Even nylons

Fig. 3. Alignment of amide dipoles in polyamide crystals (a) for a two-dimensional array of an odd nylon, nylon-7, (b) for a one-dimensional array of an odd—odd nylon, nylon-5,7 (c) for one-dimensional arrays of polyamides containing even segments an even nylon, nylon-6 an even—even nylon, nylon-6,6 ...

A number of fiber/fabric materials including industrial felting,untreated fiber glass cloth and "ballistic nylon" have been used in attempts to produce a soft armor material for interior door panels, headliners, seatbacks, etc. Even nylon, the most successful, isn t worth a damn. None of these attempts at "soft armor" can be expected to stop anything more substantial than slow moving, fairly large pieces of shrapnel. This was made painfully clear to the Viet Nam era soldiers who mistakenly assumed that their ballistic nylon flak jackets were suitable body armor. 

Even and even-even nylons are polymorphic. Crystal habit listed is normal stable structure at room temperature. 

Nylon 6,6 is an example of an even-even nylon, sometimes just abbreviated to even . It is easy to envision other even-even polymers, such as nylon 4,4. Similarly, it is possible to think of odd-odd, or just odd , nylons, such as nylon 5,5. Although these nylons have very similar chemical properties they differ in important electrical aspects, and the nature of the chain determines whether these plastics can be used to make piezoelectric components (see Section 11.2.3). 

Figure 11.10 (a) The electric dipoles present in chains of an even nylon, nylon 6 no overall dipole moment is... 

The different crystalline forms of several polyamides have been examined by infrared spectroscopy. The bands at 936 and 1140cm have been used to measure the crystalline and amorphous contents, respectively, in nylon-6,6 [309], whereas bands at 1198 and 1181 cm were employed for measuring a and y crystalline content [310]. Fluctuations in crystallinity in nylon-6,6 were related to the ratios 1430/2910 and 930/2910 [311,312]. All even-even nylons show an a-crystal-related peak at 690 cm which appears at 725 cm in odd-odd nylons [313]. In nylon-6, there is a crystallinity band at 1260 cm a-crystalline-sensitive bands at 1028, 960, 950, 930, and 830 cm and 7-crystalline-related bands at 1120, 990, and 970 cm [314—316]. A band at 979 cm has been attributed to the amorphous phase [315]. 

Fig. 4. Schematic depiction of hydrogen-bonded sheets showing dipole directions in the crystal lattices of (a) even (nylon-4) and (b) odd polyamides (nylon-5).

Here, if x s 2, then the corresponding nylon will be identified as nylon-3 because it has three carbon atonu in a repeating unit. Similariy, if the number of x is 3, 4, 5. 6, 7, 8. 9, 10, or 11, then the corresponding polymers are nylon-4, nylon-S, nylon-6, nylon-7, nylon-8, nylon-9, nylon-10, nylon-11, and nylon-12, respectively. Nylons with an even number of carbon atoms arc called even nylons, whereas nylons having an odd number of carbon atoms in the repeating unit arc known as odd nylons. The typical synthetic route for a nylon can also be expressed as follows ... 

The reactioo between 1,9-diaminooooane and l,9< iaocyanaloiioiiane yields polyurea-9 as shown la Figure 24. Here 9 indicates the number of caiboo atoms between urea bonds. If tUs number is even, the pyroelectrk and piezoelectric activities do not ipear. because the alternative urea bond dipoles orient in the antiparallel direction and the dipole moment as a whole is cancelled out The situation is similar to that for even nylons. 

Figure 2.20 Minimum energy conformations of chains of nylon n (a) and nylon n,m in the crystals under the geometrical constraints imposed by the indicated line repetition gronps. The identity period c includes two constitutional repeating units for even nylons (a) and even-odd and odd-even nylons (b), and a single constitutional repeating unit for odd nylons (a) and even-even and odd-odd nylons (b).

Figure 2.21 Projections of sheets of hydrogen bonded chains in a plane parallel to chain axes of even nylons (a, b) and even-even nylons (c) in the a form with chains in a zigzag planar conformation [122] (a, c) and in the y form with chains in a twisted less extended conformation [122] (b). The cases of a and /forms of nylon 6 are shown in (a) and (b) respectively, whereas the case of the a form of nylon 66 is shown in (c). The unit cell is monoclinic for nylon 6 with a = 9.56 A, b (chain axis) = 17.24 A, c = 8.01 A, and p = 67.5° for the a form [123] and a = 9.33 A, b (chain axis) = 16.88 A, c = 4.78 nm, and P = 121° for the Y form [123] and triclinic for the a form of nylon 66 with a = 4.9 A, = 5.4 A, c (chain axis) = 17.2 A, a = 48.5°, and p = 77°, /= 63.5° [118]. The distance of chain axes in the sheets (a - c) is =4.8 A regardless of crystalhne polymorph (a or /forms) and the numbers n and/ or m of carbon atoms per constitutional unit in the chains of nylon n and nylon n,m. The directionality of the chains of nylon 6 in (a) and (b) is indicated by arrows pointing up or down according to the direction of N-C(=0) bonds along the chains. Adjacent chains are anticlined in (a) and isoclined in (b). (See color insert.)...

As shown in Figure 2.21a, in the case of nylon 6, even nylons with n less than 8 normally crystallize in the a form [122]. The chains are in a zigzag planar conformation,... 

Even-even nylons generally crystallize in the a form with chains in a zigzag planar conformation (Fig. 2.21c) [118,122,125], whereas odd-odd nylons do not crystallize in the a form because frans -planar chains are not able to form 100% bonds (Fig. 2.22b). Odd-odd nylons, instead, tend to crystallize in the /form with chains in twisted conformation forming pleated sheets and 100% hydrogen bonds (Fig. 2.22b ) [122]. 

Figure 2.21 Projections of sheets of hydrogen bonded chains in a plane parallel to chain axes of even nylons (a, b) and even-even nylons (c) in the ot form with chains in a zigzag planar conformation [122] (a, c) and in the 7 form with chains in a twisted less extended conformation [122] (b). (See text for full caption.)...

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