Organic/inorganic-polyimide nanohybrid materials for advanced ...

More documents

Recommendations

Info

5. POLYIMIDE THIN FILM POLARIZERLinear polarizing films made of poly(vinyl alcohol) films dyed with iodine (I 2 ) are widely used for liquid crystallinedisplays (LCDs), but this type of films cannot be used for optical communication applications because iodine does notexhibit anisotropic absorptions in the near-infrared (NIR) region. Instead, inorganic polarizer fabricated by uniaxialelongation of silver or copper nano-particles dispersed in inorganic glasses are used in fiber-optic devises such as opticalisolators. Metal nano-rods show significant polarizing properties in the visible-NIR regions, though glass matrix havedrawbacks in the thickness (150~200 μm), fragility, processability, and mass-production. Hence, we tried to fabricatethin film polarizers based on silver nano-particles dispersed in colorless and transparent PIs having rigid-rodstructures. 42-45 As shown in Fig. 10, a new series of copolymers consisting of a sulfur-containing PAA having highaffinity to noble metals and a fluorinated PAA having low affinity were prepared, and silver nitrate was dissolved in aDMAc solution of the PAA. Due to the large difference in hydrophobicity between the two components, the dried PAAfilms exhibited typical micro-phase separation, and energy dispersive spectroscopy (EDS) clarified that nanometer-sizedsilver particles were selectively concentrated and precipitated in the sulfur-rich domain. During simultaneous curing anduniaxial drawing at higher temperatures (>350°C), the PI films showed apparent polarizing properties in the visible-NIRregions due to the deformed shapes of silver nano-particles. Since it has been reported that the melting temperatures (T m )of metal nano-particles are significantly lower than those of bulk (T m of bulk silver is 689°C), such nano-particles can beelongated at appropriate conditions (temperature, atmosphere, and tensile stress). We developed a new apparatus for thein-situ and real-time observation of polarized transmission spectra to monitor and control the temperature and drawingconditions. The silver nano-rods thus precipitated in the PI blend film show significantly elongated shapes (50−300 nm)with perfect orientation along the drawing direction (Fig. 10). The WAXD pattern of the film shows that these particlesare nano-crystallites of zero-valent silver, and the PI film thus obtained shows very good polarizing property (dichroicratio: >300 at 1550 nm) with high optical transmission (>70 %) as shown in Fig. 11. In addition, the PI film is only 15μm-thick (ca. ~1/10 of inorganic polarizers), tough, and flexible, which leads to great advantages in processability andmass-production required for the applications in micro-optics and waveguide devises such as optical isolator.PI-1PI-2(High Affinity to Silver)Ag +Ag nanorodTEMMagnifiedPoly(amic acid)AgNO 3Uniaxialdrawing&HeatingHeatingSilverO O CF 3NONO CF 3PMDA/TFDB (PI-1)nhtONONSO 3 HO O SO 3 HPMDA/BDSA (PI-2)htnPolyimide20Fig. 10 Schematic representation of spontaneous condensation and precipitation of silver nano-particles having elongatedshapes in the sulfur-containing phase in PI blend films (left) and TEM micrographs of the PI film with silver (right).100Transmittance (%)80604020OpticalcommunicationwavelengthT ⊥T ||Thin FilmWaveplate00.5 1.0 1.5 2.0 2.5Wavelength (0m)Thin Film PolarizerGLICABASE U rca ow$ILQONf$T)Fig. 11 Polarized optical transmission spectra of a PI film containing silver nanoparticles having elongated shapes, andan example of applications of PI thin film polarizer and half-waveplates for a waveguide-type optical isolator.Proc. of SPIE Vol. 7213 72130B-6
6. POLYIMIDE THIN FILMS WITH HIGH THEMAL CONDUCTIVITYThermally stable polymers with high thermal conductivity are strongly needed for efficient heat transfer in electronicapplications. Polymers hybridized with metal oxide particles have been widely studied, but their improvement in thermalconductivity is limited due to the large interfacial thermal resistance between polymer and fillers. We have proposed anew strategic design for effective increase in thermal conductivity based on the selective precipitation of silver nanoparticlesin micro phase-separated PI blends as illustrated in Fig. 12. 46 PI blend films (sBPDA/SDA-TFDB) consisting oftwo-types of PIs having different affinities to silver ions were prepared. Silver nitrate (AgNO 3 ) dissolved in PAAsolutions was thermally reduced to metallic silver during imidization, which forms silver nano-particles in the films. Fig.12 shows the FE-SEM micrographs of the hybrid films, in which ‘double percolation’ structures orienting along the outof-planedirection were clearly formed, and the average size of each phase is in ~10 μm order. Energy-dispersive X-rayspectroscopy (EDS) also showed that silver nano-particles were preferentially precipitated in the S-PI phase due to thehigh affinity with noble metals. The thermal diffusivity along the out-of-plane direction (a ⊥ ) were measured with an ACtemperature-wave analyzer (thermal conductivity is the product of a and a specific heat). At 20 mol% of AgNO 3 , the PIblend films of SDA:TFDB=50:50 (mol%) exhibit a higher thermal diffusivity than the pristine PI having homogeneouslydispersed silver nanoparticles and the other PI blends having typical sea-island structures. This clearly implies that thedouble percolation structures thus generated function as thermal conductivity pathways along the out-of-plane direction.Small sea-island30μmLarge sea-island30μmDouble percolationS-PI(30)/F-PI(70)/AgNO 3 (20) S-PI(40)/F-PI(60)/AgNO 3 (20) S-PI(50)/F-PI(50)/AgNO 3 (20)30μmα = 17.6α = 17.9α = 20.9(x10 -8 m 2 /s)S-PINOOOONAgAgSAgAgxNOOOONCF 3F 3 C100-xF-PIFig. 12 Cross-sectional FE-SEM photographs of the micro phase-separated PI hybrid films consisting of sulfurcontainingPI (S-PI) and fluorine-containing PI (F-PI). The phase separation and silver precipitation areschematically illustrated below the photos. The S-PI phase containing silver nano-particles are seen as white parts.7. SUMMARYOrganic/inorganinc PI nanohybrid materials having optical functionalities, such a) high refractive indices (n=1.81), b)low refractive indices (n=1.47), c) controlled thermo-optical property (|dn/dT| >350 ppm/K) and its anisotropy, d) highpolarizing property (dichroic ratio > 25 dB), and e) good thermal conductivity have been developed by the nanohybridizationtechniques based on the pyrolytic reactions (reduction, oxidation, and decomposition) of organo-solublemetallic precursors followed by spontaneous precipitation of metal/inorganic nano-particles in solid PI films. Thistechnique enables us to develop novel opt0-electronic materials having a variety of functionality with maintaining theinherent excellent properties of PIs.Acknowledgement : The author acknowledges that the results reviewed in this paper have been obtained by fruitfulcollaborations with Prof. Mitsuru Ueda, Yuji Shibasaki, Sho-ichi Matsuda, Yoshiharu Terui, Akiko Matsumura, YasuoSuzuki, Nam Ho You, Atsuhisa Suzuki, Yasuhiro Nakamura (Tokyo Tech), Jin-gang Liu (present: the Chinese academyof science), and Claudio A. Terraza (present: Pontificia univ. Catolica de Chile).Proc. of SPIE Vol. 7213 72130B-7
Page 1 and 2: Invited PaperOrganic/Inorganic-Poly
Page 3: Calculated Absorption (arb.unit)Cal

Organic/inorganic-polyimide nanohybrid materials for advanced ...

Create successful ePaper yourself

Delete template?

Save as template?