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A series of poly(ether ketone) copolymers were prepared by nucleophilic aromatic polymerization reactions of 4-fluoro-4′-hydroxybenzophenone, 2, in the presence of varying percentages of ABn monomers based on a triarylphosphine oxide platform, 1a (2F), 1b (4F), and 1c (6F), where A=OH and B=F. As expected, the crystallinity of the samples decreased with an increasing ABn content. However, the tetrahedral geometry of the phosphine oxide-based ABn monomers proved to be much more efficient at lowering the melt temperature of the copolymers than was the corresponding ketone-based ABn monomer, 3,5-bis(4-fluorophenylbenzoyl)phenol, 4, that possesses a structure more similar to that of 2. Polymerization of 2 in the presence of as little as 5 mol% of bis-(3,4,5-trifluorophenyl)-(4-hydroxyphenyl)phosphine oxide, 1c (6F), afforded a completely amorphous polymer with a glass transition temperature of 168 °C that was soluble in hot NMP and DMSO. The copolymers also exhibited excellent thermoxidative stability with a number of samples displaying 5% weight loss temperatures, in air, well in excess of 500 °C.

Multiwalled carbon nanotubes (MWNTs) (diameter range, 10−20 nm) were functionalized with 4-methoxybenzoic acid and 4-ethoxybenzoic acid via a Friedel−Crafts reaction in polyphosphoric acid to afford methoxybenzoyl- and ethoxybenzoyl-functionalized MWNTs. As-received MWNT, methoxybenzoyl-functionalized (MeO-MWNT), and ethoxybenzoyl-functionalized (EtO-MWNT) nanotubes were dispersed in ethylene glycol (EG). Because of the structural similarity, the mixture of EtO-MWNT (0.4 wt %) and EG was a homogeneous dispersion, whereas MeO-MWNT and pristine MWNT were dispersed in EG rather heterogeneously at the same loading. In situ polycondensation of EG and terephthalic acid in the presence of pristine MWNT, MeO-MWNT, or EtO-MWNT was carried out to generate the corresponding MWNT/PET, MeO-MWNT/PET, and EtO-MWNT/PET nanocomposites. High molecular weight poly(ethylene terephthalates) (PETs), with intrinsic viscosity range 0.6−0.7 dL/g (o-chlorophenol at 30 ± 0.1 °C), were obtained in all cases. In comparing the images ...

Linear A- sp 3 -D molecular conjugates as 9,9-dialkyl-2-diphenylaminofluorene (DPAF-C,)-C 60 monoadducts were demonstrated using methanoketo unit as a linker to bridge the DPAF donor moiety and the fullerene acceptor chromophore together within a short distance of roughly 2.0 A. Target products of C 60 -diphenylaminofluorene dyads C 60 (>DPAF-C 2 ) and C 60 (>DPAF-C 18 ) were synthesized using Bingel cycloproanation reaction from the corresponding 7- α -bromoacetyl-9,9-dialkyl-2-diphenylaminofluorene precursors. Both dyads C 60 (>DPAF-C 2 ) and C 60 (>DPAF-C 18 ) were characterized by spectroscopic methods and simultaneous two-photon excitation measurements, showing large two-photon absorption cross-sections in the nanosecond regime.

We recently described an improved method to synthesize poly(ether-ketones) or PEKs. It utilized an optimized mixture of polyphosphoric acid (PPA) and P 2 O 5 that is not only milder, less corrosive and less expensive than super-acid media, but also can play the multiple roles of solvent, Friedel–Crafts catalyst and dehydrating agent. The as-prepared PEKs from such a highly viscous reaction medium displayed unexpected, thermally induced relaxation exotherms regardless of the amorphous or semi-crystalline nature of polymer. This thermal behavior was not observed for the formally identical polymers [viz. p PEK or poly(oxy-1,4-phenylenecarbonyl-1,4-phenylene] which is normally semi-crystalline) that were separately prepared in a much less viscous mixture of methanesulfonic acid and P 2 O 5 (Eaton's reagent) or N -methyl-2-pyrrolidinone (NMP). Such an observation was first made when both samples of p PEKs were subjected to the same thermal history and the thermal relaxation exotherms were observed only for the p PEK sample that was prepared in PPA/P 2 O 5 medium. Further confirmation of viscosity-and-shear-induced strain stored in the as-synthesized PEKs was provided by a systematic annealing study with differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) analysis and Fourier-transform infrared spectroscopy (FT-IR). Based on these data, p PEK was successfully compression-molded in the temperature range of 230–250 °C which is more than 100 °C below its melting temperature.

One mono‐adduct and three novel multi‐adduct analogs of C60‐diphenylaminofluorene C60(>DPAF) n derivatives were synthesized and characterized by various spectroscopic methods. Optical absorption of these samples indicated a systematic increase in relative intensity of the fluorene band centered at 415 nm as the number of DPAF addend increases. Fluorescence of DPAF chromophore in all C60(>DPAF) n derivatives 4 (n = 1), 5 (n = 2), 6 (n = 3), and 7 (n = 4) in o‐dichlorobenzene and chloroform was found to be efficiently quenched as the direct covalent bond attachment of DPAF moieties to the fullerene cage facilitates efficient intramolecular electron or energy transfer processes.

In-situ polymerization of 3-phenoxybenzoic acid in the presence of various amounts (1−30 wt %) of vapor-grown carbon nanofibers (VGCNF) was carried out in poly(phosphoric acid)/phosphorus pentoxide (PPA/P2O5; 1:4 w/w) medium. 3-Phenoxybenzoic acid polymerizes via Friedel−Crafts acylation in PPA to form poly(oxy-1,3-phenylenecarbonyl-1,4-phenylene) or mPEK. The resulting mPEK-g-VGCNF products that contained less than 10 wt % of VGCNF were soluble in N-methyl-2-pyrrolidinone and had intrinsic viscosity values ranging from 1 to 1.73 dL/g in methanesulfonic acid (MSA) at 30 ± 0.1 °C. The overall evidence based on the data from elemental analysis (EA), thermogravimetric analysis (TGA), and Fourier transform infrared spectroscopy (FT-IR) as well as scanning electron and transmission electron microscopies of the resulting materials implicates that VGCNF remained more or less structurally intact under the mildly acidic, relatively high-shearing and hot polymerization conditions. It is also evident that under thes...

