WO2012146703A1

WO2012146703A1 - Process for the preparation of a conductive polymer composition

Info

Publication number: WO2012146703A1
Application number: PCT/EP2012/057735
Authority: WO
Inventors: Cornelis Eme Koning; Evgeniy Tkalya; Paul SCHOOT VAN DER
Original assignee: Stichting Dutch Polymer Institute
Priority date: 2011-04-27
Filing date: 2012-04-27
Publication date: 2012-11-01

Abstract

The present invention relates to a process for the preparation of a polymer composition, comprising the following steps: A) providing a dispersion of non-conductive particles having an aspect ratio of at least 10 in an aqueous medium comprising a first, water-soluble or dispersible conductive polymer; B) mixing the resulting product from step A) with either an aqueous latex of a second polymer, or with (a) water-soluble precursor(s) of the second polymer; C) removing water from the so obtained mixture; D) heating the product from step C) to a temperature at which the second polymer flows or where the second polymer is formed from its precursor(s); and E) processing and/or solidifying the product of step D) into a desired form.

Description

PROCESS FOR THE PREPARATION OF A CONDUCTIVE POLYMER

COMPOSITION

The present invention relates to a process for the preparation of a conductive polymer composition and the polymer composition obtainable by this process.

In recent years, much effort has been put into the incorporation of carbon nanotubes in polymer matrices. The composites obtained are interesting materials, since they have enhanced electrical and mechanical properties at very low loading due to the specific nanotube characteristics, such as their high aspect ratio and electrical conductance. However, dispersion of carbon nanotubes (CNTs) in highly viscous polymers is difficult and has often been attempted by functionalizing the nanotubes, leading to attractive interactions between the nanotubes and the polymer. In addition, dispersing exfoliated single nanotubes has been found to be a challenge, since nanotubes are highly bundled as a result of strong van-der-Waals interactions.

WO2004/072159 discloses a process for the preparation of a CNT reinforced polymer. In the examples, a conductive polymer composition was obtained from an aqueous dispersion of CNTs and sodium dodecyl sulphate (SDS) and an aqueous latex of polystyrene (PS).

WO2009/033933 discloses a process for the preparation of a conductive polymer composition. In the examples, a conductive single wall nanotubes dispersion stabilized with a commercially available aqueous dispersion of a mixture of poly (3,4 ethylene dioxy thiophene) (PEDOT) and polystyrenesulphonate (PSS) is used to make a composite with a polystyrene latex.

Conductive polymer compositions comprising graphene are also known. WO2010/086176 discloses a process for the preparation of a conductive polymer composition comprising a polymer and graphene. Use of a water-soluble surfactant for the stabilization of the graphene dispersion is mentioned. As an example of the water-soluble surfactant, a commercially available dispersion of a mixture of PEDOT and PSS is mentioned.

Known processes provide a conductive polymer composition, but tend to be costly. Furthermore, the composition and the articles obtained by the known processes are not suitable for some applications.

There is a constant need for compositions which overcome the above-mentioned and/or other problems. It is an object of the present invention to provide such polymer compositions and a process to make such polymer compositions.

The present invention provides a process for the preparation of a polymer composition, comprising the following steps:

A) providing a dispersion of non-conductive particles having an aspect ratio of at least 10 in an aqueous medium comprising a first, water-soluble or dispersible conductive polymer;

B) mixing the resulting product from step A) with either an aqueous latex of a second polymer, or with (a) water-soluble precursor(s) of the second polymer;

C) removing water from the so obtained mixture;

D) heating the product from step C) to a temperature at which the second polymer flows or where the second polymer is formed from its precursor(s); and E) processing and/or solidifying the product of step D) into a desired form.

The term 'aspect ratio' (AR) is herein understood to mean the ratio of the longer dimension to its shorter dimension of the particles. The ratio can be determined by e.g. optical or electronic microscopy. For the particles which are elongated in substantially one dimension such as fibers and whiskers, e.g. CNTs, the aspect ratio is understood as the ratio of the length to the diameter of the particles. For the particles which are substantially elongated in two dimensions such as graphene platelets, the aspect ratio is understood as the ratio of the width of the particles to the thickness of the particles. The aspect ratio of the non-conductive particles is at least 10, preferably at least 30, preferably at least 40, more preferably at least 50, more preferably at least 60, even more preferably at least 100.

