Crystallinity viscoelastic behaviour

Crystalline and amorphous phases glass transition temperatures, crystalline melting points levels of crystallinity viscoelastic behaviour... 

A preliminary stndy on the viscoelastic behaviour of polyolefin foam sheets with different chemical (PE and PP) and cellular structure by DMA, in the low freqnency and low compression ranges, is presented. DSC and SEM are also used to determine the morphological parameters of the samples. A connection between the morphological properties (apparent degree of crystallinity), type of cellular structure, homogeneity, cell size and shape, cell wall thickness) and the viscoelastic behavionr, a basic key for the development of mechanical and insnlating applications, has been established. 9 refs. 

The possibility for the existence of mesophase in a rubbery state 36,46), typical only for macromolecular compounds with their natural ability to display big reversible deformations, reveals interesting prospects from the viewpoint of creation of new types of liquid-crystalline materials in the form of elastic films, as well as for development of the theory of viscoelastic behaviour of such unusual elastomers. 

Non-covalent interactions can result in the formation of supramolecular polymers even from small organic molecules. Typically multiple, strong directional interactions are involved and interesting materials with liquid crystalline, viscoelastic or gel-type behaviour are observed. 

The purpose of this paper is to explore various aspects of the rheological behaviour of lyotropic liquid crystalline systems. Lyotropics are often used as model systems for thermotropics because their viscoelastic behaviour seems to be quite similar (1) and solutions are much more easier to handle and can be studied more accurately than melts. The emphasis is on transient data as these are essential for verifying viscoelastic models but are hardly available in the literature. Transient experiments can also provide insight in the development of flow—induced orientation and structure. The reported experiments include relaxation of the shear stress and evolution of... 

Takayanagi. M.. Viscoelastic behaviour of crystalline polymers. Prc. 4th Inti Congress Rheology 1. 161 187 (1965). 

The models have been developed mainly for semi-crystalline polymers, which in general show the largest mechanical anisotropy, but some of the discussion is equally relevant to oriented non-crystalline polymers. Although an oriented polymer is strictly a non-linear viscoelastic solid (see Chapters 10 and 11) the present discussion is restricted to theoretical models which represent linear elastic or linear viscoelastic 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. 

Guezala et al. [420] studied the viscoelastic behaviour of solutions of the thermotropic main chain liquid crystalline X7G (see Fig. 160), Le. a copolyester (M 20 kg/mol) of poly(hydroxy benzoic acid) (60%) and poly(ethylene tere-phthalate) (40%) in m-cresoi the material used is non-random with respect to chain structure and domains of poly(hydroxy benzoic acid) have been reported. Solutions were studied, varying in concentration from 2.5 to 45 wt%. Special care was taken to avoid water absorption, because in its presence degradation and/or trans terification might occur. 

An introduction to the extensive experimental studies of linear viscoelastic behaviour in pol3miers falls conveniently into three parts, in which amorphous polymers, crystalline pol3miers and temperature dependence are discussed in turn. 

Viscoelasticity is one of the important mechanical properties of blended materials. Viscoelastic behaviour is the intermediate character between liquid and solid states that combines the viscous and elastic responses under mechanical stress. When a force is applied to blended materials, they can flow in the same as being liquids. The natural rubber blended materials do not stretch, but they will only gradually return to their original shapes when the force is released. This property depends on temperature, pressure, time, chemical composition, molecular weight, distribution, branching, crystallinity, and the composite of blending conditions and systems. ... 

Although the viscoelastic behaviour of semi-crystalline polymers gives some indication of the four characteristic regions that can be identified for amorphous polymers, they are much less clearly defined, as is illustrated in Figure 7.5, which shows data for... 

Two principal approaches have been used to model the yield behaviour of polymers. The first approach addresses the temperature and strain-rate dependence of the yield stress in terms of the Eyring equation for thermally activated processes [39]. This approach has been applied to many amorphous and crystalline polymers (see Section 12.5.1) and links have been established with molecular relaxation processes determined by dynamic mechanical and dielectric measurements and with non-linear viscoelastic behaviour determined by creep and stress relaxation. The Eyring approach assumes that the yield process is velocity controlled, i.e. the yield process relates to existing thermally activated processes that are accelerated by the application of the yield stress to the point where the rate of plastic deformation reaches the applied macroscopic strain rate. This approach has... 

Boltzmann and his students showed that the superposition principle was valid for inorganic glasses. Leaderman carried out experiments to check its validity for oriented fibres such as rayon, silk and nylon 6,6. He found marked deviations from the Boltzmann principle under many conditions. He correctly attributed this behaviour to the fact that these crystalline substances when subjected to a load often undergo further orientation and crystallization and at the end of the loading experiment they are structurally different from the starting material. The Boltzmann principle can only be expected to apply to such materials at extremely small loads, and temperatures at which further crystallization does not occur. On the other hand, it has been found that many polymeric materials, particularly non-crystalline systems, do show linear viscoelastic behaviour at small strains. 

Figure 4. Five typical regions of viscoelastic behaviour of a synthetic partially crystalline polymer and stand for glass transition and melting temperature, respectively (modified from [27])...

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 semicrystalline, we shall use these simpler materials to illustrate some general characteristics of relaxation behaviour. 

The values of ri and tj" for HA as a function of shear stress are shown in Figures 4.9 and 4.10. The biopolymer studied shows the behaviour typical for viscoelastic materials with the crystalline order. Viscoelasticity of HA disappears at higher shear stresses. Viscosity r and r " significantly decreases at higher shear stresses [25] (Fignre 4.10). 

Singh S (2000) Phase transitions in liquid crystals. Phys Rep 324 107-269 Susanne EM, Gleissle W, Mckinley GH, Buggisch H (2002) The normal stress behaviour of suspensions with viscoelastic matrix fluids. Rheol Acta 41 61-67 Tai-Shung C (1986) The recent developments of thermotropic liquid crystalline polymers. Polym EngSci26(13) 901-919... 

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