Uni‐ and biaxial orientation of polymer films and sheets

Abstract

Abstract Uni‐ and biaxial stretching of various polymer films has been studied under well‐defined conditions of temperature and elongational strain rate in order to determine the relationship between stress and recoverable strain for both modes of deformation. The extent of molecular orientation has been investigated with the aid of stress optical methods: In amorphous polymers birefringence was found to be directly proportional to the frozen‐in internal entropic stress irrespective of the latter's relationship with recoverable strain. In a first approximation, molecular orientation can be understood in terms of deformation of a rubberlike network with temporary junction points. The total internal stress in an oriented glassy polymer may be significantly larger than the entropic stress. Experimental methods based on retractive force measurements, able to distinguish between internal stresses of different nature, are described. In uni‐ and biaxially drawn films of polyethylene terephtalate preferred planar orientation of (100) planes has been studied by means of X‐ray diffraction and measurement of the three principal refractive indices. The intrinsic birefringence of completely oriented PET films has been estimated from these measurements with the aid of the Lorentz‐Lorenz theory. The effect of draw ratio and temperature on the rate of crystallization and its consequences (increase of stretching stress and density, decrease of shrinkage) has also been studied. Finally, the effect of molecular orientation on various mechanical properties: modulus, tensile strength, impact resistance, creep compliance, is discussed for both amorphous and semicrystalline polymers, with special emphasis on the predominant influence of amorphous phase orientation.