A comprehensive one-photon photophysical study has been carried out on AF455, a known two-photon absorbing dye. AF455 is composed of an electron-accepting center with three arms that consist of a π-conjugated group with an electron-donating group at the terminal end of each arm. The objective of this work is to understand the one-photon excitation photophysical properties so that this knowledge will be carried into understanding the two-photon absorption properties. This was done by utilizing steady-state absorption, steady-state and time-resolved emission, femtosecond pump−probe, and nanosecond laser flash photolysis techniques. Through this study it was determined that AF455 undergoes an excited-state intramolecular charge transfer (ICT) upon absorption of a single photon. The extent of ICT stabilization is dependent on the solvent polarity, with increasing ICT stabilization in more polar solvents. The formation of the triplet excited state is small (

A series of small dendritic structures containing one of two efficient multiphoton absorbing dyes at the periphery and a nile red derivative at the core have been synthesized. These molecules display efficient (>96%) fluorescence resonance energy transfer (FRET) from the periphery to the core on selective excitation of the two-photon absorbing chromophore by either UV (linear absorption) or high-intensity IR (nonlinear absorption) radiation. In addition, a significant increase in core emission is observed on excitation of the peripheral chromophores, compared to direct excitation of the core. This “antenna effect” essentially doubles between increasing dendrimer generations within a series. The combination of the ability of the peripheral chromophores to absorb high-intensity IR radiation, coupled with a very efficient energy transfer process and a significant increase in the fluorescence of the acceptor chromophore, makes these molecules potentially useful for a variety of applications, including optical...

Degenerate two-photon absorption (TPA) spectral properties of five AFX chromophore solutions have been studied using a single and spectrally dispersed sub-picosecond white-light continuum beam. In a specially designed optical configuration, optical pathways inside the sample solution for different spectral components of the focused continuum beam were spatially separated from each other. Thus, the nondegenerate TPA processes coming from different spectral components can be eliminated, and the direct nonlinear absorption spectrum attributed to degenerate TPA processes can be readily obtained. Using this new technique, the complete TPA spectra for these five highly two-photon-active compounds (AF-380, AF-350, AF-295, AF-270, and AF-50) were obtained in the spectral range from 600 to 950 nm on an absolute scale of TPA cross section. The relationship between the molecular structures and their TPA spectral behaviors are discussed. In general the measured TPA spectra are not identical with the linear absorption spectra on the scale of absorbed photon(s) energy. Moreover, for some sample (such as AF-380), the TPA spectrum is totally different from the linear spectrum, which implies the difference of molecular transition pathways and selection rules for one- and two-photon excitation processes. At high excitation intensity levels (or=15 GW/cm(2)), the saturation behavior of TPA transition can be observed obviously in AF-350 and AF-380 solutions that exhibit much higher nonlinear absorptivity than the other chromophores investigated.

Electrophilic functionalization of vapour-grown carbon nanofibers (VGCNF) was accomplished via Friedel–Crafts acylation with 2,4,6-trimethylphenoxybenzoic acid in polyphosphoric acid using the improved conditions that we previously described. The progress of the reaction was conveniently monitored with FT-IR spectroscopy following the growth of the keto-carbonyl band at 1664 cm−1 associated with the product. In addition to scanning electron microscopic and UV-vis spectroscopic data, the combined results from the elemental analysis and thermogravimetric analysis further suggested that there were 3 arylcarbonyl groups covalently attached to the nanotube structure for every 100 carbon sites. Because of the presence of significant hydrogen content in the starting VGCNF, the covalent attachment of the arylcarbonyl groups most probably occurred at the sp2C–H sites.

Three novel two-photon absorbing (TPA) chromophores with 1,3,5-triazine as the π-electron deficient core, dialkylfluorene as aromatic bridges, and diphenylamino groups as the electron-donating end-groups were prepared. Designated as AF-450 (2,4,6-tris[7-(diphenylamino)-9,9-didecylfluoren-2-yl]-1,3,5-triazine), AF-455 (2,4,6-tris[9,9-bis(3,7-dimethyloctyl)-7-(diphenylamino)-fluoren-2yl]-1,3,5-triazine), and AF-457 (2,4,6-tris[(7-(diphenylamino)-9,9-diprop-2-enylfluoren-2-yl]-1,3,5-triazine), their overall molecular structure and local symmetry (D3h) are similar to those of previously reported three-armed AF-350 (N,N,N-tris[4-{7-(2-benzothiazolyl)-9,9-diethylfluoren-2-yl}phenyl]amine) and AF-380 (N,N,N,-tris[7-(2-benzothiazolyl)-9,9-diethylfluoren-2-yl]amine). Among the family of AFX chromophores previously reported by us, AF-450 possesses one of the largest effective TPA cross-sections (σ2‘ = 39 500 × 10-50 cm4-sec/photon-molecule, or 39 500 GM) as determined by nonlinear transmission method in the nanosec...

Several strategies for using enzymes to catalyze reactions leading to the synthesis of relatively simple substituted picolinic acids have been described. The goal of the work described here was to synthesize a more complex molecule, 6-phenylacetylene picolinic acid [6-(2-phenylethynyl)pyridine-2-carboxylic acid], for use as a potential endcapping agent for aerospace polymers. We screened 139 toluene-degrading strains that use a variety of catabolic pathways for the ability to catalyze oxidative transformation of diphenylacetylene. Acinetobacter sp. strain F4 catalyzed the overall conversion of diphenylacetylene to a yellow metabolite, which was identified as a putative meta ring fission product (2-hydroxy-8-phenyl-6-oxoocta-2,4-dien-7-ynoic acid [RFP]). The activity could be sustained by addition of toluene at a flow rate determined empirically so that the transformations were sustained in spite of the fact that toluene is a competitive inhibitor of the enzymes. The overall rate of transformation was limited by the instability of RFP. The RFP was chemically converted to 6-phenylacetylene picolinic acid by treatment with ammonium hydroxide. The results show the potential for using the normal growth substrate to provide energy and to maintain induction of the enzymes involved in biotransformation during preliminary stages of biocatalyst development.

Two new diphenylquinoxaline-containing AB2 monomers, 2,3-bis(4-aminophenyl)quinoxaline-6-carboxylic acid, 5, and 2,3-bis[4-(4-aminophenoxy)phenyl]quinoxaline-6-carboxylic acid, 9, were prepared and...

