Isotropic unoriented

Hence, the extension of an isotropic unoriented partially crystalline polymer leads to the formation of a highly organized material with a characteristic fibrillar structure. The anisotropy of the sample as a whole is expressed by a higher modulus, tenacity and optical anisotropy. It would seem that the increase in strength in the drawing direction suggests that the oriented samples consist of completely extended chains. However, while the strength of such perfect structure for polyethylene has been evaluated as 13000 MPas), the observed values for an oriented sample are 50 to 30 MPa. 

Film Isotropy Anisotropic (oriented) Isotropic (unoriented)... 

Here X ,ax is the single chain limiting extension ratio in the isotropic, unoriented polymer with the same entanglement weight. A complication is that the process of orientation above Tg may result in a loss of entanglement constraint, effectively increasing and One of the ways this loss can take place in the current versions of the tube model is by tube relaxation whereby the process of tube... 

The coefficient of linear thermal expansion (3 is another useful quantity that is commonly quoted in the literature. It simply equals one third of the coefficient of volumetric thermal expansion for an isotropic (unoriented) material ... 

Poisson s ratio v is defined by Equation 11.5 for an isotropic (unoriented) specimen. It describes the effect of the application of a deformation (strain) in one direction (i.e., along the x axis) on the dimensions of the specimen along the other two directions (i.e., the y and z axes) perpendicular to the direction of the applied deformation. The fractional change of volume dV/V of the specimen is given by Equation 11.6 in terms of the strains dex, d y and d z along... 

The preceding discussion was limited to isotropic unoriented systems in which the scattering power (such as polarizibily) was a scalar and where there was spherical symmetry. The effect of anisotropy (where, for example, polarizabilty is a tensor quantity) will be discussed laterm. 

Similar to fibers, films are often oriented. The simplest case of orientation is called uniaxial orientation, when the macromolecular segments are oriented in one preferred orientation. But a film may also be oriented in two directions (called biaxial orientation) having no fiber analog. Isotropic, unoriented film would correspond to the as-spun fiber (both having no preferred molecular orientation). Thus, DSC experiments carried out on unoriented films give results similar to those obtained from experiments performed on chips, powders, or as-spun fibers, but with minimum external thermal resistance. 

Fig. 31 Structural formation model for the initial stage of polymer crystallization [19], N G nucleation and growth of oriented domains, SD spinodal decomposition into oriented and unoriented domains, Tb, Ts, and Tx bimodal, spinodal, and crystallization temperatures, respectively I isotropic, N smectic, and C crystalline...

In general, there are three kinds of moduli Young s moduli E, shear moduli G, and bulk moduli K. The simplest of all materials are isotropic and homogeneous. The distinguishing feature about isotropic elastic materials is that their properties are the same in all directions. Unoriented amorphous polymers and annealed glasses are examples of such materials. They have only one of each of the three kinds of moduli, and since the moduli are interrelated, only two moduli are enough to describe the elastic behavior of isotropic substances. For isotropic materials... 

Similarly with low-molecular nematics is manifested in that the nematic polymers may form equally well schlieren texture, typical for low-molecular nematics (Fig. 18a) (polymers B.3.3-B.3.4, Table 9). The enthalpy of transition from LC state to isotropic melt is also close to that for low-molecular nematics. At the same time, there also exist definite structural differences. X-ray patterns of the same polymers, even in unoriented state, display certain elements of structural ordering in the arrangement of side branches (a weak diffuse halo at small angles), which could indicate a sibotactic nematic type of ordering. These differences are most distinct for oriented polymer films. As an example Fig. 18b, c, present X-ray patterns of unoriented and oriented samples of one and the same nematic polymer 121 l24. In fact two sharp small angle... 

A / is the absorbance of the sample when the light is polarized parallel to a reference axis, and Aj is the absorbance of light which is polarized perpendicular to this axis. The strength of the absorption depends on the orientation of the electric field vector of the light and the transition moment of the chromophore - parallel orientation results in maximum absorption whereas perpendicular orientation leads to zero absorption. By dividing the LD value by the absorbance of the unoriented sample under isotropic conditions (Aiso), the reduced linear dichroism (LDr), i.e. the wavelength-dependent LD, is obtained (Eq. 7) [36]. 

Flexible molecules, such as those in Fig. 2-7, permit rotational motions of one bond about another, so that a combinatorially huge number of configurations is accessible (Flory 1969). On length scales of tens or hundreds of such monomers, the details of the distribution of allowed bond angles average out, producing in the melt a configuration distribution equivalent to that of a random walk (see Fig. 2-8). Because of the flexibility of these molecules, even in the densely packed melt state, they remain unoriented, or isotropic, at equilibrium. 

When the polymer is unoriented, so that its properties are isotropic, the radial distribution function depends only on the magnitude and not on the direction of r. The intensity function is then also isotropic and depends only on the magnitude of q. Equation (4.14) can now be written (cf. Section B.5) as... 

The poorly crystaUine carbons are thought to contain unassociated or unoriented carbon atoms. The hexagonal layers are not perfectly arranged, as shown in Figure 39.2. Properties of individual crystallites seem to be highly anisotropic. However, if the crystallites are randomly dispersed, the aggregate becomes isotropic [Park and Lakes, 1992]. 

The initial published reports on high density polyethylene were dynamic mechanical studies, but before considering them it is necessary to compare the mechanical relaxations in isotropic material with those observed in unoriented low density polyethylene. From the schematic curve of tan S v. temperature f Fig. 7(b)] it can be seen that the p relaxation, which was ascribed to branch point mobility, is not present, and that the high temperature relaxation is frequently resolvable into a and a peaks. 

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