高压下尼龙6的拉曼光谱研究

e-Polymers2011, no. 049
ISSN 1618-7229
A Raman study of nylon 6 annealed under high pressure Weijie Xu , Dahu Cao,*Qiang Yu
*School of Materials Science & Engineering, Changzhou University, Changzhou, China; fax: 86-0519-********; e-mail: caodh2003@.
(Received: 02 July, 2010; published: 17 May, 2011)
Abstract:Using a cubic-anvil high pressure apparatus, nylon 6 samples were
annealed under 400 MPa to 1.5 GPa. The structure changes of nylon 6 are
studied by the methods of wide-angle X-ray diffraction (WAXD), differential
scanning calorimetry (DSC) and Raman spectra. Compared with sample prepared
at ambient pressure, high pressure treatment makes the hydrogen-bonded sheets
pack more closely and thus enhanced CH2vibration out of the hydrogen-bonded
sheets. After high pressure treatment, it is observed that all changes of C-C related
vibration modes in nylon 6 are similar with ones in nylon series when the CH2
sequence number increased, from which it is deduced that high pressure treatment
reduces the effects of amide group on the chain conformation of nylon 6.
Introduction
High pressure treatment often causes drastic structure, morphology and physical property changes in polymers. Studies have shown that polyethylene would convert into hexagonal phase from orthorhombic structure at moderately high temperature and above 350MPa [1]. A different kind of polymer morphology, called extended-chain crystals [2-5], can be formed under high pressure instead of spherulites crystals obtained at ambient pressure.
So far, most of the high pressure researches on polymer treatment are focused on the preparation of different kinds of extended-chain crystals [6-8] and their formation mechanism [9-12] motivated by the excellent strength of extended-chain crystals predicted in theory. It is reasonable to deduce from researches on proteins [13],which also have long chain conformation, that high pressure should have more subtle effects on the polymers, such as structure changes of groups, conformation and intermolecular interaction, however, these effects were not studied extensively except for polyethylene [14,15].
Nylon 6, widely used as an important engineering plastic, like other polymers [6-8] forming extended-chain crystals under high pressure [16,17], has linear molecular structure composed of symmetrical molecular backbones, whose vibration bands often show strong peaks in Raman spectra [18-19], a powerful test method to analyze the changes of the polymer skeleton [20].
The first synthesis of nylon 6 extended-chain crystals and its FTIR spectra study were reported by Gogolewski et al. [21] in 1973. So far, the effects of pressure on the melting point and crystallinity in nylon 6 had been studied by a few researchers [16,17, 21], and there is no Raman spectra study on high pressure treated nylon 6.
The present paper describes in detail the microstructure changes of nylon 6 annealed under high pressure with Raman spectra and other methods. The results show that
pressure has prominent effects on intermolecular forces of nylon 6 as well as crystal structure.
Results and Discussion
Analysis of WAXD
In Fig. 1 the WAXD spectra of different nylon 6 samples are presented and there are two main peaks at about 20 degree and 24 degree, which is the typical feature of α phase with a monoclinic symmetry. After annealing, the background scattering and full width at half maximum (FWHM) decreases, which indicates that the size of crystals grown larger.
With the annealing pressure increasing, the intensity of (200) peak decreases, while (202+002) peak becomes weak progressively. The relative height of (202+002) peak is two times larger than that of (200) peak, similar to the results published by Lu et al.
[22] and attributed by Gogolewski [16] to the formation of thicker crystals.
2/ (degree)(202+002)
1618202224262830
untreated
400 MPa
600 MPa
800 MPa
1.5 GPa
(200)
Fig. 1. WAXD curves for nylon 6 annealed under different pressure.
Additionally, after high pressure annealing, (202+002) peak shifts to high angles, since α-type nylon 6 has a monoclinic symmetry, and (001) planes are made up of full-extended antiparallel chains which were connected by the hydrogen bonds (Fig.
