FIELD OF THE INVENTION
The present invention relates to a method of making a polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE) co-polymer for use in printed memory. More specifically, the present invention relates to a method of exchanging an end group of a PVDF-TrFE copolymer in order to obtain a low molecular weight PVDF-TrFE co-polymer with ferroelectric properties and/or performance that are comparable to those of a high molecular weight polymer.
DISCUSSION OF THE BACKGROUND
Printed memory technology using PVDF-TrFE as a ferroelectric material is well known. Many reports describe ferroelectric properties for different compositions of PVDF-TrFE co-polymers, more specifically the ratio between VDF (vinylidene fluoride) and TrFE (trifluoroethylene). But the importance of the molecular weight is often neglected.
Experimental data has shown that electrical properties in memory devices that include PVDF-TrFE are improved when the molecular weight of the PVDF-TrFE is increased. For instance, the memories may be switched using a lower electric field.
The draw-back with high molecular weight polymers is that they are more challenging to synthesize and process. A very high molecular weight polymer is, for example, less soluble than a lower molecular weight polymer of the same chemical structure, and a solution of the high molecular weight polymer has a higher viscosity than that of a lower molecular weight polymer of the same chemical structure in the same solvent, which could be a disadvantage during purification (e.g., filtration) and coating. Therefore, there is a commercial interest in a PVDF-TrFE co-polymer with a lower molecular weight that exhibits ferroelectric properties similar to a high molecular weight PVDF-TrFE co-polymer.
One example of post-polymerization end group exchange is the treatment of a carboxyl-terminated PVDF-TrFE co-polymer (e.g., U.S. Pat. Nos. 6,809,166
, assigned to Solvay Solexis S.p.A.) using F2
and UV light. This reaction exchanges the carboxyl functionality to a fluorine atom. This reaction may not be suitable for polymers other than substantially perfluorinated polymers and copolymers due to the extremely high reactivity of the fluorine gas.
 U.S. Pat. No. 4,158,678
discloses segmented fluoropolymers having an iodine end group. The iodine end group may be replaced by hydrogen, fluorine or chlorine.
 JP-A-H05 320224
discloses reacting an iodine-containing fluoropolymer with chlorine to replace terminal iodine atoms by chlorine atoms.
SUMMARY OF THE INVENTION
The present invention relates to a method of making an improved ferroelectric polymer that alleviates the above-mentioned drawbacks. Furthermore, it is an object to provide a method of improving the ferroelectric properties of low molecular weight PVDF-TrFE co-polymers.
According to the present invention, the ferroelectric properties of PVDF-TrFE copolymers may be improved by exchanging the end functionalities (or terminal substituents or groups) of the polymer. The molecular weight of the PVDF-TrFE co-polymer may therefore be lowered without adversely affecting the ferroelectric properties of the PVDF-TrFE copolymer, such as the degree or extent of crystallization and the drive voltage of a memory cell including the PVDF-TrFE co-polymer.
The present invention provides a method of exchanging an end group of a PVDF-TrFE co-polymer, comprising exchanging an iodine end group on a PVDF-TrFE co-polymer with hydrogen, and isolating or purifying the PVDF-TrFE co-polymer having the hydrogen end group, wherein exchanging the iodine end group comprises reacting the PVDF-TrFE copolymer having the iodine end group with a tin hydride complex. The tin hydride complex may be tributyltin hydride. The PVDF-TrFE co-polymer may have a molecular weight of from 100 kDa to 800 kDa, preferably of from 100 kDa to 300 kDa. The PVDF-TrFE co-polymer may be a block copolymer, a random copolymer, or a combination or blend thereof.
A difference between low and high molecular weight polymers is that the relative concentration and/or effect of the end group is higher for a low molecular weight polymer. A long polymer provides a high molecular weight, and a long polymer may be desired for applications in printed electronics technology. Since the Van der Waals volume of hydrogen is small and the electrochemical properties are different between the end functionality and the VDF/TrFE monomer units in PVDF-TrFE copolymers having an iodine end group, the effect of the end group on for example the crystallinity and polarization of the polymer is not negligible. Large end groups may be expected to impair the crystallization process and result in larger disorder.
The present invention seeks to treat PVDF-TrFE copolymers having a relatively bulky iodine end group with a source of hydrogen atoms in order to exchange the end group with a smaller atom. The present invention may provide an end group transformation scheme that is less harsh than that disclosed in U.S. Pat. Nos. 6,809,166
, and that is suitable for PVDF-TrFE copolymers.
