Recent Progress in Sulfur-Containing High Refractive Index Polymers for Optical Applications

Abstract

The development in the field of high refractive index materials is a crucial factor for the advancement of optical devices with advanced features such as image sensors, optical data storage, antireflective coatings, light-emitting diodes, and nanoimprinting. Sulfur plays an important role in high refractive index applications owing to its high molar refraction compared to carbon. Sulfur exists in multiple oxidation states and can exhibit various stable functional groups. Over the last few decades, sulfur-containing polymers have attracted much attention owing to their wide array of applications governed by the functional group of sulfur present in the polymer repeat unit. The interplay of refractive index and various other polymer properties contributes to successfully implementing a specific polymer material in optical applications. The focus on developing optoelectronic devices induced an ever-increasing need to integrate different functional materials to achieve the devices’ full potential. Several devices that see the potential use of sulfur in high refractive index materials are reviewed in the study. Like sulfur, selenium also exhibits high molar refraction and unique chemical properties, making it an essential field of study. This review covers the research and development in the field of sulfur and selenium in different forms of functionality, focusing on the chemistry of bonding and the optical properties of the polymers containing the heteroatoms mentioned above. The strategy and rationale behind incorporating heteroatoms in a polymer matrix to produce high-refractive-index materials are also described in the present review.

1. Introduction

Sulfur-containing polymers have been extensively studied for several decades. This important class of polymers has attracted the attention of researchers since the mid-1990s.^1,2 The more recent studies have focused on the incorporation of sulfur into the polymer backbone to enhance specific polymer properties such as thermal resistance, mechanical strength, processability, solubility, and flame resistance of polymers. With the discovery of conducting polymers in the early 2000s, the field of polymer science has garnered enough attention from the scientific world. Sulfur allows for an extended conjugated electron system when incorporated in a polymer chain or attached to a side group. This feature opens up the possibility of fabricating devices with sustainable and efficient display technology. Figure 1 shows the number of publications on this topic in the past few decades.

Figure 1.

Scientific publications with “sulfur-containing polymers” as a keyword from 1970 until date (Web of Science).

Sulfur-containing polymers have found uses in a wide range of applications, e.g., as cathode materials in lithium-ion batteries,³⁻⁷ memory devices,^8,9 conducting polymers,¹⁰⁻¹³ and optical materials.¹⁴⁻¹⁸ Furthermore, sulfur-containing polymers with high industrial importance are polysulfones and polysulfides.¹⁹⁻²¹ Sulfur-containing polythiophene films were recently demonstrated with electrochromism, chemochromism, or sensory characteristics.²²⁻²⁴ The combination of conjugation and high molar refraction of sulfur gives polythiophenes superiority in terms of RI. Poly(3-alkythiophenes) (P3AT) provide immense scope in organic field effect transistors (OFETs) and photovoltaic devices by virtue of high field effect mobilities. Polarized elecctroluminisence has been boosted because of the strong anisotropic structure of the conjugated polyalkylthiophenes leading to ease of processability.²⁵⁻²⁷ Along with the strong in-plane and out-of-plane orientations of polythiophenes, studies show that, in P3AT structures, the alkyl side-chain length,²⁸ solvent,²⁹ casting method,³⁰ and thermal history³¹ play a vital role in field effect mobility. The numerous applications of sulfur-containing polymers are so immense that it is not possible to summarize them under one topic. An illustration of the various application fields of sulfur-containing polymers is given in Figure 2.

Figure 2.

Sulfur-containing polymers in various applications.

For achieving optical devices with advanced features in recent times, polymer materials have attracted great attention, owing to their diverse applicability along with comparatively low installation cost. As compared to their inorganic counterparts, organic polymers have the added advantage of being lightweight, possessing wide area coverage, and being easy to process into thin films.³²⁻³⁴ Polymers with high refractive index (n) have a wide range of applications, such as optical elements made by nanoimprinting, substrates with high performance in display technologies,³⁵ optical coatings in organic light emitting diodes (OLED),^36,37 antireflective coatings for lenses,^38,39 microlens modules for image sensors, and immersion fluids and resists in 193 nm immersion lithography technologies.^40,41 The relevant applications are described in Section 1.2. This review elaborates on various sulfur-containing polymers, their different synthetic procedures, and the types of incorporation of various sulfur moieties in the polymer backbone with a special focus on the optical properties of the sulfur-containing polymers.

1.1. Designing a High Refractive Index Material

The applicability of a particular polymer material in an optical device depends on the combination of certain factors such as the inherent refractive index of the repeat unit in a polymer, the wavelength dispersion of the material, denoted by the Abbe number, and the effect of the orientation of the polymer repeat units, i.e., birefringence and transparency.

1.1.1. Refractive Index, n

The refractive index of a polymer can be calculated by the Lorentz–Lorenz equation (eq 1) as given below

1

where n is the RI, ρ is the density, N_A is the Avogadro number, M_w is the molecular weight, α is the linear molecular polarizability, [R] is the molar refraction, and V₀ is the molecular volume of the polymer repeat unit.

Solving the above equation gives eq 2

2

Thus, we see that the n values of a polymer increase with increasing substituents having high molar refractions and low molar volumes. Table 1 lists the molar refraction values of some of the most frequently used groups.

Table 1. Molar Refraction Values of Some Common Groups.

Atom, bond	[R]	Atom, bond	[R]
—H	1.100	(C)—N=(C)	4.10
—Cl	5.967	N—N=(C)	3.46
(—C=O)—Cl	6.336	>C<	2.418
S—Br	8.865	—CH2—	4.711
—I	13.900	—CN	5.415
—O—(H)	1.525	—NC	6.136
—O—	1.643	C=C	1.733
=O	2.211	C≡C	2.336
—O—O—	4.035	—C=S—	7.97
(C)—S(II)—(C)	7.80	—S=S—	8.11
(C)—S(IV)—(C)	6.98	Se	11.17
Phenyl	25.463	Naphthyl	43.00

It can be seen from Table 1 that the molar refraction, [R], increases with the incorporation of sulfur as well as selenium and aromatic rings.

1.1.2. Abbe Number, ν_D

The change in the refractive index of a material with the wavelength can be approximately measured using the parameter called the Abbe number (ν_D). In the visible spectrum, this property of the material’s dispersion can be expressed as ν_D = (n_D – 1)/(n_F – n_C), where n_D, n_F, and n_C denote the refractive indices of the material at 589, 486, and 656 nm.⁴² The Abbe number can also be expressed in terms of molar refraction [R] as given in eq 3

3

where [ΔR] is the molar dispersion. From the above equation, we can infer that, with an increase in the Abbe number, there is a decrease in the dispersion of the refractive index. The chromatic dispersion is illustrated in Figure 3.

