Intrinsic High Refractive Index Polymers

By Emily K. Macdonald and Michael P. Shaver*

Keywords: High refractive index polymer, Intrinsic, Optical materials, Heteroatom polymers, Metallopolymers

Abstract

As the ubiquity and complexity of optical devices grows, our technology becomes more dependent on specialized functional materials. One area of continued interest is in high refractive index polymers as lightweight, processable and inexpensive alternatives to silicon and glass. In addition to a high refractive index, optical applications require these polymers to be transparent and have a low dispersion. Both nanocomposite and intrinsic high refractive index polymers offer particular advantages and disadvantages. While nanocomposite high refractive index polymers have refractive indices above 1.80, the nanoparticle type, content and size can negatively affect storage stability and processability. Alternatively, intrinsic high refractive index polymers are prepared by introducing an atom or substituent with a high molar refraction into a polymer chain; the resultant polymers are easier to store, transport, tune and process. Polymers containing aromatic groups, halogens (except fluorine), phosphorus, silicon, fullerenes and organometallic moieties have all shown significant promise. Many factors can affect intrinsic high refractive index polymer performance including molecular packing, molar volume, chain flexibility and substituent content. This mini-review summarizes the principles behind and recent developments in intrinsic high refractive index polymers.

Introduction

Continuing advances in optical devices are married to advances in high refractive index materials.1, 2 The refractive index (RI) of a material is a measure of how light propagates through that medium, as compared to a vacuum, and when light hits an interface between two materials with different refractive indices, the light will change speed and direction.3 Functional materials with higher refractive indices are better suited for use in modern photonic devices because, with a higher RI, the material can be thinner (Figure 1). Polymers are advantageous over other materials with high RIs (i.e. silicon and glass): they are light weight, easy to process and have a high level of mechanical strength.4, 5 High refractive index polymers (HRIPs) have a wide range of applications including lenses,5 antireflective coatings,6 ophthalmic applications,7 encapsulates for organic light emitting diodes and image sensors.8 Polymers typically have a refractive index in the range of 1.3-1.79 (see Table 1 for refractive indices for commonly used materials). Optical dispersion is another key property for HRIPs and measures how refractive index changes with wavelength of light in Abbe numbers.3 The Abbe number is calculated using the refractive index at three different wavelengths: the Fraunhofer lines. HRIPs need a low dispersion, correlating with higher Abbe numbers; Abbe numbers are also provided for selected materials in Table 1. The two main classes of HRIPs are intrinsic and nanocomposite. This mini-review will focus on the development of intrinsic high-refractive index polymers, highlighting some key advances in the field and giving a broad overview of the state-of-the art. It is intended to serve as a guide for those new to the field rather than being a comprehensive review.

Figure 1: Representation of refractive index versus required lens thickness.

Table 1: Comparison of refractive indices and Abbe numbers for selected materials.

Material

Refractive index, ɳ

Abbe number, VD

Crystalline Silicon

3.49710

N/A

TiO2 (rutile)

2.57111

9.8711

Diamond

2.41710

55.3010

Sapphire

1.77112

72.2012

Polycarbonate

1.57913

27.5613

Polystyrene

1.57713

29.1213

Quartz

1.53714

69.6914

Display Glass

1.50810

50.7410

Pyrex

1.52410

65.4010

Poly(methyl methacrylate)

