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High-refractive-index polymer

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Introduction

A high refractive index polymer is a polymer chain that has a measured refractive index 1.50 or higher. [1]

Figure 1: Depiction of Refractive Index

Due to recent developments in the optoelectronics field, a great interest in high refractive index polymers (HRIP) has come about.[2] Development of photonic devices such as organic light emitting diode devices (OLED’s), anti-reflective coating, and image sensors have created a new demand for materials with a high refractive index.[1][3] The refractive index of a polymer is based on several factors which include polarizability, chain flexibility, molecular geometry, and the polymer backbone orientation.[4][5]


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 1.80.[1] 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 introduced.[6]

Properties of HRIP

There are various physical and chemical properties that determine whether a polymer is considered a high n-refractive index polymer. These properties include low birefringence, high optical transparency, and thermal stability.[3]

Refractive Index

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 refractivity and low molar volumes increase the refractive index of the polymer.[1]

Optical Properties

Optical dispersion is also an important property to look at when trying to make a high refractive index polymer. This is measured using the Abbe number.[7] 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 lenses.[7] 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 different 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 space. The refractive index, Abbe Number, and birefringence can be measured using a refractometer.[6]

Figure 2: Example of birefringence

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 materials.[1] 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 monomer.[8] Depending on the application, an optimal transparency wavelength will change. However, for optical application, a wavelength of 400-900nm is generally used.[6]

Thermal Stability

When looking at thermal stability, the typical variables measured include glass transition, initial decomposition temperature, degradation temperature, and the melting process window.[3] 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 it is needed for and type of polymer it is. An example is polyesters, which are considered thermally stable with a degradation temperature of 410 degrees Celsius. It has been shown that having longer alkyl substituents will decrease the thermal stability. The decomposition temperature will change depending on the substituent that is attached to the monomer used in the polymerization of the high refractive index polymer.[6]

Solubility

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 toluene.[6] Polyimides are also ideal since they are also soluble in many organic solvents at room temperature.[3]

Disadvantages

The use of high refractive index polymers have led to many great new materials and applications in the scientific world. However, 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 many stability issues. For instance, because of the constant aggregation, they suffer from storage stability over a period of time. They also have synthetic disadvantages. For example, because of how they are synthesized, they also suffer from optical loss over time. This is because of bad dispersion.[1]

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 polymerization. This synthesis was first done by Crivello with polyimidothiethers, and it produced high refractive index and optically transparent polymers.[3] Polycondensation reactions are also common to make high refractive index polymers. An example of this synthetic route is found for making high refractive index polyesters.[6] A condensation reaction and a michael reaction are common organic synthetic reactions.

Figure 3: Example of a Michael polyaddition
Figure 4: Example of a Polycondensation

Types of HRIP

High refractive indices have been achieved either by introducing substituents with high molar refractions to make intrinsic HRIPs or by combining high-n nanoparticles with polymer matrixes to make HRIP nanocomposites.

Intrinsic HRIP

Sulfur-containing substituents including linear thioether and sulfone, cyclic thiophene, thiadiazole, and thianthrene are the most commonly used groups for increasing a polymer’s refractive index. Recently, systematic work about sulfur-containing polyimides (PIs) revealed the influence of sulfur groups on the refractive indices and optical dispersion of polymers.[9][10][11] Polymers with sulfur-rich thianthrene and tetrathiaanthrene moieties exhibit the higher n values than 1.72 and the degree of molecular packing of PIs also affects their n values. Figure 5 is an example of sulfur-containg PIs with high refracrive index.

Figure 5. Sulfur-containing PIs with high refractive index


Halogen elements, especially bromine and iodine, were the earliest utilized components for developing HRIPs. In 1992, Gaudiana et al reported a series of polymethylacrylate compounds containing lateral brominated and iodinated carbazole rings whose refractive indices are ranged from 1.67-1.77 depending on the components and numbers of the halogen substituents.[12] On the other hand, recently, applications of halogen elements in microelectronic devices have been severely limited by the WEEE Directive and RoHS legislation promulgated by the European Union due to their potential pollution of the environment.[13] Although much attention have not been paid to their applications in photonic devices, there still exists the potential possibility of prohibition in some optical fields, such as the high-n encapsulant for LEDs.

