Semi-crystalline polymers relaxation behaviour

The Takayanagi model was developed to account for the viscoelastic relaxation behaviour of two phase polymers, as recorded by dynamic mechanical testing. " It was then extended to treat both isotropic and oriented semi-crystalline polymers. The model does not deal with the development of mechanical anisotropy on drawing, but attempts to account for the viscoelastic behaviour of either an isotropic or a highly oriented polymer in terms of the response of components representing the crystalline and amorphous phases. Hopefully, comparisons between the predictions of the model and experimental results may throw light on the molecular processes occurring. 

It must be emphasised that the site model is applicable only to relaxation processes showing a constant activation energy, examples being those associated with localised motions in the crystalline regions of semi-crystalline polymers. The temperature dependence of the glass transition relaxation behaviour of polymers does not fit a constant activation energy model, and where this has appeared to be true it is probably a consequence of the limited range of experimental frequencies that were available. 

The interpretation of the relaxation behaviour of semi-crystalline polymers can be extremely difficult, because of their complex two-phase structure (Section 4.2). It is sometimes possible to consider the crystalline and amorphous regions as separate entities with the amorphous areas... 

We shall discuss the assignment of viscoelastic relaxations in a molecular sense to different chemical groups in the molecule, and in a physical sense to features such as the motion of molecules in crystalline or amorphous regions. Because amorphous polymers exhibit fewer structure-dependent features than those that are semi-crystalline, we shall use these simpler materials to illustrate some general characteristics of relaxation behaviour. 

The case of amorphous poly(ethylene terephthalate) (PET), where there are no side groups, is considered in Section 10.3.2 in conjunction with semi-crystalline forms of this polymer. Worthy of mention is the use of antiplasticisers to show that the p relaxation is composite [8,9]. In studies of bisphenol A (and tetramethyl bisphenol A), polycarbonate intramolecular cooperativity associated with the carbonate residue was shown by the introduction of the methyl groups on the phenyl rings [10], and intermolecular cooperativity was shown by the dielectric behaviour of blends [11]. Further evidence for intermolecular contributions to the ring motions was obtained from the observed pressure dependence of the proton NMR transverse relaxation time [12]. There is a key conclusion for polymers... 

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