Patent Application
20120178880
The present invention relates to multifunctional active polymeric blends having improved electromechanical properties and in particular polymer blends exhibiting elevated electrical field induced strain level, and elevated elastic energy density and elastic modulus. The materials can be used in electromechanical devices such as actuators and sensors which convert electrical energy into mechanical energy or convert mechanical energy into electrical energy. The electromechanical actuator devices can be used as, but not limited to, diaphragms for fluid pumps, solid state actuators for auto-focusing of camera lens, for precision position control, and for micro-steering of medical catheters.
In recent years, several fluoropolymers, especially poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)), poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene) (P(VDF-TrFE-CTFE)) and other related P(VDF-TrFE) based electrostrictive terpolymers, have been developed which exhibit very high strain under electrical field (for example, 5% strain under 150 MV/m). Examples of such fluoropolymers are described in U.S. Pat. No. 6,787,238 which is incorporated herein by reference. Furthermore, these terpolymers also show high elastic energy density, e.g., higher than 0.5 J/cm3.
However, the high electromechanical properties of terpolymers are generally reported from the thickness strain, which is the strain along the direction of the applied electrical field (see
Further, many terpolymers with high electromechanical properties have relatively low elastic modulus. Efforts to improve the modulus of these materials can in turn adversely affect the high electromechanical properties. Accordingly, a need exists to provide polymeric materials that have high electromechanical properties and high elastic modulus, particular for polymeric materials used in electromechanical devices.
The present disclosure relates to polymer materials in which a high strain polymer is blended with another polymer to increase the overall elastic modulus of the material without materially adversely affecting the electromechanical strain of the polymers. Preferably, the blend can still exhibit the same or similar levels of transverse strain response as the neat high strain polymer.
These and other advantages are satisfied, at least in part, by a polymer blend comprising at least one electrostrictive terpolymer, e.g., poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) or a derivative thereof, and at least one fluoropolymer, e.g., PVDF or derivative thereof such as PVD-TrFE. Advantageously the polymer blend has a transverse strain, i.e., a strain perpendicular to the applied electric field, that is about 1.5% or higher, e.g. 2% or higher, (as measured at 100 MV/m) while also having an elastic modulus of no less than about 400 MPa, e.g., no less than about 500 MPa, (as measured at 30° C. or lower, e.g. at about 25° C., and 1 Hz by dynamic mechanical analyzer).
The electrostrictive terpolymer can be selected from the group consisting of:
Advantageously the polymer blend has a transverse strain that is no less than about 1.5%, e.g., no less than about 2%, (as measured at 100 MV/m) while also having an elastic modulus of no less than about 400 MPa, e.g., no less than about 500 MPa (as measured at 30° C. and 1 Hz by dynamic mechanical analyzer).
In one embodiment of the present disclosure, the general chemical formula of the electrostrictive terpolymer is P(VDFx-2nd monomery-3rd monomer1-x-y) where the 2nd monomer is selected from TrFE, TFE, and the 3rd monomer is selected from CFE, CDFE, CTFE, HFP. The variables x and y are not limited but can be from 0.50 to 0.75 for x and 0.2 to 0.4 for y. The fluoropolymer can be selected from the group consisting of P(VDFz-CTFE1-z), P(VDFz-CFE1-z), P(VDFz-HFP1-z), P(VDFz-CDFE1-z), P(VDFz-TrFE1-z), P(VDFz-TFE1-z), P(VFz-CTFE1-z), [VF=vinyl fluoride] P(VFz-CFE1-z), P(VFz-HFP1-z), P(VFz-CDFE1-z), P(VFz-TrFE1-z), and P(VFz-TFE1-z), the variable z is not limited but can range from z of 0.7 to 1. Preferably, the fluoropolymer is a copolymer and has a dielectric constant higher than 8, measured at 1 kHz and 25° C. More preferably, the copolymer has an elastic modulus larger than 0.8 GPa at room temperature (20 to 25° C.).
In an embodiment of the present disclosure, the blend comprises a composition of the terpolymer and fluoropolymer where the fluoropolymer comprises up to about 15 weight percent (wt %) of the total weight of the two components. Additional polymers can be added to the blend. The blends can be prepared as films such as by co-extrusion, solution cast, spin cast or any method to produce a blend film of two or more polymers.
In one aspect of the disclosure, the transverse strain is the strain along the film surface. The polymer blend in the form of films can be used as just prepared, biaxially stretched, or uniaxially stretched.
