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Polymer International
Polym Int 48:1085±1090 (1999)
Optimization of holographic polymer dispersed
liquid crystal for ternary monomers
YH Cho,1 BK Kim1* and KS Park2
1
Department of Polymer Science and Engineering, Pusan National University, Pusan 609-735, Korea
Samsung Advanced Institute of Technology, Yongin, Kyungki 449-900, Korea
2
Abstract: Polarized optical micrography (POM) images of gratings and UV±visible spectra of
holographic polymer dispersed liquid crystals (HPDLC) are reported for a ternary monomer system
composed of dipentaerythritol hydroxypentaacrylate/trimethylolpropanetriacrylate/N-vinylpyrrolidone (DPHPA/TMPTA/NVP) = 7/2/1 by weight. Gratings were written by irradiation with an argon ion
laser (l = 488 nm) at various intensities (20±200 mW cmÿ2) on monomer/liquid crystal (LC) composite
®lms of various compositions (75/25, 70/30, 65/35, 62/38, 60/40). Re¯ection ef®ciency±irradiation
intensity±®lm composition relationships are obtained in three dimensional plots which show that
maximum re¯ection moves from high LC content (38%) at low irradiation intensity (20 mW cmÿ2) to
low LC content (25 wt%) at high irradiation intensity (200 mW cmÿ2).
# 1999 Society of Chemical Industry
Keywords: holographic polymer dispersed LC; re¯ection; monomer
INTRODUCTION
A polymer/liquid crystal composite ®lm, often referred
to as polymer dispersed liquid crystal (PDLC), is a
thin composite ®lm composed of micrometre-sized
droplets of a nematic liquid crystal (LC) dispersed in a
polymer matrix,1±5 typically made of UV-curable
acrylates because of their high optical clarity and other
adjustable properties.12±16 PDLC has the potential for
a variety of electrooptic applications ranging from
directly driven windows to active matrix driven
information displays. A number of important reviews
regarding the methods of preparation, materials,
modes of operation, device applications, etc, have
become available.6±11
Recently the laser grating technique has been
applied to fabricate PDLCs with controlled architectures of phase-separated LC domains.17±23 This type
of PDLC is called a holographic polymer dispersed
liquid crystal (HPDLC) in the literature.22 This new
technique combines polymerization-induced phase
separation (PIPS) and Bragg's law (eqn (1)).
l
Lˆ
2 sin …1†
where l is the wavelength of light, L is the grating
spacing, and 2y is the interbeam angle outside the ®lm.
The grating spacing, and hence the component of
incident light which can be re¯ected by the grating
spacing, can be easily controlled by varying the
interbeam angle using the same laser wavelength.23
This periodic structure of multilayers has very
promising optical properties because only a speci®c
component of the incident light is re¯ected by the LC
layers, due to the difference in refractive indices of the
polymer and LC.
In HPDLC, light scattering is virtually minimized
because the domain sizes are of the order of nanometres, so that this device operates on light re¯ection
or transmittance, and its ef®ciency is electrically
controlled via the refractive index of the LC molecules.17,23
In the PIPS method, the phase separation is driven
by the polymerization reaction. The rate of polymerization given below for a photoinitiated radical mechanism largely governs the phase separation and hence
the domain size.
Ri ˆ 2I0 ‰1 ÿ exp…ÿ2:3e‰AŠb†Š
eI0 ‰AŠb 0:5
Rp ˆ kp ‰MŠ
kt
…2†
…3†
Ri and Rp are the initiation and polymerization rates,
respectively and details of their application are available from the pioneering works by Decker.24,25 In
these eqns (2) and (3), f, e, I0 and b are, respectively,
initiation ef®ciency, molar absorptivity, incident light
intensity, and sample thickness. [M] and [A] are the
* Correspondence to: BK Kim, Department of Polymer Science and Engineering, Pusan National University, Pusan 609-735, Korea
Contract/grant sponsor: Korean Ministry of Commerce, Industry and Energy
Contract/grant sponsor: Korean Ministry of Science and Technology
(Received 15 September 1998; revised version received 5 March 1999; accepted 13 May 1999)
# 1999 Society of Chemical Industry. Polym Int 0959±8103/99/$17.50
1085
YH Cho, BK Kim, KS Park
Figure 1. Chemical structures of photopolymerizable monomers.
concentrations of monomer and species which undergo photoexcitation, kp and kt are the rate constants for
propagation and termination reactions, respectively.
With a suf®ciently rapid rate of polymerization, phase
separation cannot follow the polymerization kinetics
because the time-scale for polymerization is smaller
than that of phase separation due to the slower
diffusion of LC molecules in highly viscous media.
Therefore, monomers to be used for HPDLC should
have higher functionality than those for conventional
PDLC.
As a continuation of our efforts in PDLC,26±29 we
optimized holographic PDLC with regard to ®lm
composition and irradiation intensity. Ternary monomer mixtures with various monomers/LC compositions have been irradiated with an argon ion
(l = 488 nm) laser at various intensities. Three dimensional plots of re¯ection ef®ciency±laser intensity±®lm
composition are presented showing contours for
maximum re¯ections.