Figure 3.

Illustration of the chromatic dispersion of light.

Sultanova et al. published a report on the dispersion properties of different optical polymers and optical glass, explaining that any material’s dispersion is specific to the visible or the NIR (near-infrared) region.⁴³ From eq 3, it is seen that there is a general trade-off between the n_D and ν_D values for conventional optical polymers. Therefore, it is critical to design polymers with high n_D and ν_D values because, conventionally, materials with high n show low ν values.⁴⁴ You et al. reported on the synthesis of a series of poly(thioester)s with sulfide and aliphatic rings in the main polymer backbone. The careful design of the polymer repeat unit led to controlling the Abbe number of the polymers.⁴⁵

1.1.3. Birefringence, Δn

A birefringent material is capable of splitting an incident light ray into two with different velocities, giving rise to two different refractive indices, namely, in-plane RI and out-of-plane RI. Birefringence is an effect of preferential molecular orientation in the case of polymers having asymmetric structure and often arises due to processing history including injection molding or extrusion. Birefringence increases with increasing combination of components with a high inherent polarizability, morphology with rod-like structure, and high orientation.⁴⁶ Thus, it is evident that birefringence cannot be decreased if the polymeric chains are not perfectly isotropic. A birefringent medium often causes the rays emerging out of it to have a phase difference, that degrades the performance of an optical device.^47,48 Birefringence is expressed as, Δn = n_∥ – n_⊥, where n_∥ and n_⊥ denote the in-plane and out-of-plane refractive indices (Figure 4). A recent work reports on the synthesis of vapor-deposited polymer films with an ultrahigh refractive index of 1.97 and Δn < 0.0010, which demonstrates the importance of anisotropy in structure in lowering the birefringence.⁴⁹

Figure 4.

Illustration of birefringence in a medium.

1.1.4. Optical Transparency, %T

For a polymer to be successfully applied in optical devices, it needs, along with the above-mentioned features, also to be transparent in the wavelength range of ∼400–800 nm. If the polymer itself imparts color in the visible, it might deteriorate the device’s performance. Films of aromatic polymers often exhibit color because of the formation of charge-transfer complexes (CTCs), as a result of linearly stacked polymer chains. Any effect that disturbs the formation of CTCs contributes to the lowering of the absorption coefficient of the material. The introduction of branching often results in increasing the optical transparency, by lowering the regularity of polymer chains.⁵⁰

1.2. Applications of HRI Polymers

1.2.1. Nanoimprinted Optical Circuits

Nanoimprint lithography (NIL) technology has made massive advancements since first being reported by Chou et al.^51,52 The fabrication of semiconductor integrated circuits has been revolutionized by NIL. A typical procedure for NIL includes a polymerizable monomer over a suitable substrate, which is then pressed using a structured mold usually made of SiO₂, followed by imprinting and then removal of the mold. Figure 5 shows a schematic of the working principle of the NIL technology.

Figure 5.

Schematic of NIL.

In this context, high refractive index polymers are essentially important to achieve compact photonic circuits.⁵³ Tang et al. reported on an episulfide-thiol-based polymer with an RI of 1.707 at 590 nm. The intermolecular chain packing was suppressed as a result of the aliphatic and thioether moieties in the polymer structure, leading to a very high transparency of the episulfide-thiol-based polymer. The structure of the episulfide monomer and the cross-linked polymer is shown in Figure 6. Furthermore, there was only 2% shrinkage induced on cross-linking of the episulfide-thiol resin, which secures pattern fidelity during imprinting.^54,55 Nakabayashi et al. synthesized a set of three sulfur-containing polymers via thiol–ene click chemistry. The UV–NIL processability of the prepared photopolymerizable monomer mixture was demonstrated. Here the flexible fluorene moiety was employed in the structure of the polymer repeat unit, resulting in high transparency and low birefringence of the polymer films. However, the refractive indices were in the range of 1.60–1.64 with good thermal stability.⁵⁶ The structures of the polymers and their respective RIs are listed in Figure 7.

Figure 6.

Schematic of the formation of thiol-containing cross-linked polymer.

Figure 7.

Structure of polythioethers prepared using thiol–ene click chemistry. Adapted with permission from ref (56). Copyright 2018 John Wiley and Sons.

1.2.2. OLEDs

A high refractive index (RI) polymer finds use as an encapsulant and specific outcoupling layer for OLEDs to increase the external outcoupling efficiency of the device. An operational OLED consists of various layers each having different refractive indices as shown in Figure 8. The photons generated in the emissive layer travel through all the layers before coming out of the device, giving us the perception of light.³⁵ OLEDs are semiconducting light sources that emit light when activated by an electric current. OLEDs are mostly fabricated on glass or plastic substrates. They comprise several organic layers sandwiched between two electrodes that have similar work functions for the injection of electrons and holes. Modern sophisticated OLEDs generally include the following layers.⁵⁷

• Hole injection layer (HIL)- For injection of holes from the anode.

• Hole transportation layer (HTL)- For transportation of holes from the HIL to the EML.

• Emission layer (EML)- For transportation of electrons/holes and their recombination to form excitons, resulting in light emission.

• Electron transportation layer (ETL)- For injection of electrons from the cathode and their transportation.

• Electron injection layer (EIL)- For injection of electrons from the cathode.

Figure 8.

Schematic representation of light extraction loss.

To improve the outcoupling efficiency of the OLEDs, external as well as internal light outcoupling layers have been developed to be introduced in the device. Specifically, in the device, these layers are fabricated between the transparent anode and the substrate. Research on several outcoupling layers has resulted in the development of commercial external layers, which can be deposited on the substrate with much efficiency and are devoid of any degradation to the performance of the device. The internal light outcoupling layer poses certain disadvantages as it can cause the path of movement of current to change. Yet, internal light extraction layers with protruding surfaces are more useful to avoid the propagation of light laterally in the wave-guided mode. At the same time, these kinds of featured protruding surfaces may deteriorate device performance and shorten the lifetime as a result of accelerated aging. Bocksrocker et al. performed the fabrication of white OLED (WOLED) over a single layer consisting of SiO₂ microspheres.⁵⁸ Koo et al. exploited the presence of defects in hexagonal close-packed spheres of SiO₂ and were successful in showing that the defects serve well as internal light out-coupling layers.⁵⁹

A high n and high ν_D polymeric material having good optical transparency and high thermal stability is extremely desirable for OLED applications. With the advent of procedures to control the polymer backbone, it has been made possible to synthesize such high-performance polymeric materials which also show good optical properties. Several polymer groups such as poly(arylene ether)s, polyimides, polyamides, and polytriazoles fall in the category of high-performance polymers by their excellent thermal and thermos oxidative stability and chemical resistance. The introduction of heteroatoms, such as fluorine, to the polymer backbone has been known to decrease the refractive index of the material.⁶⁰⁻⁶³ The addition of sulfur and selenium to the polymer backbone or as a pendant moiety, on the other hand, resulted in the increase of the refractive index (n) by many folds. Several groups have been motivated to prepare novel polymers containing sulfur and selenium groups to optimize the refractive indices and Abbe number of the device.^{18,54,64−66} However, transparency of the materials over the visible range is an integral part of applications in optical devices, which is still a challenge to be obtained, owing to the presence of π-conjugation in most structures.

