In-process ultrasonic polling of 3D printed crystalline/semi-crystalline electroactive polymers

Abstract

Methods and systems for producing a structure having selectable piezoelectric properties via additive manufacturing. Such methods can include coupling an ultrasound generating device to a print head of the additive manufacturing apparatus; transmitting acoustic energy from the ultrasound generating device to the print head to vibrate the print head in an oscillatory manner; extruding a feed material from the print head; moving the print head in at least one dimension relative to a substrate on which the structure is being manufactured; and dispensing layers sequentially on top of each other to form the structure. Such systems can include an additive manufacturing apparatus comprising a print head movable in at least one dimension relative to a base configured to support the structure being produced; and an ultrasound generating device that is connected to the print head.

Claims

1. A method of producing a structure having selectable piezoelectric properties using an additive manufacturing apparatus, the method comprising: coupling an ultrasound generating device to a print head of the additive manufacturing apparatus; transmitting acoustic energy from the ultrasound generating device to the print head to vibrate the print head in an oscillatory manner; extruding a feed material comprising a polymeric material comprising polyvinylidenefluoride (PVDF) and/or a co-polymer thereof from the print head; moving the print head in at least one dimension relative to a substrate on which the structure is being manufactured; and dispensing layers sequentially on top of each other to form the structure, wherein the dispensing comprises: dispensing portions of the structure while the ultrasound generating device is in an on state so that the portions of the structure have piezoelectric properties; and dispensing other portions of the structure while the ultrasound generating device is in an off state so that the other portions of the structure do not have piezoelectric properties and act as insulators.

2. The method of claim 1, wherein the polymeric material is heated by a heater block within a hot-end section of the print head to form a polymer melt.

3. The method of claim 2, wherein the acoustic energy causes an alignment and/or relaxation of polymeric chains within the polymer melt.

4. The method of claim 3, wherein the alignment and/or relaxation of the polymeric chains causes the structure to have piezoelectric properties.

5. The method of claim 4, comprising cooling the polymer melt after extrusion from the print head to fix the polymeric chains within the polymer melt, such that the alignment and/or relaxation of the polymeric chains is maintained by a hardening of the polymer melt during the cooling thereof.

6. The method of claim 1, wherein the polymeric material comprises poly[vinylidenefluoride-co-trifluoroethylene] (P(VDF-TrFE)).

7. The method of claim 1, wherein transmitting acoustic energy comprises selectively applying acoustic energy to the print head by changing the ultrasound generating device between an on state and an off state.

8. The method of claim 1, wherein each layer being dispensed has a shape corresponding to a portion of a cross-section of the structure being manufactured.

9. The method of claim 1, wherein vibrating the print head in the oscillatory manner comprises moving the print head in a direction orthogonal to a longitudinal axis of the print head and/or in a direction parallel to the longitudinal axis of the print head.

10. The method of claim 2, wherein the polymer melt is extruded from a nozzle of the print head, the nozzle being coaxial to a longitudinal axis of the print head.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1A and 1B are depictions of the α phase and of the β phase of PVDF, respectively.

(2) FIG. 2 depicts an example embodiment of an apparatus for ultrasound-assisted FFF of PVDF and ABS.

(3) FIGS. 3A and 3B are Fourier Transformed Infrared Spectrum (FTIR) spectra of Homo-PVDF and P(VDF-TrFE) printed in control conditions and in ultrasound-assisted FFF conditions, respectively.

(4) FIGS. 4A and 4B show an example embodiment for an electrostatic polymer device testing configuration and the results therefor, respectively.

(5) FIG. 5 shows an example electroactive polymer device design.

(6) FIG. 6 is a schematic illustration of an example embodiment of an ultrasound-assisted additive manufacturing apparatus.

(7) FIGS. 7A-7F show results of tests of example specimens conducted at several conditions

DETAILED DESCRIPTION

(8) The presently disclosed subject matter relates to methods that can produce significant amounts of crystalline beta phase in PVDF and its co-polymers such as P(VDF-TrFE) to allow functional electroactive properties in Fused Filament Fabrication (FFF) 3D printed components. This technique will enable new methods for 3D printing of piezoelectric/electroactive polymeric materials, including exploring particulate effects for composite-based filaments, directed towards producing superior mechanical and electroactive properties compared to conventionally FFF-3D printed counterparts. In some embodiments, appropriate applications can extend beyond corrosion-resistant components to high-performance soft sensors and actuators such as electronic skins, mechanical energy harvesters, and artificial muscles.

