Conducting Polyheterocycle Composites Based on Porous Hosts

Authored by: Jun Seo Park , Eli Ruckenstein

Concentrated Emulsion Polymerization

Print publication date:  April  2019
Online publication date:  March  2019

Print ISBN: 9780367134556
eBook ISBN: 9780429026577
Adobe ISBN:

10.1201/9780429026577-3

 

Abstract

Conducting composites based on porous substrates (cotton fiber, non-woven polypropylene mat and porous crosslinked polystyrene) have been prepared by a two-step imbibition technique. First, the substrate was imbibed with a solution of monomer (pyrrole or bithiophene) in acetonitrile, followed by partial drying. Subsequently, the substrate was again imbibed, this time with an oxidant dissolved in a suitable solvent. The polymerization of the monomer inside the host in the presence of the oxidant and the doping of the polymer with the oxidant leads to the conducting composite. The highly hydrophobic and porous crosslinked polystyrene, prepared by the concentrated emulsion polymerization method, is the most efficient. The solvent employed for the oxidant plays a major role. A FeCl3-methanol system and porous crosslinked polystyrene lead to conductivities of polythiophene- and polypyrrole-based composites of 3.63 and 0.65 S/cm, respectively. Copper perchlorate and iron perchlorate are also suitable oxidants. The environmental and thermal stabilities of polypyrrole-based composites are lower than those of polythiophene-based composites. The thermal stability of polypyrrole-based composites can be enhanced by including a small amount of an organic antioxidant, such as amides or substituted phenols, in the composites.

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Conducting Polyheterocycle Composites Based on Porous Hosts

1.2.1  Introduction

The synthesis of polyheterocycles, such as polythiophene (PTP) and polypyrrole (PPY), has received a great deal of attention for the past decades due to their reasonably high conductivity and environmental stability. 1–3 The conductivity of polyheterocycles has reached values as high as 102 S/cm without a noticeable decrease over several months of standing in atmosphere. These characteristics might lead to important applications in a wide variety of electrical and electronic devices.4,5

One of the inherent difficulties with the intrinsically conducting polyheterocycles is the inability to process them into useful large articles. 1–5 A number of methods have been suggested to overcome this drawback by combining strong insulating materials with conducting polyheterocycles. 6–12 First, such conducting composites have been prepared by incorporating an electrochemically or chemically synthesized polyheterocycle in a non-porous polymer matrix. 6–8 The inclusion was carried out either by exposing an insulating sheet imbibed with an oxidant to the monomer or its vapor, or by electrochemical oxidation of a monomer-swollen matrix coated as a film on an electrode. However, the low penetration of the monomer into the non-porous matrix, which can be attributed to the low diffusivity of the monomer into the matrix, can produce only a very thin film of conducting polymer composite. Subsequently, conducting polymer composites have been prepared by first synthesizing a soluble conducting polymer by incorporating appropriate side chains into the monomer and then mixing this polymer with an insulating polymer by solution casting or melt-processing techniques. 9,10 The uniform mixing of the conducting polymer with the insulating polymer at the molecular level and/or the high loading of conducting polymer to reach the percolation threshold are essential in the preparation of highly conducting polymer composites. Thin porous materials were also employed as host substrates for composites in order to facilitate the penetration of polyheterocycles in the matrix. 11,12 Conducting composites based on filter paper and fabric were prepared by impregnation with an oxidant solution and subsequently by contacting them with the monomer.

In this laboratory thick and large conducting composites, based on polyheterocycles in combination with a porous matrix, have been prepared via chemical oxidative polymerization. 13,14 The highly porous crosslinked polystyrene, which was employed as host, was prepared by the concentrated emulsion polymerization method. 15 It was synthesized by heating at 50°C a concentrated emulsion of water dispersed in a mixture of styrene and divinylbenzene containing an initiator and a dispersant. In the concentrated emulsion the volume fraction of the dispersed phase is large, as large as 0.99, and the continuous phase is in the form of a network of thin liquid films that separate the polyhedral cells of the continuous phase. The concentrated emulsion has the appearance of a gel. The stability of the gel-like concentrated emulsion is ensured by the dispersant adsorbed upon the interface between the two phases as an oriented interfacial film. 16 A porous crosslinked polystyrene was obtained after the removal of water from the polymer by heating at 100°C. Conducting polyheterocycles were deposited on the inside surface of the porous host by first saturating the host with a monomer solution, followed by partial drying, and then by imbibing the partially dried host with an oxidant solution for chemical oxidative reaction to occur. PPY- or PTP-based conducting composites were prepared by employing this imbibition technique. 13,14 The conductivity of these composites was investigated by changing the initial compositions of the reactants, the nature of the oxidant and the imbibition order. 13,14

