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Dynamic two-dimensional infrared spectroscopy. Part II: Morphological assignments in melt-crystallized nylon 11 spectra
VIBRATIONAL SPECTROSCOPY ELSEVIER
Vibrational Spectroscopy 13 (1996) 75-82
Dynamic two-dimensional infrared spectroscopy. Part II: Morphological assignments in melt-crystallized nylon 11 spectra Anu Singhal, Leslie J. Fina * Department of Chemical and Biochemical Engineering, College of Engineering, Rutgers University, Busch Campus. P.O. Box 909. Piscataway, NJ 08855, USA
Received 6 December 1995; accepted6 May 1996
Abstract
Melt-crystallized and deuterated nylon 11 are studied using dynamic two-dimensional FTIR spectroscopy in the region below 1500 cm I of the infrared spectrum. The splittings observed in the methylene rocking-amide V (760-640 cm- 1) and methylene bending (1500-1400 cm - I ) regions are explained on the basis of morphological features. These morphological assignments are corroborated by correlating methylene rocking-amide V and methylene bending regions with the morphological assignments of amide I-amide II (1680-1500 cm-1) region of nylon 11 established in an earlier work. Keywords: Nylon 11; 2D-IR spectroscopy;Morphology
1. Introduction
In a preceding paper [1], two-dimensional infrared spectroscopy was used to study in detail the N - H stretching and amide I - a m i d e II regions of meltcrystallized nylon 11. 2D-IR correlation spectroscopy confirmed the splitting of the amide I region on the basis of ordered and disordered components and on different strengths of hydrogen bonding between the amide groups. The amide II region also splits into ordered and disordered peaks. With a cross-correlation of the N - H stretching and amide I-amide II regions, it was found that the prominent dynamic peak observed in the N - H stretching region is similar in morphological character to the ordered component (1636 cm -1) of the amide I band. The Corresponding author.
goal of this study is to analyze in detail the region below 1500 cm -1 in melt-crystallized nylon 1 1 and to correlate this with the established assignments of the amide I-amide II region using 2D-IR spectroscopy. Results are presented for the CH 2 rocking-amide V (760-640 cm -I ) and CH 2 bending (1500-1400 cm -1) regions. A detailed study of the methylene bending region of the infrared spectrum of different nylons was presented by Matsubara and Magill [2]. This region shows a sensitivity to the crystal phase in all diaciddiamine and w-amino acid nylons. Four peaks are observed in the a-phase nylon 11 at 1475, 1465, 1440 and 1420 c m - 1 and two peaks in the y-phase at 1460 and 1440 cm -1. In amorphous nylon 11 the sharp crystalline methylene bending bands are replaced by two overlapping bands at 1460 and 1430 c m - 1. In addition to crystal phase assignments, some
0924-2031/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0924-2031(96)00031-8
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A. Singhal, L.J. Fina / Vibrational Spectroscopy 13 (1996) 75-82
of the peaks in this region have also been assigned to methylene groups vicinal to the amide groups. Heidemann and Zahn [3] assigned the 1475 cm -~ and 1416 cm -~ bands to the bending vibrations of the N-vicinal and the CO-vicinal CH 2 groups, respectively. They concluded that the bending vibrations of the N-vicinal and the CO-vicinal CH 2 groups coincide at 1435 cm -~ upon rotation of the - C H 2C O N H - C H 2- group out of a t r a n s planar conformation. The 760-640 c m - ~ region of the infrared spectra of nylon 11 contains the CH 2 rocking fundamental peak at about 721 cm-~ and the amide V absorption at 690 cm -~. Like the methylene bending region, this region is phase sensitive [4-6]. When the a or /3 structure predominates, the amide V band is located at 690 c m - l and when the y structure predominates, the amide V and VI peaks are at 715 and 630 cm -~, respectively. The peaks at 721 cm -~ and 690 cm-~ originate from the crystalline regions. The amide V band of the amorphous region exists below 680 cm -1 because in molten nylon the bands at 730-680 c m - 1 diminish and the position of the peak shifts to a lower frequency. These assignments are explored in this work.
