The Relation between Stress, Strain and Birefringence in Some High Polymers

Simultaneous measurements of the birefringence, stress and strain in crystalline and amorphous Terylene have been made at different temperatures. The results are compared with those obtained for polyethylene and Perspex where the birefringence is a single-valued function of strain. It is found that, for both the crystalline and amorphous forms, the birefringence is a function of both stress and strain and, for small strains, can be represented by an equation of the form Δn = Aσ + B, where Δn is the birefringence, σ the stress, the strain and A and B are constants. The behaviour of these polymers is discussed in terms of this equation.