Introduction

In this chapter an attempt to answer the following questions is made.

1. (i) What kind of theoretical models, or deformation schemes can be envisaged for predicting the distribution of molecular orientations in a drawn polymer?

2. (ii) How do the predictions of these models agree with experimental results for the orientation averages such as 〈P2(cos θ)〉?

3. (iii) How can experimental or theoretical values of the orientation averages be used to predict the properties of the drawn polymer and with how much success?

These questions form the topics of discussion for sections 11.2, 11.3 and 11.4, respectively. In extremely highly drawn material, a great deal of modification of the original structure of the polymer must have taken place and most of the chains are essentially parallel to the draw direction, so that 〈cos2 θ〉 and 〈P2(cos θ)〉 are both close to 1. It is difficult to describe theoretically and in detail how molecular orientation takes place under these circumstances and thus to predict the small departures from unity for such materials. Discussion of the prediction and use of orientation averages is therefore limited to materials of moderate draw ratios. Models used for interpreting the properties of highly oriented polymers are discussed in sections 11.5 and 11.6.