User:Cddwumich/sandbox

Objectives

We plan to create a new site for high refractive index polymers. Within this site, we would like to give an introduction and some basic background of this class of polymer, along with practical applications of these materials.

Introduction

A high refractive index polymer is a polymer chain that has a measured refractive index 1.50 or higher . Once an n is greater than or equal to 1.50, the polymer is considered a high refractive index polymer.

Due to recent developments in the optoelectronics field, a great interest in high refractive index polymers (HRIP) has come about3. Development of photonic devices such as organic light emitting diode devices (OLED’s), antireflective coatings, and image sensors have created a new demand for materials with a high refractive index1,5. The refractive index of a polymer is based on several factors which include polarizability, chain flexibility, molecular geometry, and the polymer backbone orientation8,9.

As of 2004, the highest refractive index measured for a polymer was 1.76 by Nitto Denko. However, many optoelectronic devices require much higher refractive indexes for polymers. The current limitation for refractive index measurement as of current is just below 1.801. In order to try to increase high refractive index measurements in polymers, substituents with high molar fractions or high-n nanoparticles with polymer matrices have been introduced4.

Properties of HRIP

There are various physical and chemical properties that determine whether a polymer is considered a high n-refractive index polymer. A high refractive index material should also have various other properties which include low birefringence, high optical transparency, and thermal stability5.

A typical polymer has a refractive index of 1.30-1.70. A higher refractive index, exceeding 1.7, is often required for specific applications. In order to predict whether a polymer can be considered a high refractive index polymer, the refractive index must be represented and related to the molecular refraction, weight, and volume of the monomer. This is done using the Lorentz-Lorenz equation. In general, high molar refractions and low molar volumes increase the refractive index of the polymer1.

Optical dispersion is also an important property to look when trying to make a high refractive index polymer. This is measured using the Abbe number11. The larger the Abbe number, the smaller the dispersion is in the material. A high refractive index material will generally have a large Abbe Number, or a small optical dispersion. This is helpful, specifically, when making high refractive index materials such as thin optical plastic lenses11. A low birefringence has been required along with a high refractive index for many applications. Having a low birefringence means that the polymer has little or no effect to the visible light. A low birefringence can be acquired by using difference functional groups in the initial monomer used to make the high refractive index polymer. Not only do aromatic monomers increase refractive index, but the aromatic group will create a decrease in optical anisotropy, which will create a low birefringence. This is due to the aromatics occupying a different plane in space4. The refract index, Abbe Number, and birefringence can be measured using a refractometer4.

A high clarity is desired in a high refractive index polymer. This is because many applications for the high refractive index polymers include using them for optically active materials1. Optical transparency is important for an optical material since the properties of the film will change with a change in clarity. The clarity is dependent on the refractive index of the polymer itself and the refractive index of the initial monomer10. Depending on the application, an optimal transparency wavelength will change. However, for optical application, a wavelength of 400-900nm is generally used4.

When looking at thermal stability, the typical variables measured include glass transition, initial decomposition temperature, degradation temperature, and the melting process window5. The thermal stability can be measured by thermogravimetric analysis and differential scanning calorimetry. A high refractive index polymer will be considered thermally stable depending on the application is it needed for and type of polymer it is. For example, for polyesters, they are considered thermally stable with a degradation temperature of 410 degrees Celsius. It has been shown that having longer alkyl subsituents will decrease the thermal stability4. The decomposition temperature will change depending on the substituent that is attached to the monomer used in the polymerization of the high refractive index polymer4.

Since high refractive index polymers are long carbon chains, viscosity and solubility issues can change the effectiveness and properties of the material. For most applications, it is favorable to have the polymer be soluble in as many solvents as possible. This is why high refractive polyesters are being researched. At room temperature, they are soluble in all ordinary organic solvents. These include dichloromethane, methanol, hexanes, acetone, and toluene4. Polyimideas are also ideal since they are also soluble in many organic solvents at room temperature5.

Synthesis of HRIP

Each high refractive index polymer can have a different synthetic route depending on what type it is. For a polyimide, the Michael polyaddition is used. The Michael polyaddition is used because it can be carried out at room temperature, and it can used for step growth polymers. This synthesis was first done by Crivello with polyimidothiethers, and it produced high refractive index and optically transparent polymers5. Polycondensation reactions are also common to make high refractive index polymers. An example of this synthetic route is found for making high refractive index polyesters4. A condensation reaction and a michael reaction are common organic synthetic reactions.

Figure 1: Example of a Michael polyaddition

Figure 2: Example of a Polycondensation

Issues of HRIP

While the nanocomposite materials have the ability to create better high refractive index polymers, the combination of the inorganic nanoparticles with the polymer matrix suffers from stability issues. Because of constant aggregation, they suffer from storage stability. Because of how they are synthesized, they also suffer from optical loss over time because of bad dispersion1.

Applications

In information processing, a key material to have is a microlens array. This can then be used for optoelectronics, optical communications, and displays. If the microlens can be made of a polymer material, it could be easier to make and be more flexible than the current materials used. However, the material would need to have a high refractive index value. This microlens arrays have been produced using high refractive index polymers in the application to CMOS image sensors (CIS). CIS sensors use less power, are smaller in size, and cost less to produce. Using the high-n microlens array, the CIS is able to better focus on the optical signals and increase the sensitivity of the instrument1.

Figure 3: Image of a CMOS image sensor

Another application of a high refractive index polymer is in the research and development of immersion lithography. Immersion lithography is a new technique that is being investigated for circuit manufacturing. This method uses both photoresists and high refractive index fluids. In order to create a high refractive index with a low absorbance, a series of high refractive index polymers were looked into in order to create a better photoresist system. A photoresist needs to have an n value of greater than 1.90. It has been shown that non-aromatic, sulfulr containing high refractive index polymers are the best materials to use to create an optical photoresist system1.

References

1. Jin-gang Liu and Mitsuru Ueda, High refractive index polymers: fundamental research and practical applications, J. Mater. Chem., 2009, 19, 8907-8919 doi:10.1039/B909690F

2. T.W. Kelly, P.F. Baude, C. Gerland, D.E. Ender, D. Muyres, M.A. Haase, D.E. Vogel and S.D. Theiss, Recent Progress in Organic Electronics: Materials, Devices, and Processes, "Chemsitry of Materials," 2004, 16, 4413–4422

3. Wehrspohn, R.B. et.al. Nanophotonic Materials. Germany:Weinheim, Wiley-VCH, Inc., 2008.

4. Seto, Polymer, 51, 4744-4749

5. Yen, J. Mater. Chem., 20, 4080-4084 (2010)

6. Beercroft, L. L.; Ober, C. K. J.M.S.-Pure Appl. Chem. 1997, A34(4), 573

7. Flaim, T.; Wang, Y.; Mercado, R. SPIE Proceedings of Optical Systems Design, 2003.

8. Li, Polymer, 51, 3851-3858 (2010)

9. Han, J. Appl. Polym. Sci, 77, 2172-2177 (2000)

10. Nolan P., Tillin M. and Coates D. (1993), Liquid Crystals, 14:2, 339-344

11. T. Matsuda, Y. Funae, M. Yoshida, T. Yamamoto and T. Takaya, J. Appl. Polym. Science, 2000, 76, 50.