The Problem of Isotropic Polymers

In any discussion on orientation, the isotropic material cannot be ignored, since it forms the starting point for the operation, which means that the properties of the oriented material must be latent or inherent in the isotropic polymer. However, it may seem illogical to end this chapter with a discussion of the isotropic polymer, rather than to begin at this point. The reason why the present pattern has been adopted is quite simple—the isotropic material represents a much more complex system, and can best be regarded as made up of the structural elements which have already been discussed. 

Although the isotropic polymer may be amorphous or semi-crystalline, it will suffice if we confine the discussion to the latter type. Since semi-crystalline polymers include an amorphous component, they may be regarded as subsuming the class of amorphous polymers as a consequence. 

For convenience, we can regard an isotropic semicrystalline polymer as being made up of an isotropic polycrystalline phase and an isotropic amorphous phase, as shown in Fig. 6, which is purely diagrammatic (it shows the classical fringed micelle model for simplicity). From a geometrical standpoint, we cannot discriminate a priori between two continuous interpenetrating phases, a dispersion of a crystalline phase in an amorphous phase, or of an amorphous phase in a crystalline phase. The distinction may depend upon the volume fractions. 

The modulus of an isotropic semi-crystalline polymer can then be described by the following general expression  

The actual modulus of a material made up of two randomly arranged phases will fall somewhere between these two limits or bounds, and it becomes necessary to estimate values for pj and ,. 

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