High-refractive-index polymer

Last updated June 10, 2024

A high-refractive-index polymer (HRIP) is a polymer that has a refractive index greater than 1.50.^[1]

Such materials are required for anti-reflective coating and photonic devices such as light emitting diodes (LEDs) and image sensors.^[1]^[2]^[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 for a polymer was 1.76.^[6] Substituents with high molar fractions or high-n nanoparticles in a polymer matrix have been introduced to increase the refractive index in polymers.^[7]

Properties

Refractive index

A typical polymer has a refractive index of 1.30–1.70, but a higher refractive index is often required for specific applications. The refractive index is related to the molar refractivity, structure and weight of the monomer. In general, high molar refractivity and low molar volumes increase the refractive index of the polymer.^[1]

Optical properties

Optical dispersion is an important property of an HRIP. It is characterized by the Abbe number. A high refractive index material will generally have a small Abbe number, or a high optical dispersion.^[8] A low birefringence has been required along with a high refractive index for many applications. It can be achieved by using different functional groups in the initial monomer to make the HRIP. Aromatic monomers both increase refractive index and decrease the optical anisotropy and thus the birefringence.^[7]

Example of birefringence Calcite.jpg — Example of birefringence

A high clarity (optical transparency) is also desired in a high refractive index polymer. The clarity is dependent on the refractive indexes of the polymer and of the initial monomer.^[9]

Thermal stability

When looking at thermal stability, the typical variables measured include glass transition, initial decomposition temperature, degradation temperature and the melting temperature range.^[2] The thermal stability can be measured by thermogravimetric analysis and differential scanning calorimetry. Polyesters are considered thermally stable with a degradation temperature of 410 °C. The decomposition temperature changes depending on the substituent that is attached to the monomer used in the polymerization of the high refractive index polymer. Thus, longer alkyl substituents results in lower thermal stability.^[7]

Solubility

Most applications favor polymers which are soluble in as many solvents as possible. Highly refractive polyesters and polyimides are soluble in common organic solvents such as dichloromethane, methanol, hexanes, acetone and toluene.^[2]^[7]

Synthesis

The synthesis route depends on the HRIP type. The Michael polyaddition is used for a polyimide because it can be carried out at room temperature and can be used for step-growth polymerization. This synthesis was first succeeded with polyimidothiethers, resulting in optically transparent polymers with high refractive index.^[2] Polycondensation reactions are also common to make high refractive index polymers, such as polyesters and polyphosphonates.^[7]^[10]

Example of a polycondensation Polycondensation.png — Example of a polycondensation

Types

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

Intrinsic HRIP

A sulfur-containing polyimide with high refractive index PIs.png — A sulfur-containing polyimide with high refractive index

Sulfur-containing substituents including linear thioether and sulfone, cyclic thiophene, thiadiazole and thianthrene are the most commonly used groups for increasing refractive index of a polymer.^[11]^[12]^[13] Polymers with sulfur-rich thianthrene and tetrathiaanthracene moieties exhibit n values above 1.72, depending on the degree of molecular packing.

A halogen-containing polymethacrylate Wiki-halogen.png — A halogen-containing polymethacrylate

Halogen elements, especially bromine and iodine, were the earliest components used for developing HRIPs. In 1992, Gaudiana et al. reported a series of polymethylacrylate compounds containing lateral brominated and iodinated carbazole rings. They had refractive indices of 1.67–1.77 depending on the components and numbers of the halogen substituents.^[14] However, recent applications of halogen elements in microelectronics have been severely limited by the WEEE directive and RoHS legislation adopted by the European Union to reduce potential pollution of the environment.^[15]

A polyphosphonate Polyphosphonate.tif — A polyphosphonate

Phosphorus-containing groups, such as phosphonates and phosphazenes, often exhibit high molar refractivity and optical transmittance in the visible light region.^[3]^[16]^[17] Polyphosphonates have high refractive indices due to the phosphorus moiety even if they have chemical structures analogous to polycarbonates.^[18] Shaver et al. reported a series of polyphosphonates with varying backbones, reaching the highest refractive index reported for polyphosphonates at 1.66.^[10] In addition, polyphosphonates exhibit good thermal stability and optical transparency; they are also suitable for casting into plastic lenses.^[19]

