High refractive index polymers
Encyclopedia
A high-refractive-index polymer (HRIP) is a polymer
that has a refractive index
greater than 1.50.
Such materials are required for anti-reflective coating
and photonic devices such as light emitting diodes (LEDs) and image sensor
s. The refractive index of a polymer is based on several factors which include polarizability
, chain flexibility, molecular geometry
and the polymer backbone orientation.
As of 2004, the highest refractive index for a polymer was 1.76. Substituents with high molar fractions or high-n nanoparticle
s in a polymer matrix have been introduced to increase the refractive index in polymers.
. A high refractive index material will generally have a small Abbe number, or a high optical dispersion. 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.
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.
, initial decomposition temperature, degradation temperature and the melting
temperature range. 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.
, methanol
, hexanes, acetone
and toluene
.
. This synthesis was first succeeded with polyimidothiethers, resulting in optically transparent polymers with high refractive index. Polycondensation reactions are also common to make high refractive index polymers, such as polyesters.
-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. Polymers with sulfur-rich thianthrene and tetrathiaanthrene moieties exhibit n values above 1.72, depending on the degree of molecular packing.
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. 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.
Phosphorus
-containing groups, such as phosphonate
s and phosphazene
s, often exhibit high molar refractivity and optical transmittance
in the visible light region. Polyphosphonates have high refractive indices due to the phosphorus
moiety even if they have chemical structures analogous to polycarbonates. In addition, polyphosphonates exhibit good thermal stability and optical transparency; they are also suitable for casting
into plastic lenses.
Organometallic components result in HRIPs with good film
forming ability and relatively low optical dispersion. Polyferrocenylsilanes and polyferrocenes containing phosphorus 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 of their intermediate optical dispersion between organic polymers and inorganic glass
es.
the hybrid technology between inorganic and organic components. The refractive index of a nanocomposite can be estimated as
, 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. 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. 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), ZrO2 (n=2.10), amorphous silicon
(n=4.23), PbS (n=4.20) and ZnS
(n=2.36). 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.
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.
. It is a new technique for circuit manufacturing that uses 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.
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.
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
that has a refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
greater than 1.50.
Such materials are required for anti-reflective coating
Anti-reflective coating
An antireflective or anti-reflection coating is a type of optical coating applied to the surface of lenses and other optical devices to reduce reflection. This improves the efficiency of the system since less light is lost. In complex systems such as a telescope, the reduction in reflections also...
and photonic devices such as light emitting diodes (LEDs) and image sensor
Image sensor
An image sensor is a device that converts an optical image into an electronic signal. It is used mostly in digital cameras and other imaging devices...
s. The refractive index of a polymer is based on several factors which include polarizability
Polarizability
Polarizability is the measure of the change in a molecule's electron distribution in response to an applied electric field, which can also be induced by electric interactions with solvents or ionic reagents. It is a property of matter...
, chain flexibility, molecular geometry
Molecular geometry
Molecular geometry or molecular structure is the three-dimensional arrangement of the atoms that constitute a molecule. It determines several properties of a substance including its reactivity, polarity, phase of matter, color, magnetism, and biological activity.- Molecular geometry determination...
and the polymer backbone orientation.
As of 2004, the highest refractive index for a polymer was 1.76. Substituents with high molar fractions or high-n nanoparticle
Nanoparticle
In nanotechnology, a particle is defined as a small object that behaves as a whole unit in terms of its transport and properties. Particles are further classified according to size : in terms of diameter, coarse particles cover a range between 10,000 and 2,500 nanometers. Fine particles are sized...
s in a polymer matrix have been introduced to increase the refractive index in polymers.
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.Optical properties
Optical dispersion is an important property of an HRIP. It is characterized by the Abbe numberAbbe number
In physics and optics, the Abbe number, also known as the V-number or constringence of a transparent material, is a measure of the material's dispersion in relation to the refractive index...