The scope of this article is to study the effect of the porous substrate, reaction medium and antioxidants on the conductivity of the composite in order to improve its electrical properties as well as its long-term environmental stability. As porous hosts, cotton fiber, non-woven polypropylene mat and porous crosslinked polystyrene are employed. Ferric and cupric salts are used as oxidants for the chemical oxidative reaction, because they are known to provide the best performance. 13,14,17 It is known that solvent-related factors for the ferric salt, such as solubility, solvent basicity, the dielectric constant and the Fe+3/Fe+2 redox couple affect the morphology of the polyheterocycles deposited on the inner surface of the substrate and the conductivity of the composite. 11,13,14,17 The effect of various solvents for the oxidant, which acts both as initiator and dopant, is examined. One reason for the lack of significant technical applications of intrinsically conducting polymers is their rapid aging. Positive effects on the stability of conducting polymers to aging were obtained when oxidation was carried out in the presence of some selected antioxidants. 18 Such antioxidants are therefore examined in order to improve the stability of conducting composites.

1.2.2  Experiment

1.2.2.1  Chemicals

Styrene (Aldrich), pyrrole (Aldrich) and divinylbenzene (Polysciences) were purified before use. 2,2-bithiophene (BT, Aldrich), azobisisobutyronitrile (AIBN, Alfa), 3-hydroxybenzoic acid (Alrich), 3-nitrophenol (Aldrich), phthalic acid (Aldrich), acetamide (Aldrich), pyrogallol (Aldrich), sorbitan monooleate (Span 80, Fluka), acetonitrile (Aldrich), acetone (Aldrich), methanol (Aldrich), 1-propanol (Aldrich), ether (Aldrich), ferric chloride (Aldrich), copper perchlorate hexahydrate (Aldrich) and iron perchlorate hydrate (Aldrich) were used as-received. Water was deionized and distilled.

1.2.2.2  Porous Host Substrates

Cotton fiber and non-woven polypropylene (PP, Aldrich) were used as-received. Porous crosslinked polystyrene (PS) was prepared by polymerizing the concentrated emulsion as reported in previous papers. 13–15 A concentrated emulsion of water (25 mL) in a mixture of styrene (5 g) and divinylbenzene (1 g) containing AIBN (0.05 g) as initiator and Span 80 (1 mL) as dispersant was prepared at room temperature by dropwise addition of water under vigorous stirring to a small amount of the mixture. This gel-like, concentrated emulsion was additionally packed by mild centrifugation to remove the air bubbles trapped in the emulsion. The host porous polymer was prepared by polymerizing the concentrated emulsion sandwiched between two clean glass plates (100 × 150 cm) at 50°C for 1 day, and by drying the material thus obtained at 100°C for 3 days.

1.2.2.3  Preparation of the Conducting Composite

A well-dried host was first saturated for 10 min with a solution of the monomer (pyrrole or bithiophene) in acetonitrile and then partially dried, in order to facilitate the penetration of the oxidant solution, by exposure to air for 5 min for the fiber and 30 min for the non-woven PP mat and the crosslinked PS. The durations of the partial drying times have been selected on the basis of the experiments on drying presented later in the paper. The partially dried substrates were imbibed again, by immersing them into an excess amount of oxidant solution for 10 min. The materials thus obtained were dried in air at room temperature for 6 hr and at 50°C for 1 day before measuring the conductivity.

1.2.2.4  Absorption Test

The absorption capacity of the substrates (fiber, 0.08 (diameter) × 10 cm; PP mat and crosslinked PS, 0.15 × 2 × 3 cm) was determined by immersing them in various solvents at room temperature for 20 min. The rate of desorption of the solvent from the substrates to air was measured at room temperature. A Mettler balance was used to measure the weight changes of the samples.

