GENERATION AND RELAXATION OF THE MOLECULAR-ORIENTATION DURING HOT-STRETCHING OF AMORPHOUS THERMOPLASTICS

Between the mechanical properties and the molecular orientation of hot-stretched amorphous polymers there exist definite and univalent relationships which are independent of the stretching conditions. It, therefore, seemed of great interest to find out whether the orientation and thereby the mechanical properties of such materials can be predicted from their thermomechanical stretching history. In order to check this conjecture stress relaxation, tensile, creep, creep recovery, and free shrinkage tests were performed on polystyrene at temperatures between 105 and 140 °C. During these experiments the stress (or the strain) and the birefringence (as a measure of the orientation) were measured simultaneously. If polystyrene above Tg would behave like an ideal rubber the true stress σ (force per actual cross section) and the birefringence Δn should increase proportional to (λ2 -λ-1) (λ = draw ratio). Furthermore the stress optical coefficient Δn/σ should be strain-independent. In reality, the measured σ- and Δn-values increase more slowly than they would do in accordance with the rubber theory. In addition, both quantities are not only strain- but also time-dependent. They relax already during the stretching process. Nevertheless, the stress optical coefficient remains constant (i. e. strain- and time-independent as well) with the exception of the values obtained at temperatures below 120 °C. At these low temperatures the stress contains a considerable energy elastic part and consequently the quotient of Δn and σ is reduced. Furthermore it turned out that σ and Δn as functions of the Cauchy strain (λ - 1) formally follow the theory of linear viscoelasticity over a strain range up to about 600%. That means, if σ and Δn are presented in isochronous plots, they prove to be linear functions of (λ - 1) in this range. According to additional experiments carried out on some other amorphous polymers the mentioned findings turned out to be true not only for polystyrene but also for high impact polystyrene, PVC, and PMMA over a strain range the upper limit of which is to be found between 100 and about 600%. The formally linear viscoelastic behaviour of these polymers above Tg leads to the following consequences:- In principle it is possible to use the whole well-known formalism of the theory of linear viscoelasticity to determine the orientation from an arbitrary stretching history of the tested amorphous polymers. - In addition, the possibility is given to convert the effects of a certain strain history (e. g. creep etc., into effects of another history by using, for instance) the approximative formula given by Schwarzl. - Since it is possible to predict the orientation of hotstretched amorphous polymers, the mechanical properties can be predicted, too, owing to the univalent relationships existing between the orientation and the mechanical properties. © 1979 Dr. Dietrich Steinkopff Verlag.