Patent Publication Number: US-2021178732-A1

Title: Electroactive polymers, methods of manufacture, and structures formed thereof

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a division patent application of co-pending U.S. patent application Ser. No. 15/556,696, filed Sep. 8, 2017, which claims priority to International Patent Application No. PCT/US16/21778, filed Mar. 10, 2016, which claims the benefit of U.S. Provisional Application No. 62/132,556, filed Mar. 13, 2015. The contents of these prior applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to polymeric structures. The invention particularly relates to methods for producing polymer materials having electroactive properties and structures formed thereof. 
     A group of polymeric materials known as electroactive polymers (EAP) have been considered for various applications due to their ability to convert electrical energy into mechanical motion through a process of deformation. Notable examples include actuators in which motion can be induced by the application of electrical energy to an EAP material. EAPs can be divided into two subcategories, ionic and electric EAPs. Dielectric EAP materials are a subclass of electric EAPs that are viscoelastic and exhibit properties similar to dielectric materials of capacitors when positioned between two conductive electrodes. When a sufficient electrical potential is applied to the electrodes, Coulomb forces cause electrostatic stresses to occur that cause the viscoelastic EAP material to reallocate its volume, forcing it to constrict in thickness and expand (strain) in the in-plane (length and width) directions. This deformation brings the oppositely charged electrodes into closer proximity relative to one another. When the electromagnetic field is removed, the EAP material substantially returns to its original state. 
     Electroactive polymer materials, particularly dielectric elastomers, exhibit improved performance in the form of increased deformation when exposed to an electric field if the materials are pre-strained prior to implementation of an electric field. In most applications, pre-strain is applied using a mechanical stretcher and the polymer is retained on the stretcher. 
     U.S. Patent Application Publication No. 2015/0091254 discloses actuators and methods utilizing electrical properties of polymer materials, including but not limited to sealing systems, elements and methods. Such actuators may comprise a multilayer structure that includes electrodes formed of electrically-conductive polymer materials, and an electroactive polymer layer therebetween formed of a dielectric elastomer. The electroactive polymer layer is bonded to the electrodes so as to have a thickness dimension therebetween, and an electric potential applied to the electrodes causes the electroactive polymer layer to expand. 
     U.S. Patent Application Publication No. 2015/0091254 also discloses methods of fabricating such actuators by inducing a strain memory state in the electroactive polymer material using a pre-straining technique that expands the electroactive polymer material and then releases the electroactive polymer material to allow the electroactive polymer material to substantially shrink to its pre-strained dimensions. In contrast to the aforementioned mechanical pre-straining methods, a particular example is a chemical pre-straining technique that is believed to be particularly suitable for certain EAP materials, including a fluorocarbon-based FKM EAP material. 
     There is an ongoing desire for improved methods of producing EAP materials and actuators formed therefrom. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention provides methods for pre-straining polymer materials having electroactive properties, and producing layered structures from such materials, a nonlimiting example of which includes actuators. 
     According to one aspect of the invention, a method includes providing a cured dielectric polymeric layer formed of a first volume of a dielectric polymeric material, doping a second volume of the dielectric polymeric material with conductive particulates to yield a conductive polymeric material, partially curing the conductive polymeric material, solvating the conductive polymeric material to create a conductive polymeric paste, applying the conductive polymeric paste to a surface of the dielectric polymeric layer wherein the conductive polymeric paste includes a chemical that causes the dielectric polymeric layer to swell, drying the conductive polymeric paste to form a conductive polymeric layer on the dielectric polymeric layer, allow the dielectric polymeric material to shrink, induce a pre-strain in the dielectric polymeric layer, and yield a layered structure comprising the conductive polymeric layer and the dielectric polymeric layer, and then curing the conductive polymeric layer to bond the conductive polymeric layer to the dielectric polymeric layer. 