Another aspect of the disclosure includes electromechanical devices comprising at least one layer of the polymer blend film. For example, an electromechanical device comprising multilayered polymer blend films such as the Braille display actuator is schematically illustrated in
Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the invention is shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.
Reference is made to the attached drawings, wherein elements having the same reference numeral designations represent like elements throughout and wherein:
The present disclosure is directed to polymer blends having improved electromechanical properties while also having improved mechanical properties, such as increased elastic modulus. For electromechanical applications, besides the strain level, a high elastic modulus is also highly desirable for polymer actuators among other components in devices.
However, when combining high strain terpolymers with other polymers which possess higher elastic modulus, it is possible and likely that the strain level will be reduced. This is because besides the elastic consideration, one major reason is the low dielectric constants of other insulation polymers. Terpolymers with high electrostrictive strain response (>3%) possess a high dielectric constant at room temperature (>40 at 1 kHz frequency), which is in fact the highest among insulation polymers with low dielectric loss (<10%) near room temperatures. The dielectric constants of other insulation polymers are mostly below 4 or even 3. Consequently, when these polymers are blended with the terpolymers, they will often reduce the real electrical field in the terpolymer region and reduce the strain under a fixed external electric field.
To overcome this problem, we investigated blends of P(VDF-TrFE-CFE) terpolymer with several fluoropolymers including P(VDF-CTFE) and P(VDF-HFP) (CTFE: chlorotrifluoropolymers; and HFP: hexafluoropropylene). These fluorocopolymers have dielectric constants higher than about 10 and also exhibit a high electric field induced polarization. As has been shown in earlier studies, the electrical field induced strain response originates from the electrical field induced molecular conformation change between the non-polar phase and polar phase in the terpolymer. Hence, it is believed that the strain response is from the crystalline region of the polymer. Further, P(VDF-TrFE-CFE) and other similar terpolymers are semicrystalline polymers (see
In one embodiment of the present disclosure, a polymer blend comprises at least one electrostrictive terpolymer and at least one fluoropolymer. Advantageously the polymer blend has a transverse strain, i.e., a strain perpendicular to the applied electric field, that is about 1.5% or higher, e.g., a transverse strain of about 2%, 2.2%, 2.5%, 3% or higher (as measured at 100 MV/m). Further, the polymer blend also has an elastic modulus of no less than about 400 MPa, e.g. no less than about 450, 500, 550, or 600 MPa (as measured at 30° C. or lower, e.g. at about 25° C., 20° C., 15° C., or 10° C., and 1 Hz by dynamic mechanical analyzer). In one aspect of the disclosure, the polymer blend has a transverse strain that is 2% or higher, as measured at 100 MV/m, and an elastic modulus of no less than about 0.5 GPa, as measured at 25° C.
In an embodiment of the present disclosure, the polymer blend comprises a composition of the terpolymer and fluoropolymer where the fluoropolymer comprises up to about 15 wt %, e.g., up to about 10 wt % of the total weight of the two components. In one aspect of the present disclosure, the fluoropolymer comprises up to about 5 wt % of the total weight of the two components. Additional polymers can be added to the blends. The polymer blends can be prepared as films such as by co-extrusion, solution cast, spin cast, or any method to produce a blend film of two or more polymers.
The electrostrictive terpolymer can be selected from the group consisting of:
In one aspect of the present disclosure, the terpolymer can be expressed by the formula of P(VDFx-2nd monomery-3rd monomer1-x-y), where the 2nd monomer is selected from TrFE, TFE, and the 3rd monomer is selected from CFE, CDFE, CTFE, HFP. The variables x and y are not limited but can be for x from about 0.50 to 0.75, e.g., about 0.55 to 0.70, and for y from about 0.2 to 0.4, e.g., about 0.25 to 0.35.
The fluoropolymer can be selected from the group consisting of P(VDFz-CTFE1-z), P(VDFz-CFE1-z), P(VDFz-HFP1-z), P(VDFz-CDFE1-z), P(VDFz-TrFE1-z), P(VDFz-TFE1-z), P(VFz-CTFE1-z), P(VFz-CFE1-z), P(VFz-HFP1-z), P(VFz-CDFE1-z), P(VFz-TrFE1-z), and P(VFz-TFE1-z), the variable z is not limited but can range from about 0.7 to 1, e.g., about 0.85 to 0.99. Preferably, the fluoropolymer has a dielectric constant higher than 8, measured at 1 kHz and 25° C. More preferably, the fluoropolymer has an elastic modulus larger than about 0.8 GPa at room temperature (20 to 25° C.).
