B. Synthesis in a Host Polymer

polystyrene as a host for blending with polypyrrole, and these two polymers

were then dissolved in tetrahydrofuran. Conductivity of the resulting films was

about 0.05 S/cm, although the films were stable in the atmosphere for more

than 650 days. Chen et al. (74) produced a blend of polypyrrole and low-density

polyethylene with the melting technique as well as solution casting and found

that the processing method greatly influenced morphology and electrical conductivity of the blends. Solution casting produced blends with higher conductivities,

which was also confirmed by Omastova et al. (75).

D. Absorption Method

In the absorption method, a host polymer is first contacted with a monomer

solution and then with an oxidant solution or vice versa. Adequate time is allowed for the monomer and oxidant to impregnate the host substrate, where in

situ polymerization to the conducting polymer and simultaneous blend formation occur. Ruckenstein and Park (76), Wu and Chen (60), Mano et al. (77),

and Pigois-Landureau (78) have shown that thick conducting composites can

be formed using this method. Films with conductivity as high as 0.65 S/cm

were obtained using porous, cross-linked polystyrene as the host, polypyrrole

as the conducting polymer, and ferric chloride as the oxidant (76). Impregnating the host polymer with ferric chloride first and then with the pyrrole

solution yielded higher conductivities than the reverse process. High conductivities were also achieved when using nonaqueous solvents. Morsli et al. (79)

also used porous, cross-linked polystyrene as a host polymer in their studies in order to increase the penetration of the conducting polymer into the

host.

Thieblemont et al. (56) employed several host substrates in forming polypyrrole blends including woven glass, polyvinyl chloride, and polyester. Their

process consisted of dipping these host polymers into a solution of pyrrole in

ethanol followed by dipping in an aqueous solution of ferric chloride. Electrical

conductivity of the blends was found to be as high as 120 S/cm. However, they

also found that water and oxygen caused a loss of electrical conductivity over

time. Polyvinyl chloride was employed as a host substrate for blending with

polypyrrole by Mano et al. (77) The blend was investigated under a transmission electron microscope, which showed that the polypyrrole layer was only

0.1 µm thick. The blend also exhibited conductivity as high as 10−1 S/cm.

Balci et al. (80) polymerized polypyrrole in a copolymer of polyvinyl chloride and polyvinyl acetate. Films were prepared by first introducing pyrrole into

the polymer matrix, followed by treatment with ferric chloride. Tetrahydrofuran

was used as the solvent in this case, and conductivities were in the range of

10−4 –10−3 S/cm. The blend was not very stable in air and lost its electrical

conductivity in one week.



Copyright 2002 by Marcel Dekker. All Rights Reserved.



E. Other Methods

Faguy et al. (81) employed spectroscopy to study the polymerization of pyrrole

in montmorillonite. An aqueous solution of pyrrole was added to a slurry of

montmorillonite clay particles impregnated with ferric chloride, and polypyrrole

was formed on the accessible surfaces of the clay particles. The resulting conductivity was on the order of 10−4 –10−6 S/cm. Polypyrrole was blended into

several host substrates, such as polypropylene, polyethylene, polyvinyl chloride,

and terpoly(acrylonitrile-butadiene-styrene) by Lenz et al. (1). These blends were

synthesized by placing host substrates in a monomer and oxidant solution, and

they exhibited conductivities in the range of 10−3 –10−4 S/cm. The unusual

aspect of this research is that the conducting blends were coated with copper

in order to prepare metallized films. Polypropylene pellets were used in the

formation of a blend with polypyrrole by Omastova et al. (63). A coating of

polypyrrole on the surface of these pellets was obtained. The pellets were then

compression-molded into films that were 0.2 mm thick. Electrical conductivity

attained values from 4 × 10−10 to 10 × 10−3 S/cm, which was much higher

than values reached by mechanical mixing of the two polymers. Yin et al. (82)

used polyethylene spheres and nylon fibers to form blends with polypyrrole.

The host particles were dispersed in a pyrrole solution and then in an aqueous ferric chloride solution, resulting in a coating of polypyrrole. The particles

were then hot-pressed into composite films. Conductivity reached as high as

10−3 S/cm, but the polypyrrole content was high (20%), resulting in low mechanical strength. Lascelles et al. (61) investigated the coating of polypyrrole

onto polystyrene latexes. The latex particles were formed with ferric chloride,

and then a solution containing the pyrrole monomer was added. The dimensions of the resulting particles were in the submicrometer range, and the coating

thickness of polypyrrole was controlled to vary the electrical conductivity. The

highest conductivities were found to be 10 S/cm, but this was only demonstrated

using particles with a polystyrene latex center and not films.

Blending of polythiophene with polymer hosts has been studied by a number of research groups (22,44,49,79,83), who have used a variety of hosts as

well as different thiophenes. Ruckenstein and Park (83) employed porous, crosslinked polystyrene as the host substrate and formed polythiophene blends using several different oxidants, including copper perchlorate hydrate, iron perchlorate hydrate, ferric chloride, and ferric chloride hexahydrate. All of these

oxidants yielded composites with good conductivities. The host polymer was

impregnated with an oxidant solution and then with a monomer solution. The

resulting blends exhibited electrical conductivity as high as 4.8 S/cm and were

stable for more than 52 days. The best results were obtained with acetonitrile

as the solvent. Samir et al. (68) also obtained composites of polythiophenes

with porous, cross-linked polystyrene that exhibited conductivities as high as

2 S/cm.



Copyright 2002 by Marcel Dekker. All Rights Reserved.