EXPERIMENTAL
Materials
A eutectic mixture of four cyanobiphenyl and cyanoterphenyl components with TKN = ÿ 10 °C, TNI =
60.5 °C, ek = 19.0, and e? = 4.2 (E7, Merck) was used
as LC. Three types of photopolymerizable monomers
(Fig 1) (dipentaerythritol hydroxypentaacrylate
(DPHPA,
functionality
( f ) = 5),
trimethylolpropanetriacrylate (TMPTA, f = 3), and N-vinylpyrrollidone (NVP, f = 1) were used in appropriate
combinations to prepare the host polymers upon laser
irradiation.
DPHPA and TMPTA have very high reactivity
together with high viscosity because of their high
molecular weight, and provide the polymers with
extensive crosslinkings, whereas monofunctional NVP
simply extends the chains at a much slower rate.
However, the use of monofunctional monomers is
often essential to reduce the viscosity of LC/monomer
mixtures and to make the starting mixture homogeneous. Otherwise, polymerization-induced phase
separation starts with the heterogeneous reaction
mixture and the morphology of the composite ®lm
becomes out of control.
A dye, Rose Bengal (RB), was used as photoinitiator
for holographic recording with an argon ion laser,
because it displays a broad absorption spectrum with a
peak molar extension coef®cient of about
104 Mÿ1 cmÿ1 at about 490 nm.17 To this, a millimolar
amount of N-phenylglycine (NPG) was added as
coinitiator. In this experiment 3 10ÿ6 M of RB and
1.2 10ÿ4 M of NPG were used.
Formulation
Gratings were formulated with different ®lm compositions (monomers/LC) which were irradiated at various
laser intensities. Basic formulations of our ternary
DPHPA/TMPTA/NVP systems are given in Table 1.
The monomer composition was ®xed at DPHPA/
TMPTA/NVP = 7/2/1 by weight, and the effects of
®lm composition and irradiation intensity were
examined.
Gratings
The holographic recording system used is schematically shown in Fig 2. An argon ion laser (l = 488 nm)
was used as light source. The beam passes through a
spatial ®lter, a beam expander, and is split in two
beams of identical intensity. These two beams are
subsequently passed through a collimator and only the
central portions are re¯ected from the mirrors to
impinge normally on the cell from the opposite side.
The cell was constructed by sandwiching the monomers/LC between two indium±tin-oxide (ITO) coated
Table 1. Formulation to prepare HPDLC from DPHPA, TMPTA and NVP
Monomer
Pentaacrylate = DPHPA
Triacrylate = TMPTA
Monofunctional = NVP
Penta-:tri-:mono- = 7:2:1
1086
LC: Monomer Rose Bengal (wt%) NPG (wt%) Intensity (mW cmÿ2) Space thickness (mm)
25:75
30:70
35:65
38:62
40:60
0.5
2.5
20
50
100
150
175
200
14
Polym Int 48:1085±1090 (1999)
Optimization of HPDLC for ternary monomers
Figure 2. Experimental set-up for holographic PDLC.
glass plates, with a gap of 14 mm adjusted by a bead
spacer.30 Interference of the two beams establishes the
periodic interference pattern according to the Bragg
law; this is approximately 488 nm in our case. The
laser intensity was varied from 20 to 200 mW/cmÿ2,
with typical exposure times of 30±120 s.
Figure 3. POM micrographs of HPDLC
versus film composition (monomers/LC):
(a) 75/25; (b) 70/30; (c) 65/35; (d) 62/38
and (e) 60/40 (I0 = 100m W cmÿ2).
Polym Int 48:1085±1090 (1999)
1087
YH Cho, BK Kim, KS Park
Measurements
The morphology of the composite ®lm was studied by
polarized optical microscopy (POM). The re¯ection of
a speci®c wavelength by the composite ®lm was
analysed using a UV±visible spectrometer (Perkin
Elmer, Lambda 20). The re¯ection ef®ciency was
estimated from the spectral data.
RESULTS AND DISCUSSION
Polarized optical microscopy (POM) images of our
gratings for various ®lm compositions are shown in Fig
3. The width variation of the grating is very small
throughout the ®lm compositions. In general, spatial
variation of the grating width is observed due to
inhomogeneity of the laser spot. This can be minimized by using a more square beam pro®le, which is
obtained by expanding and collimating the Gaussian
beams and passing only the central portion through an
aperture in contact with the front plate of the
sample.19 This procedure was used in our experiments. With the increase of LC content in the ®lm, the
thickness of the LC lamella (dark area) increases while
the Bragg spacing remains constant.
UV±visible spectra of the ®lms are shown in Fig 4.
