WO1993023495A1

WO1993023495A1 - Electrically conducting liquid-crystal polymers and process for preparation thereof

Info

Publication number: WO1993023495A1
Application number: PCT/FI1993/000216
Authority: WO
Inventors: Ko-Shan Ho; Kalle Levon
Original assignee: Neste Oy
Priority date: 1992-05-20
Filing date: 1993-05-21
Publication date: 1993-11-25
Also published as: EP0641371A1

Abstract

The present invention concerns an electrically conducting liquid-crystal polymer, process for its preparation and fibers and films comprised at least to some extent of said polymer. According to the invention, the liquid-crystal polymer comprises a main chain incorporating conjugated double bonds, the monomer units of said main chain being linked to side chains which, together with the main chain, render the polymer liquid-crystal properties at elevated temperatures. The main chain is treated with an electron-donor or corresponding electron-acceptor type of dopant to make it conducting. In an advantageous embodiment of the invention, the conducting liquid-crystal polymer is prepared by polymerizing 3-dodecylthiophene monomers, after which the polymer obtained in this manner is doped using ferric chloride or iodine. The polymer can be processed into fibers or films using conventional processing methods of plastics such as die extrusion, injection molding, ram molding or film blowing.

Description

ELECTRICALLY CONDUCTING LIQUID-CRYSTAL POLYMERS AND PROCESS FOR PREPARATION THEREOF

The present invention relates to polymers possessing both liquid-crystal and conducting properties. The invention also concerns a method for the production of electrically conducting liquid-crystal polymers and fibers and films incorporating these polymers.

Electrically conducting polymers are subjected to intensive research around the world. They can be prepared from organic polymers having long conjugated chains of double bonds. The ^-electrons of the double bonds are disturbed by doping the polymer with certain dopants which may be electron acceptors or donators. Then, the doping produces vacancies or alternatively excess electrons in the polymer chain that facilitate current flow along the conjugated chain. The electrical conductivity of the polymer can be controlled by varying the dopant concentration so as to cover nearly the entire spectrum of conductivity from insulators to metals.

The above-mentioned polymers are suited to several interesting fields of application. For instance, they can be utilized to replace metallic conductors and semiconductors in multiple applications such as batteries, photo-electric components, printed-circuit boards, antistatic packaging and electromagnetic interference (EMI) suppression materials.

In comparison with metals, the potential advantages of electrically conducting polymers are their light weight, mechanical properties, stability in corrosive environments and lower costs in synthetic preparation and processing. It must be noted, however, that the processing and stability properties of most intrinsically electrically conducting polymers do not today permit their use in the above-described applications.

Polymers that can be modified electrically conducting by doping include poly- acetylene, poly-para-phenylene, polypyrrole, polythiophene and polyaniline. Liquid-crystal polymers are also being investigated widely. Their spinning and tenacity characteristics in the liquid-crystal state offer optimal prerequisites for the processing of fibers with high mechanical performance. The orientation of polymer chains in the liquid-crystal state under the influence of an electrical or magnetic field also provides a possibility of using said polymers in the optical systems of display terminals and other applications utilizing optical effects.

The actual liquid crystal in liquid-crystal polymers is formed by a rigid structural unit called a mesogen. Mesogens are generally formed by two or more linearly substituted ring units linked to each other via a short, rigid bridging group known as a spacer. Liquid-crystal polymers can be divided into two main categories, viz. main-chain and side-chain liquid-crystal polymers, depending on whether the the mesogenic groups are located in the main chain or in the side chain. Main-chain liquid-crystal polymers are generally polymers of highly rigid character whose stable crystalline structure can only be melted by using abundant energy. Therefore, their melting occurs at high temperatures, the melting being accompanied by thermal decomposition. Side-chain liquid-crystal polymers in turn are determined by the liquid-crystal behavior of the side-chain substituent irrespective of the main-chain structure. The substituent (mesogen) in the side chain frequently has the chemical structure of a biphenyl-cyano group.

