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Synthesis and crystallization behavior of nylon 12 14. II: chain-folded lamellar crystals and crystalline transformation behavior of nylon 12 14.

INTRODUCTION

Nylons are very important polymeric materials because of their special properties. Generally, most even-even nylons are polymerized by the method of polycondensation of diamine and diacid (1). The investigation of nylon lamellar crystals can provide insight into the relationship between the structure and properties of these polymers. Single crystals of a number of even-even nylons have been grown from dilute solutions, and their morphologies and their structures have also been studied (2--7). Hydrogen bonds between NH and CO groups in adjacent molecular chains play an important role in the crystal structure of nylons. The relatively strong hydrogen-bonds between polyamide chains result in a sheet structure of hydrogen-bonded molecular chains. These hydrogen-bonded sheets can stack together, either progressively displaced, termed [alpha]-phase by Bunn and Garner, or with an alternating shear, referred to as [beta]-phase (8). Each [alpha] and [beta] phase generates two characteristic diffraction spacings of 0.44 nm and 0.37 nm when measured with wide-angle X-ray diffraction (WAXD). The two reflections represent a projected inter-chain distance within a hydrogen-bonded sheet and the intersheet distance, respectively.

Some even-even nylons show a crystal-to-crystal transformation on heating, known as the Brill transformation (9--12). Two models were proposed to explain the transition process: one suggested (11, 12) that the two-dimensional hydrogen-bonded [alpha] or [beta] sheets present at room temperature change to a three-dimensional hydrogen-bonded network at high temperature; another (13, 14) states that the polymer keeps the [alpha] or [beta]-phase with hydrogen bonds arranged in sheets at high temperature, but the projection along the c-axis takes on a hexagonal symmetry.

Nylon 12 14 is a newly synthesized polymer in this laboratory (15). It is a member of the 2N 2(N+ 1) nylon family (16-20), which has the same number of meth-ylene groups in both the diacid and diamine alkane segments, so more chain arrangements are accessible to form sheets via linear hydrogen bonds. Meanwhile, nylon 12 14 has very long alkane methylene units between amide groups, which is long enough to re-entry fold without strain. Therefore, the crystal structure and the crystal transformation of nylon 12 14 have received much attention. In this work, the structure and morphology' of the chain-folded lamellar crystals of nylon 12 14 and the crystalline transformation behavior of melt-crystallized and dilute solution-crystallized nylon 12 14 are reported.

EXPERIMENTAL

Materials

The polyamide used in the present work was synthesized by melt polycondensation of an organic salt of 1,12-dodacanediamine and 1,12-dodacanedicarboxylic acid. The detailed polymerization method and the characterization data will be published elsewhere (15). The material used in this work shows an intrinsic viscosity of 0.84 dL/g, which corresponds to an average molecular weight of about 18,000 when the viscosimetric equation of nylon 6 6 is applied for calculation.

Sample Preparation

The melt-crystallized samples were obtained by the following procedure: A film of nylon 12 14 about 1 mm thick for the WAXD measurements was heated and pressed between two glass slides at 200[degrees]C, and then the sample was quenched to room temperature. More pressure was applied to obtain a film about 20 [micro]m thick for infrared (IR) measurements.

Single Crystal Preparation and Transmission Electronic Microscopy (TEM) Observation

A solution of nylon 12 14 in 1,4-butanediol (0.02% w/v) was prepared by the so-called "self-seeding" technique. Crystals were grown Isothermally at 125[degrees]C for 36 h after seeding at 175[degrees]C. A drop of this suspension was deposited onto carbon-coated copper grids and the solvent removed by evaporation in a vacuum at 100[degrees]C for 24 h. Then the sample was shadowed with platinum-carbon at an angle of 14[degrees]. A JEOL JEM-2010 Ex instrument operating at 200 KV for bright field and electron diffraction (ED) modes, respectively, was used throughout this work. ED patterns were recorded under a minimum flux of electrons in the selected area and were internally calibrated with gold. Crystal mats to be studied by WAXD were recovered by centrifugation and were washed repeatedly with n-butanol.

