Thin Flexible Linear Polarizer of Silver-Dispersed Polyimide Blend ...

Thin Flexible Linear Polarizer of Silver-Dispersed
Polyimide Blend Film Exhibiting
High Extinction Ratios in Near-IR Region
S. Matsuda and S. Ando
Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Tokyo, Japan
smatsuda@polymer.titech.ac.jp
Abstract: Flexible thin-film (19 µm-thich) polarizers consisting of silver nanoparticles having
rodlike shapes dispersed in polyimide blend films were prepared. Linear polarizing character with
extinction ratios >20dB and transparencies >75% were obtained in a wide-range of wavelengths
(1.4-1.8 µm).
©2005 Optical Society of America
OCIS codes: (160.1190) Anisotropic optical materials; (160.5470) Polymers
1. Introduction
Conventional thin-film polarizers for optical communication devices are made of stretched inorganic glass containing
silver (Ag) particles [1,2], periodic metal-silica layers [3], and elongated Ag islands embedded in glasses [4]. Although
such inorganic materials exhibit high optical extinction ratios and good thermal stability, they are not easy to make
into thin plates or small components due to their brittleness. Conventional polymeric polarizers containing iodine or
dichroic dyes have good flexibility and tractability, however, the polymeric polarizers exhibit no polarizing character
in the near-infrared (IR) region (>0.7 µm) and no thermal stability above 100°C. In addition, the thicknesses of
polymeric polarizers are larger than 50 µm, which should result in considerable excess losses when they are inserted
into waveguide gaps. We have reported that distinct optical anisotropies in the visible and near-IR region (0.6−0.9
µm) are generated in a precursor films of rigid-rod fluorinated polyimide (PI) dissolving silver nitrate (AgNO 3
) during
thermal curing and uniaxial drawing [5]. In addition, we have developed thin-film PI polarizers containing silver
nanoparticles having spheroidal shape [6,7]. The PI polarizers have good tractability and high thermal stability (>200°C),
but the polarizing efficiency is not sufficiently high above 1.0 µm. Hence, there has been a demand for a 10–20 µmthick
polarizer applicable to optical communication wavelength region (1.3−1.7 µm) while having good processability
and tractability. In this study, we investigate the precipitation behavior of silver nanoparticles and the generation of
optical anisotropy in uniaxially drawn PI blend films. In addition, by optimizing the preparing conditions, we
successfully fabricated novel thin-film PI blend polarizers having high extinction ratios (22dB) and transparency
(77%) at 1.55 µm.
2. Experimental
Fig. 1 schematically shows a method for fabricating a PI blend polarizer. A rigid-rod fluorinated PI, PMDA/TFDB
(PI-1) synthesized from pyromellitic dianhydride (PMDA) and 2,2'-bis-(trifluoromethyl)-4,4'-diaminobiphenyl (TFDB)
was used as a continuous phase, and a rigid-rod sulfonated PI, PMDA/BDSA (PI-2) synthesized from PMDA and
2,2’-benzidinedisulfonic acid (BDSA) was used as a dispersed phases in the PI blend system. These two PIs show
macro-phase separation. AgNO 3
(Aldrich) was used as a metallic dopant. A poly(amic acid) (PAA) of PI-2, a precursor
Ag nanoparticles
PI-1
+
PI-2 + Ag
//
Heating
&
Drawing
Heating
⊥
(a)
(b)
(c)
Fig. 1. A fabrication scheme of a thin-film PI blend polarizer. The fluorinated PI-1 and the sulfonated PI-2 exhibit macrophase
separation in the film, and nanocrystalline Ag-particles are selectively precipitated in the PI-2 phase due to the difference
in polarity and the specific interactions between Ag + ions and sulfonic acid. The PI-2 phase works as nanoreacters for Ag +
reduction and anisotropic growth of Ag particles.