(9) FIG. 2 shows a first example embodiment of an apparatus, generally designated 100, of a modified open-source polymer 3D printer used to produce a 25 mm (L)×25 mm (W)×0.3 mm (D) specimen 20, which can also be referred to herein as a “structure,” which can include a single-layer HOMO-PVDF and/or Copolymer P(VDF-TrFE), using an FFF technique. In this embodiment, the apparatus 100 includes a print head, generally designated 120, in which a polymer feed material is fed into the print head 120, heated by a heater block 126 of a hot-end section 122 of the print head 120, and extruded over a substrate 10 through a nozzle 122 attached at an outlet of the print head 120 to form the specimen 20 on top of the substrate 10. In some embodiments, the hot-end section 122 is a melt chamber. In some embodiments, the substrate 10 can be an inert material to which the specimen 20 is not bonded during its formation. In other embodiments, the substrate 10 can be a previously deposited layer of a multi-layer specimen of a same or different material, such that the specimen 20 is bonded to the substrate 10 as the feed material is extruded from the nozzle 122 of the print head 120. An ultrasonic transducer 160 is attached, directly in the example embodiment shown, but indirectly in other embodiments, to the hot-end section 124 of the print head 120 to induce vibration energy into the feed material while it is being heated by the heater block 126 as it passes through the print head 120 to be extruded from the nozzle 122 to form the specimen 20. The print direction X is substantially aligned with the longitudinal axis of the transducer 160.

(10) In order to validate the subject matter disclosed herein, control specimens and specimens using transverse vibration mode ultrasound acoustic energy at 42 kHz were produced using identical print parameters on the same apparatus 100. Other parameters of production include a 0.3 mm layer thickness of the specimen 20, a print temperature of 220° C. for the feed material exiting the nozzle 122, a print speed of 200 mm/min, and an extrusion multiplier of 1.02.

(11) The characterization of PVDF against the presence of the β-phase can be performed using Fourier Transformed Infrared Spectrum (FTIR) to examine the absorption of the polymer at 839 cm.sup.−1 (CH.sub.2 rocking, skeletal C—C stretching, and CF.sub.2 stretching) as well as the peak at 1279 cm.sup.−1 (Trans band)) [12, 16, 52]. Shown in FIGS. 3A and 3B are two sets of initial experimental results where the FTIR spectra and 839 cm.sup.−1 and the 1279 cm.sup.−1 peaks were obtained, respectively, from 3 homo-PVDF and copolymer P(VDF-TrFE) specimens FFF-printed with the assistance of ultrasound energy and controls (e.g., no ultrasound). In FIG. 3A, the absorption for homopolymer without ultrasound is shown in solid line with squares and the absorption for homopolymer with ultrasound is shown in broken line with circles. In FIG. 3B, the absorption for co-polymer without ultrasound is shown in solid line with squares, the absorption for co-polymer-1 with ultrasound is shown in broken line with circles, and the absorption for copolymer 2 with ultrasound is shown in dot-dash-dot broken line with triangles. Qualitative evidence of an increased presence of β-phase was observed in both homo-PVDF and copolymer P(VDF-TrFE) produced using the ultrasound-assisted additive manufacturing apparatus 100 of FIG. 2 in a transverse mode (e.g., oscillations of the print head 120 are oriented orthogonal to the direction of extrusion of the feed material from the print head 120). This increased presence of β-phase is demonstrated by the prominence of both the 840 cm.sup.−1 and the 1279 cm.sup.−1 peaks, along with simultaneous suppression of the peak at 749 cm.sup.−1 in HOMO-PVDF.

(12) In a second example embodiment, specimen fabrication was accomplished in separate steps on a custom 3D printer system configured to allow polymer FFF 3D printing and Metal Directed Acoustic Energy Deposition (DAED) processes. The specimen was designed such that the d13 piezoelectric properties of the P(VDF-TrFE) could be measured and quantified. FIG. 4A shows a top view of the actual specimens tested. FIG. 4B shows testing results from the specimens in terms of measured charge in the metal features in the device when the device was compressed in the thickness direction to induce varying amounts of stress on the specimens. In FIG. 4B, the voltage response results for a PVDF-TrFE device with 1 mm compression are shown in solid line with squares and the voltage response results for a PVDF-TrFE device with 1.5 mm compression are shown in broken line with circles.

