Uni‐ and biaxial orientation of polymer films and sheets

Abstract

SYNOPSISUni-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 ( 1 00) 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.

The effect of hot multistage drawing on molecular structure and optical properties of polyethylene terephthalate fibers

“…The intrinsic birefringence values for the crystalline and amorphous phases were assumed to be . The Gaylord's theoretical analysis 26 of stress-induced crystallization is in good agreement with various experimental data 27,28 . The crystalline orientation factor f c was evaluated by using the following Equation:…”

Opto-thermal properties of fibers. XVI. Macrostructural and optical properties of cold-drawn, quenched PET fibers

“…The relation between the density and the average refractive indices of drawn uniaxially stretched polyethylene (PET) was given by de Vries and his coworkers. 3 The density of the crystalline regions in a polymer is greater than that of the amorphous component; in some cases, the difference may amount to as much as 10%, so the determination of density constitutes an accurate method for determining percentage crystallinity, and density measurements can be used to follow the continuous changes during the crystallization of a sample.…”

“…with expressed in kg cm 3 . This relation is independent of the degree of crystallinity and the level of orientation.…”

“…with c ϭ 1.457 ϫ 10 Ϫ3 kg cm 3 and a ϭ 1.336 ϫ 10 Ϫ3 kg cm 313 The volume fraction of amorphous material was determined by the following relation:…”

Optothermal properties of fibers. XIV. Investigation of some optical structural parameters of annealed and cold drawn polyester fibers

“…19, 32 The densities of the drawn samples were estimated by using the formula of de Vries and coworkers 33 by the following relation:…”