2). The decrease of the distance between (001) planes means that the hydrogen bonded sheets are packed more closely and thus Van der Waals forces between these sheets become stronger compared with untreated samples.
Fig. 2. Hydrogen bonded sheets of nylon 6.
Analysis of DSC Tab. 1 compares the melting point and crystallinity of different samples. After annealing, the crystallinity of nylon 6 has increased dramatically. The melting point of all samples except one annealed under 1.5 GPa is increased somehow compared with source material, a sign that the lamellae thickness become lager in these samples with short time of annealing [17, 22] , which is in agreement with the result of WAXD. The relation between melting point and the lamellae thickness can be described by the Gibbs-Thomson equation:
l
h T T e o m m 21
where l is the thickness of a lamellar crystal, o m T denotes the equilibrium melting
point, e is the surface free energy and h is the heat of fusion of unit volume of
crystal. Assuming o m T =270 °C [23], e =65 erg/cm 2 [24], h =230 J/cm 3 [25], the
lamella thickness of sample with m T =225 °C is about 69 Å, which is very close to the thickness (70 Å) observed by SEM [16]. In Tab. 1, we can see that the lamellae thicknesses of the samples are much smaller than that of extended-chain crystals (0.2 m). This can be attributed to the relative short annealing time comparing to the Gogolewski’s studies (longer than 60 hours) [17].
The lamella thickness of sample annealed under 1.5 GPa remains unchanged, though its crystallinity increased greatly. In other words, there is no lamella thickening in it when pressure is too high, since most of the lamellae thickening of nylon 6 are realized by transamidation and the conformation adjustment which becomes more difficult under high pressure [26]. Although the lamella thickness of the1.5 GPa sample is not changed significantly, its change in Raman spectra is similar to those in the samples with thicker crystals, or even more obvious. Therefore it is inferred that
the Raman differences between original nylon 6 and high pressure treated ones can not be attributed to the increase of lamella thickness.
Tab. 1.Melting point and crystallinity of different nylon 6 samples.
Sample Melting point (°C)Crystallinity (%) Thickness of
lamella (Å) Untreated 222 27.0 65
400 MPa 227 63.5 70
600 MPa 231 65.7 79
800 MPa 227 63.6 70
1.5 GPa 222 6
2. 8 65
Analysis of Raman spectra
After high pressure treatment, in Raman spectra (Fig. 3), all of the peaks, including weak and shoulder peaks in source material, become sharper compared with the ambient pressure sample. Similar phenomenon is also observed in Raman spectra of extended-chain poly(vinyl chloride) [19].
1.5 GPa
800 MPa
600 MPa
400 MPa
untreated
110012001300140015001600260028003000
wavenumber/(cm-1)
Fig. 3. Raman spectra of nylon 6 treated by different conditions.
The reason is that high pressure annealing improves the crystallinity and reduces the entanglement of molecular chains of the samples. For example, in the Raman spectra
[27] of nylon 6 prepared at ambient pressure, with sample crystallinity a little higher than in present experiment, absorption peaks assigned to C-C stretching such as one at 1 109 cm−1 are quite weak or ambiguous. In order to investigate the high pressure effects on nylon 6, especially on its backbone, three regions of the Raman spectra are discussed in detail in the following analysis.
C-C stretching modes
Compared with untreated sample, the intensity of the Raman peak at 1 130 cm−1 which is assigned to the skeletal vibration, becomes the strongest in all the peaks, and its frequency shifts to larger wave number slightly, which implies a stronger C-C interaction after high pressure treatment. This is distinguished from all of Raman spectra reports of nylon 6 prepared at ambient pressure. In fact, all C-C stretching bands at 1 042 cm−1, 1 064 cm−1, 1 109 cm−1, 1 130 cm−1 become stronger or at least not weaker after high pressure treatment except the peak at 1 077 cm−1.