     According to another aspect of the invention, a method includes chemically pre-straining a cured dielectric polymeric layer formed of a first volume of a dielectric polymeric material by contacting the dielectric polymeric layer with a chemical that causes the dielectric polymeric layer to swell, and then removing the dielectric polymeric layer from contact with the chemical to allow the dielectric polymeric layer to shrink, induce a pre-strain in the dielectric polymeric layer, and yield a pre-strained dielectric polymeric layer. The method further includes doping a second volume of the dielectric polymeric material with conductive particulates to yield a conductive polymeric material, partially curing the conductive polymeric material, solvating the conductive polymeric material to create a conductive polymeric paste, applying the conductive polymeric paste to a surface of the pre-strained dielectric polymeric layer, drying the conductive polymeric paste to form a conductive polymeric layer on the pre-strained dielectric polymeric layer and yield a layered structure comprising the conductive polymeric layer and the pre-strained dielectric polymeric layer, and then curing the conductive polymeric layer to bond the conductive polymeric layer to the dielectric polymeric layer. 
     Technical effects of the methods described above preferably include the ability to provide, manufacture, and use electroactive polymer actuators entirely composed of polymer-based materials for their conductive and dielectric components. The use of the same polymeric base material for the dielectric and conductive components allows for the dielectric and conductive components to be cross-linked together, ensuring better electrical contact therebetween. In addition, use of the same polymer base material reduces inhibition of deformation by the conductive components. 
     Other aspects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2  are plan and cross-sectional views, respectively, that schematically represent a planar multilayer configuration suitable for use as an actuator in accordance with certain embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides methods for pre-straining polymer materials having electroactive properties, and producing layered structures from such materials. In particular, the methods may be used to manufacture actuators whose movement is controlled by the application of an electrical field to an electroactive polymers (EAP) material. Conversely, it is possible to generate an electrical field with an actuator containing an EAP material by physically actuating the actuator. One aspect of the invention is a construction of an actuator using only EAP materials. 
     Preferred EAP materials for use with the present invention include, but are not limited to, dielectric elastomers whose strain is nominally proportional to the square of the activating electric field. Various dielectric elastomers are known and can be used with the present invention, nonlimiting examples of which include VHB 4905 and 4910 acrylic-based materials commercially available from 3M. Other notable EAP materials include fluoropolymers, particularly FKM (hexafluoropropylene vinylidene fluoride copolymer), commercial sources of which include Parker Seals, Inc. 
       FIGS. 1 and 2  schematically represent a nonlimiting actuator  10  configured in accordance with a nonlimiting embodiment of the invention. As represented in  FIGS. 1 and 2 , the actuator  10  has a layered (multilayer) structure  12  comprising a layer  14  of an EAP material disposed between a pair of electrodes  16  and  18 . An optional fourth layer  20  is represented in  FIGS. 1 and 2  that, if present, is preferably also formed of an EAP material. In the configuration represented in  FIGS. 1 and 2 , the EAP material layer  14  between the electrodes  16  and  18  is referred to as an active layer because it is subjected to an electrical field applied as a result of an electrical potential being applied across the electrodes  16  and  18 . When an electromagnetic field is applied through the electrodes  16  and  18 , the EAP material of the layer  14  reallocates its volume, compressing in the thickness direction and expanding on the plane transverse to the thickness direction. The fourth layer  20  can be considered to be an inactive layer of the actuator  10 , and its primary role is to prevent shorting between the electrode  16  and surrounding components of a system in which the actuator  10  is installed. Alternatively, the fourth layer  20  may also be an active layer and the circuit may be repeated, that is, multiple actuators  10  may be stacked in order to increase the force and actuation potential. The layers  14 ,  16 ,  18  and  20  contact and are bonded to each other so that the layers  14 ,  16 ,  18  and  20  expand and contract largely in unison, primarily in the plane parallel to the layers  14 ,  16 ,  18  and  20 . For this purpose, adhesives or compression molding techniques may be used, though in preferred embodiments the electrodes  16  and  18  are applied to the EAP layer  14  by roll to roll processes, additive manufacturing, or screen printing processes and the layers  14 ,  16 ,  18  and  20  are bonded together by co-curing. Various co-curing techniques are foreseeable and within the scope of the invention. In the embodiment represented in  FIGS. 1 and 2 , the electrodes  16  and  18  are contacted by metal leads  22  through which the electrical potential can be applied to the electrodes  16  and  18 . A wide variety of flexible and substantially rigid conductive materials can be used to form the leads  22 . 