1.2.2.5  Instrumentation

The standard four-point probe method was employed to measure the conductivity of the specimens (0.3 × 1.2 × 3 cm). The morphologies of the fractured porous substrates and conducting composites were investigated by scanning electron microscopy (SEM, Hitachi S-450). A thin layer of gold was deposited on the surface of the samples before investigation.

1.2.3  Results and Discussion

1.2.3.1  Absorption of Liquids by the Porous Hosts and their Evaporation

The amounts of various liquids absorbed by the hosts are listed in Table 1.2.1, which shows that the crosslinked PS is the most hydrophobic among the three hosts and can absorb a relatively large amount of toluene. The cotton fiber and the non-woven PP each absorb comparable amounts of water and organic solvents (including toluene).

Table 1.2.1   Absorption of Various Liquids by the Host Substrates. The Host Was Immersed in the Liquid for 20 Min at Room Temperature

Absorption (g liquid/g host substance)

Water

Ethanol

Acetonitrile

Toluene

Cotton fiber

2.00

1.83

2.01

1.83

Non-woven PP

2.68

2.15

2.38

2.62

Crosslinked PS

1.88

2.94

2.56

5.06

The amount of absorbed liquid that remains in the host during exposure to air, plotted in Figure 1.2.1, was measured in order to evaluate the time of partial drying. It shows that the liquid absorbed by the fiber was evaporated in a few minutes and that the evaporation of liquids from the non-woven PP mat lasted longer than that from the fiber. The time in which crosslinked PS liberated the liquid was the longest because it has a large internal area and absorbs a greater amount of liquid.

Amount of liquid against time during drying in air at ambient temperature. Δ and ○ refer to acetonitrile and toluene, respectively. -----, —.— and — denote cotton fiber, non-woven PP and crosslinked PS, respectively.

Figure 1.2.1   Amount of liquid against time during drying in air at ambient temperature. Δ and ○ refer to acetonitrile and toluene, respectively. -----, —.— and — denote cotton fiber, non-woven PP and crosslinked PS, respectively.

1.2.3.2  Electron Microscopy Study

The SEM pictures (Figures 1.2.2 through 1.2.4) present the morphologies of the various hosts and composites. The morphologies of the cotton fiber (Figure 1.2.2) and non-woven PP mat (Figure 1.2.3), which are composed of many thin fibers, can be contrasted with that of the crosslinked PS. The crosslinked PS (Figure 1.2.4) contains uniform poles of about 10 μm, which were formed during the drying of the polymerized concentrated emulsion. The similarity between substrates and composites suggests that the conducting polymer coats with some uniformity the inner surface of the substrates. A close examination of the SEM pictures with higher magnification (Figure 1.2.5) reveals that the conducting polymer covers almost uniformly the fibers of the non-woven PP mat. The substrate in the composite has a double role. It provides a macroscopic structure in which the conducting polymer generates a network of conducting thin films; it also provides mechanical strength as a reinforcing agent.

Scanning electron micrographs of (a) host cotton fiber, (b) polypyrrole-based conducting fiber and (c) polythiophene-based conducting fiber. The conducting composites were prepared by employing 2.98 mol/L of pyrrole and 1.23 mol/L of BT in acetonitrile as the monomer solution and 1.23 mol/L of FeCl

Figure 1.2.2   Scanning electron micrographs of (a) host cotton fiber, (b) polypyrrole-based conducting fiber and (c) polythiophene-based conducting fiber. The conducting composites were prepared by employing 2.98 mol/L of pyrrole and 1.23 mol/L of BT in acetonitrile as the monomer solution and 1.23 mol/L of FeCl3 in methanol as the oxidant solution. The drying time for the monomer solution saturated host fiber was 5 min.

