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Investigation of odd-odd nylons based on undecanedioic acid. 2: crystal structures.

INTRODUCTION

Aliphatic polyamides, named nylons, present three basic crystal structures at room temperature according to the differences in the disposition of the amide groups and the methylene concentration in the repeat unit, based on the classification by Kinoshita in 1959 [1]. The first is the so-called [alpha]-form (triclinic unit cell), which consists of a stack of hydrogen-bonded sheets that progressively shear parallel to the plane of the sheet. The all-trans conformation in the methylene sequence is adopted in the intrasheet and the nature of the intersheet shear is mainly controlled by Van der Waals interactions. Therefore, the interchain hydrogen bonds are confined to the plane and displaced along the unique generalized direction. The linear hydrogen bonds in the intrasheet are all saturated. This kind of crystal structure was found in even-even, odd, and most of the even nylons, which gives a pair of strong diffractions at 0.44 nm in their electron diffraction (ED) patterns [2-4]. Another phase is the so-called [beta]-form (monoclinic unit cell), in which the staggering arrangement of the hydrogen-bonded sheets is the alternative shear instead of the progressive one. The 2N 2(N+1) family of polyamides exhibits this phase, which is characteristic of one pair of strong diffractions at 0.44 nm and two pairs of strong diffractions at 0.37 nm in their ED patterns [5, 6]. The third crystalline phase is the so-called [gamma]-form, corresponding to the near-hexagonal unit cell [7-10]. In this phase the chain repeat values were reduced by ~0.04 nm per amide unit from that of the extended conformation. Furthermore, the reduction of the chain values is independent of the length of the methylene segments, which indicates that these sequences arrange with the all-trans conformation [9]. Kinoshita reported that the [gamma]-form is the crystal structural characteristic of the even polyamides with a high aliphatic content and the polyamides containing the odd-number methylene units. As for the odd-odd, even-odd and odd-even nylons, the linear hydrogen bonds between the adjacent chains cannot be formed based on the extended conformation [1, 7]. In this model the amide groups are permitted to contort relative to the chain axis in order to constitute the saturated hydrogen bonds. The various interpretations of the [gamma]-form have been reported, which focuses on the arrangement of the intrasheet hydrogen bonds.

The odd-odd nylons are the attractive family because of their ferroelectric and piezoelectric activity [11, 12]. Recently, a series of novel odd-odd nylons with high aliphatic content were prepared by polycondensation of undecanedioic acid with various diamines and their crystal structures are necessary for study [13]. According to the report by Kinoshita [1], these nylons should be the hexagonal phase characteristic of the strong diffraction at 0.42 nm. However, nylons 5 6, 6 5, 9 2, 6 9, 3 5, 5 7, and 5 5 were studied again recently and monoclinic or orthogonal phases were found in their crystal structures instead of the hexagonal lattices [9, 14-18]. As a kind of novel polyamide with high aliphatic content, their crystal structures need to be measured. The goal of this work is to measure the specific crystal structures of the odd-odd nylons under study. Both solution-crystallized samples and melt-crystallized samples were taken into account.

EXPERIMENTAL

Material and Preparation

Nylons 3 11 (N3 11), 5 11 (N5 11), 7 11 (N7 11), 9 11 (N9 11), and 11 11 (N11 11) were synthesized by melt polycondensation of undecanedioic acid with various diamines. The molecular weights of the synthesized polyamides are between 4,900 and 22,000, determined by a Ubbelohde (Paragon Scientific, UK) viscometer [13].

The solution-crystallized samples of the novel odd-odd polyamides were obtained by crystallizing isothermally from dilute solution (0.05% (w/v)) in 1,4-butanediol. The polymers were dissolved completely at [T.sub.1], then cooled to [T.sub.2] to crystallize isothermally for 24 h. After that, the crystals were treated by two different methods: 1) The solution was hot-filtered at [T.sub.1] and cooled to room temperature, then the crystals were washed three times by ethanol to remove the remaining butanediol. The mats of crystals obtained were desiccated in a vacuum oven for the measurement of X-ray diffraction. 2) The chain-folded lamellar crystals for transmission electron microscopy (TEM) were stored in a butanediol suspension until use. Detailed information is listed in Table 1.