Two peaks are observed at about 480 and 580 nm,
which correspond to the re¯ection by the holographic
grating and absorption by the dye Rose Bengal,
respectively. The Bragg spacing is slightly smaller than
the incident laser wavelength (488 nm), presumably
Figure 4. Irradiation intensity dependence of UV–visible spectra of HPDLC films (monomers/LC): (a) 75/25; (b) 70/30; (c) 65/35; (d) 62/38 and (e) 60/40.
1088
Polym Int 48:1085±1090 (1999)
Optimization of HPDLC for ternary monomers
because of the shrinkage of the mixture upon
polymerization. As mentioned above, the 480 nm peak
will be approximated as re¯ection by the gratings
because the scattering is small, with nanometre sized
domains. At low LC content (monomer/LC = 75/25),
the re¯ection intensity is maximum for high irradiation
power, and at high LC content (60/40), the peak
intensity is maximum for low irradiation power; at
intermediate composition (62/38), the maximum
re¯ection is obtained for intermediate irradiation
power.
In HPDLC, the peak intensity should depend on the
perfectness of holographic gratings. Obviously, more
perfect gratings give higher peak intensity. Then, what
is governing the perfectness of the grating? The answer
should be the proper LC±polymer phase separation,
where nanometre sized LC domains imbedded in
polymer layers are separated by almost LC-free
polymer layers. Phase separation in the polymerizing
system is regarded as a liquid±liquid demixing process
where spinodal decomposition prevails.31,32 Elementary Flory±Huggins theory is often used to obtain the
interaction energy which increases with the progress of
the polymerization reaction.
Following Tanaka et al,22 small LC droplets of high
density give higher re¯ection ef®ciency. Our results
indicate that an optimum polymer±LC phase separation may exist. According to the reaction kinetics of
photoinitiated radical polymerization described
earlier, it has been noted that both Rp and Ri increase
with irradiation intensity I0, although the effect of I0 on
Ri is more pronounced. Rp also increases linearly with
monomer concentration, which corresponds to the
monomer content of the composite ®lm. Therefore,
high monomer content directly gives a high reaction
rate and hence augments the crosslinking density of
host polymers.
The polymerization rate is highest when the ®lms of
highest monomer content are irradiated at the highest
laser intensity; conversely, it is lowest when the ®lms of
lowest monomer content are irradiated at the lowest
laser intensity. When the polymerization rate is too
fast, the rate of phase separation cannot follow the rate
of network formation and LC domains are entrapped
within the polymer nets leading to imperfect gratings.
Also, coalescence of LC domains into larger ones
becomes less plausible with a highly viscous host
polymer matrix, and this also retards phase separation.
However, when the polymerization rate is too low,
phase separation cannot take place thermodynamically. Therefore, there exists an optimum monomer
content for the desired maximum re¯ection, depending on the irradiation intensity.
The re¯ection ef®ciency±irradiation power relationship is shown in Fig 5 for various LCs. One observes a
monotonic increase, then an asymptotic increase, and
®nally a maximum when the LC content increases
from 25 to 40 wt%. It seems that some monomers
necessitate more powerful irradiation.
The same data were replotted for re¯ection
Polym Int 48:1085±1090 (1999)
Figure 5. Reflection efficiency versus laser intensity of HPDLC films for
various LC contents.
ef®ciency±®lm composition in Fig 6. Regardless of
the irradiation power, the re¯ection ef®ciency shows a
maximum for a given LC content, its value increasing
as the irradiation power increased. However, the LC
content of maximum re¯ection decreased with increasing irradiation power.
A three-dimensional representation of the re¯ection
ef®ciency±irradiation intensity±®lm composition relationships is shown in Fig 7. This plot shows that the
maximum re¯ection depends on ®lm composition and
laser intensity, and the optimum set of conditions is
much narrower than with binary monomers.33 The
maximum re¯ection moves from high LC content
(38 wt%) at low irradiation intensity (20 mW cmÿ2) to
low LC content (25 wt%) at higher irradiation
intensity (200 mW cmÿ2).
CONCLUSIONS
The re¯ection ef®ciency of holographic polymer
dispersed liquid crystals (HPDLCs) for ternary
monomers has been studied as a function of ®lm
composition and irradiation the intensity. Regardless
of the irradiation power, the re¯ection ef®ciency
showed a maximum with regard to the LC content;
the maximum value generally increased with increased
irradiation power. In contrast, higher irradiation
power gave maximum ef®ciency at lower LC content,
1089
YH Cho, BK Kim, KS Park
implying that optimum irradiation power should
depend on the amount of monomers to be cured.
ACKNOWLEDGEMENTS
The research is in the program of G7 project which has
been supported by the Korean Ministry of Commerce,
Industry and Energy, and the Korean Ministry of
Science and Technology. The ®nancial support is
gratefully acknowledged.
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Figure 6. Reflection efficiency versus LC content of HPDLC films irradiated
at various laser intensities.
Figure 7. Reflection-efficiency–LC-content–laser-intensity relationships of
HPDLC films (NVP/TMPTA/DPHPA = 1/2/7).
1090
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Polym Int 48:1085±1090 (1999)
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