(13) FIG. 5 is a schematic illustration of the specimen design, testing configuration, and testing apparatus, generally designated 105. In this embodiment, the specimen 20 has metallic features 30, e.g., wires, tabs, or layers formed within the specimen 20 made from aluminum or copper, embedded within the specimen 20. The specimens 20 are made of a polymeric material, for example, HOMO-PVDF or Copolymer P(VDF-TrFE). The metallic features 30 are connected to a data acquisition system 50 configure to measure voltage induced in the specimen 20. A compression ram 40 is provided over the specimen 20 to compress the specimen 20 and generate a voltage that can be measured and recorded by the data acquisition system 50. To generate the data shown in FIG. 4B, the compression ram 40 is moved in the direction D to compress the specimen 20 by 1 mm and 1.5 m, respectively.

(14) FIG. 6 is a schematic illustration of an embodiment of a validated additive manufacturing apparatus, generally designated 101. In this example embodiment, a 40 kHz piezoelectric crystal-based transducer 160 was rigidly connected to a heater block 126 in the hot-end section 124 of the print head, generally designated 120, using a connecting rod 150, which can be made of any suitable metal, including, for example, 304 stainless-steel, and threaded fasteners (e.g., screws). A nozzle 122 through which the polymer material is to be extruded after being heated and vibrated within the hot-end section 124 of the print head 120 is connected at the outlet of the hot-end section 124. The connecting rod 150 couples vibrations generated by the transducer 160 to the print head 120, while reducing heat transfer from the heater block 126 within the print head 120 into the transducer 160. With the transducer 160 connected to the hot-end section 124 of the print head 120 by the connecting rod 150, the overall resonant frequency of the apparatus 101 was measured to be 34.4 kHz using a single point laser-Doppler vibrometer. A frequency-adjustable ultrasonic signal generator was adjusted to the resonant frequency of the apparatus 101 and connected to the transducer 160 to provide excitation signals that cause the nozzle 122 to vibrate at the measured resonant frequency.

(15) To prepare each specimen (e.g., “component,” “part,” or “structure”) printed with ultrasound assistance, the transducer 160 was activated prior to, or simultaneous with, deposition of the top layer 20 over the substrate 10. The print conditions and parameters in all specimens created using ultrasound assistance were identical to those in the control specimens, which were produced without the use of any ultrasound energy from the transducer 160.

(16) FIGS. 7A-7F show empirical results of example specimens conducted at several conditions in the testing apparatus 105 shown in FIG. 5. In FIGS. 7A-7F, it can be observed that the charge accumulation generated in the metal features 30 as a result of the piezoelectric response of the specimen, for example, P(VDF-TrFE) polymer, under compression was captured by the data acquisition system 50. It can be seen that the amount and rate of charge accumulation was dependent on the compression distance, as well as the rate of compression. At the highest tested compression rate of 500 mm/min at a compression distance of 1.5 mm, the charge (e.g., voltage) generated within the specimen 20 produced ˜100 mV of open circuit voltage in the metal features 30. At the lowest tested compression rate of 50 mm/min at a compression distance of 1 mm, the charge (e.g., voltage) generated within the specimen 20 produced ˜30 mV of open circuit voltage. It can clearly be seen that the rate of charge accumulation is dependent on the compression rate in the specimen 20. However, the charge dissipation rate from the metal features 30 showed little dependence and was observed to be somewhat invariant of testing parameters.

(17) In another embodiment of the subject matter disclosed herein, an additive manufacturing method of producing a polymeric structure so as to have selectable piezoelectric properties is provided. The method includes steps of: coupling an ultrasound generating device to a print head (e.g., 120, FIGS. 2, 6) of the additive manufacturing apparatus (e.g., 100, FIG. 2, or 101, FIG. 6); transmitting acoustic energy from the ultrasound generating device (e.g., transducer 160, FIGS. 2, 6) to the print head (e.g., containing a feed material 22, FIG. 6) to vibrate the print head in an oscillatory manner; extruding a feed material from the print head; moving the print head in at least one dimension relative to a substrate (e.g., 10, FIGS. 2, 6) on which the structure is being manufactured; and, optionally, dispensing layers sequentially on top of each other to form the structure. In some embodiments, the feed material is a polymeric material, the method further including heating the feed material by a heater block (e.g., 126, FIGS. 2, 6) within a hot-end section (e.g., 124, FIGS. 2, 6) of the print head to form a polymer melt. In some embodiments, the method includes extruding the polymer melt from a nozzle of the print head, the nozzle being coaxial to the longitudinal axis of the print head.