The melt-crystallized samples were prepared by pressing the melt polyamides at 220[degrees]C and cooling them to room temperature naturally. The stretched sample was obtained by drawing the melt-crystallized sample at room temperature with a ratio of 3:1.

Differential Scanning Calorimetry (DSC)

Melting temperatures of the polyamides were measured on a Perkin-Elmer (Norwalk, CT) Q10 V7.3 Build 249 differential scanning calorimeter at a heating rate of 10[degrees]C/min. The samples were heated until melt in order to remove the effect of the preparation condition. Then they were cooled to room temperature and heated to melt again at the rate of 10[degrees]C/min.

Transmission Electronic Microscopy

Samples for TEM were prepared by depositing drops of the lamellar crystal suspension in butanediol onto the carbon-coated, newly cleaved mica. After the solvent was evaporated, the resulting lamellar crystals supported by carbon film were floated off on the distilled water surface and transferred to the 400-mesh copper grids. Both imaging and electron diffraction observations were used to study the lamellar crystals of the novel even-odd polyamides using a JEOL (Japan) JEM-2010Ex electron microscope operated at 200 kV.

Variable-Temperature WAXD

Wide-angle X-ray diffraction (WAXD) measurements were carried out on a Rigaku (Tokyo, Japan) Dmax-rc X-ray diffractometer at 40 kV and 100 mA. Ni-filtered CuK[alpha] radiation was used. The imaging plate photographs were taken with a camera length of 75 mm.

RESULTS AND DISCUSSION

For the novel odd-odd nylons derived from undecanedioic acid, the crystal structures of both the solution-crystallized samples and the melt-crystallized samples were investigated by WAXD and ED.

Crystal Structures of the Solution-Crystallized Samples of the Polyamides Studied

For the solution-crystallized samples of the novel odd-odd nylons, two different kinds of crystal structures were found. Figure 1 shows the TEMs of nylons 11 11, 9 11, 7 11, and 3 11, which are typical morphologies of the odd-odd nylons under study. Nylon 11 11 crystallizes from 1, 4-butanediol as the elongated and multilayered lath-shaped crystalline lamellae (Fig. 1a), which are similar to those reported for many even-even nylons [2-6]. The dimensions of these lamellar crystals are several microns in length and several hundred nanometers in width. Nylon 9 11 gives the multilayered and irregular crystalline aggregate (see Fig. 1b). These complex aggregates are composed of near-hexagonal lamellar crystals. It was worth noting that the regular edges in these lamellae form angles close to 120[degrees]. The dimensions of these lamellar crystals are several hundreds of nanometers. Nylon 7 11 gives the multilayered and oval lamellar crystals seen in Fig. 1c. Nylon 5 11 showed similar crystals as nylon 9 11. As for nylon 3 11, the multilayered and irregular lamellar crystals are shown in Fig. 1d.

