CRYSTAL GROWTH & DESIGN
Crystal Structure of Novel Polyamides with Long Diacid Segment: Polyamides 2 16, 4 16, 6 16, 8 16, 10 16, and 12 16
2003 VOL. 3, NO. 4 531-534
Weihua Li and Deyue Yan* College of Chemistry and Chemical Technology, Shanghai Jiaotong University, 800 Dongchuan Road, Shanghai, 200240, People’s Republic of China Received March 10, 2003
ABSTRACT: The morphologies of a series of novel polyamides with a long diacid segment were carefully studied using transmission electron microscopy (TEM), while their crystal structure was investigated using both TEM and wide-angle X-ray diffraction (WAXD). It was seen that the solution-grown crystals of all polyamides in this work were lath-like and crystallized as chain-folded, hydrogen-bonded sheet lamellae from 1,4-butanediol. The electron diffraction of these samples reveals that two crystalline phases are present including both Rp and βp in all polyamides under consideration except for polyamide 2 16, which only consists of one crystal form, Rp. In addition, sedimented mats of these polyamide crystals and stretched film samples were examined by WAXD. The unit cell parameters for these crystals were calculated thereinafter. Introduction Linear aliphatic polyamide, usually known as nylon, is a member of a polymer material family that plays an important role in engineering plastics. Since nylon 6 6 was first synthesized in 1934,1,2 many polyamides have been commercialized owing to their excellent physical properties. The unique crystal structure in polyamides provides them higher melting points as compared with some other kinds of semicrystalline polymers, such as polyethylene. There is a probable tendency that polyamide performs more like polyethylene if the density of amide groups (-NHCO-) along molecular chains becomes very low. As a result, studies on long alkane segment polyamides have attracted much attention in recent years.3-7 The prominent characteristic in polyamide crystals lies in hydrogen bonds originated from the amide groups between adjacent molecular chains. The requirement that all hydrogen bonds should be linear is a basic rule for polyamides to form the crystal structure. Accordingly, the arrangement pattern of hydrogen bonds is a function of the stereochemistry for a particular polyamide.8 For even-even nylons, the crystals are composed of chain-folded, hydrogen-bonded sheets, in which the chains may shear progressively and alternatively, termed p and a, respectively. The progressive shear is the preference for most even-even nylons except for the nylons of 2N 2(N + 1) family. The latter have an equal number of methylene units in both diacid segments and diamine segments,4,9 which results in two shear patterns of the chains. Similarly, there are also two ways to stack the hydrogen-bonded sheets, namely R phase for progressive shear and β phase for alternative shear. In consequence, four possible crystalline forms (i.e., Rp, βp, Ra, and βa) can be present for even-even nylons, which consist of a triclinic or monoclinic unit cell. As a matter of fact, Rp and βp are the two predominant crystal * Corresponding author. Fax: +86-21-54741297. E-mail: dyyan@ mail.sjtu.edu.cn.
phases in all polyamides, while Ra and βa have occasionally been found in the nylons of 2N 2(N + 1) family due to the reason mentioned above.4,9-11 Pseudohexagonal structure is another crystalline form for polyamides, which is called γ phase sometimes12 and normally observed at high temperature. Both R and β crystal patterns give two diffraction signals in wide-angle X-ray diffraction (WAXD) at spacing of 0.44 and 0.37 nm, which represent the projected interchain distance within hydrogen-bonded sheet and the intersheet distance, respectively. These two diffraction signals are commonly considered as characteristic for even-even nylons at room temperature since they have been indexed as a 100 and 010/ 110 pair for nylon 6 6 by Bunn and Garner.13 In contrast, pseudohexagonal structure is characterized by one strong reflection with spacing of 0.42 nm. In this paper, the chain-folded lamellar crystals of a series of novel polyamides with a rather long diacid segment were grown from dilute solution, and their morphologies and crystal structure were investigated by transmission electron microscopy (TEM) and WAXD measurement. Experimental Procedures Sample Preparation. Polyamides 2 16, 4 16, 6 16, 8 16, 10 16, and 12 16 were synthesized with the molecular weights between 5.5 × 103 and 2.0 × 104. Single crystals of these polymers were grown from the solutions in 1,4-butanediol (0.02% w/v) by seeding at a temperature (T1) and crystallizing at another temperature (T2) as shown in Table 1. The sediTable 1. Preparation Conditions of Single Crystals for the Polyamides polyamide
