J. I. Scheinbeim

High‐pressure crystallization of poly(vinylidene fluoride)

The pressure dependence of the melting and crystallization temperatures of poly(vinylidene fluoride) is determined by high‐pressure differential thermal analysis (DTA). These results are used in the investigation of the polymorphic crystal form obtained by pressure quenching molten poly(vinylidene fluoride): The resulting crystal form depends on both the initial and final pressures. The pressure‐quenching experiments were performed in a high‐pressure piston‐cylinder system and in a high‐pressure DTA system; a comparison is made of the results obtained by both methods.

Piezoelectric activity and field‐induced crystal structure transitions in poled poly(vinylidene fluoride) films

Unoriented and biaxially oriented phase‐II poly(vinylidene fluoride) films were poled at very high fields up to 4.0×106 V/cm at room temperature. Changes in molecular orientation and crystal structure induced by the high‐field poling were studied using x‐ray and DSC methods. The piezoelectric coefficients d31 and e31 together with the dielectric response (e′ and e″) were determined using a Piezotron Model U (Toyo Seiki, Tokyo) for the poled films. The piezoelectric activity did not increase steadily with poling field, but showed four distinct regions depending on poling field. At low fields (Ep<106 V/cm), when x‐ray and DSC data indicated that the films were predominantly in the phase‐II structure, the piezoelectric activity was small (e31<2×10−3 C/m2). For poling fields between 1×106 and 2×106 V/cm the piezoelectric activity was observed to rise rapidly with poling field until a value of e31∼14.5 C/m2 was observed. In this region the x‐ray scans indicated that phase‐II crystallites were being transformed...

Piezoelectricity in nylon 11

At the present time, only poled, drawn poly (vinylidene fluoride) (PVF2) films give evidence of sufficiently high piezoelectric response to be useful in device applications, and for this reason the great majority of research has centered around this polymer. As in the case of PVF2, many odd nylons crystallize in a polar space group with a large net dipole moment in the unit cell. On the basis of the understanding now reached of the properties of poled PVF2 films, it would appear that the odd nylons have the potential to make films with high piezoelectric activity. Piezoelectricity and pyroelectricity in nylon 11 films have been studied previously. The piezoelectric strain constants found were larger than most polymers, but still two orders of magnitude less than the corresponding activity found in poled oriented PVF2 (d31∼20 pC/N). Studies carried out in this laboratory have shown that by appropriate variation of poling conditions and sample microstructure, relatively large piezoelectric constants can be ...

Piezoelectric properties and ferroelectric hysteresis effects in uniaxially stretched nylon‐11 films

Oriented nylon‐11 films [initially quenched (δ′ form) or slow cooled (α′ form) from the melt] subjected to a series of static positive and negative poling fields were found to show a hysteresislike behavior of the piezoelectric strain constant d31 and the piezoelectric stress constant e31. This behavior was very similar to that observed in poly(vinylidene fluoride) (PVF2) and attributed to ferroelectric dipole switching, in a previous study. No change in the x‐ray diffraction patterns of the δ′‐form films was observed after poling. Changes in the x‐ray diffraction patterns of the α′‐form films after poling were consistent with dipole reorientation in the crystalline regions. In both cases the magnitude of the piezoelectric response of the oriented films was considerably higher than that of the unoriented films. The melting points of the poled films increased significant ly relative to that of the unpoled films with the same thermal history, which may suggest field‐induced annealing effects.

A study of piezoelectric and mechanical anisotropies of the human cornea

The piezoelectric and dynamic mechanical properties of human cornea have been investigated as a function of drying time. As expected, the piezoelectric coefficient, d(31), and the Youngs modulus, Y, were found to be extremely sensitive to water content. d(31) decreased with dehydration of the corneal tissue and Y increased with dehydration. While these results are significant, the discovery of the unprecedented mechanical and electromechanical anisotropy exhibited by the cornea are the major findings of this study and indicate that the collagen fibrils comprising the cornea are highly oriented. The piezoelectric responses of corneas observed in this study are: diagonally cut samples starting at an average piezoelectric coefficient value of 2250 pC/N, followed by the vertically cut samples, with an average starting value of about 600 pC/N and finally the horizontally cut samples with an average starting value of about 200 pC/N.