Two different types of selected area electron diffraction patterns were obtained from the room-temperature lamellar crystals of the prepared odd-odd nylons with the beam normal to the lamellar surface. Nylon 11 11 brings out the selected area ED pattern shown in Fig. 2, in which the strong diffractions at 0.42 nm (100) and 0.40nm (010) are observed. They are similar to the triclinic phases of the even-even nylons 2 Y [3-4, 19]. However, a pair of weak diffractions at 0.24 nm (2-10) were also found. These results are different from the triclinic forms found in the crystals of even-even nylons. Taking the hydrogen-bond structures of the odd-odd nylons into account, the monoclinic form structures may be the crystal structures. This is in good agreement with previous research on nylons containing the odd number C[H.sub.2] units [9-18]. The detailed spacings of the observed diffractions for nylon 11 11 are tabulated in Table 2. Nylons 9 11, 7 11, 5 11, and 3 11 display similar hexagonal patterns in Fig. 3. In Fig. 3a the typical ED pattern of the hexagonal lattice of nylon 9 11 was observed, in which three pairs of strong diffractions at 0.42 nm are present. In Fig. 3b, the electron diffraction pattern of two overlapped crystals rotating around each other by a few degrees was found. Nylon 5 11 shows the arced hexagonal patterns in Fig. 3c, indicating that the diffraction did not occur from an isolated crystal, but rather a group of crystals with a similar orientation. This is easily understood because of its irregular and small crystalline lamellae. The results of the ED patterns indicate that the crystal structures of the solution-crystallized samples of nylons 9 11, 7 11, 5 11, and 3 11 were hexagonal lattices. The detailed spacings from the ED results were calculated and are listed in Table 3.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Figure 4 presents the WAXD patterns of the solution-crystallized samples of the odd-odd nylons. Nylon 11 11 presents a different WAXD pattern from other odd-odd nylons under study. In Fig. 4a, two strong diffractions at 0.42 and 0.40 nm were observed for nylon 11 11, which can be indexed as 100 and 010 planes based on the monoclinic form. Nylons 9 11, 7 11, 5 11, and 3 11 show one strong diffraction at 0.42 nm in Fig. 4b, which is typical of the hexagonal one. The d-spacings calculated are consistent with the data of their ED patterns (see Tables 2, 3). From the results of WAXD and ED, the [gamma]-form structures (hexagonal phase) of the solution-crystallized samples of the odd-odd nylons suggested by Kinoshita [1] is proper for the novel odd-odd nylons. However, the solution-crystallized sample of nylon 11 11 shows the monoclinic lattice that gives the two strong diffractions at about 0.42 and 0.40 nm in the ED and WAXD patterns. The lattice parameters for the crystal structures of the solution-crystallized samples of the odd-odd nylons are calculated in Tables 2 and 4. From these results it can be seen that the angle [gamma] of nylon 11 11 is 119.5[degrees], which was close to the 120[degrees] adopted by other odd-odd nylons. The difference between the crystal structures of the novel odd-odd nylons lies in the chain axis c, which increases as the C[H.sub.2] units between the amide groups increase.

[FIGURE 3 OMITTED]

Crystal Structures of the Melt-Crystallized Samples of the Polyamides Studied

Besides of the solution-crystallized samples of the novel odd-odd nylons, the crystal structures of the melt-crystallized samples were also examined by WAXD (Fig. 5). The diffractions of the melt-crystallized samples for the odd-odd nylons show a strong diffraction at 0.42 nm in the WAXD patterns. The results indicate that the melt-crystallized samples of the novel odd-odd nylons adopt a hexagonal lattice. Figure 6 displays the typical 2D WAXD pattern of the odd-odd nylons under study. Only one pair of strong diffractions in the equatorial direction was observed at 0.42 nm, which is in good agreement with the results in Fig. 5. The d-spacings of the melt-crystallized samples of the novel odd-odd nylons are tabulated in Table 3. The calculated lattice parameters are listed in Table 4. It was found that the lattice parameters of the hexagonal form possessed by the melt-crystallized samples were almost the same except for the c axis. The value of the c axis increases with the length of the methylene sequence between the amide groups.

[FIGURE 4 OMITTED]

For the series of odd-odd nylons, nylon 11 11 exhibits two different crystal structures under the solution- and melt-crystallized conditions. In order to clarify the mechanism for this crystallization condition dependence, structures of the nylon 11 11 samples isothermally crystallized from melt at various temperatures were measured by the WAXD method. Figure 7 shows their corresponding WAXD patterns. A single strong diffraction was found in WAXD patterns for all the samples under study, indicating that nylon 11 11 crystallizes into the [gamma]-form from melt independent of crystallization temperature. For nylon 11 11, the monoclinic lattice obtained from the solution-crystallized samples is more compact that the [gamma]-form unit cell in the melt-crystallized samples. Under the solution-crystallized condition, the molecular chains have enough time to arrange themselves regularly and form a compact monoclinic lattice due to the very dilute solution. However, in the melt-crystallized state the macromolecular chains were entangled with each other and difficult to be arranged orderly. Therefore, the less-ordered [gamma]-form structure was obtained.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Structural Model Analysis