In the present invention, exchanging the iodine end group comprises reacting or treating the PVDF-TrFE copolymer having the iodine end group with a tin hydride. To exchange the iodine end group with a hydrogen atom, the iodine-terminated PVDF-TrFE copolymer is treated with a tin hydride, such as tributyltin hydride, and optionally a radical initiator, such as azobisisobutyronitrile (AIBN), to provide a polymer with a hydrogen end group.
The PVDF-TrFE co-polymer obtained by the method of the invention may be used as ferroelectric, electromechanical, piezoelectric or dielectric material in the manufacture of electronic devices.
These and other advantages of the present invention will become readily apparent from the detailed description of various embodiments below.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a diagram of drive voltage as function of PVDF-TrFE polymer thickness and molecular weight.
FIG. 2 shows an exemplary end group transformation according to embodiments of the invention.
Reference will now be made in detail to various embodiments of the invention, examples of which are illustrated in the accompanying drawings.
The advantages of the present invention will become readily apparent from the detailed description of various embodiments below.
FIG.1 is a graph 10 that illustrates how the drive voltage that switches the state of a ferroelectric memory including a PVDF-TrFE co-polymer depends on the molecular weight of the PVDF-TrFE co-polymer. As shown by lines 12, 14 and 16, a memory using a polymer with high molecular weight (e.g., 600 kDa, line 16) can be switched at a lower drive voltage than a polymer with low molecular weight (e.g., 370 kDa, line 12, or 450 kDa, line 14). This lower drive voltage switching is desirable, but a high molecular weight PVDF-TrFE copolymer has disadvantages, such as being less soluble, and a solution of the polymer has higher viscosity at the same mass loading. A high viscosity is a disadvantage during filtration and possibly during coating (e.g., there may be problems such as lower uniformity of the film thickness when forming the memory using a coating method).
Thus, an object of the present invention is to provide PVDF-TrFE copolymers with a smaller end group, thereby enabling use of a smaller molecular weight PVDF-TrFE copolymer at lower drive voltages for switching the state of a ferroelectric memory. However, as is known in the art, PVDF-TrFE copolymers have other uses as well (e.g., as transparent protective films, waterproofing agents, UV- and graffiti-resistant paints, low-temperature gaskets in aeronautical and aerospace applications, core and cladding materials in fiber optics), and such other uses may also benefit from a smaller molecular weight PVDF-TrFE copolymer with a H atom end group in place of a bulky end group (which is believed to improve crystallinity of the copolymer).
A "high molecular weight polymer" may refer to a polymer having a molecular weight of at least 600 kDalton. A "low molecular weight polymer" may refer to a polymer having a molecular weight of less than 600 kDalton. The present invention may be used to improve the ferroelectric properties of both low and high molecular weight polymers, but in particular it may significantly improve the properties for polymers with a molecular weight of less than 800 kDalton. Thus, the present PVDF-TrFE copolymer may have a molecular weight in the range of 100-800 kDa (e.g., 200-600 kDa, 100-300 kDa, or any value or range of values within the range of 100-800 kDa).
One aspect of the present invention relates to a method of making a PVDF-TrFE copolymer comprising a plurality of vinylidene fluoride units, a plurality of trifluoroethylene units, and a hydrogen end group. The PVDF-TrFE co-polymer may be a block copolymer, random copolymer, or a combination or blend of block and/or random copolymers. The monomers in any such block or random copolymer can be linked in head-to-head, head-to-tail or tail-to-tail orientations.
The PVDF-TrFE co-polymer may further contain additional alkene and/or cycloalkene monomers that include one or more fluorine atoms. For example, the additional monomers may have the formula Cp
, where p is 3 or more (e.g., 3 to 8 when the additional monomer is an alkene, and 5 or 6 when the additional monomer is a cycloalkene), r is from 1 to 2p, and (q + r) = (2p - 2) or 2p. For example, the additional monomers can include perfluoro-propene, 1,1,1,3,3-pentafluoroprop-2-ene, 1,1,1-trifluoroprop-2-ene, 1,1,2-trifluoroprop-1-ene, 1,1-difluoropropene, 1,1,1,2,2-pentafluorobut-3-ene, and perfluorocyclopentene. In general, the PVDF-TrFE co-polymer can be any fluorinated polyalkene having a ratio of F atoms to H atoms > 1:1 and < 3:1 (e.g., from 7:5 to 2:1, or any other value or range of values between 1:1 and 3:1).
As illustrated in FIG. 2, the method according to an embodiment of the present invention exchanges or converts a bulky end group of a PVDF-TrFE copolymer to an H atom. In the present invention, an I end group is replaced with H as shown in the second reaction of FIG. 2 generally as follows.
