Amorphous polymers mechanical anisotropy

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. 

The heterogeneity of the crystalline polymer solid is accentuated still more in the case of mechanical properties by the enormous mechanical anisotropy of the crystals and the large difference in the elastic moduli of the crystalline and amorphous components. With polyethylene, the elastic modulus of the crystals is 3452 or 2403 X 1010 dynes/cm2 in the chain direction (E ) and 4 X 1010 dynes/cm2 in the lateral direction (E ) (2, 3). The elastic modulus of the amorphous component (Ea) of polyethylene is 109-1010 dynes/cm2 (4). This is significantly less than Eu and Ebut at least 10 times the elastic modulus of a rubber that has about five monomers in the chain segments between the crosslinks. This is quite surprising since room temperature is far above the glass transition temperature of polyethylene (Tg is either —20°C or — 120°C), and therefore one would expect a fully developed rubbery... 

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. 

We shall begin with a brief and simplified discussion of the main features of experimental observations and proceed to consider the interpretation of these features. Three polymers are selected as paradigms PET, which can exist in the wholly amorphous state but also as a partially crystalline polymer polyethylene, which is a high crystalline polymer and a liquid crystalline polymer, the thermotropic copolyester whose mechanical anisotropy was discussed in Section 7.5.4 above. 

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]. 

Drawing causes molecular alignment so that the drawing stress (often called the flow stress) is increased. This is a general phenomenon, true for both crystalline and amorphous polymers. (Note that the theories of mechanical anisotropy developed in Sections 8.6 and 8.7 apply to the final drawn material and do not relate directly to the strainhardening effect.)... 

At a nanoscale, semicrystalline polymers can be seen as lamellar crystals, which are embedded by amorphous layers. This morphology imparts locally a high mechanical anisotropy, especially when the amorphous phase is in the rubbery state, like in polyolefins. 

The LSCEs represent a new class of macromolecular systems distinguished by macroscopically uniform anisotropy. The concept of LSCE can be furthermore extended to other synthesis routes and to the densely crosslinked systems. Macroscopic uniaxially oriented films can be formed by mechanical force, alignment surface [30], magnetic and electric fields [31], polarized light [32], etc., and then crosslinked to form an anisotropic network (LC network is abbreviated as LCN) if the mixture components contain polymerizable or crosslink-able bifunctional monomers (Figure 9.13). Alternatively, amorphous or liquid crystalline side-group and/or main-chain polymers incorporating additional... 

One recent work illustrates a good example of an orientational polymer study [21]. The molecular mechanisms involved in the residual stress in relation to shape memory effects in glassy amorphous starch has been investigated using a combination of synchrotron-based Wide Angle X-ray Scattering (WAXS) and polarized SR infrared micro-spectroscopy. The aim of this study was to analyse and better understand the structural anisotropy... 

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