CRYSTAL GROWTH & DESIGN
Crystal Structures of Polyamides X 18 Made from Long Alkyl Dicarboxylic Acid Weihua Li and Deyue Yan* College of Chemistry and Chemical Technology, Shanghai Jiaotong UniVersity, 800 Dongchuan Road, Shanghai, 200240, People’s Republic of China
2006 VOL. 6, NO. 9 2182-2185
ReceiVed February 16, 2004; ReVised Manuscript ReceiVed June 1, 2005
ABSTRACT: Morphologies and crystal structures of a series of even-even polyamides X 18 have been investigated using transmission electron microscopy (TEM) and wide-angle X-ray diffraction (WAXD), where X varies from 2 to 12. It is shown that solutiongrown crystals of these polyamides are lath-like lamellae, which consist of chain-folded hydrogen-bonded sheet. Selected-area electron diffraction reveals that both Rp phase and βp phase present in these samples except for polyamide 2 18, while the latter only shows Rp crystal structure. Moreover, the unit cell parameters of all polyamides under consideration are obtained based on electron diffraction and WAXD results. Introduction Since nylon 6 6 was first synthesized in 1934, many polyamides have been commercialized during the past 70 years. Polyamides are widely used due to their excellent toughness, high modulus and strength, good abrasion resistance, and temperature resistance. It is the peculiar crystal structure that provides polyamides such unique comprehensive performances. Polyamides present higher melting points than polyethylene owing to the hydrogen bonds formed by amide groups between adjacent molecular chains. The density of hydrogen bonds plays such an important role in the physical properties of polyamides that many studies have been focused on novel polyamides with both short and long alkyl segments recently.1-5 Generally, stereochemistry determines the final crystal structure in polyamides because all hydrogen bonds should be linear or close to linear when forming crystal. It is known that all even-even polyamides form lamellar crystals at room temperature, which are composed of chain-folded hydrogen-bonded sheets. There are four possible patterns in the crystals of eveneven polyamides if both the arrangement of molecular chains and the combining of H-bonded sheets are considered, named Rp, βp, Ra, and βa. In R phase, H-bonded sheets stack progressively, while in β phase the sheets stack alternately. Similarly, a represents an alternating shear between hydrogen-bonded chains, while p describes a progressive shear. The progressive staggering is the preference for most even-even polyamides except for the 2N 2(N + 1) family. The latter has the same length between dicarboxylic acid segments and diamine segments,6-9 and accordingly all crystal phases may present. Both R phase and β phase give typical diffraction signals at 0.44 and 0.37 nm in wide-angle X-ray diffraction (WAXD),10 which correspond to the projected hydrogen-bonded interchain distance and the intersheet distance, respectively. In addition, pseudohexagonal structure has also been found in even-even polyamides. This crystal phase normally exists at high temperature and is characterized by a strong single peak at spacing of 0.42 nm in WAXD. In this paper, a series of even-even polyamides X 18 have been prepared based on 1,16-octadecane dicarboxylic acid, where X varies from 2 to 12. Their morphologies and crystal * Fax: +86-21-54741297. E-mail:
[email protected].