1.48413

52.6013

Water

1.32715

73.0015

Nanocomposite HRIPs

Nanocomposite HRIPs are inorganic/organic hybrid materials which comprise polymer chains tethered to or intertwined with inorganic nanoparticles of high refractive index (>1.8). The first reports of these materials appeared in the early 1990s16 and their performance has improved dramatically alongside parallel advances in nanotechnology. The refractive index of a material is additive of each component, taking into account volume fraction. Titania is one of the most common nanoparticles used,17-25 with a refractive index of 2.450 as anatase24 or 2.571 as rutile.11 While rutile would appear to be the better choice for composites, it is challenging to synthesize the required small nanoparticles; particles above 50 nm give undesirable scattering effects.26, 27 Increasing the TiO2 content can increase the refractive index but may also induce cracks on the surface of the nanocomposite.20, 28 Increasing the content of the inorganic nanoparticle also increases the rigidity and fragility of the composite, however this can be counteracted by increasing the flexibility of the polymer chains.29 Recently, graphene has been used as the nanoparticle in nanocomposite HRIPs, resulting in a promising refractive index of 2.058.30 ZnS has also become a popular choice as the inorganic component31, 32 and a range of polymer chains have been attached to the nanoparticle surface, including polyimides,18, 19, 23 methacrylates22 and sulfur-containing materials.33 High performing nanocomposites contain polymers with high refractive indices and low molar volumes, combined with the optimal content level of small nanoparticles. However, these nanocomposite materials can lead to aggregation, which results in poor stability and processability.34 All nanocomposites suffer from this same limitation in processability: if lenses or devices are to be fabricated using high temperature extrusion or injection moulding, nanocomposites are not ideal. While intrinsic HRIPs do not, and will not, meet the RI performance of these nanocomposites, they offer significant advantages in tunability, stability and processability.

Intrinsic HRIPs

Intrinsic HRIPs incorporate an atom or functional group with a high refractive index directly into the polymer chain. The Lorentz-Lorenz equation (Eq. 1) can be used to predict the refractive index of a substituent:35

(1)

where R is the molecular refraction, M the molecular weight and V the molecular volume of the repeat unit. R/M can also be represented as molar refraction (Rm) and M/V as the reciprocal of molar volume (Vm). Accordingly, a substituent with a high molar refraction and low molar volume will increase the refractive index of a polymer. Some common functional groups with their molar refractions are shown in Table 2.

Table 2: Comparison of molar refraction of selected substituents.

Substituent

Rm /(cm3mol-1)

Substituent

Rm /(cm3mol-1)

H

1.100

C≡C

2.398

C

2.418

C=C

1.733

O (in OH)

1.524

4-membered ring

0.400

O (in C=O)

2.211

Phenyl

25.463

O (in ether)

1.643

Naphthyl

43.000

Cl

5.967

S (S-H)

7.691

Br

8.865

S (S-S)

8.112

I

13.900

PH3

9.104

From Table 2, aromatic groups, sulfur and the higher halogens all possess a high molar refractivity. Molar refraction is related to the polarizability and density of the material, with higher molar refractivity values obtained with more polarizable, higher density atoms/moieties. As a beam of light enters a medium, it causes a disruption of electron density, slowing the electromagnetic wave. More polarizable materials slow the wave more, hence increasing the RI. Aside from the selected groups in Table 2, metallic and π-conjugated systems are also effective at increasing the RI of the polymer.

Most intrinsic HRIPs are synthesized by either step growth polymerizations, via Michael polyaddition or polycondensation reactions, or by radical polymerizations. A Michael addition is the attack of a nucleophile on an α,β-unsaturated carbonyl compound; in this case the Michael donor is a bis-nucleophile and the α,β-unsaturated carbonyl compound is a Michael acceptor, resulting in polymerization. Scheme 1 shows one of the more recent examples of such a polymer: a polyimidothioether synthesized by successive Michael additions, with the high RI of 1.665 derived from the many key aromatic and sulfur functionalities.8

Scheme 1: A polyimidothioether prepared via Michael addition from commercially available monomers.

Polycondensations, whereby a small neutral molecule is eliminated from a bi-functional monomer, are also a popular synthetic strategy in the synthesis of intrinsic HRIPs. The example shown in Scheme 2 is of an unusual polymer with a fullerene-substituted side-chain which benefits from the very high molar refractivities of the polyaromatic fullerene units and possesses one of the highest reported RIs for an intrinsic HRIP (RI = 1.793).36

Scheme 2: Polycondensation of click-derived fullerene monomer to prepare an intrinsic HRIP.

A radical polymerization is a chain polymerization where the chain propagator is a reactive radical, with polymer formation occurring through addition of this free radical to an unsaturated monomer unit, extending the chain and forming a new radical moiety. A carbazole phenyoxy-based methacrylate homopolymer was synthesized by McGrath et al. by radical polymerization.37 As illustrated in Scheme 3, this free radical polymerization can be initiated either thermally or photochemically, yielding a polymer with an RI of 1.631.