To complete the survey, it should be mentioned that conventionally known are also polymers in which the liquid-crystal properties are combined with electrical conductivity. These compositions are invariably of the side-chain type described above. For instance, the published JP Patent Application No. 2,227,425 discloses a polymerized thiophene derivative having a mesogenic biphenylester substituent as its side chain. Said group is also claimed to have ferroelectric properties. A similar polythiopheπe polymer is also known from the published GB Patent Application 2,225,008 also having mesogenic cyanobiphenyl derivatives as the substituents of the thiophene ring; The EP Patent Specification No. 0 106 175 as well discloses an electrically conducting liquid-crystal composite in which the backbone exhibiting liquid-crystal behavior is directly or indirectly linked to a complex group capable of electron transfer.

The most essential disadvantage of above-described electrically conducting liquid- crystal polymer alternatives is their high cost of synthesis due to the complicated, grafting of the biphenylcyano group to the side chain.

It is an object of the present invention to overcome the drawbacks associated with the prior-art technology and to achieve an entirely novel polymer possessing both liquid-crystal and conducting properties.

Our invention aims at achieving electrical conductivity by way of employing conventional techniques for doping a polymer having conjugated double bonds is its main chain with a suitable dopant of the electron-acceptor, or alternatively- electirø- donor type. The polymer selected for doping is of a kind having rigid, mesogenic groups in its main chain. The double bonds can be located in said mesogenic groαps in particular. It has been found that many unsubstituted polymers having the above- mentioned rigid basic structure are difficult in processing due to their high melting points which in turn prohibit processing. Therefore, the invention is based on the concept of attaching to the main chain of the polymer appropriate side chains to obtain transition of above-mentioned rigid polymers into the liquid-crystal phase at relatively low temperatures. This phenomenon is evidently related to the fact that the substituent groups (i.e. the side chains) prevent the contiguous aggregation of the main chains of the polymers. Thus, the substituents have a dissolving effect, thereby allowing the polymers to attain their liquid-crystal properties at such temperatures which still are compatible with easy processing.

In summary, the polymer according to the invention comprises a main chain containing conjugated double bonds, the monomer units of said main chain being linked to side chains which, together with the main chain, render the polymer liqtώi- crystal properties. The polymer is treated with a dopant to make it conducting. According to the invention, the polymers are prepared by al ylating a monomer compound, which in the polymerized state provides a chain incorporating conjugated double bonds, by an alkyl group containing a minimum of 9 carbon atoms. The alkylated monomer compound thus obtained is reacted in a polymerization reaction and the polymer thus obtained is treated with a dopant.

Within the scope of the present application, the term "liquid-crystal polymers" designates polymers, which in their molten state are in an intermediate phase between the solid and the liquid states, called the mesophase or isotropic phase.

The term "mesogen" or "mesogenic" structural unit in this application refers to such rigid structural units which are principally responsible for the liquid-crystal properties of substituted polymer chains. In addition to those groups mentioned above, mesogens can be, e.g., the thiophene monomers of polythiophene.

The term "room temperature" refers to a temperature of approx. 15 °C to approx. 25 °C.

A "hydrocarbyl" group is a group containing carbon and hydrogen atoms in a linear or branched configuration, whereby said group can be saturated or unsaturated. It may also contain functional groups.

As mentioned above, the main chain of the polymer according to the invention com¬ prises mesogenic structural units and the mesogenic structural groups are substituted by side chains which prevent the contiguous aggregation of several main chains of the polymers. The crystalline structure of the polymers according to the invention is dependent on the chemical structure of the main chain, the interaction between the chains and the dimensions of the monomer segment. Typically, the alkyl side chains of a side-chained polymer crystallize in a hexagonal lattice, irrespective of the rigidity of the side chain and the formation of the liquid-crystal state. Therefore, the side chains of the polymers according to the invention may at least partially be in a crystallized form at room temperature. The crystallized side chains melt with an increase in temperature. The side chains do not, however, form crystalline structures in the liquid-crystal state, but rather, the mesogens are responsible for the formation of such structures.