Figure 6. Halogen-containing polymethacrylates

Phosphorus-containing groups, such as phosphonates and phosphazenes, often exhibit high molar refractions and good optical transmittance in the visible light region.[14][15] Thus, phosphorous-containing polymers are being researched as potential HRIPs that can be used for optical applications. Polyphosphonates have high refractive indices due to the phosphorus moiety even if they have chemical structures analogous to polycarbonates.[16] In addition, polyphosphonates exhibit good melt stability, good optical transparency, and good fire retardant property. Thus, they are suitable for casting into plastic lenses for customer use.

Figure 7. Polyphosphonates

Organometallic components have proven to be effective in developing HRIPs with good film formability and relatively low optical dispersion. Polyferrocenylsilanes[17] and polyferrocenes containing phosphorous spacers and phenyl side chains show unusually high n values (n=1.74 and n=1.72). They might be good candidates for all-polymer photonic devices because they exhibit moderate optical dispersion between organic polymers and inorganic glasses.

Figure 8. Organometallic HRIP

HRIP nanocomposite

Hybrid techniques which combine an organic polymer matrix with highly refractive inorganic nanoparticles could achieve much higher n values. The factors affecting the refractive index of a high-n nanocomposite include the characteristics of the polymer matrix, nanoparticles, and the hybrid technology between inorganic and organic components. The refractive index of a nanocomposite can be approximately estimated by the equation , where , , and stand for the refractive indices of the nanocomposite, nanoparticle, and organic matrix, respectively. and represent the volume fractions of the nanoparticles and organic matrix, respectively.[18] In order to achieve high n nanocomposite with a definite type of nanoparticle, refractive of organic polymer should be high. Polyimides are often utilized as the matrix to combine with high n nanoparticles due to their inherent high n nature. Control of nanoparticle load is also important in designing HRIP nanocomposites for optical applications because an overload of nanoparticles often increases the optical loss and decreases the processability of the nanocomposites. On the other hand, the choice of nanoparticle is often influenced by its size and surface characteristics. In order to achieve good optical transparency and avoid Rayleigh scattering of the nanocomposite, the diameter of the nanoparticle should be below 25 nm.[19] In addition, the direct mixing of nanoparticles with the polymer matrix facilitates the aggregation of nanoparticles. Thus, in practice, the surface of nanoparticles is often modified in order to fit the mixing process. The most commonly used nanoparticles for HRIPs include TiO2 (anatase, n=2.45; rutile, n=2.70),[20] ZrO2 (n=2.10),[21] amorphous silicon (n=4.23), PbS (n=4.20),[22] and ZnS (n=2.36).[23] Polyimides (PIs) are often utilized as the matrix to combine with high-n nanoparticles due to their inherent high-n nature. Nanocomposite having PI moiety exhibits a tunable refractive index ranging from 1.57 to 1.99.[24]

Figure 9. High-n polyimide nanocomposite

Applications

Figure 10: Image of a CMOS image sensor

Image Sensors

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 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 instrument.[1]

Lithography

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 system.[1]

OLED'S

Figure 11: LEDs of the 5mm diffused type

Light-emitting diodes (LEDs) are thought to be the most promising solid-state light source for general lighting and have the potential to replace traditional incandescent bulbs and fluorescent lamps. To be widely used, high-brightness LEDs (HBLEDs) should be developed. The technical hurdles lie in the relatively low light extraction efficiency (LEE) of HBLEDs, mainly coming from the mismatch of the refractive indices between inorganic LED dies (GaN, n=2.5) and the organic encapsulants (epoxy or silicone, n=1.5). As the refractive index of the encapsulant increases, the LEE ratio increases rapidly. If an encapsulating resin with an index higher than 1.80 can be achieved, the LEE will be greatly enhanced.[25]