It is noted that the vibration band at 1 077 cm−1only belongs to the “fingerprint” of nylon 6 and was replaced by another peak at higher wavenumber as the CH2 sequence length increased in nylon 7 to nylon 12 and finally changed into 1 130 cm−1 in polyethylene according to Tab. 2 [28]. For example, the vibration at 1 077 cm−1 was replaced by a peak at 1 087 cm−1 in nylon 7, a peak at 1 107 cm−1 both in nylon 11 and nylon 12.
Tab. 2. Raman spectra compare of series nylons C-C stretch.
Assignment Nylon
3
Nylon
4
Nylon
6
Nylon
7
Nylon
8
Nylon
11
Nylon
12
PE
C-C stretch 1110 1067
1063
1076
1063 1063 1063 1063
1062
1087 1093 1107 1107
1123 1123 1123 1120 1120 1128
From the published results, the absorption at 1 107(1 109 in this study) cm−1 in nylon 6 is usually “very weak”[27],in fact, invisible in most of cases [29, 30],and only grows stronger and stronger with the increase of CH2 group numbers in nylon 7 to nylon 12. So the intensity increase of 1 107 cm−1 in high pressure treated sample is in contrast with most of published papers.
Ambient Raman studies in the nylon series, with the same kinds of changes observed at 1 077 cm−1, 1 109 cm−1, 1 130 cm−1 and other C-C vibration modes just like in this work, are interpreted as a continually deduced effect of amide groups on the conformation of the chains of nylons with the CH2sequence number increasing. Therefore it is reasonable to infer that high pressure treatment reduces the effects of amide group on the chain conformation of nylon 6, and thus increases the skeleton vibration and makes high pressure treated nylon 6 behaves more like nylons with longer CH2 sequence.
Amide III
The increase of Amide III band at 1 283 cm−1is partially caused by the increased crystallinity [27]. Besides that, a large part of contribution to this vibration comes from C-C bending according to the latest calculation by S. K. Shukla et al. [31] and less part is assigned to the C-N stretching and N-H bending modes. It explains the increase of Amide III band while the effects of amide group on C-C are reduced.
CH2
It is hard to present all the effects of the pressure treatment on the CH2 group from the Raman spectra and we limit our discussion to the band at 1310 cm−1 which shows a slight shift to a higher frequency. Since it is assigned to the CH2 twisting in and out of (001) planes, we tentatively attribute the reason of this frequency increase to the reduced distance between the (001) sheets. The Van der Waals interaction between (001) planes increased with its distance shorten, therefore CH2groups need to overcome a stronger repulsion to move in and out of (001) sheets.
Conclusions
Samples of nylon 6 are annealed under different high pressure from 0.4 to 1.5 GPa. High pressure treatment does not change the monoclinic symmetry of α-form, but reduces the distance between the hydrogen bond sheets, therefore increases the Van der Waals forces between them and thus enhances the CH2 out-of-plane vibration. High pressure also changes the conformation of the backbone slightly by reducing the effects of amide group on C-C stretching, and then increases the skeletal vibration.
Experimental Part
Materials
The raw material used in present study was nylon 6, 1013B produced by Japan's Ube Kosan Co., Ltd. Before the high pressure experiments, nylon 6 was purified by technique described by Gogolewski [17]. According to the infrared analysis, there was no caprolactam remaining in the samples.
Annealing under pressure
The purified material was melted, pressed into plate with 2 mm in thickness and cut into pellet with a diameter of 16 mm. The polymer pellet was wrapped with silver foil and put into the high pressure cell as described in Fig. 4.
Fig. 4. The diagram of sample assembly for high pressure annealing.
The annealing experiments were carried out by a cubic-anvil high pressure apparatus. The practical annealing pressure on samples was pre-calibrated using a Manganin
wire pressure sensor, and temperature was measured by a K-type thermocouple. During the treatment, the pressure was first increased to the required value, then the sample heated at a rate of 25 °C·m in−1 to the annealing temperature (270 °C). After 30 min, the sample was cooled to room temperature, and then pressure was released. Characterization of the samples
The effect of pressure on the annealing of nylon 6 was evaluated by several techniques such as wide-angle X-ray diffraction, differential scanning calorimetry, infrared and Raman spectra.