Organometallic HRIP Wiki-organometallic2.png — Organometallic HRIP

Organometallic components result in HRIPs with good film forming ability and relatively low optical dispersion. Polyferrocenylsilanes^[20] and polyferrocenes containing phosphorus spacers and phenyl side chains show unusually high n values (n=1.74 and n=1.72).^[21] They might be good candidates for all-polymer photonic devices because of their intermediate optical dispersion between organic polymers and inorganic glasses.

HRIP nanocomposite

Hybrid techniques which combine an organic polymer matrix with highly refractive inorganic nanoparticles could result in high 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 estimated as ${n_{comp}}={\Phi _{p}}{n_{p}}+{\Phi _{org}}{n_{org}}$ , where ${n_{comp}}$ , ${n_{p}}$ and ${n_{org}}$ stand for the refractive indices of the nanocomposite, nanoparticle and organic matrix, respectively. ${\Phi _{p}}$ and ${\Phi _{org}}$ represent the volume fractions of the nanoparticles and organic matrix, respectively.^[22] The nanoparticle load is also important in designing HRIP nanocomposites for optical applications, because excessive concentrations increase the optical loss and decrease the processability of the nanocomposites. The choice of nanoparticles is often influenced by their size and surface characteristics. In order to increase optical transparency and reduce Rayleigh scattering of the nanocomposite, the diameter of the nanoparticle should be below 25 nm.^[23] Direct mixing of nanoparticles with the polymer matrix often results in the undesirable aggregation of nanoparticles – this is avoided by modifying their surface. The most commonly used nanoparticles for HRIPs include TiO₂ (anatase, n=2.45; rutile, n=2.70),^[24] ZrO₂ (n=2.10),^[25] amorphous silicon (n=4.23), PbS (n=4.20)^[26] and ZnS (n=2.36).^[27] Polyimides have high refractive indexes and thus are often used as the matrix for high-n nanoparticles. The resulting nanocomposites exhibit a tunable refractive index ranging from 1.57 to 1.99.^[28]

High-n polyimide nanocomposite Wiki-nano.png — High-n polyimide nanocomposite

Applications

A CMOS image sensor Matrixw.jpg — A CMOS image sensor

Image sensors

A microlens array is a key component of optoelectronics, optical communications, CMOS image sensors and displays. Polymer-based microlenses are easier to make and are more flexible than conventional glass-based lenses. The resulting devices use less power, are smaller in size and are cheaper to produce.^[1]

Lithography

Another application of HRIPs is in immersion lithography. In 2009 it was a new technique for circuit manufacturing using both photoresists and high refractive index fluids. The photoresist needs to have an n value of greater than 1.90. It has been shown that non-aromatic, sulfur-containing HRIPs are the best materials for an optical photoresist system.^[1]

LEDs

Light-emitting diodes (LEDs) are a common solid-state light source. High-brightness LEDs (HBLEDs) are often limited by the relatively low light extraction efficiency due to the mismatch of the refractive indices between the LED material (GaN, n=2.5) and the organic encapsulant (epoxy or silicone, n=1.5). Higher light outputs can be achieved by using an HRIP as the encapsulant.^[29]

Related Research Articles

$<span class="mw-page-title-main">Refractive index</span> Ratio of the speed of light in vacuum to that in the medium$

In optics, the refractive index of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.

$<span class="mw-page-title-main">Birefringence</span> Property of materials whose refractive index depends on light polarization and direction$

Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are described as birefringent or birefractive. The birefringence is often quantified as the maximum difference between refractive indices exhibited by the material. Crystals with non-cubic crystal structures are often birefringent, as are plastics under mechanical stress.

<span class="mw-page-title-main">Polydiacetylenes</span>

Polydiacetylenes (PDAs) are a family of conducting polymers closely related to polyacetylene. They are created by the 1,4 topochemical polymerization of diacetylenes. They have multiple applications from the development of organic films to immobilization of other molecules.

