Odd-numbered nylons

Odd-numbered nylons, the next most widely investigated semicrystalline piezoelectric polymers, have excellent piezoelectric properties at elevated temperatures but have not yet been embraced in practical application. Other semicrystalline polymers including polyureas, liquid crystalline pol5oners, biopolymers, and an array of blends have been studied for their piezoelectric potential and are summarized in the following section. The chemical repeat imit and piezoelectric constants of several semicrystalline pol3uners are listed in Table 3. 

Kawai s (7) pioneering work almost thirty years ago in the area of piezoelectric polymers has led to the development of strong piezoelectric activity in polyvinylidene fluoride (PVDF) and its copolymers with trifluoroethylene and tetrafluoroethylene. These semicrystalline fluoropolymers represent the state of the art in piezoelectric polymers. Research on the morphology (2-5), piezoelectric and pyroelectric properties (6-70), and applications of polyvinylidene fluoride 11-14) are widespread in the literature. More recently Scheinbeim et al. have demonstrated piezoelectric activity in a series of semicrystalline, odd numbered nylons (75-77). When examined relative to their glass transition tenq>erature, these nylons exhibit good piezoelectric properties (dai = 17 pCTN for Nylon 7) but have not been used commercially primarily due to the serious problem of moisture uptake. In order to render them piezoelectric, semicrystalline polymers must have a noncentrosynunetric crystalline phase. In the case of PVDF and nylon, these polar crystals cannot be grown from the melt. The polymer must be mechanically oriented to induce noncentrosynunetric crystals which are subsequently polarized by an electric field. In such systems the amorphous phase supports the crystalline orientation and polarization is stable up to the Curie temperature. 

Piezoelectric and ferroelectric polymers have been recognized as a new class of electroactive materials when the significant piezoelectricity in polyvinyhdene fluoride (PVDF or PVF2) was discovered by Kawai (1969). Since then, a variety of new piezoelectric polymers have been developed including copolymers of vinylidene fluoride and trifluoroethylene, P(VF2-TrFE), odd-numbered nylons, composite polymers, etc. These materials offer options of material selections for sensor and actuator technologies that need lightweight electroactive materials. 

The alpha-phase crystal structure of nylon 11, suggested by Slichter (1959), is polar. The molecular conformation of the proposed structure is all-trans, and for an odd-numbered nylon, this entails a net dipole moment per chain. Figure 4 shows the molecular sfructures of odd-numbered nylons and even-numbered nylons (Lee 1991 Su et al. 1995). As can be seen, in odd-numbered nylons, the electric dipoles formed by amide groups (H-N-C=0) with a dipole moment of 3.7 Debyes are sequenced in a way that all the dipoles are in the same direction synergistically. Therefore, the net dipole moment can be formed in one direction. While in even-numbered nylons, the amide group dipoles are in two ways if one is in one direction, the next one will be in the opposite direction, alternately. This results in an intrinsic cancellation of the dipole moments, as demonstrated schematically in the figure. 

The remnant polarization of the ferroelectric odd-numbered nylons is a function of the number, which is decisive to the electric dipole density. The smaller the number, the higher the dipole density. This is because tire number represents the... 

Fig. 4 Schematics of molecular stmctures of odd-numbered nylons and even-numbered nylons (Adapted from J. Su)...

Table 3 Dipole density and remnant polarization of odd-numbered nylons...

Takase et al. also reported, in 1991, that die annealing Ireatment of die poled nylon 11 and nylon 7 demonstrated both enhanced piezoelectric sliain coefficient and improved piezoelectric stability. These make die odd-numbered nylons possess high and stable piezoelectric properties at high temperatures (up to 200 °C), which is... 

The existenee of the two classes of ferroelectric and piezoelectric polymers, PVDF and its eopolymers and odd-numbered nylons, makes it possible to develop polymer-polymer eomposite systems. Ferroelectric and piezoelectric PVDF-nylon 11 bilaminates and nylon 11-PVDF blends were studied by Su (1992) and Gao et al. (1999), respectively. Significant enhancements in ferroelectric and piezoelectric properties were observed in both polymer-polymer composite systems. Interfacial polarization plays an important role for the enhancement. 

The development of the two ferroelectric and piezoelectric polymers - PVF2 and odd-numbered nylons - provides a possibility to make all polymer ferroelectric and piezoelectric composite system. Using PVDF and nylon 11, Su et al. developed nylon ll-poly(vinylidene fluoride) bilaminates by a co-melt-pressed-stretched process in 1992 (Su et al.). The bilaminate exhibits a typical ferroelectric D-E hysteresis... 

Lee (1991) Ferroelectricity and piezoelectricity of odd-numbered nylons. PhD Thesis, Rutgers, The State University of New Jersey... 

Scheinbeim J et al (1991) Effect of water content on the piezoelectric technical report properties of nylon 11 and nylon 7. J Polym Sci Part B Polym Phys 24 1791 Scheinbeim J, Newman B (1993) Electric field-induced changes in odd-numbered nylons. Trip 1 394... 

TaMa 1 Chemical Strocture and Thennal Properties of Odd-Numbered Nylons (10-12)... 

Yetnani and Boyd [25] reported frequency-dependent dielectric constant and loss of oriented and unoriented nylon-77 and nylon-11 films at several temperatures. The normalized relaxation strengths in odd-numbered nylons were higher and approached a com-... 

Figure 5 shows the polar hydrogen-bonded sheet stmaures for odd-numbered nylons and even-numbered nylons. As can be seen from the molecular dipole alignment, in odd-numbered nylons there exists a net polarization in hydrogen-bonded sheets. In nylon-7, all the N—H- 0 KT hydrogen bonds point in the same direction, and this arrangement leads to a polar structure (Fig. 5a). In contrast, the N—hydrogen bonds point in the opposite direction to each other in nylon-6, which causes a cancellation oif the...

The spectroscopic analysis and various physical, dielectric, and electrical properties of odd-numbered nylons have been sumtxtarized. Odd-numbered nylons sb polymorphism. Nylon-11 shows at least live different crystal forms. The y form seems to be the most common polymorph in many odd-numbered nylons. The piezoelectric and ferroelectric behaviors fA odd-numbered nylons such as nylon-7 and nykm-11 are comparable or belter than for PVDF and depend on the crystal form. The desired stability of the piezoelectric response in odd-munbered nylons is related to the ferroelectric reorientation of the amide group dipoles followed by densely packed hydrogen-bonded sheets in the crystalline regions induced by poling and annealing. 

The author is grateful to Professor Shigeru Tasaka of Shizuoka University for generously providing information m advance of publication on fluorinated odd-numbered nylons. 

S. Ihsaka. M. Obtani, K. Maeda, and N. Inagaki. Electrical properties of odd number nylons. Polymer Preprints Japan 40 1227 (1991) (in Japanese ... 

The water absorption of nylons generally decreases as the length of the carbon chain increases. Thus we see from Table 7 that the higher nylons absorb less water than the lower nylons. Also, the even-numbered nylons absorb less water than the odd-numbered nylons. Absorption of moisture results in a slight decrease in tensile strength, a major increase in ultimate elongation, and a corresponding decrease in flexural modulus, as shown in Table 8. 

It is interesting to note that nylon-3 has a monoclinic unit cell which is unlike other ordinary odd-numbered nylons such as nylon-7, -9 or -11, which display a triclinic or pseudohexagonal unit cell. The amide concentration in the polymer might be a deteremining factor in the formation of a monoclinic, triclinic, or pseudohexagonal structure. 

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