The term 'non-conductive particles' is herein understood to mean particles with an intrinsic conductivity below 10² S/m. In the present invention, non- conductive particles with an intrinsic conductivity below 10² S/m, below 10 S/m, below 1 S/m, below 10^"1 S/m, below 10^~2 S/m, below 10^"3 S/m, below 10^"4 S/m, below 10^"5 S/m, below 10^~6 S/m or below 10^~7 S/m may also be used.

The term 'conductive polymer' is herein understood to mean a polymer having an intrinsic conductivity of at least 10² S/m. In the present invention, conductive polymer with an intrinsic conductivity of at least 10² S/m, or e.g. at least 5 x 10² S/m, at least 10³ S/m, at least 5 x 10³ S/m or at least 10⁴ S/m may be used.

Surprisingly, the inventors have found that the polymer composition obtained by the process according to the present invention shows a high electrical conductivity despite the use of the non-conductive particles. Although not wishing to be bound by a theory, the inventors predict that the anisotropic non-conductive particles form a percolating network which serves as a template for the conductive first polymer. Surprisingly, a high electrical conductivity was obtained at low loadings of the non- conductive particles and the conductive first polymer. The invention is extremely advantageous in that inexpensive materials can be used in place of the expensive conductive materials such as CNTs.

The addition of the non-conductive particles significantly lowers the percolation threshold of the expensive conductive polymer such as PEDOT-PSS, thus lowering the amount of the conductive polymer used. The inventors have

experimentally shown that a high conductivity and a low percolation threshold can be obtained by the simultaneous presence of a low amount of the anisotropic non- conductive particles and a low amount of the first, conductive polymer in the second, matrix polymer. The particles of the latex of the second polymer, after brought to flow by e.g. compression molding, form a continuous matrix in which the non-conductive particles and the first, conductive polymer are dispersed.

In principle, any types of non-conductive hydrophobic fillers and hydrophylic fillers may be used as the non-conductive particles, as long as they have an aspect ratio of at least 10. In the cases where the non-conductive particles are hydrophobic, step A) may be performed by contacting the non-conductive particles with (an aqueous latex or solution of) the first polymer which acts as a surfactant for obtaining an aqueous dispersion of the non-conductive particles. In the cases where the non-conductive particles are hydrophilic, the aqueous dispersion of the non- conductive particles may also be provided by first providing an aqueous dispersion of the non-conductive particles and subsequently contacting it with (an aqueous latex or solution of) the first polymer.

Examples of the non-conductive particles include inorganic nanofibers of titanium dioxide, silicon dioxide, zirconium dioxide, aluminum oxide, lithium titanate, titanium nitride or platinum. Sepiolite nanofibers, halloysites nanotubes, boehmite whiskers and crystalline needles of inorganic salts and organic salts are also suitable examples. Sepiolite is a clay mineral, a complex magnesium silicate, a typical formula for which is Mg₄Si₆0i₅(OH)2-6H₂0. Sepiolite fibers are available from Sivomatic and can have a length of a few hundred nm and a diameter of 10-30nm (see D.P.N. Vlasveld "Fibre reinforced nanocomposites" ISBN-10: 90-9019883-0, page 100 and 102). Halloysite is a 1 :1 aluminosilicate clay mineral with the empirical formula

AI₂Si₂0₅(OH)4. Halloysite nanotubes of 1 -5 micron in length with a diameter of 100nm or less are commercially available from NaturalNano. Boehmite from e.g. Sasol exfoliate into nano needles, with LJD between 5 and 20/30 particualrly in polar matrices.

Other examples include organic nanowhiskers and nanofibers, such as cellulose nanowhiskers and polyamide nanofibers. Cellulose nanowhiskers have an advantage that they are bio-based. Polyamide nanofibers have an advantage that they allow use in high temperature applications. These non-conductive particles mentioned above have an advantage that they are inexpensive, except for platinum. Further, these particles give a light, whitish color to the resulting composition, unlike CNTs and graphene which give a black color to the resulting composition. This is advantageous in that the color of the resulting composition can be easily controlled, since the color of white compositions can be tuned by suitable colorants.

The cellulose nanowhiskers may be surface modified. For example, the surface of the nanowhiskers may have sulphonate or Na-sulphonate groups. They may be used in the process having the surface modification, or they may be neutralized before being mixed with the other components in step A).

Preferably, the first polymer is selected from poly(3,4- ethylenedioxythiophene) poly(styrenesulfonate), polyphenylene-vinylenes,

polythiophenes, polyaniline, polypyrrole or polyacetylene. Each of these polymers can be used in combination with suitable non-conductive particles and second polymer to improve the dispersibility of the non-conductive particles in the second polymer which acts as the matrix. An example of the combination is poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate), cellulose nanowhiskers and polystyrene.