In organic chemistry, an imide is a functional group consisting of two acyl groups bound to nitrogen. The compounds are structurally related to acid anhydrides, although imides are more resistant to hydrolysis. In terms of commercial applications, imides are best known as components of high-strength polymers, called polyimides. Inorganic imides are also known as solid state or gaseous compounds, and the imido group (=NH) can also act as a ligand.

<span class="mw-page-title-main">Polyimide</span> Class of polymers

Polyimide is a polymer containing imide groups belonging to the class of high-performance plastics. With their high heat-resistance, polyimides enjoy diverse applications in roles demanding rugged organic materials, such as high temperature fuel cells, displays, and various military roles. A classic polyimide is Kapton, which is produced by condensation of pyromellitic dianhydride and 4,4'-oxydianiline.

A dielectric mirror, also known as a Bragg mirror, is a type of mirror composed of multiple thin layers of dielectric material, typically deposited on a substrate of glass or some other optical material. By careful choice of the type and thickness of the dielectric layers, one can design an optical coating with specified reflectivity at different wavelengths of light. Dielectric mirrors are also used to produce ultra-high reflectivity mirrors: values of 99.999% or better over a narrow range of wavelengths can be produced using special techniques. Alternatively, they can be made to reflect a broad spectrum of light, such as the entire visible range or the spectrum of the Ti-sapphire laser.

Ormosil is a shorthand phrase for organically modified silica or organically modified silicate. In general, ormosils are produced by adding silane to silica-derived gel during the sol-gel process. They are engineered materials that show great promise in a wide range of applications such as:

<span class="mw-page-title-main">Nanocomposite</span> Solid material with nano-scale structure

Nanocomposite is a multiphase solid material where one of the phases has one, two or three dimensions of less than 100 nanometers (nm) or structures having nano-scale repeat distances between the different phases that make up the material.

Heptacene is an organic compound and a polycyclic aromatic hydrocarbon and the seventh member of the acene or polyacene family of linear fused benzene rings. This compound has long been pursued by chemists because of its potential interest in electronic applications and was first synthesized but not cleanly isolated in 2006. Heptacene was finally fully characterized in bulk by researchers in Germany and the United States in 2017.

Polymer nanocomposites (PNC) consist of a polymer or copolymer having nanoparticles or nanofillers dispersed in the polymer matrix. These may be of different shape, but at least one dimension must be in the range of 1–50 nm. These PNC's belong to the category of multi-phase systems that consume nearly 95% of plastics production. These systems require controlled mixing/compounding, stabilization of the achieved dispersion, orientation of the dispersed phase, and the compounding strategies for all MPS, including PNC, are similar. Alternatively, polymer can be infiltrated into 1D, 2D, 3D preform creating high content polymer nanocomposites.

A silsesquioxane is an organosilicon compound with the chemical formula [RSiO_3/2]_n. Silsesquioxanes are colorless solids that adopt cage-like or polymeric structures with Si-O-Si linkages and tetrahedral Si vertices. Silsesquioxanes are members of polyoctahedral silsesquioxanes ("POSS"), which have attracted attention as preceramic polymer precursors to ceramic materials and nanocomposites. Diverse substituents (R) can be attached to the Si centers. The molecules are unusual because they feature an inorganic silicate core and an organic exterior. The silica core confers rigidity and thermal stability.

A thermoset polymer matrix is a synthetic polymer reinforcement where polymers act as binder or matrix to secure in place incorporated particulates, fibres or other reinforcements. They were first developed for structural applications, such as glass-reinforced plastic radar domes on aircraft and graphite-epoxy payload bay doors on the Space Shuttle.

Fire-safe polymers are polymers that are resistant to degradation at high temperatures. There is need for fire-resistant polymers in the construction of small, enclosed spaces such as skyscrapers, boats, and airplane cabins. In these tight spaces, ability to escape in the event of a fire is compromised, increasing fire risk. In fact, some studies report that about 20% of victims of airplane crashes are killed not by the crash itself but by ensuing fires. Fire-safe polymers also find application as adhesives in aerospace materials, insulation for electronics, and in military materials such as canvas tenting.

