Optical, mechanical, and photoelastic anisotropy of biaxially stretched polyethylene terephthalate films studied using transmission ellipsometer equipped with strain camera and stress gauge

Abstract

The anisotropic properties of polyethylene terephthalate film resulting from its manufacturing process are quantitatively investigated in terms of its optical, mechanical, and photoelastic aspects. Transmission ellipsometers and a Jones-matrix-based analysis software together with a 4 × 4 Berreman-matrix-based analysis software are adopted to determine the wavelengthdependent in-plane birefringence, the principal refractive indices, and the orientation of the optical axis. Mechanical anisotropy is characterized in terms of the elastic compliance tensor components using the measured azimuthal angle-dependent Young's modulus and Poisson's ratio. From the measured variation of the wavelength-dependent in-plane birefringence as a function of tensile stress, the dispersive photoelastic coefficients are obtained for a few sample azimuthal angles, and the components of the photoelastic tensor are determined.

Laser-Inscribed Stress-Induced Birefringence of Sapphire

“…Controlling the width of t1, its period t1+t2 and depth d (along the light propagation) are essential parameters to an engineer’s optical elements for polarization control. Stress-induced birefringence is a well-used phenomenon to create artificial birefringent materials and used as a method for material inspection and characterization [19,20]. Generally, the inverse permittivity tensor is Δ(1/εij)=Pijkl∂kul, where Pijkl is the fourth-rank photoelasticity tensor, ∂kul is the gradient of the displacement from equilibrium with ul being the linear displacement from equilibrium and ∂k denotes differentiation with respect to the Cartesian coordinates.…”

Optical Anisotropy of Polyethylene Terephthalate Films Characterized by Spectral Means

“…The polymeric chains mostly lie in-plane during the fabricating process [ 34 ] and a supplementary applied uniaxial tension has a slight impact on the structure of the molecular chains situated in the out-of-plane direction [ 29 ]. In this way, the out-of-plane optical parameters are supposed to remain unchanged and only the in-plane optical properties modifications along the stretching direction are considered [ 29 , 30 ].…”

“…According to this, and due to the very close values of the two extraordinary refractive indices [ 35 ], it can be considered that, from an optical point of view, PET films act as uniaxial materials, and there is a single direction governing the optical anisotropy. This direction is called the optical axis of the material, and it coincides with the main axis of stress [ 34 ].…”

“…Birefringence can be measured using various techniques: optical microscopy using cross-polarized light, interference techniques—measurement of the phase difference between two polarized components of light that pass through a birefringent material, ellipsometry [ 29 , 30 , 34 , 35 , 36 , 37 , 38 , 39 ]. The latter allows for measuring the change in polarization of light as it passes through a material.…”

Effect of Chain Orientation on Coupling of Optical and Mechanical Anisotropies of Polymer Films

“…This polarizability dictates how light propagates through the material and, in turn, affects the refractive indices. Aromatic backbone polymers have a difference in their in-and out-of-plane polarizability, and aligning these molecular units (e.g., by stretching) creates anisotropy or directional-dependent properties of the bulk film [14]. Birefringence is the difference of the refractive indices in two or more mutually orthogonal directions and is typically observed in optically anisotropic materials.…”