The transition temperature of the liquid-crystal structure state drops as the length of the side-chain substituent is increased. Therefore, it has been found advantageous according to the invention to utilize such hydrocarbyl substituent which contain 9 to 15 carbon atoms, for instance. If the substituent contains less than 9 carbon atoms, the liquid-crystal phase cannot be realized as is evident from Example 3 to be described below.

The main chain can be formed by, e.g., polythiophene, polyaniline, polyacetylene, polypyrrole or polyparaphenylene.

According to one preferred embodiment, the conducting liquid-crystal polymer is a

3-substituted thiophene polymer doped with an appropriate dopant of the electron- donor or electron-acceptor type.

According to a particularly preferred embodiment of the invention, the conducting liquid-crystal polymer is poly(3-dodecylthiophene).

The above-described conducting liquid-crystal polymers are prepared by, e.g., polymerizing a monomer substituted by a suitable long-chain substituent using conventional techniques, after which the polymer obtained in this manner is treated with a desired dopant.

According to an embodiment of the invention, a monomer compound comprising in its polymerized state an essentially linear chain containing conjugated double bonds is alkylated with an alkyl group, preferably a linear alkyl group, having a minimum of 9 carbon atoms. The alkylated monomer compound is reacted in a polymerization reaction and the obtained polymer is treated with a dopant. According to an advantageous embodiment of the invention, thiophene is alkylated with a Grignard reagent of the form

R-Mg-X wherein R is an alkyl group containing from 9 to 15 carbon atoms and X is a halogen atom, the formed 3-al ylthiophene is halogenated with, e.g., elemental halogen, the obtained 2,5-dihalogen-3-alkylthiophene is polymerized to form poly(3-alkylthiophene) and, after an optional processing step, the obtained polymer is contacted with an agent of the electron-donor or electron-acceptor type in order to impart conductive properties to the polymer.

The polymer according to the invention can be processed in both its liquid-crystal and isotropic state using, e.g., die extrusion, injection molding, ram molding or film blowing. Preferably, the polymer is brought in the liquid-crystal state prior to its processing and/or processing to attain a polymer product.

Rapid cooling of the polymer from a liquid-crystal temperature to room temperature provides a polymer which even at room temperature remains in the liquid-crystal state. This is evidently caused by the fact that the rapid crystallization of the side chains prevents the crystallization of the main chain, whereby the main chain remains in the liquid-crystal state.

The doping of the polymer with dopants of the electron-acceptor type can take place either chemically or electrochemically.

According to an advantageous embodiment of the invention, the polymer is treated with a carrier containing ferric chloride, FeCl₃. The carrier can be a suitable organic solvent such as nitromethane, for instance, or any other convenient solvent or suspension-forming agent which does not deleteriously interfere with the doping process by, e.g., dissolving poly(3-alkylthiophene). Typically, such organic solvents are applicable that^" dissolve the required salt of the dopant and simultaneously cause swelling of the matrix resin so as to make doping possible. After doping, the prepared polymer such as a film, for instance, is rinsed free from excess dopant using a suitable solvent, preferably of the same kind as used during doping, and the polymer product is dried.

An alternative advantageous dopant is iodine, which can be used as such for elevating the conductivity of poly(3-alkylthiophene).

The electrical conductivity characteristics of the polymer can be altered by varying the dopant concentration, doping time and temperature. Typically, the electrical conductivity of the prepared polymer can be varied in the range from 10^~10 to

100 S/cm.

The liquid-crystal state of the polymers prepared according to the invention can be determined from the thermal phase transitions from the crystalline state to the liquid- crystal state and from the liquid-crystal state to the isotropic state.

The invention offers significant benefits. For instance, the liquid-crystal polymers prepared in the above-described manner are suited to applications requiring conducting properties, applications requiring liquid-crystal properties and to such applications that require both conducting and liquid-crystal properties. The drawing of a polymer in its liquid-crystal state is easier than that of a material in the isotropic state. Moreover, the effect of electrical and magnetic fields on the alternating, reversible orientation can be utilized. The mechanical properties of fibers drawn in the liquid-crystal state are excellent, since the length-to-diameter ratio of the fiber can be minimized by virtue of the applicability of the maximum drawing parameters.