. A high refractive index material will generally have a small Abbe number, or a high optical dispersion. 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
Monomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
to make the HRIP. Aromatic
Aromaticity
In organic chemistry, Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article by August...
monomers both increase refractive index and decrease the optical anisotropy and thus the birefringence.
Thermal stability
When looking at thermal stability, the typical variables measured include glass transitionGlass transition
The liquid-glass transition is the reversible transition in amorphous materials from a hard and relatively brittle state into a molten or rubber-like state. An amorphous solid that exhibits a glass transition is called a glass...
, initial decomposition temperature, degradation temperature and the melting
Melting
Melting, or fusion, is a physical process that results in the phase change of a substance from a solid to a liquid. The internal energy of a substance is increased, typically by the application of heat or pressure, resulting in a rise of its temperature to the melting point, at which the rigid...
temperature range. The thermal stability can be measured by thermogravimetric analysis
Thermogravimetric analysis
Thermogravimetric analysis or thermal gravimetric analysis is a type of testing performed on samples that determines changes in weight in relation to change in temperature. Such analysis relies on a high degree of precision in three measurements: weight, temperature, and temperature change...
and differential scanning calorimetry
Differential scanning calorimetry
Differential scanning calorimetry or DSC is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference is measured as a function of temperature. Both the sample and reference are maintained at nearly the same temperature...
. 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
Polymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
of the high refractive index polymer. Thus, longer alkyl substituents results in lower thermal stability.
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 dichloromethaneDichloromethane
Dichloromethane is an organic compound with the formula CH2Cl2. This colorless, volatile liquid with a moderately sweet aroma is widely used as a solvent. Although it is not miscible with water, it is miscible with many organic solvents...
, methanol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
, hexanes, acetone
Acetone
Acetone is the organic compound with the formula 2CO, a colorless, mobile, flammable liquid, the simplest example of the ketones.Acetone is miscible with water and serves as an important solvent in its own right, typically as the solvent of choice for cleaning purposes in the laboratory...
and toluene
Toluene
Toluene, formerly known as toluol, is a clear, water-insoluble liquid with the typical smell of paint thinners. It is a mono-substituted benzene derivative, i.e., one in which a single hydrogen atom from the benzene molecule has been replaced by a univalent group, in this case CH3.It is an aromatic...
.
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 used for step-growth polymerizationStep-growth polymerization
Step-growth polymerization refers to a type of polymerization mechanism in which bi-functional or multifunctional monomers react to form first dimers, then trimers, longer oligomers and eventually long chain polymers. Many naturally occurring and some synthetic polymers are produced by step-growth...
. This synthesis was first succeeded with polyimidothiethers, resulting in optically transparent polymers with high refractive index. Polycondensation reactions are also common to make high refractive index polymers, such as polyesters.
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
SulfurSulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
-containing substituents including linear thioether
Thioether
A thioether is a functional group in organosulfur chemistry with the connectivity C-S-C as shown on right. Like many other sulfur-containing compounds, volatile thioethers have foul odors. A thioether is similar to an ether except that it contains a sulfur atom in place of the oxygen...
and sulfone
Sulfone
A sulfone is a chemical compound containing a sulfonyl functional group attached to two carbon atoms. The central hexavalent sulfur atom is double bonded to each of two oxygen atoms and has a single bond to each of two carbon atoms, usually in two separate hydrocarbon substituents.-IUPAC name and...
, cyclic thiophene
Thiophene
Thiophene is a heterocyclic compound with the formula C4H4S. Consisting of a flat five-membered ring, it is aromatic as indicated by its extensive substitution reactions. Related to thiophene are benzothiophene and dibenzothiophene, containing the thiophene ring fused with one and two benzene...