Wide-Angle X-ray Diffraction

The WAXD measurements were performed using a Rigaku III Dmax 2500 diffractometer with Cu radiation (35 KV, 25 mA). The equipment was fitted with a high temperature attachment. The sample was placed in a platinum block sample holder and heated at 5[degrees]C/min to the desired temperature and held there for 2 mm before commencing the data collection.

Variable-Temperature Infrared Spectroscopy

IR spectra were collected at a resolution of 4 [cm.sup.-1] using a Bruker Equinox-55 FT-IR spectrometer equipped with a temperature variable cell. The sample was heated from room temperature to 180[degrees]C at a heating rate of 5[degrees]C/min. Sets of 8 scans were used for signal averaging. The curve fitting procedure (Lorentzian method) was used to resolve selected IR bands and determine the area beneath the peaks.

RESULTS AND DISCUSSION

Chain-Folded Lamellar Crystals

Morphological details of these "self-seeded" crystals may be observed in the bright-field electron micro-graph shown in Fig. 1, which was taken from a Pt-C shadowed specimen. The crystals are "lath-shaped" with the long sides, slightly bent, and the tips showing clearly visible serrated edges. Their dimensions are approximately 0.5 [mu]m in width and about ten times greater in length. The thickness ranges between 6 and 7 nm, as estimated from their shadows on the electron micrographs.

Two kinds of ED patterns are observed when a different area was selected for diffraction as shown in Figs. 2 and 3. Only one pair of diffraction spot, with a spacing of 0.44 nm, is observed in Fig. 2a. The diffraction pattern is similar to that obtained from nylon 6 6 [alpha]-phase crystals. The diffraction signal can be indexed as the 100 on the Bunn and Garner nylon 6 6 [alpha]-phase crystallographic unit cell. Furthermore, from orientation of the patterns with respect to the lamellae it can be concluded that 100 hydrogen bond directions are parallel to the long axes of the crystals. The absence of other reflections in the electron diffraction patterns supports an [alpha]-phase structure, where molecular chains are tilted to the lamellar surface. If we tilt the lamellae along the [alpha] axis by 41[degrees], three pairs of diffraction signals can be observed as shown in Fig. 2b which can be indexed as 100, 010, 110, respectively.

The second single crystal electron diffraction pattern can be observed in Fig. 3. The diffraction pattern is similar to that obtained from nylon 6 10 [beta] phase crystals. This pattern shows a strong diffraction signal at 0.44 nm (indexed as 100) and diffraction signals at 0.37 nm, indexed as the combined 020 and 120 on the Dunn and Garner nylon 6 10 [beta]-phase crystal unit cell. For [beta] phase. the electron beam Is parallel to the molecular chain (c axis) because the degree of [alpha] was set at 90[degrees]. Therefore, three pairs of diffraction spots can be observed.

The WAXD pattern for a sample of nylon 12 14 mats displays two strong reflections corresponding to the d spacings of 0.44 and 0.37 nm (as shown in Fig. 4a). Similar spacings were also present in the powder diagrams recorded from samples crystallized from the melt (Fig. 4b). The WAXD patterns confirm the data obtained from the electron diffraction experiments. The crystal structure of nylon 12 14 is typical triclinic form with two strong reflections at room temperature, the characteristic projected inter-chain and inter-sheet diffraction signals at spacing 0.44 and 0.37 nm, respectively. In addition, another distinct diffraction peak at 2[theta] = 6.9[degrees] can be observed, which corresponds to 002 signal of [alpha] crystal phase.