1.2.3. Image Sensors

High refractive index polymers find special applications in the microlens for complementary metal-oxide-semiconductor (CMOS) image sensors. The main purpose of an image sensor is to produce electrical charge from photons. A CMOS converts charge into voltage directly in the pixels. The advancement in the display technology of smartphones is directly related to the development of pixel technology. In the past few years, there has been an immense decrease in pixel size, leading to more picture clarity in modern devices. Microlenses require polymers due to their superiority in being lightweight and their ease of fabrication. Polymers with a RI of >1.8, having a thickness in the range of a few micrometers, are most suitable for applications in microlenses. Tomikawa et al. designed a photosensitive polyimide with an RI of 1.78 at 633 nm. The high RI was attributed to the addition of an inherent HRI nanosol to the pristine polyimide. In general, pristine sulfur-containing polymers exhibit a RI of <1.7; however, by incorporation of nanoparticles in the polymer matrix, the RI can be further increased. The effect of nanoparticle addition in a polymer matrix is discussed in detail in Section 5.

1.2.4. Antireflective (AR) Coatings

Advanced optical and optoelectronic devices require reduced reflectance, as well as reduced glare. Antireflective coatings on top of glass substrates result in increased transmittance and reduced glare.⁶⁷ In typical modern-day liquid crystal display (LCD) technologies, polymers such as poly(ethylene terephthalate) (PET) and triacetyl cellulose (TAC) are used as the protective layer. However, the difference in RI of these polymers and air is high due to the fact that a huge portion of the light falling in the polymer/air interface gets reflected into the device, reducing the efficiency. A stack of polymer thin films with gradually decreasing RI works best as an AR coating, which can be fabricated over the polymer protective layer to decrease the RI difference compared to air. Optical transparency is another vital property for the coatings to be applied as AR coatings. Several groups have reported on the synthesis of transparent polymers with high RI that are potential candidates for AR applications.^68,69 Zhang et al. reported two high RI photopolymers containing diphenyl sulfide in the polymer chain (Figure 9). These photopolymers exhibited excellent transmittance of >95% above 400 nm and RI of >1.63, which was tunable concerning the structure. The additional hydrogen bonding resulted in an increased RI in AOI-UV as compared to EA-UV. Furthermore, these photopolymers showed only 1% weight loss above 200 °C, enabling the use of nanoimprinting for the formation of a microstructured prismatic grating film as well as nanoimprinted optical devices.⁷⁰

Figure 9.

Structures and schemes of sulfur-containing photopolymers.

2. General Synthetic Procedures

The basic mechanisms in which polymers are built up are classified as chain-growth and step-growth polymerization according to modern-day terminology. The main difference between chain and step-growth polymerizations lies in (i) the species that react to one another and (ii) the development of polymer molecular weight with the extent of conversion.⁷¹ In chain-growth polymerization, a reactive species reacts with a large number of monomers, generating a new active site. Successive addition of the reactive site to a large number of monomer molecules results in polymerization. In conventional free radical polymerization, the molar mass of the polymer chain rapidly increases at lower conversion and levels at higher conversion. In a controlled type of chain growth, molar mass increases linearly with conversion. Depending on the type of reaction, the reactive species could either be anionic, cationic, or a free radical.⁷²

Step-growth polymerizations advance via stepwise reactions to form a dimer, trimer, tetramer, and so on, which continue due to the reactions between the different functional groups in the reactants. Step-growth polymerization is the most studied pathway for the preparation of sulfur-containing high RI polymers. Several polysulfones, polyethers, polythioethers, and polyimides are preferably synthesized via step polymerizations following reactions of functional groups such as dithiols, dihalides, and diamines.^65,73−77 The different polymers are reviewed in the following sections.

Inverse vulcanization is a unique and comparatively recently studied method for the preparation of high-resolution (RI) polymers having sulfur in their backbones. Pyun et al. reported on the exploitation of dynamic S–S bonds which are responsive to heat, light, and mechanical forces. Chalker and co-workers contributed an outlook citing the various polymers synthesized using inverse vulcanization.⁷⁸ Polymeric samples with high transparency and tunable RI to sulfur content were reported via two-step reactions (Figure 10). First, ring-opening polymerization of elemental sulfur was performed, resulting in molten polysulfide, which was then copolymerized with 1,3-diisopropenylbenzene (DIB). The monomer feed ratio played an important part in controlling the RI, wherein the copolymer with 50 and 80% sulfur content showed an RI of 1.765 and 1.865 at 633 nm, respectively.⁷⁹⁻⁸¹

Figure 10.

Synthesis scheme for the copolymerization of elemental sulfur with DIB following inverse vulcanization. The RI data is reported as the average of values for light polarized parallel and perpendicular to the film surface as reported in Griebel et al.⁸¹ Adapted with permission from ref (81). Copyright 2014 John Wiley and Sons.

An analogous thiophene-containing monomer, 2,5-diisopropenylthiophene (DIT), was reported by Tavella et al., as a comonomer for inverse vulcanization of sulfur.⁸² Copolymers prepared using DIT exhibited RI values as high as 1.93 and 1.84 at 375 and 780 nm. The effect of various reaction parameters, such as temperature, base, and phase transfer catalyst, on the reaction yield was also studied, indicating higher reaction yield at stronger basic medium.