Organically modified layered silicates (OLS) with high thermal stability are critical for synthesis and processing of polymer layered silicate nanocomposites (PLSN). In the current study, the non-oxidative thermal degradation chemistry of alkyl and aryl quaternary phosphonium-modified montmorillonites (P-MMT) was examined using TGA combined with pyrolysis/GC-MS. The morphology evolution at elevated temperature was investigated using in-situ high-temperature X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The onset decomposition temperature via TGA of these P-MMTs ranged from 190 to 230 °C. The initial degradation of the alkyl P-MMTs follows potentially two reaction pathways - ‚-elimination [E‚] and nucleophilic displacement at phosphorus [SN(P)] reflecting the multiple environments of the surfactant in the silicate. Aryl P-MMT decomposition proceeds via either a reductive elimination through a five-coordinate intermediate or radical generation through homologous cleavage of the P-phenyl bond. Overall, the interlayer environment of the montmorillonite has a more severe effect on stability of the phosphonium surfactant than previously reported for ammonium-modified montmorillonite (N-MMT). Nonetheless, the overall thermal stability of P-MMT is higher than that of N-MMT. These observations indicate that, in addition to their conventional purpose as stabilizers, phosphonium salts offer unique opportunities for melting processing polymer layered silicate nanocomposites.

The degenerate two-photon absorption (TPA) spectrum has been studied for a novel organic chromophore (AF389) that exhibits extremely high TPA cross-section values around the ∼800 nm range in both nanosecond and femtosecond domains. To measure the direct degenerate TPA spectrum of AF389 in the femtosecond regime, we have developed a new technique that is based on the use of a single continuum-generation beam as an intense coherent white-light source for TPA excitation. The different spectral components of the input continuum beam are spatially dispersed and then pass the sample through different pathways so that nondegenerate TPA processes among different spectral components can be avoided. Our results show that the TPA spectrum of AF389 is different from the linear absorption spectrum on the scale of absorbing photon energy. There is a significant blue shift of the absorption peak position for the TPA spectrum with respect to the linear absorption spectrum, indicating that the transition pathways are diff...

While aromatic polyimides have found widespread use as high-performance polymers, the present work addressed the need for organosoluble pre-imidized materials through the use of a hyperbranching scheme. The AB2 monomer, N-[3,5-bis(4-hydroxybenzoyl)benzene]-4-fluorophthalimide, was prepared from 4-fluoroisophthalic anhydride and 3,5-bis(4-hydroxybenzoyl)aniline. The latter was synthesized in three steps starting from commercially available 5-nitroisophthalic acid. The AB2 monomer was then polymerized via aromatic fluoride-displacement reaction to afford the corresponding hydroxyl-terminated hyperbranched polymer, HT-PAEKI, which was then functionalized with allyl and propargyl bromides as well as epichlorohydrin to afford allyl-terminated AT-PAEKI, propargyl-terminated PT-PAEKI, and epoxy (glycidyl)-terminated ET-PAEKI, in that order. All hyperbranched poly(ether−ketone−imide)s were soluble in common organic solvents. Intrinsic viscosities of HT-, AT-, PT-, and ET-PAEKI in NMP were 0.13, 0.08, 0.08, and 0....

Adaptive soft materials exhibit a diverse set of behaviors including reconfiguration, actuation, and locomotion. These responses however, are typically optimized in isolation. Here, the interrelation between these behaviors is established through a state space framework, using Nylon 6 thin films in a humidity gradient as an experimental testbed. It is determined that the dynamic behaviors are a result of not only a response to but also an interaction with the applied stimulus, which can be tuned via control of the environment and film characteristics, including size, permeability, and coefficient of hygroscopic expansion to target a desired behavior such as multimodal locomotion. Using these insights, it is demonstrated that films simultaneously harvest energy and information from the environment to autonomously move down a stimulus gradient. Improved understanding of the coupling between an adaptive material and its environment aids the development of materials that integrate closed loop autonomous sensing, actuation, and locomotion.

The accumulation of photoinduced deformation in azobenzene functionalized polymers has received a significant amount of attention in recent years. Critically, the induced photomechanical deformation in these systems experiences varying degrees of relaxation. Control over the persistence of photomechanical strains is vital to the broader utility of these materials in shape programmable systems including soft robotics and engineered origami. Furthermore, investigations of relaxation in light responsive polymer systems triggered by UV light are more prominent than those triggered by blue-green light. In this study, the impact of chain mobility and initially induced photostrain on the relaxation dynamics of azobenzene-functionalized polyimides after irradiation with blue light is examined. A modeling effort coupling chromophore population dynamics to material strain is carried out to further explore the relationship between material structure, relaxation dynamics, and macroscopic deformation. The implications for controlling strain persistence are highlighted by simulating one example of a photoprimed bistable actuator.

Popularly utilized oxidation media, via nitric acid/sulfuric acid mixtures, are too corrosive and oxidizing to preserve structural integrity of highly ordered graphitic materials (carbon nanotubes (CNTs) and graphene). Here, for the most commonly used oxidation method, the important advantages of defect/edge-selective functionalization of carbon materials (CNTs/graphene/graphite) in a polyphosphoric acid (PPA)/phosphorous pentoxide (P2 O5 ) medium are elucidated. The optimized PPA/P2 O5 medium is a mild acid that is not only less corrosive than popularly utilized oxidation media, but also has a strong capability to drive Friedel-Crafts acylation by covalently modifying carbon materials. With a broader spectrum of functional groups accessible, the PPA/P2 O5 -driven Friedel-Crafts acylation offers more options for tailoring the properties and processing of carbon materials.

