CA1248263A

CA1248263A - Polymer composition

Info

Publication number: CA1248263A
Application number: CA000433570A
Authority: CA
Inventors: Malcolm G. Cross; Robert Lines
Original assignee: Raychem Ltd
Current assignee: Raychem Ltd
Priority date: 1982-08-02
Filing date: 1983-07-29
Publication date: 1989-01-03
Also published as: GB2124635A; EP0104726B1; EP0104726A2; EP0104726A3; GB2124635B; GB8320646D0; DE3377308D1; US5378402A

Abstract

ABSTRACT

Polymer Composition Electrically conductive polymers, especially polypyrroles, are formed with a charged polymeric dopant suplying the necessary counter -ions to stabilise the charged conductive form of the conductive polymer. The physical properties of the conductive polymer can thus be usefully modified in ways different from those achieved by non-polymeric dopants or by forming the conductive polymers in pre-existing bodies of non dopant (i.e. uncharged) polymers.

Description

8~i3

- 2 - RK172 This invention relates to compositions comprising an electrically conductive polymer.

Known electrically conductive polymers include polysulphur nitrides, polyanilines, polyacetylenes and polypyrroles. Although some polymers e.g. (SN)X are intrinsically conductive, many polymers require oxi-dation (or reduction) to render them significantly electrically conductive. For example, polyacetylenes require oxidation or reduction, polypyrroles require oxidation. The process of oxidation (or reduction) is often called doping and a counter-ion is required to associate itself with the oxidised (or red") polymer to balance the ionic charges therein. This counter-ion is often referred to as a dopant. Examples of dopants include BF4 , p-toluene sulphonate, Br .

In the interests of clarity, the phrase "electric-ally conductive polymer" will be used herein to mean any polymer or oligomer inherently or intrinsically capable of electrical conductivity or semi-conductivity, (hereinafter both included in the terms "electrical conductivity" or "electrically conductive"), regardless of whether or not oxidation/ reduction of the polymer and/or the presence of a "dopant" is or are required or present actually to render it electrically conductive.
Thus, the aforesaid phrase includes inorganic polymers such as the polysulphur nitrides, and organic polymers such as the polypyrroles, and particularly includes the organic polymers whether in the "doped" electrically more conductive state, or in the "undoped" or "de-doped" electrically less conductive state.

- 3 - RK172 The terms "doping" and "de-doping" will be taken to mean the oxidaton/reduction processes used to convert the polymers between their less conductive and more conductive states; the terrn "dopants" will refer to the aforementioned counter-ions and to the materials used to provide the counter-ions which stabilise the more conductive form of the polymers; and the terms "un-doped", "doped" and "de-doped" will be understood accordingly.

The present invention provides a polymer compos-ition comprising an electrically conductive polymer associated with a polymeric dopant which stabilises the polymer in an electrically conductive state.

It will be understood that the "polymeric dopant"
specified in accordance with the present invention is a material in which the necessary counter-ions are provided by a polymer or oligomer molecule having charged groups in the polymer backbone and/or as side chains or pendant groups, examples of such a polymeric dopant including polymers and oligomers carrying ionisable sulphonate, carboxylate, or phosphonate groups, polyelectrolytes, and ionenes (generic name for ionic amines).

The use of a polymeric dopant according to the present invention leads to very considerable advantages over the dopants used in the past. For example, the physical and/or chemical characteristics of the poly-meric dopant can be used to modify those of the conduc-tive polymer itself. Furthermore because the modifying polymer is also the dopant, it can be drawn into the conductive polymer during formation of the latter in a ~2f~2$~3

- 4 - RK172 solution of the dopant to form a modified polymer film which will have different properties from those which can be achieved by forming a conventionally doped conductive polymer in a solvent~swollen pre-formed film of another polymer. Also, because the polymeric dopant will tend to interpenetrate with the conductive poly-mer, it can provide an intimate combination of the materials which will be highly resistant to undesired loss of dopant and de-doping.

