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.
1
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.
2
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
3
(>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.
4
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
𝑛2 −1
𝑛2 +2
𝑅
𝑀
𝑅
= 𝑀 𝑥 𝑉 = 𝑉𝑀
𝑀
(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
5
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 πconjugated 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
6
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
7
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.
8
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.
9
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
10
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 chainchain 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.
11
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
12
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.
13
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.
14
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.
15
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
1.618-1.620
Br
1.644-1.646
I
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.
16
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.
17
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
18
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
19
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
Si
Ge
Sn
P
R/R1
R = CH3; R1 = CH2CH2CF3
R = CH3; R1 = CH2CH3
R = R1 = CH3
R = CH3; R1 = C6H5
R = R1 = CH3
R = R1 = tBu
R = R1 = Mes
R = R1 = Nap
R = C6H5; X = absent
R = C6H5; X=S
ɳ
1.60
1.66
1.68
1.68
1.69
1.64
1.66
1.82
1.74
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.
20
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.
21
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.
22
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