Synthesis of several C(sub 60-)(antenna)x conjugates was performed to demonstrate high flexibility in the design of organic nonlinear optical (NLO) nanostructures showing broadband characteristics with capability to absorb light over a wide range of wavelengths. It was achieved by covalent attachment of a hybrid combination of two types of light-harvesting fluorescent antenna chromophores on a C(sub 60) cage. Ultrafast photoresponsive intramolecular Foester resonance energy transfer among antenna units and shared excited energy-accepting C(sub 60) cage is proposed as a plausible mechanism to enhance the broadband NLO ability. Characterization of the branched triad C(sub 60)(DPAF-C(sub 18))( CPAF-C(sub 2M)) and the tetrad C(sub 60)(DPAF-C(sub 18))( CPAF-C(sub 2M))2 was carried out by various spectroscopic techniques. These compounds showed approximately equal extinction coefficients of optical absorption over 400-550 nm that corresponds to near-IR two-photon based excitation wavelengths at 780-1100 nm. These nanomaterials may be utilized in NLO coatings for achieving efficient light-transmittance reduction at the same NIR wavelengths.

Three two-photon absorption (2PA) dyes (donor-π-donor (DPA2F), donor-π-acceptor (AF240), and acceptor-π-acceptor (BT2F); specifically, D is Ph2N-, A is 2-benzothiazoyl, and the π-linker is 9,9-diethylfluorene) are examined in a variety of aprotic solvents. Because the 2PA cross section is sensitive to the polarity of the local environment, this report examines the solvent-dependent linear photophysics of the dyes, which are important to understand before probing more complex solid-state systems. The symmetrical dyes show little solvent dependence; however, AF240 has significant solvatochromism observed in the fluorescence spectra and lifetimes and also the transient absorption spectra. A 114 nm bathochromic shift is observed in the fluorescence maximum when going from n-hexane to acetonitrile, whereas the lifetimes increase from 1.25 to 3.12 ns. The excited-state dipole moment for AF240 is found to be 20.1 D using the Lippert equation, with smaller values observed for the symmetrical dyes. Additionally, the femtosecond transient absorption (TA) spectra at time zero show little solvent dependence for DPA2F or BT2F, but AF240 shows a 52 nm hypsochromic shift from n-hexane to acetonitrile. Coupled with the solvatochromism in the fluorescence and large excited-state dipole moment, this is attributed to formation of an intramolecular charge-transfer (ICT) state in polar solvents. By 10 ps in AF240, the maximum TA in acetonitrile has shifted 30 nm, providing direct evidence of a solvent-stabilized ICT state, whose formation occurs in 0.85-2.71 ps, depending on solvent. However, AF240 in nonpolar solvents and the symmetrical dyes in all solvents show essentially no shifts due to a predominantly locally excited (LE) state. Preliminary temperature-dependent fluorescence using frozen glass media supports significant solvent reorganization around the AF240 excited state in polar solvents, and may also support a twisted intramolecular charge-transfer (TICT)-state contribution to the stabilization. Finally, time-dependent density functional theory calculations support ICT in AF240 in polar media and also allow prediction of the 2PA cross sections in the 0-0 band, which are much larger for AF240 than the symmetrical dyes.

Light responsive azobenzene functionalized polymer networks enjoy several advantages as actuator candidates including the ability to be remotely triggered and the capacity for highly tunable control via light intensity, polarization, wavelength and material alignments. One signi cant challenge hindering these materials from being employed in applications is their often relatively slow actuation rates and low power densities, especially in the absence of photo-thermal e ects. One well known strategy employed in nature for increasing actuation rate and power output is the storage and quick release of elastic energy (e.g., the Venus ytrap). Using nature as inspiration we have conducted a series of experiments and developed an equilibrium mechanics model for investigating remotely triggered snap-through of bistable light responsive arches made from glassy azobenzene functionalized polymers. After brie y discussing experimental observations we consider in detail a geometrically exact, planar rod model of photomechanical snap-through. Theoretical energy release characteristics and unique strain eld pro les provide insight toward design strategies for improved actuator performance. The bistable light responsive arches presented here are potentially a powerful option for remotely triggered, rapid motion from apparently passive structures in applications such as binary optical switches and positioners, surfaces with morphing topologies, and impulse locomotion in micro or millimeter scale robotics.

Four new bis(2-thienyl)naphthalene monomers were synthesized and their absorption, emission and redox properties were studied. Thus, 1,5-dimethoxy-2,6-bis(2-thienyl)naphthalene (2,6-BTDMN), 1,5-dimethoxy-4,8-bis(2-thienyl)-naphthalene (4,8-BTDMN), and 1,4,5,8-tetramethoxy-2,6-bis(2-thienyl)naphthalene (2,6-BTTMN) were synthesized by Stille coupling reaction between the respective dibromides and 2-(tributylstannyl)-thiophene while 2,6-bis(2-thienyl)-naphthalene (2,6-BTN, previously prepared from dibromo precursor) and 1,5-bis(2-thienyl)-naphthalene (1,5-BTN) were prepared similarly between the corresponding di-triflates and 2-(tributylstannyl)thiophene. Monomers 1,5-BTN and 4,8-BTDMN reveal only one peak in both absorption and emission spectra, while monomers 2,6-BTN, 2,6-BTDMN, and 2,6-BTTMN show multiple bands for absorption and a single doublet for emission. The oxidation potential values for the monomers range from 0.95 V, for monomer 1,5-BTN to 0.59 V, for monomer 2,6-BTTMN. Electropolymerized monomers, with the exception of 2,6-BTTMN, form good polymeric films on ITO coated glass substrate. They are pale greenish in color in their oxidized states and relatively transmissive and pale yellow in their reduced state. Spectro-optoelelectrochemistry revealed a band gap range of 2.4–2.9 eV. Iodine-doped polymer from chemically polymerized monomer 2,6-BTTMN has a conductivity of 5×10 −2 S/cm.

A new series of linear, asymmetrical diphenylaminofluorene-based chromophores (AFX) with various strong π-electron acceptors were synthesized and evaluated for two-photon absorptivity. These chromophores were studied to determine a suitable replacement for 2-(4-vinyl)pyridine, the π acceptor for our previously reported AFX series, which contains a photochemically and thermo-oxidatively unstable olefinic unit. In addition to the benzoyl group (AF-370), these π-electron acceptors include 2-benzothiazolyl (AF-240), 2-benzoxazolyl (AF-390), 2-(N-phenyl)benzimidazolyl (AF-386), and 2-(3,4-diphenyl)imidazolyl (AF-385) moieties (five-membered heterocycles) and the 2-quinoxalinyl (AF-260) group (six-membered heterocycle). From nanosecond nonlinear transmission measurements, these new chromophores have effective two-photon cross sections (σ2‘) at 800 nm, spanning from 3.87 × 10-48 cm4 s/(photon molecule) for AF-385 to 97.46 × 10-48 cm4 s/(photon molecule) for AF-240. Two of them, 2-benzothiazolyl-end-capped AF-240...