3. Polymers Containing Sulfur in Their Backbone

3.1. Sulfur-Containing Polyethers

Poly(arylene ether sulfide), poly(p-phenylene sulfide) (PPS), and poly(arylene sulfide sulfone)s together form the class of poly(arylene sulfide)s. The polymer backbone of this class mainly consists of multiple phenylene groups linked with sulfide bonds, which makes them very good choices for applications in optical devices, with high refractive indices, and good mechanical and thermal properties. The study of thermoplastics containing sulfur, to be utilized as optical materials, is a comparatively new field of research owing to the complexity and challenges of their chemical synthetic procedures. An extensive amount of work has been done in the field of poly(arylene ether)s having ether linkages (−O−) that are flexible and have a tendency to form isotropic structures.^83,84 Much attention has also been given to sulfonated poly(arylene ether)s to study the effect of sulfonation on the physical and chemical features of the polymers.⁸⁵⁻⁹³ Furthermore, the introduction of sulfide linkages (−S−) to the main polymer chain opens a new horizon for the preparation of high-performance polymers. The effect of introducing sulfide bonds in a polymer backbone on the physicochemical properties of a polymer has been a subject of investigation.^94,95 A series of sulfur-containing poly(arylene thioether)s and poly(arylene ether)s was recently reported by our group.⁷⁴ The polymers were prepared via nucleophilic substitution reactions between bis(3-(trifluoromethyl)phenyl) thiophene-based bisfluoro monomer and commercially available dithiol and bis-hydroxy compounds. The linear polymers exhibited a linear relationship of the refractive index with their respective sulfur content. However, the combined effect of aromatic rings along with the sulfur content on RI was more profound in the case of the hyperbranched poly(thioether)s. A maximum RI of 1.71 was obtained for polymers with the highest sulfur content and aromatic content among the series, and here, the strategy to employ a branching unit to increase the solubility of the highly aromatic polymers was very effective. We calculated the theoretical refractive indices for the linear polymers using the group contribution to molar volumes and polymer densities at 589 nm. Although there is a difference in the predicted and calculated values of RI, which may arise due to several factors, such as packing density, the orientation of the side groups, etc., the trend of the experimental refractive index values (RI_expt) was similar to that of the theoretical values (Figure 11). Figure 12 show the wavelength-dependent refractive index of the polymers in the series, while the structure of the polymers is shown in Figure 13.

Figure 11.

Plot of calculated and experimental RI versus sulfur content of linear polymers. Adapted with permission from ref (74). Copyright 2022 American Chemical Society.

Figure 12.

Wavelength-dependent refractive indices of thin films of (a) linear polymers and (b) hyperbranched polymers. Experimental data were fitted using Cauchy’s equation.

Figure 13.

Structure of linear and hyperbranched poly(arylene thioether)s and poly(arylene ether)s.⁷⁴ Adapted with permission from ref (74). Copyright 2022 American Chemical Society.

The introduction of sulfide linkages to conventional PEEK polymers has been found to increase the refractive index⁹⁶ and processability and led to good mechanical and thermal stability, better electrical insulation, and excellent flame resistance.⁷³ Several poly(arylene ether sulfone)s have been explored by Bottino et al.⁹⁷ Their work focused on the synthetic route to prepare polysulfones with ether and naphthalene linkages in the polymer backbone via condensation polymerization. A series of polyether sulfones were prepared by the nucleophilic substitution reaction between 1,5- and 2,6-bis(4-fluorosulfonyl)naphthalene and different commercially available dihydroxy compounds, as shown in Figures 14 and 15. The polymers were end-capped with fluoro groups. Although the thermal and mechanical characterizations of the polymers have been examined following the content of sulfone and naphthalene moiety in the main chain, a correlation between structure and optical properties is up for discussion. The linear polymers prepared exhibited excellent solubility in several polar aprotic solvents.

Figure 14.

Synthesis of polymer with ethynyl-terminated fluorinated poly(arylene ether) sulfide.⁹⁵ Adapted with permission from ref (95). Copyright 2006 American Chemical Society.

Figure 15.

Synthesis of polymer with an alkyne end group.⁹⁶ Adapted with permission from ref (96). Copyright 2001 American Chemical Society.

You et al. demonstrated the design and synthesis of polymers containing sulfide bonds along with heteroatomic rings containing —C—N=C— bonds, which have high molar refractions, and thus, the final polymer showed a greater refractive index and better optical transparency as well. Linear polysulfides were synthesized by condensation polymerization between a dithiol moiety and a triazine monomer containing sulfur.⁹⁸ Even though the polymer prepared was linear, the flexible sulfur linkages resulted in low birefringence. The poly(arylene sulfide) prepared showed one of the highest refractive indices of 1.744 at 633 nm. A polycondensation reaction using a similar triazine moiety was performed by Fu et al. They modified the structure of the triazine moiety by introducing O— and NH— linkages. The polymers all showed high refractive index, and their birefringence values could be directly correlated with their chemical structure. For example, the polymers where an extensive H-bonding was formed showed a lower birefringence, since H-bonding leads to isotropic structure.⁹⁹ Furthermore, the polymers exhibited excellent solubility in most common solvents. The design and control of the side chain and main chain structures resulted in good solubility, keeping the high refractive index of the polymers unperturbed.

Another polycondensation reaction between dithiol monomers and dichloro pyrimidine units to prepare poly(phenylene thioether)s was carried out by Nakabayashi and the group. They employed comparatively lower temperatures (80–100 °C) to obtain nearly colorless polymers with high molecular weights. A high refractive index of 1.678 at 633 nm of the polymer thin films with 95% transparency at 380 nm was reported. The structure of the polymers was based on thioketal linkage and fluorene moiety (structure below), due to which their polymer backbone has relatively good isotropy which led to these polymers having a low birefringence, the difference in refractive index between in-plane and out-of-plane orientation being less than 0.005.¹⁰⁰ In addition to linear polysulfides, significant work in cyclic polysulfides has been done by Takashima et al.¹⁰¹ It has been well demonstrated that cyclic polymers have different properties than their linear analogues having similar compositions and molecular weights.^102−104 The main reason for this is the nonavailability of an end group and restricted conformational mobility. A facile method to achieve ring expansion polymerizations was established involving heating macro-monomers under concentrated conditions, without the requirement of the addition of catalysts or initiators.¹⁰⁵

Okutsu et al. in 2008 prepared a thermoplastic poly(thioether) sulfone which has a high refractive index as well as a high Abbe number. They started with a monomer having a high sulfur content and an alicyclic ring structure (DSDT). On performing the Michael addition reaction between the prepared monomer and divinyl sulfone (DS) they were successful in obtaining a poly(thioether) sulfone, which showed a refractive index of 1.686 at 589 nm and an Abbe number of 48.6. However, the polymer showed a certain absorption in the visible range due to a compact linear structure¹⁰⁶ (Figure 16).

Figure 16.

Polymerization via Michael addition. Adapted with permission from ref (106). Copyright 2008 American Chemical Society.

