High-density polyethylene relaxation

Fig. 2.7. Drop off of doubled extinction angle 2"/ during stress-relaxation after cessation of steady shear flow according to Wales (59). Measurements on the melt of a high-density polyethylene (Marlex 6002) at a measurement temperature of 147° C. Shear rate of the steady shear flow q = 0.06 sec-1...

For example, Figs. 2.43 and 2.44 present the measured [55] viscosity and first normal stress difference data, respectively, for three blow molding grade high density polyethylenes along with a fit obtained from the Papanastasiou-Scriven-Macosko [59] form of the K-BKZ equation. A memory function with a relaxation spectrum of 8 relaxation times was used. 

Relaxation time distribution breadth 2,4-dimethylpentad ienyl environmental stress crack resistance high-density polyethylene high-load melt index Janzen-Colby... 

Here A is a time constant and is often interpreted as the relaxation time of the polymer, particularly in the filled composites. It significantly increases with wood flour content in filled high density polyethylene (HDPE) [4], The authors suggested that it is a result from the increased relaxation time of the reduced amount of polymer in the filled composite. 

Obtain DSC thermal curves of several semicrystaUine polymers such as polymethylmethacrylate (PMMA), polystyrene, polycarbonate, high-density polyethylene, low-density polyethylene and look for the glass transition in these polymers. The DSC run may need to be repeated twice with rapid cooling between runs. Many as received polymers will show a small peak on top of the glass transition on the first mn due to relaxation effects in the polymer. The second run should not show this peak , but only a step change in the baseline. Compare your values of Tg to literature values. Deviations may indicate the presence of plasticizers or other additives in the polymer. 

In parallel research to that ofTakayanagi, McCrum and Morris " studied the a and a relaxations in high density polyethylene. They proposed that the a relaxation should be attributed to slip at the boundaries of the lamellae, and put forward a model similar to that of Iwayanagi" in which elastic lamellae are separated by a viscous liquid. To ensure recoverability, the lamellae are pinned at points along their length. The composite solid then shows linear viscodastic behaviour, with a characteristic relaxation which depends on structural parameters. 

The cross-over point in high density polyethylene occurs above the a-relaxation, which it is proposed is an inter-lamellar shear relaxation. This highlights a major difference between high density polyethylene and low density polyethylene, where the a-relaxation is attributed to the c-shear process which gives rise to the anomalous mechanical anisotropy. 

The original analysis of the relaxation mechanism by Stachurski and Ward was qualitative, and based on simplifying assumptions such as all lamellae at 45° to the draw direction. More recently Davies et have presented a quantitative theory for mechanical isotropy of cylindrically symmetric specimens based on an inter-lamellar shear mechanism. As will be discussed later the a relaxation in high density polyethylene shows a similar anisotropy to the /) relaxation in low density polyethylene, and is therefore proposed as being a consequence of inter-lamellar shear. 

The initial published reports on high density polyethylene were dynamic mechanical studies, but before considering them it is necessary to compare the mechanical relaxations in isotropic material with those observed in unoriented low density polyethylene. From the schematic curve of tan S v. temperature f Fig. 7(b)] it can be seen that the p relaxation, which was ascribed to branch point mobility, is not present, and that the high temperature relaxation is frequently resolvable into a and a peaks. 

Although the mechanical mechanisms of the p relaxation in low density polyethylene and the a relaxation in high density polyethylene are similar, this does not imply that both relaxations are associated with the same molecular processes. There are considerable structural differences between the materials not only does Rigidex have a higher crystallinity than Alkathene, but the unordered fraction might be rather a defect region than an amorphous component These factors, plus the... 

From B. Na, Y. Wang, Q. Zhang, Q. Fu, Shish and its relaxation dependence of re-crystallization of isotactic polypropylene from an oriented melt in the blends with high-density polyethylene. Polymer 45 (18) (2004) 6245-6260. 

A TSC study of isotactic and atactic polypropylene, high density polyethylene, ethylene-propylene block copol)uners, ethylene-propylene rubbers and blends thereof [8] in the y and p relaxation range helped to identify the molecular origin of the relaxation peaks. Two 3 processes were observed in the copolymer samples, one (p,) around —5°C, the other one (Pj) at about -50°C. The former is attributed to the PP chains, while the latter is ascribed to microscopically random ethylene-propylene rubber segments. 

Various workers have discussed aspects other than those mentioned above in studies of the viscoelastic properties of polymers. These include PVOH [62], hydroxy-terminated polybutadiene [63], styrene-butadiene and neoprene-type blends [64], and polyamidoimides [65]. Other aspects of viscoelasticity that have been studied include relaxation phenomena in PP [66] and methylmethacrylate-N-methyl glutarimide copolymers [67], shear flow of high-density polyethylene [68], Tg of PMMA and its copolymers with N-substituted maleimide [69] and ethylene-vinyl acetate copolymers [70], and creep behaviour of poly(p-phenylene terephthalate) [71] and PE [72]. 

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