Polymer Selection for Premium Optical Quality in Displays
By: Radu Reit, Ph.D.
As displays begin to take on new shapes and form-factors, the materials used to build these devices have had to also adapt, becoming thinner, lighter and more robust. One important need for increasing the functionality of these complex devices is an increase in the quality of optical films. These materials should provide excellent transparency in the visible spectrum, have minimal haze, minimal color and interact as little as possible with the emitted light from the display. When looking for these materials, it is useful to understand how the micro- and macroscopic properties of optical films affects each of these factors and how to select a materials system that will satisfy your optical needs.
1. Visible Light Transparency
Visible light transparency (VLT) through a polymer is heavily dependent on the backbone structure of the material in question. The fastest way to determine if a polymer will have a high VLT will be to understand if the backbone has a potential for forming crystallites. Semi-crystalline polymers, or those with some degree of order due to secondary bonding, come in a variety of degrees of crystallinity. Polyester films can be manufactured with low degrees of crystallinity (< 5%) to stabilize the mechanical performance of the films above the glass transition (Tg). Conversely, fluoropolymer films such as polytetrafluoroethylene naturally are produced with high degrees of crystallinity (>80%) due to the strong secondary interaction imparted by the carbon-fluorine bonds. In cases where crystallinity is present, photonic scattering at interfaces between randomly oriented crystallites in the material and the remaining amorphous region (Figure 1) produces losses in VLT that are dependent on the degree of crystallinity present in the polymer.
Another important consideration of the materials’ total VLT is whether the backbone of the material is conjugated – that is, whether there is a presence of delocalized electrons in the backbone, typically seen through the presence of alternating double-bonds. This can be seen in both cyclic (e.g. benzene) or acyclic (e.g. 1,3-butadiene) backbone constituents, which can absorb select visible light photons. This absorption leads to an excitation of delocalized electrons and subsequent emission of higher-wavelength photons as the electrons fall back down to their primary energy level. While this phenomenon is a boon for the emitting components of OLEDs, optically-clear films need to eliminate this undesirable interaction with light.
2. Haze, Reflectance, Color and Birefringence
Aside from VLT, there are a number of other factors to consider when evaluating films for optical applications including haze, reflectance, color and birefringence. Haze can be understood as the scattering of light due to the interaction of visible light photons with imperfections in the material – this can be both backbone ‘imperfections’ such as crystallites which will scatter photons, but also manufacturing defects such as surface roughness from extrusion and other processing-related defects (e.g. bubbles, blisters, etc.) Minimizing these process-related defects by using films that are manufactured using low-tension, roll-to-roll processing or using coated resin-based materials is one way to remove the manufacturing induced-haze in optical films. Reflectance is another source of VLT loss, wherein incident light is turned away (reflected) from the surface of the film – this can be one or a combination of specular reflection (perfect reflection of a ray at a single angle) or diffuse reflection (multi-angle reflection due to irregular surfaces of materials). While coated films such as hard-coat acrylics and polysulfide thermosets can be quite reflective due to a high level of specular reflectance, extruded thermoplastic films such as polyesters or polycarbonates will have combined optical losses due to both specular and diffuse reflection of incident rays (Figure 2).
Color extrinsic to the polymer backbone is another factor to consider when selecting an optical film. Disregarding filled polymers which use additives (e.g. pigments, dyes, etc.) to intentionally impart color to a film and color due to conjugation from the backbone of the material, color can come from a variety of damage to the backbone to the material. Thermo- and photo-oxidation are the two primary methods of color change due to environmental factors, and both can be directly addressed by the use of additives such as antioxidants and light absorbers. In both cases, these small molecules prevent the unwanted side-reactions that occur within organic films under high temperatures or long-term exposure to high-energy photons. These additives must be carefully chosen to prevent an unwanted appearance of color due to other color-imparting factors described above.
Finally, the intrinsic birefringence of the film should be considered for applications relying on polarized light exiting from the emissive layer. Birefringence can be loosely understood as the different speed of propagation of light depending on the polarity of light for the incoming rays. For optically-demanding applications where the polarization of light from the emissive layer needs to pass through the frontplane unchanged, the birefringence of frontplane materials should be as low as possible. In polymer films, this birefringence is primarily controlled by the backbone of the material itself, as well as how the film was processed during manufacturing. In the case of the polymer backbone, highly cyclical structures (both with or without conjugation) which can form ordered structures in the backbone (that may or may not introduce crystallinity) are the primary cause of birefringence. Regardless of semi-crystallinity, this order can interfere with polarized light via optical retardation (i.e. the directionally-dependent speed of propagation for an incoming polarized ray) through the thickness of the film and manifests itself in a polymer film in the form of a high birefringence constant. This same order can be artificially induced in even the least intrinsically birefringent films due to manufacturing. For thermoplastic films specifically, the film extrusion process is particularly problematic as the forced extrusion will naturally align even the least birefringent materials. To prevent this, manufacturing methods which do not rely on high-pressure resin extrusion (e.g. slot-die coating) help reduce processing-related backbone order and can yield low birefringence materials.
Ares’ has developed a variety of polymers which combine many of the design principles discussed in this article to achieve highly transparent, superior quality optical-grade films. Our Pylux films are amorphous, unconjugated polysulfide thermoset films specifically designed for optically-demanding applications in information display devices. The films are processed via slot-die coating (or similar techniques) of liquid resins which are 100% solid-content solutions and have no solvent-evaporation related defects during manufacturing. Additionally, this processing allows our low backbone-order materials to have ultra-low haze and birefringence as a factor of the manufacturing process. Finally, color stability is achieved by a careful blend of additives that have been validated by countless accelerated aging tests.
Have any further questions about transparent polymers or Pylux-brand materials? Contact us anytime at firstname.lastname@example.org