T1 (°C)
T2 (°C)
Tm (°C)
2 16 4 16 6 16 8 16 10 16 12 16
205 190 190 175 170 210
150 150 140 130 128 125
234 224 200 191 177 171
10.1021/cg034038+ CCC: $25.00 © 2003 American Chemical Society Published on Web 04/18/2003
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Figure 2. Examples of TEM electron diffraction patterns for polyamide lamellar crystals. (a) R structure of polyamide 10 16 with the electron beam parallel to the lamellar normal; (b) R structure of polyamide 10 16 with the electron beam parallel to the chain axis; (c) β structure of polyamide 6 16 with the electron beam directed parallel to the lamellar normal (see Supporting Information). Figure 1. Transmission electron micrographs of lamellar crystals for polyamides (a) 12 16; (b) 10 16; (c) 8 16; (d) 6 16; (e) 4 16; and (f) 2 16. The scales are the same, and the magnification is 1:8000. mented mats were prepared by hot filtering the crystal suspensions at the crystallization temperatures, followed by annealing in a vacuum oven at 100 °C for 24 h. The stretched films were obtained by processing the samples at 90 °C with a maximum draw-ratio of 3.6. The melting temperatures of the crystal mats measured by Perkin-Elmer Pyris-1 differential scanning calorimeter (DSC) are also listed in Table 1 as a comparison (see Supporting Information). Transmission Electron Microscopy. Lamellar crystals in our study were examined in both imaging and electron diffraction modes, using a JEOL-2010 TEM operating at 200 kV. The samples for TEM measurements were prepared by placing drops of the crystal suspension on copper grids coated with carbon and drying in a vacuum oven at 80 °C. Some crystals were shadowed with Platinum to enhance the contrast of the images, while others were lightly decorated with Aurum to calibrate the diffraction patterns. Wide-Angle X-ray Diffraction. WAXD patterns were obtained on D/max 2500V PC X-ray diffractometer at 40 kV and 250 mA. Silicon was used for calibration purposes. WAXD measurements were carried out with the λCuKR of 0.154 nm. Ni filter was used and imaging plate photographs were taken with the camera diameter of 75 mm.
Results and Discussion Microscopy and Electron Diffraction. The morphologies of polyamide lamellar crystals were observed by TEM imaging mode. It is shown that all polymer crystals grown from solution of 1,4-butanediol are lathlike and elongated (see Figure 1), which have some
Table 2. Tilt Angles of Polymer Chain from the Lamellar Normal for the Polyamides polyamide
2 16
4 16
6 16
8 16
10 16
12 16
tilt angle (deg)
30.2
42.7
40.2
42.1
40.8
42.5
similarity with the even-even nylons reported previously.8,9,11 The polyamide crystal lamellae in our work normally exist in the form of multilayers, which are several microns in length and several hundred nanometers in width. The thickness of the crystal can be calculated through the width of shadow, which is around 5-20 nm. The selected-area electron diffraction of the single crystals in this paper suggests that two distinct crystal structures are present in the samples. One is the typical Rp phase, which has been obtained with the electron beam directed parallel to the lamellar normal and characterized by a single pair of diffraction spots at spacing of 0.44 nm (see Figure 2a). This diffraction should be indexed as a 100 reflection according to Bunn and Garner’s13 triclinic crystal structure for nylon 6 6. Provided that the electron beam is parallel to the chain axis (c axis) of the Rp phase (i.e., when the lamellae are tilted 40.8° by rotating around (100) direction), all the 100, 010, and 110 diffractions can be observed (see Figure 2b). This kind of crystal structure has been reported for many even-even nylons, for example, nylons 4 4, 6 4, 8 4, 10 4, 12 4,9 and 12 10.14 Moreover, when the electron beam is parallel to the lamellar normal, the diffraction pattern reveals that another crystal structure is present in the sample. In this case, the diffraction signals observed (see Figure 2c) should
Crystal Structure of Novel Polyamides
Figure 3. WAXD photographs of sedimented mats and stretched sample for polyamides. (a) Sedimented mats of polyamide 4 16. (b) Drawn film of polyamide 8 16 (see Supporting Information).
