Optical Properties

Light can permeate material and excite its electrons to produce an induced electric field. The interaction between the light and the induced electric field causes the light to refract. The comparison of the speed of light when permeating a material to the speed of light in a vacuum is called the refractive index. Different directions of light can permeate through different parts of the materials with a different refractive index. This can cause the speed of the light to differ depending on the surface it excites. This phenomenon is known as birefringence.

This birefringence phenomenon has a negative effect on optical materials such as those used in compact discs, plastic lenses, and optical film. For polymers in an irregular state such as an amorphous state, there is no directionality, and so the polymer becomes a uniform medium to the light, thus avoiding birefringence. However, when material is in a melted state for injection molding etc., this will cause polymer molecular orientation. This oriented state will cause birefringence (orientation birefringence) due to the difference in the refractive index of direct polarized light in moving toward the oriented structures at a particular direction versus the refractive index of the direct polarized light that bisects each other towards the oriented structure.

When light permeates a material that has birefringence properties, the light will separate into two beams that oscillate in the vertical direction each other, but will have differing speeds. The will cause the imaging point not to focus properly, thus reducing imaging performance. For this reason, when such polymers are used as optical devices for raw material such as compact discs and optical film, it is necessary to control the optical properties of these polymers-most importantly the arrangement of the polymer chain which has a direct influence on birefringence (orientation birefringence).

Here we have used the molecular dynamics engine COGNAC to compute orientation birefringence by calculating polymer uniaxial elongation properties. The substances used for these calculations were polystyrene for its negative birefringence properties, and polycarbonate for its high birefringence properties during orientation.

Figure 1 Polystyrene Uniaxial Elongation Simulation
Benzene Ring Plane Orients Perpendicular to the Direction of Elongation

Figure 2 Polycarbonate Uniaxial Elongation Simulation
Benzene Ring Plane Orients Parallel with the Direction of Elongation

Looking at the structure of the elongated polymers, we can see that for both polymers stretch along the elongation path (Z-axis). With the polystyrene, the benzene ring plane orients perpendicularly to the Z-axis. However, with the polycarbonate, the benzene ring plane orients parallel with the Z-axis. As benzene rings contain ƒÎ electrons that move easily, the refractive index of the ring plane direction is high. The refractive index is thought to decrease if the ring plane is set to a perpendicular direction. This is why polystyrene shows a negative birefringence as the benzene ring plane orients perpendicularly to the elongation direction. Polycarbonate, with its benzene ring plane that is parallel to the direction of elongation, shows a positive orientation birefringence.

Figure 3 Orientation Birefringence of Various Plastics

Figure 3 shows the orientation birefringence with respect to the cell elongation ratio for PC (polycarbonate), PS (polystyrene), PC and PS blends, as well as PMMA (polymethylmethacrylate). PC shows a positive value, PS shows a negative value, and we obtained a value of nearly zero with PMMA. These results show tendencies that nearly replicate experimental results.
A compatible PC and PS blend system also obtained a value near zero. By creating a compatible polymer blend that has both a positive birefringence and negative birefringence, we can reduce the birefringence properties.

Click here to see an animation [1170K]

As the Figure below shows, the orientation birefringence has been evaluated by using 3 types of derivatives to change the structure of the PC side chain moiety. Figure 5 shows the calculation results. This shows that derivative 1 with a benzene ring in the side chain reduces the birefringence value.

Figure 4 Polycarbonate Derivatives

Figure 5 Orientation Birefringence of Polycarbonate Derivatives

Reference
[1] Miyazaki, Togawa, Masubuchi, Molding Process '05, IV-312, pp 267-268, (2005)
[2] Iwashimizu, Okubo, Ozawa, Kimizuka, Molding Process Symposium '06, C-201, pp 107-108, (2006)

Please feel free to contact us if you have any questions or comments.

HOME > Case Studies > Optical Properties