CONDUCTIVE POLYMER COMPOSITION HAVING HIGH VISCOSITY AND CONDUCTIVITY

The present invention relates to a conductive polymer composition having high viscosity and high conductivity, and more particularly, to a conductive polymer composition having excellent electrical conductivity and stability by adding a thixotropic agent, which is dissociated in an aqueous solution to generate negative charges, to PEDOT.

DETAILED DESCRIPTION

The above and other aspects, features, and advantages of the present invention will become apparent from the detailed description of the following embodiments. However, it should be understood that the present invention is not limited to the following embodiments and may be embodied in different ways, and that the embodiments are provided for complete disclosure and thorough understanding of the present invention by those skilled in the art. The scope of the present invention will be defined only by the claims and equivalents thereof.

A conductive composition according to the present invention will now be described in more detail.

Conductive Polymer Composition

A conductive polymer composition according to the present invention includes: (a) an aqueous solution of a polythiophene-based conductive polymer, and (b) a thixotropic agent dissociated in the aqueous solution to generate negative charges.

The polythiophene-based conductive polymer (a) may be i) PEDOT (poly(3,4-ethylenedioxythiophene)) represented by Formula I, or ii) a mixture of the PEDOT and PSS (poly(4-styrenesulfonate)) represented by Formula II.

(wherein n and m are independently an integer ranging from 5 to 10000)

In the present invention, the polythiophene-based conductive polymer is an aromatic polymer, such as polythiol or polyaniline, and representatively PEDOT. The PEDOT may be used alone or in combination with PSS. PEDOT:PSS is most preferred.

The thixotropic agent (b) contained in the conductive polymer composition is dissociated in the aqueous solution and generates negative charges. In the present invention, the thixotropic agent (b) may be a linear or cross-linked polyacrylic acid.

The polyacrylic acid dissolved in water is dissociated into polymer ions and lower molecular weight ions to generate negative charges, whereby the polyacrylic acid can be maintained in a swollen state by van der Waals force between the negative charges. Thus, the polyacrylic acid may control rheological properties of PEDOT or PEDOT:PSS and provides thixotropy, thereby improving resistance stability in liquid phase and electrical conductivity.

The thixotropic agent such as the polyacrylic acid is different from a binding agent. The thixotropic agent swells due to generation of charges in a main chain thereof, thus exhibited increased viscosity together with increased specific surface, and permits reversible tangling or stretching of the main chain.

Unlike the thixotropic agent, a binding agent has viscosity which increases due to increase in specific surface area or no movement between chains by chemical bonding between the chains, and permits irreversible tangling or stretching of the main chain.

If the thixotropic agent swells due to charge generation, thus having an increased specific surface area such that the amount of effective charges of the polymer ions is increased, the lower molecular weight ions are attracted by the polymer ions and then fixed to the polymer. As a result, the amount of the effective charges of the polymer ions is reduced and electric repulsion between homogeneous ions is weakened, whereby there is a tendency to be deflected like a skein.

Consequently, the polymer ions reach an equilibrium state between stretching and tangling, and the specific surface area of polymer chains causes a reversible viscosity change

The thixotropic agent may be present in an amount of 0.00001 to 2 parts by weight, preferably 0.00001 to 1 parts by weight, based on 100 parts by weight of the aqueous solution of the polythiophene-based conductive polymer. If the amount of the thixotropic agent is less than 0.00001 parts by weight, it is difficult to obtain desired viscosity suitable for screen printing. If the amount of the thixotropic agent exceeds 2 parts by weight, the viscosity can be increased, but there are problems such as agglomeration of the conductive polymer, significant increase in surface resistance, particularly, variation in viscosity over time, and the like. Particularly, if the amount of the thixotropic agent exceeds 1 part by weight, there are problems of variation in viscosity over time and rapid change in conductivity.

The conductive polymer composition according to the present invention may further include a binding agent to increase binding force between chains of the polythiophene-based conductive polymer. Here, HPC (hydroxypropylcellulose) is preferred as the binding agent.

As described above, unlike the thixotropic agent, the binding agent, HPC, serves to increase viscosity while improving specific surface area through chemical bonding between chains.

The binding agent may be present in an amount of 0.001 to 10 parts by weight based on 100 parts by weight of the aqueous solution of the polythiophene-based conductive polymer. If the amount of the binding agent is below this range, the binding agent does not properly function in the conductive polymer. If the amount of the binding agent exceeds this range, the conductive polymer suffers from increase in surface resistance.

In addition, the conductive polymer composition according to the present invention may further include a cros slinking agent. The cros slinking agent may be selected from linear or cross-linked isocyanate compounds.

The crosslinking agent may be present in an amount of 0.00001 to 2 parts by weight based on 100 parts by weight of the aqueous solution of the polythiophene-based conductive polymer. If the amount of the cross-linking agent is below this range, there is a limit in increasing viscosity due to insufficient binding force. If the amount thereof exceeds this range, the conductive polymer suffers from increase in surface resistance.

The conductive polymer composition according to the present invention may further include a polar solvent. The polar solvent may include at least one selected from the group consisting of dimethylformamide (DMF), dimethylsulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), methyl alcohol, ethyl alcohol, isopropyl alcohol, propanol, butanol, 4-methylphenol, ethylene glycol, cyclohexanone, tetrahydrofuran (THF), N-nitromethane, toluene, propylene glycol monomethylether acetate, ethyl-3-ethoxypropionate and hexanol.

