B. A. Newman

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.

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.

Effects of plasticizer on the mechanical and ferroelectric properties of uniaxially oriented β-phase PVF2

The ferroelectric, piezoelectric, and dynamic mechanical properties of uniaxially oriented films of poly(vinylidene fluoride) PVF 2 , undoped and doped with a plasticizer, tricresyl phosphate (TCP), were investigated. X-ray diffraction studies were carried out and show that for uniaxially oriented films, as dopant concentration increased, the percentage of nonpolar α-phase increased. These results also confirmed previous studies that indicate that the dopant resides in the amorphous regions. The piezoelectric coefficients (d 31 and e 31 ), pyroelectric coefficient (p y ), and remanent polarization (P r ) were observed to be enhanced by doping with TCP. Measurements of the pyroelectric coefficient, p y , showed that the dopant produced a large increase in p y from 19.6 to 28.5 μC/m2/K, suggesting an increase in the thermal expansion coefficient, α 3 . The data also suggest that a small amount of dopant in the noncrystalline regions greatly enhanced the high-temperature stability of the remanent polarization.

High pressure melting and crystallization of nylon-11

Differential thermal analysis (DTA), high pressure differential thermal analysis (HP-DTA), and high temperature X-ray studies are combined to elucidate the origin of the two melting peaks in Nylon-11. The results of the studies suggest that two species of crystals are involved in the melting of Nylon-11 for samples crystallized at atmospheric pressure or when the environmental pressure is below 4 kbar. At atmospheric pressure, the high melting species is predominant. However, under hydrostatic pressures, the high melting species undergoes phase transition to the low melting species before melting. The amount of the material involved in the transition depends on the pressure. At pressures of 4 kbar or greater, the entire high melting species transforms to the low melting species. The melting behaviour, at atmospheric pressure, of samples crystallized at high pressures also shows two melting peaks if the crystallization pressure is below 4 kbar. The amount of the low melting species increases with increasing pressure and, at 4 kbar or higher, only melting of the low melting species is observed. The X-ray photographs taken at room temperature suggest that samples crystallized between atmospheric pressure and 3 kbar contain both theα-form and theδ′-form crystals but the samples crystallized at 4 kbar and higher contain only theα-form crystal. However, it appears from X-ray scans taken at high temperatures near melting that the low melting species is of theδ-form and the high melting species of the δ′-form crystals for samples crystallized below 4 kbar. Theδ-form crystals result from theα-δ transition that occurs at 95° C. Moreover, the melting at high pressures (<4 kbar) of samples crystallized at atmospheric pressure also appears to involve aδ′-δ transition. These results suggest that both the crystal forms,δ andδ′, are stable at high temperatures, if the environmental pressure is below 4 kbar, and that only theδ-form crystals are stable up to melting at pressures greater than 4 kbar.

Ferroelectricity and piezoelectricity of nylon 11 films with different draw ratios

The first studies by Shuford et al. (Polym Eng Sci 1976, 16, 25) of the effect of draw ratio on the piezoelectric properties of poled poly(vinylidene fluoride) films concluded that the effect of molecular orientation due to draw prior to film polarization was to increase greatly the piezoelectric response. In fact, unoriented films (being crystallized in the nonpolar crystal form, phase II) showed negligible piezoelectricity. It was clear that the effect of increased draw ratio prior to film polarization was to increase the measured piezoelectric coefficients, because draw gave rise to a phase transition to a polar crystal phase, phase I, and molecular chain orientation in the plane of the film, subject to high electric poling fields perpendicular to the plane of the film, resulted in a preferred dipole orientation perpendicular to the plane of the film by a rotation about molecular stems. A detailed study of nylon 11 films and other odd nylons has shown that these polymers also can exhibit ferroelectricity, piezoelectricity, and pyroelectricity of comparable or greater magnitude than that exhibited by poly(vinylidene fluoride) with a greater thermal stability. For these polymers, the effect of draw on piezoelectricity and ferroelectricity is not complicated by the effect of crystal phase transitions. For this reason, a study of the effect of draw ratio on the ferroelectric and piezoelectric properties of nylon 11 was initiated in order to provide some insight into the direct effect of molecular orientation (without accompanying phase changes) on piezoelectric and ferroelectric properties. Some melt-quenched nylon 11 films were uniaxially drawn to draw ratios, ranging from 1.5 : 1 to 3.5 : 1 in increments of 0.5, at room temperature. Both undrawn and drawn films were found to exhibit an electric displacement vs. electric field hysteresis loop, characteristic of ferroelectric materials. As the draw ratio increased from 1 : 1 to 3.5 : 1, the remanent polarization increased from 25 to 53 mC/m2 and the coercive field decreased from 73 to 63 MV/m. The piezoelectric response and electromechanical coupling constant were also measured at temperatures of 25°C and 120°C under dry conditions. As the draw ratio of the films increased from 1 : 1 to 3.5 : 1, the piezoelectric strain coefficient d′31 measured at 25°C remained unchanged at ∼ 1.1 pC/N but the piezoelectric stress coefficient e′31 increased from 2.1 to 4.9 mC/m2. At 120°C, d′31 increased from 3.8 to 20 pC/N and e′31 from 0.5 to 27 mC/m2. d′31 showed a linear relationship with the remanent polarization Pr at both 25°C and 120°C for all draw ratios but e′31 showed this relationship only at 25°C. The electromechanical coupling constant k′31 increased from 0.42% to 0.61% at 25°C and from 0.42% to 1.56% at 120°C as the draw ratio increased to 3 : 1.