Dali Yang

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.

Characterization of reaction intermediate aggregates in aniline oxidative polymerization at low proton concentration.

Aggregates of reaction intermediates form during the early stages of aniline oxidative polymerization whenever the initial mole ratio of proton concentration to aniline monomer concentration is low ([H(+)](0)/[An](0) <or= 1.0). Detailed characterization is carried out on those aggregates. The intermediate aggregates show a UV-Vis absorption peak at around 410 nm when dispersed in aqueous solution, whereas the peak is centered on 370 nm when dissolved in an organic solvent such as N-methylpyrrolidone. The electronic band gap decreases when the intermediates aggregate to form a solid, and thus, the absorption peak is red-shifted. Gel permeation chromatography (GPC) shows the aggregates contain a major low molecular weight peak with a long tail. The oligoanilines with low molecular weights consistently show a UV-Vis absorption peak at around 370 nm. Mass spectrometry confirms that the intermediate aggregates contain mainly a component with mass number 363 (M + H(+)), likely a tetramer. UV-Vis, GPC, mass spectrometry, NMR, FTIR, and XRD characterization results are presented and chemical structures for the tetramer are proposed. The major components of the intermediate aggregates are likely highly symmetric phenazine- and dihydrophenazine-containing structures. These particular organic compounds have not been identified before as intermediates. The aggregation and precipitation of the tetramers apparently stabilizes these intermediates. The aggregates are highly crystalline, as evidenced by powder X-ray diffraction. A new reaction mechanism for the formation of these intermediates is proposed.

Polyaniline emeraldine base in N-methyl-2-pyrrolidinone containing secondary amine additives: B. Characterization of solutions and thin films

Abstract Secondary amines are added to emeraldine base (EB)/ N -methyl-2-pyrrolidinone (NMP) solutions to stabilize the EB solution. However, amines can interact with the EB molecule either by physical interaction, such as with the hydrogen bond (H-bond), or by chemical reaction. Several amines with different basicity and geometry were investigated. The UV–VIS–NIR, FT-IR, GPC, and TGA results reveal that secondary amines, due to their different basicity and molecular size, can chemically degrade EB in two different ways. Secondary amines with strong basicity and small size, such as azetidine (AZ) and pyrrolidine (PY), attack both the quinoid ring and the NC site in the EB backbone immediately. The attacked EB not only loses its conjugated structure due to ring substitution, but also loses its molecular weight because the chain breaks down. Secondary amines with moderate or strong basicity and large geometry, such as heptamethyleneimine (HPMI), reduce most of the quinoid ring to form a ring-substituted material. The final product loses its conjugated structure, but its molecular weight does not appreciably change. However, amines with weak basicity, such as 2-methyl-aziridine (2MA), form H-bonds with EB but do not appreciably degrade the EB structure.

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.

Kinetic hysteresis in gas adsorption behavior for a rigid MOF arising from zig-zag channel structures

A new porous MOF, Zn(TBC)2·{guest}, is synthesized and studied by the single crystallography, N2 isothermal adsorption and GC separation of CO2 from air. This MOF shows large hysteresis on N2 adsorption at 77 K up to a P/Po of 0.9, which arises from the unique zig-zag channel structures of the framework. The MOF shows promising separation ability for CO2 from air.

New methods for determining the molecular weight of polyaniline by size exclusion chromatography

Aggregation of polyaniline emeraldine base dissolved in polar aprotic solvents containing lithium chloride salt results in distorted non-Gaussian elution peaks in the size exclusion chromatogram. This results in significant error for the calculation of the weight average and number average molecular weights measured by size exclusion chromatography. We compare the effectiveness of LiCl, LiBr, and LiBF 4 , for limiting polymer aggregation in a high molecular weight sample of polyaniline, and thereby improving chromatographic peak shape. Weight average and number average molecular weight values were most accurately determined for dilute polyaniline in NMP solutions containing LiBF 4 .

