Relaxation processes polyethylene

Fig. 3 Mechanical a relaxation process in polyethylene. Drawn after Boyd [20]...

Figure 10. 5 The o-Ps lifetime (x3) and intensity (I3) for polyethylene as a function of temperature. The temperature ranges corresponding to mechanical relaxation processes (a,P and y) are shown in the figure [44].

Study of relaxation processes in polyethylene and polystyrene by positron annihilation . J. Poly. Sci. PartB Poly. Phys. 24,2145. 

In polyethylene the ac-relaxation process (see Section 3.4) enables the movement of chains into and out of the crystalline lamellae. Theoretical treatments have demonstrated that it most probably proceeds by propagation of a localized twist (180° rotation) about the chain axis extending over 12 CH2 units (Fig. 6.14). As the twist defect travels along the chain, it rotates and translates the chain by half a unit cell (i.e, by one CH2 unit) - this is termed the c-shear process (Mansfield and Boyd, 1978). The activation energy for this process is about HOkJmoF1, corresponding to the extra energy required to introduce the twist defect into the crystal. Once formed, the twist can freely... 

Values for AH 30 kcal/mol were obtained, which are in the range for the a-relaxation process in polyethylene, when studio by dynamic mechanical measurements. 

Taking into account that the volume of a unit cell of polyethylene, with four units of CH2 inside, is 0.254 x 0.736 x 0.492 = 0.1 nm, V is a volume containing nearly 160 polymer carbon atoms. The value of AE is typical of a deformation process associated with polymers. Although it cannot usually be related to any molecular relaxation process, AE near Tg may approach the activation barrier energy of the a molecular relaxation. 

In crystalline polymers, the principal relaxation process is associated with melting. In polyethylene, a (1-. and 7-transit ions have been identified and, particularly in higb-tlensity polyethylene, the a-transition has been sub-divided into a and a. In ethylene-based polymers, the y-transitions at —120°C is generally associated with the amorphous phase, in particular, with crankshaft motion of methylene sequences. " However, based upon studies of solution grown lamellae, it has also been suggested that this may llien be associated with... 

A Ithough perhaps the most extensively studied polymer, polyethylene (PE) is still the subject of controversy with respect to the nature of its thermal relaxation processes, especially the glass transition (Tg). The transitional phenomena of this polymer are the subject of an immense quantity of literature and several excellent reviewes 1, 2, 3,4). 

Polymers with a flexible chain, such as polyethylene (PE), polypropylene, poly(tetrafluorethylene), or poly(methylene oxide), exhibit relaxation processes directly related to the presence of their crystalline Action, For PE, by far the most important system in this context, such processes may be dielectrically active, provided that the sample is "decorated" widi a few C-Cl or C=0 dipoles, by chlorination or oxidation [166]. 

These may be carried out in extension, torsion or flexure, and are concerned with alternating strains, normally over a range of frequencies and temperatures. Such methods are particularly suited to the study of molecular relaxation processes and have not been applied very extensively to oriented materials, where the studies to date have been more concerned with the static arrangement of structural elements in the materials. However, Takayanagi, and Stachurski and Ward have used dynamic tensile and torsion modulus to study the anisotropy of relaxations in oriented polyethylene. 

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. 

Temperature dependence of the shear modulus G and logarithmic deaement A of linear (LPE) and branched (BPE) polyethylene. Measurements by torsion peixjulum at frequency 1 Hz. The relaxation processes are labelled a for that at the highest temperature, j3 for that at the next highest temperature, and so on. The obsen/ation of the o-process in aeep for LPE is shown in Figure 4.4 (after Flocke). 

Boyd RH (1985) Relaxation Processes in Crystalline Polymers Experimental Behavior Molecular Interpretation—A Review. Polymer 26 323-347 and 1123-1133. Wunderlich, B (1997) The Basis of Thermal Analysis. In Turi E ed. Thermal Characterization of Polymeric Materials 2" edn. Academic Press, New York, pp 387-389. Wunderlich B (1962) Motion in Polyethylene. III. The Amorphous Polymer. J Chem Phys 37 2429-2432. 

In both electric and mechanical cases, the loss tan, 6, is defined as the inelastic component normalized to the elastic component. In principle, dielectric and mechanical loss angles (6) should be the same if the relaxation processes are the same and if both electric field and mechanical load are acting at the same dipole field. At least at low temperatures and low frequencies, the relaxation processes are the same if the same modes are activated. The activation can be different. For the dielectric losses, the net dipole moments are decisive. For many polymers with a certain regularity of structure, dipole moments are cancelled to some extent, thus yielding a lower dielectric loss. These materials are known as nonpolar ones. Polyethylene and Teflon are two examples. 

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