Ferroelectricity and piezoelectricity of nylon 11 films with different draw ratios

Abstract

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. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2737–2746, 1999