Conclusions Macroscopic Models and Molecular Structure

Other chapters in this book are devoted to the texture of oriented polymers, and to a detailed discussion of experimental studies of low strain anisotropy (Chapters 3 and 9 respectively). 

However, at present it has not proved possible to construct a quantitative model for such complex structures, and generalised models which do not take account of fine structure at the molecular level are needed as working approximations. Of such models the Ward aggregate model and the Takayanagi two phase model have been most widely used. 

The aggregate model takes no account of the way in which crystalline and amorphous material are interrelated, but assumes that this relation does not change substantially as a result of stretching. Where the theory gives a reasonable fit to experimental data it is strongly suggestive that orientation does occur principally in the simple manner proposed. If the theory is inappropriate then light may be shed on the alternative mechanisms that may occur. 

Investigation of cold drawn specimens of low density polyethylene by low angle X-ray scattering indicates that there is little clearly defined lamellar structure in this material, in contrast with the strongly lamellar 

Structure deduced from the distinct four point pattern observed in samples which were annealed after drawing. For cold drawn polymers the assumption of a single phase structure implicit in the aggregate theory may therefore be valid. Some of the materials to which the aggregate theory has been applied were hot stretched (i.e. drawn above the glass transition temperature), but the annealing in these cases would be far from complete, since the temperature was maintained for a relatively short time, and the filaments were always under tension. Thus again a distinct two phase structure may not have been well developed. 

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