As an alternative method for processing polyaniline (PANI) from its conducting (protonated) state, vacuum casting of PANI from a methanesulfonic acid (MSA) solution provided films with electrical conductivity values of about 130–150 S/cm. In addition, we similarly prepared blended films of PANI · MSA and poly(p-phenylene benzobisthiazole) (PBZT). This process eliminated the need for a subsequent protonation step and had the additional advantage that the conjugated PBZT may provide alternative conducting pathways. Conductivity values of the composite films ranged from 100 pS/cm to 124 S/cm, and the films displayed critical concentration behavior with a PANI threshold concentration of 2.75% and a critical exponent of 4. Transmission electron micrographs displayed phase-separated regions with PANI forming a continuous network at high concentrations. Thermogravimetric analysis results demonstrated the thermal and thermooxidative stability advantage of the blends due to the PBZT component. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2539–2548, 2001

Two-photon processes have recently received considerable attention, as they offer opportunities for both fundamental research and technological applications. In this paper, we illustrate both of these opportunities by reporting on a study of two-photon properties and discussing one specific application of a new chromophore, tris[4-(7-benzothiazol-2-yl-9,9-diethylfluoren-2-yl)phenyl]amine (AF-350). This new compound exhibits a large two-photon absorptive cross section and, more importantly from the application point of view, a high photochemical/photothermal stability. The nonlinear optical properties of an AF-350 solution were studied with ∼800-nm laser pulses in both nanosecond and femtosecond regimes. The two-photon excited fluorescence spectrum and temporal behavior were compared with the corresponding results obtained for one-photon excitation. There is an ∼11-ps delay between an ultrashort pump pulse and the first peak of the two-photon induced fluorescence signal, whereas no delay was measured betwe...

Thermotropic liquid crystalline behaviors of dihydoxyphenylene benzobisthiazole with alkyl moieties of three different lengths at both ends were studied by d.s.c. polarizing optical microscope. and X-ray scattering. All three samples showed tilted smectic phases. Higher transition temperature and wider Sc interval were noted in DiOH-PBT-Cn. The alkoxy chain length lowered the transition temperature and temperature interval.

Oxidatively stable and electrically insulating rigid-rod polymer poly(p-phenylenebenzobisthiazole) (PBT), was incorporated with electroactive triarylamino (TPA) moieties, (R)TPA-PBT with R=NMe2 and Me. Copolymers or homopolymers were synthesized with various TPA fractions changing the polymers to segmentally rigid configuration, and fabricated into films. On cast films, x-ray scattering revealed that the copolymers were random copolymers, microscopy showed “template effect” from the rigid segments. Iodine activation of the incorporated polymers led consistently to more than five order-of-magnitude increases in electrical conductivity. The activated conductivity also increased with TPA fraction. A conductivity value as high as 4.1×10−4 S/cm was realized for the activated (NMe2)TPA-PBT homopolymer. However, temporal stabilities were observed for the conductivity, which seemed to depend on the polymer composition and film morphology. Spectroscopic examination detected no new electronic state generated by iod...

New aromatic benzobisthiazole copolymers containing 10-70 mol % of 4-N,N-dimethylamino-triphenylamine functionality were prepared from the respective dinitrile or dicarboxylic acid monomers, terephthalic acid, and 2,5-diamino-1,4-benzene-dithiol dihydrochloride in polyphosphoric acid. At the first approximation, the copolymers containing 10 mol % or less of the triarylamino moieties in the polymer chains still preserve the capability to form anisotropic (nematic) solutions at 10 wt % polymer concentration. This is an important requirement for processing the copolymers into fibers and films with good to excellent mechanical properties. Films with good mechanical integrity were cast from the dilute methanesulfonic acid solutions of the copolymers under reduced pressure. They showed electrical conductivity values of the order of 10 -11 -10 -10 S/cm in pristine state, with four to seven orders of magnitude increase upon exposure to mild oxidizing agents such as iodine vapor. On the contrary, the parent polymer, poly(p-phenylene benzobisthiazole) is an insulator with conductivity of less than 10 -12 S/cm, and its conductivity does not improve at all with exposure to iodine vapor.

Thermally stable, nonrigid-rod poly(benzobisthiazoles), (R)TPA-PBZT, where R = H, Me, NMe2, and OH, and poly(benzobisoxazoles), (R)TPA-PBO, where R = Me, NMe2 containing electron-rich triarylamine groups with various para-substituents (Rs) on the pendent phenyl ring, were synthesized from either 2,5-diamino-1,4-benzenedithiol dihydrochloride or 2,4-diamino-1,5-benzenediol dihydrochloride and the respective triarylamine-based dinitrile or diacid monomer in polyphosphoric acid. Whereas (R)TPA-PBZT polymers were obtained in moderate molecular weights, analogous (R)TPA-PBO polymers were only prepared in low molecular weights. No lyotropic behaviors, characteristic of the unmodified rigid-rod benzazole polymers, as evidenced by the absence of either stir opalescence or birefringence under crosspolarizers, were observed for these homopolymers at about 10 wt % polymer concentration. Among these polymers, only (Me)TPA-PBZT and (NMe2)TPA-PBZT formed cast films with good mechanical integrity. In their pristine state, their film conductivity values were in the range of 10−10–10−9 S/cm at room temperature. Upon exposure to iodine vapor, their conductivities were increased to the maximal values of 5.0 × 10−5 S/cm ((Me)TPA-PBZT) and 4.1 × 10−4 S/cm ((NMe2)TPA-PBZT). © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 1909–1924, 1997

Photomechanical effects in polymeric materials and composites transduce light into mechanical work. The ability to control the intensity, polarization, placement, and duration of light irradiation is a distinctive and potentially useful tool to tailor the location, magnitude, and directionality of photogenerated mechanical work. Unfortunately, the work generated from photoresponsive materials is often slow and yields very small power densities, which diminish their potential use in applications. Here, we investigate photoinitiated snap-through in bistable arches formed from samples composed of azobenzene-functionalized polymers (both amorphous polyimides and liquid crystal polymer networks) and report orders-of-magnitude enhancement in actuation rates (approaching 102 mm/s) and powers (as much as 1 kW/m3). The contactless, ultra-fast actuation is observed at irradiation intensities <