Scanning electron micrographs of (a) host non-woven PP, (b) polypyrrole-based composite and (c) polythiophene-based composite. The conducting composites were prepared by employing 2.98 mol/L of pyrrole and 1.23 mol/L of BT in acetonitrile as the monomer solution and 1.23 mol/L of FeCl

Figure 1.2.3   Scanning electron micrographs of (a) host non-woven PP, (b) polypyrrole-based composite and (c) polythiophene-based composite. The conducting composites were prepared by employing 2.98 mol/L of pyrrole and 1.23 mol/L of BT in acetonitrile as the monomer solution and 1.23 mol/L of FeCl3 in methanol as the oxidant solution. The drying time for a monomer solution saturated host non-woven mat was 30 min.

Scanning electron micrographs of (a) host crosslinked PS, (b) polypyrrole-based composite and (c) polythiophene-based composite. The conducting composites were prepared by employing 2.98 mol/L of pyrrole and 1.23 mol/L of BT in acetonitrile as the monomer solution and 1.23 mol/L of FeCl

Figure 1.2.4   Scanning electron micrographs of (a) host crosslinked PS, (b) polypyrrole-based composite and (c) polythiophene-based composite. The conducting composites were prepared by employing 2.98 mol/L of pyrrole and 1.23 mol/L of BT in acetonitrile as the monomer solution and 1.23 mol/L of FeCl3 in methanol as the oxidant solution. The drying time for a monomer saturated host crosslinked PS was 30 min.

Scanning electron micrographs of (a) host non-woven PP and (b) polythiophene-based composite, (a) and (b) are higher magnifications of (a) and (c) of

Figure 1.2.5   Scanning electron micrographs of (a) host non-woven PP and (b) polythiophene-based composite, (a) and (b) are higher magnifications of (a) and (c) of Figure 1.2.3, respectively.

1.2.3.3  Conductivity of the Polymer Composites

Table 1.2.2 lists the conductivities of the composites and the amount of conducting polymer loaded in the composites for the three different kinds of porous substrates employed. The initial concentrations of reactants employed in these experiments were selected (on the basis of the results reported in previous papers 13,14 ) to be large enough to achieve the highest conductivity of the composites. Only above an initial concentration of monomer, corresponding to a percolation threshold, is a network generated in the host. A high conductivity is, however, achieved only when a sufficiently high ratio of oxidant to monomer concentration is also employed. The oxidant acts both as initiator and dopant. With the exception of the fiber, the polythiophene-based composites had higher conductivity than the polypyrrole-based composites. Somewhat higher conducting composites have been obtained with the crosslinked PS than with the non-woven PP mat. The amount of conducting polymer loaded in the crosslinked PS-based composites was also the largest. As expected, the experimental results indicate that the morphology and the nature of the substrate affect both the conductivity and the amount of conducting polymer loaded in the composites. Perhaps this is a result of the interactions between the host and the monomer which facilitate the spreading of the latter and hence the connectivity among the formed conducting films. We have observed for the cotton fiber that the attachment of the conducting polymer to the fiber is poor. For this reason, the conducting polymer comes out easily from the fiber and the conductivity of the composite fiber decays in time.

Table 1.2.2   Conductivity of Composites Based on Different Porous Substrates. The Composites Were Prepared by Employing 1.23 mol/L of BT and 2.98 mol/L of Pyrrole in Acetonitrile as the Monomer Solution and 1.23 mol/L of FeCl3 in Methanol as the Oxidant Solution

Host

Cond. Polym.

Cotton Fiber

Non-woven PP Mat

Cross-linked PS

PPY

PTP

PPY

PTP

PPY

PTP

Conductivity (S/cm)

0.30

0.22

0.13

0.30

0.65

3.63

g cond. polymer per g host

1.01

0.72

0.65

0.86

0.89

1.41

1.2.3.4  Effect of the Solvent on the Conductivity of the Composite

In previous experiments, 13,14 we noted that aqueous solutions of ferric or cupric ions are suitable oxidant systems in the synthesis of PPT- and PPY-based composites.