In 2012, Suzuki and co-workers prepared two poly(thioether) sulfones via a similar Michael polyaddition reaction between dithiol monomer and monomers containing a divinyl sulfone moiety. They reported the refractive indices of the polymers as 1.6052 and 1.6228 at 589 nm and Abbe numbers of 48.0 and 45.8. The polymers prepared were of high thermal stability and exhibited transparency of 98% at 400 nm.¹⁰⁷

Thiol–ene click chemistry, under synthetic simplicity, is a choice of pathway for preparing polymers with tunable properties, especially in the field of optical applications. Thiol–ene chemistry has been employed to prepare polymers as high refractive index materials,^108,109 optical waveguides,¹¹⁰ and LED-encapsulants.¹¹¹ McClain et al. prepared a series of thiol–ene polymers using a common tetra thiol moiety and different divinyls and tetravinyls.^112,113 The polymers were optically characterized to reveal that they have a transparency of about 75% in the visible to near IR region. Also, the refractive indices were found to be in the ranges of 1.51 and 1.56 at 636 nm. Although the refractive indices were quite low compared to those reported for other similar polymers, they could be nicely correlated with the polarizability of the element involved in the polymer backbone. The polymer containing highly polarizable Sn was found to have the highest refractive index at one particular wavelength.

In another study, they prepared a set of poly(phenylene thioether)s via thiol–ene polyaddition reactions.¹¹⁴ The refractive indices were reported between 1.6553 and 1.6751 at 633 nm, with the birefringence being in the range of 0.0014 and 0.0030. These polymers were essentially structured on a fluorene-based cardo moiety. The presence of bulky groups as the fluorene component at 90° with the polymer chain resulted in a nearly isotropic structure, and thus, the birefringence was low.

3.2. Sulfur-Containing Polyimides

Polyimides constitute high-performance polymers by their superior thermal, chemical, and mechanical stability.^115,116 Polyimides can be utilized in various forms such as fiber, membrane, nanofiber, foam, adhesive, or coating in different applications.¹¹⁷ By their thermal stability and inherent high refractive index, sulfur-containing polyimides are good contenders for applications in optical devices.¹¹⁸ However, the limited solubility and hence difficulties in processing impart a challenge to the researchers. The above hurdles can be overcome by engineering the polymer structure in such a way that causes internal attractions to cease. This can be done by introducing bulky groups in perpendicular orientation to the polymer chain or flexible linkages which results in an effective reduction of molecular packing and can thus increase the solubility of the polymer (Figure 17).^119,120 Also, the transparency of a polymer film usually imposes a challenge for its application in the optical field. However, a lot of investigations in recent times have shown how the control of molecular structure and reaction conditions can be effective in inducing particular characteristics to the polymer specific for its applications.^121−123

Figure 17.

Synthesis of linear (2), hyperbranched (4), and branched (5a–i) poly(ether imide)s,¹¹⁹ with branching improving solubility. Adapted with permission from ref (119). Copyright 2000 American Chemical Society.

In 2007 a new series of polyimides were prepared by Liu et al.; new sulfur-containing diamines were prepared, and these were reacted with various commercially available dianhydrides to obtain high refractive index polyimides that showed low birefringence (Figure 18).¹²⁴

Figure 18.

Synthesis scheme for polyimides with an average refractive index of more than 1.74.¹²⁴ Adapted with permission from ref (124). Copyright 2007 Springer Nature.

It was found in this study that two different polymers having the same sulfur content exhibited varied differences in their respective refractive indices. This observation pointed to the fact that the sulfur content is a deciding factor in the case of refractive index values along with molecular packing. The PI-1 solid structure exhibits a much more densely packed polymeric backbone with restricted chain mobility, whereas the PI-2 molecular structure has a loosely packed structure, resulting in a decreased value of the average refractive index. The refractive indices were in the range of 1.712–1.760 at 633 nm, and the birefringence was as low as 0.007. The synthetic procedure involved preparing a poly(amic acid) initially and then proceeding with thermal imidization. However, the polymer films in all of these cases were slightly colored and had an absorption peak in the visible spectrum near 500 nm. This is mostly due to the characteristic coloration of polyimides, which is attributed to the formation of charge transfer complexes (CTCs). In addition, it could also be due to partial degradation and oxidation of the polymers during the imidization process under a high temperature of 300 °C. However, this can be avoided by carrying out the imidization under an inert atmosphere. The same group attempted the preparation of another series of polyimides with a new diamine-containing thianthrene moiety that was synthesized by a two-step procedure.⁷⁵ However, the isolation of the monomer was difficult, and the yield was less than 25%, due to its difficult solubility in common laboratory solvents. This was mainly because of the rigidity of the chemical structure with two −S– linkages. However, the bent structure of the thianthrene-containing monomer helped to achieve polyimides with a high transparency and very low birefringence. The irregularity in the polymer chain structure resulted in their isotropic nature, and thus, the films prepared were suitable for applications in optical devices.

A series of similar works has been carried out by the group of Liu,¹²⁵ where the synthetic procedures involved preparing a diamine with high sulfur content and reacting them with dianhydrides via polycondensation. The refractive index was more than 1.71, but the polyimides faced the challenge of imparting coloration in the visible region. The contribution of the thiophene and dibenzothiophene moiety on the refractive index was explored, and as expected from the Lorentz–Lorenz equation, the increase in aromatic rings in the polymer repeat unit led to an increase in the refractive index.

The effect of the addition of the thiophene moiety to the molecular structure of the polymer was also explored in another work by Fukuzaki and colleagues, where they used the thiophene moiety to design a diamine with high sulfur content. At 633 nm they reported the refractive index of their polyimide films to be in the range of 1.7228–1.7677 with low birefringence as well. However, the polyimides showed a certain absorption around 500 nm.¹²⁶

Another method of preparing polyimide was introduced by Nakagawa in 2010 when they employed Michael’s addition between two sulfur-containing monomers, one bismaleimide, and a dithiol moiety. The RI was found to be 1.7272 at 632.8 nm. Choosing a polymer backbone with a high sulfur content helped to achieve a refractive index. The problem of color in the visible was also attended to in this work, and a transparency of 87% was achieved above 400 nm.¹²⁷

More recently, many groups used the conventional technique of preparing polyimides; i.e., at the onset, they started with the synthesis of different sulfur-containing diamines and then a two-step polymerization process with dianhydrides via the formation of polyamic acid. The refractive indices have been reported above 1.7 at 633 nm, but the current challenge remains the same, which is to gain transparency in the visible region.^128−130

A more recent work of our group involved the study of the effect of sulfur content and branching on the refractive index of a series of polyimides and polytriazoles. The synthesis of the polyimides progressed via the preparation of poly(amic acid)s and consecutive thermal imidization. The polymers prepared in this series showed a very high refractive index of 1.75 at 589 nm, although the high sulfur content of the polymers imparted a yellowish coloration to the free-standing polymer films. The effect of structural modification on the birefringence of the material was also studied, and it was found that the more irregular structure induced a decrease in birefringence. Figure 19 shows the wavelength-dependent refractive indices of the polyimides (PIs).⁶⁵ In another work, polytriazoles were prepared using Cu(I) assisted click polymerization reactions. It was shown that the introduction of −CF₃ resulted in an increase of transparency by interrupting the formation of change transfer complexes.¹³¹

Figure 19.