Scheme 3: Free radical or UV photo-polymerization of a functionalized methacrylate monomer to afford a HRIP with RI of 1.631.

Halogen-rich HRIPs

Halogens are effective in increasing the RI of polymers, with the exception of electronegative fluorine which is not polarizable and thus decreases the RI. Guadiana et al. were one of the first to systematically investigate halogen-functionalized polymers, reporting the polymerization of a series of unsaturated monomers with pendant halogenated carbazole substituents to produce HRIPs.38 Free radical polymerization of the substituted (meth)acrylates afforded the desired HRIPs, as depicted in Scheme 4. The polymerization can be carried out in the melt with reaction times from minutes to hours.

Scheme 4: Synthesis of halogen-substituted poly(meth)acrylates by radical polymerization, showing linker group Z, where X1, X2 and X3 are chlorine, bromine or iodine. Y1, Y2, Y3, Y4 and Y5 are hydrogen, chlorine, bromine or iodine and R is hydrogen or methyl.

The RI of the resultant polymer varied depending on the halogen incorporated (I > Br > Cl), correlating with their polarizability. In this specific example, the RIs ranged from 1.67-1.77,38 with the highest RI obtained with the periodated carbazoles. Tuning could be quite precise by controlling the number and type of halogens present to obtain specific polymer properties. The linker group can also affect melting and glass transition temperatures, as well as RI, with longer linker groups resulting in a decrease in RI, melting temperature (Tm) and glass transition temperature (Tg). Lower temperatures and linker flexibility can help in the manufacture and processing of these polymers.

Sulfur-rich HRIPs

Sulfur-containing polymers are the most extensively investigated intrinsic HRIPs and have incorporated various moieties including thioethers,39 thianthrenes,40 sulfones,41 and many other functionalities. Highlights include the work of Ueda et al. who synthesized and characterized a number of sulfur-containing aromatic polyimides by a two-step reaction. The process involved a polycondensation reaction followed by a thermal imidization from the parent dianhydrides and diamines39-44 and their results confirmed that polymers with the highest sulfur content per repeat unit had the highest RIs. However, they also noted a significant contribution from molecular packing, tuned by controlling the steric bulk present in the polymer backbone. Chain flexibility was also investigated through the synthesis of a series of aromatic polyimides containing either meta or para linkages, with the meta substituted polymers giving HRIPs with better optical transparency, as there are less chain-chain electronic interactions.39, 41 One study highlighted the importance of low molar volume, with replacement of a sulfonyl (O=S=O) substituent by a thioether (-S-) resulting in an increase in RI by 0.015; the oxygen increases the molar volume and reduces the polarisability of the sulfur atom.41 Bent structures using thianthrene rings and flexible thioether linkages gave HRIPs with high transparency and low birefringence (high Abbe number).40 Furthermore, it was reported that fluorene bridges increased transparency by preventing molecular packing, but incorporating more than one fluorene group could reduce the RI due to the considerable increase in molar volume.44 Table 3 illustrates some of the best performing sulfur-rich polymers and their refractive indices, using the general structure shown in Figure 2:

Figure 2: General structure of the sulfur-rich polyimides presented in Table 3.

Table 3: Structure and refractive index of best-performing sulfur-rich polyimides.

R1

R2

ɳ

1.735

1.719

1.746

1.740

1.716

1.760

1.755

1.737

1.769

1.742

1.721

1.737

1.695

1.726

1.702

1.726

More recently Ueda et al. have also synthesized poly(thioether sulfones) HRIPs by Michael polyaddition,45 producing polymers with an RI of 1.686 and a high Abbe number. In addition to homopolymers, copolymers have also been produced via Michael polyadditions, including co-poly(thioether sulfone)s, with a top RI of 1.651 and high Abbe numbers.46 Yang et al. combined the effects of flexible thioether linkers and highly conjugated rings to produce polymers with ultra-high refractive indices of up to 1.796.47 Recently, polyamides featuring thioether and sulfone substituents have been reported with RIs up to 1.725, with the heterocycle and thioether units also imparting improved solubility in polar aprotic solvents.48