Thus, further aspects of the present invention provide (1) a method of making an electrically conduc-tive polymer, comprising bringing the polymer from an electrically less conductive state to an electrically more conductive state in the presence of a polymeric dopant which stabilises the polymer in the more con-ductive state; (2) a method of making an electrically conductive polymer comprising polymerising an appro-priate monomer in the presence of a polymeric dopant capable of stabilising the electrically conductive polymer in an electrically conductive state; (3) a method of making an electrically conductive polymer comprising polymerising an appropriate monomer and bringing it into an electrically conductive state in a solution of a polymeric dopant capable of stabilising the polymer in the electrically conductive state; and (4) a method of making an electrically conductive polymer comprising providing the conductive polymer in association with a monomer which is polymerisable to - produce in situ a polymeric dopant capable of stabilis -ing the conductive polymer in an electrically conduc-tive state, and preferably including the step of polymerising the said monomer to produce the dopant.

- 5 - RK172 The specific choice of polymeric dopant will depend on the nature of the conductive polymer to be formed. Positively charged dopants may be appropriate when negatively charged conductive polymers such as polyacetylenes are involved. ~xamples oE positively charged polymeric dopants are ionenes of formula (I), (II), or (III) in the accompanying formula drawings;
acrylamide copolymers such as copoly(acrylamide)(3-acrylamido-3-metylbutyltrimethyl ammonium chloride) of formula (IV) on the accompanying formula drawings; and polymers of 2-methylene ammonium or 2-methylenesulph-onium salts of 1,3-butadiene, of formula (V) in the accompanying formula drawings. In the formula draw-ings, R represents suitable organic substituent groups and n,x,y are integers of suitable value, suitable groups and integers being readily identifiable by persons familiar with this field of technology.

Negatively charged dopants, however, are more likely to be of interest, especially in connection with polypyrroles, which are a preferred class of electric-ally conductive polymers in view of their resistance to degradation on aging. An outstanding advantage of the present invention is the possibility of producing flexible self-supporting films of polypyrroles, which tend to be undesirably brittle and difficult to handle.
Examples of negatively charged polymeric dopants are ionisable polysulphonates such as salts of poly(2-acrylamido-2-methyl-propane sulphonic acid)or ots copolymers with acrylamides or alkyl methacrylates, sulphonated polystyrene, sulphonated polyepichloro-hydrin, sulphonated poly(2,5-dimethyl-phenylene oxide), sulphonated polyphenylether sulphones (or ketones), and sulphonated polyethylene, of formulae (VI) to (XI)

- 6 - RK172 respectively in the accompanying Eormula drawings;
polyvinylsulphonic acid sodium salt (m.w=2000), poly-acrylic acid (m.w=90,000), polymaleic acid, sulphonated EPDM, being an elastomer, could also advantageously be used; polycarboxylates such as salts of polyacrylic acid and copolymers of acrylic acid; and phosphonated polymers such as that indicated by formula (XII) in the accompanying formula drawings. The negatively charged groups may be on the polymer backbone and/or on pendant side chains, and the polymers may be aromatic and or aliphatic in natureO Copolymers and/or mixtures and blends of the above dopants can be used.

It should be noted that the polymeric dopant itself may provide a certain amount of ionic conduc-tivity, but this is usually clearly distinguishablefrom the intrinsic electronic conductivity of the conductive polymer.

As aforesaid, an especially interesting class of electrically conductive polymers comprises polypyrrole and its derivatives, which may be prepared from pyrrole or suitably substituted derivatives thereof, as generally indicated by formula IXIII) in the accompany-ing formula drawings, sufficient positions being left unsubstituted to permit the required polymerisation.
Electrically conductive polymers are generally believed to operate by way of a conjugated pi-electron system, and it will be understood that some substituents or combinations of substituents may interfere with the conjugated system so as to detract from or destroy the desired electrical conductivity, acceptable substi-tuents being readily determinable by simply testing the conductivity of the resulting polymer.

- 7 RK172 Other classes of electrically conductive polymer which may benefit from the use of polymeric dopants according to the present invention include polymers of anilines and naphthylamines of formula (XIV) and (XV);
and conjugated polymers such as polyacetylenes and polyphenylenes (XVI).