Table 1.2.3 shows the results of a set of experiments carried out with ferric chloride as oxidant in various solvents. The rate of reaction of pyrrole with ferric chloride depends on the kind of solvent employed for the oxidant. The reaction of pyrrole with ferric chloride in acetone is rapid (since the color of the host changes to black rapidly). It is also associated with a high exothermic heat effect which reduces the conductivity of the composite. The lower conductivity of the composite obtained at high temperatures might be partially attributed to the lower degree of conjugation in the conducting polymer backbone. 19 The highest conductivities for PPY were obtained with alcohol or water as solvents for the oxidant. It is interesting to note that an acetonitrile-water mixture used as solvent for the oxidant increases the conductivity of the composites in comparison to that provided by acetonitrile alone. The reaction of pyrrole with the oxidant is rapid and takes place during the immersion in the oxidant solution. In contrast, the reaction of BT with the oxidant is slower and occurs particularly during the drying step at room temperature that follows the immersion in the oxidant solution. The reaction of BT with FeCl3-acetone system started during immersion and continued during the drying process (as revealed by the slow change in the color of the host), and produced a low conducting composite. The oxidation reaction of BT with a FeCl3-water system was very slow and occurred mostly during drying, and the conductivity was 2.84 S/cm. Composites with conductivities of 3.63 and 3.39 S/cm were obtained by employing methanol or ether as solvents for the oxidant; the reaction occurred slowly, during drying.

Table 1.2.3   Effect of the Solvent Employed for the Oxidant on the Conductivity of the Composite. The Conducting Composites Were Prepared by Employing 1.23 mol/L of BT and 2.98 mol/L of Pyrrole in Acetonitrile as the Monomer Solution and 1.23 mol/L of FeCl3 in Various Solvents as the Oxidant Solution. The Porous Crosslinked PS Is Used as Host

Solvent for Oxidant

Cond. Polym.

Acetone

Water

Acetonitrile

90 wt% Acetonitrile in Water

Ether

1-Propanol

Methanol

PPY-based Composite

Conductivity (S/cm)

«10−5

0.26

1.7 × 10−2

0.15

0.14

0.53

0.65

g cond polym. per g host

1.36

1.09

1.00

1.03

1.32

1.06

0.89

PTP-based Composite

Conductivity (S/cm)

9.4 × 10−2

2.84

0.69

2.4

3.39

2.07

3.63

g cond. polym. per g host

1.25

1.26

1.16

1.17

1.30

1.37

1.41

Copper perchlorate was reported to be a strong oxidizing agent in the one-step chemical synthesis of polythiophene. 20,21 Experimental results for conducting composites prepared with copper perchlorate or iron perchlorate as oxidants are listed in Table 1.2.4. The rate of reaction of pyrrole with copper perchlorate was strongly affected by the nature of the solvent employed. In the chemical oxidative reaction of pyrrole with copper perchlorate, only acetonitrile and acetonitrile-water mixtures were effective solvents for the oxidant, the reactions being fast and the conductivities high. In the chemical oxidative reaction of BT with copper perchlorate in various solvents, the oxidative reaction proceeded only in acetonitrile and in mixtures of acetonitrile and water. The conductivity of the polythiophene-based composite prepared with a copper perchlorate–acetonitrile solution was 0.63 S/cm. The performance of various solvents used for iron perchlorate was similar to that for copper perchlorate but the reaction rate was higher. One of the highest conductivities of polypyrrole-based composites prepared with iron perchlorate was 0.22 S/cm and was achieved with a mixture of acetonitrile and water as solvent for the oxidant. Ferric ions in an aqueous acid medium were also effective, producing a polypyrrole-based conducting composite with a conductivity of 0.23 S/cm. The highest conductivity of polythiophene-based composites prepared with iron perchlorate was 0.78 S/cm and was obtained with acetonitrile as solvent for the oxidant.

Table 1.2.4   Effect of the Solvent Employed for the Oxidant on the Conductivity of the Composite. The Conducting Composites Were Prepared by Employing 1.23 mol/L of BT and 2.98 mol/L of Pyrrole in Acetonitrile as the Monomer Solution and 1.10 mol/L of Copper Perchlorate and 1.10 mol/L of Iron Perchlorate in Various Solvents as the Oxidant Solution. The Crosslinked PS Is Used as Host

Oxidant

Cond. Polym.