Wavelength-dependent average refractive indices of PIs fitted using the Cauchy equation. Adapted with permission from ref (65). Copyright 2022 American Chemical Society.

3.3. Polyvinylsulfides as High Refractive Index Polymers

An earlier work related to the introduction of a dithiane moiety for improving RI by Okubo et al. involved the preparation of poly(vinyl sulfide)s. Over the years of 2014 and 2016, the Voit group made use of hyperbranched polymers to increase the refractive index and also ensure better transparency concerning visible light. A series of linear as well as hyperbranched polymers was prepared by employing the selectivity of thiol radicals to add to the phenylacetylene groups to only one carbon center of the triple bond, i.e., a monoaddition reaction.¹¹⁵ The refractive indices were between 1.682 and 1.756, with the highest Abbe number being 14.3. But the highlight of this work is the aspect of transparency of the material. A gradual study of the UV–visible spectrum for the polyvinylsulfides was performed which showed that, on introduction of branching units, the absorption peak shifts to a lower wavelength region, thus enhancing the transparency of the material. The presence of branching also resulted in good solubility of the prepared polyvinylsulfides,^132−135 and first photonic crystals and Bragg reflectors could be prepared efficiently by those materials.¹³⁶

Yao et al. reported the synthesis of polyvinylsulfides in a catalyst-free thiol–yne click polymerization under mild conditions. It was found that, with an increase in reaction temperature, the molar mass of the polymers produced increased; however, the dispersity also increased with an increase in temperature. An optimized temperature of 30 °C resulted in significantly high molar mass and dispersity values between 1.5 and 2. The effect of higher conjugation and high polarizability of heteroatoms present in the polymer backbone was studied, and the highest RI achieved in this work was 1.771 at 633 nm. Also because of little preferential orientation of the polymer chains, the polyvinylsulfides exhibited little to no birefringence.¹³⁷

In 2016 another set of polyvinylsulfides that were hyperbranched and with increasing the aromatic content using naphthalene acetylenic groups together with dithiolbenzene was prepared by the Voit group.¹³⁸ The synthetic procedure utilized similar selectivity of thiol–yne polyaddition reactions. The polymers differed from each other concerning the ratio of the monomer feed, which ensured different molar ratios of functional groups, thus giving rise to different molecular weights. The highest molecular weight was found to be with the thiol and alkyne moieties in stoichiometric concentrations. The highest refractive index of 1.7839 at 589.3 nm was measured with the Abbe number being 13.1, matching the refractive index of common low molar mass substances (N,N′-di(naphthalene-1-yl)-N,N′-diphenylbenzidine). The polymers also showed good compatibility with the OLED preparation. To test this, the polymers were used in cooperation with the Reineke group as a polymeric outcoupling layer in a cost-efficient spin-coating process in monochrome OLEDs, and the external quantum efficiency was reported to be greater than 20%.¹³⁸

3.4. Sulfur-Containing Polyamides

Polyamides with an aromatic backbone have been found to exhibit excellent thermal and mechanical properties. Zhang et al. in 2010 and 2011 prepared a series of novel polyamide materials via the conventional polycondensation reaction between diacid monomer and diamine monomer. The monomers in each case had a high percentage of sulfur, and the polymers had a significant number of thioether linkages. The refractive indices were found to nicely correlate with the sulfur content of each polymer repeat unit. The polyamide films exhibited refractive indices from 1.699 to 1.716 at 632.8 nm with the lowest birefringence of 0.006. The films were of considerable transparency, with absorption above 400 nm being less than 20%. The flexible sulfur linkages in the polymer backbone are mainly attributed to the high refractive index and low difference in directional refractive index, i.e., low birefringence.¹³⁹

In 2013 Javadi and co-workers synthesized a diamine monomer that had an inherently high sulfur content and followed polycondensation reactions with diacids to prepare polyamides. The highlight of this particular work is the exceptionally high refractive indices of 1.7414 to 1.7542 at 632.8 nm with a significantly low birefringence of 0.0067, along with 85% transmittance above 450 nm.⁶⁶

In another instance, Sun et al. have used the abundant elemental sulfur to bring about direct polymerization in diamines with aromatic structures, thus highlighting the economical aspect of research. This work introduces an innovative approach for preparing diamines having a −NH₂ group at the α position from an alkyl or a phenyl group. The reaction mechanism proceeds via the stepwise formation of benzydamine followed by the formation of the thiobenzamide linkage, during which the role of sulfur is that of an oxidizing agent. The polymers thus prepared had desirable properties with a refractive index over 1.77 at 589.3 nm and an Abbe number of around 13 but with a detrimental effect on transparency with most of the films showing an intensive coloration at 500 nm.¹⁴⁰

3.5. Sulfur-Containing Poly(amide imide)s

Achieving a set of soluble polymers without compromising their thermal and mechanical properties is a sought-after goal. Aromatic poly(amide imide)s are good candidates for the same, which can be utilized to increase the scope of high-performance polymers.^141−143 During the early years, Ray et al. prepared a set of heat-resistant poly(amide imide)s that were processable and contained sulfonamide linkages.¹⁴⁴ The polymers were prepared via polycondensation reactions between 2-sulphoxy-l,3-dioxoisoindoline-5-carboxylic acid and various diamines. The reaction was performed at low temperatures and was thionyl chloride activated. The poly(amide imide)s formed all had very high solubilities in polar solvents. This is attributed to the fact that, during the process of polycondensation, the backbone of the polymer loses its crystallinity and chain symmetry, which results in decreased chain interactions and thus higher solubility. Since all the polymer repeat units had varied aromatic group content as well as different sulfur content, the study of their refractive index is expected to be a subject of interesting findings.