     As in U.S. Patent Application Publication No. 2015/0091254, methods for producing the multilayer structure  12  utilize a chemical pre-straining technique to produce the EAP layer  14  that does not require the strain to be retained in its EAP material. Such a chemical treatment process generally entails contacting a sheet or film of a cured EAP material with a chemical that causes the cured EAP material to swell. The treatment is carried out until the EAP material has sufficiently swelled, as a nonlimiting example, as evidenced by a linear dimensional increase of about 100 percent or more. Thereafter, the EAP material is removed from contact with the chemical and allowed to dry, resulting in the EAP material substantially shrinking back to its original dimensions, for example, within about 5% of its original dimensions. 
     In a particular but nonlimiting example, the EAP material can be a fluoropolymer (FKM), and the chemical pre-straining technique uses a methyl ethyl ketone (MEK), which is known to cause cured FKM to swell due to an amine reaction. This reaction causes intercalation of the solutes into the polymer matrix of FKM. In one investigation, a volume of FKM was cured to form a layer of cured FKM that was then placed in MEK for about five minutes, resulting in the FKM at least doubling in volume. If the FKM was not cured prior to contact with MEK, the MEK would dissolve the FKM. The FKM was then allowed to dry for at least twelve hours, which allowed the FKM to return to roughly its original size. The swelling reaction was determined to be greater than 95% reversible. Furthermore, the MEK caused the volume of FKM to swell evenly in all directions, allowing for more uniform strain when compared to mechanical stretching. 
     While not wishing to be held to any particular theory, with this process the EAP layer  14  and, optionally, the EAP layer  20  appeared to retain a strain memory, allowing for the electrodes  16  and  18  to be attached thereto while the layers  14  and  20  are not in the process of being pre-strained or are in a physically pre-strained condition. Instead, the electrodes  16  and  18  can be attached to the EAP layers  14  and 20 after pre-strain has been released (i.e., their EAP materials are no longer swelled). Such a technique is in contrast to prior practices that entail intentionally retaining a pre-strain in an EAP material during application of the electrodes, for example, with a stiffened region surrounding a pre-strained region of an EAP material to continuously apply a strain to the pre-strained region while the electrodes are being attached. Consequently, the present invention encompasses a method of fabricating an actuator that entails pre-straining an EAP material, and then releasing the strain to induce a strain memory in the EAP material prior to application of electrodes thereto, and until such a time as the resulting actuator ( 10 ) is activated by the application of an electric field with the electrodes ( 16  and  18 ). Though investigations leading to the invention induced strain memory in an EAP material through a chemical treatment that caused the material to swell, it is foreseeable that strain memory could be induced in a variety of EAP materials through the use of other pre-straining techniques that expand the EAP material and then release the EAP material to allow the material to shrink and return or nearly return to its pre-strained dimensions. Suitable techniques for pre-straining the EAP layers  14  and  20  include mechanical, electrical, radiation, and thermal techniques of types known in the art. For example, pre-straining of the EAP layers  14  and  20  can be mechanically induced with the use of unidirectional, bidirectional, and omnidirectional stretching equipment. 