T //
100
80
(a) 30 min
(b) 60 min
60
T T
40 ⊥
⊥
T
20 //
20
T //
T //
0
100
80
(c) 90 min
(d) 120 min
60
40 T⊥
T ⊥
0
0.4 0.8 1.2 1.6 0.4 0.8 1.2 1.6
Wavelength (µm)
Fig. 2. Polarized transmission spectra of Ag-dispersed PI blend films with varying t H
= (a)30, (b)60, (c)90, and (d)120 min at
T f
=345°C for normal incident light polarized parallel (T //
) and perpendicular (T ⊥
) to the drawing direction.
of PI, was added to a PAA of PI-1 (in DMAc solution, 10 wt%, 300 Poise) and stirred for 12 h at room temperature,
followed by addition of AgNO 3
to the solution and stirring for 12 h. Most of Ag + ions are expected to be transferred
and included in the PI-2 phase due to the difference of polarity between the two PIs and the specific interactions
between Ag + and sulfonic acid [8]. The stirred solution were spin-coated onto a 4-inch silicon wafer, followed by
drying in N 2
at 70°C for 1 h. PAA films peeled from substrate were uniaxially drawn up to 1.7 times of draw ratio
during thermal curing with a constant tensile load using a thermomechanical analyzer (TMA, Sinku-Riko TM-7000)
or an automatic thermal drawing machine equipped in our laboratory. The PAA films were heated to the final curing
temperature (T f
) at 10°C/min, kept at the temperature for a constant holding time (t H
), and then cooled to room
temperature. The thermal curing was carried out in air. Ag nanoparticles are precipitated in PI blend films during
thermal curing. Polarized optical transmittance spectra of the PI blend films thus obtained were measured by rotating
a Glan-Taylor prism (polarizer) inserted into the light path of a spectrophotometer (Hitachi U-3500, λ= 0.3-2.5 µm).
Cross-sectional transmission electron micrographs (TEMs) were taken with JEOL 100CX TEM. The uniaxially
drawn PI blend films were embedded in epoxy resin and then sectioned into 60 nm thicknesses with a Reichert-Jung
Werke ultramicrotome. The sectioning directions were parallel to the drawing direction.
3. Results and Discussion
Fig. 2 shows the polarized transmission spectra of PI blend films with varying the holding time (t H
=30, 60, 90, and 120
min) at a final curing temperature (T f
= 345°C). The molar ratios of PI-1, PI-2, and AgNO 3
in the blend films were
100/0.95/6. During thermal curing of a uniaxially drawn PI blend film, the spectral shapes along the two polarizations
(parallel and perpendicular to the drawing direction) were gradually changed due to the precipitation of Ag nanoparticles.
Strong extinctions with a small anisotropy, which is caused by the plasmon resonance of spherical Ag nanoparticles,
are observed around λ=0.5 µm for the film cured for t H
=30 min. As t H
increases, an extinction peak in T //
was gradually
shifted to a longer wavelength and the peak-top finally reaches to λ=1.0 µm at t H
=120 min. On the other hand, T ⊥
gradually decreased in a wide wavelength region (0.4 < λ < 1.6 µm) until t H
= 90 min and then increased at t H
= 120
min. As a result, high polarizing efficiency (i.e. high extinction ratios) was obtained in the region of ca. 0.9-1.2 µm for
the film cured for t H
=120 min. The wavelength region with high extinction ratios is located at much longer than that
had been obtained for a uniaxially drawn neat PI-1 film containing Ag nanoparticles [7]. In addition, a similar
Fig. 3. Cross-sectional TEM images of the Ag-dispersed PI blend film cured for 120 min at T f
= 345°C. Arrows denote the
drawing direction. The inset shows the magnified image of a typical Ag nanoparticle.

(a)
100
80
60
40
T ⊥
22 dB
@1.55 m
(b)
30
20
T //
0
0.5 1.0 1.5 2.0 2.5
0 0.5 1.0 1.5 2.0 2.5
Wavelength (µm)
Wavelength (µm)
Fig. 4. (a) Polarized transmission spectra of uniaxially drawn Ag-dispersed PI blend film exhibiting a large extinction ratio at
1.55 µm and (b) the spectrum of extinction ratio calculated from the polarized transmission spectra. The inset shows the
aspect of the film.
phenomenon was observed in case of the films cured with varying T f
values. The desirable t H
value for obtaining high
polarizing efficiency decreases as the T f
value increases, whereas the extinction ratio increases.
The cross-sectional TEM images of a PI blend film cured for t H
=120 min at T f
=345°C are shown in Fig. 3. Ag
nanoparticles having rodlike shapes whose longer axes are directed to the drawing direction are observed (see inset
magnified image). The diameters of the Ag nanoparticles along their longer axes are 100∼200 nm, and their aspect
ratios are ca. 4. In our previous study, the corresponding values were ca. 30 nm and 1.5, respectively for uniaxially
drawn neat PI-1 films [7]. This indicates that considerably larger and more elongated Ag nanoparticles are precipitated
in the PI blend film. These results strongly suggests that Ag + ions are selectively included in the PI-2 phase during
stirring of PI blend solution, and Ag nanoparticles are mainly precipitated in deformed (elliptic-shaped) PI-2 islands
containing concentrated Ag + ions.
Fig. 4 shows the polarized transmission spectra of a PI blend film fabricated with the optimized preparing conditions,
i.e. the mol ratio of PI-1/PI-2/AgNO 3
(100/1.5/9.75), T f
(420°C) and t H
(5 min). High polarizing efficiency with
extinction ratios larger than 20dB (22dB at λ = 1.55 µm) and transparencies higher than 75% (T ⊥
= 77% at λ = 1.55
µm; including surface reflection) was obtained in a wide wavelength region (1.4 < λ < 1.8 µm). The thickness of the
film is sufficiently thin as 19 µm and shows good flexibility and tractability (see the inset in Fig. 4).
4. Summary
The precipitation behavior of Ag nanoparticles in uniaxially drawn PI blend films and their optical anisotropy were
investigated, and we have successfully fabricated a novel flexible thin-film polarizer for optical communication
wavelengths. The Ag nanoparticles precipitated in a PI blend film are much larger and more elongated than those
precipitated in a neat PI film, which results in the significant optical anisotropy with high extinction ratios. The PI
blend film fabricated under the optimized condition exhibits high polarizing efficiency in a wide wavelength region
(1.4 < λ < 1.8 µm). In addition, the thickness of 19 µm is sufficiently thin for reducing any excess losses caused by
insertion of film polarizers into optical devices. The film is tough and flexible, easy for insertion. Hence, the material
system based on novel PI blends and the fabrication procedures designed in this study are promising for fabricating
flexible thin-film polarizers for optical waveguide devices.
References
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extinction ratio”, Opt. Lett. 16, 964-966 (1991).
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[6] S. Matsuda, S. Ando, and T. Sawada, “Thin flexible polariser of Ag-nanoparticle-dispersed fluorinated polyimide”,
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[7] S. Matsuda and S. Ando, “Anisotropy in optical transmittance and molecular chain orientation of silver-dispersed
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[8] S. Horiuchi, T. Fujita, T. Hayakawa, and Y. Nakao, “Three-dimensional nanoscale alignment of metal nanoparticles
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