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Keywords:

  • cyclopolymerization;
  • fluoropolymers;
  • high performance polymers;
  • hybrid composites;
  • optics;
  • perfluorocyclobutyl (PFCB) aryl ether polymer;
  • polymer light emitting diode (PLED);
  • proton exchange membrane (PEM)

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. POLYMER SYNTHESIS AND PROPERTIES
  5. APPLICATIONS
  6. TECHNOLOGY OUTLOOK
  7. Acknowledgements
  8. REFERENCES AND NOTES
  9. Biographical Information

This article highlights the preparation of perfluorocyclobutyl (PFCB) aryl ether polymers for a multitude of commercial technologies that are of academic and commercial global interest. In this account, the synthesis of various aryl trifluorovinyl ether (TFVE) monomers tailored for specific applications is discussed. The preparation of PFCB aryl ether polymers and their properties is then presented. Topics of PFCB aryl ether polymers and their applications include photonics, polymer light emitting diodes (PLEDs), proton exchange membranes (PEMs) for fuel cells, atomic oxygen (AO) resistant coatings, and hybrid composites. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5705–5721, 2007


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. POLYMER SYNTHESIS AND PROPERTIES
  5. APPLICATIONS
  6. TECHNOLOGY OUTLOOK
  7. Acknowledgements
  8. REFERENCES AND NOTES
  9. Biographical Information

Since the serendipitous discovery of polytetrafluoroethylene (PTFE) by researchers at DuPont in 1938, polymers possessing fluorine have pioneered the global development of many high performance materials for automotive, aerospace, biomedical, and military technology corridors.1 Fluoropolymers are highly desired because of their unique combination of thermal stability, chemical resistance, low surface energy, low refractive index, and high insulating ability. However, because of their high degree of crystallinity, many fluoropolymers are often difficult to solution- or melt-process and thus industrial manufacturing costs can be high. Indeed, successful examples by modification of polymer structure of low crystalline fluoropolymers include polychlorotrifluoroethylene (PCTFE), Teflon®-AF, Cytop®, and various copolymers of PTFE.

Perfluorocyclobutyl (PFCB) aryl ether polymers are an emerging class of semi-fluorinated polymers introduced by Dow in the early 1990s that have demonstrated analogous properties compared with commercial fluoropolymers, yet are entirely amorphous and solution processable. PFCB aryl ether polymers (2) are commonly prepared by free radical-mediated [2 + 2] thermal cyclodimerization of aryl trifluorovinyl ether (TFVE) containing monomers (1) at temperatures of 150−200 °C (Scheme 1).2–4 They can also be prepared by classic condensation polymerization using reactive 1,2-bishexafluorocyclobutyl aryl ether monomers.2 The first example of thermal cyclodimerization of fluoroolefins for advancing molecular weight was reported by DuPont in 1965 producing PFCB alkyl ether oligomers for thermosets.5 The only other successful reported linear polymerization of an aryl TFVE required extensive heating under very high pressure in the presence of gamma radiation.6

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Scheme 1. Thermal [2 + 2] cyclodimerization of aryl TFVE monomers to PFCB aryl ether polymers.

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PFCB rings were first discovered in 1947 in the thermal decomposition products of PTFE.5 Because of the high strain energy of the fluoroolefin, formation of the less sterically strained cyclobutane ring is thermodynamically favored.7 For oxygen-containing fluoroolefin ethers (R[BOND]O[BOND]CF[DOUBLE BOND]CF2), the activation energy for cycloaddition is further reduced by 15−20 kcal/mol as compared with alkyl fluoroolefins (R[BOND]CF[DOUBLE BOND]CF2) or halogen-containing fluorocarbons (X[BOND]CF[DOUBLE BOND]CF2).8–10 The thermal cyclodimerization of aryl TFVE aryl ether monomers produces stereo-random PFCB rings in primarily 1,2 fashion (i.e., head-to-head). The resulting extension of isomers produces an amorphous PFCB aryl ether polymer with excellent solubility and processability. It is this combination of thermal stability and solution processability that makes PFCB aryl ether polymers attractive for a multitude of new applications often inaccessible using commercial fluoropolymer preparative routes. A review by Ameduri and coworkers11 and consolidated accounts by Babb2, 3 have illustrated the general utility of PFCB alkyl/aryl ether polymers. Many research groups globally including our own are active in the preparation of new PFCB aryl ether polymers tailored for a broad range of applications. This article highlights this exciting class of highly processable, semi-fluorinated PFCB aryl ether polymers encompassing the most recent contributions from the worldwide field of researchers. Areas of technology utilizing PFCB aryl ether polymers that will be discussed include photonics, atomic oxygen (AO) resistant coatings, hybrid composites, proton exchange membranes for fuel cells, and liquid crystals.