The invention can be applied to optical devices, films and display terminals in which the reversible or alternating orientation of the liquid-crystal component in the polymer under the effect of an electrical or magnetic field can be employed.

In respect to an advantageous embodiment of the invention (that is, poly(3-dodecyl- thiophene polymer), it must be noted that fabricability of polythiophene in its isotropic state has in prior investigations on polythiophene been attained only after melting of the polymer. At the melting temperature, however, starts the thermal disintegration of the polythiophene and the oxidative disintegration of the alkyl side chain. In contrast to this, the present invention makes it possible to fabricate the polymer at lower temperatures in which the polymer exists in its liquid-crystal state.

In the following the invention is described in greater detail with the help of exemplifying embodiments, which are not to be taken as limiting in any way. Reference is made in Example 2 to the appended drawing which shows the results of thermal analyses performed on poly(3-dodecylthiophene) using a Differential

Scanning Calorimeter (DSC).

Example 1

Preparation of poly(3-dodecylthiophene)

A. Monomer preparation

3-dodecylthiophene was prepared according to EP-203 as follows: Magnesium (dried, 1.6 mol) and dodecyl bromide (dried, 1.5 mol) were mixed in 500 ml dried diethyl ether to obtain a corresponding Grignard reagent. The magnesium and the ether were hereby transferred to a reaction vessel containing an argon atmosphere and the argonated dodecyl bromide was added slowly. An iodine crystal was added to initiate the reaction.

The concentration of the obtained reagent was determined as follows: A 10 ml aliquot of the sample was taken and added to^" 150 ml of distilled water. The indicator was added and the solution was titrated with 0.2 M NaOH at 70 °C.

The reagent was transferred to another reaction vessel, to which in an argon atmosphere was^" slowly added a molar quantity corresponding to the reagent concentration of 3-bromothiophene and dichloro-[l,3-bis(diphenyl-phosphino)-pro- pane]nickel(II) as a catalyst. To initiate the reaction, the reaction vessel was heated. The solution was refluxed for 4 hours. After this the reaction flask was placed in an ice bath and the mixture was acidified with 1.0 N HC1. The mixture was washed in a separatory funnel with water (three times), with saturated NaHCO₃ (three times) and dried with CaCl₂. The mixture was distilled and the product was 3-dodecyI- thiophene.

2,5-diiodo-3-dodecylthiophene was prepared as follows: 250 ml of dichlorσmethaπe, a 0.4 mol aliquot of the above-prepared 3-dodecylthiophene and 0.5 mol of iodine were transferred to a reaction vessel (argon atmosphere). A 1: 1 mixture of 90 mi nitric acid and water was slowly added, and the temperature of the reacted mixture was slowly elevated to 45 °C. The solution was refluxed for 4.5 hours. After this the reacted mixture was washed three times with water, three times with 10 % NaOH and again two times with water. Filtration and purification of the product were performed in a column (silica + hexane). The product was 2,5-dϋodo-3- dodecylthiophene.

B. Polymerization

Poly(3-dodecylthiophene) was prepared as follows: a 0.3 mol aliquot of the above- prepared 2,5-dϋodo-3-dodecylthiophene, 0.3 mol of magnesium and 200 ml of tetrahydrofurane (THF) were added in a reaction vessel and refluxed for 2 hours. A 0.001 mol .aliquot of a catalyst (dichloro-(l,3- bis(diphenyl-phosphino)-propane)- nickel(II)) was added. The reaction vessel was cooled to 20 °C prior to the addition of the catalyst. The temperature was elevated to 70 °C and the solution was refluxed for 20 hours. The obtained product was poured in methanol (1200 ml methanol + 5