Two C60-(antenna)x analogous compounds having branched hybrid triad C60(>DPAF-C18)(>CPAF-C2M) and tetrad C60(>DPAF-C18)(>CPAF-C2M)2 nanostructures were synthesized and characterized. The structural design was intended to facilitate the ultrafast fs intramolecular energy-transfer from photoexcited C60[>1(DPAF)*-C18](>CPAF-C2M)1or2 or C60(>DPAF-C18)[>1(CPAF)*-C2M]1or2 to the C60> cage moiety upon two-photon pumping at either 780 or 980 nm, respectively. The latter nanostructure showed approximately equal extinction coefficients of optical absorption over 400–550 nm that corresponds to near-IR two-photon based excitation wavelengths at 780–1100 nm for broadband nonlinear optical (NLO) applications. Aside from their enhanced two-photon absorption (2PA) activity at 780 nm, we also demonstrated ultrafast photo-responses at 980 nm showing 2PA cross-section (σ2) values of 995–1100 GM for the hybrid tetrad. These σ2 values were correlated to the observed good efficiency in reducing fs light-transmittance down to 35% at the light intensity of 110 GW/cm2. Accordingly, 2PA characteristics of these nanostructures at multiple NIR wavelengths provided support for their suitability in uses as broadband NLO nanomaterials at 600–1100 nm that includes the 2PA ability of two antenna, DPAF (700–850 nm) and CPAF (850–1100 nm), and the fullerene cage at shorter wavelengths (600–700 nm).

Electronic circuits alone cannot fully meet future requirements for speed, size, and weight of many sensor systems, such as digital radar technology and as a result, interest in integrated photonic circuits (IPCs) and the hybridization of electronics with photonics is growing. However, many IPC components such as photodetectors are not presently ideal, but germanium has many advantages to enable higher performance designs that can be better incorporated into an IPC. For example, Ge photodetectors offer an enormous responsivity to laser wavelengths near 1.55μm at high frequencies to 40GHz, and they can be easily fabricated as part of a planar silicon processing schedule. At the same time, germanium has enormous potential for enabling 1.55 micron lasers on silicon and for enhancing the performance of silicon modulators. Our new effort has begun by studying the deposition of germanium on silicon and beginning to develop methods for processing these films. In initial experiments comparing several common chemical solutions for selective etching under patterned positive photoresist, it was found that hydrogen peroxide (H 2 O 2 ) at or below room temperature (20 C) produced the sharpest patterns in the Ge films; H 2 O 2 at a higher temperature (50 C) resulted in the greatest lateral etching.

A crosslinkable polymer having repeating units of the formula: ##STR1## wherein Ar is selected from the group consisting of ##STR2## These polymers are useful in the preparation of molecular composites with rigid-rod polymers, including para-oriented benzobisazole polymers, such as benzobisthiazole, benzobisoxazole and benzobisimidazole polymers.

Two new diamines containing three nitriles are synthesized via a 3-step route. They are polymerized with four commercial dianhydrides (i.e. 6FDA, OPDA, BTDA and PMDA) in N,N-dimethylacetamide (DMAc) to afford poly(amic acid)s, which are thermally cured at temperatures up to 300 °C to form tough, creasable films. Most of these polyimides are soluble in common solvents. Their glass transition temperatures range from 216 to 341 °C. The polyimides are stable up to 400 °C. The dielectric constants of these OPDA-based polyimides increase from 2.9 (CP2) to 4.7 as measured by the D-E loops.

We fabricated highly magnetic γ-FeOx@AuNP core-shell nanoparticles with deposition of fullerosome array of C60(>DPAF-C9) at the outer shell. It formed a configuration of three-layered core-shell (γ-FeOx@AuNP)@[C60(>DPAF-C9)]n 1. These surface-stabilized soluble NPs were found to be capable of photoinducing surface plasmonic resonance (SPR) effect in the inner Au layer by white LED light and subsequently transfer the accumulated PR energy at the interface to the outer, partially bilayered 1-derived fullerosome membrane layer in a near-field (~1.5 nm). The phenomena led to large amplification of dielectric properties associated with the photoswitching effect and the new unusual phenomenon of delayed photoinduced capacitor-like (i.e. electric polarization) behavior at the frequency range of 0.5‒4.0 GHz. The effect may be suitable for designing photoresponsive dielectric-switchable nanomaterial toward applications of GHz frequency devices.

We applied a novel molecular wrapping approach to fullerenyl hybrid triads to minimize solid aggregation in coating thin-film applications. Several C60-(antenna)x and C70-(antenna)x analogous compounds having branched hybrid triads C60(>DPAF-C18)(>CPAF-C2M) and C70(>DPAF-C2)(>CPAF-C2M) nanostructures, for example, were applied in the study. The structural design was intended to facilitate the ultrafast fs intramolecular energy-transfer from photoexcited antenna to the C60> cage moiety upon two-photon pumping at either 780 or 980 nm, respectively, for broadband nonlinear optical applications. 2PA characteristics at multiple NIR wavelengths provided support for their suitability in uses that includes the 2PA ability of two antenna, DPAF (700‒850 nm) and CPAF (850‒1100 nm), and the fullerene cage at shorter wavelengths (600‒700 nm). The phenomena can be enhanced by the helical polymer wrapping technique.

We have been studying exciplex formation in nonlinear optical materials containing a high concentration of 2PA chromophores (AFX dyes) as a means to enhance the nonlinear optical properties. A number of dipolar AFX dyes having various electron-accepting moieties (π-excess and π-deficient examples) and three bisimide compounds having substituents with varying electron-withdrawing power were synthesized to study exciplex formation in their solid state blends. In substrate supported thin films of various equimolar blends containing an AFX dye and bisimide, the dye monomer emission was severely quenched, and a new emission, red-shifted by up to 110 nm, appeared. The emission energies were consistent with the charge recombination energy calculated from the energy levels of the donor and acceptor present in the blend, which confirmed the emission was from an exciplex. Time resolved emission measurements also indicated the presence of much longer lived transients in the blends, consistent with exciplex formation. Spectroelectrochemistry confirmed that the radical cations of these dyes had strong absorption in the NIR region, so exciplex formation is a means to enhance nonlinear optical absorption of the dyes in this spectral region.