POLYMER SYNTHESIS AND PROPERTIES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. POLYMER SYNTHESIS AND PROPERTIES
  5. APPLICATIONS
  6. TECHNOLOGY OUTLOOK
  7. Acknowledgements
  8. REFERENCES AND NOTES
  9. Biographical Information

Monomer and Polymer Synthesis

Traditionally, aryl TFVE containing monomers (1) are efficiently prepared in a two step process starting from phenolic precursors (3) via alkylation with 1,2-dibromotetrafluoroethane,12 followed by zinc mediated dehalogenation of the dibrominated intermediate 4 (Scheme 2). In 1993, Babb et al.4 at Dow first reported the synthesis of 3-trifluorovinyloxy-α,α,α-trifluorotoluene, 1,3-bis(trifluorovinyloxy)benzene, 4,4′-bis(trifluorovinyloxy)biphenyl, and 1,1,1-tris(4-trifluorovinyloxyphenyl)ethane from commercial phenols. This report demonstrated thermal chain extension of bis- and trisfunctionalized aryl TFVE monomers to PFCB aryl ether polymers 5 for thermoplastics (Fig. 1) and 9 R = CH3 for thermosets (Fig. 2). The thermal cyclopolymerization proceeds either as a melt or in high boiling solvents (e.g., diphenyl ether or mesitylene) without initiator or evolution of condensate. To date, a variety of mono-, bis-, and tris-functional aryl TFVE monomers have been prepared simply by choosing the appropriate phenolic starting materials.

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Figure 1. Thermoplastic PFCB aryl ether polymers prepared by aryl TFVE monomers synthesized from commercial bisphenols.

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Figure 2. Network PFCB aryl ether polymers prepared from trisphenols.

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Scheme 2. Preparation of aryl TFVE monomers from functionalized bisphenols.

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Other PFCB aryl ether thermoplastics containing hexafluoroisopropylidene (6),13 α-methylstilbene (7),14 and isomers of naphthalene (8)15 have been recently reported. Included in the family of thermosetting materials, a trifluoromethyl-functionalized PFCB aryl ether network 9 R = CF3 has also been recently prepared (Fig. 2).16 The ability to modestly increase the degree of fluorination has profound optical affects as will be demonstrated in this report. Many of the aryl TFVE monomers and PFCB aryl ether polymers have now been commercialized by Tetramer Technologies, L.L.C. and are commercially available by Oakwood Chemicals.

A methodology was established employing p-bromo (trifluorovinyloxy)benzene 10 as a versatile intermediate for the preparation of aryl TFVE monomer systems tailored for a myriad of applications (Scheme 3).17–19 The aryl TFVE-functionalized compound 11 can be easily prepared from inexpensive 4-bromophenol in high yield utilizing the aforementioned alkylation–reduction methodology. The reactive intermediate is prepared from 10 using well-established Grignard or organolithium halogen-metal exchange chemistry with the fluorinated olefin intact. Using this methodology, readily transformable functional groups have been installed on the aryl TFVE. These include carboxylic acid, acyl chloride, aldehyde, alcohol, and amine groups that can undergo further coupling or condensation. An important derivative is the boronic acid and stannyl aryl TFVE from which numerous bisfunctionalized aryl TFVE monomers were prepared by Pd-mediated Suzuki and Stille coupling. Phosphine-containing aryl TFVE monomers have also been prepared for use as Horner–Witting intermediates.

