Crystallinity / Degree of Crystallinity - Training & Know-how

Crystallinity refers to the degree of structural order of a solid. In a crystal, the arrangement of atoms or molecules is consistent and repetitive. GlossaryCrystallinity/DegreeofCrystallinity Crystallinityreferstothedegreeofstructuralorderofasolid.Inacrystal,thearrangementofatomsormoleculesisconsistentandrepetitive.Manymaterialssuchasglassceramicsandsomepolymerscanbepreparedinsuchawayastoproduceamixtureofcrystallineandamorphousregions.However,evenforcompletelycrystallinematerials,thedegreeofstructuralperfectioncanvary.Forexample,mostmetallicalloysarecrystalline,butusuallycomprisemanyindependentcrystallineregions(grainsorcrystallites).Invariousorientationsseparatedbygrainboundaries,theyalsocontainothercrystallographicdefects,suchasdislocations.Thatreducesthedegreeofstructuralperfection.Themosthighlyperfectcrystalsaresiliconboulesproducedforsemiconductorelectronicswhicharelargesinglecrystals(i.e.,theyhavenograinboundaries);thesearenearlyfreeofdislocationsandhavepreciselycontrolledconcentrationsofdefectatoms.CrystallizationCrystallizationisthephysicalprocessofhardeningduringtheformationandgrowthofcrystals.Duringthisprocess,heatofcrystallizationisreleased.Crystallizationofpolymerscanbeobservedinsomethermoplastics.Whenthemeltsolidifies,partialalignmentofthemolecularchainsinthepolymeroccurs.BasedonCrystallizationCrystallizationisthephysicalprocessofhardeningduringtheformationandgrowthofcrystals.Duringthisprocess,heatofcrystallizationisreleased.crystallizationnuclei,themolecularchainsfoldtogetherandformorderedregionscalledlamellae.LabContactDegreeofCrystallinityThepropertiesofplasticsaresignificantlyinfluencedbytheirdegreeofcrystallization.Thehigherthedegreeofcrystallization,thestifferandstronger,butalsomorebrittleamoldedpartis.Thedegreeofcrystallizationisinfluencedbythechemicalstructureandthermalhistory,suchasthecoolingconditionsduringprocessingorpost-thermaltreatment.Fordeterminationofthedegreeofcrystallization,K,themeasuredmeltingenthalpy∆Hmeasissetinrelationtotheliteraturevalue∆Hlitforcompletelycrystallinematerial.K=∆Hmeas/∆HlitThermalhistory:Thethermalormechanicalhistoryisshowninthe1stheatingcurveofaDSCmeasurement.The2ndheatingcurveservesfordeterminationofthematerialpropertiesundergivendynamicconditions.Thedegreeofcrystallinityhasasignificantinfluenceonhardness,DensityThemassdensityisdefinedastheratiobetweenmassandvolume.density,transparencyanddiffusion.However,thepropertiesarenotdeterminedonlybythedegreeofcrystallinity,butalsobythesizeofthestructuralunitsorthemolecularorientation.ApplicationExamplesDeterminationofthedegreeofcrystallinityofpolymersThemeltingbehavioroflow-densitypolyethylene(LDPE),high-densitypolyethylene(HDPE)andpolypropylene(PP)isdepictedinfigure1.Peaktemperatureshelpinidentifyingpolymersandpolymermixtures.Thepeakareasrepresentthemeltingenthalpies.Basedonthesevaluesfromtheexperiment,thedegreeofcrystallinitycanbedeterminedbymeansofthestandardProteus®evaluationsoftware.Themeltingenthalpyfor100%crystallinePEis293J/gandis207J/gforPP.ThisyieldsacalculatedcrystallinityforLDPEof46.5%,forHDPEof74.2%andforPPof48.8%.Fig.1:DeterminationofthedegreeofcrystallizationfordifferentpolymersbetweenRTand200°Cataheatingrateof10K/mininanitrogenatmosphere(40ml/min);themeasurementswerecarriedoutwiththeDSC204F1Phoenix®®employingaluminumcrucibleswithpiercedlids.DeterminationofthedegreeofCrystallinityduringPostCrystallizationThecalculationofthedegreeofcrystallinity,K,ofPETisdepictedinfigure2 below.Atthispoint,theenthalpyareaofthemelt,∆hm,here42.83J/g,issubtractedfromtheenthalpyareaofthePostCrystallization(ColdCrystallization)Thepostcrystallizationofsemi-crystallineplasticsoccursprimarilyatelevatedtemperaturesandincreasedmolecularmobilityabovetheglasstransition.post-crystallization,∆hc,here40.29J/g,anddividedbytheliteraturevalueof140J/g.Thisresultsinadegreeofcrystallization,K,of1.8%forPET.ApplicationNoteFig.2:CalculationoftheDegreeofCrystallizationCrystallizationisthephysicalprocessofhardeningduringtheformationandgrowthofcrystals.Duringthisprocess,heatofcrystallizationisreleased.Crystallization,K,duringpost-crystallizationofPETinthetemperaturerangebetweenRTand300°C.Thesamplewasheatedataheatingrateof10K/mininasealedaluminumcrucibleswithpiercedlid.RelatedMethods DifferentialScanningCalorimetry(DSC)