Generally speaking, odd-odd polyamides are assumed to be in the [gamma]-form at room temperature, characterized by one signal or two close diffractions at spacings in the range of 0.41-0.42 nm. This phase is observed for the melt-crystallized samples of the odd-odd nylons in our work. However, both the electron and X-ray diffraction patterns for the lamellar crystals of polyamide 11 11 present two strong diffractions at 0.42 and 0.40 nm (see Table 2, Figs. 2 and 4a). Nylon 11 11 cannot generate a fully saturated and optimum hydrogen-bonding scheme with all-trans conformation in the wholly planar polymeric backbone (Fig. 8a). Thus, the crystal structure of the lamellar crystals of nylon 11 11 is not only different from the [alpha] or [beta] phases found for the even-even polyamides, but is similar to the [gamma]-phase model [1-4].

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

The strong diffractions at 0.42 and 0.40 nm indicate that a different crystal structure existed in the solution-crystallized sample of nylon 11 11. Indexing the diffractions in the X-ray and electron diffraction patterns leads to a monoclinic unit cell with parameters: a = 0.482 nm, b = 0.456 nm, c = 5.72nm, and [gamma] = 119.5[degrees]. This is consistent with the crystal structures of nylons 5 5 and 13 13 reported previously [18, 20]. Since the odd-odd nylons cannot establish the saturated hydrogen bonds between the neighboring molecular chains with the all-trans conformation, another hydrogen bonding structure must be formed. According to the report by Kinoshita [1, 7], the NH and CO groups need to be tilted to form adequate hydrogen bonds. Figure 8b shows the scheme of the hydrogen-bonding structure built in the intrasheet adopted by nylon 11 11. In this scheme, the NH and CO groups must be rotated relative to the all-trans alkane segments. Diacid and diamine amide groups must be tilted in opposite directions in order to make perfect hydrogen bonds. However, the neighboring NH and CO groups can be tilted in the same or opposite directions, which leads to the two different hydrogen-bonding arrangements, including the alternative mode (left) or the progressive mode (right). In both cases the chain axis is orthogonal to the lamellar surface, indicating the coincidence of the c and c* axes. Further study is needed to clarify which is preferably adopted in the crystal structure of nylon 11 11. The hydrogen bonds were built up within the ac-plane with a length of 0.42 nm, indicating that the amide groups are tilted about 30[degrees] to match each other. The intersheet hydrogen bonds were excluded from the scheme of hydrogen-bonding structure because the lengths of the diagonal cannot match up with the hydrogen-bonding length. Therefore, hydrogen bonds constituted in the intersheet in the crystal structures of nylons 5 6, 6 5, 9 2, and 6 9 were not adopted in the case of nylon 11 11 [14-18].

For the melt-crystallized samples, the hexagonal form was chosen. In this phase, hydrogen bonds are still formed in the intrasheet, illustrated by taking the melt-crystallized sample of nylon 11 11 as an example. Comparing the monoclinic lattice with the hexagonal one of nylon 11 11, some difference was found including the parameters b and [gamma]. The intersheet distance b of the hexagonal lattice is larger than that of the monoclinic one. This increment of the intersheet distance makes the unit cell angle [gamma] change from 119.5[degrees] in the monoclinic lattice to 120[degrees] in the hexagonal unit cell. Other odd-odd nylons under study present a similar hydrogen-bonding structure as nylon 11 11. Summarizing the above discussion, the crystal structures of the novel odd-odd nylons were obtained. For the solution-crystallized samples, nylon 11 11 has a monoclinic lattice while other odd-odd nylons show a hexagonal unit cell. For the melt-crystallized samples, all the nylons present the hexagonal form.

CONCLUSION

In this work, the crystal structures of the novel odd-odd nylons were determined by various analysis technologies. From the results of TEM, it was found that nylon 11 11 crystallizes as elongated lamellar crystals, while other odd-odd nylons show multilayered lamellae. Through investigation of the crystal structures of the different samples of the novel odd-odd nylons, two different crystal structures were found. For the solution-crystallized samples, nylon 11 11 has a monoclinic crystal structure, while other odd-odd nylons under study show a hexagonal lattice. For the melt-crystallized ones, all the prepared odd-odd nylons have hexagonal unit cells. The hydrogen-bonding structure in the crystal of nylon 11 11 was analyzed. The neighboring molecular chains can form perfect hydrogen bonds by rotating the amide groups relative to the methylene segment. This mode of hydrogen bond can also be used to explain the hexagonal structure possessed by other odd-odd nylons.

