Relaxation processes in polyethylene

We must conclude that a, p and y relaxations occur in all forms of polyethylene. The first stage in the process of analysis is to determine whether a given relaxation is related to the crystalline or the amorphous component, or to an interaction involving both phases. Next, one deduces a process that is able to account for each relaxation. Finally, an attempt must be made to model a molecular mechanism that can cause the process to occur. 

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 

Sample P relaxation activation energy (kj/mol) a relaxation activation energy (kJ/mol) 

In the mechanical situation, the translational component of the crystal process can lead to reorganisation of the crystal surface and hence modify the connections of amorphous 

The p relaxation is very broad compared with that in completely amorphous polymers due to the immobilising effect of the crystals on the amorphous fraction. Boyd speculates that the shortest relaxation times may be associated with motions of very loose folds and relatively non-extended tie chains conversely tight folds are unable to relax. The relative prominence of the p relaxation in LDPE compared with HDPE is enhanced by the lower value of the relaxed p process modulus in LDPE, which will increase the relative intensity of the p and decrease that of the a. On a molecular basis, the branching of LDPE gives a more loosely organised amorphous component, capable of relaxing to a lower limiting rabbery modulus. 

The identification of the a process as a c-shear relaxation and the process as interlamellar shear in a drawn and annealed LDPE sheet was nicely confirmed by measurements of the anisotropy of dielectric relaxation [27], Pure polyethylene 

The /3 relaxation is very broad compared with that in completely amorphous polymers due to the immobilizing effect of the crystals on the amorphous fraction. 

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

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

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. 

Several years ago, Kakizaki and Hideshima °> reported the influence of diluent like tetrachloroethylene on the transition temperatures associated with relaxation processes in polyethylene. They showed that the temperature associated with p-relaxation shifts downward in the presence of the diluent and that the transition temperature for y-relaxation shifts upward. These phenomena were interpreted in terms of an increase of the relative content of the amorphous part for the p-relaxation process and some freezing effect for y-relaxation. 

Maxwell, A.S., Monnerie, L. and Ward, I.M. (1998) Secondary relaxation processes in polyethylene terephthalate-additive blends 2. Dynamic mechanical and dielectric investigations. Polymer, 39, 6851. 

Figure 21 Activation energy plots for various relaxation processes in 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. 

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. 

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. ... 

Some polymers, notably low-density polyethylene, show clearly resolved a, P and y processes. The high-temperature a relaxation is frequently related to the proportion of crystalline material present, the fi process is related to a greatly broadened glass-rubber relaxation and the y relaxation has been associated, at least in part, with the amorphous phase. Other materials an example to be discussed shortly being that of PET - show only two relaxation processes. In these cases the a relaxation is akin to the process in polymers where all three relaxations are evident. 

Therefore, if distorted region is assumed to be a sphere with similar dimensions as the lattice constants of polyethylene crystals, the relaxation time t can be estimated from the value of D determined from the data of the decay reaction. The rdaxation time is considered to be a time constant of the molecular motion causing a slight distortion associated with the diffusion of the free radical. Thus, the relaxation time of the molecular motion associated with the decay reaction can be estimated. In order to validate this procedure, the diffusion constant was estimated from the known relaxation time obtained in dynamic mechanical studies of polyethylene within the temperature region of the so-called relaxation process in a crystalline phase and... 

How can these different facts be reconciled and brought together in one common picture The answer is The a-process in polyethylene has a composite nature. The mechanical relaxation indeed originates firom an additional shear of the amorphous regions. The prerequisite for this shear, however, is a chain transport through the crystallites. By this intracrystalline motion the... 

Figure 7.50 Dielectric ot relaxation process in linear polyethylene. The smooth twist consisting of 12 carbon atoms out of register propagates through the crystal in the direction of the arrow. Drawn after Boyd (1985b).

Other low temperature relaxation processes are also detected in polymers (142,144,158-162). The interpretation of these transitions is, however, highly ambiguous, as regard to the kinetic units of motion involved in each relaxation process. For instance, for polyethylene (PE) the observed relaxation transitions in 150 to 170 K (S- or /-transitions) is most frequently attributed to a local rotation event within about two monomer units in the vicinity of the defects in crystallites, or in amorphous region, while the relaxation processes in the temperature... 

Figure 28 Schematic diagram showing a, a, p and y relaxation processes in low-density polyethylene (LDPE) and high-...

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