Piezoelectric And Ferroelectric Behavior In Odd-odd Nylon 11,11

Piezo- and ferroelectric polymers have been widely used in sensors and actuators as they offer many advantages over piezoelectric ceramics, such as higher piezoelectric voltage constant, higher dielectric breakdown strength, and lower density.Piezoelectric polymers are readily manufactured into large areas, and can be cut and formed into complex shapes. They can offer the ability to pattern electrodes on the film surface, and pole only selected regions. However, the ferroelectric machnism is unclear for ferroeletric polymers. Moreover, traditional piezoelectric polymers have some disadvantages. For example, Polyvinylidenefluoride(PVDF) is unstable at high temperature; Poly(vinylidenefluoride-trifluoroethylene) [P(VDF-Tr FE)] is poizonous;other piezoelectric polymers have too much low piezoelectric response. This limits their applications in narrow areaes. So it is meaningful to investigate the new, cheap and green odd-odd nylon 11,11 from the practical and theoretical point of view. In this paper, we study the crystalline structure and crystal transition of long-alkyl-chain odd-odd nylon 11,11 which are prepared by our lab. We investigate the dielectric,piezoelectric and ferroelectric behaviors of nylon 11,11. And we also prepare Nylon11,11/PVDF blend films and discuss the ferro- and piezoelectric properties.The data of crystalline structure and unit cell parameters for nylon 11,11 are obtained by WAXD. Nylon 11,11 has two main crystal forms: the monoclinic α-form:a=0.485 nm, b=0.456 nm, c=5.70 nm, α=β=90°, γ=119.5°, the hexagonal γ-form:a=b=0.482 nm, c=5.61 nm,α=β=90°,γ=120°. The effects of drawing and annealing on the crystal transition are studied. The results indicate that there is a competition between the stretching inducement and the thermal inducement, that the former is beneficial to form the γ-form crystal, while the latter is favorable to producing theα-form crystal. It is the first time to find the Brill trasition undr stretching for odd-odd nylons, and the Brill transition stretching ratio n=1.5 form α to γ-form for nylon 11,11.The γ-form will transform into the α-form under high temperature annealing conditions, and the Brill temperature is about 145°C.By means of the dielectric relaxation spectroscopy(DRS) above room temperature, the dielectric behaviors of nylon 11,11 have been discussed. Theexperimental dielectric data are analyzed within the formalisms of dielectric permittivity(ε*), complex impedance(Z*), and complex electric modulus(M*). The results show that there are three relaxation processes above room temperature for nylon 11,11, which are α relaxation, MWS relaxation, and electrode polarization with the temperature increasing. The α relaxation corresponds to the chain segments motion of nylon 11,11; the MWS relaxation is related to charge carrier movement accumulated the interface between the amorphous and crystalline regions; and the electrode polarization corresponds to charge carrier movement accumulated the interface between the smple and the electrode. The charge carrier transport is governed by the motion of the nylon 11,11 chain segments above Tg.For the first time, we confirm the ferroelectricity in long-alkyl-chain odd-odd nylons. Ferroelectric films of nylon 11,11 are obtained by hot-pressing, stretching,evaporating electrode, and then polarizing. The remanent polarization, Pr, decreases with increasing test frequency, while the coercive electric field, Ec, increases. With the increase of maximum applied electric field, E0, both Pr and Ec increase. Ec is about 75MV/m, and Pr is about 47 m C/m2 at 0.1Hz under 150MV/m.The piezoelectricity in odd-odd nylons with long alkane segments is successfully detected for the first time. The analysis of poling conditions reveals that the piezoelectric strain constant, d33, increases rapidly in the first 20 min and then remains unchanged. And d33 of nylon 11,11 will increase as poling field and poling temperature increase. Under optimal polarization conditions, the d33 and g33 values as high as-3.9p C/N and-169 m V?m?N-1, respectively, are obtained.The structural change of nylon 11,11 samples before and after poling is investigated by WAXD, DSC, and FTIR. The ferroelectric mechanism is analyzed on the basis of the structural change and piezoelectric and ferroelectric measurements.The results indicate that the nature of the ferroelectricity originates from amide group dipoles in the γ-form crystal regions. Hysteresis behavior appears to result from the crystallites reversal mechanism.In the paper, the effects of stretching and annealing on the piezo- and ferroelectricity in nylon 11,11 are discussed. The remanent polarization, Pr, increases;the coercive electric field, Ec, decreases; and the piezoelectric strain constant, d33,remains almost unchanged with increasing stretching ratios. When the stretching ratio is 4, the coercive electric field, Ec, the remanent polarization, Pr, and the piezoelectric strain constant, d33, are about 74MV/m, 31 m C/m2 and-4.0p C/N, respectively. Below125°C, the coercive electric field, Ec, will increase, while the remanent polarization,Pr, and the piezoelectric strain constant, d33, decrease with increasing annealing temperature. When annealed above 165°C, non-ferroelectric α-form, bigger crystal grain size, and higher degree of crystallinity will be produced.Finally, Nylon11,11/PVDF blend films are fabricated in order to improve the piezoelectricity for nylon 11,11. The structures of blends films are analyzed based on measurements of SEM、WAXD、DSC、FTIR and DRS, and ferro- and piezoelectricity in blend films are discussed. The results show that there are dipolar interactions between nylon 11,11 and PVDF. Compared to PVDF, nylon 11,11 has comparable remament polarization(Pr≈39m C/m2), higher coercive electric field. The remament polarization increases after blending and a Pr value as high as 53 m C/m2 has been obtained in 50/50 blend film. The piezoelectric strain coefficient, d33, of Nylon11,11/PVDF blends will increase when the content of PVDF increases. It is a effective way to improve the piezoelectricity for nylon 11,11 by blending with PVDF.