Solvent for Oxidant

Acetone

Water

Acetonitrile

90 wt Acetonitrile in Water

0.1 M HCl Aqueous Solution

1-Propanol

Methanol

Copper Perchlorate

PPY

conductivity (S/cm)

7.84 × 10−4

6.15 × 10−4

0.11

0.14

9.75 × 10−4

5.1 × 10−2

1.25 × 10−3

g cond. polym. per g host

1.50

0.52

1.00

0.77

0.52

0.59

0.53

PTP

conductivity (S/cm)

a

a

0.63

2.89 × 10−4

a

a

a

g cond. polym. per g host

a

a

1.16

1.16

a

a

a

Iron Perchlorate

PPY

conductivity (S/cm)

2.22 × 10−3

7.14 × 10−2

7.38 × 10−2

0.22

0.23

9.67 × 10−2

9.80 × 10−2

g cond. polym. per g host

1.81

1.24

1.17

1.27

1.13

1.03

1.13

PTP

conductivity (S/cm)

«10−5

2.18 × 10−3

0.78

4.4 × 10−3

3.8 × 10−3

3.1 × 10−3

4.0 × 10−3

g cond. polym. per g host

1.81

1.42

2.16

1.78

1.39

1.81

1.26

Notes:

a  No reaction observed

As indicated by Tables 1.2.3 and 1.2.4, the conductivity of the composites is affected by the presence of water in the oxidant solution. For this reason the effect of the presence of water in acetonitrile and methanol was investigated in some detail. Table 1.2.5 summarizes the experimental results. The effect of water in the reaction medium was more pronounced on the conductivity of PPY-based composites than on that of PTP-based composites. The conductivity of PPY-based composites reached the value of 0.75 S/cm with a mixture of methanol and water as the solvent for the oxidant.

Table 1.2.5   Effect of Water in Mixtures with Acetonitrile or Methanol on the Conductivity of the Composites. The Composites Were Prepared by Employing 1.23 mol/L of BT and 2.98 mol/L of Pyrrole in Acetonitrile as the Monomer Solution and 1.10 mol/L of Copper Perchlorate and 1.10 mol/L of Iron Perchlorate in the Solvent as the Oxidant Solution. The Porous Crosslinked PS Was Used as Host

Solvent

Cond. Polym.

Concentration of Solvent in Aqueous Solution (wt%)

100

90

70

50

0

Acetonitrile

PPY

conductivity (S/cm)

1.70 × 10−2

0.15

0.49

a

0.26

g cond. polym. per g host

1.00

1.03

1.08

a

1.09

PTP

conductivity (S/cm)

0.69

2.40

1.58

a

2.84

g cond. polym. per g host

1.16

1.17

1.12

a

1.26

Methanol

PPY

conductivity (S/cm)

0.65

0.61

0.75

0.44

0.26

g cond. polym. per g host

0.89

1.13

1.10

0.97

1.09

PTP

conductivity (S/cm)

3.63

3.96

3.39

3.60

2.84

g cond. polym. per g host

1.41

1.29

1.19

1.23

1.26

Notes:

a  Phase separation occurred at this concentration when FeCl3 was introduced in the aqueous acetonitrile solution.

1.2.3.5  Stability of Conducting Composites

Even though very stable in comparison with polyacetylene, PPY becomes unstable after long-term exposure to air at elevated temperatures. 22 Figure 1.2.6 presents the time dependence of the conductivity of composites which were exposed to air for a certain number of days at different temperatures. The conductivity of PPY-based composites decreases slightly at room temperature regardless of the oxidant used as initiator and dopant, while no discernable decrease in the conductivity of PPT-based composites was observed during the same period. With the increase in the aging temperature, a sharp decrease in the conductivity of the PPY-based composites was observed. This thermal instability of the conducting polymers is a result of a thermally driven undoping process in which conformational changes in the polymer backbone are activated at elevated temperature. 23 The conductivity of PPY-based composites decreases much more rapidly than that of PTP-based composites at higher aging temperatures. In PPY-based composites the rate of decrease of the conductivity of the composites prepared with copper perchlorate was faster than that of composites prepared with FeCl3. This indicates that the stability of the conducting polymer can be influenced by the nature of the oxidant employed during preparation. 24

