Houran Shafiee

Investigation of optical and thermal properties of N-(alkyl-substituted) maleimides for use in zero-zero-birefringence polymer.

N-(alkyl-substituted) maleimides (RMIs) were proposed as materials useful for the development of a zero-zero-birefringence polymer that exhibits no birefringence. We analyzed the optical and thermal properties of poly(RMI)s, such as the refractive index, birefringence, and glass transition temperature. The characteristics of the obtained polymers varied significantly because the shift of the density and polarizability derived from the change of the substituent structure influenced the optical properties, and the bulkiness of the substituents influenced the thermal properties. We also designed a zero-zero-birefringence polymer using N-ethyl maleimide (EMI) as a comonomer, and the obtained copolymer had no birefringence, relatively high heat resistance, and high transparency.

Control of the Birefringence Dispersion of an Optical Polymer by Doping with an Inorganic Crystal

The birefringence dispersion of a polymer film was controlled by doping with an inorganic birefringent crystal. Nanosized crystals of strontium carbonate (SrCO3) with a needlelike shape of average length of 192 nm were oriented during the preparation of the film by solvent casting, and the contributions of the SrCO3 crystals to the birefringence and to the birefringence dispersion of the composite film were analyzed. A SrCO3-doped polymer film with reverse birefringence dispersion was fabricated, and the birefringence dispersion of this film was shown to be successfully controlled by the presence of the crystals.

The effect of trans-stilbene unit in the compensation of birefringence of poly(methyl methacrylate) in the random copolymerization method and anisotropic molecule dopant method

We investigated the effects of trans-stilbene unit in compensating birefringence in the random copolymerization method and the anisotropic molecule dopant method. In the random copolymerization method, trans-stilbene methacrylate (TSMA) containing the trans-stilbene unit in the side chain was polymerized with methyl methacrylate (MMA) in solution polymerization to compensate the photoelastic birefringence and the orientational birefringence of poly(methyl methacrylate) (PMMA). In the anisotropic molecule dopant method, trans-stilbene was added to PMMA. In the both methods, the photoelastic birefringence and the orientational birefringence shifted from the negative side to the positive side with an increase in the concentration of trans-stilbene unit. 0.8 mol% of TSMA almost eliminated the photoelastic birefringence. Also, we demonstrated that poly(MMA/TSMA) exhibited no orientational birefringence with 1.9 mol% of TSMA. 2.0 mol% of trans-stilbene almost eliminated the photoelastic birefringence of PMMA. Similarly, we demonstrated compensating orientational birefringence with 2.0 mol% of trans-stilbene. Based on the results, the effects of trans-stilbene unit in compensation of orientational birefringence are almost the same in the two methods. However, in compensation of photoelastic birefringence, the trans-stilbene unit had 2.5 times higher effect in the random copolymerization method than that in the anisotropic molecule dopant method. Photoelastic birefringence is caused in elastic deformation below Tg, in which the side chains are mainly orientated while the polymer main chains are scarcely orientated. Therefore, we concluded that addition of trans-stilbene unit to the side chain enhanced the effect for compensating photoelastic birefringence.

Design of temperature-independent zero-birefringence pressure sensitive adhesives (Conference Presentation)

Liquid crystal displays (LCDs) are composed of two glass substrates, two polarizers and some optical films. These components are laminated by pressure sensitive adhesives (PSAs). When a polarizer shrinks by humidity or the heat from a backlight of LCDs, stress appears and deforms PSAs. PSAs tend to exhibit birefringence due to applied stress and temperature change, which causes light leakage degrading image quality of LCDs. PSAs are consisted of main chain polymers and cross-linkers. To evaluate birefringence of PSAs at room temperature is difficult because PSAs easily plastically deform at the temperature. The purpose of this article is to design temperature-independent zero-birefringence PSAs (TIZBPSAs) exhibiting almost no birefringence even during stress-induced deformation over a wide temperature range. Butyl acrylate (BA) and phenoxyethyl acrylate (PHEA) were selected as the monomers of main chain polymers and an isocyanate-type cross-linker was added. Trilaminar films were prepared in which PSAs were sandwiched between two supporting films. We successfully evaluated birefringence and temperature dependence of birefringence of PSAs for the first time by using temperature-independent zero-birefringence polymers (TIZBPs) as the supporting films. TIZBPs, designed in our group, show almost no orientational birefringence even when the polymer main chain is in an oriented state and almost no temperature dependence of orientational birefringence over a wide temperature range. We have proposed a novel method to design PSAs having desirable the birefringence properties by determining the contributions of BA, PHEA and the cross-linker to birefringence and temperature dependence of PSAs quantitatively. Furthermore, we have designed TIZBPSAs by the proposed method.