As mentioned in the Introduction, the 2N 2(N+1) family of nylons is unique within the even-even nylon series, since the diamine and diacid alkane segments are of equal length, this allows these polyamides to form two different types of chain-folded hydrogen bonded sheet structures: p-sheets or a-sheets. The requirement that the hydrogen bonds between adjacent chains should be linear is a crucial feature in determining the crystal structure adopted by aliphatic polyamides. If nylons form the a-sheets, diamine alkane segments should be juxtaposed diacid alkane segments and intrasheet adjacent re-entry folding can only happen when amide groups are located at the apex of the folds; thus this part of amide groups will not be hydrogen-bonded. That is to say, if nylons undertake this a-sheet, only some of the amide groups are hydrogen-bonded. However in p-sheets, to form a linear hydrogen bond and undertake intrasheet adjacent re-entry folding in crystals, the fold can occur via an alkane segment, either diamine alk ane or diacid alkane, if the length of alkane segments is long enough for folding. In this chain arrangement patterns, all of the amide groups are hydrogen-bonded. Therefore, the p-sheet structures have lower energy as compared with the a-sheet structures if the alkane segments folding is not strained. Atkins et al. (18, 19) have shown that unstrained folds are not possible in alkane segment with less than four methylene units. Nylon 12 14 has twelve methylene units between the amide groups and no difficulties to fold via an alkane segment. Therefore, both [alpha] and [beta]-phase of nylon 12 14 undertake the p-sheet structure.

Based on the above mentioned analysis, the and [[alpha].sub.p] and [[beta].sub.p] crystal structures are used for nylon 12 14 crystallized at 125[degrees]C from the dilute 1,4-butanediol solution. The crystal unit cells of the two kinds of crystal structures are shown in Table 1. The unit cells were determined as follows: the a is set at 0.49 nm in accordance with the requirement that the hydrogen bonds should be linear within the sheets. The [beta] value is 77[degrees] as the chains progressively shear by 13[degrees] parallel to the chain axis within the hydrogen-bonded sheet. The [alpha] value was set to be 90[degrees] for [beta]-phase crystals because of the alternating stacking of the hydrogen-bonded sheets. The value of c was set at (0.125 N-0.02) nm, where N is the number of backbone bond consistent with an all-trans conformation for the nylon chains. 0.02 nm is subtracted because of the inclusion of one nitrogen atom in each backbone repeat. The rest of the unit cells could then be calculated from the measured [d.sub.100], [d.sub.010], [d.sub.110], [d.sub.020] and [d.sub.120] spacings.

Crystalline Transformation Behavior

The WAXD patterns for melt-crystallized and dilute solution-crystallized nylon 12 14 at room temperature are shown In Fig. 4. The melt-crystallized sample shows three strong reflections at 0.44 nm, 0.42 nm, and 0.37 mm, respectively. The strong reflections at spacing 0.44 and 0.37 nm are the characteristic reflections of [alpha] or [beta] phase, which represent the projected inter-chain distance within the hydrogen bonded sheet and the inter-sheet distance, respectively. Generally, the reflection at 0.42 nm signal can be considered as the characteristic of high temperature pseudohexagonal phase, normally found above the Brill temperature for even-even nylons. The diffraction patterns show that melt-crystallized nylon 12 14 contain some pseudohexagonal phase crystals at room temperature, which is much different from that of solution-crystallized samples.

The WAXD patterns taken as a function of temperature for melt-crystallized and solution-crystallized nyion 12 14 are shown in Fig. 5. On increasing temperature, the two strong reflections in the WAXD pattern of melt-crystallized nylon 12 14 are found to converge and then merge into the intermediate reflection at about 90[degrees]C, and it is the Brill temperature ([T.sub.B]) for melt-crystallized nylon 12 14. In other words, the [alpha] or [beta] crystals of nylon 12 14 transform into pseudohexagonal crystals at about 90[degrees]C. The [T.sub.B] is much lower than that of the melt-crystallized nylon 6 6, nylon 10 10, nylon 10 12, and nylon 12 12. The changes in spacings of the characteristic diffraction signals as a function of temperature for both melt-crystallized and solution-crystallized samples are shown in Fig. 6. The 100 spacing, [d.sub.(100)], i.e., the distance between 100 planes, which is mainly fixed by hydrogen bonds and hence less sensitive to temperature variation, shows a slight decrease durin g the transformation. On the contrary, [d.sub.(010/110)] (intersheet distance) shows a dramatic increase with temperature. Finally they converge into one peak at the Drill temperature for the melt-crystallized sample. The reflection of 0.42 nm shows that almost no changes can be observed before the transformation, which means that there is almost no change for pseudohexagonal crystals in the melt-crystallized nylon 12 14. Above [T.sub.B], d spacing slightly increases as a result of normal thermal expansion. The process of cooling the sample shows that the Drill transformation is totally reversible, but it has a lower temperature on cooling than on heating. On the other hand, though the change of the two characteristic diffraction signals for the solution-crystallized sample has the same trends with the melt-crystallized nylon 12 14, i.e., the two characteristic diffraction signals converge gradually on increasing temperature, no Brill temperature can be observed. In fact, most of the long alkane segments nylo ns crystallized from dilute solution have no [T.sub.B] (21) like nylon 1214.