The Birch reduction is an organic reaction that is used to convert arenes to 1,4-cyclohexadienes. The reaction is named after the Australian chemist Arthur Birch and involves the organic reduction of aromatic rings in an amine solvent with an alkali metal and a proton source. Unlike catalytic hydrogenation, Birch reduction does not reduce the aromatic ring all the way to a cyclohexane.

Polymers with the ability to kill or inhibit the growth of microorganisms such as bacteria, fungi, or viruses are classified as antimicrobial agents. This class of polymers consists of natural polymers with inherent antimicrobial activity and polymers modified to exhibit antimicrobial activity. Polymers are generally nonvolatile, chemically stable, and can be chemically and physically modified to display desired characteristics and antimicrobial activity. Antimicrobial polymers are a prime candidate for use in the food industry to prevent bacterial contamination and in water sanitation to inhibit the growth of microorganisms in drinking water.

<span class="mw-page-title-main">Polyfluorene</span> Chemical compound

Polyfluorene is a polymer with formula (C₁₃H₈)_n, consisting of fluorene units linked in a linear chain — specifically, at carbon atoms 2 and 7 in the standard fluorene numbering. It can also be described as a chain of benzene rings linked in para positions with an extra methylene bridge connecting every pair of rings.

Oxycarbide glass, also referred to as silicon oxycarbide, is a type of glass that contains oxygen and carbon in addition to silicon dioxide. It is created by substituting some oxygen atoms with carbon atoms. This glass may contain particles of amorphous carbon, and silicon carbide. SiOC materials of varying stoichiometery are attractive owing to their generally high density, hardness and high service temperatures. Through diverse forming techniques high performance parts in complex shapes can be achieved. Unlike pure SiC, the versatile stoichiometry of SiOC offers further avenues to tune physical properties through appropriate selection of processing parameters.

Itaconic anhydride is the cyclic anhydride of itaconic acid and is obtained by the pyrolysis of citric acid. It is a colourless, crystalline solid, which dissolves in many polar organic solvents and hydrolyzes forming itaconic acid. Itaconic anhydride and its derivative itaconic acid have been promoted as biobased "platform chemicals" and bio- building blocks.) These expectations, however, have not been fulfilled.

β-Butyrolactone is the intramolecular carboxylic acid ester (lactone) of the optically active 3-hydroxybutanoic acid. It is produced during chemical synthesis as a racemate. β-Butyrolactone is suitable as a monomer for the production of the biodegradable polyhydroxyalkanoate poly(3-hydroxybutyrate) (PHB). Polymerisation of racemic (RS)-β-butyrolactone provides (RS)-polyhydroxybutyric acid, which, however, is inferior in essential properties (e.g. strength or degradation behaviour) to the (R)-poly-3-hydroxybutyrate originating from natural sources.

3,3’,4,4’-Benzophenone tetracarboxylic dianhydride (BTDA) is chemically, an aromatic organic acid dianhydride. It may be used to cure epoxy-based powder coatings. It has the CAS Registry Number of 2421-28-5 and a European Community number 219-348-1. It is REACH and TSCA registered. The formula is C₁₇H₆O₇ with a molecular weight of 322.3.