, thiadiazole and thianthrene
Thianthrene
Thianthrene is a sulfur-containing heterocyclic chemical compound. It is the sulfur analog of dibenzodioxin . Unlike dioxins, the shape of thianthrene is not planar...
are the most commonly used groups for increasing refractive index of a polymer. Polymers with sulfur-rich thianthrene and tetrathiaanthrene moieties exhibit n values above 1.72, depending on the degree of molecular packing.
Halogen
Halogen
The halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...
elements, especially bromine
Bromine
Bromine ") is a chemical element with the symbol Br, an atomic number of 35, and an atomic mass of 79.904. It is in the halogen element group. The element was isolated independently by two chemists, Carl Jacob Löwig and Antoine Jerome Balard, in 1825–1826...
and iodine
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
, were the earliest components used for developing HRIPs. In 1992, Gaudiana et al. reported a series of polymethylacrylate
Polymethylacrylate
Polymethylacrylate is a synthetic polymer used to absorb water in diapers, pads, etc. It can absorb up to 400 times its own mass.Polyalkylmethylacrylate is a synthetic polymer used to improve the high temperature viscosity of industrial lubricants....
compounds containing lateral brominated and iodinated carbazole
Carbazole
Carbazole is an aromatic heterocyclic organic compound. It has a tricyclic structure, consisting of two six-membered benzene ring fused on either side of a five-membered nitrogen-containing ring...
rings. They had refractive indices of 1.67–1.77 depending on the components and numbers of the halogen substituents. However, recent applications of halogen elements in microelectronics have been severely limited by the WEEE directive and RoHS legislation adopted by the European Union
European Union
The European Union is an economic and political union of 27 independent member states which are located primarily in Europe. The EU traces its origins from the European Coal and Steel Community and the European Economic Community , formed by six countries in 1958...
to reduce potential pollution of the environment.
Phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
-containing groups, such as phosphonate
Phosphonate
Phosphonates or phosphonic acids are organic compounds containing C-PO2 or C-PO2 groups . Bisphosphonates were first synthesized in 1897 by Von Baeyer and Hofmann. An example of such a bisphosphonate is HEDP . Since the work of Schwarzenbach in 1949, phosphonic acids are known as effective...
s and phosphazene
Phosphazene
Phosphazenes are a class of chemical compounds in which a phosphorus atom is covalently linked to a nitrogen atom by a double bond and to three other atoms or radicals by single bonds. While other substitutions produce relatively persistent compounds, in organic synthesis the term largely refers to...
s, often exhibit high molar refractivity and optical transmittance
Transmittance
In optics and spectroscopy, transmittance is the fraction of incident light at a specified wavelength that passes through a sample. A related term is absorptance, or absorption factor, which is the fraction of radiation absorbed by a sample at a specified wavelength...
in the visible light region. Polyphosphonates have high refractive indices due to the phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
moiety even if they have chemical structures analogous to polycarbonates. In addition, polyphosphonates exhibit good thermal stability and optical transparency; they are also suitable for casting
Casting
In metalworking, casting involves pouring liquid metal into a mold, which contains a hollow cavity of the desired shape, and then allowing it to cool and solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process...
into plastic lenses.
Organometallic components result in HRIPs with good film
Film
A film, also called a movie or motion picture, is a series of still or moving images. It is produced by recording photographic images with cameras, or by creating images using animation techniques or visual effects...
forming ability and relatively low optical dispersion. Polyferrocenylsilanes and polyferrocenes containing phosphorus 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 of their intermediate optical dispersion between organic polymers and inorganic glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
es.
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 andthe hybrid technology between inorganic and organic components. The refractive index of a nanocomposite can be estimated as
, 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. 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 scatteringRayleigh scattering
Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...
of the nanocomposite, the diameter of the nanoparticle should be below 25 nm. 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
Anatase
Anatase is one of the three mineral forms of titanium dioxide, the other two being brookite and rutile. It is always found as small, isolated and sharply developed crystals, and like rutile, a more commonly occurring modification of titanium dioxide, it crystallizes in the tetragonal system; but,...
