Polymeric optical waveguides are becoming a high priority as technology forges ahead. Traditional inorganic crystals and glasses are continuing to hold steady decades after their first introduction. Recently, polymers are beginning to move into the communication field as optical networks and computing systems move into homes.52 Planar waveguide technology, in particular, is being researched as a solution to the traffic jam from long haul, oversea delivery of information to the short, metropolitan connection.53 Polymers including polyacrylates,54, 55 polyimides,56, 57 polyethers,58, 59 and polysiloxanes60, 61 fill this requirement over inorganic counterparts because of their well-known optical properties, ease of processing, cost effectiveness, flexibility, toughness, tunability, and long term stability. However, in such systems CH vibration becomes the most significant role of attenuation in the near infrared (NIR) region. Attempts to replace the hydrogen have shown to reduce or remove the vibrations and overtones entirely in the communication wavelengths.62 Therefore, fluoropolymers have gained the most consideration because of their extended strength and chemical, temperature, and oxygen resistance.
Therefore, PFCB aryl ether polymers possess the desirable properties for optical waveguide applications such as excellent processability, low dielectric constant, low moisture absorption, good oxidative resistance, good thermal stability, high dn/dT value, low birefringence, and excellent optical transparency.30, 51, 63 Fischbeck et al.64 reported the first polymer waveguide using PFCB aryl ether polymers exhibiting both thermal stability in excess of 250 °C as well as transmission losses of <0.3 dB/cm in the range of 1535−1565 nm. Wong et al.,65 demonstrated a variety of PFCB aryl ether polymers with a combination of good thermal and chemical stability, controllable refractive index, low birefringence, and low optical loss. Their group also introduced siloxane-containing structural units into PFCB aryl ether polymers because of their chemical, thermal, and oxidative resistance and flexibility at low temperatures.66 Ma et al.,67, 68 achieved low optical loss, excellent processability, high thermal stability, high solvent resistance, and low surface roughness by synthesis and polymerization of a crosslinkable fluorinated dendritic PFCB aryl ether polymer. Kim et al.,69 described synthesis, analysis, characterization, and applications of fluoropolymers containing PFCB aryl ether rings. Their group also looked into the isomeric effect by synthesizing a variety of naphthalene-based PFCB aryl ether polymers.15, 70 Lee et al.,71 synthesized a sol–gel hybrid material containing PFCB aryl ether groups with the ability to change the refractive index (1.4568–1.4876), birefringence (–0.0003 to 0.0029), and optical loss (0.34–0.47 dB/cm at 1550 nm) by changing the comonomer feed ratio.
Nanocrystals and clusters such as quantum dots (QDs) are chemically synthesized semiconducting nanometer-sized crystalline particles, which exhibit unique optical and spectroscopic properties, such as broad absorption, narrow and tunable emission, resistance to photobleaching, efficient luminescence, and long luminescent lifetimes.72 They are promising building blocks for a range of photonic applications,73–75 as well as the immerging fields of medicinal and biological imaging applications.76 While the particles easily disperse in various solvents, to utilize their functionality in practical integrated photonic systems, they need to be encapsulated into a more robust but highly functional matrix. To meet this need, Riman and coworkers77–79 reported the fabrication of PFCB aryl ether polymer nanocomposites containing infrared-emitting nanocrystals. Rare-earth doped nanocrystals were incorporated into the PFCB aryl ether polymer matrix via solvent blending. This method provides a versatile platform for the integration of these nanometer-sized crystals as planar optical amplifiers. As another example, Fuchs and coworkers80 fabricated PFCB aryl ether polymer waveguides containing infrared-emitting QDs including PbSe, InAs, InAs/ZnSe, and CdSe/ZnS. PFCB aryl ether polymer films containing InAs QDs were shown for the first time to exhibit an optically induced population inversion and gain, based on a three-beam pump-probe technique.81 Chen et al.82 functionalized the QD capping ligands compatible with the chemical structure of the polymer, thereby allowing loading levels as high as 20 wt %. This is shown in Figure 6 with a TEM image of 10 wt % PbS QDs uniformly dispersed in PFCB aryl ether polymer 5.
Our group has focused on structure-property tunability of PFCB aryl ether thermoplastics and thermosets,48 thereby leading to a combination of physical, chemical, and optical properties easily fabricated into low loss waveguide structures.83 We have also prepared submicron optically diffractive line gratings (580 nm) for waveguide structures by direct micro transfer molding using only a silicon master on 100-μm thick PFCB aryl ether polymer film (Fig. 7).46, 84, 85 In collaboration with Nordin and coworkers,86 we have reported the design, fabrication, and measurement of highly efficient, compact 45° single air interface bends (SAIBs) using standard microfabrication techniques using PFCB aryl ether polymers on silicon substrates (Fig. 8). They show theoretically and experimentally the optical efficiency of waveguide bends and splitters can be increased when using air trench structures.87, 88 Recently, they have developed an anisotropic high aspect ratio (18:1) etch for PFCB aryl ether polymers with trenchs as narrow as 800 nm; thereby allowing their air-trench bends to reduce thearea required by a factor of 20, as compared with conventional arrayed waveguide gratings.89, 90 Jiang et al.91, 92 from the Communications Research Center in Canada, used PFCB aryl ether polymers as core and cladding materials to fabricate compact wide-band wavelength division multiplexers, based on arrayed waveguide grating structures, for coarse wavelength division multiplexing (Fig. 9).