See also

References

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  2. ^ Ralf B. Wehrspohn, Heinz-Siegfried Kitzerow and Kurt Busch (2008). Nanophotonic Materials. Germany: Wiley-VCH Inc. ISBN 978-3-527-40858-0.
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  4. ^ Cheng Li, Zhuo Li, Jin-gang Liu, Xiao-juan Zhao, Hai-xia Yang and Shi-yong Yang (2010). "Synthesis and characterization of organo-soluble thioether-bridged polyphenylquinoxalines with ultra-high refractive indices and low birefringences". Polymer. 51: 3851. doi:10.1016/j.polymer.2010.06.035.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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  7. ^ a b Tatsuhito Matsuda, Yasuaki Funae, Masahiro Yoshida, Tetsuya Yamamoto and Tsuguo Takaya (2000). "Optical material of high refractive index resin composed of sulfur-containing aromatic methacrylates". J. Appl. Polym. Science. 50: 50. doi:10.1002/(SICI)1097-4628(20000404)76:1<50::AID-APP7>3.0.CO;2-X.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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  10. ^ Jin-Gang Liu, Yasuhiro Nakamura, Yuji Shibasaki, Shinji Ando and Mitsuru Ueda (2007). "Synthesis and characterization of highly refractive polyimides from 4,4′-thiobis[(p-phenylenesulfanyl)aniline] and various aromatic tetracarboxylic dianhydrides". J. Polym. Sci., Part A: Polym. Chem. 45: 5606. doi:10.1002/pola.22308.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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  13. ^ Emma Goosey (2006). "Brominated flame retardants: their potential impacts and routes into the environment". Circuit World. 32: 32. doi:10.1108/03056120610683603.
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  16. ^ H. K. Shobha, H. Johnson, M. Sankarapandian, Y. S. Kim, P. Rangarajan, D. G. Baird and J. E. McGrath (2001). "Synthesis of high refractive-index melt-stable aromatic polyphosphonates". J. Polym. Sci., Part A: Polym. Chem. 39: 2904. doi:10.1002/pola.1270.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  17. ^ Ian Manners (2002). "Polyferrocenylsilanes: metallopolymers for electronic and photonic applications". J. Opt. A: Pure Appl. Opt. 4: S221. doi:10.1088/1464-4258/4/6/356.
  18. ^ Lorenz Zimmermann, Martin Weibel, Walter Caseri, Ulrich W. Suter and Paul Walther (1993). "Polymer nanocomposites with "ultralow" refractive index". Polym. Adv. Tech. 4: 1. doi:10.1002/pat.1993.220040101.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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  20. ^ Akhmad Herman Yuwono, Binghai Liu, Junmin Xue, John Wang, Hendry Izaac Elim, Wei Ji, Ying Li and Timothy John White (2004). "Controlling the crystallinity and nonlinear optical properties of transparent TiO2–PMMA nanohybrids". J. Mater. Chem. 14: 2978. doi:10.1039/b403530e.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  21. ^ Naoaki Suzuki, Yasuo Tomita, Kentaroh Ohmori, Motohiko Hidaka and Katsumi Chikama (2006). "Highly transparent ZrO2 nanoparticle-dispersed acrylate photopolymers for volume holographic recording". Opt. Express. 14: 012712. doi:10.1364/OE.14.012712.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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  23. ^ Changli Lü, Zhanchen Cui, Zuo Li, Bai Yang and Jiacong Shen (2003). "High refractive index thin films of ZnS/polythiourethane nanocomposites". J. Mater. Chem. 13: 526. doi:10.1039/B208850A.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  24. ^ Chih-Ming Chang, Cheng-Liang Chang and Chao-Ching Chang (2006). "Synthesis and optical properties of soluble polyimide/titania hybrid thin films". Macromol. Mater. Eng. 291: 1521. doi:10.1002/mame.200600244.
  25. ^ Frank W. Mont, Jong Kyu Kim, Martin F. Schubert, E. Fred Schubert and Richard W. Siegel (2008). "High-refractive-index TiO2-nanoparticle-loaded encapsulants for light-emitting diodes". J. Appl. Phys. 103: 83120. doi:10.1063/1.2903484.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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