The electrically conductive polymers with poly-meric dopants according to the present invention may be prepared in several different ways.

Electrochemical methods of preparing electrically conductive polymers can be used as follows to produce the polymer-doped materials of the present invention.
In general, an electrochemical cell is used, wherein two electrodes (made from, for example, stainless steel,platinum or Indium oxide coated glass) are immersed in an electrolyte mixturet suspension or solution. The electrolyte includes a polymer carrying groups which are ionisable into an appropriately charged ionic dopant species. The solvent or mixture of solvents may be chosen from protic and aprotic solvents, for example acetonitrile, tetrahydrofuran, dimethyl formamide, water, methanol. To the electro-lyte mixture is added the monomer species that is to be oxidatively polymerised to form the desired electric-ally conductive polymer. Stirring of the resultingmixture may be required to achieve a homogeneous mixture. Upon operation of the cell, an electrically conducting film is formed on the appropriate elec-trode. ~ith positively charged polymers such as polypyrroles, the film is formed on the anode and is

- 8 - RK172 partially oxidised and contains a negatively charged polymer dopant to provide charge neutrality to the film.

Chemical preparative methods can also be used, in which the monomer and the polymeric dopant are mixed with an oxidising agent in a suitable liquid vehicle.
The oxidising agent can advantageously be carried by the polymeric dopant of the present invention, for example by forming the ferric salt of a dopant such as sulphonated polystyrene, thereby bringing about simul-taneous oxidative polymerisation and doping with the negatively charged polymeric dopant. This ingeniously avoids or reduces the degree of doping with inorganic anions, which would occur if ferric chloride were used as the oxidising agent.

Some specific examples of the present invention will now be described, in which electrical conductivity was measured by a 4-probe method similar to that described in "Organic Semiconducting Polymers", Ed.
J.E. Katon, (Marcel Dekker 1968).

Sulphonated polymeric dopants were obtained as follows, unless otherwise stated in the Examples.

Sulphonated Pol~ethylene Commercially available chlorosulphonated poly-ethylene (density 1.28) containing 1.1% by wt. S, which indicates a degree of sulphonation of approx. 2% by weight, was hydrolysed with NaOH to convert the S02Cl groups to -S03 Na+ (see Formula XI) and used in this form as the electrolyte in the electrochemical dPing experiments.

- 9 ~ RK172 Sulphonated Pol~styrene Polystyrene (Average molecular not 70,000) was treated with chlorosulphonic acid and the product was subsequently hydrolysed to give an acidic ionisable polymer, that contained 3.23% by wt. S (degree of sulphonation approxO 12% by wt.).

Sulphonated poly(2,5-dimethylphenylene oxide) Commercially available poly(2,5-dimethylphenylene oxide) was treated with chlorosulphonic acid and sub-sequently hydrolysed to give an acidic ionisable poly-mer, containing 3.54~ by wt. S (degree of sulphonationapprox. 16% by wt.).

Preparati _ of Sulphonated Poly(vinyl alcohol) 24.8g Chlorsulphonic acid added slowly with rapid stirring to 100 mls pyridine. This solution was added to a suspension of 8.8g poly(vinyl alcohol) of molecular weight 14000 in 100 ml. pyridine. The mixture was heated at 90C for one hour with continuous stirring, allowed to cool to room temperature and solids were removed by vacuum filtration. The solids were re-dissolved in 200 mls distilled water; 100 mls of water were then removed on a rotary evacuator, and the remaining solution was acidified with 50 mls lM HCl.
The polymer was precipitated by addition of methanol, redissolved in water and reprecipitated with methanol, washed with methanol and dried over P205 under vacuum.

Degree of sulphonation = 25% by weight determined by sulphur content.

