ABSTRACT:Tensilestressandbirefringenceinbothrealandmodelamorphouspolymermeltshavebeenmeasuredduringconstantrateuniaxialelongationalflow.Wefocusoninvestigationswheredeviationsfromthelinearstress-opticalbehaviorarepronounced.Arate-dependentcontributiontothestresswhichisnotdirectlyrelatedtotheintramolecularconformations(“stressoffset")isdetectedforbothtypesofmacromolecularfluids.Independentoftheflowhistory,duringrelaxationalinearstress-opticalbehaviorisrevealed.Nonequilibriummoleculardynamics(NEMD)computersimulationsonthemultibeadanharmonicspringmodelareshowntoprovideinsightintothemolecularmechanismsunderlyingtheviscoelasticbehavior:duringrelaxationtheintermolecularinteractionsbecomedominantincorrelationwithlinearstress-opticalbehavior;thestressoffsetshowntobeverysimilartothestressarisinginthecorrespondingsimplefluid;thetotalstresscanwellbeapproximatedbyasumofthreepartswhicharebasedonsingle-particleandsingle-linkdistributionfunctionsonly;theyieldpointbehaviorathighelongationratesreflectsthetransitionfromaffinetononaffinemotionofbondsandisunderstoodwithoutreferencetostronginhomogeneitiesresultingfromlocalplasticstrainproduction;distinctmicroscopicstresscontributionsunderelongationandsubsequentrelaxationsuchasinter-andintramolecular,attractiveandrepulsive,kineticandpotentialcontributionsareresolved.