Table 1. Crystallization Temperatures and Melting Points of Polyamides X 18 polyamide
T1 (°C)
T2 (°C)
Tm (°C)
2 18 4 18 6 18 8 18 10 18 12 18
195 195 195 170 170 210
150 150 145 145 140 135
233 218 199 184 176 172
structures have been carefully studied using transmission electron microscopy (TEM) and WAXD. Experimental Procedures Sample Preparation. Polyamides 2 18, 4 18, 6 18,11 8 18, 10 18, and 12 18 were synthesized through polycondensation based on octadecane dicarboxylic acid and various diamines. Single crystals of these polyamides were grown from dilute solutions of 1,4-butanediol (0.02% w/v). The polymer solution was first seeded at a temperature T1 and then crystallized at another temperature T2 as shown in Table 1. The crystal suspensions were filtered at crystallization temperatures to obtain the sedimented mats, which were later annealed at 100 °C for 24 h. The stretched films were prepared by drawing the samples at 90 °C. Melting temperatures (Tm) of polyamides involved are also listed for comparison (measured by Perkin-Elmer Pyris-1 differential scanning calorimeter). Transmission Electron Microscopy. Both imaging and electron diffraction modes were used to study lamellar crystals of polyamides X 18 on a Hitachi 800 TEM at 175 kV. The samples for TEM measurement were prepared by dropping crystal suspensions on carboncoated copper grids to be dried in a vacuum oven at 80 °C. Wide-Angle X-ray Diffraction. WAXD measurements were carried out on a D/max 2500V PC X-ray diffractometer at 40 kV and 250 mA. Ni-filtered Cu KR radiation was adopted, and silicon was used for calibration purposes. The imaging plate photographs were taken with a camera diameter of 75 mm.
Results and Discussion Morphologies of Single Crystals of Polyamides. The morphologies of single crystals were observed with TEM imaging mode. It is seen that all polyamides in our study crystallize into elongated and lath-like lamellae with several micrometers in length and hundreds of nanometers in width (see Figure 1). The morphologies are similar to other even-even polyamides,7,9,11,15,16 which suggests that hydrogen-bonded sheets run along the length of the crystals.12,13
10.1021/cg049934f CCC: $33.50 © 2006 American Chemical Society Published on Web 08/08/2006
Crystal Structures of Polyamides X 18
Crystal Growth & Design, Vol. 6, No. 9, 2006 2183
Figure 1. Transmission electron micrographs of polyamide lamellar crystals: (a) 2 18; (b) 4 18; (c) 6 18; (d) 8 18; (e) 10 18; (f) 12 18. The magnification is 1:8000 in all photographs.
Selected-Area Electron Diffraction of Polyamides X 18. The crystal structures of polyamides X 18 were investigated using selected-area electron diffraction mode. The diffraction patterns obtained reveal that two different phases exist in these lamellar crystals. When the electron beam is directly parallel to the lamellar normal, a single pair of diffraction spots is observed at spacing of 0.44 nm, which represents the characteristic Rp phase of even-even nylons. Figure 2a is here given as an example for Rp phase of polyamide 2 18, in which the 100 reflection can be indexed based on the triclinic crystal structure of nylon 6 6 studied by Bunn and Garner.10 When the
lamellae are tilted a certain angle by rotating around the 100 direction, that is, when the electron beam is parallel to the chain axis, all 100, 010, and 110 reflections can be observed (Figure 2b). The tilt angles aforementioned for polyamides X 18 are all close to 41.8° for nylon 6 6 lamellar crystal,10 except for polyamide 2 18 (Table 2). The speciality of polyamide 2 18 arises from the short alkyl of the diamine segment, which introduces a perturbation into the all-trans chain conformation.14 Moreover, when the electron beam is parallel to the lamellar normal, the diffraction pattern suggests that βp phase also exists in the lamellar crystals (see Figure 2c). In this case, the
2184 Crystal Growth & Design, Vol. 6, No. 9, 2006
Li and Yan Table 2. Tilt Angles of Molecular Chain from Lamellar Normal for Polyamides X 18 polyamide
2 18
4 18
6 18
8 18
10 18
12 18
tilt angle (deg)
30.6
42.6
40.1
42.0
41.3
41.7
All polyamides in this paper present both Rp phase and βp phase in the crystal samples, except for polyamide 2 18. The peculiar structure observed in polyamide 2 18 will be discussed later. A further observation demonstrates that the intensity of 100 diffraction is much stronger compared with 020 and 120 reflections in TEM, which suggests that Rp phase accounts for the majority in the lamellar crystals. These results are in full agreement with the studies for even-even nylons14-16 4 4, 6 4, 8 4, 10 4, 12 4,7 12 10,13 6 6, 8 6, 10 6, 10 8, 10 10, 12 6, 12 8, 12 10, 12 12,8 12 16,15 and 12 22.16 Furthermore, no evidence of other crystal form has been detected in these samples. WAXD of Sedimented Mats and Stretched Samples. Two WAXD photographs are displayed in Figure 3 as examples for sedimented mats and stretched films. Because no orientation priority exists in crystal mats, homocentric diffraction circles are observed in WAXD photographs for sedimented samples (see Figure 3a). In contrast, in stretched films, 100 and 010/ 110 diffractions are seen along the equator, while the 001 diffraction is shown perpendicular to the tension direction (see Figure 3b). Moreover, no significant crystal transformation has occurred due to the tension since d spacings are almost the same in these two samples.
