0izmabPialitsy, winh oicrhgacnainc bmeoalcehcuielvees dcahne rbee. realized by an increase of the number of movable electrons, e.g., by the extension of delocalized π-electrons as given in the dopant phenanthrene. In addition, a 3.2. OpticalPropertiesofUncuredandCuredSystems further interaction—e.g., by forming charge–transfer complexes—can lead to a light absorption (yel- lowinWg)it ihn rthesep veicstibtloe etqhueivfaalbernict attoi otnhe oimf pagoilnyamrye rpawrat voef gthueid ceosmupslienxg redfrifafcetrievnet insdheaxp.i nWgitmh reethspoedcst, ttoh eteacdhjunsictmal eanptpolfictahteiornefsr aocf ttihvee innedwic mesaotferthiaelsc—oree.agn.,d asc lwadadvienggufiodrews—avaeng aubidsoinrgptiisocnr uinc itahl.e Ivnisaidbdleit hioans, tkon boew slterdicgtelyo afvthoeidoepdt.i cFaiglutrraen 6s mshitotwansc feoirs aelsl stehnet iinalvfeosrtilgoawtedlo ssyssmteamtesr iaa lchsealnegctei oinn .the refractive index with mixture composition and dopant concentration. The most important correlations found were: 3.2.1. RefractiveIndex • Due to the lower density of the liquid monomer mixtures, the refractive index is lower in general Therealpartofthecomplexrefractiveindex,typicallydenotedasrefractivityorreactiveindex, than the related ones in the solidified state. •is amAelal smuereasoufrtehde vinaltuereasc atiroen cloofsteh teoegleetchteror,m thaeg nriestei cinfi ethlde orefftrhaectiinvceo imndinegx wcaanv beew aittthribthueteedle mctraoinnliyc to the increasing phenanthrene moiety. • As found in other systems, an almost linear correlation of the refractive index with the phenan- threne concentration can be derived [18,19]. • In the case of DTTA-based systems, the use of larger DTTA amounts are favorable in the liquid state. • A clear correlation of the polyfunctional acrylate monomer structure with the refractive index values cannot be found. Polymers2018,10,337 8of14 situation in matter, especially the polarizability in the affected molecules [26]. An increase of the polarizabilityinorganicmoleculescanberealizedbyanincreaseofthenumberofmovableelectrons, e.g., bytheextensionofdelocalizedπ-electronsasgiveninthedopantphenanthrene. Inaddition, a further interaction—e.g., by forming charge–transfer complexes—can lead to a light absorption (yellowing)inthevisibleequivalenttotheimaginarypartofthecomplexrefractiveindex. Withrespect totechnicalapplicationsofthenewmaterials—e.g.,aswaveguides—anabsorptioninthevisiblehas tobestrictlyavoided. Figure6showsforalltheinvestigatedsystemsachangeintherefractiveindex withmixturecompositionanddopantconcentration. Themostimportantcorrelationsfoundwere: • Duetothelowerdensityoftheliquidmonomermixtures,therefractiveindexisloweringeneral thantherelatedonesinthesolidifiedstate. • Allmeasuredvaluesareclosetogether,theriseintherefractiveindexcanbeattributedmainlyto theincreasingphenanthrenemoiety. • As found in other systems, an almost linear correlation of the refractive index with the phenanthreneconcentrationcanbederived[18,19]. • In the case of DTTA-based systems, the use of larger DTTA amounts are favorable in the liquidstate. • Aclearcorrelationofthepolyfunctionalacrylatemonomerstructurewiththerefractiveindex valuescannotbefound. Polymers 2018, 10, x FOR PEER REVIEW 8 of 13 FiFgiugruer6e. 6R. eRfreafrcaticvtievien idnidceicseosf oafl lailnl ivnevsetisgtaigteadtemd mixtiuxtruesrepsr piorrio(rli q(luiqidu)ida)n adnadf taefrtecru cruinrgin(gs o(lsiodl)i.d). 3.2.2. Optical Transmittance in the Visible Range 3.2.2. OpticalTransmittanceintheVisibleRange Exemplarily, the influence of the copolymer composition on the optical transmittance properties Exemplarily,theinfluenceofthecopolymercompositionontheopticaltransmittanceproperties were investigated applying the DTTA-based mixtures. Without dopant, the influence of the increas- wereinvestigatedapplyingtheDTTA-basedmixtures. Withoutdopant,theinfluenceoftheincreasing ing DTTA content was negligible; an influence was found for low amounts of DTTA in combination DTTAcontentwasnegligible;aninfluencewasfoundforlowamountsofDTTAincombinationwitha with a moderate phenanthrene load of 10%. Higher DTTA moieties in combination with 10% dopant moderatephenanthreneloadof10%. HigherDTTAmoietiesincombinationwith10%dopantresulted resulted in some higher absorbance (Figure 7a). The combination of low amounts of DTTA with in- insomehigherabsorbance(Figure7a). ThecombinationoflowamountsofDTTAwithincreasing creasing phenanthrene contents up to 20% produced only a small drop in the transmittance values phenanthrenecontentsupto20%producedonlyasmalldropinthetransmittancevalues(Figure7b). (Figure 7b). (a) (b) Figure 7. Optical transmittance of DTTA-containing mixtures: (a) Influence of DTTA content without and with 10% phenanthrene; (b) Influence of phenanthrene content at constant DTTA amount. Mixtures consisting of TMPTA or PETA in combination with phenanthrene showed comparable transmittance behaviour due their similar chemical structure without functional groups absorbing in the visible range. The main result of this subsection can be summarized as follows: large refractive indices, gener- ated with huge dopant concentrations, have the drawback of the reduced optical transmittance of the used doped copolymer. The amount of polyfunctional acrylate monomer has a certain influence on Polymers 2018, 10, x FOR PEER REVIEW 8 of 13 Figure 6. Refractive indices of all investigated mixtures prior (liquid) and after curing (solid). 