A relatively low molecular weight iodine-terminated copolymer 22 is dissolved (e.g., optionally with heating) in a suitable solvent (e.g., one that generally does not react with radical initiators, even when heated up to its boiling point). The amount of PVDF-TrFE copolymer 22 can be any that is soluble in the solvent but a typical amount is from 1% to 20% by weight (e.g., 5%). The solvent may be one or more of C6-C12 alkanes (which may be linear, branched or cyclic), C6-C10 arenes (which may be substituted with one or more fluorine atoms and/or C1-C4 alkyl groups, each of which may be substituted with one or more fluorine atoms) such as benzene or toluene, a C1-C4 alkyl ester of a C1-C4 alkanoic acid such as ethyl acetate, a dialkyl or cyclic alkylene ether having from 4 to 10 carbon atoms (e.g., diethyl ether, methyl t-butyl ether, tetrahydrofuran [THF], dioxane), etc. The mixture may be heated at any temperature greater than room temperature, up to the boiling point of the solvent. However, solvents having a boiling point of at least 50 °C, up to about 150°C, may be particularly suitable. Also, a solvent having some polarity (e.g., esters, cyclic ethers) may be preferable to one that is nonpolar (e.g., alkanes, unsubstituted arenes). In one example, the PVDF-TrFE copolymer 22 is dissolved in ethyl acetate at 70 °C.
The solution of copolymer 22 may be cooled and optionally filtered through a 0.2 µm filter (to remove insoluble matter). The solution of copolymer 22 is then reacted with a hydrogen source being a tin hydride complex, such as tributyltin hydride (Bu3
SnH), and optionally a radical initiator, such as AIBN, as shown in FIG. 2.
For example, a solution of iodine-terminated PVDF-TrFE copolymer 22 (e.g., in a relatively polar solvent such as ethyl acetate) is reacted with excess Bu3
SnH and an amount of AIBN effective to initiate the iodine extraction at 70 °C for a length of time sufficient to substantially complete the hydrogen exchange (e.g., 2 h). The reaction mixture may then be cooled (e.g., to room temperature or 0 °C), and triturated with a nonpolar solvent (e.g., an alkane such as n-heptane). The resultant polymer precipitate is collected by filtration. Any remaining reactants and byproducts may be removed by heating the polymer precipitate in a small amount of a mixture of a relatively polar solvent (e.g., ethyl acetate) and a nonpolar solvent (e.g., n-heptane). The ratio of polar solvent to nonpolar solvent may be at least 1:1 by volume (e.g., 1 part of polar solvent to 2 or more parts of nonpolar solvent by volume). The remaining polymer is removed by filtration. After any such polymer washing is completed, the polymer may be re-dissolved (e.g., in a dialkyl ester such as ethyl acetate) and filtered with a 0.2 µm filter to give the desired H-terminated polymer 24 as a solution. Thus, to increase the crystalline content and/or improve the ferroelectric properties of a low molecular weight, iodine-terminated PVDF-TrFE copolymer 22, the iodine end group is replaced with a hydrogen group. This is done using a hydrogen source such as R'3
Sn-H and an initiator such as azobisisobutyronitrile (AIBN), which provides a polymer 24 having a hydrogen end group (e.g., of the formula R-(CH2
The obtained PVDF-TrFE copolymer has improved ferroelectric properties. In general, the smaller the molecular weight of the PVDF-TrFE copolymer, the greater the improvement.
The obtained fluorinated co-polymer material may be used in an electronic device having a lower drive voltage than a device using a similar or identical polymer material in which the bulky end groups have not been exchanged.
Procédé pour échanger un groupe terminal d'un copolymère de poly(fluorure de vinylidène)-trifluoroéthylène (PVDF-TrFE), comprenant :
l'échange d'un groupe terminal iode sur un copolymère de poly(fluorure de vinylidène)-trifluoroéthylène (PVDF-TrFE) avec de l'hydrogène, et
l'isolation ou la purification du copolymère de PVDF-TrFE ayant le groupe terminal hydrogène,
dans lequel l'échange du groupe terminal iode comprend la réaction du copolymère de PVDF-TrFE ayant le groupe terminal iode avec un complexe d'hydrure d'étain.
2. Procédé selon la revendication 1, dans lequel le complexe d'hydrure d'étain est l'hydrure de tributyl-étain.
3. Procédé selon la revendication 1 ou 2, dans lequel le copolymère de PVDF-TrFE a une masse moléculaire de 100 kDa à 800 kDa.
4. Procédé selon l'une quelconque des revendications 1 à 3, dans lequel le copolymère de PVDF-TrFE a une masse moléculaire de 100 kDa à 300 kDa.
5. Procédé selon l'une quelconque des revendications 1 à 4, dans lequel le copolymère de PVDF-TrFE est un copolymère séquencé, un copolymère statistique, ou une combinaison ou un mélange de ceux-ci.