Figure 2. Electron diffraction patterns of polyamide lamellar crystals: (a) R phase of polyamide 2 18, obtained with the electron beam parallel to the lamellar normal; (b) R phase of polyamide 2 18, obtained with the electron beam parallel to the chain axis; (c) β phase of polyamide 10 18, obtained with the electron beam parallel to the lamellar normal.
diffractions should be indexed as 100, 020, and 120, respectively (b-axis should be doubled) according to the βp unit cell parameters for nylon 6 10 by Bunn and Garner.10
Figure 3. Examples of WAXD photographs for sedimented mats and stretched film: (a) sedimented mats of polyamide 4 18; (b) drawn film of polyamide 12 18.
Crystal Structures of Polyamides X 18
Crystal Growth & Design, Vol. 6, No. 9, 2006 2185 Table 4. Unit Cell Parameters for Polyamides 2 18, 4 18, 6 18, 8 18, 10 18, and 12 18 polyamide parameter a ( 0.005 (nm) b ( 0.005 (nm) c ( 0.01 (nm) R ( 0.5 (deg) β ( 1 (deg) γ ( 0.5 (deg) a ( 0.005 (nm) b ( 0.005 (nm) c ( 0.01 (nm) R ( 1 (deg) β ( 1 (deg) γ ( 0.5 (deg)
Figure 4. Diffraction patterns of polyamides X 18. Table 3. Spacings for the Polyamides X 18 Measured by Electron Diffraction and X-ray Diffraction electron diffraction (nm)
X-ray diffraction (nm)
polyamide
100
010/110
100
010/110
001
002
2 18 4 18 6 18 8 18 10 18 12 18
0.43 0.44 0.44 0.44 0.44 0.44
0.39 0.37 0.37 0.37 0.37 0.37
0.427 0.445 0.446 0.445 0.441 0.447
0.391 0.371 0.371 0.372 0.368 0.371
2.39 2.19 2.47 2.59 2.80 2.97
1.32 1.40 1.49
The diffraction patterns of all polyamides in our study are shown in Figure 4, from which the d spacings can be easily read. It is seen that two spacings of polyamide 2 18 are much closer than those of other polyamides. Atkins14 has attributed such an unusual structure to the diamine segments in the chains instead of the diacid segments since the dimethylenediamine segments are restricted in movement during crystallization. Therefore, polyamide 2 18 presents an intermediate structure between the normal Rp form and the pseudohexagonal phase, which is similar to other members of polyamide 2 Y family.14 Crystal Structures of Polyamides X 18. Spacings measured by both electron diffraction and X-ray diffraction are listed in Table 3. The determination of the unit cell parameters for polyamides is as follows.6,11,15,16 For R phase, since the sheets are fixed by hydrogen bonds, the values of a and β are set at 0.49 nm and 77° respectively. The value of c is set at (0.125N - 0.02) nm, in which N represents the number of backbone bonds. This is in accordance with an all-trans conformation of polyamide chains, where 0.02 nm is subtracted due to the inclusion of a nitrogen atom in each backbone repeat unit. Then the values of b, R, and γ can be calculated from the measured d100, d010, d110 and d001 spacings. As for β phase, by setting R ) 90°, β ) 77°, a ) 0.49 nm, and c ) (0.125N - 0.02) nm, the values of b and γ can be obtained from the measured d100, d020 and d120 spacings. The unit cell parameters obtained are displayed in Table 4 for lamellar crystals of polyamides X 18. It is concluded that