References

1 2 3 4 5 Jin-gang Liu; Mitsuru Ueda (2009). "High refractive index polymer: fundamental and practical applications". J. Mater. Chem. 19 (47): 8907. doi:10.1039/B909690F.
1 2 3 4 Hung-Ju Yen; Guey-Sheng Liou (2010). "A facile approach towards optically isotropic, colorless, and thermoplastic polyimidothioethers with high refractive index". J. Mater. Chem. 20 (20): 4080. doi:10.1039/c000087f. S2CID 55614500.
1 2 Macdonald, Emily K.; Shaver, Michael P. (2015-01-01). "Intrinsic high refractive index polymers". Polymer International. 64 (1): 6–14. doi:10.1002/pi.4821. hdl: 20.500.11820/49b9098d-0731-4f39-8695-6ed2eb313f97 . ISSN 1097-0126.
↑ Cheng Li; Zhuo Li; Jin-gang Liu; Xiao-juan Zhao; Hai-xia Yang; Shi-yong Yang (2010). "Synthesis and characterization of organo-soluble thioether-bridged polyphenylquinoxalines with ultra-high refractive indices and low birefringences". Polymer. 51 (17): 3851. doi:10.1016/j.polymer.2010.06.035.
↑ Kwansoo Han; Woo-Hyuk Jang; Tae Hyung Rhee (2000). "Synthesis of fluorinated polyimides and their application to passive optical waveguides". J. Appl. Polym. Sci. 77 (10): 2172. doi:10.1002/1097-4628(20000906)77:10<2172::AID-APP10>3.0.CO;2-9.
↑ Naoki Sadayori and Yuji Hotta "Polycarbodiimide having high index of refraction and production method thereof" US patent 2004/0158021 A1 (2004)
1 2 3 4 5 Ryota Seto; Takahiro Kojima; Katsumoto Hosokawa; Yasuhito Koyama; Gen-ichi Konishi; Toshikazu Takata (2010). "Synthesis and property of 9,9-spirobifluorene-containing aromatic polyesters as optical polymers with high refractive index and low birefringence". Polymer. 51 (21): 4744. doi:10.1016/j.polymer.2010.08.032.
↑ Tatsuhito Matsuda; Yasuaki Funae; Masahiro Yoshida; Tetsuya Yamamoto; Tsuguo Takaya (2000). "Optical material of high refractive index resin composed of sulfur-containing aromatic methacrylates". J. Appl. Polym. Science. 76: 50. doi:10.1002/(SICI)1097-4628(20000404)76:1<50::AID-APP7>3.0.CO;2-X.
↑ P. Nolan; M. Tillin; D. Coates (1993). "High on-state clarity polymer dispersed liquid crystal films". Liquid Crystals. 14 (2): 339. doi:10.1080/02678299308027648.
1 2 3 Macdonald, Emily K.; Lacey, Joseph C.; Ogura, Ichiro; Shaver, Michael P. (2017-02-01). "Aromatic polyphosphonates as high refractive index polymers". European Polymer Journal. 87: 14–23. doi:10.1016/j.eurpolymj.2016.12.003.
↑ Jin-gang Liu; Yasuhiro Nakamura; Yuji Shibasaki; Shinji Ando; Mitsuru Ueda (2007). "High refractive index polyimides derived from 2,7-Bis(4-aminophenylenesulfanyl)thianthrene and aromatic dianhydrides". Macromolecules. 40 (13): 4614. Bibcode:2007MaMol..40.4614L. doi:10.1021/ma070706e.
↑ Jin-Gang Liu; Yasuhiro Nakamura; Yuji Shibasaki; Shinji Ando; 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 (23): 5606. Bibcode:2007JPoSA..45.5606L. doi:10.1002/pola.22308.
↑ Nam-Ho You; Yasuo Suzuki; Daisuke Yorifuji; Shinji Ando; Mitsuru Ueda (2008). "Synthesis of high refractive index polyimides derived from 1,6-Bis(p-aminophenylsulfanyl)-3,4,8,9-tetrahydro-2,5,7,10-tetrathiaanthracene and aromatic dianhydrides". Macromolecules. 41 (17): 6361. Bibcode:2008MaMol..41.6361Y. doi:10.1021/ma800982x.
↑ Russell A. Gaudiana, Richard A. Minns and Howard G. Rogers "High refractive index polymers" U.S. patent 5,132,430 (1992)