Controlling macrovoid formation in wet-spun polyaniline fibers

The mesoscopic morphology of wet-spun polyaniline fibers determines their mechanical strength. Macrovoid formation in the coagulation bath is responsible for poor mechanical properties of these fibers. The effects of polymer concentration, coagulation bath temperature, polymer molecular weight and coagulant on the morphology of wet-spun polyaniline fibers have been investigated. The fibers were spun from concentrated solutions of low/medium and medium molecular weight emeraldine base dissolved in N-methyl-2- pyrrolidinone containing heptamethyleneimine as a gel inhibitor. The impact of the fiber morphology on the mechanical properties of the fibers prepared under different conditions is studied. A wet-spinning method, which minimizes macrovoid formation in the polyaniline fiber, is reported, and consequently the strength of the unstretched polyaniline fibers increased dramatically.

Molecular weight dependence of the physical properties of protonated polyaniline films and fibers

Polyaniline, (PANI) in the form of emeraldine base, was synthesized by polymerizing aniline in acid solutions at different sub-zero temperatures to give a range of molecular weights between 100,000 and 300,000 gmol-1. Molecular weights were measured using gel permeation chromatography (GPC). The polymers were formed into solvent-cast films using an acid processing technique, involving 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPSA) as the solvating/protonating acid group and dichloroacetic acid (DCA) plus formic acid (FA) as the solvent. The dried, free-standing films were stretched by drawing over a hot pin to align the polymer chains. Fibers were prepared by spinning more concentrated solutions into a butanone coagulation bath. Conductivity measurements were then made on the drawn films and fibers, and tensile test measurements performed to determine the peak stress and modulus of the drawn films and fibers. The reaction conditions under which the different polyanilines were synthesized, and their molecular weight, were found to have a definite effect upon both the electrical and mechanical properties of the drawn films and fibers. The drawn films and fibers can be used as mechanical actuators.

Rheokinetic Study of Concentrated High Molecular Weight Emeraldine Base in N-Methyl-2-Pyrrolidinone Solutions Containing 2-Methyl-Aziridine

Abstract Concentrated solutions of ultra-high molecular weight (HMW) emeraldine base (EB) are obtained when near stoichiometric quantities of 2-methyl-aziridine (2MA) per polymer repeat unit are co-dissolved in N-methyl-2-pyrrolidinone (NMP). Hydrogen bonds are formed between the secondary and tertiary EB nitrogen atoms found in the polymer repeat unit, and both the 2MA additive and the NMP solvent molecules. The rheological behavior of such solutions is quite different from that of EB/NMP solutions without 2MA. The principals of rheokinetic analysis are used to investigate the mechanism of the EB·2MA complex formation in the concentrated EB/NMP/2MA solution systems. The reaction rate, equilibrium constant and activation energy associated with the complexation are determined. Further, the stability of variable 2MA/EB molar composition and the kinetic effects due to variable temperature are reported for 20% (w/w) EB solutions.

Electrically Conductive Polyaniline Fibers Prepared by Dry-Wet Spinning Techniques

Publisher Summary The concept of gel-inhibitory agents that improve the solution processing parameters of the emeraldine base is introduced in the chapter. These agents are quite sensitive to the concentration range and the gel inhibitor/emeraldine base (GI/EB) molar ratio between 1 and 4. The preparation of highly concentrated EB solutions utilizing high molecular weight polyaniline is easily achieved with this processing advantage. The solutions are stable for periods of time sufficient to dry–wet spin fiber. The fiber moduli, strength, and DC conductivity are improved by stretch alignment. Mineral acids tend to weaken the fibers following doping protocols, while organic acids such as benzene phosphinic acid (BPA) preserve good mechanical and conductive properties. In the study described in the chapter, all of the doped samples showed temperature- dependant DC conductivity. This data can be fit to disorder models such as the quasi-one- dimensional variable-range hopping (VRH) model. Acetic acid is an anomalously poor dopant, and for polyaniline BPA is a good dopant.