Synthesis and evaluation of high-birefringence polymethacrylate having a diphenyl-diacetylene LC moiety in the side chain

Article abstract of DOI:10.1039/c2jm32489j

We synthesized polymethacrylate with a diphenyl-diacetylene moiety in the side chain. It formed a nematic LC, which appears in the wide temperature region and vitrified at room temperature. The polymer exhibited a high-birefringence of ～0.3 at 550 nm at room temperature.

Full text of DOI:10.1039/c2jm32489j

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Synthesis and evaluation of high-birefringence polymethacrylate having
a diphenyl-diacetylene LC moiety in the side chain†
*
Yuki Arakawa, Shunpei Nakajima, Sungmin Kang, Gen-ichi Konishi and Junji Watanabe
*
Received 20th April 2012, Accepted 25th May 2012
DOI: 10.1039/c2jm32489j
reactions, the degree of substitution was 30–70%.11 On the other
We synthesized polymethacrylate with a diphenyl-diacetylene
moiety in the side chain. It formed a nematic LC, which appears in
the wide temperature region and vitrified at room temperature. The
polymer exhibited a high-birefringence of ꢀ0.3 at 550 nm at room
temperature.
hand, the direct radical polymerization of the DPDA based mono-
mer was attempted by Canizal et al., however, it failed because
diacetylene units (–C^C–C^C–) trapped radicals to interrupt free-
radical polymerization.12 Therefore, to the best of our knowledge, the
optical properties of LCPs containing DPDA moieties have not yet
been reported in detail.
In recent years, liquid crystal displays (LCDs) have been widely used
for manufacturing flat panel displays employed in devices such as
televisions, personal computers, and mobile phones. This, in turn, is
leading to an increasing use of high-birefringence (Dn) liquid crystals
(LCs) for fabricating telecommunication devices,1 cholesteric LC
(ChLC) laser emission films,2 organic light-emitting diodes
(OLEDs),3 optical fibres,4 and photostorage devices.5
In this study, we synthesized a high-Dn LC polymethacrylate
containing dialkoxy DPDA as a long mesogenic core in the side
chain, by anion polymerization using n-butyllithium (n-BuLi). The
synthesized polymers contained a DPDA moiety in all repeated units.
The thermal and optical properties (Dn) of these polymers were
investigated in detail. The results of our study should prove useful in
the design of new LCPs for realizing high-Dn materials.
The use of high-Dn materials in such applications is advantageous
because it reduces the response time of the devices by reducing the cell
gaps and film thickness. It is preferable for high-Dn materials to have
a conjugation along the long molecular axis. In order to extend the
conjugation length, multiple bonds or unsaturated rings are
commonly employed. Various high-Dn compounds such as tolane,6
bistolane,7 and diphenyl-diacetylene (DPDA)8 have been reported
thus far. Among these, DPDA derivatives have been widely studied
as low-molar-mass LCs with high Dn and low viscosity. Very
recently, we have reported that DPDAs having alkoxy tails at both
ends (so-called DPDA-OCm) exhibit the nematic LC in a wide
temperature region and high Dn values.9 DPDA-OCm and its similar
homologues thus possess great potential for use as high-Dn LC
materials in various optical applications such as the fabrication of
optical phase difference films, laser emission films, and optical rota-
tory plates.
Polymer synthesis
The synthetic route for side-chain-type (S) of polymethacrylate (PM)
with DPDA-OC6 (so-called SPM-DPDA-OC6) is shown in
The second target is then a LC polymer with a DPDA moiety. LC
polymers (LCPs) offer a wider range of phases10 than LC monomers;
in addition, owing to vitrification, they offer a stably oriented and
transparent film. Active studies of various LCPs have shown them to
be excellent materials for optical applications. Previously, Lu et al.
synthesized LC polysiloxanes and polymethacrylates with DPDA
moieties in the side chain through the polymer reactions; in these
Department of Organic and Polymeric Materials, Tokyo Institute of
Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan.