The resulting product from step A) can contain up to 75 weight% of the particles. In step A), the mass ratio of the first polymer to the particles can range from 0.05 to 20, preferably between 0.1 and 10.

The temperature at which this step A) is performed is not critical. Temperatures between room temperature and 75°C are very well suited.

The residence time needed for an effective exfoliation of the particles can be easily determined by a man skilled in the art. Residence times below 1 hour have proven to be sufficient for that purpose.

Step B): The product resulting from step A) is brought into contact with either an aqueous latex of a second polymer, or with (a) water-soluble precursor(s) of the second polymer. This second polymer is the polymer which constitutes the matrix of the final polymer composition, in which the particles are well-dispersed. Every aqueous polymer latex known to the skilled man can be used. Preference is given to a polymer being selected from the group comprising polyacrylates, styrene-based (copolymers, butadiene-based (co-)polymers, polycarbonate, acrylonitrile-based (copolymers, (halogen-containing) polyolefins (like polyethylene or polypropylene), polyvinylchloride, polyurethanes and polyamides.

Also (a) precursor(s) for such a second polymer can be used, as they are, or in the form of an aqueous solution thereof which can be converted to the polymer via an emulsion polymerization. Preference can be given, for instance when a nylon is used as the polymer, to the use in this step B) of either the monomer of said polymer (like ε-caprolactam when using nylon 6 as the final matrix material), or to the use of a salt of adipic acid and hexamethylene diamine, or diaminobutane, when nylon 6,6 or nylon 4,6 as the matrix material is aimed at. The skilled man is aware of the precursor(s) needed for such a second polymer. A preference is given to the use in this step B) of ((a) precursor(s) of) a polyamide or a polystyrene based polymer.

The temperature of this step B) generally lies between 10 and 150°C. The pressure is generally atmospheric, but may be increased in order to accommodate for processability in this step B) or in the following step C). The residence time for this step B) is not critical, and generally does not exceed 1 hour.

Although both thermoset polymers as well as thermoplastic polymers can be used as the matrix of the polymer composition, the preference is given to the use of a (semi-) crystalline or amorphous thermoplastic polymer.

Step C): the mixture obtained in step B), according to the present invention, is treated to remove (substantially all of the) water. There are different physical methods available to the skilled man to achieve this removal.

Out of these methods, a preference is for performing step C) by means of evaporation, freeze-drying, or flash-drying.

Step D): is intended to realize a homogeneous dispersion of the particles in the second polymer. When in the preceding steps use is made of (a) precursor(s) for this second polymer, this step D) is also intended to form the polymer from this/these precursor(s). In the case that the polymer is a thermoplastic polymer, the temperature in this step D is chosen such that it is 10-100°C above the melting point (in case of a (semi-)crystalline second polymer), or above the glass point (in case of an amorphous polymer). In the case that the polymer is a thermoset polymer, the temperature in this step D) is chosen such that this polymer can be formed from its precursor(s), during which formation also step E) of the process of the present invention is applied. It was found that performing step D) at a higher temperature generally results in a lowering of the percolation threshold. Step D) may be performed under an elevated pressure, e.g. under a pressure of 1 -100 MPa. It was also found that performing step D) for a longer time at a certain temperature generally results in a lowering of the percolation threshold.

In all cases, the man skilled in the art is aware of the process conditions under which this step D) is to be performed, depending on the nature of the polymer.

Step E) of the process of the present invention is the processing and/or solidification of the product of step D) in a desired form. This step E) can be a molding step, a pelletizing step, an injection or compression molding step, or any known step to form a solidified polymer object.

The process of the present invention results in a polymer composition in which the particles are hardly or not damaged, as a result of which they retain their original length as well as their original aspect ratio (AR). The anisotropic non- conductive particles presumably, although not wishing to be bound by any theory, form an (almost) percolating network. This network presumably serves as a template for the conductive surfactant to form a percolating network.

The invention also relates to a polymer composition obtainable by the process of the present invention. With the (process of the) present invention polymer compositions are obtainable comprising a conductive polymer and a non-conductive polymer, wherein the conductive polymer is PEDOT/PSS and the polymer composition has a conductivity percolation threshold below 2.3 wt.% of PEDOT/PSS. PEDOT/PSS particles themselves percolate at 2.3 wt% in a PS matrix. The addition of the anisotropic particles lowers the percolation threshold of PEDOT/PSS in a PS matrix to a much lower level, e.g. to 2.0 wt%, more preferably to 1 .5 wt%, even more preferably to 1 .0 wt%, even more preferably to around or below 0.75 wt%.