Giant electrostrictive response in poly(vinylidene fluoride-hexafluoropropylene) copolymers

Electrostrictive strains were measured in three different polymeric materials: a low modulus polyurethane elastomer, previously studied by Scheinbeim et al. (1994), and two higher modulus random copolymers of poly(vinylidene fluoride-hexafluoropropylene) [P(VDF-HFP)] with 5% and 15% HFP content. Measurements at increasing voltage (electric fields ranging from 0 to 60 MV/m) were taken using an air gap capacitance system and then converted to sample thickness. Copolymer samples with different thermal histories were compared, ice water quenched, air quenched, and slow cooled, for both compositions. The ice water-quenched 5% P(VDF-HFP) copolymer exhibited the highest strain response (>4%) with a dielectric constant of 13.9. The previously studied polyurethane elastomer exhibited the second highest strain response, >3%, with the lowest dielectric constant, 8.5. The ice water-quenched 15% HFP copolymer exhibited the lowest strain response among the three polymeric materials tested, /spl ap/3%, with a dielectric constant of 12.2. The strain energy density of the 5% HFP ice water-quenched copolymer, /sup 1///sub 2/ YS/sub m//sup 2/ (/sup 1///sub 2/ Youngs modulus, Y, times the maximum electrostrictive strain, S/sub max/ squared), is the largest known for any semi-crystalline polymer: 0.88J/cm/sup 3/.

The ferroelectric behavior of odd-numbered nylons

Abstract The combined results of the present investigation of Nylon 5 and Nylon 9 and the previous investigation of Nylon 11 and Nylon 7 reveal that the odd-numbered nylons exhibit classic ferroelectric J-E and D-E hysteresis behavior at room temperature. In addition, the remanent polarization, P r, of Nylon 5 and Nylon 9 is found to be 125-130 mC/m2 and 65-70 mC/m2, respectively. The remanent polarization of the odd nylon series is observed to increase linearly with increasing dipole density as does the melting point. The coercive field also shows a linear increase with dipole density. The present study also shows that both uniaxially oriented and unoriented odd-numbered nylons exhibit ferroelectric hysteresis behavior, and that the ferroelectric response can be significantly enhanced by uniaxial orientation.

Effect of Water Content on the Piezoelectric Properties of Nylon 11 and Nylon 7

A study of the effect of absorbed water on the piezoelectric properties of Nylon 11 and Nylon 7 films has been carried out. Films were prepared by slow cooling from the melt or by quenching rapidly into ice water. Poled films were soaked in distilledde-ionized water and the piezoelectric strain coefficientd31, piezoelectric stress coefficiente31, dielectric constantɛ, and elastic modulusc measured continuously as the films dried out in a stream of dry nitrogen gas. The results obtained show that the piezoelectric response of poled Nylon 11 and Nylon 7 films is sensitive to adsorbed water, the sensitivity being greater for Nylon 7 than for Nylon 11. These observations are discussed in terms of the effect of water on molecular relaxation of these polymers.

Ferroelectric properties of nylon 11 and poly(vinylidene fluoride) blends

Both poly(vinylidene fluoride) (PVF2) and nylon 11 are ferroelectric polymers, and have been extensively studied over the past two decades. Blend films were made from mixed powders of these two polymers, which were then melt pressed and cold drawn. The ferroelectric properties of these blend films were investigated. The remnant polarization, Pr, was found to vary with composition, and to be 60% larger than that of either component at a 50/50 (by weight) composition where Pr exhibited a maximum of about 90 mC/m2. The magnitude of the coercive field, Ec, also exhibited a maximum at this composition. Both Pr and Ec are also observed to change significantly with the draw ratio. The results are discussed based on a two-phase dielectric composite model.

Piezoelectricity in γ‐form Nylon 11

X‐ray studies of unpoled and poled γ‐form Nylon 11 films indicate that the polarization mechanism is not due to a field‐induced crystal transformation from the γ to the α form, suggesting the existence of a polar γ form. The temperature dependence of d31 shows no decrease in polarization at the α‐γ transition temperature (∼95 °C), indicating that the hydrogen bonds in the poled film are not able to randomize. These results suggest the possibility that our understanding of the γ‐form structure of Nylon 11 is incomplete.