Figure 7. Scanning electron micrographs of line gratings micromolded into PFCB aryl ether polymer films by a silicon master. (From D.W. Smith, Jr., et al., Adv Mater 2002, 14, 1585−1589, © 2002 Wiley-VCH Verlag GmbH & Co. KGaA, reprinted with permission).
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Figure 8. Microscope image taken with a DIC filter through a ×50 objective focused at the waveguide plane of SAIBs in good alignment (left). Scanning electron micrograph of finished SAIB (right). The rounded edges were introduced to reduce stress. The dotted line shows the waveguide core location. (From S. Kim, et al. Proc SPIE 2005, 5729, 264−276, © 2005 Society of Photographic Instrumentation Engineers, reprinted with permission.)
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Figure 9. Scanning electron micrograph showing arrayed waveguide grating. (From J. Jiang, et al., J Lightwave Tech 2006, 24, 3227−3234, © 2006 Institute of Electrical and Electronics Engineers (IEEE), reprinted with permission.)
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Polymer Light Emitting Diodes
The discovery of efficient electroluminescence in organic and conjugated polymer thin films generated a tremendous research effort toward the design and fabrication of organic or polymeric light-emitting diodes (OLEDs or PLEDs).93–95 Polymeric emissive displays produce high contrast images while eliminating the cost, space, weight, and high power consumption of inorganic-based devices. However, OLEDs suffer from poor lifetimes and low quantum efficiencies. Thus, there is an increasing need for high performance organic polymers as substrates for light emitting devices. Recently, the use of fluorinated polymers revealed the elimination of excimer or exciplex formation and thus the enhancement of the EL brightness at reduced turn on voltage (because of the hindrance of F atoms).96 PFCB aryl ether polymers offer a series of advantages for use as emissive materials. To date, these semi-fluorinated polymers have only been used as hole-transport layers, due to their ease of processing and thermal polymerization and chemical resistance; as compared with the conventional PEDOT:PSS layer. Jen and co workers97–105 showed that PFCB aryl ether polymers functionalized with hole-transport components, for example 21, has identical or better performance, as well as high transmission in the entire visible spectrum. One noteworthy example utilizing crosslinked PFCB aryl ether polymer such as 23 was as a gate dielectric for organic thin film transistors (OTFTs), showing a high on/off ratio.21 Their high Tg, high thermal stability, superior optical clarity, and broad tailorability help to produce efficient devices with long lifetimes.97
Hexa-peri-hexabenzocoronene (HBC) networks, pioneered by Müllen,106, 107 have advanced in materials applications such as liquid crystals, hole-transporting layers, and photonics.108 Poor solubility is a limitation of HBCs, resulting in limited processing methods. As shown in Figure 3, HBC core structures were functionalized with aryl TFVE moieties.26 Thermal polymerization yields processable PFCB aryl ether polymers with polyaromatic core shells (17) that exhibit unique photoluminescence. Our group is currently active in preparing light-emissive PFCB aryl ether polymers with tunable photoluminescence by incorporating chromophore segments of fluorene, thiophene, and p-phenylvinylene.109
As we move further into the future, demands on our communication and computing technologies have dramatically increased. Telecommunications, in particular, have bandwidth expansion growing at an exceeding rate and must increase even more rapidly; limitations of electronics in signal propagation and switching speeds brings the role of photonics to the forefront of research and development. Nonlinear optical (NLO) materials have been growing in interest and attention for almost two decades now.110–113 Second-order NLO properties come from the non-centrosymmetric alignment of the NLO chromophores; guest-host doping, covalently bound side-chains, or self-assembly.114, 115 To achieve wide spread commercial use of electro-optic (EO) applications, NLO materials must have low dielectric constants, constant refractive index over wide wavelength (telecommunications) and temperature ranges, high dipole (μ) and hyperpolarizable (β) chromophores, long-term thermal stability, low attenuation, and be processable for the preparation of photonic devices.116–119
PFCB aryl ether polymers encompass excellent properties suitable for EO applications. Incorporating second-order NLO chromophores into PFCB aryl ether polymers gives the composite material high EO coefficients, low optical loss, high glass transition temperatures, and good processability. Other research groups as well as our own are currently studying PFCB aryl ether polymers for possible EO applications.120–129 Our group is currently investigating PFCB aryl ether polymers functionalized with a series of triaryl amino-based chromophores that possess unprecedented hyperpolarizability values (20,000 × 10−30 esu measured at 1.6μm).130–132
Liquid Crystalline Polymers
Liquid crystalline (LC) polymers are used as displays in electro–optic devices and require interfacial ordering as well as thermal stability; fluoropolymers, in particular, are quite advanced in this respective area.108 Attempts to improve on LC materials were envisioned using low surface area fluorine-containing polymers to influence bulk properties. Specifically, the mesogenic nature of α-methylstilbene containing polymer 7 imparts LC ordering similar to LC epoxy thermoset polymers.14, 133, 134 The polymer obtained by thermal cyclopolymerization forms lyotropic lamellar mesophases over a wide range of temperatures and molecular weights. Birefringent textures observed by optical microscopy are consistent with a nematic phase. Slight shear was essential in order to induce the self assembly (Fig. 10). Furthermore, film studies using neutron-reflectometry show self-assembly, producing phase separation of fluorinated segments from the non-fluorinated segments.134
Figure 10. Optical microscope images of PFCB aryl ether polymer 7 without shear (left) and with shear (right). (Reprinted from Ref.133, with permission from American Chemical Society.)