The polymer composition of the present invention can be used for several applications in which the conductivity properties can be exploited. Reference can be given to shielding applications (like electromagnetic interference shielding); high modulus conducting, in-line paintable body panels for the automotive industry with a better surface appearance than glass fibre filled polymers; nano-electric devices (such as thin-film transitors), and others.

According to another aspect of the present invention, use of cellulose nanowhiskers for lowering the conductivity percolation threshold of a polymer composition, such as a PS based polymer composition, is provided.

The invention is illustrated by the following non-limiting examples and comparative experiment.

Figure 1 shows the result of conductivity measurements for a comparative example in which the PS matrix comprises PEDOT:PSS;

Figure 2 shows the result of conductivity measurements for an example of the present invention in which the PS matrix comprises PEDOT:PSS and cellulose whisker and

Figure 3 shows the result of conductivity measurements for a further example of the present invention in which the PS matrix comprises PEDOT:PSS and cellulose whisker.

Materials and techniques

Sisal cellulose nanowhiskers made according to the method described in Lis et.al., Cellulose (2006) 13:261 -270 were used. The nanowhiskers have the geometric average diameter and length of approximately 4 ± 1 and 250 ± 100 nm (an aspect ratio of around 60).

A polystyrene latex was synthesized via conventional free radical emulsion polymerization. Emulsion polymerization runs were carried out in an oxygen free atmosphere. 252 g styrene was mixed with 712 g water in the presence of 26 g SDS surfactant and 0.7 g Na₂C0₃ buffer. The reaction mixture was degassed, by purging with argon, for 30 min. The reaction was initiated by adding 0.45 g SPS (sodium per sulphate) dissolved in 10g of water which was also preliminary degassed. The polymerization took place at constant temperature of 70 'C. Batch emulsion polymerization experiments were performed in a glass stirred reactor equipped with four baffles and with external jackets for heating and cooling (allowed by circulation of water). A 45° pitched four-bladed Teflon impeller was used. The obtained SDS- stabilized polystyrene latex had a peak molar mass around 1 ,000,000 g/mol, with a 28 % solid content. The average size of the latex particles was about 90 nm.

Baytron®P was purchased from H C Starck (Bayer(R)) and contains

0.4 wt% poly(3,4-ethylened[iota]oxyth[iota]ophene) (PEDOT) and 1 wt% poly(styrene sulfonate) (PSS) (relative to the total composition including water). The average size of the latex particles was about 80 nm.

Sonication was performed using a Sonics Vibracell VC750 horn sonicator with a 10mm tip diameter. The sonication power was maintained at 20 W during the exfoliation, and the solution was cooled in an ice-bath to prevent any excessive nanofiller damage. Solution volumes for the sonication process were limited to 25 ml_ and all weight percentages of the cellulose whiskers within these solutions were kept at 0.1 wt% ((relative to the total composition including water))

Freeze-drying of PS/Baytron®P and PS/Baytron®P/cellulose mixed colloids was performed on a Christ Alpha 2-4 drier. Samples were typically dried at 0.25 mbar for 12 hours.

Compression moulding of dried composite powders (remaining after freeze-drying) was performed on a 300G Collins Press. The powders were heated to a temperature between 160 ^ to leO' within 20 min between two PET sheets, degassed (by opening and closing the compression mold) and twice pressed together for 20 seconds at a pressure of 4 MPa, followed by pressing the powder with 10 MPa for 2 minutes at 180°C. They are allowed to cool to room temperature at atmospheric pressure.

Conductivity measurements are performed via a standard 4-point technique. Graphite contacts are painted on the surface of the compression moulded films, and volume resistivites are measured using a Keithley 6512 programmable electrometer and a Keithely 220 Programmable current source.

Comparative example 1

Blends of purchased Baytron®P and prepared PS were prepared. The Baytron®P latex was simply mixed with the PS latex such that the weight percentage of Baytron®P in the final dried polymer blend was between 0 and 6 wt%, or 30 wt%.

The mixed colloid (PS and Baytron®P particles in water) was freeze- dried and then the resultant powder was compression molded (see method in Materials and techniques) into a homogeneous film. One standard PS latex (prepared via emulsion polymerization, see Materials and techniques) was used for all the composites The number average particle size according to data from the supplier of Baytron®P is in the range of 80 nm.

