Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Nitrogen-containing reactant
Reexamination Certificate
2000-09-22
2002-08-06
Yoon, Tae H. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Nitrogen-containing reactant
C528S492000, C252S500000, C524S236000
Reexamination Certificate
active
06429282
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the preparation of solutions of polyaniline and, more particularly, to the preparation of concentrated solutions (15%-40% w/w) having molecular weight averages (M
w
)≧120,000 and number averages (M
n
)≧30,000 in the pernigraniline, emeraldine, and leucoemeraldine base forms of polyaniline using certain primary and secondary amines as gel inhibitors, and the preparation of solutions (>20% w/w) having molecular weight averages (M
w
)<120,000 and number averages (M
n
)<30,000 in these forms of polyaniline using certain primary, secondary and tertiary amines as gel inhibitors, in polar aprotic organic solvents, which solutions may be processed into films, coatings, and fibers that are highly electrically conducting after subsequent exposure to acid.
BACKGROUND OF THE INVENTION
Dopable &pgr;-conjugated polymers (CPs) possess alternating double and single bonds along the polymer main chain repeat units, such as those found in the family of polymers known as polyaniline, show potential for a variety of commercial applications such as chemical separations, electromagnetic interference shielding, protection of metals from corrosive environments, antistatic coatings, and current carrying fibers. Polyaniline is a commercially attractive polymer since, unlike many other dopable &pgr;-conjugated polymers, it is both environmentally stable and can be made electrically conducting by acid treatment.
Electrical conductivity (&sgr;) of CPs is possible due to electron mobility along (intrachain) and between (interchain) polymer chains in a solid state article. The magnitude of the conductivity depends upon the number of charge carriers (n) which is determined by the extent of doping with oxidizing or reducing chemical agents (or in the special case of polyaniline, with an acid), the charge on these carriers (q), and on the combined interchain and intrachain mobilities (&mgr;). These relationships are related by: &sgr;=nq&mgr;. In order to obtain high conductivities, n is usually maximized by a chemical doping process (generation of electrons or holes on the polymer chain), so that conductivity becomes dependent on the mobility of the carriers. At the maximum doping levels, it is the mobility of the charge carriers which must be increased to obtain higher conductivity. Mobility of charge carriers in some cases depend upon the polymer's morphology once it is “frozen” into a non equilibrium glassy solid state article determined by processing conditions. Interchain mobility depends upon the statistical distribution of conformational features such as bond and torsion angles, interchain distances, packing density, orientation, fractional crystallinity, free volume, etc. By contrast, intrachain mobility depends upon the degree and extent of &pgr;-conjugation and defects along the polymer chains, and the polymer chain conformations. It is therefore desirable to develop improved processing procedures which allow control over the factors governing mobility in order to generate higher conductivities in polyaniline.
Optimally doped CPs contain approximately equal numbers of carriers, but exhibit order of magnitude differences in conductivity depending on sample preparation methods. For instance, stretch-aligned optimally doped transpolyacetylene films exhibit conductivities of the order of 10
5
S/cm, while identically doped nonstretched films are in the range of 10
3
S/cm. Structural features which favor enhanced carrier mobility, e.g., chain alignment through mechanical stretching, are important for obtaining high transport coefficients and 3-dimensional metallic transport. Interchain mobility depends upon the statistical distribution of conformational features such as bond and torsion angles, interchain distances, packing density, orientation, fractional crystallinity, free volume, and the spatial distribution and ordering of dopants. By contrast, intrachain mobility depends upon the degree and extent of &pgr;-conjugation, molecular weight, number of defects along the polymer chains, and the polymer chain conformations themselves. Processing procedures which exert control over the structural factors which govern mobility must be utilized in order to achieve metallic-like conductivities in solid-state &pgr;-conjugated polymers.
However, despite efforts to develop viable, processing routes for polyaniline (PANI), the proccessing barriers intrinsic to this material have not yet been overcome: (a) production of practical, high-quality fibers with adequate strength; and, simultaneously (b) achievement of metallic state conductivity predicted by theory. Melt extrusion is not feasible since this polymer, like most conducting polymers (CPs), decomposes before melting. Solution processing of PANI into film, fiber, or coatings is extraordinarily difficult due to: (a) extremely poor solubility in solvents; (b) rapid polymer gelation times at low (>5% w/w) total solids content; and, (c) strong aggregation tendency due to interchain attractive forces, e.g., hydrogen bonding. Furthermore, these problems prevent utilization of high molecular weight polyaniline at concentrations exceeding 15% w/w which are generally required to produce strong fibers by dry-jet wet spinning techniques or impact resistant coatings, or films by conventional rolling techniques.
There are three oxidation states for polyaniline (PANI): a) the fully oxidized form known as pernigraniline base; b) the intermediate form called emeraldine base; and c) the fully reduced form which is given the name leucoemeraldine base. The general formula describing each of these three primary oxidation states for PANI is: [(C
6
H
4
—NH—C
6
H
4
—NH—)
1−x
][(C
6
H
4
—N=C
6
H
4
=N—)
x
], where x ranges from 0 to 1. When x=1, the polymer is in the fully oxidized (pernigraniline) form and each nitrogen of the polymer repeat unit is a tertiary amine, i.e., all are imine nitrogens. When x=0, the polymer is in the fully reduced (leucoemeraldine) oxidation state and every nitrogen of the polymer repeat unit is a secondary amine. However, when x=0.5, the polymer is in an intermediate (emeraldine) oxidation state with equal numbers of amine and imine nitrogens in the polymer repeat unit. These structures were deduced by Green and Woodhead early in this century.
Emeraldine base is the well-known form of PANI and this “A-B” base polymer exhibits the structure:
where the repeat unit is fully planar and has one quinoid (Q) and three benzenoid (B) ring monomers (tetrameric repeating units each containing two secondary amine and two tertiary imine nitrogen atoms).
The untreated EB is itself an electrical insulator. When powders of EB are treated with acid solutions, the imine nitrogen atoms extract protons from solution with the acid counterion associating with the polymer chain to maintain overall charge neutrality. When less than 50% of the available imine nitrogens are coordinated to form quaternary iminium salt complexes; that is, immersion in solutions having pH values between 2 and 7, the polymer becomes a semiconductor and is called a bipolaron (see
FIG. 1
b
hereof), since charge carriers delocalized along the &pgr;-conjugated polymer backbone are spinless. Immersion in more concentrated acid solutions (pH<2) generates polarons (see
FIG. 1
c
hereof) since, due to self-localized reorganization of electronic states, the mobile charge carriers are now sufficiently delocalized to produce mobile spins. Thus, treatment of EB (which has a conductivity of less than 10
−10
Siemen/cm [S/cm]) with an excess of concentrated acid solution (pH<1) results in an electrically conductive polymer having a conductivity of about 1 S/cm. Under these latter doping conditions, the maximum number of charge carriers (n) have been generated on the polymer since all of the nitrogen atoms, available as protonation sites, are occupied. Thus, the conductivity of EB can be increased by over ten orders of magnitude (<1
Mattes Benjamin R.
Wang Hsing-Lin
Freund Samuel M.
The Regents of the University of California
Yoon Tae H.
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