2. Experimental The nylon 11 powder used in this study is provided by the Rilsan Corporation (Glen Rock, NJ). Melt-crystallized films are prepared by melting the powder between aluminum foil sheets in a hot press at 210°C for 5 min followed by slow cooling to room temperature. Previous work has shown that this sample preparation treatment does not lead to polymer chain orientation [10]. Deuterated samples are prepared by annealing the melt-crystallized nylon 11 films at 150°C with deuterium oxide (D20) in an acid digestion bomb (Parr Instruments) under 1800 psi pressure for 4 h and then cooling to room temperature. Before scanning, each sample is sanded to remove the interference fringe effects. A BioRad FTS-60A FTIR spectrometer is used for all measurements. The nylon 11 film used for scanning is approximately 2.5 cm long and 1.5 cm wide. A phase modulation frequency of 400 Hz with an amplitude of 2AlaeNe (1.26 /zm) is used. In these
experiments, a 25 mm diameter sapphire low-pass filter with an undersampling ratio of 4 (2 AHeNedata spacings) is used to scan the 1800-400 cm-1 region of the nylon 11 film. A room temperature triglycine sulphate (TGS) photoconductive detector is used to detect the infrared beam. After detection, the signal is first demodulated by the first of two lock-ins (PAR, model 5209) referenced to the phase modulation frequency. The output of this lock-in produces the reference interferogram. It is equivalent to the 'normal' infrared transmission spectrum and used to normalize the dynamic spectra produced from the second lock-in amplifier (PAR, model 5210). The dynamic spectra thus obtained show only changes in the infrared absorption which are associated with the reorientation of the transition dipoles in response to the mechanical perturbation. The spectral resolution is set at 16 cm l in order to be compatible with past work [1]. A scan rate of 0.25 Hz is used, and a total of 16,000 digital samples are collected from each of the three output channels: lock-in l, lock-in 2 (inphase) and lock-in 2 (quadrature) outputs. Twelve scans are coadded to reduce the noise in the spectra. Each sample is scanned in four separate sets and the reproducible features are reported here. In order to determine the possible influence of sample thickness changes on the dynamic and two-dimensional spectra, nylon 11 is also scanned in a triple modulationdemodulation experiment where polarization modulation was added in the optical train. Polarization modulation tends to minimize the observation of sample thickness changes. The results reported herein are based on observations repeatable in both double and triple modulation experiments.
3. Results Fig. 1 shows the 1500-500 cm -1 region of the normal absorbance spectrum, and the in-phase and quadrature dynamic spectra of a melt-crystallized nylon 11 film. From the dynamic spectra it can be observed that a very small response is observed in the quadrature spectrum relative to the in-phase spectrum, which establishes that the polymer film in this region is responding mostly in-phase to the applied perturbation.
A. Singhal, L.J. Fina / Vibrational Spectroscopy 13 (1996) 75-82
77
3.1. CH 2 rocking-amide V region In the 760-640 cm -1 range of the normal absorbance spectrum (Fig. 1), two peaks are observed at 721 and 675 cm -1. The peak 721 cm -1 has been assigned to CH 2 rocking vibrations and the band at 675 cm-1 to N - H out-of-plane deformation vibrations [4]. The latter has been termed the amide V band. Fig. 2 shows the synchronous correlation plot in this region generated from the raw dynamic infrared spectra shown in Fig. 1. More information on the theory and basis for the correlation analysis can be found elsewhere [7]. In the synchronous correlation plot, diagonal peaks are referred to as autopeaks. Their presence represents the overall extent of dynamic fluctuations of spectral intensity with respect to the intensity of the reference spectrum. Off-diagonal peaks are called crosspeaks and their presence indicates that the transition dipole moment of the peaks move in-phase with each other. The synchronous plot in Fig. 2 shows that the bands at 721 and 675 cm t are moving in-phase with each other in response to the applied stretch. No frequency shifting or splittings as a result of stress-induced effects are visible in this region, or in the other regions presented. Fig. 1 also indicates that the methylene rocking and amide V bands move in-phase with the dynamic stretch as seen by their large in-phase response. Fig. 3 shows the asynchronous
740 ~ / / ~
~___~
640 "~', ~ 640 660 680
700
, "/ 720 740
Wavenumbers
Fig. 2. 2D-FT1R synchronous correlation plot of a melt-crystallized nylon 11 film in the CH 2 rocking-amide V region.