(18) According to the method, it is advantageous to couple the ultrasound generating device to the print head and transmit the acoustic energy to the print head because the acoustic energy causes an alignment and/or relaxation of polymeric chains within the polymer melt after it had been melted within the print head (e.g., by the heater block). One of the primary advantages associated with the use of the acoustic energy is the alignment and/or relaxation of the polymeric chains, which in turn causes the resultant structure to have piezoelectric properties. Once the polymeric chains are aligned and/or relaxed by the acoustic energy, the arrangement of the polymeric chains can be fixed within the structure by cooling the polymer melt after extrusion from the print head, such that the alignment and/or relaxation of the polymeric chains is maintained by a hardening of the polymer melt during the cooling thereof.

(19) In some advantageous embodiments, the polymeric material comprises polyvinylidenefluoride (PVDF) and/or a co-polymer thereof, including poly[vinylidenefluoride-co-trifluoroethylene] (P(VDF-TrFE)).

(20) The method further includes the ability to make only designated portions of the structure have the piezoelectric properties disclosed herein, such that certain predefined regions of the structure can have piezoelectric properties, while other predefined regions of the structure will not have piezo electric properties, instead acting as an insulator. The selective piezoelectric properties can be accomplished by selectively transmitting acoustic energy to the print head by changing the ultrasound generating device between an on state and an off state. As such, when the apparatus is producing a portion of the structure that is to have piezoelectric properties, the ultrasound generating device is activated (e.g., turned on), such that the polymeric chains within the polymer melt to be deposited to form the portion of the structure that is to have piezoelectric properties are aligned and/or relaxed. Then, when the apparatus is producing a different portion of the structure that is not specified as having piezoelectric properties, but is instead to act as an insulator, the ultrasound generating device is deactivated (e.g., turned off), such that the polymeric chains within the polymer melt to be deposited to form the portion of the structure that is to have piezoelectric properties are not aligned and/or relaxed, at least not to any degree beyond which such polymeric chains are naturally aligned and/or relaxed during the melting of such polymeric material in the absence of ultrasonic acoustic energy.

(21) Thus, in order to provide the aforementioned discrete regions of the structure with piezoelectric properties, the method includes the steps of: dispensing portions of the structure while the ultrasound generating device is in the on state so that the portions of the structure have piezoelectric properties; and dispensing other portions of the structure while the ultrasound generating device is in the off state so that the other portions of the structure do not have piezoelectric properties and act as insulators.

(22) In forming the structure, each dispensed layer has a shape corresponding to a portion of a cross-section of the structure being manufactured.

(23) According to the method, the acoustic energy transmitted to the print head advantageously has a frequency that is the same as, or is substantially similar to, the natural frequency of the apparatus, such that the step of vibrating the print head in the oscillatory manner includes moving the print head in a direction orthogonal to a longitudinal axis of the print head and/or in a direction parallel to the longitudinal axis of the print head.

REFERENCES

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(25) While the subject matter has been described herein with reference to specific aspects, features, and illustrative embodiments, it will be appreciated that the utility of the subject matter is not thus limited, but rather extends to and encompasses numerous other variations, modifications and alternative embodiments, as will suggest themselves to those of ordinary skill in the field of the present subject matter, based on the disclosure herein. For example, such barriers may be used as an enclosure for patios, driveways, driveway entrances, fences, docks, and the like.

(26) Various combinations and sub-combinations of the structures and features described herein are contemplated and will be apparent to a skilled person having knowledge of this disclosure. Any of the various features and elements as disclosed herein can be combined with one or more other disclosed features and elements unless indicated to the contrary herein. Correspondingly, the subject matter as hereinafter claimed is intended to be broadly construed and interpreted, as including all such variations, modifications and alternative embodiments, within its scope and including equivalents of the claims.