A series of similar works has been carried out by Patil et al.¹⁴⁵ and Shockravi et al.,^146−148 to prepare sulfur- and sulfoxide-containing poly(amide imide)s. A three-stage reaction procedure was followed to synthesize the diimide-dicarboxylic acid monomer, which consisted of a nucleophilic displacement reaction of diol and 2-chloro-5-nitrobenzotrifluoride in the presence of K₂CO₃ in DMSO. For the synthesis of the diamine monomer, catalytic reduction of the intermediate dinitro compound by zinc-ammonium chloride in refluxing methanol was done. The diimide-dicarboxylic acid was obtained by the condensation of a diamine monomer with trimellitic anhydride. The studies suggested that, with the introduction of bulky pendent groups on the polymer backbone, their solubility was substantially increased and the poly(amide imide)s were soluble in common laboratory solvents along with highly polar solvents.

4. Reflections on Comparison among Different Sulfur-Containing Polymers

Table 2 records the repeat unit structures of different sulfur-containing polymers reviewed in this paper and their corresponding refractive index.

Table 2. Repeat Unit Structures of Some of the Sulfur-Containing Polymers along with Their RI Values.

Figure 20 represents the plot of the refractive index obtained for some of the different classes of polymers studied by us, which include poly(arylene thioether)s, poly(arylene ether)s, polyimides, and polytriazoles. Along with high thermal and mechanical stability, the polyimides show a higher RI at 589 nm than poly(arylene thioether)s, poly(arylene ether)s, and polytriazoles probably due to the higher order of additional polarizability arising from nitrogen atoms in the polymer backbone and higher conjugation. From our understanding, we found poly(arylene thioether)s and poly(arylene ether)s to be more solution processable. This property can be attributed to the presence of flexible sulfide and oxide linkages. The polytriazoles showed intermediate properties. Among other sulfur-containing polymers studied by our group, the polyvinylsulfides exhibit excellent solution processability along with RI greater than 1.7 at 589 nm; however, they have limited mechanical properties and this class of polymers could be extremely desirable for nanostructured applications.^133−136

Figure 20.

Comparison of RI among poly(arylene thioether)s, poly(arylene ether)s, polyimides, and polytriazoles. Please note that the data points marked are according to the structures in Table 2.

5. Polymers Containing Selenium in Their Backbone

Selenium is yet another fascinating element that has attracted research quite recently by its unique stimuli-responsive properties which have found wide applications in the field of biomedical research,^149−155 along with applications in organic photovoltaics.^156,157 It is a semimetallic element in group XVI of the periodic table and is an essential trace element for the human body. Selenium also has a significant amount of similarity with respect to chemical properties with sulfur. Additionally, it has the features of a bigger atomic radius and weaker electronegativity than sulfur. This results in the dynamic nature of C–Se bonds or Se–Se bonds, which makes selenium-containing polymers applicable as responsive materials to physiological changes as well. Se shows a molar refraction even greater than that of sulfur, [R]_Se = 11.17, which makes it a preferable entity for high refractive index applications.^158,159 The study of selenium-containing polymers is comparatively new compared to that of sulfur;¹⁶⁰ nevertheless polymers with Se provide a wide array of studies for the research world. The use of selenium in the field of optoelectronic devices and as conducting polymers has been reported in the 1980s,^161,162 and we would like to bring this aspect also into this article, reporting on selenium-containing polymers and their synthetic methods with the main focus on refractive index effects. Pyun and group established that the refractive index of a polymer material could be increased significantly with the incorporation of Se along with S.¹⁶³ They demonstrated the preparation of a chalcogenide inorganic–organic polymer (CHIP) hybrid system with Se and S, exhibiting a very high RI of 1.96 (Figure 21), prepared by the inverse vulcanization of S₈ with a vinylic comonomer. They also demonstrated the preparation of Bragg reflectors by spin-coating alternating layers of CHIP and cellulose acetate, which showed a reflectance of more than 90%.

Figure 21.

Repeat unit structure of high RI CHIPs. Adapted with permission from ref (163). Copyright 2018 American Chemical Society.

Ueda et al. prepared selenophene-containing high refractive index polymers by studying the similarity between the selenophene moiety and thiophene moieties in combination with a sulfur-containing comonomer.⁶⁴ The selenium-containing polyimides (Figure 22) showed a higher refractive index than the polyimides containing only sulfur due to their higher molar refraction. Also, the transparency and the birefringence of the Se-containing polyimides were found not to deteriorate compared to the pure sulfur-containing polymers.

Figure 22.

Repeat unit structure of sulfur and selenophene containing high refractive index polymers. Adapted with permission from ref (64). Copyright 2009 John Wiley and Sons.

Li et al. reported on the synthesis of a series of high Se content polyimides by the polycondensation reaction in the amine-anhydride system. The polyimides exhibited an ultrahigh refractive index owing to the presence of high Se content, up to 1.968 at 633 nm. The birefringence in the highest selenium-containing polymer was found to be the lowest, as a result of more random orientation. The presence of selenium in the polymer backbone also ensured a high Abbe number, an indication that the incorporation of selenium can open new arrays in the field of optical applications for organic materials.¹⁶⁴ The presence of selenium, not only in the main polymer chain but also as a pendant moiety, has a significant effect on the optical properties of the polymer. Li et al. prepared a series of polymers containing maleimide, with Se as a pendant moiety, and obtained a very high RI, more than 1.8 at 633 nm and an Abbe number greater than 34.¹⁶⁵ Lu et al. reported hyperbranched polymers with Se as a pendant moiety. The polymer with ∼30% Se exhibited a refractive index of 1.7 at 589 nm and an Abbe number of 31, demonstrating the tunable optical properties of the prepared polymers according to the polymer structure.¹⁶⁶ Jiang et al. reported a Se-modified PGMA polymer. The modification was performed by the reaction between the epoxy group in PGMA and a Se-agent, PhSeZnCl. The presence of Se did not hinder the optical transparency of the modified polymer, and it was found to have similar transparency to that of PGMA. Beyond 10% (w/w) Se-loading in the polymer, the Abbe number decreased. The control of RI was influenced by Se-loading.¹⁶⁷Table 3 contains the list of polymers containing selenium in their repeat units with their respective RIs. Another work by Kim et al. demonstrated the influence of different chalcogenide elements on the refractive index of sulfur-containing polymers.¹⁶⁸ They studied the effect of oxygen, sulfur, selenium, and tellurium on the reference sulfur-containing polymer backbone. The heavier elements by their high polarizability impart very high molar refraction, and the polymer with Te atoms exhibited a RI of 1.778. It was also seen that the incorporation of chalcogenides in the polymer backbone had a more pronounced effect on the RI of the material than that of nanoparticles.

Table 3. Repeat Unit Structures of Some Selenium-Containing Polymers along with Their RI Values.