The polar solvent may be present in an amount of 2 to 30 parts by weight based on 100 parts by weight of the aqueous solution of the polythiophene-based conductive polymer. If the amount of the polar solvent is less than this range, there is a limit in functioning as a secondary dopant. If the amount of the polar solvent exceeds this range, there is a limit in improving electrical properties due to saturation of dopants.

EXAMPLE

Hereinafter, the present invention will be described in detail with reference to examples. It should be understood that the scope of the present invention is not limited by these examples.

A common oxidizing agent and a polymer stabilizer were mixed with EDOT (3,4-ethylenedioxythiophene) in a certain ratio, and stirred at room temperature for 24 hours to perform emulsion polymerization, thereby preparing an aqueous solution of PEDOT:PSS.

DMSO was added to the prepared aqueous solution of PEDOT:PSS in an amount of 5 parts by weight based on 100 parts by weight of the aqueous solution of PEDOT:PSS to prepare a conductive polymer solution (A).

A linear polyacrylic acid was gradually added to the conductive polymer solution (A) in an amount of 0.00001 to 2 parts by weight based on 100 parts by weight of the conductive polymer aqueous solution to prepare a conductive polymer composition including a thixotropic agent.

A conductive polymer composition was prepared in the same manner as in Example 1, except that a cross-linked polyacrylic acid was used instead of the linear polyacrylic acid.

A conductive polymer composition was prepared in the same manner as in Example 1, except that 0.002 parts by weight of an HPC binder was further mixed with the conductive polymer composition.

A conductive polymer composition was prepared in the same manner as in Example 1, except that 0.0001 parts by weight of an isocyanate crosslinking agent was further mixed with the conductive polymer composition.

Comparative Example

A common oxidizing agent and a polymer stabilizing agent were mixed with EDOT (3,4-ethylenedioxythiophene) in a certain ratio, and stirred at room temperature for 24 hours to perform emulsion polymerization, thereby preparing an aqueous solution of PEDOT:PSS.

DMSO was added to the prepared aqueous solution of PEDOT:PSS in an amount of 5 parts by weight based on 100 parts by weight of the aqueous solution of PEDOT:PSS to prepare a final conductive polymer solution.

Experimental Example: Evaluation of Resistance and Viscosity

Each of the conductive polymer solutions prepared in the examples and the comparative example was coated on a PET film, followed by evaluation as to surface resistance and viscosity.

For the conductive polymer compositions prepared in Examples 1 to 4, change in surface resistance according to the amount of the thixotropic agent was observed. In addition, change in surface resistance of the conductive polymer composition prepared in Comparative Example, which contained no thixotropic agent, was also observed. Results are shown in Table 1.

As shown in Table 1, it could be seen that the resistance was increased with increasing amount of the thixotropic agent in the examples and was rapidly increased at an amount of 1 part by weight or more, regardless of the type of thixotropic agent (linear or cross-linked) (comparing Examples 1 and 2).

Generally, preferred surface resistance of a conductive polymer composition for electric devices, particularly touch modules, ranges from 150 Ω/sq to 400 Ω/sq. Here, since the surface resistance of the conductive polymer significantly increases with increasing viscosity thereof, it is important to maintain stability of surface resistance.

As compared with Comparative Example, it could be seen that, although the surface resistance of the conductive polymer compositions prepared in the inventive examples increased with increasing viscosity due to the addition of the thixotropic agent, the conductive polymer compositions of the inventive examples had stability of surface resistance in a desirable range.

For the conductive polymer compositions prepared in Examples 1 to 4 and Comparative Example, change in viscosity according to the amount of the thixotropic agent was observed. Viscosity was measured using a Brookfield viscometer under conditions of 22° C., spindle: #4, speed: 10 rpm. Results are shown in Table 2.

As shown in Table 2, it could be seen that the viscosity was increased with increasing amount of the thixotropic agent and was rapidly increased at an amount of 1 part by weight or more, regardless of the type of thixotropic agent (linear or cross-linked).

Particularly, as compared with Comparative Example, it could be confirmed that high viscosity was obtained by adding the thixotropic agent, and despite a significant increase in viscosity, resistance increase was effectively compensated for.

According to the present invention, the addition of a thixotropic agent to PEDOT or PEDOT:PSS results in thixotropy that provides rapid increase in viscosity in a static state and decrease in viscosity upon application of stress, thereby improving resistance stability in liquid phase and electrical conductivity.

In addition, according to the present invention, the addition of HPC to the PEDOT or PEDOT:PSS results in increase in binding force between chains of the PEDOT, thereby improving stability of a molecular structure and electrical conductivity.

Further, according to the present invention, increase in surface resistance due to viscosity increase can be effectively compensated using the thixotropic agent of the present invention. Furthermore, the conductive polymer composition prepared according to the present invention exhibits high viscosity and excellent printability, and thus can be suitably used for screen printing and a transparent electrode.

Although some exemplary embodiments have been described herein, it should be understood by those skilled in the art that these embodiments are given by way of illustration only, and that various modifications, variations and alterations can be made without departing from the spirit and scope of the present invention. Therefore, the scope of the present invention should be limited only by the accompanying claims and equivalents thereof.