E-mail: konishi.g.aa@m.titech.ac.jp; skang@polymer.titech.ac.jp; Fax:
+81-3-5734-2888; Tel: +81-3-5734-2321
† Electronic supplementary information (ESI) available: Typical DCS
chart and spectral data. See DOI: 10.1039/c2jm32489j
Scheme 1 Synthesis of SPM-DPDA-OC6.
14346 | J. Mater. Chem., 2012, 22, 14346–14348
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Scheme 1. Previously, asymmetric diaryl-diacetylene derivatives were
typically synthesized by Cu-catalyzed coupling. However, in the
present study, we employed Negishi cross-coupling of the asymmetric
diacetylene (6)13 of the terminal acetylene protected with a tert-
butyldimethylsilyl (TBDMS) group (4) and 2,2-dibromoalkene (5)
prepared by the Corey–Fuchs reaction.14 Then, monomer 8 (so-called
mono-DPDA-C6) was synthesized by an E2 reaction, and depro-
tection proceeded with tetra-n-butylammonium fluoride (TBAF),
followed by esterification with methacrylate chloride.15
UV-visible spectra
The UV-visible spectra for SPM-DPDA-OC6 and mono-DPDA-
CO6 are shown in the ESI†. Their longest absorption edges are at
350 nm, indicating that the DPDA moiety is suitable as a mesogen
unit because it is transparent in the visible wavelength region.
Further, their photostability will not affect their application in the
visible to infrared region.
Thus far, radical polymerization has not been used for the poly-
merization of methacrylates containing diacetylene units (–C^C–)
because they trapped free radicals.12 Therefore, we employed anion
polymerization with n-BuLi. Polymerization was carried out under
argon atmosphere in tetrahydrofuran (THF) at 0 ꢁC for 24 h,
following which the product was quenched with excess methanol and
purified by reprecipitation. The polymerization proceeded with 40%
yield, with Mw ¼ 13 500 and Mw/Mn ¼ 1.69. The polymer consists of
a monomer of ꢀ27 units. The chemical structures of the monomer
Wavelength and temperature dependences of
birefringence
Generally, the Dn values of the high-birefringence materials have
been determined in the mixtures with the host nematic LC. Here, we
obtained actual values for the nematic LCs of pure target
compounds.
The Dn measurement was performed for a homogeneously aligned
nematic LC kept in a thin cell with the rubbed polyimide.9a The cell
gap (d) determined by the interferometric method was 4–6 mm. The
transmittance of light under cross-polarization conditions was then
observed as a function of wavelength, l, by a microscope spectro-
scopic method using a Nikon LV100 Pol optical microscope equip-
ped with a USB4000 (Ocean photonics) spectrometer. Here, the
rubbing direction, i.e. nematic director, was set by q ¼ 45ꢁ to the
polarizers. It should be noted that the viscosity of SPM-DPDA-OC6
is so low that the cell can be filled by capillary force as is in low
molecular weight nematogenic material. Thus, the uniaxial alignment
in the nematic phase could also be easily achieved by using
a commercially available polyimide alignment layer. Here, as shown
in Fig. 2, POM images of SPM-DPDA-OC6 clearly indicate
a uniaxial alignment state in the nematic phase. Typical transmittance
data are shown in Fig. S7†. The transmitted light intensity, I,
modelled using eqn (1), was curve-fitted by applying Cauchy’s
equation (eqn (2)) for Dn (refer to Fig. S7†), leading to the determi-
nation of constants, a, b and c in eqn (2):
1
and polymer were confirmed by H NMR and 13C NMR spectros-
copy and high-resolution mass spectrometry (see the ESI† for more
details).