be indexed as 100, 020, and 120, respectively (b axis effectively doubled), based on Bunn and Garner’s unit cell for the nylon 6 10 βp phase.13 This kind of crystal form has been observed previously in some even-even nylons, such as nylons 6 6, 8 6, 10 6, 10 8, 10 10, 12 6, 12 8, 12 10, and 12 12.10 Since the strength of the 100 reflection is too great as compared to the 020 and 120 diffraction signals, it could also be drawn that the Rp and βp phase coexist in the sample, while the Rp form accounts for the majority. Tilt angles of the polymer chain from lamellar normal for all nylons in the study are listed in Table 2. Except for polyamide 2 16, all values are in accordance with the angle of 41.8° reported for nylon 6 6 lamellar crystal13 and other even-even nylons. Polyamide 2 16 is special because of the relatively short methylene segment between amide groups. Wide-Angle X-ray Diffraction. A typical WAXD photograph of sedimented mats is given in Figure 3. As a contrast, the X-ray diffraction patterns of stretched film are also provided, which illustrate the corresponding 100, 010/110, 001, and 002 diffraction. The spacings in drawn samples are much closer to the sedimented mats, which supports the assumption that the crystal structure is still kept under the tension force. The diffraction patterns of all polyamides mentioned are displayed in Figure 4, from which the d spacings can be easily read. Table 3 indicates spacings from both electron diffraction and X-ray diffraction. It can be seen that these data are in agreement with each other. Additionally, the unit cells for all polyamides under consideration at room temperature can be determined according to the following method.4,8 For the R phase, the values of a and β are set at 0.49 nm and 77°, respectively, because the sheets are fixed by the hydrogen bonds in the nylon structure. The value of c is set at (0.125N - 0.02) nm, in which N represents the number of backbone bonds. This is consistent with an all-trans conformation for polyamide chains (0.02 nm is subtracted since the inclusion of one nitrogen atom in each backbone repeat). Then the b, R, and γ values could be calculated from the measured d100, d010, d110, and d001 spacings. The unit cell parameters for the β phase are obtained by setting
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Figure 4. Diffraction patterns of polyamides 2 16, 4 16, 6 16, 8 16, 10 16, and 12 16. Table 3. Comparison of Electron and X-Ray Diffraction Spacings for the Polyamides Electron Diffraction (nm)
X-ray Diffraction (nm)
polyamide
100
010/110
100
010/110
001
2 16 4 16 6 16 8 16 10 16 12 16
0.42 0.44 0.44 0.44 0.44 0.44
0.38 0.37 0.37 0.37 0.37 0.37
0.413 0.438 0.444 0.438 0.445 0.443
0.389 0.366 0.371 0.366 0.369 0.370
2.14 2.01 2.28
002
1.14 1.20 1.32 1.38
Table 4. Unit Cell Parameters for Polyamides 2 16, 4 16, 6 16, 8 16, 10 16, and 12 16 polyamide
2 16
4 16
6 16
8 16
10 16
12 16
R phase a ( 0.005 (nm) 0.490 0.490 0.490 0.490 0.490 0.490 b ( 0.005 (nm) 0.516 0.555 0.529 0.549 0.527 0.539 c ( 0.01 (nm) 2.48 2.73 2.98 3.23 3.48 3.73 R ( 0.5 (deg) 60.5 45.9 49.0 46.7 48.7 47.7 β ( 1 (deg) 77 77 77 77 77 77 γ ( 0.5 (deg) 58.0 61.0 63.5 61.0 64.5 65.0 a ( 0.005 (nm) b ( 0.005 (nm) c ( 0.01 (nm) R ( 1 (deg) β ( 1 (deg) γ ( 0.5 (deg)
β phase 0.490 0.490 0.490 0.490 0.490 0.797 0.777 0.799 0.792 0.798 2.73 2.98 3.23 3.48 3.73 90 90 90 90 90 77 77 77 77 77 66.0 66.5 66.0 66.0 66.0
R ) 90°, β ) 77°, a ) 0.49 nm, and c ) (0.125N - 0.02) nm. Then the values of b and γ could be calculated from the measured d100, d020, and d120 spacings. Furthermore, no evidence proves that a pseudohexagonal phase exists in our measurement. Table 4 displays the unit cell parameters for all polyamide crystals in our paper. It is demonstrated that chain-folded lamellar crystals grown from solution for polyamides 4 16, 6 16, 8 16, 10 16, and 12 16 consist of different ratios of the Rp and βp phases, with the Rp form in the majority. As for polyamide 2 16, the Rp structure is the only crystalline form found in a single crystal. Acknowledgment. This work is subsidized by the Special Funds for Major State Basic Research Projects of China (G1999064802). Supporting Information Available: DSC curve and photographs of single crystal polyamides. This material is available free of charge via the Internet at http://pubs.acs.org.
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CG034038+
