Amorphous polymers anisotropy

Anisotropic materials have different properties in different directions (1-7). 1-Aamples include fibers, wood, oriented amorphous polymers, injection-molded specimens, fiber-filled composites, single crystals, and crystalline polymers in which the crystalline phase is not randomly oriented. Thus anisotropic materials are really much more common than isotropic ones. But if the anisotropy is small, it is often neglected with possible serious consequences. Anisoiropic materials have far more than two independent clastic moduli— generally, a minimum of five or six. The exact number of independent moduli depends on the symmetry in the system (1-7). Anisotropic materials will also have different contractions in different directions and hence a set of Poisson s ratios rather than one. 

Furthermore, it is not surprising that the thermal conductivity of melts increases with hydrostatic pressure. This effect is clearly shown in Fig. 2.3 [19]. As long as thermosets are unfilled, their thermal conductivity is very similar to amorphous thermoplastics. Anisotropy in thermoplastic polymers also plays a significant role in the thermal conductivity. Highly drawn semi-crystalline polymer samples can have a much higher thermal conductivity as a result of the orientation of the polymer chains in the direction of the draw. 

The optical properties of semicrystalline polymers are often anisotropic. On the other hand, amorphous polymers are normally isotropic unless directional stresses are frozen in a glassy specimen during fabrication by a process such as injection molding. Anisotropy can often be induced in an amorphous polymer by imposing an electric field (Kerr effect), a magnetic field (Cotton-Mouton effect), or a mechanical deformation. Such external perturbations can also increase the anisotropy of a polymer that is anisotropic even in the absence of the perturbation. 

Arruda, E. M. and Boyce, M. C. (1993) Evolution of plastic anisotropy in amorphous polymers during straining, Int. J. Plasticity, 9, 697-720. 

E. M. Arruda and M. C. Boyce, Evolution of Plastic Anisotropy in Amorphous Polymers during Finite Straining , Int. J. Plast. 9, 697—720 (1993). 

E. M. Arrudaand M. C. Boyce, EvolutionofPlastic Anisotropy in Amorphous Polymers... 

In die late nineteenth century, scientists quickly adopted flie seminal publications of the Curie brothers. Consequently, piezoelectricity and electrostriction were first discovered and investigated on inorganic, mono- or polycrystalline materials (Katzir 2006). Therefore, the theoretical treatment of tire relevant electromechanical properties has been based on the physics and in particular on the structure and the anisotropy of crystals (Newnham 2005 Tichy et al. 2010). Semicrystalline or amorphous polymers are usually less anisotropic flian crystals, and the symmetry... 

There are relatively few measurements on amorphous polymers, where the degree of mechanical anisotropy is much less than in crystalline polymers. Early studies include those of Hennig [92] on polyvinyl chloride, polymethylmethacrylate and polystyrene and Robertson and Buenker [93] on bisphenol A polycarbonate. The results are summarised in Table 8.8. Hennig s measurements on 533 and 5n were obtained from dynamic testing at... 

For amorphous polymers, Ward et al. [96] and Kausch [88] and later Rawson and Rider [95] are in agreement that the mechanical anisotropy can be discussed very satisfactorily by the aggregate model. Moreover, the development of anisotropy with draw ratio can often be described by the pseudo-affine deformation scheme [94]. 

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