Viscoelasticbehaviourandtime±temperaturecorrespondenceof HDPEwithvaryinglevelsofprocess-inducedorientation J.F.Mano a, * ,R.A.Sousa a,b ,R.L.Reis a ,A.M.Cunha a ,M.J.Bevis b a Department of Polymer Engineering, University of Minho, Azure Âm Campus, 4800-058 Guimara Äes, Portugal b Wolfson Centre for Materials Processing, Brunel University, Uxbridge, Middlesex UB8 3PH, UK Received12October2000;receivedinrevisedform26January2001;accepted26January2001 Abstract Shear controlled orientation in injection moulding SCORIM) is a non-conventional injection moulding technique that allows for the enhancement of the mechanical properties of semi-crystalline polymers. In this work the ¯exural mechanical feature, namely their time- dependence, of conventionally injection-moulded high-density polyethylene HDPE) PEc) and SCORIM-processed HDPE PEs) are compared. The differences should be attributed to the appearance of highly oriented structures such as shish-kebab morphologies in the lattercase.Thequasi-staticperformanceofHDPEshowsa59%increaseinthe¯exuralmodulusfollowingSCORIMapplication.Master curvesofthefrequency-dependenceofthestoragemodulusandofthetime-dependenceofstrainwereconstructedsuccessfullyintheregion ofthe a-relaxation.TheviscoelasticdataarewelldescribedbytheCole±Colemodel.Thewidebroadnessofthedistributionofrelaxation time observed for both materials is ascribed to the molecular mobility of the amorphous phase involved in the relaxation process. Two approacheswereusedtoinvestigatethecurvatureobservedintheArrheniusplots.Inthe®rstcase,twoseparateprocesseswereassumedto contributetothe a-relaxation,withactivationenergiesof150±170and180±210kJmol 21 ,respectivelydynamicdata).However,thetwo a- processes, aIand aII,oftenreportedintheliterature,didnotappearexplicitlyintheoriginaldata,whichshowedasinglepeakinan E 00 vs T plot.Therefore,auniqueprocessforthe a-relaxationwasalsoassumedandwaswelldescribedtakingintoaccountchangesinthereduced volume whose temperature-dependence obeys a general equation of state. This concept enabled a good agreement between creep and dynamicexperiments. q 2001ElsevierScienceLtd.Allrightsreserved. Keywords:Polyethylene;Orientation;Relaxation 1. Introduction Thequestforpolymericmaterialswithhighmechanical performance has resulted in several complementary approaches. The synthesis of new materials and the chemicalforexample,grafting,branchingorcross-linking) and/or physical for example, blending, compounding or annealing) modi®cation of existing polymeric materials are used for the development of polymers with enhanced mechanical properties. Other attempts rely on structure manipulation during melt processing. In fact, the control ofthethermomechanicalconditionsimposeduponthepoly- mer provides for considerable enhancement of the aniso- tropy of the polymer with positive cost and engineering implications.Thesolidstateengineeringprocessessuchas tensiledrawing[1,2],diedrawing[3]andhydrostaticextru- sion [4,5] of polyethylene PE) are examples where the application of this principle to a semi-crystalline thermo- plasticmaterialishighlyeffective. Itisalsopossibletoconferastronganisotropicbehaviour toinjection-mouldedPEbymeansofprocessingtechniques such as high-pressure injection moulding [6] and shear- controlled orientation in injection moulding SCORIM) [7±11]. The former technology is based on the combined application of very high pressures and special mould designs [6]. By contrast, the SCORIM technology relies on the application of shear stress ®elds to the melt/solid interfaces during the packing stage by means of hydrauli- cally actuated pistons [9]. After the ®lling of the cavity mould, the molten polymer is continuously sheared as the solidi®cation progressively occurs from the mould wall to themouldingcorepart.AsaresultofSCORIMprocessing, a highly sheared crystalline phase of PE can be obtained, exhibitingasigni®cantimprovementofmechanicalperfor- mance[7,8].Themicrostructuretextureandthemolecular orientationleveldevelopedunderprocessingalsodetermine thetime-responseforagivenmechanicalexcitation.Infact, Polymer422001)6187±6198 0032-3861/01/$-seefrontmatter q 2001ElsevierScienceLtd.Allrightsreserved. PII:S0032-386101)00090-8 www.elsevier.nl/locate/polymer * Correspondingauthor.Tel.: 1351-5360-4451;fax: 1351-2535-10249. E-mail address: jmano@dep.uminho.ptJ.F.Mano).