Jehuda Greener

Stress relaxation of polymer films in bending

Abstract Stress-relaxation functions in bending and in tension were obtained for several polymer films with widely different microstructures and relaxation characteristics. For all the films tested, non-linear viscoelastic response is observed at strains exceeding 0.8%, with the relaxation time being generally lower the higher the strain. It is also observed that, within the linear viscoelastic range, the relaxation spectra of the bending and tensile moduli are qualitatively different, with the bending modulus relaxing generally more slowly than the tensile modulus. This difference is attributed mainly to a temporal change in Poissons ratio, which can be estimated directly from the bending and tensile relaxation functions. Other possible causes for the observed differences in relaxation response are discussed.

Optical properties of solvent-cast polarizer films for liquid-crystal displays: A viscoelastic modeling framework

Abstract— Many of the films used in polarizer assemblies in LCDs (e.g., triacetyl cellulose or TAC) are produced by a solvent-casting process, which is known to impart optical anisotropy to the film expressed as finite out-of-plane birefringence. This feature of the film could have a significant impact on the optical performance of the display and it needs to be accounted for in any compensation scheme for the LC cell. This paper reviews the origin of this optical anisotropy, and it presents a viscoelastic model that links this property to the solvent-casting process and to some key material parameters. The model results are compared with experimental data generated for polystyrene films cast from toluene, and generally good agreement is demonstrated.

P-108: Birefringence Dispersion of Polymer Films As Applied to Optical Compensation in LCD

Minimizing color shift effects in a liquid crystal display (LCD) requires close attention to the optical dispersion of compensation layers used to increase the contrast ratio and viewing angle of the display. This paper describes our attempt to systematize and characterize the birefringence dispersion of solvent-cast polymer films and relate it to the “native” out-of-plane birefringence (Δnth) of the polymer and its structure. Results for a wide range of polymers show general correlation between the dispersion parameter DPΔn [≡Δn(450)/Δn(590)] and Δnth. for all positively birefringent polymers, DPΔn is always > 1, suggesting “normal” dispersion at any level of Δnth, while for negatively birefringent polymers DPΔn > 1 only when |Δnth| > ∼0.003. Below this critical value, DPΔn < 1, i.e., the polymer is reverse-dispersive. The results are explained in terms of the Cauchy coefficients for the corresponding polymers, and they suggest that the dispersion of birefringence cannot be controlled independently of the birefringence of the polymer film.

P-120: Optical Properties of Solvent-Cast Polarizer Films for Liquid Crystal Display: A Viscoelastic Modeling Framework

The optical properties of polarizer films in LCD are critical to the optical performance of the display. Solvent-cast cellulosic films (e.g., triacetyl cellulose or TAC) are used in conventional polarizer assemblies, in part, for their high transmission and relatively low optical retardation. However, even low-birefringent materials such as TAC film possess some finite level of residual out-of-plane retardation that needs to be accounted for in various compensation schemes for the display. This paper reviews the origin of this optical anisotropy, and it describes a viscoelastic model that links this property to the solvent-casting process.

Effect Of Birefringence In Polycarbonate Substrates On Dynamic Performance Of Magnetooptical Disks

The effect of birefringence in polycarbonate substrates on the dynamic performance of magnetooptical disks has been investigated. We have observed a monotonic decrease of the carrier-to-noise ratio with increase in the normal retardation for retardation levels greater than about 15 nm.