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Scheme 3. Halogen–metal exchange or Grignard reagent of 10 followed by substitution of electrophiles affords a diverse pool of reactive aryl TFVE functionalized intermediates for the preparation of aryl TFVE monomers.

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Scheme 3 demonstrates the utility of intermediate 11 from the aforementioned halogen-metal exchange strategy producing an abundance of bis- and tris-functionalized aryl TFVE monomers. Condensation of aryl TFVE dimethylchlorosilane [R = Si(CH3)2H] via dehydrogenative hydrolysis/methanolysis yielded the first siloxane-containing aryl TFVE monomer for the preparation of PFCB aryl ether fluorosilicones (12).19 Disilanol monomers containing PFCB aryl ether rings have also been prepared to produce fluorosilicones via condensation polymerization.20 Similarly, silicon-enriched PFCB aryl ether network polymers (23) were prepared by Kim and Heeger using phenethylsilane trifunctional aryl TFVE monomer prepared by the condensation of lithiated 10 with alkylchlorosilanes.21 Trapping the lithium intermediate of 10 with FSO2Cl followed by amination afforded sulfonimide aryl TFVE monomer (R = SO2NH2) that produces various PFCB aryl ether polymers with fluorosulfonimide backbones similar to 16.22 Triaryl phosphine oxide functionalized PFCB aryl ether polymers (14) were prepared from the respective aryl TFVE monomer by quenching the Grignard of 10 (R = MgBr) with phosphorus trichloride or phenylphosphonic dichloride.23 Pd-catalyzed Suzuki coupling of the boronic acid or ester 11 [R = B(OH)2 or B(OR)2] with halogen-containing heteroaromatics afforded the respective aryl TFVE monomers used to prepare monomers for PFCB aryl ether polymers containing pyrimidine (22),24 terphenyl (13),25 triarylamine (20). Similarly, Sonogashira Cu-coupling of intermediate 9 was used to prepare hexa-peri-hexabenzocoronene PFCB aryl ether networks (17).26 Harris and Choi27 showed the construction of imide-functionalized PFCB aryl ether polymers (15) can be prepared starting from the amine aryl TFVE monomer (R = NH2). The ability to couple the aryl TFVE carboxylic acid of 11 (R = COOH) with primary alcohols was used for the preparation polyhedral oligomeric silesquioxanes (POSS) PFCB aryl ether copolymers (19).28 Similarly, Jen has shown coupling with the aryl TFVE carboxylic by anchoring chromophore-functionalized oligomers via postesterfication to afford polymer 21 and other analogous dendrimers.29 The aryl TFVE carboxylic acid monomer has also been used to chelate rare-earth metals as shown with PFCB aryl ether network (18).30 PFCB aryl ether polymers functionalized with crown ether vertebrae (24) have also been reported utilizing the condensation with the acyl chloride aryl TFVE (R = COCl) to prepare the starting aryl TFVE monomer.31 The PFCB aryl ether polymers shown in Figure 3 are not all-encompassing examples prepared to date; rather each polymer are representative examples spanning a broad range of technological applications. The polymers related to their applications will be discussed in the later portion of this report.

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Figure 3. Diverse family of PFCB aryl ether polymers from aryl TFVE monomers prepared utilizing the halogen−metal exchange intermediate strategy.

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Several groups have reported a step-wise classical polymerization in conjunction with thermal PFCB aryl ether polymerization. As shown in Figure 4, Neilson etal.32 prepared a series of poly(alkyl/aryl)phosphazenes prepolymers with reactive pendant aryl TFVE moieties (25) by employing condensation polymerization of N-silylphosphoranimines. PFCB aryl ether-based ABA triblock copolymers with poly(styrene) (26)33 and methyl methacrylate34 have also been prepared using atom transfer radical polymerization (ATRP). These examples demonstrate the utility of incorporating aryl TFVE groups that still possess latent reactivity to form PFCB aryl ether polymers. As another example, Ameduri and coworkers11 published the radical copolymerization with fluoroolefins such as vinylidene fluoride.