correlation plot for the same spectral region. The presence of asynchronous crosspeaks indicates that the movement of the corresponding chemical groups is out-of-phase with each other. From the figure it can be observed that the CH 2 rocking band at 721 cm-~ splits into two bands at 725 and 715 cm -~, and the amide V band at 675 cm-~ splits into three bands at about 690, 675, and 660 c m - ~. The analysis of the crosspeaks shows that the middle amide V peak (675 c m - 1) is out-of-phase with the 725 c m - J. These results are consistent with 2D-IR studies of
740
Absorbance
700
i
E 680
<
Quadrature 641
1500
1300
It00 900 Wavenumbers
700
660
680
700
720
740
500
Fig. 1. Static absorbance spectrum and in-phase and quadrature step-scan dynamic spectra of a melt-crystallized nylon 11 film.
Wavenumbers
Fig. 3. 2D-FTIR asynchronous correlation plot of a melt-crystallized nylon 11 film in the CH 2 rocking-amide V region.
78
A. Singhal, L.J. Fina / Vibrational Spectroscopy 13 (1996) 75-82
740 7
2
0
~
(~
v
v
720.~ ~
7 0 0 ~ - - ~
~ ~.
~680 ~
,
~
E
~
700680
660 660 640 640
660
680
700
720
740
t/ti \
640 1480 1520 1560 1600 1640 1680 i
Wavenumbers Fig. 4. 2D-FT1R synchronous correlation plot of a deuterated melt-crystallized nylon 11 film in the CH 2 rocking-amide V region.
nylon 11 films upon deuteration of the nitrogen in the amide group. Fig. 4 and Fig. 5 show the synchronous and asynchronous correlation plots of the deuterated melt-crystallized nylon 11 film. Deuteration shifts the amide V band from 685 to 486 cm-1 in the infrared spectrum [8]. In the synchronous plot, the response of the CH 2 rocking modes are observed (autopeak at ~721 cm-~). Note that 675 cm -1 peak is missing due to the deuteration. In the asynchronous plot (Fig. 5) a weak response in the amide V compared to the methylene bending region is also
Wavenumbers
Fig. 6. 2D-FTIR synchronous correlation crossplot between the CH 2 rocking-amide V and amide I-amide II regions of a meltcrystallized nylon 11 film.
seen, which implies that the nitrogen in the meltcrystallized nylon 11 is not completely deuterated. Fig. 6 and Fig. 7 show the synchronous and asynchronous crossplots between the CH 2 rockingamide V and amide I-amide II regions. In the previous work with the amide I and II region [1,9,10], the 1636 cm -1, 1560 cm -I and 1525 cm -1 peaks have been assigned as primarily crystalline and the 1640 cm -~ as a peak resulting from structures of
740
740-
720
720.
700
700-
680
680
660
660
t-
640 640
660
680
700
720
740
6401480 1520 1560 1600 1640 1680
Wavenumbers
Wavenumbers
Fig. 5. 2D-FTIR asynchronous correlation plot of a deuterated melt-crystallized nylon 11 film in the CH 2 rocking-amide V region.
Fig. 7. 2D-FTIR asynchronous correlation crossplot between the CH 2 rocking-amide V and amide I-amide II regions of a meltcrystallized nylon 11 film.
79
A. Singhal, L.J. Fina / Vibrational Spectroscopy 13 (1996) 75-82
1500 1480
/
1460
E
E 1440 1420
/
1400 1400 1420 1440 1460 1480 1500
1480 15'20 15'60 16'00 16'40 1680 Wavenumbers
Wavenumbers Fig. 8. 2D-FTIR synchronous correlation plot of a melt-crystallized nylon 11 film in the CH 2 bending region.
intermediate order between crystalline and true amorphous. The synchronous crossplot (Fig. 6) shows that the CH 2 rocking and amide V bands are in-phase with the 1525 and 1560 cm -l bands of the amide H peak and the 1636 cm -1 peak of the amide I vibrations. The relationship between the splittings of the CH 2 rocking and amide V bands and amide 1-amide II is further elucidated in the asynchronous plot shown in Fig. 7, where it can be seen that the bands at 725, 690 and 660 cm-~ are out-of-phase with the bands at 1560 and 1636 cm 1. Also, the bands at
Fig. 10. 2D-FTIR synchronous correlation crossplot between the CH 2 bending and amide l-amide II regions of a melt-crystallized nylon 11 film.