As presented in
On the other hand, the elastic modulus Y of the blends is increased as the P(VDF-CTFE) wt % increases (see
In an embodiment of the present disclosure, the blend comprises a composition of the terpolymer to fluoropolymer where the fluoropolymer comprises up to about 15 wt %, e.g., up to about 10 wt % of the total weight of the two components. In one aspect of the present disclosure, the ratio of terpolymer to fluoropolymer can be expressed as terpolymer 1-b /fluoropolymer b, where b is in the range of about 15 wt % to about 0.5 wt %, preferably between about 5 wt % and about 1 wt %, and more preferably between about 5 wt % and 2.5 wt %. The elastic modulus of the blend can be higher than about 400 MPa as measured at about 30° C. or lower, e.g. at about 25° C., 20° C., 15° C., or 10° C. Additional polymers can be added to the blend. The blends can be prepared in the form of films such as by co-extrusion, solution cast, spin cast, or any method to produce a blend film of two or more polymers. As films, the polymer blends can be used in actuators such for cameras, and cell phones in place of the materials typically used for such devices.
In another aspect of the disclosure, the transverse strain is the strain along the film surface (in the direction perpendicular to the applied field such as S1 in
In one embodiment of the present disclosure, the polymer blend is in the form of a film which is uniaxially stretched and has a drawing ratio of more than 2 times along the film drawing direction. Preferably the film has a transverse strain of 1.5% or higher under a 100 MV/m electrical field and an elastic modulus higher than 0.5 GPa, measured at room temperature and 1 Hz, or an elastic modulus higher than 0.4 GPa, measured at 40° C. and 1 Hz.
For actuator and electromechanical transducer materials, the elastic energy density Um=YS2/2, where Y is the elastic modulus and S is the strain, is another important parameter. Although the blends may not improve the energy density at room temperature, (Um is 0.71 J/cm3 for the transverse strain S1 of the neat P(VDF-TrFE-CFE) terpolymer of 70/30/8 mol %, and is 0.71 J/cm3, 0.73 J/cm3, and 0.62 J/cm3 for the blends with about 2.5 wt %, 5 wt % and 10 wt % of P(VDF-CTFE) 91/9 mol % copolymer), the blends increase the elastic energy density at higher temperatures. For example, for the transverse strain S1 at 40° C. and under 140 MV/m Um for the neat terpolymer is 0.226 J/cm3 while for the blend films with 2.5 wt % and 5 wt % and 10 wt % P(VDF-CTFE), U is increased to 0.33 J/cm3 and 0.37 J/cm3 and 0.323 J/cm3 at 40° C. and 140 MV/m.
In electromechanical applications, the electromechanical coupling factor k31 measures the energy conversion efficiency in converting electric energy and mechanical energy. For electrostrictive materials, the electromechanical coupling factor can be expressed as
where s is the elastic compliance (s=1/Y, Y elastic modulus) and Ps is the saturation polarization. In ferroelectric based electrostrictive materials such as the terpolymer blends, the dependence of P on applied electric fields E can be approximated by P=Pstanh(kE), where k is a constant. By fitting the experimental P-E curves of the blends with this equation, Ps and k can be obtained. For the blends with 0, 2.5%, 5% copolymer, Ps is 93, 104, 97 mC/m2, and k is 8.1, 6.9, 7.3×10−9m/V, respectively. The electromechanical coupling factor k31 thus obtained for the blends is presented in
Another aspect of the disclosure includes electromechanical devices comprising at least one layer of the polymer blend film. For example, an electromechanical device comprising multilayered polymer blend films is schematically illustrated in
Only the preferred embodiment of the present invention and examples of its versatility are shown and described in the present disclosure. It is to be understood that the present invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein. Thus, for example, those skilled in the art will recognize, or be able to ascertain, using no more than routine experimentation, numerous equivalents to the specific substances, procedures and arrangements described herein. Such equivalents are considered to be within the scope of this invention, and are covered by the following claims.
This application claims the benefit of U.S. Provisional Application No. 61/225,722, filed 15 Jul. 2009, the entire disclosure of which is hereby incorporated by reference herein.
This invention was made with government support under Grant No. 5R01EY018387-01 and 5R01EY018387-02, awarded by the National Institutes of Health. The Government has certain rights in the invention.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US10/42118 | 7/15/2010 | WO | 00 | 4/2/2012 |
| Number | Date | Country | |
|---|---|---|---|
| 61225722 | Jul 2009 | US |