% Hcl). The mixture was allowed to stabilize for 2 hours. After this, the mixture was filtered and washed with hot water and methanol. Extraction was performed with methanol and drying in a vacuum. The product was poly(3-dodecylthiophene). Example 2

Thermal analysis of poly(3-dodecylthiophene)

The thermal behavior of the obtained poly(3-dodecylthiophene) was investigated sep- arately using DSC calorimetry (Perkin Elmer 7). The appended diagram shows a thermogram which has three endothermic phase transition shifts. The first, T_l5 occurs at 35 °C, which is the melting point of the alkyl side chain. T₂, the second transition temperature, represents the phase change from the crystalline state to the liquid-crystal state, which is a comb-like layered structure, in which the interlayer spacing varies according to the length of the side chains. This transition is also detectable by an x-ray diffractometer, in which the scattering occurring in the wide- angle diffractogram of the crystalline state vanishes at the transition temperature. The only remaining scattering is related to the narrow-angle scattering characteristic of the liquid-crystal state, caused by the scattering due to the interlayer spacing.

In this liquid-crystal state the material possesses a degree of order resembling that of a solid material as indicated by the x-ray diffraction and thermal analysis results, yet exhibiting a fluid creep property that facilitates processing, for instance, within this temperature range. T₃ is the phase transition temperature from the liquid-crystal state to the isotropic state, in which the material behaves like a solution lacking any degree of order.

Example 3 - Reference example Polyoctylthiophene

Starting from octyl bromide and 3-bromothiophene, poly(3-octyIthiophene) was prepared in the manner described in Example 1. The polymer was subjected to thermal analysis as described in Example 2. The thermogram contained two endo¬ thermic phase transitions, of which the first represented the melting of the crystallized octyl group and the second the isotropic melting, respectively. Corre¬ spondingly, this polymer does not exhibit liquid-crystal properties in the molten state, which is evidently related to the fact that the alkyl side group having a length of 8 carbon atoms is not sufficiently long to accomplish the required dissolving effect.

Example 4 Polymer preparation using a melt blending apparatus

Using a melt blending apparatus (Brabender extruder), a poly(3-dodecylthiophene) sample was prepared. The blending temperature was 170 °C, at which temperature the isotropic state (T >T₃) was attained. The blending time was 10 min and the rotation speed 30 r/ in.

Example 5

Polymer preparation using a melt blending apparatus

Using the Brabender extruder mentioned in the previous example, a poly(3- dodecylthiophene) sample was prepared. The blending temperature was 130 °C (liquid-crystal state, T₂ < T < T₃), the blending time 20 min and the rotation speed 30 r/min.

Example 6

Processing of sheets

The polymer samples prepared in Examples 4 and 5 were processed into sheet pieces by injection molding in either the liquid-crystal state or the isotropic state. The injection time was 5 min. the temperature either 130 °C or 170 °C and the pressure

100 bar. Example 7 Processing of films

The polymer samples prepared in Examples 4 and 5 were ground into granulates, which were further processed by blowing into polymer films. The processing was carried out in the Brabender apparatus, whose block temperatures were in the range from 150 °C to 170 °C.

Example 8 Doping of the polymer

The polymer samples prepared in Examples 4 and 5 were doped. The samples were immersed in concentrated FeCl₃-nitromethane solution (dry argon atmosphere). After doping for one hour, the samples were washed with nitromethane and vacuum dried. The electrical conductivity of the polymer samples was measured to be 1 S/cm.

Example 9

Doping of the processed polymers

The films processed according to Examples 6 and 7 were doped with iodine vapor in a vacuum. The electrical conductivity of the doped films was measured to be 1 S/cm.

Example 10 Liquid-crystal properties of processed polymers at room temperature

The polymer sheets processed according to Examples 6 and 7 were pressed at 130 °C to attain the liquid-crystal state. The samples were cooled in an icewater bath to achieve side-chain crystallization. The side-chain crystallization was hereby capable of preventing the liquid-crystal state from reverting to the crystalline state.

