Benjamin R. Mattes

Formation of conductive polyaniline fibers derived from highly concentrated emeraldine base solutions

Abstract The high molecular weight (M n > 30,000, M w >120,000) form of emeraldine base (EB) may be dissolved at concentrations exceeding 20% (w/w) in organic solvents for periods of time sufficient to dry-wet spin solid fibers. This result is due to the combination of a gel-inhibitor (GI) with the polymer/solvent solution. The GI/EB complex disrupts hydrogen bond formation between EB chains in the concentrated EB solutions and thereby lowers the solution viscosity and increases the gelation time. The as spun fiber exhibits good mechanical strength and high conductivities, and both physical properties are further improved by drawing the fiber to four times its initial length. Doping the fibers with inorganic mineral acids significantly reduces fiber strength, while doping with certain organic acids, such as benzene phosphinic acid, preserves mechanical properties and high conductivity values. All of the fully doped fibers show temperature activated conductivity. No detectable crystallinity was observed by measurements with a wide angle XRD technique.

Effect of processing conditions on the properties of high molecular weight conductive polyaniline fiber

Polyaniline- emeraldine base (EB) fiber with excellent mechanical and electrical properties have been spun from highly concentrated (20% w/w), EB/N-methyl- 2-pyrrolidinone (NMP)/2-methylaziridine (2 MA) solution. These solutions had gelation times, which varied from hours to days depending on the molar ratio of 2 MA to EB tetramer repeating unit in the N-methyl-2-pyrrolidinone (NMP) solvent. To better compare the mechanical and electrical properties, dense films were also prepared by thermal evaporation of less concentrated solution (1% w/w). Both fibers and films were amenable to thermal stretching with maximum draw ratios o f4:1a ndthese stretched samples exhibited the greatest tensile strength overall. Wide-angle X-ray diffraction (WAXD) of as-spun and 4-times stretched fiber showed a completely amorphous struc- ture. Fiber subjected to heat treatment at 250 °C under N2 flux for 2 h displayed further improvements in mechanical properties because of crosslinking between the polymer chains. Fibers and films were later doped by immersion in a variety of aqueous acid solutions. Room temperature DC conductivities for the doped samples ranged from 6 3 10 24 to 45 S/cm depending on the specific choice of acid. Scanning electron micros- copy of fiber samples shows the presence of macrovoid formation during fiber spinning. Continued refinement of the processing parameters and fiber post-treatment, to en- hance chain alignment and increase fiber density, will likely lead to additional improve- ments in the fiber mechanical and electrical properties. Characterization of emeraldine base (EB) powder, solution, films, and fibers by UV-Vis, DSC, TGA, and WAXD were also performed.

Enhanced optical limiting in derivatized fullerenes

We have observed enhanced optical limiting behavior in solutions of a derivatized fullerene (phenyl-C(61)-butyric acid cholesteryl ester) from 532 to 700 nm. Transient absorption measurements determined the spectral and temporal regions of interest for optical limiting in C(60) and in C(60) derivatives that are due to a reverse saturable absorption mechanism and predicted enhanced limiting at longer wavelengths. Intensity-dependent transmission measurements made at several wavelengths confirmed these results. The increased solubility and the broadened ground-state absorption of the functionalized C(60) make it suitable for use as an optical limiter in the red and the near infrared.

Ultrafast holography using charge-transfer polymers

Abstract Charge-transfer polymers are a new class of nonlinear optical materials which can be used for generating femtosecond holographic gratings. Using semiconducting polymers sensitized with varying concentrations of C 60 , holographic gratings were recorded by individual ultrafast laser pulses; the diffraction efficiency and time decay of the gratings were measured using non-degenerate four-wave mixing. Using a figure of merit for dynamic data processing, the temporal diffraction efficiency, this new class of materials exhibits between two and 12 orders of magnitude higher response than any other class of material. The high quantum efficiency and metastability of the charge transfer enables tuning of the decay dynamics by controlling the concentration of acceptors in the blend.

Investigation of gel inhibitor assisted dissolution of polyaniline: A case study for emeraldine base, 2-methyl-aziridine, and N-methyl-pyrrolidone

Abstract The secondary amine 2-methylaziridine (2MA) is used as an additive together with N-methyl-2-pyrrolidone (NMP), to inhibit the gel formation of highly concentrated high molecular weight solutions of polyaniline (PANi) in the emeraldine base (EB) oxidation state for up to 29 hours room temperature. Rheological properties of concentrated EB (~ 20 wt%) solutions are studied by means of rotational viscometry as function of 2MA/EB molar ratio, time, shear rate, and temperature. Due to the formations of hydrogen bonds between the amine proton of 2MA and to lone electron pair from the imine nitrogens in the EB repeat unit. 2MA and NMP prevents the interactions between polymer chains which less to enhanced solubility, dramatic prolongation of times to gelation, and reductions in initial solution viscosity. The temperature effects on the gelation time and the activation energy for viscous flow are reported.