Phosphorus-Rich HRIPs

Phosphorus has a high polarizability due to its electronic structure, with the polarizability comparison to nitrogen remaining one of the classic components of undergraduate inorganic curricula. Figure 3 shows atomic energy levels: the 3s-3d promotional energy for phosphorus is 17 eV compared to 23 eV for nitrogen.49 The contribution of higher energy levels (4s, 4p, 5s) to stabilize electronic distortions is greater in phosphorus because the energy gap is smaller, leading to greater polarizability and hence a higher RI. This energy gap is even smaller in, for example, metallic chromium: this is why transition metal nanoparticles have such success in nanocomposite HRIPs. Phosphorus-containing functionalities also tend to have good transmission in the visible region of the electromagnetic spectrum, making them a good choice to incorporate into HRIPs.

Figure 3: Atomic energy levels of nitrogen, phosphorus and chromium.

McGrath et al. synthesized aromatic polyphosphonates through polycondensation reactions,50 using the organocatalysts N-methyl imidazole and 4-(dimethylamino)pyridine. The RI of polyphosphonates is higher than the analogous polycarbonate systems by 0.02. RI can also be increased 0.04 by conjugating rings in a biphenol system, compared to a bisphenol-A system. In addition to these modest RI increases, the phosphorus-rich polymers absorb at a much lower wavelength than the polycarbonate systems, a beneficial property for optical applications. Scheme 5 shows the top-performing polyphosphonate thus far reported, with an RI of 1.61.

Scheme 5: Polycondensation synthesis of poly(phenylbiphenylphosphonate).

Allcock et al. reported a series of polyphosphazenes with high RIs synthesized via ring opening polymerization (Scheme 6).51, 52 The phosphazene backbone gives the polymer a high RI and is optically transparent in the visible region. With pendant naphthyl functionalities, the polymers showed a shorter cut off point in the UV, limiting their utility. However, biphenyl systems showed refractive indices as high as 1.755, and several also had low optical dispersion, making them promising HRIP targets.52 The RIs for the polyphosphazenes are given in Table 4, using the general formula in Scheme 6.

Scheme 6: Ring-opening polymerization of phosphazenes to prepare polyphosphazene HRIPs substituted by various R groups (Table 4).

Table 4: Refractive indices of substituted polyphosphazenes.

R

X = H

Br

I

1.618-1.620

1.644-1.646

1.710-1.715

1.662-1.664

1.686-1.688

1.750-1.755

1.632-1.634

1.646-1.648

1.682-1.684

1.650-1.652

1.660-1.662

1.664-1.666

Allcock’s group also investigated the ring-opening polymerization of sulfur-substituted cyclic phosphazenes,53 with an RI as high as 1.616 with an ethylthio substituent. Scheme 7 shows the range of cyclic phosphazenes prepared.

Scheme 7: Preparation of substituted cyclotriphosphazenes.

Silicon-Rich HRIPs

Recently, intrinsic HRIPs have been extended to those containing silicon and heavier main group compounds.54, 55 Polymers containing these highly polarizable main group elements, including silicon, germanium, tin and sulfur, can be synthesized by a slow reaction between a main group vinyl or allyl compound and a multi-functional thiol. As an example, the reaction shown in Scheme 8 is the thiol-ene coupling reaction between tetravinylgermane and 1,2-ethanedithiol. The reaction can use virtually any vinyl or allyl substituted main group monomer and a dithiol monomer, with selected examples shown in Figure 4.

Scheme 8: Preparation of branched HRIP from the poly(thiol-ene) reaction of tetravinylgermane and 1,2-ethanedithiol.

Figure 4: Vinyl, allyl and dithiol monomers used in thiol-ene coupling reactions.

The resulting polymers were highly cross-linked, improving their mechanical strength. High refractive indices were obtained from the incorporation of the polarizable main group elements and the absence of highly electronegative, low-polarizability atoms such as nitrogen or oxygen, common in many other high RI polymers. The refractive indices varied significantly over the range of 1.590-1.703.55 Copolymers incorporating silicon have also been synthesized via hydrosilylation, as shown in Scheme 9, obtaining a maximum RI of 1.605.56

Scheme 9: Hydrosilation of poly(siloxane) macromonomers to prepare branched HRIPs.