1.2.3.6  Effect of Antioxidants on the Composite

A set of experiments was carried out to investigate the effect of an antioxidant on the stability of conducting composites. Figure 1.2.7 presents experimental results regarding the aging of composites at 80°C, when an antioxidant is introduced in the porous substrate before the imbibition with the monomer. Except for pyrogallol, which reacts with the oxidant, positive effects on the stability of composites to aging were found in the presence of antioxidants. The composite treated with pyrogallol degraded rapidly at 80°C. When the antioxidants were introduced after the preparation of the composites (Figure 1.2.8), some of them, such as 3-hydrobenzoic acid, phthalic acid and acetamide, were as effective as when they had been included before the imbibition with the monomer solution. Pyrogallol was effective when included after the formation of the composite.

Conductivity of composites against aging time at room and elevated temperatures. Conducting composites were prepared by employing 2.98 mol/L of pyrrole and 1.23 mol/L of BT in acetonitrile as the monomer solution and 1.23 mol/L of FeCl

Figure 1.2.6   Conductivity of composites against aging time at room and elevated temperatures. Conducting composites were prepared by employing 2.98 mol/L of pyrrole and 1.23 mol/L of BT in acetonitrile as the monomer solution and 1.23 mol/L of FeCl3 in methanol, 1.10 mol/L of copper perchlorate in acetonitrile and 1.10 mol/L of iron perchlorate in acetonitrile as the oxidant solution. Porous crosslinked PS was used as host. The aging samples (0.3 × 1.5 × 3.0 cm) were exposed to air. Δ and ▲ denote polypyrrole-based composites prepared by employing FeCl3 in methanol and copper perchlorate in acetonitrile as the oxidant solution, respectively. ○ and ● denote polythiophene-based composites prepared by employing FeCl3 in methanol and copper perchlorate in acetonitrile as the oxidant solution, respectively. -, ----- and --- denote aging temperatures of 25°C, 50°C and 80°C, respectively.

Conductivity of composite against time at 80°C. The composites were prepared by employing 2.98 mol/L of pyrrole in acetonitrile as the monomer solution and 1.23 mol/L of FeCl

Figure 1.2.7   Conductivity of composite against time at 80°C. The composites were prepared by employing 2.98 mol/L of pyrrole in acetonitrile as the monomer solution and 1.23 mol/L of FeCl3 in methanol as the oxidant solution. Antioxidants (0.1 mol% in acetonitrile) were used to treat the host crosslinked PS before imbibition with the monomer solution. ● denotes the blank sample. ○, Δ, □, ◊ and ∇ denote the samples treated with 3-hydrobenzoic acid, 3-nitrophenol, phthalic acid, acetamide and pyrogallol, respectively.

Conductivity of composite against time at 80°C. Composites were prepared by employing 2.98 mol/L of pyrrole in acetonitrile as the monomer solution and 1.23 mol/L of FeCl

Figure 1.2.8   Conductivity of composite against time at 80°C. Composites were prepared by employing 2.98 mol/L of pyrrole in acetonitrile as the monomer solution and 1.23 mol/L of FeCl3 in methanol as the oxidant solution. Antioxidants (0.1 mol% in acetonitrile) were used to treat the composites. ● denotes the blank sample. ○, Δ, □, ◊ and ∇ denote the samples treated with 3-hydrobenzoic acid, 3-nitrophenol, phthalic acid, acetamide and pyrogallol, respectively.

1.2.4  Conclusion

Conducting composites, in which conducting polyheterocycles coat the inner surface of porous substrates, are prepared by a two-step imbibition technique. A porous substrate is first imbibed and saturated with a monomer-acetonitrile solution, followed by partial drying. Subsequently, the partially dried system is imbibed with an oxidant solution. The monomer polymerizes in the presence of the oxidant and the polymer is doped with the oxidant. The structure of the porous host substrates affects the conductivity of the composite. While three different substrates have been employed, higher conducting composites have been obtained by employing porous crosslinked PS. This hydrophobic crosslinked PS was prepared by the concentrated emulsion polymerization method and contains uniform voids of about 10 μm.

