Large electrostrictive strain at gigahertz frequencies in a polymer nanoactuator: Computational device design

Abstract

Using molecular dynamics with a first-principles-based force field ͑denoted MSXX͒, we show that large electrostrictive strains ͑ϳ5%͒ at extremely high frequencies ͑over ϳ10 9 Hz͒ can be achieved in a poly͑vinylidene-fluoride͒ nanoactuator if the packing density of the polymer chains is chosen appropriately. We control the packing density by assembling the polymer chains on a silicon ͗111͘ surface with one-half coverage. Under these conditions, the equilibrium, zero electric field conformation of the polymer contains a combination of gauche and trans bonds. This structure can be transformed to an all-T conformation by applying an external electric field. Such molecular transformation is accompanied by a large deformation in the direction of the polymer chains. The device shows typical electrostrictive behavior with strain proportional to the square of the polarization. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1862343͔Materials that interconvert mechanical and electrical energy have received significant attention due to their important technological applications as actuators, sensors and transducers in a wide range of devices, such as artificial muscles, pumps, and noise reduction systems. Desired design properties include: High strain, speed, and precision, as well as good energy density and conversion factor. Within this class of materials, electroactive polymers have recently attracted large interest due to a variety of unique properties, such as good acoustic matching with biological materials and water, light weight, ease of processability, and low cost. Electroactive properties of soft matter have been long known, starting with Galvani's frog's legs in the 1780's. More recently, the discovery of large electrostrictive strain in electron-irradiated poly ͑vinylidene fluoridetrifluoroethylene͒ ͓P͑VDF-TrFE͔͒ copolymer by Zhang and collaborators 1 showed the possibility of using electric-fieldinduced and reversible phase transitions between polar and nonpolar structures of ͓P͑VDF-TrFE͔͒ to achieve large strains ͑ϳ7%͒ at high frequencies ͑up to ϳ10 kHz͒ leading to very good energy densities, similar to those in piezoceramics. 2,3 In this letter, we use atomistic computer simulations to explore the possibility of improving the electromechanical properties of PVDF-based nanoactuators by controlling and optimizing their structure at the molecular level. Nanoscale actuators and motors play a central role in nanotechnology, and atomistic modeling is a powerful tool to design and test nanodevices in a fast and cost-effective manner, postponing issues of fabrication until a promising design is found.Four crystalline polymorphs of PVDF have been well characterized experimentally 4,5 and theoretically. 6 They are generally referred to as I, II, III, and IV, and differ by the conformation of their chains and their relative orientation. [4][5][6] Phase I is formed by all-trans ͑all-I͒ bonds with chain dipoles arranged in a parallel fashion leading to a polar crystal. Phases II and IV are formed by chains with TG...

Molecular scale simulations on thermoset polymers: A review

“…The use of molecular simulations to design polymers optimized at the molecular level has been a longstanding goal of the research community and progress is being made. Such efforts involve either searching for new chemistries or optimized molecular architectures …”

Dielectric relaxation of relaxor ferroelectric P(VDF-TrFE-CFE) terpolymer over broad frequency range

“…This can be elucidated using a defect-induced "quasi-debye" mechanism, where polar domains may be treated as debye dipoles [24]. as has been shown in a recent computer simulation, nanostructured molecular chains in PVdF can follow external electric fields at microwave frequency [25]. The response of nanostructured polar-polymers leads to a dielectric constant of P(VdF-TrFE-cFE) terpolymer higher than that of P(VdF-TrFE) copolymer, as observed in this investigation.…”

Mesophase Structure‐Enabled Electrostrictive Property in Nylon‐12‐Based Poly(ether‐block‐amide) Copolymers

“…Upon application of a high electrical field, the chain conformation transforms to more or less planar zigzag, resulting in nanoactuation of the crystal along the stretching direction. [38] Additionally, strong dipolar interactions among many high ε r (>50) crystallites further squeeze the soft amorphous matrix. As a result, these RFE polymers exhibit giant electrostriction (4-7% strain) with an elastic energy density ≈1 J cm −3 and an energy conversion efficiency k 2 > 40%.…”

“…This is different from stretched P(VDF-TrFE-X) (X = CFE or CTFE) terpolymer films, where a positive S 1 was observed. [47,50,53] Namely, upon the application of electric field in the film normal direction, the stretched terpolymer film expanded longitudinally because the chain conformation in the crystals transformed from twisted at a zero field to planar zigzag at a high field (i.e., nanoactuation [38] ). Based on our previous reports, [43,48] the negative S 1 for the QS P7033 could also be attributed to chain conformation changes in the mesomorphic nylon-12 crystals.…”

“…Without any electric field, polymer chains in the crystal adopt a more or less random trans (T) and gauche (G) conformation with nanosized ferroelectric domains. Upon application of a high electrical field, the chain conformation transforms to more or less planar zigzag, resulting in nanoactuation of the crystal along the stretching direction . Additionally, strong dipolar interactions among many high ε r (>50) crystallites further squeeze the soft amorphous matrix.…”