~ 10 - RK172 Preparation of a Sulphonated Styrene (Hydrogenated) Butadiene Copol~
___ --._. __ _ __ _ _ __ __ _ _ _ _____ - 800 mls dichloromethane were placed in a 2 litre round bottomed flask fitted with two 500 ml dropping funnels, high speed stirrer, reflux condenser, N2 inlet and gas outlet attached to inverted funnel/H20 gas absorber. One of the dropping funnels contained 20g of a commercially available styrene- (hydrogenated) butadiene copolymer dissolved in 500 mls dichloro-methane the other funnel contained 3.6 mls chloro-sulphonic acid in 500 mls dichloromethane.

The reaction flask was cooled to 5C and kept at this temperature during the addition of the reactants.
The solutions of chlorosulphonic acid and the polymer were added at the same slow rate over approximately 3 hours. Rapid stirring was maintained throughout the addition. ~fter addition was complete stirring was continued and the reaction maintained at 5C for 16 hours. The temperature was allowed to rise to room temperature and the reaction left stirring for a further 24 hours. The reaction was monitored by the quantity of HCl liberated. The HCl was measured by titrating the water in the gas absorber with 1M NaOH
using phenolphthalein indicator.

The dichlormethane was removed from the precip-itated polymer by a combination of decanting, fil tration and rotary evacuation. The polymer was dis-solved in THF and precipitated by addition with rapid stirring, to distilled water. THF was removed from the water/polymer suspension by rotary evacuation and the i3 ~ RK172 polymer filtered. Polymer dried at 40C under vacuum and over NaOH C13 NMR shows addition of sulphonic acid to be on aromatic section of polymer.

Sulphur content = 4.8~. This can be altered by time of reaction.

Example 1 Two stainless steel electrodes 2cm apart were employed as the anode and cathode of an electro-lytic cell, having an electrolyte mixture consisting of 1g commercially available (Aldrich) poly(2-acrylamido-2-methyl propane sulphonic acid) in a mixture of 37cc water and 10cc acetonitrile, together with 0.06 moles of pyrrole. During the operation of the cell the mixture was stirred.

The electrolytic cell was powered by a D.C. power supply. Electrolysis of the electrolyte mixture was carried out by applying to the cell an electrical potential of 5V for a period of one hour, during which time the current density was SmA~m 2.

A black film (33 microns thick) was deposited on the anode and was removed to give a free-standing film that was tough and flexible while wet~ After drying in air the film was brittle, and could not be creased or folded in two like paper without fracturing. The "dry"
film had a room temperature conductivity (4 probe measurement) of 25 Scm~l.

Example 2 A similar procedure to that of Example 1 was followed but this time employing as the electrolyte a solution of 2g of the acid form of sulphonated poly(2, 5-dimethylphenylene oxide) in 75cc water. The cell was operated at a current density of 5mAcm ~ for 45 minutes. A black free standing film 36 microns thick was obtained that was extremely tough and flexible when wet. When dry the film was very brittle and 'nad a room temperature conductivity of 0.3 Scm 1.

Example 3 The procedure of Example 2 was repeated, but this time employing as the electrolyte a solution of 2g of the acid form of sulphonated poly(2,5-dimethylphenylene oxide) in 50 cc DMF, and the polymerisable species used was aniline (0.02 moles), at a current density of 6mAcm 2 for 45 minutes.

A black film was deposited on the anode and could be removed to give tit is believed for the first time~
a free-standing film consisting substantially only of polyaniline and dopant. The film was very brittle, and the sample was too small for conductivity to be deter-mined.

Example 4 The procedure of Example 1 was followed, but this time employing as the electrolyte 5g of the sodium salt of sulphonated polyethylene in a mixture of 1.5cc of water and 50cc of tetrahydrofuran, at a current density of 1mAcm 2 ~or a period of one hour.

A black film was deposited on the anode and on removal was found to be flexible (creaseable) and %~3 stretchable (30~ elongation to break) both when wet and when dry, with a dry conductivity of about 10 3 Scm 1.

- Example 5 The procedure of Example 1 was followed but this time employing as the electrolyte mixture, 2g of the sodium salt of sulphonated polystyrene in 50 cc acetonitrile, at a current density of 1mAcm 2 for ~5 minutes.