3.2.2. Optical Transmittance in the Visible Range Exemplarily, the influence of the copolymer composition on the optical transmittance properties were investigated applying the DTTA-based mixtures. Without dopant, the influence of the increas- ing DTTA content was negligible; an influence was found for low amounts of DTTA in combination with a moderate phenanthrene load of 10%. Higher DTTA moieties in combination with 10% dopant resulted in some higher absorbance (Figure 7a). The combination of low amounts of DTTA with in- creasing phenanthrene contents up to 20% produced only a small drop in the transmittance values (Figure 7b). Polymers2018,10,337 9of14 (a) (b) Figure 7. Optical transmittance of DTTA-containing mixtures: (a) Influence of DTTA content without Figure7.OpticaltransmittanceofDTTA-containingmixtures:(a)InfluenceofDTTAcontentwithout and with 10% phenanthrene; (b) Influence of phenanthrene content at constant DTTA amount. andwith10%phenanthrene;(b)InfluenceofphenanthrenecontentatconstantDTTAamount. Mixtures consisting of TMPTA or PETA in combination with phenanthrene showed comparable MixturesconsistingofTMPTAorPETAincombinationwithphenanthreneshowedcomparable transmittance behaviour due their similar chemical structure without functional groups absorbing in transmittancebehaviourduetheirsimilarchemicalstructurewithoutfunctionalgroupsabsorbingin the visible range. thevisiblerange. The main result of this subsection can be summarized as follows: large refractive indices, gener- The main result of this subsection can be summarized as follows: large refractive indices, ated with huge dopant concentrations, have the drawback of the reduced optical transmittance of the generatedwithhugedopantconcentrations,havethedrawbackofthereducedopticaltransmittance used doped copolymer. The amount of polyfunctional acrylate monomer has a certain influence on oftheuseddopedcopolymer. Theamountofpolyfunctionalacrylatemonomerhasacertaininfluence on the optical properties, enabling a certain flexibility in the viscosity value for use in different shapingmethods. 3.3. PhotoinitiatedCuringBehaviour Ingeneral,theradicalpolymerizationofacrylatessufferfromtheinhibitinginfluenceofoxygen ifthecuringprocessiscarriedoutunderambientconditionsinanairatmosphere. Severalnegative effectscanbeobservedduringphotopolymerization[27]: • Occurrenceofaninductionperiodpriortothechemicalreaction; • Reductionofthepolymerizationrateandfinalconversion; • Reductionofthechainlengthandconversionrate; • Formationofastickysurfaceappearanceduetooligomerformation. Differentapproachescanbeappliedtoovercometheoxygeninhibitionprocess[28–30]: • UV-curinginaninertatmosphere; • Preventingoxygendiffusionbyprotectingfilmsortransparentfoils; • Selectionofhighlyeffectivelightsourceslikevaporpressurearclamps; • Useofsuitableadditivestrappingoxygen-liketriphenylphosphine[31]; • Useofmonomerswithhugechemicalreactivity. Especiallyinthelattercaseitisrecommendedtoaddpolyfunctionalacrylatestoaphotocurable system to enhance the cure speed, reduce the oxygen inhibition effect and enhance the resulting physical properties [23,29]. Earlier investigations applying different acrylate mixtures exhibited sticky surface properties after UV-curing in air, hence a polymerization between glass plates was necessary[32]. Withrespecttotheverificationofthepositiveinfluenceofthepolyfunctionalacrylates onthecuringbehaviour[23],DTTA-containingmixtureswerecuredexemplarilybyUV-light. Two differentsetupsforthephotopolymerizationprocesswereinvestigated. First,theresincompositions Polymers2018,10,337 10of14 wereplacedbetweentwoglassplatesandUV-curedwiththeexclusionofoxygen. Second,theresins wereUV-curedunderexposuretoair. ItwasfoundthatevenatlowconcentrationsofDTTA(4wt%) bothsetupsdeliveredtack-free,hardandtransparentsurfaces,whileDTTA-freesamplesremained stickyatthesurface. Hence,asimplifiedcuringprocedureispossibleandshowsthepositiveinfluence ofthepolyfunctionalacrylatemonomer. AtlargeDTTAamounts(32wt%)andhigherphenanthrene concentrationsthesamplesbecameturbidsomeweeksafterpreparation. TheUV-curingofpureDTTA deliveredaverybrittlepolymerduetothermosetformation[23]. Theaboveresultsarealsovalidfor thetwootherpolyfunctionalacrylates,PETAandTMPTA. 