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We have also prepared a new class of thermotropic LC PFCB aryl ether polymers (13) improving the processability of poly(p-phenylene)s (PPPs) through the installation of PFCB group as a solubilizing segment into the PPP main chain.25 The polymerization induced LC formation mechanism was observed. Initially, monomer melted to form a clear isotropic liquid that does not exhibit birefringence under hot stage polarized optical microscopy. As the thermal polymerization proceeded, birefringent (nematic) droplets were observed and the droplets continued to grow coalescing into a continuous colorful birefringent film characteristic of Schlieren textures (Fig. 11).
Figure 11. Nematic droplets at the early stage (left) and final Schlieren texture (right) observed for polymerization to 13 during polymerization at 190 °C using hot stage polarized optical microscope. (From J. Jin, et al., Macromolecules 2006, 39, 4646−4649, © 2006 American Chemical Society, reprinted with permission.)
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Space Durable Materials
Polymers possessing phenylphosphine oxide (PPO) backbones have received much interest for flame resistance,135 metal complexation,136, 137 and atomic resistance.138 PPO containing polymers are observed to be amorphous in nature providing low birefringence139 and high refractive index.135 Of particular note, Connell and coworkers140 at NASA Langley have developed a series of PPO containing polymers that have shown excellent AO resistance in short-term space flight studies and ground-based spaces simulations.
To enable PFCB aryl ether polymer AO resistance, we developed a method to functionalize aryl TFVE monomers with the the PPO group and polymerize through the traditional thermal [2 + 2] cyclodimerization. To date, our group has prepared PPO functionalized PFCB aryl ether polymers with similar structure of 14 in addition to three pendent PPO containing polymers interconnected with amide, imine and ester linkages (Fig. 3).141, 142
AO durability of PPO containing PFCB aryl ether polymers was evaluated at the NASA Marshall Space Flight Center AO beam facility by mass loss caused by erosion after an AO exposure level of 2−6 × 020 atoms/cm2 [equivalent to an exposure of four months duration in low earth orbit] and with accompanied UV irradiation comparable with several hundred equivalent solar hours (ESH). Table 2 presents the data for a PPO PFCB aryl ether copolymer of 14 containing 12.5 wt % PPO segment. The copolymer experienced only 2.2 wt % mass loss compared with 33.2 wt % for homopolymer 5 (without PPO). These preliminary results clearly indicate an order of magnitude increase in AO-erosion resistance for a PFCB aryl ether copolymer containing PPO segments.
Table 2. Space Environment Simulation Data for Selected PFCB Aryl Ether Polymersa
|Material||AO Fluence(atoms/cm2)||Mass Loss (%)||Solar Absorptance (α)||Thermal Emittance (ε)|
|PPO PFCB Copolymer||2.6 × 1020||2.2 (0.85)b||0.163||0.193||0.680||0.650|
|PFCB Polymer 5 (no PPO)||3.88 × 1020||33.2 (8.6)b||0.144||0.164||0.565||0.411|
Proton Exchange Membranes (PEMs) for Fuel Cells
Polymers functionalized with acid groups have dominated as the ion-exchange components for PEMs in hydrogen-oxygen and direct methanol fuel cells.143 Perfluorinated polyethers such as DuPont's Nafion® polymer system has been one of the most widely studied for fuel cell membrane materials. However, processability, chemical resistance, and low-moisture absorption of the matrix material are requirements in order develop a practical and energy efficient fuel cell.
PFCB aryl ether polymers possess the desired environmental sustainability, yet are highly processable which makes them suitable for PEM materials. Perfluorinated sulfonimides are suitable candidates as substitutes for perfluorinated polyether-based polymers.144 DesMarteau and coworkers22 have prepared PFCB aryl ether polymers installed with perfluorinated sulfonimides representative of 16. The sulfonimide-functionalized PFCB aryl ether polymers were fabricated into moldable, transparent, free-standing films that demonstrated excellent thermal stability. Conductivities tested for these developed systems approach that of Nafion® 110 EW. As another approach, post-sulfonation of PFCB aryl ether polymers, for example 5 to produce 33, either by SO3, oleum, or chlorosulfonic acid has also been demonstrated as a potential PEM material (Scheme 5).145 Alternatively, sulfonated aryl TFVE monomers have been prepared and undergo thermal [2 + 2] cyclodimerization to afford PFCB aryl ether polymers similar to 33.144, 146