Glass Transition Temperature - Glossary

The glass transition temperature, Tg, of a material characterizes the temperature range over which this glass transition occurs. It is always lower than the ... GlossaryGlassTransitionTemperatureTheglasstransitionisoneofthemostimportantpropertiesofamorphousandsemi-crystallinematerials,e.g.,inorganicglasses,amorphousmetals,polymers,pharmaceuticalsandfoodingredients,etc.,anddescribesthetemperatureregionwherethemechanicalpropertiesofthematerialschangefromhardandbrittletomoresoft,deformableorrubbery. Manypolymers,e.g.,thermoplastics,thermosets,rubbers,etc.areusuallycomposedofbothamorphousandcrystallinestructures.Thismeansthatmanypolymersexhibitbothaglasstransitiontemperature,Tg,andaMeltingTemperaturesandEnthalpiesTheenthalpyoffusionofasubstance,alsoknownaslatentheat,isameasureoftheenergyinput,typicallyheat,whichisnecessarytoconvertasubstancefromsolidtoliquidstate. Themeltingpointofasubstanceisthetemperatureatwhichitchangesstatefromsolid(crystalline)toliquid(isotropicmelt).meltingtemperature.Theglasstransitiontemperature(Tg)islowerthanthemeltingtemperatureofacrystallinematerial.DeterminationofglasstransitiontemperaturebymeansofDifferentialScanningCalorimetry(DSC)DeterminationofglasstransitiontemperaturebymeansofDynamicMechanicalAnalysis(DMA)DeterminationofglasstransitiontemperaturebymeansofDilatometry(DIL)/ThermomechanicalAnalysis(TMA)LabContactGlassTransitionTemperatureforMaterialIdentificationDeterminationoftheglasstransitiontemperatureisatoolformaterialidentification.Theglasstransitiontemperature(Tg)alsodeterminestheapplicationfieldofamaterial.Forexample,arubbertire(car)issoftandductilebecauseatnormaloperatingtemperaturesitiswellaboveitsglasstransitiontemperature.Ifitsglasstransitiontemperaturewerehigherthanitsoperatingtemperature,itwouldnothavetheflexibilityneededtogripthepavement.Otherpolymersoperatebelowtheirglasstransitiontemperature,e.g.,astiffplastichandle.Iftheplastichandlehadaglasstransitiontemperaturebelowitsoperatingtemperature,itwouldbetooflexible.DeterminationoftheglasstransitiontemperaturebymeansofdifferentthermoanalyticalmethodsbyDifferentialScanningCalorimetry(DSC)(e.g.,ASTME1356)InDSCmeasurements,theglasstransitioncanbeobservedbyastepinthebaselineofthemeasurementcurve(Fig.1).Itischaracterizedbyitsonset,midpoint,inflectionandendsettemperature.ThestepheightcorrespondstoΔcpandisgiveninJ/(g⋅K).Theevaluationprocedureisdescribedine.g.,ASTME1356-08.DSCcanbeusedforsolids,powdersandliquids.WhatexactlyisGlassTransitionTemperatureTheglasstransitiontemperature,Tg,ofamaterialcharacterizesthetemperaturerangeoverwhichthisglasstransitionoccurs.Itisalwayslowerthanthemeltingtemperatureofthecrystallinestateofthematerial(ifoneexists).Inthetemperaturerangeoftheglasstransition,polymerschangefromahardandrigidstatetoamoreflexibleandsupplestate.Tgoccursinatemperaturerangeoverwhichthemobilityofthepolymerchainsincreasessignificantly.Thermoplasticslikepolystyrene(PS)andpoly(methylmethacrylate)(PMMA)areusuallyusedbelowtheirglasstransitiontemperature,i.e.,intheirglassystate.Elastomerslikepolyisopreneandbutadienerubber(BR)areusedabovetheirTg,wheretheyaresoftandsupple.InvestigatingtheInfluenceofMoistureontheGlassTransitionTemperatureofSorbitolApplicationInvestigatingtheInfluenceofMoistureontheGlassTransitionTemperatureofSorbitolSorbitolisusedasasugarsubstituteinmanysweets,dietproductsandmedications.Aproportionof10%waterinsorbitoleffectuatesadecreaseintheglasstransitiontemperatureofapprox.24K(midtemperatures)relativetoanhydroussorbitol.Bothsamplesremaincompletelyamorphousaftertherapidcoolingfromthemoltenstate(whichtookplacepriortothedisplayedheatingstep).Themeasurementswerecarriedataheatingrateof10K/mininnitrogenatmosphere.Thesealedsamplepansmadeofaluminumwereclosedwithapiercedlid.Thesamplemassesamountedtoapproximately12mg±1mg.byDynamicmechanicalanalysis(DMA)(e.g.,ASTM1640)TheDMAtechnique(e.g.,ASTME1640-09)isaverysensitivetechniqueforthedeterminationoftheglasstransitiontemperature(e.g.,1640-94).Itprovidesanalternativeprocedureinthedeterminationoftheglasstransitiontotheuseofdifferentialscanningcalorimetry(DSC)(ISO11357‑2).InDMAmeasurements,TgcanbeobservedintheextrapolatedonsetofthesigmoidalchangeinthestoragemodulusE’,thepeakoftheViscousmodulusThecomplexmodulus(viscouscomponent),lossmodulus,orG’’,isthe“imaginary”partofthesamplestheoverallcomplexmodulus.Thisviscouscomponentindicatestheliquidlike,oroutofphase,responseofthesamplebeingmeasurement.lossmodulusE’’andthepeakoftanδ.DMAcanbeusedforunreinforcedandfilledpolymers,foams,rubbers,adhesivesandfiber-reinforcedplastics/composites.Differentmodes(e.g.flexure,compression,tension)ofdynamicmechanicalanalysiscanbeapplied,asappropriate,totheformofthesourcematerial.TheGlassTransitionofaRubberApplicationTheGlassTransitionofaRubberDynamicMechanicalAnalysis(DMA)recordsamaterial’stemperature-dependentvisco-elasticproperties(stiffness,E’andlossmodulus,E’’,measurefortheoscillationenergy),anddeterminesitsmodulusofelasticityanddampingvalues(tanδ)byapplyinganoscillatingforcetothesample.Theglasstransitiontemperature,Tg,ofahydrogenatedacrylonitrilebutadienerubber(HNBR)wasdeterminedintensionmodebymeansofdynamicmechanicalanalysis,DMA.Themeasurementwasperformedataheatingrateof2K/min,afrequencyof1Hzandanamplitudeof±20µminthetemperaturerangebetween-90°Cand40°C.TheextrapolatedonsetdeterminedinthestoragemodulusE’,thepeakinthelossmodulusE’’andthepeakinthetanδcurveallcorrespondtotheglasstransitiontemperature,Tg,ofthisrubbermaterial(throughapplicationoftherespectiveevaluationconventions).byDilatometry(DIL)/ThermomechanicalAnalysis(TMA)(e.g.,ASTME831)Indilatometer(DIL)andthermomechanicalanalyzers(TMA,bothdescribedinASTME473–11a),theglasstransitioncorrespondstotheinflectioninthedimensionalchange(e.g.,ASTME1545.ItisrecordedastheextrapolatedonsetofkinkintheexperimentalDIL/TMAcurveanddisplayedasafunctionoftemperature.Tomakethisdefinitionreproducible,oneshouldspecifythecoolingorheatingrate.E.g.,ASTME1545describesthedeterminationoftheglasstransitionbymeansofTMA.DeterminationoftheGlassTransitionbymeansofDilatometryApplicationDeterminationoftheGlassTransitionbymeansofDilatometryDILmeasurementonanaturalrubbermaterialbetween-120°Cand20°Cataheatingrateof3K/mininheliumatmosphere.thesamplelengthamountedto2mm.Theextrapolatedonsettemperatureof-62°Ccorrespondstotheglasstransition(Tg).Inamorphousmaterialssuchasrubber,itisareversibletransition.Thematerialchangesfromahardandrelativelybrittlestateintoasoftorrubberystate.RelatedMethodsDifferentialScanningCalorimetry(DSC)DynamicMechanicalAnalysis(DMA)ThermomechanicalAnalysis(TMA)Dilatometry(DIL)