3.4. ThermomechnicalProperties Onemotivationfortheuseofpolyfunctionalacrylatesascomonomersistheirabilitytoform rigidpolymernetworksaftercuring. Itcanbeexpectedthatwithahigherpolyfunctionalacrylate contentintheresultingpolymer,enhancedthermomechanicalpropertieswillbefoundincomparison topurethermoplastics[23]. 3.4.1. GlassTransitionTemperature Figure8showstheT valuesmeasuredbyDSCofallinvestigatedmixtures,thedatafordoped g Plexit without any other comonomer was addedfor reference. Inall cases, increasing amounts of phenanthrenelowersT relativetothereferencematerial(curedPlexit,i.e.,PMMA)duetoplasticizing, g as has been reported elsewhere [17–21]. This is also valid for the use of a polyfunctional acrylate comonomer irrespective of whether phenanthrene is present or not. A more careful data analysis shows a certain order of the comonomers: PETA caused a pronounced T drop, the influence of g DTTA was moderate, and TMPTA showed the smallest decrease. Higher amounts of TMPTA generated the aspired T elevation at higher dopant concentrations in comparison to the related g Plexit/phenanthrenemixtures. Polymers 2018, 10, x FOR PEER REVIEW 10 of 13 Figure 8. Glass transition temperatures of all investigated systems after curing. Figure8.Glasstransitiontemperaturesofallinvestigatedsystemsaftercuring. At present, inconsistent Tg values for the polymerized pure polyfunctional acrylates have been Atpresent,inconsistentT valuesforthepolymerizedpurepolyfunctionalacrylateshavebeen described. While [23] lists 98 °Cg for poly(TMPTA): 62 °C, poly(PETA): 103 °C and poly(DTTA), some described. While[23]lists98◦Cforpoly(TMPTA):62◦C,poly(PETA):103◦Candpoly(DTTA),some other references provide different information. One group measured a glass transition around 88 °C otherreferencesprovidedifferentinformation.Onegroupmeasuredaglasstransitionaround88◦Cfor for a doped PETA polymer [33], while in [34] no glass transition in the temperature range from −50– adopedPETApolymer[33],whilein[34]noglasstransitioninthetemperaturerangefrom−50–220◦C 220 °C was found. The aspired effect of an increase in glass transition temperature, within one series, wasfound. Theaspiredeffectofanincreaseinglasstransitiontemperature,withinoneseries,and and with an increasing polyfunctional acrylate comonomer content and a constant dopant amount, can only be observed at large TMPTA amounts (Figure 8). This may be attributed to the flexible chem- ical structure of the comonomers allowing a more elastomeric substructure. One can assume that huge amounts of TMPTA enables the formation of a more rigid thermoset-like network due to its more compact chemical structure. This can be also deduced from the low viscosity value of the TMPTA monomer, shown in Figure 2, as an indication of low inter-molecular friction in the liquid phase due to a more compact structure. Earlier work described that the use of rigid reactive molecules, like divinylbenzene, can increase Tg if they are used as comonomer in an unsaturated polyester–resin matrix after solidification [17]. Within an acrylate-based resin matrix, the use of 1,3-butandiol dimethacrylate (BDMA) as small di- functional molecule resulted in an observed drop of Tg with increasing amounts and a clear correla- tion between BDMA-content and Tg stabilization at high plasticizer content could be detected [18]. Without any crosslinker, a pronounced Tg decay with increasing phenanthrene concentration in PMMA (polymerized Plexit) and polymerized unsaturated polyester was found [19]. 3.4.2. Vickers Hardness Previous work investigating phenanthrene/unsaturated polyester/divinylbenzene mixtures, de- scribed a significant increase in the Vickers hardness of cured specimen with increasing divinylben- zene content [17]. In the case of a comparable system consisting of phenanthrene/Plexit/BDMA, no remarkable hardness increase could be measured [19]. Exemplarily, the DTTA-based mixtures were investigated with respect to the influence of the comonomer and the phenanthrene content (Figure 9). In all cases, increasing dopant concentrations show a decline in hardness due to plasticizing.

Description:
2 of 14 for the fabrication of optical components, e.g., lenses or data storage devices such as CD/DVD/Blu. Ray disks. With respect to the fabrication of customized devices, such as polymer waveguides with passive or active optical properties, new materials with tailored features including refracti
See more

The list of books you might like

Upgrade Premium
Most books are stored in the elastic cloud where traffic is expensive. For this reason, we have a limit on daily download.
DMCA & Copyright: Dear all, most of the website is community built, users are uploading hundred of books everyday, which makes really hard for us to identify copyrighted material, please contact us if you want any material removed.
Copyright @ PDFdrive.to, 2022 | We love our users