Conducting PPY- or PTP-based composites are prepared by the chemical oxidative reaction of pyrrole or BT with an appropriate oxidant solution. The solvent for the oxidant affects both the rate of reaction and the conductivity of the composite. A FeCl3-methanol oxidant system is very effective in obtaining a good conducting composite. Copper perchlorate and iron perchlorate are most effective with an aqueous solution of acetonitrile as solvent for PPY-based composites, and with acetonitrile for PTP-based composites. Iron perchlorate with 0.1 M aqueous HCl solution as solvent is as effective for PPY-based composites as the aqueous solution of acetonitrile. Some increases in the conductivity of PPY-based composites are achieved by employing acetonitrile-water mixtures as solvents instead of acetonitrile for FeCl3.

In contrast to the PTP-based composites, the PPY-based composites are less environmentally stable at room temperature, 50°C and 80°C. The degradation of PPY-based conducting composite can be slowed down by treating the porous substrate with an antioxidant. Composites prepared with copper perchlorate are less thermally stable than those prepared with FeCl3, which indicates that the stability of the conducting composites is influenced by the nature of the oxidant used in preparing the conducting polymer.

References

1
K. K. Kanazawa , A. F. Diaz , R. H. Geiss , W. D. Gill , J. F. Kwak , J. A. Logan , J. F. Rabolt and G. B. Street , J. Chem. Soc., Chem. Comm. 854 (1979).
2
K. Kaneto , K. Yoshino and Y. Inuishi , Jpn. J. Appl. Phys. 21, L567 (1982).
3
J. E. Frommer and R. P. Chance , “Electrically conducting polymers,” in Encyclopedia of Polymer Science and Engineering, Wiley, New York, Vol. 5, p. 462 (1985).
4
F. Tourillon and F. Gamier , J. Electroanal. Chem. 161, 407 (1984).
5
M. J. Davidson , “Polymers in Electronics,” in Adv. Symp. Ser., Am. Chem. Soc. Vol. 24 (1984).
6
C. Li and Z. Song , Synth. Met. 40, 23 (1991).
7
T. Yoshikawa , S. Machida , T. Ikegami , A. Techagumpuch and S. Miyata , Polym. J. 22, 1 (1990).
8
M. Bi and Q. Pei , Synth. Met. 22, 145 (1987).
9
J. E. Österholm , L. Laakso , P. Nyholm , H. Isotalo , H. Stubb , O. Inganäs and W. R. Salaneck , Synth. Met. 28, 435 (1989).
10
D. M. Bigg and E. J. Bradbury , in Conductive Polymers, Plenum Press, New York, p. 23 (1981).
11
R. B. Bjorklund and I. Lundström , J. Electron. Mater. 13, 211 (1984).
12
R. V. Gregory , W. C. Kimbrell and H. H. Kuhm , Synth. Met. 28, C823 (1989).
13
E. Ruckenstein and J. S. Park , J. Appl. Polym. Sci. 42, 925 (1991).
14
E. Ruckenstein and J. S. Park , Synth. Met. 44, 293 (1991).
15
E. Ruckenstein and J. S. Park , J. Polym. Sci. Polym. Lett. (Part C) 26, 529 (1988).
16
E. Ruckenstein and J. S. Park , Polym. 33, 405 (1992).
17
R. E. Myers , J. Electron. Mater. 15, 61 (1986).
18
V. Bocchi , G. P. Gardini and S. Rapi , J. Mater. Sci. Lett. 6, 1283 (1987); I. M. Hodge , U.S. Patent 4,642,331 (1987).
19
M. Ogasawara , K. Funahashi and K. Iwato , Mol. Cryst. Liq. Cryst. 118, 159 (1985).
20
M. M. Castillo-Ortega , M. B. Inoue and M. Inoue , Synth. Met. 28, 65 (1989).
21
M. B. Inoue , E. F. Velazque and M. Inoue , Synth. Met. 24, 223 (1988).
22
G. L. Baker , Adv. Chem. Ser. (Am. Chem. Soc.) 218, 271 (1988).
23
Y. Wang and M. F. Rubner , Synth. Met. 39, 153 (1990).
24
J. E. Österhelm , P. Passiniemi , H. Isotalo and H. Stubb , Synth. Met. 18, 213 (1987).

Journal of Electronic Materials, Vol. 21, 205–215, (1992).

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