Silicon-based HRIPs offer exceptional stability, finding particular application in light emitting diodes. This is especially true when the polymer is cross-linked where composition pot lives reach upwards of 24 hours.46

Organometallic HRIPs

As with nanocomposite HRIPs, metal incorporation into the polymer gives excellent RIs. To overcome difficulties with solubility and processability, recent research has focused on polymers of organometallic coordination compounds, building the metals into the polymer chain. Organometallic species combine the highly refractive metal and the macromolecular nature of an intrinsic HRIP. Manners et al.47 synthesized a range of polyferrocenes with both high refractive indices and high Abbe numbers, possessing very low optical dispersions. The polymers were easily synthesized by ring opening polymerization of the strained cyclic monomer and contained main group spacers to further boost the RI, incorporating phosphorus, silicon, germanium and tin alongside a range of R groups, as shown in Table 5.

Table 5: Refractive indices of polyferrrocenes.

Repeat unit

E

R/R1

ɳ

Si

R = CH3; R1 = CH2CH2CF3

1.60

R = CH3; R1 = CH2CH3

1.66

R = R1 = CH3

1.68

R = CH3; R1 = C6H5

1.68

Ge

R = R1 = CH3

1.69

Sn

R = R1 = tBu

1.64

R = R1 = Mes

1.66

R = R1 = Nap

1.82

P

R = C6H5; X = absent

1.74

R = C6H5; X=S

1.72

Tang et al.48 produced a remarkable organocobalt polymer that had an RI of 1.813, with low optical dispersion and high optical transparency. This polymer cannot be injection moulded, but is readily spin-coated, making it an ideal candidate for optical coatings if the synthesis can be scaled. The basic structure of the polymer is shown in Figure 5.

Figure 5: Branched HRIP of the Co2(CO6) dimer with triphenylamine linkers.

Conclusions

In this mini-review, we have introduced the field of intrinsic HRIPs. Offering advantages of stability and easy processing compared to nanocomposite HRIPs, these polymers can be precisely tuned by controlling the functional group, relative position, steric bulk and flexibility in the polymer chain. A high molar refraction and low molar volume are the main considerations for substituent choice when designing an intrinsic HRIP. Most intrinsic HRIPs are produced by Michael polyaddition or polycondensation reactions, thus straightforward to manufacture on a large scale. Several simple trends give guidance to future HRIP design: a higher percent of the moiety of choice increases RI; limiting steric bulk in the polymer improves molecular packing and increases RI; and flexibility in the chain makes the polymer easier to process.

Early research focused on halogen-rich HRIPs, before a considerable effort was made in the development of sulfur-rich polymers. Recently, new intrinsic HRIPs have emerged using phosphorus and silicon building blocks. These new systems are complemented by rare examples of heavier, even more polarizable, main group elements that have been exploited in these systems. Organometallic HRIPs also give high refractive indices along with low optical dispersion and, while challenging to injection mould, are suitable for spin-coating. Of course, the key driver in the search for new intrinsic HRIPs is in the ever expanding range of applications: from encapsulants for LEDs, thin lenses such as those found in mobile devices, fibre optic communications materials with minimal or zero birefringence and advanced sensors and functional coatings. On the more fundamental side, we expect that the upper limit of intrinsic polymer refractive indices has yet to be reached. In particular, phosphorus, silicon and organometallic components remain understudied, as do the interfacial areas combining two or more of these high molar refractivity functionalities. Due to the ever present demand for improved polymer properties for use in next-generation optical devices, research will continue to push the limits of intrinsic HRIPs, targeting performance polymers that remain easy to manufacture, process and store.

Acknowledgements

We would like to thank the University of Edinburgh, EaStCHEM and the Marie Curie Actions Program (FP7-PEOPLE-2013-CIG-618372) for funding. We would also like to thank Dr Laura Allan for helpful discussions.

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