Conoscopic Measurement of Birefringence and Orientation in Biaxially Stretched Polymer Films and Sheets

Abstract

The technique for using conoscopy to determine the optical orientation and birefringences of stretched polymer films is presented here in detail. The method is nondestructive and has the capacity for implementation both off-line and online during processing. Analysis was performed on films of polyethylene naphthalate (PEN) and poly(ethylene terephthalate) (PET), with birefringence results compared to measurements using a Berek compensator. The technique is highly effective at measuring the orientation and in-plane principal birefringence, though an accurate measure for a film's out-plane birefringence requires knowledge of at least an approximate value for the material's average refractive index.

Fisheye lens conoscopy

“…For analysis of conoscopic images it proves convenient to represent I(θ,ψ = 90°) as an analytic function, for example to scale theoretical interference figures. [11] To this end, we generated a heuristic expression that describes N(θ,ψ = 90°), also indicated in Figure 10e. This function, of the form N illum (θ,ψ = 90°) = C*cos[Dθ] v , is plotted for its best fit parameters in Figure 10f (white curve), with C = 238, D = π/146, and v = 0.11.…”

“…Thus, orientations of principal axes and relative values of components of the optical dielectric tensor of samples may be determined from their conoscopic interference patterns. [11] Microscope-based conoscopy typically illuminates a small sample area, in a range between the microscope resolution and the objective aperture size. Precision mapping of a wide range of incidence angles onto an image plane typically requires use of a high quality, high-numerical aperture (NA) microscope objective and condenser.…”

Structural Evolution of Two-Phase Blends of Polycarbonate and PMMA by Simultaneous Biaxial Stretching

“…Biaxially stretched polymer films are often used as packaging materials for food and industrial products, because the mechanical and gas barrier properties can be improved using biaxial stretching in the manufacturing process [1]. Many researchers have investigated how the structures of polymer films obtained by biaxial stretching vary in a number of aspects [2,3] for crystalline polymers [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27], polymer blends [28,29,30,31,32,33,34] and composites [35,36,37]. The dimensional stability is enhanced due to the increase in the degree of the stress-induced crystallinity [4], surface morphology is changed [5,6], hardness is enhanced [7], and high toughness polylactide film can be obtained by development of highly-oriented small crystallites [8] in the crystalline polymers.…”

Structure Development in Uniaxial and Biaxial Film Stretching

“…It also should be noted that crystallization behavior can be accelerated through the development of molecular orientation, while the state of molecular orientation can be affected through the spontaneous structure formation caused by the crystallization [1]. Although there have been various publications on the uniaxial and biaxial stretching behavior of PET films [2][3][4][5][6][7][8][9][10][11][12], research on in situ measurement of stretching behavior and three-dimensional analyses of orientation development is limited [13,14].…”