References
[1] Priest, R. G. Macromol. 1985, 18, 1504.
[2] Wunderlich, B. J. Polym. Sci. 1963, 1(Pt. A), 1245.
[3] Geil, P. H.; Anderson, F. R.; Wunderlich, B.; Arakawa, T. J. Polym. Sci., Pt. A: General Papers 1964, 2, 3707.
[4] Wunderlich, B.; Arakawa, T. J. Polym. Sci., Pt. A: General Papers1964, 2, 6397.
[5] Wunderlich, B.; Melillo, L. Makromolekulare Chemie1968, 118, 250.
[6] Matsushige, K.; Takemura, T. J. Polym. Sci., Polym. Phys. Edit.1978, 16, 921.
[7] Siegmann, A.; Harget, P. J. J. Polym. Sci., Polym. Phys. Edit.1980, 18, 2181.
[8] Parker, J. A.; Bassett, D. C.; Olley, R. H.; Jaaskelainen, P. Polym.1994, 35, 4140.
[9] Hikosaka, M. Polym.1987, 28, 1257.
[10] Hikosaka, M. Polym. 1990, 31, 458.
[11] Asahi, T.; Miyamoto, Y.; Miyaji, H.; Asai ,K. Polym.1982, 23, 773.
[12] Tseng, H. T.; Phillips, P. J. Macromol. 1985, 18, 1565.
[13]Wilfried, B. H.; Isaacs, N. S. High Pressure Techniques in Chemistry and Physics:
A Practical Approach. Oxford, New York, 1996, p. 353.
[14]Priest, R. G.Macromol. 1982, 15, 1357.
[15] Yamamoto, T.Polym.1984, 25, 178.
[16] Gogolewski, S.; Pennings, A. J. Polym. 1977, 18, 647.
[17] Gogolewski, S.; Pennings, A. J. Polym.1977, 18, 654.
[18] Snyder, R. G.; Scherer, J. R. J. Chem. Phys.1979, 71, 3221.
[19] Koenig, J. L.; Druesedow, D. J. Polym. Sci., Polym. Phys. Edit.1969, 7, 1075.
[20] Hendra, P. J. Fortschritte der Hochpolymeren-Forschung1969, 6, 151.
[21] Gogolewski, S.; Pennings, A. J. Polym.1973, 14, 463.
[22] Lu, Y.W.; Huang, R.; Xie, B. H. Chin. J. Hig. Pres. Phys.1998, 12, 228.
[23] Arakawa, T; Nagatoshi, F. J. Polym. ScL (B) 1970, 8, 41.
[24] Magill, J. H. Polym.1965, 6, 367.
[25] Arakawa, T.; Nagatoshi, F.; Arai, N. J. Polym. Sci. B, 1968, 6, 513.
[26] Gogolewski, S. Polym.1977, 18, 63.
[27] Schmidt, P.; Hendra, P. J. Spectrochimica Acta, Part A: Molecular and Biomolecular Spectra 1994, 50A, 1999.
[28] Hendra, P. J.; Maddams, W. F.; Royaud, I. A. M.; Willis, H. A.; Zichy, V. Spectrochimica Acta, Part A: Molecular and Biomolecular Spectra,1990, 46A, 747. [29] Hendra, P. J.; Watson, D. S.; Cudby, M. E. A.; Willis, H. A.; Holliday, P. J. Chem. Soc. 1970,17, 1048.
[30]Stuart B. H. Polym. Bul.1994, 33, 68.
[31] Shukla, S. K.; Kumar, N.; Mishra, A. K.; Tandon, P.; Gupta, V. D. Polym. J. (Tokyo, Japan)2007, 39, 359.。