The effect of plasmonic enhancement on the two-photon absorption cross section of organic chromophores attached to polyelectrolyte-coated gold nanorods was investigated. The magnitudes of such enhancements were confirmed using single and two photon excitations of the chromophore molecules bound to polyelectrolyte-coated gold nanorods. By synthesizing two-, four-, six-, and eight-polyelectrolyte layer coated nanorods of a particular aspect ratio, the distance dependence of the evanescent electromagnetic field on molecular two-photon absorption was observed. Enhancements of 40-fold were observed for the chromophores nearest to the surface.

We demonstrated the strategy of a nanocomposite design by the incorporation of both a delocalized π-electrons system in a closely bound acceptor-donor analogue chromophore, based on charge-polarizable C(60)(DPAF-C(9)) nanostructure 1, and spin-polarized d-electrons in the form of γ-FeO(x) nanoparticles. Facile intramolecular electron transfer from the DPAF-C(9) donor moiety to the C(60) acceptor cage of 1 upon activation to the excited state with a long lifetime of the charge-separated state forms a possible mechanism to integrate semiconducting and magnetic properties in a single system. We observed an appreciable magnetocurrent (MC) of C(60)(DPAF-C(9))-encapsulated magnetic γ-FeO(x) nanoparticles in PMMA matrix upon applying a magnetic field from 0 to 300 mT at either 77 K (12% MC) or 300 K (4.5% MC). Interestingly, the detailed analysis of magnetocurrent curve profiles taken at 77 K allowed us to conclude that the measured magnetocurrent may be attributed to the contributions from magnetic field-dependent excited-state populations in semiconducting structure (density-based MC), magnetism from magnetic structure (mobility-based MC), and product of density and mobility-based MC components (π-d electronic coupling). At the higher temperature region up to 300 K, the semiconducting mechanism dominated the determining factor of measured magnetocurrent. This experimental observation indicated the feasibility of combining delocalized π electrons and spin-polarized d electrons through charge transfer to induce internally coupled dual mobility- and density-based MC through the modulation of spin polarization and excited states in semiconducting/magnetic hybrid materials.

Multi-walled carbon nanotubes (MWCNTs) are functionalized at the sp2 C–H defect sites with 3,4-diaminobenzoic acid by a “direct” Friedel–Crafts acylation reaction in a mild polyphosphoric acid/phosphorous pentoxide medium. Owing to enhanced surface polarity, the resulting 3,4-diaminobenzoyl-functionalized MWCNTs (DAB-MWCNT) are highly dispersible in polar solvents, such as ethanol, N-methyl-2-pyrrolidone, and methanesulfonic acid. The absorption and emission properties of DAB-MWCNT in solution state are qualitatively shown to be sensitive to the pH in the environment. The DAB-MWCNT is used as a stable platform on which to deposit platinum nanoparticles (PNP). The PNP/DAB-MWCNT hybrid displays high electrocatalytic activity with good electrochemical stability for an oxygen reduction reaction under an alkaline condition. Multi-walled carbon nanotubes (MWCNTs) were functionalized with 3,4-diaminobenzoic acid to produce 3,4-diaminobenzoyl-functionalized MWCNT (DAB-MWCNT). Platinum nanoparticles (PNP) were deposited to DAB-MWCNT. The resulting PNP/DAB-MWCNT hybrid displayed high electrocatalytic activity.

Anisotropic homopolymerization of 2,5-dihydroxyterephthalic acid (DHTA) and 2,5-diamino-1,4-benzenedithiol dihydrochloride (DABDT.2HCl), and copolymerization of DHTA, terephthalic acid and DABDT.2HCl in polyphosphoric acid (PPA), afforded high molecular weight rigid-rod polymer and copolymers, with two hydroxyl groups as pendants per repeat unit. In PPA, the lyotropic liquid crystalline behaviour of the homopolymer was distinctly affected by the polymerization temperature. Spectroscopic evidence strongly suggested a completely planar and ribbon-like structure, the consequence of the strong intramolecular hydrogen bonds between the OH moieties and the N atoms of the nearby benzothiazole units. The homopolymer also exhibited the unusual capability, characteristic of BBL-type ladder polymers, to form directly from its precipitate an aggregated film which displayed little or no shrinkage when dried. Anisotropic dopes of the homopolymer (dihydroxy-polybenzothiazole (PBZT)) and certain dihydroxy-PBZT/PBZT copolymers were spun into fibres with typical tensile properties for modified PBZT polymers, but marginal compressive strength (4–30 ksi (1 ksi = 6.9 MPa) for homopolymer and 16–61 ksi for the copolymers). The intrinsic conductivity for the as-spun fibre of the homopolymer was 2.2 × 10−7 S cm−1 and that for heat-treated (435°C) fibre was 5.6 × 10−8 S cm−1.

We demonstrated ultrafast femtosecond nonlinear optical (NLO) absorption characteristics of bilayered fullerosome vesicle nanostructures derived from molecular self-assembly of amphiphilic oligo(ethylene glycolated) C(60)-(light-harvesting diphenylaminofluorene antenna). Fullerene conjugates were designed to enhance photoresponse in a femtosecond time scale by applying an isomerizable periconjugation linker between the C(60) cage and diphenylaminofluorene antenna subunit in an intramolecular contact distance of only3.0 Å. Morphology of C(60)(DPAF-EG(12)C(1))-based fullerosome nanovesicles in H(2)O was characterized to consist of a bilayered shell with a sphere diameter of 20-70 nm and a chromophore shell-width of 9.0-10 nm, fitting well with a head-to-head packing configuration of the molecular length. At the estimated effective nanovesicle concentration as low as 5.5 × 10(-8) MV (molecular molar concentration of 5.0 × 10(-4) M) in H(2)O, two-photon absorption (2PA) phenomena were found to be the dominating photophysical events showing a large molar concentration-insensitive 2PA cross-section value equivalent to 8500 GM in a form of nanovesicles, on average. The observed NLO characteristics led to a sharp trend of efficient light-transmittance intensity reduction at the input laser intensity above 100 GW/cm(2).