Variable temperature IR can also be used to display the Brill temperature of nylons (14, 22, 23). Figure 7 shows the IR spectra taken as a function of temperature for the melt-crystallized nylon 12 14 in the frequency regions 1100-800 [cm.sup.-1] and 1350-1100 [cm.sup.-1], respectively. Band assignment can be performed by comparison with the spectra of nylon 66, nylon 6, nylon 10 12 and nylon 12 12. The bands at 941 and 1191 [cm.sup.-1] are associated with the crystalline phase, while the band at 1132 [cm.sup.-1] is associated with the amorphous phase. On heating, the crystalline bands become weaker and broader and disappear in the melt, while the amorphous bands remain essentially unchanged up to the melt. On the other hand, some of the weaker bands at 1062, 1083, 1119, 1202, 1246 and 1332 [cm.sup.-1] disappear abruptly between 80[degrees]C and 100[degrees]C. These bands give information of the Brill temperature for the melt-crystallized nylon 12 14 on heating. The integrated absorbances of the vibrational b ands at 1202, 941, and 1132 [cm.sup.-1] associated with Brill temperature, crystalline phase, and amorphous phase, respectively, shown In Fig. 8 also display the same change in the Brill transformation.

Now we can easily understand the existing of some pseudohexagonal phases preserved at room temperature for melt-crystallized nylon 12 14 (as shown in Fig. 4b). Nylon 12 14 transforms into the pseudohexagonal phase at high temperature, and a part of pseudohexagonal phases at high temperature has been frozen in on cooling and it is not easy to transform into the [alpha] or [beta] phases crystals because of the very low transition temperature. Therefore, a reflection at 0.42 nm can be observed for melt-crystallized nylon 12 14 at room temperature. In contrast, when crystallized from dilute solution, nylon 12 14 crystallize Into very ordered [alpha] and [beta] phase crystals and no reflection at 0.42 nm can be observed at room temperature.

CONCLUSION

Single crystals of nylon 12 14 have been crystallized from dilute solution in 1,4-butanediol. The crystals are composed of chain-folded hydrogen bond sheets, which stack, either progressively, or alternatively, to form [alpha] or [beta] crystalline phase, but the adjacent chains are sheared progressively for the two crystalline phases. That is to say, both [alpha] and [beta] crystal phases coexist at room temperature for nylon 12 14 under the present crystallization conditions.

In addition, the changes of the crystal structure as a function of temperature for melt-crystallized and solution-crystallized nylon 12 14 monitored by variable-temperature WAXD and variable-temperature IR show that the melt-crystallized sample undergoes Drill change at 80[degrees]C-90[degrees]C, but no Drill temperature can be observed before melting takes place for the dilute solution-crystallized nylon 12 14.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 7 OMITTED]
Table 1

The Crystal Structures of Nylon 12 14.

Crystal structure a (nm) b (nm) c (nm) [alpha] [beta]

[alpha]-phase 0.49 0.525 3.48 48.5[degrees] 77[degrees]
[deta]-phase 0.49 0.804 3.48 90[degrees] 77[degrees]

Crystal structure [gamma]

[alpha]-phase 64[degrees]
[deta]-phase 67.2[degrees]


ACKNOWLEDGMENTS

This work was sponsored by the Special Funds for Major State Basic Research Projects (G1999064802).

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Deyue Yan *

* To whom correspondence should be addressed. E-mail: dyyan(Talk of Florida Players)mail.sjtu.edu.cn
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Author:Zhang, Guosheng; Li, Yongjin; Yan, Deyue
Publication:Polymer Engineering and Science
Date:Feb 1, 2003
Words:3153
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