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Figure 4. Poly(methylphenyl-phosphazine) with PFCB aryl ether chain extensions (25) and ABA poly(styrene)-PFCB aryl ether triblock copolymers (26).

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As previously mentioned, an alternate and less commonly reported procedure for preparing PFCB aryl ether polymers has been performed using the PFCB aryl ether dimer intermediate 27 as a monomer (Scheme 4). Examples include condensation polymerization of intermediate 27 with 4,4′-(1-phenylethylidene)diphenol (28),2 4-phenylazoaniline (29),35 4,4′-sulfonyldiphenol (30),36 and polyethylene glycol diazides (31).37 The intermediate 27 (where X = OH) was shown to undergo facile addition to the aryl TFVE aryl ether monomers, producing a novel polymer system 32 with difluorovinylene aryl ether ([BOND]O[BOND]CF[DOUBLE BOND]CF[BOND]O[BOND]) enchainment.28 The formation of these semi-fluorinated aryl ether polymers are tunable to a variety of applications based on the substitution of the bisphenol segment such as curing additives,38 proton exchange membranes (PEMs) for fuel cells,39 light emitting polymers for PLEDs,40 and chemical sensing. In all cases, the copolymers were shown to possess similar thermal stability and processability compared with PFCB aryl ether homopolymers or copolymers using the traditional thermal [2 + 2] cyclodimerization of aryl TFVEs.

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Scheme 4. Route to PFCB aryl ether copolymers via condensation polymerization using PFCB aryl ether dimer intermediate 27.

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Polymer Properties

As a result of their molecular composition, PFCB aryl ether polymers possess excellent performance properties such as ease of processing, chemical and thermal stability, and the ability to easily tailor for specific applications (Fig. 5). Cyclodimerization of aryl TFVEs afforded a statistical head-to-head mixture of (cis-1,2; cis-1,2), (cis-1,2; trans-1,2), and (trans-1,2; trans-1,2) PFCB aryl ether rings in 1:2:1 ratio as determined by gas chromatography coupled with mass spectroscopy (GC-MS).4 In addition, less that 10% of 1,3-substituted PFCB aryl ether is observed due to head-to-tail cyclodimerization. It was later shown that isomers of PFCB aryl ether dimers can be separated by fractional crystallization and was further elucidated by X-ray crystallography.41 Cycloaddition kinetics have been studied in a variety of ways, including differential scanning calorimetry (DSC), time-resolved electron paramagnetic resonance (EPR),42 and Raman spectroscopy.43, 44 Using thermal-gravimetric analysis (TGA) coupled with mass spectrometry (MS), the products of decomposition were studied by Babb and coworkers.45 Interestingly, PFCB aryl ether did not undergo ring scission, rather the degradation mechanism results in the expulsion of perfluorobutenes from the polymer system. Hexafluorocyclobutene, 1,3-hexafluorobutadiene, and phenol were the major components of the decomposition gases observed by MS, minor amounts of aryl TFVE were detected providing evidence of thermal reversibility.

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Figure 5. Design motif of high performance PFCB aryl ether polymers. Structure−property relationships of the polymer system designed for processability, versatility, and performance.