Hydrogels capable of releasing growth factors from very precise 3D structures to be placed on neural electrodes were microfabricated by two-photon lithography in aqueous solution. Hydrophobic chromophores with a large two-photon absorption cross section were solubilized using a nonionic surfactant and the resulting complex were applied with an oil soluble photoradical initiator to fabricate hydrogel microstructures. The results show that hydrogel microstructures are formed when the initiator 2,2-dimethoxy-2-phenyl acetophenone (Irgacure 651) and the 2-photon sensitive chromophore are present in the polymerizable mixture. The presence of water in the prepolymer solution helps in controlling important properties of the resulting structure such as modulus and the pore size which can be fine-tuned for its intended application.

Vapor-grown carbon nanofibers (VGCNF) functionalized with amine-containing pendants, viz.H2N-VGCNF, reacted with 2,2-bis(phthalic anhydride)-1,1,1,3,3,3-hexafluoroisopropane, which was the dianhydride monomer used in in-situ polymerization with 1,3-bis(3-aminophenoxy)benzene to afford a series of CP2-polyimide nanocomposite films (FCNFCP2), containing 0.18-9.19 wt % of H2N-VGCNF (corresponding to 0.10-5.0 wt % of pristine VGCNF), via conventional poly(amic acid) precursor method. For comparison, another series of in situ nanocomposites containing pristine VGCNF (0.10-5.0 wt %) was also prepared similarly. While H2N-VGCNFs enabled direct formation of CP2 grafts on the nanofibers, pristine VGCNFs would result in a relatively weak interface between nanofibers and the CP2 matrix. Conducting-tip atomic force microscopy (C-AFM) showed that the electrical transport was solely through the nanofiber networks in the PCNF-CP2. In general, low-frequency ac impedance measurements followed well the percolation bond model with low percolation threshold; 0.24 and 0.68 vol % for PCNF-CP2 and FCNF-CP2, respectively. However, the design of interface is determined to be crucial for controlling the electrical behavior in four substantial ways: (i) magnitude of limiting conductivity, (ii) linearity of I-V response, (iii) magnitude and direction of temperature-dependent resistivity, and (iv) reproducibility of the absolute value of resistivity with thermal cycling. These observations are consistent with a direct CNF-CNF contact limiting transport in the PCNF-CP2 system, where the CP2 grafts on FCNF form a dielectric layer between individual CNFs, limiting transport within the FCNF-CP2 system. Furthermore, the CP2 grafts on the FCNF surface reduce local polymer dewetting at the nanofiber surfaces when the temperatures exceed the CP2 glass transition, and stabilize the structure of the percolation network and associated conductivity. The general behavior of these interfacial extremes (pristine and fully functionalized CNFs) set important bounds on the design of interface modification for CNFs when the intended use is for electrical performance at elevated temperatures or under extreme current loads. The influence of processing conditions resulting in the spread of measured conductivity by several orders of magnitude for films containing the same type and same amount of CNFs is also reported.

We present a new technique for direct measurements of degenerate two-photon absorption (TPA) spectra of two-photon absorbing materials including non-fluorescent samples. This technique is based on the use of an intense single continuum-generation beam as the coherent white-light source with specially flattened spectral distribution. The different spectral components of the continuum beam are spatially dispersed and then passed through the sample material along different pathways so that nondegenerate TPA processes among different input spectral components can be avoided. By comparing the input and transmitted continuum spectral distributions, the TPA spectrum for a given sample can be obtained. As an example, the continuous TPA spectrum (from 550 to 1000 nm) is measured for a novel two-photon-absorbing compound (AF-389) which exhibits an extremely high TPA cross-section value of ~1×10-20 cm4/GW, or ~249 GM, around ~800-nm spectral range in femtosecond regime.

We present the first experimental measurement of the geometric critical exponent $\ensuremath{\beta}$ associated with the percolation probability, the probability a metallic filler belongs to the conducting network, of an electrical composite. The technique employs conducting-tip atomic force microscopy to obtain a conducting areal density, and is demonstrated on polyimide nanocomposites containing different concentrations of carbon nanofibers. We find $\ensuremath{\beta}\ensuremath{\approx}1$ and $t$ (the exponent for bulk conductivity) $\ensuremath{\approx}3$. These values are consistent with the predictions for the Bethe lattice and larger than the values predicted in the 3D lattice percolation model. Hence, this electrical composite likely belongs to the same universality class as the Bethe lattice. The ability to measure geometric and transport critical exponents on the same material is critical to drawing this conclusion.

Single-walled carbon nanotube (SWCNT) and multi-walled carbon nanotube (MWCNT) were functionalized with 3,4-diaminobenzoic acid via “direct” Friedel-Crafts acylation reaction in PPA/P2O5 to afford ortho-diamino-functionalized SWCNT (DIF-SWCNT) and MWCNT (DIF-MWCNT). The resultant DIF-SWCNT and DIF-MWCNT showed improved solubility and dispersibility. To improve interfacial adhesion between CNT and polymer matrix, the grafting of ABPBI onto the surface of DIF-SWCNT (10 wt%) or DIF-MWCNT (10 wt%) was conducted by simple in-situ polymerization of AB monomer, 3,4-diaminobenzoic acid dihydrochloride, in PPA. The resultant ABPBI-g-MWCNT and ABPBI-g-SWCNT showed improved the mechanical and electrical properties.

The first example of ionic molecular composites, i.e. the formation of rigid-rod molecular composites via ionic interactions between the polyelectrolyte host and the reinforcing rigid-rod polymer, was provided by the physical blend of poly(2-acrylamido-2-methylpropanesulphonic acid) (HAMPS homopolymer) and poly(p-phenylene benzobisthiazole) (PBZT). HAMPS homopolymer was formed in situ from its sodium salt form (NaAMPS), which is commercially available. A 52 48 ( w / w ) PBZT/HAMPS homopolymer blend was prepared by co-dissolving the components in methanesulphonic acid. Upon precipitation in cold water, the molecular composite was obtained as an orange-yellow fibre, which became red when it was dry. This solid-state chromatic variability is reversible and was observed when the blend (polymer pulp or coagulated film) was immersed in a variety of organic solvents. In addition, coagulated film showed relatively little lateral shrinkage after vacuum drying at 110°C overnight and no phase-separated morphology was detected by scanning electron microscopy within 500 A resolution.