2 18
4 18
6 18
8 18
10 18
12 18
0.490 0.481 2.73 65.2 77 60.0
R Phase 0.490 0.490 0.484 0.521 2.98 3.23 52.3 49.6 77 77 63.5 62.0
0.490 0.508 3.48 51.7 77 65.0
0.490 0.540 3.73 47.5 77 62.0
0.490 0.534 3.98 47.9 77 65.0
β Phase 0.490 0.490 0.798 0.793 2.98 3.23 90 90 77 77 66.0 66.0
0.490 0.798 3.48 90 77 66.0
0.490 0.796 3.73 90 77 66.0
0.490 0.792 3.98 90 77 66.0
the crystals consist of chain-folded hydrogen-bonded sheets with the stacking of both progressive and alternate patterns for most polyamides in our study. In other words, both Rp structure and βp structure exist in the lamellar crystals grown from solution for polyamides 4 18, 6 18, 8 18, 10 18, and 12 18, with Rp phase in the majority. In contrast, polyamide 2 18 belongs to the 2 Y family and only presents Rp crystal structure. Acknowledgment. This work is subsidized by the National Basic Research Program of China (Grant 2005CB623800). References (1) Jones, N. A.; Atkins, E. D. T.; Hill, M. J. J. Polym. Sci., Polym. Phys. Ed. 1998, 36, 2401. (2) Li, W. H.; Yan, D. J. Appl. Polym. Sci. 2003, 10, 2462. (3) Yan, D.; Li, Y. J.; Zhu, X. Y. Macromol. Rapid Commun. 2000, 21, 1040. (4) Li, Y. J.; Yan, D.; Zhu, X. Y. Macromol. Rapid Commun. 2000, 21, 1282. (5) Li, W. H.; Zhang, G. S.; Huang, Y.; Yan, D.; Wang, J. K.; Zhou, E. L. Polym. Bull. 2003, 49, 387. (6) Jones, N. A.; Atkins, E. D. T.; Hill, M. J.; Cooper, S. J.; Franco, L. Macromolecules 1997, 30, 3569. (7) Jones, N. A.; Atkins, E. D. T.; Hill, M. J.; Cooper, S. J.; Franco, L. Macromolecules 1996, 29, 6011. (8) Jones, N. A.; Atkins, E. D. T.; Hill, M. J. J. Polym. Sci., Polym. Phys. Ed. 2000, 38, 1209. (9) Jones, N. A.; Cooper, S. J.; Atkins, E. D. T.; Hill, M. J.; Franco, L. J. Polym. Sci., Polym. Phys. Ed. 1997, 35, 675. (10) Bunn, C. W.; Garner, E. V. Proc. R. Soc. London 1947, 189A, 3297. (11) Jones, N. A.; Atkins, E. D. T.; Hill, M. J.; Cooper, S. J.; Franco, L. Polymer 1997, 38, 2689. (12) Bellinger, M. A.; Waddon, A. J.; Atkins, E. D. T.; Macknight, W. J. Macromolecules 1994, 27, 2130. (13) Franco, L.; Puiggali, J. J. Polym. Sci., Polym. Phys. Ed. 1995, 33, 2065. (14) Jones, N. A.; Atkins, E. D. T.; Hill, M. J. Macromolecules 2000, 33, 2642. (15) Li, W. H.; Yan, D. Y. Cryst. Growth Des. 2003, 3, 531. (16) Zhang, G. S.; Yan, D. Y. Cryst. Growth Des. 2004, 4, 383.
CG049934F