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As summarized in Table 1, several physical properties of PFCB aryl ether polymers have been examined. Melting endothermic transitions and subsequent exothermic polymerization events beginning near 150 °C using DSC indicate similar monomer reactivity.48 Copolymer and terpolymer reactions show likewise behavior by DSC. Most linear PFCB aryl ether polymers are solution processable in common organic solvents such as chloroform (CHCl3), tetrahydrofuran (THF), cyclopentanone, and dimethylsulfoxide (DMSO) to name a few. The highly fluorinated PFCB aryl ether polymer possessing hexafluoroisopropylidene (6) was shown to lower the pressures and temperatures needed to dissolve a given polymer in supercritical carbon dioxide and propane.49, 50 After spin coating, the cyclopolymerization is progressed by heating under inert atmosphere to the desired degree of conversion. Optical absorption for various PFCB aryl ether homopolymers were studied in the wavelength spectrum range, 200−3200 nm.51

Table 1. Selected Physical Properties of PFCB Aryl Ether Polymers
PropertyRangeRef(s)
Mechanical
 Glass transition (DSC or DMA)110–350 °C4, 46
 Thermal decomposition (TGA in N2 & air)>450 °C4
 Tensile strength50.3–66.0 MPa4
 Tensile modulus1,770–2,270 MPa4
 Flexural strength74.0–92.4 MPa4
 Flexural modulus1,779–2,320 MPa4
 Percent elongation (break)4.1–12.5%4
 Interfacial shear123–163 MPa47
 Hardness175–653 MPa47
Processing
 Cure temperature/time (°C)150–220/0.1–3.0 h46
 Molecular weight (GPC)1,200–30,000 Mw46
 Solution viscosity (RMS)0.02–100 Pa s46
 Crystallinity (WAXD)0–35%13
 Solid content50–90%46
 Patterning techniqueμ-molding, RIE, O2 plasma46
 Spin coat thickness (μm, single)1–3046
 Percent water absorption (wt %) (24 h)0.02146
Optical
 Loss (1,550 nm)<0.25 dB/cm46
 Birefringence<0.00346
 Refractive index (1,550 nm)1.442–1.50546
 dn/dt (1,550 nm)−0.7 to −1.5 (×10−4)46
Insulation
 Dielectric constant (10 kHz)2.4–2.454
 Dissipation factor (10 kHz)0.0003–0.00044

APPLICATIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. POLYMER SYNTHESIS AND PROPERTIES
  5. APPLICATIONS
  6. TECHNOLOGY OUTLOOK
  7. Acknowledgements
  8. REFERENCES AND NOTES
  9. Biographical Information

Photonics

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 C[BOND]H 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.

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Figure 6. TEM image of dispersed PbS QDs (10 wt %) in PFCB aryl ether polymer 5.

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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).

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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

Electro-Optics

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

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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).

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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
MaterialAO Fluence(atoms/cm2)Mass Loss (%)Solar Absorptance (α)Thermal Emittance (ε)
Pre-exposurePost-exposurePre-exposurePost-exposure
  • a

    Tests were carried out at the NASA Marshall Space Flight Center's AO Beam Facility.

  • b

    AO fluence normalized mass loss %.

PPO PFCB Copolymer2.6 × 10202.2 (0.85)b0.1630.1930.6800.650
PFCB Polymer 5 (no PPO)3.88 × 102033.2 (8.6)b0.1440.1640.5650.411

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

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Scheme 5. Direct sulfonation of PFCB aryl ether polymers.

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Hybrid Composites

The preparation of inorganic−organic hybrid polymers has produced nanocomposites with the ability to engineer chemical and physical properties.147 Specifically, polyhedral oligomeric silsesquioxanes (POSS) have well-defined nanometer-length scales and possess the unique ability to compatabilize with organic-based polymers enhancing the properties of the virgin polymer.148 We have utilized the synthetic versatility, excellent processability, and thermal performance of PFCB aryl ether polymers combined with well-defined POSS structures to produce covalently bound and blended composites. Specifically, our earlier work in collaboration with Laine and coworkers reported the synthesis of octa(aminophenyl)silsesquioxane (OAPS) functionalized PFCB aryl ether polymer network as a platform for new high strength nanocomposites with optical clarity (34) (Fig. 12).149 More recent work encompassed the preparation of improved thermally stable, solution processable linear POSS PFCB aryl ether polymers with POSS appended to the chain ends (19)28 and as pendants (35).150 Blending PFCB aryl ether polymers 5 or 6 with fluorinated POSS has also enhanced the water and oil repellency of the homopolymer.151 In both cases, the composites are prepared with optical integrity preserved without compromising properties of the PFCB aryl ether polymer. The preparation of PFCB aryl ether sol-gel materials was also prepared with increased thermal stability with optical clarity intact.71

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Figure 12. Examples of PFCB aryl ether polymer architectures functionalized with POSS.