The plot of the average refractive indices at 633 nm in the case of a series of sulfur-containing polymers and selenium-containing polymers from the literature and their respective sulfur and selenium content was attempted, and the plots are shown in Figure 23a and b. It was seen that, along with several factors like packing density, the orientation of side groups, the structure of the repeat unit, etc., the refractive index is highly influenced by the content of the heteroatom present in the polymer backbone.¹⁶⁹

Figure 23.

Plot of refractive indices of (a) sulfur-containing polymers and (b) selenium-containing polymers versus sulfur and selenium content, respectively.

6. Organic/Inorganic Hybrid Materials

The organic/inorganic hybrid materials or nanocomposites have been the subject of intrigue and engaging research since the 1990s.^170−173 Methodologies that result in a hybrid system by the combination of an organic polymer matrix and inorganic nanoparticles achieve enhanced properties in terms of mechanical, thermal, and optoelectronic.^174−176 It is also established that, using a nanoparticle system, the polymer properties, e.g., the refractive index, can be varied just by varying the feed ratio of the monomers, and no novel synthetic strategy is required to alter the chemical backbone of the polymer to induce a change in refractive index.¹⁷⁷ Three factors often contribute to the refractive index of a nanocomposite. These include the individual features of the polymer, the inorganic nanoparticles, and the technique by which the hybrid is formed. To prepare high RI hybrids, inherent high RI polymer matrixes are suitable choices, although any polymer matrix can be selected. The following equation (eq 4) predicts the approximate RI of a hybrid material

4

where n_comp, n_p, and n_org imply the refractive indices of the nanocomposite, nanoparticles, and organic matrix, respectively. Φ_p and Φ_org imply the volume fractions of the nanoparticles and organic matrix, respectively. Thus, from the above equation, it can be concluded that, to achieve a definite n_comp value with a particular type of nanoparticle, the higher the value of n_org, the lower the value of Φ_p should be. Although several publications have supported the above equation, it is to be noted that the effective medium theory predicts the dielectric permittivity as the average over the volume fraction of the composing materials’ permittivities and not the average of their refractive indexes.¹⁷⁸

This is significant for the design of nanocomposites with high refractive indices for optical applications because an overload of nanoparticles often increases optical loss and decreases processability. Further, an increase in nanoparticles often results in a deterioration of mechanical strength.^179−181 Moreover, the choice of nanoparticles is governed by their size and surface properties. A particle diameter size below 40 nm is often essential for achieving desirable optical transparency with minimum scattering.^182,183 Furthermore, the direct mixing of nanoparticles with the polymer matrix also results in aggregation of the particles in the matrix. Thus, the preparation of applicable HRI nanocomposites follows several optimizations.

Among the inorganic nanoparticles, TiO₂ is the most extensively used one, which can be found in the form of rutile or anatase.^{179,184−188} A work by Liu et al.¹⁸⁹ involved the incorporation of TiO₂ nanoparticles into the polymer structure. The nanoparticles were introduced after the formation of poly(amic acid) and before catalytic imidization. On one hand, the refractive indices of the polyimide films were reported to be between 1.680 and 1.713 determined at 633 nm; on the other hand, the hybrid films showed the highest refractive index of 1.810 at the same wavelength, thus elucidating the advantages of adding nanoparticles for increasing refractive index. Ireni et al.¹⁹⁰ reported the synthesis of a series of nanocomposite coatings comprising TiO₂/ poly(thiourethane urethane) urea for applications as anticorrosive materials along with NIR reflectivity and high refractive index. The maximum refractive index observed was 1.6 at 550 nm and greater than 90% NIR reflectance. The method for preparation of the nanocomposites essentially consisted of two distinct steps: (a) the sulfur-rich polymers (polyols) were first grafted on TiO₂ nanoparticles and (b) the realization of transparent HRI nanocomposites. The surface modification of the nanoparticles involved amine termination of the nanoparticle surfaces, followed by the formation of a capped moiety with a carboxylic acid. Several polymer matrixes were reported in the literature which were used to prepare polymer/TiO₂ hybrids, e.g., polysilsesquioxanes,^191,192 polyimides,^64,69 poly(methyl methacrylate), and poly(bisphenol A carbonate).¹⁹³ These polymer–titania systems have been extensively studied, which reiterate the influence of nanoparticles on various polymer properties. For example, the incorporation of titania nanoparticles via ex situ sol–gel synthesis and then using melt-compounding along with the PMMA matrix has been found to enhance the thermal and mechanical properties of PMMA/titania hybrids.^194,195 Another work demonstrated the use of 2-hydroxyethyl methacrylate as a coupling agent in the synthesis of sol–gel-derived organic–inorganic hybrid materials consisting of organic poly(methyl methacrylate) (PMMA) and inorganic titania. The refractive index was found to increase with an increase in mol % titania.¹⁹⁶ Finally, these were further grafted with the polyol by using a condensation reaction. Also, the incorporation of nanoparticles into polyimides has been investigated.^197,198

The most effective method to prepare hybrid materials is sol–gel processing.¹⁹⁹ A very well-controlled morphology in the phase separation can be ensured by the sol–gel technique, along with the generation of particles with sizes of less than 20 nm. Lü et al. showed that the refractive index increases with the amount of titania for the hybrid materials. Along with TiO₂ nanoparticles, other inorganic nanoparticles have also been explored, and their influence on the composite material was studied. Among these, ZnS and ZrO₂ are the most common ones.^200−203

7. Conclusions and Future Scope

This review article covers the various methods of synthesizing the different classes of polymers with a high refractive index and how the presence of sulfur and selenium can be used to engineer the polymers according to a particular application need. We have seen the various factors like refractive index, Abbe number, and birefringence that need to be taken into account in the design of a highly functional polymer. To be applicable in the field of optical devices, the material has to be of high refractive index, high Abbe number, low birefringence, and transparent in the visible region. The refractive index and Abbe number are inversely proportional to each other and require special research consideration to achieve a high value for both. Although we have seen several groups working toward achieving refractive indices as high as 1.8, an Abbe number as high as 45, and significantly low birefringence, there is still a considerable amount of work to be done to achieve optical transparency, which is an important criterion for optical applications. For practical applications, other properties, such as solubility and thermal, mechanical, and oxidative stability, must be fulfilled. Good mechanical properties of the polymers with the chance for tough and patternable thick polymer layers and free-standing films will open up new application avenues. Our understanding concludes that any class of polymer can be tailored according to the specific requirements of molecular engineering. And we believe the field of high refractive index polymers is of utmost importance, which will continue to motivate scientists in the future as well.

Author Present Address

^§ Technische Universität Chemnitz, Professur Polymerchemie, Str. der Nationen 62, 09111 Chemnitz, Germany

The authors declare no competing financial interest.