Polyethylene relaxation behaviour

George et al. [27] studied stress relaxation behaviour of pineapple fibre-reinforced polyethylene composites. They found stress relaxation to be decreased with an increase of fibre content due to better reinforcing effect It is also reported by George et al. [28] that properties of fibre-reinforced composites depend on many factors like fibre-matrix adhesion, volume fraction of fibre, fibre aspect ratio, fibre orientation as well as stress transfer efficiency of the interface. Luo and Netravah [29] found an increase in the mechanical properties of green composites prepared from PALFs and poly(hydroxybutyrate-co-valerate) resin (a biodegradable polymer) with the fibres in the longitudinal direction. However, the researchers reported a negative effect of the fibres on the properties in the transverse direction. 

A complete treatment of the relaxation behaviour has been given by Ferry, Stavermann and Schwarzl, and Muller. As an example, we mention the so-called y process in polyethylene. The change of location of a pair of CH2 groups gives rise to a step in the E curve and a peak in the E curve. This has been described extensively by Pechhold. ... 

The molecular weight of a polymer influences its relaxation behaviour considerably. However, these effects can be modified considerably by the crystallinity of the material. Consequently it seems convenient to consider the influence of molecular weight under the separate headings of amorphous and crystalline polymers. Two representative examples will be discussed in some detail i.e. atactic polystyrene [25] which is a typical amorphous polymer and polyethylene oxide [33, 42 ] whose crystallinity varies with molecular weight, whilst brief illustrative references will be made to a number of other materials. 

Due to its practical importance as a cable material and due to its special position as a model polymer, polyethylene has been the subject of numerous dielectric studies. In coimnon with poly(vinylidene fluoride), polyethylene shows a complicated relaxation behaviour. Since the chain is intrinsically non-dipolar the material is oxidized, dil( inated or is made by copolymerizing carbon monoxide with ethylene, so that d les can be introduced to act as a probe on the motions of the parent material. Extensive accounts of the eady eiqjerimental wodc are available (McCrum et al.,... 

The identification of the a process as a c-shear relaxation and the p process as interlamellar shear in a drawn and annealed LDPE sheet was nicely confirmed by measurements of the anisotropy of dielectric relaxation [32], Pure polyethylene shows no dielectric response, so experiments were made on specimens that had been lightly decorated with dipoles by means of oxidation, to such a small extent that the overall relaxation behaviour was not significantly affected. The dielectric relaxation data showed marked anisotropy for the relaxation, consistent with its assignment to the c-shear relaxation, but the P relaxation... 

Branched polyethylene shows a different behaviour (Fig. 7.47). At low temperatures, below the P relaxation (the glass transition of polyethylene), the behaviour is similar to that of linear polyethylene. At higher temperatures, above Tp, the modulus of the amorphous component is crystallinity-dependent. 

Figure 4 Master curve for the linear viscoelastic behaviour of entangd polymers in the terminal region of relaxation V Polystyrene, bulk (M=860000, T=190°C) Polyethylene, bulk (M=340000, T=130°C) A Polybutadiene solution (M=350000, <)) polymer=43%, T=20°C) [ om ref.4].

During the course of these and related studies, notably those concerned with the temperature dependence of the mechanical anisotropy and the identification of relaxation processes in structural terms, it became apparent that the aggregate model was successful in low density polyethylene because it described effectively the influence of the very anisotropic x-relaxation process on the mechanical behaviour. Stachurski and Ward were even able to extend the aggregate model to deal with the anisotropy of dynamic loss factor. (See Chapter 9 for further discussion.) It was, however, more in the spirit of the original conception of the aggregate model that it would deal with mechanical anisotropy in glassy polymers, where morphology was of secondary importance. 

Takayanagi s first comparison between the predictions of his model and the observed mechanical behaviour covered a wide range of crystalline polymers, including polyethylene, polyvinyl alcohol, polytetra-fluoroethylene, polyamide, polyethylene oxide, polyo. ymethylene and polypropylene. Attempts were made to define relaxation processes as associated with either the crystalline regions or the non-crystalline regions, and in the former case specific molecular mechanisms were proposed, e.cj. a local twisting mode of molecular chains around their axes and a translational mode of molecular chains along their axes. 

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. 

R.G. Matthews, I.M. Ward, and G. Capaccio, Relationship between the dynamic mechanical relaxations and the tensile deformation behaviour of polyethylene, Journal of Materials Science, 34 (12), 2781-2787,1999. 

Y.M. Boiko, W. Brostow, A.Y. Goldman, A.C. Ramamurthy, Tensile, stress relaxation and dynamic mechanical behaviour of polyethylene crystallized from highly deformed melts. Polymer, 36 (7), 1383-1392,1995. 

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]. 

---