↑ Emma Goosey (2006). "Brominated flame retardants: their potential impacts and routes into the environment". Circuit World. 32 (4): 32–35. doi:10.1108/03056120610683603.
↑ Michael Olshavsky; Harry R. Allcock (1997). "Polyphosphazenes with high refractive indices: Optical dispersion and molar refractivity". Macromolecules. 30 (14): 4179. Bibcode:1997MaMol..30.4179O. doi:10.1021/ma961628q.
↑ Toshiki Fushimi; Harry R. Allcock (2009). "Cyclotriphosphazenes with sulfur-containing side groups: refractive index and optical dispersion". Dalton Trans. (14): 2477–81. doi:10.1039/B819826H. PMID 19319392.
↑ H. K. Shobha; H. Johnson; M. Sankarapandian; Y. S. Kim; P. Rangarajan; D. G. Baird; J. E. McGrath (2001). "Synthesis of high refractive-index melt-stable aromatic polyphosphonates". J. Polym. Sci., Part A: Polym. Chem. 39 (17): 2904. Bibcode:2001JPoSA..39.2904S. doi: 10.1002/pola.1270 .
↑ US 20160289391,Jung, Hoe-Chul; Shaver, Michael Patrick& Macdonald, Emily Kate,"Polyphosphonate, and lens and camera module including the same",published 2016-10-06
↑ Ian Manners (2002). "Polyferrocenylsilanes: metallopolymers for electronic and photonic applications". J. Opt. Soc. Am. A. 4 (6): S221–S223. Bibcode:2002JOptA...4S.221M. doi:10.1088/1464-4258/4/6/356. S2CID 122033801.
↑ Bellas, Vasilios; Rehahn, Matthias (2007). "Polyferrocenylsilane-Based Polymer Systems". Angewandte Chemie International Edition. 46 (27): 5082–104. doi:10.1002/anie.200604420. PMID 17587204.
↑ Lorenz Zimmermann; Martin Weibel; Walter Caseri; Ulrich W. Suter; Paul Walther (1993). "Polymer nanocomposites with "ultralow" refractive index". Polym. Adv. Tech. 4: 1–7. doi:10.1002/pat.1993.220040101.
↑ H. Althues; J. Henle; S. Kaskel (2007). "Functional inorganic nanofillers for transparent polymers". Chem. Soc. Rev. 36 (49): 1454–65. doi:10.1039/b608177k. PMID 17660878.
↑ Akhmad Herman Yuwono; Binghai Liu; Junmin Xue; John Wang; Hendry Izaac Elim; Wei Ji; Ying Li; Timothy John White (2004). "Controlling the crystallinity and nonlinear optical properties of transparent TiO₂–PMMA nanohybrids". J. Mater. Chem. 14 (20): 2978. doi:10.1039/b403530e.
↑ Naoaki Suzuki; Yasuo Tomita; Kentaroh Ohmori; Motohiko Hidaka; Katsumi Chikama (2006). "Highly transparent ZrO₂ nanoparticle-dispersed acrylate photopolymers for volume holographic recording". Opt. Express. 14 (26): 12712–9. Bibcode:2006OExpr..1412712S. doi: 10.1364/OE.14.012712 . PMID 19532163.
↑ Fotios Papadimitrakopoulos; Peter Wisniecki; Dorab E. Bhagwagar (1997). "Mechanically attrited silicon for high refractive index nanocomposites". Chem. Mater. 9 (12): 2928. doi:10.1021/cm970278z.
↑ Changli Lü; Zhanchen Cui; Zuo Li; Bai Yang; Jiacong Shen (2003). "High refractive index thin films of ZnS/polythiourethane nanocomposites". J. Mater. Chem. 13 (3): 526. doi:10.1039/B208850A.
↑ Chih-Ming Chang; Cheng-Liang Chang; Chao-Ching Chang (2006). "Synthesis and optical properties of soluble polyimide/titania hybrid thin films". Macromol. Mater. Eng. 291 (12): 1521. doi:10.1002/mame.200600244.
↑ Frank W. Mont; Jong Kyu Kim; Martin F. Schubert; E. Fred Schubert; Richard W. Siegel (2008). "High-refractive-index TiO₂-nanoparticle-loaded encapsulants for light-emitting diodes". J. Appl. Phys. 103 (8): 083120–083120–6. Bibcode:2008JAP...103h3120M. doi:10.1063/1.2903484.