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TECHNOLOGY OUTLOOK

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. POLYMER SYNTHESIS AND PROPERTIES
  5. APPLICATIONS
  6. TECHNOLOGY OUTLOOK
  7. Acknowledgements
  8. REFERENCES AND NOTES
  9. Biographical Information

Since the seminal publication on PFCB aryl ether polymers by Babb in 1993, considerable attention from the academic as well as the industrial community has been drawn to these unique semi-fluorinated polymers. The ability to easily functionalize aryl TFVE monomers from commercial phenols has produced PFCB aryl ether polymers tailored toward specific applications. Clearly, this has been demonstrated in this report where technology areas utilizing PFCB aryl ether polymers have enhanced performance capabilities in photonics, space durable materials, fuel cell membranes, and other high performance applications. In spite of these notable advancements, the author feels PFCB aryl ether polymers can address other performance limitations associated with traditionally-fielded polymer systems in addition to new opportunities that encompass the need for processable thermoplastic or thermosetting materials with high thermal stability.

Acknowledgements

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. POLYMER SYNTHESIS AND PROPERTIES
  5. APPLICATIONS
  6. TECHNOLOGY OUTLOOK
  7. Acknowledgements
  8. REFERENCES AND NOTES
  9. Biographical Information

Acknowledgment is made to the National Science Foundation (NSF) and Defense Advanced Research Projects Agency (DARPA) for financial support. D.W. Smith, Jr. expresses thanks to John Ballato (CU), Stephen Foulger (CU), Darryl DesMarteau (CU), Dvora Perahia (CU), Gregory Nordin (BYU), Richard Riman (Rutgers), Joseph Mabry (Air Force Research Laboratory), and Claire Callender [Communications Research Centre (CRC)] for continued collaboration. He also acknowledges Earl Wagener and Chris Topping from Tetramer Technologies, L.L.C. for their contribution.

REFERENCES AND NOTES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. POLYMER SYNTHESIS AND PROPERTIES
  5. APPLICATIONS
  6. TECHNOLOGY OUTLOOK
  7. Acknowledgements
  8. REFERENCES AND NOTES
  9. Biographical Information

Biographical Information

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. POLYMER SYNTHESIS AND PROPERTIES
  5. APPLICATIONS
  6. TECHNOLOGY OUTLOOK
  7. Acknowledgements
  8. REFERENCES AND NOTES
  9. Biographical Information
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Mr. Iacono

Mr. Iacono received a B.S. (Chemistry) from Louisiana State University in 2000 and M.S. (Chemistry) from The University of Texas at Austin in 2002. He was then commissioned as an officer in the U.S. Air Force and assigned as a research chemist at the Air Force Research Laboratory, Space and Missile Propulsion Division, Edwards AFB, CA. There he was a program manager and project leader for synthesizing new high-yield carbon fiber precursors for solid rocket motor components. He was selected for an Air Force educational sponsorship beginning his Ph.D. graduate work at Clemson University in the fall of 2005. Under the advisement of Dr. Dennis W. Smith, Jr., he is currently investigating inorganic-organic hybrid fluoropolymers for hydrophobic coatings, polymer light emitting diodes (PLEDs), chemical sensors, and latent crosslinkable fluoroelastomers. He was selected as a finalist for the 2007 ICI Student Award in Applied Polymer Science sponsored by the Polymer Education Committee of the ACS Divisions of Polymeric Materials: Science and Engineering and Polymer Chemistry. He has currently published nine refereed journal publications and has 36 published conference proceedings.

