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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