Thermal properties
The thermal properties of mono-DPDA-OC6 and SPM-DPDA-
OC6 were investigated by differential scanning calorimetry (DSC)
and polarizing optical microscopy (POM). Their phase transition
temperatures and enthalpy changes were measured during both
cooling and heating scans performed at 10 ꢁC minꢂ1. The DSC
curves, shown in Fig. 1, exhibit an enantiotropic phase transition for
both monomer and polymer. The liquid crystal was found to be
nematic from the observation of characteristic Schlieren texture. The
transition temperatures and enthalpies obtained by the cooling
process are as follows: Cr 49.3 (22.8) N 98.6 (0.86) Iso: Tm ¼ 74.9 ꢁC
for mono-DPDA-CO6 and G 70.3 (ꢂ) N 141.3 (ꢂ) Iso for SPM-
ꢀ
ꢁ
ꢁ
DPDA-OC6. The isotropic point (Ti) of the polymer (141.3 C) is
pDnd
I ¼ I0 sin2
sin2 q
(1)
(2)
higher than that of the monomer (98.6 ꢁC), indicating that the
nematic phase of the polymer is wider than that of the monomer. In
addition, SPM-DPDA-OC6 showed a glass transition temperature
(Tg) at 70.3 ꢁC, at which the nematic liquid crystal is vitrified, in other
words, the transparent nematic monodomain is perfectly retained at
room temperature. Thus, we can conclude that DPDA is useful as
a nematic mesogen in a side-chain-type LCP and that the polymer is
a candidate material for optical LC films.
l
b
c
Dn ¼ a þ
þ
l2 l4
Fig. 3 shows the wavelength length dependence of Dn measured at
various temperatures for SPM-DPDA-OC6. At high nematic
temperature near Ti, the polymer exhibits the relatively low bire-
fringence. With decreasing the temperature, the birefringence
increases. This trend can be explained by the temperature dependence
Fig. 2 POM images of uniaxially aligned SPM-DPDA-OC6 in nematic
phase in which the rubbing direction is (a) parallel and (b) 45 degrees to
the polarization direction.
Fig. 1 DSC thermogram of SPM-DPDA-OC6 and mono-DPDA-OC6.
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Table 1 Values of Dn0 and b estimated by curve fitting with eqn (3)
and (4)
Sample
Dn0
b
SPM-DPDA-OC6
Mono-DPDA-OC6
DPDA-OC6
0.41
0.45
0.46
0.24
0.19
0.16
containing a DPDA moiety in the side chain should be useful for
optical applications such as the fabrication of optical phase difference
films, laser emission films, and optical rotation plates.
Fig. 3 Wavelength dependence of birefringence for SPM-DPDA-OC6
at three different temperatures.
Notes and references
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of the order parameter (S) of nematic LC. In Fig. 4, the Dn values
collected at 550 nm are plotted against the temperature. This
temperature dependence is well described by following Haller’s
approximation:16
Dn ¼ Dn0S
(3)
(4)
S ¼ (1 ꢂ T/Ti)b
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gence (S ¼ 1) of the nematic LC and b is the material constant
characteristic of the nematic LC. As observed in Fig. 4, the fitting is
well done and the determined values of n0 and b are listed in Table 1.
Although some deviation is observed in the lower temperature region
below 70 ꢁC, it is due to the vitrification of the nematic LC. The value
of n0 is 0.41, which is as high as those (0.45–0.46) of mono-DPDA-
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DPDA moiety.9a Further, the actual value for a glassy film at room
temperature is still a high value of 0.3 and higher uniaxial orientation
is also maintained from S ¼ 0.66, holding the promise for application
to the fabrication of various types of optical films.
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In conclusion, we successfully polymerized methacrylate contain-
ing a DPDA moiety by anion polymerization with n-BuLi. The
obtained polymer has a wider nematic phase than the monomer, and
it vitrified under a nematic orientation. We determined the wave-
length and temperature dependence of the nematic birefringence. The
polymer exhibited a high-birefringence, with the highest value of ꢀ0.3
obtained at room temperature. Our results show that a polymer
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Fig. 4 Temperature dependence of birefringence measured at a wave-
length of 550 nm for SPM-DPDA-OC6 and mono-DPDA-CO6. The
dotted curves are based on eqn (3).
14348 | J. Mater. Chem., 2012, 22, 14346–14348
This journal is ª The Royal Society of Chemistry 2012