Blends of poly(3-alkylthiophenes) have been formed by Pomerantz et al.

(49). Chloroform was used as the solvent, and the blends were prepared by

melting and by fiber spinning. The melt blends were nonconducting, whereas

the fiber blends had conductivities of 10−1 S/cm. Copolymers and blends of

poly(3-decylthiophene) with polyethylene were also formed with conductivities up to 5 S/cm. Sun and Ruckenstein (44) formed composites of poly(3methylthiophene) and rubber. An inverted emulsion of rubber was mixed with

ferric chloride, and 3-methylthiophene was introduced to the emulsion. Conductivities as high as 1.3 S/cm were obtained. This method for producing composites is preferable to direct mixing and also showed that 3-methylthiophene was

a more suitable monomer for obtaining conductive composites than thiophene

or 2,2 -bithiophene. Ho et al. (72) produced blends of poly(3-dodecylthiophene)

with EVA using a solution casting method. However, the electrical conductivity

was not reported.

An electrochemical method was employed by Vatansever et al. (84) to

polymerize thiophene on a polyamide-coated electrode at a constant potential.

A uniform blend was obtained, but the method is difficult to scale up.



VII. BLENDING IN SUPERCRITICAL CARBON DIOXIDE

Since the solvent has a major effect on electrical conductivity (Tables 1 and 2),

the use of supercritical CO2 as both a solvent and a reaction medium in blending

is of considerable interest. However, CO2 has not been employed in polymer

blending until recently, although its use in polymerization reactions (85–89) is

quite common. A new route to polymer blends was proposed by Watkins and

McCarthy (32), who carried out the polymerization of styrene in supercritical

fluid–swollen host polymers. The monomer was allowed to diffuse into several

host substrates and then polymerized in situ in the host polymer impregnated

with an initiator. The resulting polymers were not soluble in carbon dioxide

and not miscible with the host matrices, poly(chlorotrifluoroethylene) or poly(4methyl-1-pentene), so that discrete phases of the two polymers were obtained

in the composite. The use of supercritical fluids in this process offers several

advantages because of the increased diffusion rates of penetrants dissolved in

the supercritical fluid. Swelling of the polymer substrate, as well as partitioning

of reagents between the swollen polymer phase and the supercritical phase,

can be adjusted by manipulating the temperature and pressure. Also, the most

common supercritical fluid (carbon dioxide) is a gas at atmospheric pressure and

can be rapidly dissipated upon release of pressure. Carbon dioxide also causes

plasticization, changes in surface properties, and nucleation of voids in the host

polymer, which facilitates formation of the blend.

Watkins and McCarthy (32) employed their method of blending to diffuse

styrene into several substrates and to subsequently polymerize the styrene in situ



Copyright 2002 by Marcel Dekker. All Rights Reserved.



in the host polymer. The polymerization involved a free-radical reaction and the

amount of polystyrene incorporated in the host was controlled via the reaction

time and monomer concentration. Watkins and McCarthy (32) concluded that

the blend composition is not limited by the solubility of the monomer in the host

polymer but by the solubility of carbon dioxide in the formed polystyrene. They

later repeated this work with styrene in poly(chlorotrifluoroethylene) (PCTFE)

(90). Diffusion rates in swollen PCTFE were sufficiently high to produce highmolecular-weight polystyrene. These studies emphasize the role of solubility

and diffusion of carbon dioxide in the polymers.

Supercritical carbon dioxide is generally a poor solvent for polymers (91).

However, it does have the capacity to swell many polymers (92,93), and this can

be of considerable advantage in blend formation. Watkins and McCarthy (32)

found that the solubility of carbon dioxide in PCTFE reaches a maximum at a

temperature of 313 K and a pressure of 10.4 MPa. This maximum represents

a mass gain of 4.5% carbon dioxide in the host substrate PCTFE. Wissinger

and Paulaitis (94) found that PMMA swells by about 20 vol % after reaching

equilibrium in carbon dioxide at a temperature of 305.7 K. However, it is important to note that PMMA undergoes foaming with supercritical carbon dioxide

treatment, as observed by Shieh et al. (95,96).

Supercritical carbon dioxide tends to lower the glass transition temperature of glassy polymers, by as much as 60◦ C in some cases. This makes the

polymer more rubbery and flexible and allows carbon dioxide to diffuse into

these polymers. Shieh et al. (95) reported that amorphous polymers absorb carbon dioxide to a greater extent than glassy polymers and are therefore subject

to increased plasticization. Carbon dioxide diffusivities have been measured to

be on the order of 10−6 –10−7 cm2 /s in rubbery polymers (97), which is of

considerable advantage when solutes must be impregnated into host polymers.

The diffusivity of penetrants into host substrates can be enhanced by several orders of magnitude in the presence of supercritical carbon dioxide (98).

Nealey et al. (99) found that the diffusivity of a polybutadiene oligomer in

polystyrene increased by a factor of 10 at a temperature slightly above the glass

transition temperature of the host polystyrene, which had been plasticized with

compressed argon. Also, Raleigh scattering measurements of tracer diffusion

coefficients showed that the diffusion of azobenzene in glassy polystyrene was

enhanced near its glass transition temperature (100).



VIII. RECENT STUDIES INVOLVING IN SITU BLEND

FORMATION USING SUPERCRITICAL

CARBON DIOXIDE

We have studied the polymerization of pyrrole and 3-undecylbithiophene and

blend formation with several host polymers using supercritical carbon dioxide



Copyright 2002 by Marcel Dekker. All Rights Reserved.