Density of drawn polyethylene

An estimation of the changes in the level of chain defects (branches) accommodated within crystals of undrawn and drawn branched polyethylene, subsequently annealed, is carried out in the light of unit cell expansion data. The results reveal that annealing affects the estimated crystalline defect concentration differently depending on whether the material was drawn or undrawn. Thus, annealing of drawn polyethylene with a defect concentration of 3 % results in a healing of crystal defects, due to a preferential rejection of branches from the crystals. In contrast, annealing of melt crystallized branched polyethylene does not influence the average concentration of chain defects within the crystal despite the increase in lamellar thickness. The branches are trapped into the much wider lamellar crystals with fewer grain boundaries available and defect sequences cannot diffuse out of the crystals.

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“…On the other hand, the increase in V with increasing draw ratio substantiates earlier results of Glenz et al. [25].…”

Section: Resultssupporting

confidence: 92%

Changes in defect concentration within polymer crystals: annealing of isotropic and drawn polyethylene

Calleja

Rueda

Cabarcos

1986

Colloid & Polymer Sci

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“…On the overall, except minor strain-induced crystallization facts reported in some papers (e.g. Wade Adams et al [27] for PE), most authors (including us) have demonstrated that stretching semi-crystalline polymers globally leads to a decrease of crystallinity [28,29]. Active fragmentation of the crystallites occurs while the fibrillar microstructure is progressively formed and crystallized chains are readily transferred into amorphous clusters during this destruction process.…”

Section: Microstructural Compaction Mechanismsmentioning

confidence: 99%

Volume Variation Process of High-Density Polyethylene During Tensile and Creep Tests

Addiego¹,

Dahoun²,

G’Sell³

et al. 2006

Oil & Gas Science and Technology - Rev. IFP

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Résumé -Processus de la variation volumique du polyéthylène à haute densité au cours d'essais de traction et de fluage -Des éprouvettes de polyéthylène à haute densité ont été soumises à des essais de traction et des tests de fluage à l'aide d'un dispositif d'essais mécaniques à pilotage vidéo-métrique (VidéoTraction ® ). L'évolution du volume spécifique de ce polymère semi-cristallin est caracté-risée en fonction de la déformation vraie. Dans le stade élastique, on mesure une expansion hydrostatique puis, dans le stade plastique, on note une compétition entre un effet de compaction et un phénomène de dilatation. La compaction, sans doute surestimée dans les expériences, est toutefois un mécanisme important dû à l'orientation des chaînes amorphes au cours de l'étirage. Ce phénomène est attesté par des mesures de diffraction des rayons X qui montrent une réduction de la distance moyenne entre les chaînes amorphes. Le processus de dilatation s'explique par la diminution du taux de cristallinité et par la formation, la croissance et la coalescence de craquelures au sein et entre les sphérolites. L'observation d'échan-tillons déformés par microscopie électronique révèle ces défauts. La compétition compaction/dilatation est dictée par la mobilité de la phase amorphe influencée elle-même par la température et le temps. Abstract -Volume Variation Process of High-Density Polyethylene During Tensile and Creep Tests -Samples of high-density polyethylene have been subjected to tensile tests and creep experiments by means of a video-controlled testing system (VidéoTraction © ). The evolution of specific volume in this semi-crystalline polymer is determined versus true strain. In the elastic stage, we measure a hydrostatic expansion and then, in the plastic stage, we observe a competition between a compaction effect and a dilatation phenomenon. Although compaction is probably overestimated in the present testing technique, it represents a pertinent mechanism that is ascribed to the orientation of the amorphous chains during stretching. This phenomenon is characterized by X-ray diffraction measurements that show a reduction of average distance between amorphous chains. Dilatation process is explained by the diminution of crystallinity and by the formation, growth and coalescence of crazes inside and between spherulites.Electron microscopy reveals these defects. The competition between compaction and dilatation, controlled by the mobility of the amorphous phase, depends on temperature and time.

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“…and the mechanical data by Meinel and Peterlin. 6 The density of the drawn samples of HDPE was studied among others by Fisher et al7t8 and Glenz et al 9 The results obtained do not differ substantially from those obtained in conventional HLDPE. The elongation strain before the neck formation is 100% corresponding to X = 2.…”

mentioning

confidence: 90%

Mechanical and transport properties of drawn low‐pressure low‐density polyethylene

Candia¹,

Perullo²,

Vittoria³

et al. 1983

J of Applied Polymer Sci

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The new low density polyethylene (LDPE) obtained under low pressure in the reactor, the so-called low pressure low density polyethylene (LLDPE), with a great many short branches seems to replace the conventional LDPE with long branches obtained in the reactor under high pressure (high pressure LDPE = HLDPE). It has a higher tensile strength, impact strength, elastic modulus, elongation at break, and resistance to heat and stress cracking. The replacement goes on in film extrusion, blow molding, and wire and cable coating. The material seems also to be a good mixing component in polymer blends. ' The most obvious physical differences between the short and long branch LDPE are the higher crystallinity and the narrower molecular weight distribution, higher melting temperature (122O instead of 108"), and a narrower melting range in the former than in the latter case.The thicker and more perfect crystals are the main cause of these changes. The higher melting point, however, means a higher processing temperature that may be more harmful to the material and require a higher energy input.The increased technical interest in this type of material prompted us to investigate the mechanical and transport properties of Dowlex 2045, a Dow Chemical made LLDPE with a density 0.9139 g/cm3 that corresponds to a, = 0.414. This is less than the density 0.9152 g/cm3 of the formerly investigated conventional HLDPE with a, = 0.423. The ratio 10-35 of side chain ends CH3 per 1000 CH2 on the backbone and side chains of the molecule indicates a large number of side chains. According to the producer the side chains are much shorter than in the conventional HLDPE. (Certain commercial material is identified in this paper in order to adequately specify the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material identified is necessarily the best available for the purpose.)The enormous elastic modulus of the crystals as compared with that of the amorphous component makes the mechanical properties of the semicrystalline polymer depend primarily on the fraction of taut tie molecules crossing the amorphous layers and on the mechanical properties of these layers. Although they are in the rubbery state, the amorphous molecules are so intimately bound to the crystals that the properties of the amorphous component substantially deviate from those of a completely amorphous rubber. In particular the specific volume of the amorphous component changes upon straining. Instead of the shear modulus G one must introduce the about 1000 times higher expansion modulus K of the rubber.The best method for the investigation of the amorphous component is the measurement of the transport properties. In first approximation they depend on the fractional free volume. It increases with the deformation if the specific volume does the same. The plastic deformation of drawing. however, reduces quite substantially the specific volume and hence the transport pr...

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Density of drawn polyethylene

Cited by 71 publications

References 15 publications

Changes in defect concentration within polymer crystals: annealing of isotropic and drawn polyethylene

Changes in defect concentration within polymer crystals: annealing of isotropic and drawn polyethylene

Volume Variation Process of High-Density Polyethylene During Tensile and Creep Tests

Mechanical and transport properties of drawn low‐pressure low‐density polyethylene

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