On the in¯uence of molecular weight and crystallisation condition on the development of defect in highly drawn polyethylene Taweechai Amornsakchai * , Ponusa Songtipya Department of Chemistry, Faculty of Science, Mahidol University, Rama 6 Road, Phyathai, Bangkok 10400, Thailand Received 5 November 2001; accepted 14 March 2002 Abstract Longitudinal morphology of highly drawn high density polyethylenes has been investigated. Three grades of HDPEs with different molecularweightswereused.Twodifferentinitialmorphologieswereobtainedbyquenchingandslowcoolingfromthemelt.Thesamples were drawn at 75 8C close to their breaking points and their longitudinal morphologies examined under SEM after etching. Two types of defectswereobserved.Thesearethestructurecontainingalargenumberofparallellongitudinaletchedpocketslaidintheregionsbetween almostparallelcontinuoustransversebandsor`Pisa'structurerecentlyreportedandtransversecracks.Theappearanceofthesedefectswas found to correlate with molecular weight and thermal history of the samples. For all quenched samples, both Pisa structure and transverse crackswereobserved.Thenumberoftransversecrackswasfoundtoincreasewithincreasingmolecularweight.Apparently,thetransverse cracks,whichthoughttolimitthedrawingtohighdrawratios,canberetardedbydrawingathighertemperature100 8C).Thissuggeststhat theformationoftransversecracksrelatestochainmobilityanddrawabilityofthematerials.ThePisastructurewasfoundtodisappearfrom slowly cooled low molecular weight samples. For slowly cooled high molecular weight sample, the Pisa structure became less salient. It is suggestedthattheformationofPisastructureisdeterminedbymolecularentanglement.AnalysisofthebandseparationofthePisastructure showsthatthereseemstobeacharacteristicvaluewhichdependsondrawingtemperature. q 2002ElsevierScienceLtd.Allrightsreserved. Keywords: Polyethylene; Deformation; Defect formation 1. Introduction Mechanicalbehaviourofmaterialsisdeterminedbytheir internal structures. Drawing of polymers, especially those with semi-crystalline nature, is known to change their internal structures signi®cantly. Mechanical properties are alsoimprovedsigni®cantlyasaresultofdrawing.Thechar- acterisation of the internal structure of the ®bre is therefore important in the understanding of their mechanical beha- viour.Thisisparticularlysoforthedependenceofstrength on ¯aws [1]. For highly oriented ®bres ¯aws of various types have been reported [2±9]. These include disc shape defectsofabout10nmindiameter,crackswiththeirmajor axis perpendicular to drawing direction with size of several micronsorevenlongitudinalsplitting.Duetotheirpseudo- brittle characteristic, the existence of ¯aws in highly drawn polyethylenes has been shown to result in the size or diameter) effect on their strengths [10±12]. Therefore, in the speci®c case of ®bre strength, the dependence on internal ¯aws and their origins is particularly important. Recently, a very distinct longitudinal morphology was observed in highly drawn polyethylenes [13,14]. This morphologyisremarkablyregularconsistingofanalternat- ing entire bands and row of etched pockets, which has been termed `Pisa' structure. Similar morphology was also observed in highly drawn polypropylene and polyoxy- methylene [9,15,16] but less distinct. The knowledge with regard to its formation is rather limited. In the previous work, this Pisa structure is formed only in a particular type of polyethylene [13]. The draw ratio at which this Pisa structure was observed also varies depending on the starting materials. It was suggested that this type of defect would be formed near the breaking point. It is this type of defect which we are concentrating in this paper. Further investigation into factor controlling the formation of this Pisa structure will be presented. 2. Experimental The materials used were commercially available HDPEs grade Thai-Zex 1600J, 2208J, and 6200B obtained from a local manufacture, Bangkok Polyethylene, Thailand. The Polymer 43 2002) 4231±4236 0032-3861/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII:S0032-386102)00208-2 www.elsevier.com/locate/polymer * Corresponding author. Tel.: 166-2-201-5163; fax: 166-2-245-8332. E-mail address: sctam@mahidol.ac.th T. Amornsakchai).