The conductivity was measured for the series of PS/Baytron®P films (see method in Materials and techniques) The conductivity profile is given in figure 1 , and a percolation threshold can be identified. The observed percolation threshold is around 2.3 wt%.

Example 1

Blends of cellulose nanowhiskers, purchased Baytron®P and prepared PS were prepared. The cellulose nanowhiskers were dispersed in water by 15 minutes sonication, with energy supplied about 30000 Joules to 25 mg of whiskers in 25 ml of Water. To this dispersion the Baytron®P was added such that the weight ratio of the whiskers to Baytron®P was 1 :4. The mixture was sonicated for additional 5 minutes for better mixing.

The PS latex was added drop-wise to an appropriate volume of the dispersion (such that loading of cellulose in final composite, i.e. polymer, surfactant and cellulose only, is between 0.1 and 6.0 wt%). These mixed colloids were freeze-dried and compression molded in the same way as the comparative experiment 1 , giving a resultant homogeneous film.

The conductivity profile is given in figure 2. The observed percolation threshold is around 0.4 wt% of cellulose loading, which means 1 .6 wt% of Baytron P. Also the maximum conductivity is high. Example 2

Example 1 was repeated except for that such that the weight ratio of the whiskers to Baytron®P was 1 :1 .

The conductivity profile is given in figure 3. The observed percolation threshold is around 0.7 wt% of cellulose loading, which means 0.7 wt% of Baytron P. Also the maximum conductivity is very high.

Claims

Process for the preparation of a polymer composition, comprising the following steps:

C) removing water from the so obtained mixture;

D) heating the product from step C) to a temperature at which the second polymer flows or where the second polymer is formed from its precursor(s); and

E) processing and/or solidifying the product of step D) into a desired form. Process according to claim 1 , wherein the non-conductive particles are chosen from the group consisting of cellulose nanowhiskers and polyamide nanofibers, sepiolite nanofibers, halloysites nanotubes, boehmite

nanowhiskers, nanofibers of titanium dioxide, silicon dioxide, zirconium dioxide, aluminum oxide, lithium titanate, titanium nitride or platinum, crystalline needles of inorganic salts and crystalline needles of organic salts. Process according to claim 1 or 2, wherein step A) involves contacting an aqueous dispersion of the non-conductive particles with an aqueous latex or solution of the first polymer.

Process according to anyone of the claims 1 -3, wherein the first polymer is selected from poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate), polyphenylene-vinylenes, polythiophenes, polyaniline, polypyrrole or polyacetylene.

Process according to anyone of claims 1 -4, wherein the second polymer is selected from the group comprising polyacrylates, styrene-based (co-) polymers, butadiene-based (co-) polymers, polycarbonate, acrylonitrile-based (co-) polymers, (halogen-containing) polyolefins, polyvinylchloride, polyurethanes and polyamides.

Process according to claim 5, wherein the second polymer is (originating from

(a) precursor(s) of) a polyamide or a polystyrene-based polymer.

Process according to anyone of claims 1 -6, wherein step C) is performed by means of evaporation, freeze-drying, or flash-drying.

Process according to anyone of claims 1 -7, wherein the temperature in step

D) is 10-100°C above the melting or glass point of the second polymer.

Polymer composition obtainable by the process according to anyone of claims

1 -8.

Polymer composition comprising a conductive polymer and a non-conductive polymer, further comprising non-conductive particles having an aspect ratio of at least 10.

Polymer composition according to claim 9 or 10, wherein the polymer composition has a conductivity percolation threshold below 2.3 wt.% of the conductive polymer.

Polymer composition according to any one of claims 9-1 1 , wherein the non- conductive particles are chosen from the group consisting of cellulose nanowhiskers and polyamide nanofibers, sepiolite nanofibers, halloysites nanotubes, boehmite nanowhiskers, nanofibers of titanium dioxide, silicon dioxide, zirconium dioxide, aluminum oxide, lithium titanate, titanium nitride or platinum, crystalline needles of inorganic salts and crystalline needles of organic salts.

Polymer composition according to claim 9-12, wherein the conductive polymer is selected from poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate), polyphenylene-vinylenes, polythiophenes, polyaniline, polypyrrole or polyacetylene.

Use of the conductive polymer composition according to any one of claims 9- 13 in shielding applications, films, in automotive industry or in nano-electric devices.

Use of cellulose nanowhiskers for lowering the conductivity percolation threshold of a polymer composition.