, n=2.45; rutile
Rutile
Rutile is a mineral composed primarily of titanium dioxide, TiO2.Rutile is the most common natural form of TiO2. Two rarer polymorphs of TiO2 are known:...
, n=2.70), ZrO2 (n=2.10), amorphous silicon
Amorphous silicon
Amorphous silicon is the non-crystalline allotropic form of silicon. It can be deposited in thin films at low temperatures onto a variety of substrates, offering some unique capabilities for a variety of electronics.-Description:...
(n=4.23), PbS (n=4.20) and ZnS
ZNS
ZNS or ZnS may refer to:In entertainment:* ZNS , a popular entertainer in China and Chinese speaking parts of the world...
(n=2.36). 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.
Image sensors
A microlensMicrolens
A microlens is a small lens, generally with a diameter less than a millimetre and often as small as 10 micrometres . The small sizes of the lenses means that a simple design can give good optical quality but sometimes unwanted effects arise due to optical diffraction at the small features...
array is a key component of optoelectronics, optical communications, CMOS
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
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.
Lithography
Another application of HRIPs is in immersion lithographyImmersion lithography
Immersion lithography is a photolithography resolution enhancement technique for manufacturing integrated circuits that replaces the usual air gap between the final lens and the wafer surface with a liquid medium that has a refractive index greater than one. The resolution is increased by a factor...
. It is a new technique for circuit manufacturing that uses 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.
LEDs
Gan
Gan may refer to:-Computing and telecommunications:*.gan, the file extension for documents created by GanttProject*Generic Access Network formerly known as Unlicensed Mobile Access *Global Area Network- Mythology :...
, n=2.5) and the organic encapsulant (epoxy
Epoxy
Epoxy, also known as polyepoxide, is a thermosetting polymer formed from reaction of an epoxide "resin" with polyamine "hardener". Epoxy has a wide range of applications, including fiber-reinforced plastic materials and general purpose adhesives....
or silicone, n=1.5). Higher light outputs can be achieved by using an HRIP as the encapsulant.
See also
- Refractive indexRefractive indexIn optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
- RefractometerRefractometerA refractometer is a laboratory or field device for the measurement of an index of refraction . The index of refraction is calculated from Snell's law and can be calculated from the composition of the material using the Gladstone-Dale relation....
- Abbe numberAbbe numberIn physics and optics, the Abbe number, also known as the V-number or constringence of a transparent material, is a measure of the material's dispersion in relation to the refractive index...
- OptoelectronicsOptoelectronicsOptoelectronics is the study and application of electronic devices that source, detect and control light, usually considered a sub-field of photonics. In this context, light often includes invisible forms of radiation such as gamma rays, X-rays, ultraviolet and infrared, in addition to visible light...
- PolarizabilityPolarizabilityPolarizability is the measure of the change in a molecule's electron distribution in response to an applied electric field, which can also be induced by electric interactions with solvents or ionic reagents. It is a property of matter...
- BirefringenceBirefringenceBirefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...
- Lorentz-Lorenz equation
- DispersionDispersionDispersion may refer to:In physics:*The dependence of wave velocity on frequency or wavelength:**Dispersion , for light waves**Dispersion **Acoustic dispersion, for sound waves...
- Optical anisotropy
- NanocompositeNanocompositeA nanocomposite is as a multiphase solid material where one of the phases has one, two or three dimensions of less than 100 nanometers , or structures having nano-scale repeat distances between the different phases that make up the material...
- Image sensorImage sensorAn image sensor is a device that converts an optical image into an electronic signal. It is used mostly in digital cameras and other imaging devices...
- Immersion lithographyImmersion lithographyImmersion lithography is a photolithography resolution enhancement technique for manufacturing integrated circuits that replaces the usual air gap between the final lens and the wafer surface with a liquid medium that has a refractive index greater than one. The resolution is increased by a factor...
- Organic light emitting diode (OLED)