Consequently, the sample remained in the liquid-crystal state even at the room temperature. The sample was doped with iodine vapor. The electrical conductivity of the sample was measured to be 1 S/cm.

Example 11 - Reference example Polyundecyl isocyanate

Analogously to Example 1, polyundecyl isocyanate was prepared and subjected to analysis using the apparatus described in Example 2. The obtained thermogram indicated that polyundecyl isocyanate - as well as poly(3-undecylthiophene) — has three transitions (T, to T₃), of which T₂ is the transition temperature from the crystalline state to the liquid-crystal state and T₃ is the transition temperature from the liquid-crystal state to the isotropic state. T is melting point of the alkyl side chain. Consequently, polyundecyl isocyanate is a liquid-crystal polymer according to the application. The liquid-crystal state can be either a nematic or smectic layered structure. Doping of said polymer, however, did not invoke any electrical conductivity properties.

Claims

Claims:

1. A conducting liquid-crystal polymer, comprising a main chain containing conjugated double bonds, the monomer units of said main chain being linked to side chains which, together with the main chain, render the polymer liquid-crystal properties, said polymer having been treated with a dopant to make it conducting.

2. The polymer of claim 1, wherein the main chain contains mesogenic structural units and — the mesogenic structural units are substituted by side chains which prevent the contiguous aggregation of several main chains.

3. The polymer of claim 1, wherein at least a part of the side chains are comprised of alkyl radicals containing a minimum of 9 carbon atoms.

4. The polymer of claim 1, wherein the side chains are comprised of linear alkyl radicals essentially containing 9 to 15 carbon atoms.

5. The polymer of claim 1, wherein the main chain of the conducting polymer is selected from the group of polythiophene, polyaniline, polyacetylene, polypyrrole amd polyparaphenylene.

6. The polymer of claim 1, wherein ferric chloride, FeCI₃, has been used as a dopant.

7. The polymer of claim 1, wherein iodine has been used as the dopant.

8. The polymer of claim 1, having an electrical conductivity in the range from 10-¹⁰ to 100 S/cm.

9. The polymer of claim 3, wherein its alkyl side chains are in at least partially crystallized phase at room temperature.

10. The polymer of claim 1 , wherein it is in the liquid-crystal phase at room temperature.

11. The polymer of claim 1, wherein the polymer is poly(3-dodecylthiophene).

12. A process for preparing a conducting polymer, comprising

- alkylating a monomer compound, which in polymerized state forms an essentially linear chain containing conjugated double bonds, by an alkyl group having at least 9 carbon atoms to form an alkylated monomer compound,

- polymerizing said alkylated monomer compound to form a polymer, and

- treating said polymer with a dopant to render it conducting.

13. The process of claim 12, comprising

- alkylating thiophene by a compound of the formula

R-Mg-X wherein R represents a linear alkyl having 9 to 15 carbon atoms and X is a halogen atom to form a 3-alkyltiophene compound, - halogenating said 3-alkylthiophene thus obtained with an elemental halogen to form a dihalogenated 3-alkyltiophene derivative,

- polymerizing said derivative to form poly(3-alkylthiophene), and

- after an optional processing step, contacting the obtained polymer with a doping agent in order to make the polymer conducting.

14. The process of claim 12, wherein the polymer is brought to the liquid-crystal state prior to its processing into a polymer product.

15. The process of claim 14, wherein the polymer is rapidly cooled after processing from the liquid-crystal state temperature to room temperature in order to provide a polymer product which remains in the liquid-crystal state at room temperature.

16. The method of claim 12, wherein the polymer is shaped by using die extrusion, injection molding, ram molding or film blowing.

17. The method of claim 12, wherein the polymer is doped by reacting the polymer with a dopant of the electron-acceptor type.

18. The method of claim 17, wherein the polymer is doped chemically.

19. The method of claim 18, wherein the polymer is doped by ferric chloride, FeCl₃.

20. The method of claim 17, wherein the polymer is doped electrochemically.

21. Polymer fibers containing a polymer of claim 1.

22. Polymer films contaning a polymer of claim 1.