A shiny black film was formed on the anode and when dry was very brittle. (Conductivity not deter-mined.) Example 6 Sulphonated Poly(vinyl alcohol) 0.03 moles of pyrrole were dissolved in 30 mls of a solution of 2g 25% sulphonated poly(vinyl alcohol) in 50 mls DMF and 20 mls water. This solution was elec-trolysed for one and a half hours at a current density of 10 mA cm2 on stainless steel electrodes as in Example 1. A black film of polypyrrole doped with sulphonated polyvinyl alcohol was removed from the anode. The film was 0.2~ mm thick and had a conduct-ivity of 2.56 Scm 1. The dry film was tough but brittle.

- 14 - RKl72 Example 7 Poly(methacrylic acid) 0.045 moles of pyrrole were dissolved in 50 mls of a solution of 10g poly(methacrylic acid) supplied by BDH in 40 mls water and 50 mls methanol. This solution was electrolysed for one and a half hours at a current density of 10 mA cm2 on stainless steel electrodes.
A black film of polypyrrole doped with poly(methacrylic acid) was removed from the anode. The film was 0.09 mm thick and had a conductivity of 1.70 Scm 1.

The dry film was brittle.

Example 8 Sulphonated Sytrene (Hydrogenated) Butadiene Copolymer -An electrolysis solution was prepared by dissolv-ing pyrrole (5~ v) in a solution of the sulphonated polymer (2% wt.) in tetrahydrofuran:nitrobenzene (75/25 v/v). This solution was electrolysed in a cell con-sisting of a solid stainless steel anode and a stain-iess steel gauze cathode separated by 5mm. A constant current of lmAcm was passed for 20 minutes. Stirring of the electrolyte was maintained throughout the electrolysis. The polypyrrole film was peeled from the anode and washed in tetrahydrofuran and then dried under vacuum at 50C. Film thickness was 0.4 mm.

~sical Properties -The film was smooth on one side (electrode side) and very rough on the other (solution side). The smooth side had a conductivity of 5 X 10 3 Scm 1 while the rough side was essentially con-conductive. Examin-ation of the film under a microscope revealed a lamin-ate structure. The film could be moulded readily under heat and pressure to give a completely smooth film.
Also, by folding the film in half and hot pressing a film conductive on both sides was obtained.

Film as prepared: thickness 0.4mm, ultimate elongation 410~, stress at failure 18 MPa.

Pressed film: thickness 0.27rnm, ultimate elong-ation 390%, stress at failure 32 MPaO

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AM EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A polymer composition comprising an electrically conductive polymer associated with a polymeric dopant which stabilizes the po-lymer in an electrically conductive state.

2. A composition according to claim 1, wherein the conductive polymer is positively charged and the dopant is negatively charged.

3. A composition according to claim 2, wherein the conductive polymer is a polyaniline.

4. A composition according to claim 2, wherein the conductive polymer is a polypyrrole.

5. A composition according to claim 3 or 4, wherein the poly-meric dopant is an ionisable polysulphonate.

6. A composition according to claim 3 or 4, wherein the dopant is sulphonated polyethylene.

7. A composition according to claim 3 or 4, wherein the dopant is sulphonated styrene (hydrogenated) butadiene copolymer.

8. A composition according to claim 3, which is in the form of a self-supporting film.

9. A composition according to claim 4, which is in the form of a self-supporting film.

10. A composition according to claim 8 or 9 having a greater concentration of the conductive polymer at one of its surfaces and a greater concentration of the dopant at the other of its surfaces.

11. A method of preparing an electrically conductive polymer, comprising polymerising an appropriate monomer in the presence of a polymeric dopant capable of stabilising the electrically conductive polymer in an electrically conductive state.

12. A method according to claim 11, wherein the conductive polymer requires oxidation to bring it into an electrically conductive state, and such oxidation is carried out during and/or after the polymerisation.

13. A method according to claim 12, wherein the oxidation is effected by means of an oxidising agent bonded to the polymeric dopant.