REFERENCES

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11. J.W. Lee, Y. Takase, B.A. Newman, and J.I. Scheinbeim, J. Polym. Sci., Part B: Polym. Phys., 29, 279 (1991).

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Xiaowen Cui, Weihua Li, Deyue Yan

College of Chemistry and Chemical Technology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai, 200240, P.R. China

Correspondence to: D. Yan, e-mail: Dyyan@sjtu.edu.cn

Contract grant sponsor: National Natural Science Foundation of China; contract grant numbers: 20274024, 50233030.
TABLE 1. Conditions for preparing the single crystals of odd-odd nylons
under study.

 [T.sub.m]
Nylons [T.sub.1] ([degrees]C) [T.sub.2] ([degrees]C) ([degrees]C)

N11 11 170 110 182
N9 11 170 110 187
N7 11 180 120 198
N5 11 180 120 209
N3 11 180 120 194

TABLE 2. Diffraction spacings and lattice parameters of nylon 11 11.

Observed spacings (nm)
 WAXD
 Solution- Melt- Calculated
 crystallized crystallized spacings
Index ED sample sample (nm) Intensity

004 -- 1.43 1.39 1.43 (a) 1.39 (b) Strong
008 -- 0.716 -- 0.715 -- Medium
0014 -- 0.407 -- 0.408 -- Medium
100 0.42 0.420 0.420 0.419 0.420 Very strong
010 0.40 0.397 -- 0.397 -- Very strong
110 -- -- 0.241 -- 0.241 Weak
2-10 0.24 0.241 -- 0.241 -- Medium

(a) Calculated from the monoclinic lattice with parameters: a = 0.482
nm, b = 0.456 nm, c = 5.72 nm, [alpha] = [beta] = 90[degrees],
[gamma] = 119.5[degrees].
(b) Calculated from the hexagonal unit cell with parameters:
a = b = 0.484 nm, c = 5.56 nm, [alpha] = [beta] = 90[degrees],
[gamma] = 120[degrees].

TABLE 3. Diffraction spacings of nylons 9 11, 7 11, 5 11, and 3 11.

 Electronic
 diffraction
 (nm) X-ray diffraction (nm) (a)
Nylons [d.sub.100] [d.sub.110] [d.sub.100] [d.sub.110]

N9 11 0.42 0.42 0.419 (vs) 0.241 (w)
N7 11 0.42 0.42 0.420 (vs) 0.240 (w)
N5 11 0.42 0.42 0.420 (vs) 0.241 (w)
N3 11 0.42 0.42 0.420 (vs) 0.241 (b) (w)

 X-ray diffraction (nm) (a)
Nylons [d.sub.002] [d.sub.004]

N9 11 1.28 (s)
N7 11 1.19 (s)
N5 11 1.07 (s)
N3 11 1.85 (s) 0.927 (s)

(a) Intensities visually estimated and denoted as vs = very strong,
w = weak, s = strong.
(b) The diffractions are same for both the solution-crystallized samples
and the melt-crystallized samples.

TABLE 4. Lattice parameters of nylons 9 11, 7 11, 5 11, and 3 11. (a)

 [alpha] [beta] [gamma]
Nylons a (nm) b (nm) c (nm) ([degrees]) ([degrees]) ([degrees])

N9 11 0.484 0.484 5.12 90 90 120
N7 11 0.484 0.484 4.76 90 90 120
N5 11 0.484 0.484 4.28 90 90 120
N3 11 0.484 0.484 3.70 90 90 120

(a) The lattice parameters are calculated based on the hexagonal unit
cell.
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Author:Cui, Xiaowen; Li, Weihua; Yan, Deyue
Publication:Polymer Engineering and Science
Geographic Code:1USA
Date:Dec 1, 2005
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