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Mr. Budy

Mr. Budy received a B.S. (Chemistry) from Humboldt State University in 2000 and a M.S. (Chemistry) from Syracuse University in 2003 working on bacteriorhodopsin for holographic gratings. He joined Clemson University in 2004 to pursue his Ph.D. where he has been working on perfluorocyclobutyl (PFCB) aryl ether polymers for optical applications, including silica-based fiber coating and hollow fibers. He has also been focusing on “self-healing” polymers with thermally reversible crosslinking capabilities; including polystyrenes, polycarbonates, polyesters, and polyurethanes. He was awarded many scholarships and fellowships including a Carl Storm Underrepresented Minority Fellowship and a Gordon Conference Travel Award. He is also a member and recipient of the Southeast Alliance for Graduate Education and the Professoriate (SEAGEP) Fellowship. He has published three refereed journal papers and presented his work at over 15 conferences.

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Dr. Jin

Dr. Jin received a B.Sc. (Chemistry) from Dalian University of Technology in 1996, M.Sc. (Macromolecular Science) from Fudan University in 1999 and a Ph.D. (Chemistry) from Clemson University in 2005 under the supervision of Dr. D.W. Smith, Jr. and Dr. S.H. Foulger. His dissertation focused on the synthesis, characterization, and application of perfluorocyclobutyl (PFCB) aryl ether polymers. He then joined Promerus Corporation, where he worked as an industrial postdoctoral researcher in the synthesis of norbornene-based polymers for microelectronic packaging. In 2006, he joined Tetramer Technologies, L.L.C. where he has been actively involved in the development of quantum dot encapsulation, gas separation membrane, proton exchange membranes, renewable resource derived materials. He has published 16 refereed journal publications. In 2004, he received an Excellence in Polymer Graduate Research Award by American Chemical Society Polymer Chemistry Division.

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Dennis W. Smith

Professor Dennis W. Smith, Jr. received his B.S. (1988) from Missouri State University and his Ph.D. (1992) from the University of Florida under the guidance of Prof. Ken Wagener on the scope and mechanism of acyclic diene metathesis (ADMET) polymerization. He was a Rhone Poulenc Graduate Research Fellow in Lyon, France pursuing novel silicone elastomers, and a Dow Chemical Postdoctoral Fellow (1993) with Dr. Raymond König in Rheinmünster, Germany exploring fundamental aspects of epoxy networks. Dr. Smith joined The Dow Chemical Company Central Research Laboratory as Sr. Research Chemist (1993), and later was promoted to Project Leader (1996) working primarily on the synthesis and characterization of high performance thermosets for thin film microelectronics applications. He served as National Chemistry Week (NCW) Coordinator (1996) and Chair of the Brazosport Section of the American Chemical Society (1997). He was a recipient of the 1997 Dow Chemical Central Research Inventor of the Year Award before joining Clemson in 1998. He is a recipient of a National Science Foundation Early Faculty CAREER Award, 3M Pre-Tenured Faculty Award, Clemson University Award for Faculty Achievement in the Sciences, Cottrell Scholar of Research Corporation, and was named 2005 Outstanding Faculty Member by the Graduate Student Association. Dr. Smith is a founding member and Associate Director of Clemson's Center for Optical Materials Science & Engineering Technologies (COMSET). He has also served the American Chemical Society Division of Polymer Chemistry as Councilor, Founder and Organizer of the international conference, Fluoropolymer 2000, 2002, 2004, and 2006, and in 2006 he was elected Vice Chair of the Polymer Division. He is the current Editor of the journal, Polymer Bulletin, and serves on the editorial board for High Performance Polymers, Polymers for Advanced Technology. Dr. Smith's research interest includes synthesis, mechanism, structure/property relationships, and applications of polymeric materials and composites including (1) fluoropolymers from aryl trifluorovinyl ethers, (2) polyarylenes from bis-ortho-diynyl arenes (BODA), and (3) renewable polymers based on lactide and rubber recycling.