     Unlike U.S. Patent Application Publication No. 2015/0091254, which used electrically-conductive polymer materials that exhibit flexibility comparable to the EAP layer  14 , such as mixtures of conductive grease mixed with graphite, silver inks or paints, mixtures of silicone and graphite, and electrically-conductive silicone-based rubber materials, preferred embodiments of the present invention form the electrodes  16  and  18  from the same EAP material used for manufacturing the EAP layer  14 . In accordance with a nonlimiting embodiment of the invention, one or both of the electrodes  16  and  18  may be produced by doping a volume of a dielectric polymeric material having the same composition as that of the EAP layer  14  with conductive particulates to yield a conductive polymer material having the same base polymeric composition as that of the EAP layer  14 . The conductive polymer material is partially cured, which enables the material to be at least partially solvated to create a conductive polymeric paste or paint. This paste may be applied to opposite sides of the cured EAP layer  14  (in which case the layer  14  serves as a substrate) and then dried to form the layers  16  and  18  on the pre-strained EAP layer  14 . Application of the paste to the EAP layer  14  may induce additional pre-strain in the EAP layer  14 . If layer  20  is desired, it may be attached at this point. Preferably, the layers  16  and  18  are then bonded to the layers  14  and  20  by curing the layers  16  and  18 . 
     For example, the aforementioned investigations further involved the application of a conductive layer to a pre-strained dielectric FKM layer formed as described above. In this process, a second volume of FKM was doped with conductive particulates and partially cured. The partially-cured, doped FKM material was then solvated using MEK, creating a paste which was then painted onto a surface area of the pre-strained dielectric FKM layer. The paste was then allowed to dry to form a conductive layer of the partially-cured, doped FKM material. The MEK in the conductive layer caused additional pre-strain in the pre-strained dielectric FKM layer, thereby improving deformation potential. The resulting layered structure was then cured at about 425° F. The use of the same polymeric base material for the dielectric and conductive layers of the layered structure allowed for a more durable design because all layers (dielectric and conductive) were cross-linked together, ensuring better electrical contact between the dielectric and conductive layers. In addition, use of the same polymeric base material drastically reduced inhibition of deformation by the conductive layer. 
     In the above processes, the cured EAP layer  14  is pre-strained by the chemical straining process prior to application of the conductive polymeric paste thereto, which may cause additional pre-straining in the EAP layer  14 . As an alternative, the conductive polymeric paste may be applied to the cured EAP layer  14  prior to any pre-straining of the EAP layer  14 , and a chemical in the conductive polymeric paste (for example, MEK) may be utilized to cause swelling of the EAP layer  14  and create pre-straining therein. In yet another alternative method, conductive polymeric layers and dielectric polymeric layers may be contacted and co-cured to form the layers  14 ,  16 , and  18  (optionally layer  20 ), and then the stack may be contacted with a chemical to cause swelling and pre-strain in the EAP layer  14 . 
     In view of the above, electroactive polymer actuators can be fabricated whose conductive and dielectric components are entirely composed of polymer-based materials, and preferably the very same polymeric base materials. Actuators can be fabricated by curing a layered structure comprising solvated conductive layers applied to opposite surfaces of a pre-strained dielectric layer as described above. Such a chemical pre-strain and electrode application method can be used for many materials other than FKM. As nonlimiting examples, ethylene propylene rubber (EPDM) and silicone swell in petroleum oil and fuel, nitriles (for example, nitrile rubber (NBR)) swell in toluene, and VHB acrylics (3M) swell in solvents, and therefore are candidates for the chemical pre-straining technique described above. Preferred polymeric materials for a given application will depend on desired material properties of a layered structure. 
     Applications for layered structures as described above include, but are not limited to, seals having the ability to reseal a leak, and actuators for use in such varied applications as automotive systems (for example, to alert drivers to hazards) and medical applications (for example, to promote circulation, control urinary incontinence, pumps, etc.). 
     While the invention has been described in terms of specific embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, the physical configuration of the actuator  10  could differ from that shown, and materials and processes/methods other than those noted could be used. Therefore, the scope of the invention is to be limited only by the following claims.