Femtosecond to nanosecond dynamics in fullerenes: Implications for excitedstate optical nonlinearities

We compared detailed dynamics of the excited-state absorption for C60 in solution, thin films, and entrapped in an inorganic sol-gel glass matrix. Our results demonstrate that the microscopic morphology of the C60 molecules plays a crucial role in determining the relaxation dynamics. This is a key factor for applications in optical limiting for nanosecond pulses using reverse saturable absorption. We find that the dynamics of our C60-glass composites occur on long (ns) timescales, comparable to those in solution; thin film samples, by contrast, show rapid decay (<20 picoseconds). These results demonstrate that C60-sol-gel glass composites contain C60 in a molecular dispersion, and are suitable candidates for solid-state optical limiting. Multispectral analysis of the decay dynamics in solution allows accurate determination of both the intersystem crossing time (600±100ps) and the relative strengths of the singlet and triplet excited-state cross sections as a function of wavelength from 450–950 nm. The triplet excited-state cross section is greater than that for the singlet excited-state over the range from 620–810 nm.

Gas transport and sorption in polyaniline thin film

Abstract The permeability of gases through thin polyaniline EB film depends upon the combined mobility and solubility of the gases at a given temperature and pressure. The gas transport behavior of an emeraldine base (EB) film was studied in detail. Permeability measurements were made using the manometric time-lag method of analysis for He, H2, CO2, O2, N2 and CH4, all as functions of varying temperature (15° to 40° C). These measurements allowed us to determine the activation energies for permeation, diffusion, and heat of solution for each gas tested. Permeability of the gases is temperature dependent, permeability increases with increasing temperature and permselectivity of the gas pairs He/N2, O2/N2, CO2/CH4 decreases with increasing temperature.

Optical limiting and excited-state absorption in fullerene solutions and doped glasses

We report the ground-state and excited-state optical absorption spectra in the visible and near infrared for several substituted fullerenes and higher fullerenes in toluene solutions. Based on these measurements, broadband predictions of the optical limiting performance of these molecules can be deduced. These predictions are then tested in the wavelength range from 532 nm to 700 nm in intensity-dependent transmission measurements. We observe optical limiting in all fullerenes measured; higher fullerenes show the greatest potential for limiting in the near infrared (650 - 1000 nm), while substituted C60 shows optimal limiting in the visible (450 - 700 nm). We observe dramatically reduced limiting for solid forms of C60 (thin films and C60-doped porous glasses), indicating that efficient optical limiting in fullerenes requires true molecular solutions.

C60-doped silicon dioxide sonogels for optical limiting

We report the successful preparation of a solid solution of C60 in a silicon dioxide (SiO2) glass matrix by means of sol-gel chemistry. Raman spectroscopy and x-ray diffraction were used to verify that our synthetic route produced glasses with a homogeneous dispersion of intact fullerenes. The vibrational spectrum of C60 is preserved in the C60/SiO2 gel glass. Raman and X-ray diffraction data confirm that the C60 is microscopically dispersed, and does not form detectable phase-separated, microcrystalline regions. We report preliminary observations of optical limiting in these gels, with intensity and concentration dependence consistent with that observed for C60 in solution.

Femtosecond excited-state absorption dynamics and optical limiting in fullerene solutions, sol-gel glasses, and thin films

We compare detailed dynamics of the excited-state absorption for C60 in solution, thin films, and entrapped in an inorganic sol-gel glass matrix. Our results demonstrate that the microscopic morphology of the C60 molecule plays a crucial role in determining the relaxation dynamics. This is a key factor for applications in optical limiting for nanosecond pulses using reverse saturable absorption. We find that the dynamics of the C60-glass composite occur on long timescales, comparable to that in solution; thin film samples, by contrast, show rapid decay. These results demonstrate that the C60-sol-gel glass composites contain C60 in a molecular dispersion, and are suitable candidates for solid-state optical limiting. Multispectral analysis of the decay dynamics in solution allows accurate determination of both the intersystem crossing time and the relative strengths of the singlet and triplet excited-state cross sections as a function of wavelength from 450-950 nm. The triplet excited-state cross section is greater than that for the singlet excited-state over the range form 620-810 nm.