715 and 675 c m - ' are out-of-phase with the bands at 1540 and 1640 cm -~. Further analysis of these relationships is covered in Section 4. 3.2. CH 2 bending region The infrared absorption spectrum shown in Fig. 1 shows the presence of four peaks in the methylene bending region. The synchronous correlation plot in the CH 2 bending region depicted in Fig. 8 shows
1500 -
~
1500
1480 ~
~
~
~
\
E
1480-
)
E
1460 1440 1420 1400 1480 1520 1560 1600 1640 1680
1400 1420 1440 1460 1480 1500
Wavenumbers Fig. 9. 2D-FTIR asynchronous correlation plot of a melt-crystallized nylon l I film in the CH 2 bending region.
Wavenumbers Fig. 11. 2D-FTIR asynchronous correlation crossplot between the CH 2 bending and amide I-amide II regions of a melt-crystallized nylon 11 film.
A, Singhal, L.J. Final VibrationalSpectroscopy 13 (1996) 75-82
80
740
.g
1400 1420 1440 1460 1480 1500 Wavenumbers Fig. 12. 2D-FTIR synchronous correlation crossplot between the CH 2 bending and CH 2 rocking-amide V regions of a melt-crystallized nylon 11 film.
autopeaks at 1415 and 1460 cm -1 and the presence of crosspeaks between them. The asynchronous plot in the same region (Fig. 9) shows that the absorption in this region splits into four bands at 1475, 1460, 1430, and 1415 cm -1. This region is crossed with the amide I-amide II region in Fig. 10 and Fig. 11, the synchronous and asynchronous crossplots, respectively. The synchronous crossplot in Fig. 10 shows that the CH 2 bending peaks at 1460 and 1415 cm -~ are in-phase with the bands at 1636, 1560, and
"760 ~
,
?
°
/ I 1400 1420 1440
1460
1480 lg00
Wavenumbers Fig. 13. 2D-FTIR asynchronous correlation crossplot between the CH 2 bending and CH 2 rocking-amide V regions of a melt-crystallized nylon 11 film.
1525 (very low intensity) cm -1. The asynchronous plot in Fig. 11 shows that the CH 2 bending bands at 1475 and 1430 are out-of-phase with the bands at 1636 and 1560 cm -1 and the other two CH 2 bending bands at 1460 and 1415 cm -1 are out-of-phase with the bands at 1640 and 1540 cm -I. Figs. 12 and 13 show the synchronous and asynchronous crossplots between the CH 2 bending and C H 2 rocking-amide V regions. The synchronous plot (Fig. 12) shows that the CH 2 bending bands at 1460 and 1415 cm -1 are in-phase with the bands at 721 and 678 cm -l and asynchronous plot (Fig. 13) shows that the former are out-of-phase with the bands at 721,690 and 660 c m - ) . Also, the bands at 1475 and 1430 cm- ~ are out-of-phase with the bands at 715 and 675 cm -~.
4. Discussion The asynchronous plot in Fig. 3 shows that the C H 2 rocking peak, observed in the normal infrared
spectrum at 721 cm -) splits into two bands at 725 and 715 cm -1. There could be two reasons for the observed split, a crystal phase split (a-y phases) or morphological phase split (order-disorder). A splitting due to bond stretching is unlikely since this effect will show up in the synchronous correlation spectra rather than the asynchronous. The y-phase results in an amide VI band at 630 cm -1 in the absorbance spectrum of nylon 11 [4]. The absence of a 630 cm -~ peak in the absorbance spectrum of melt-crystallized nylon 11 in the films studied here (Fig. 1) suggests that the y-phase is not present. Therefore, an explanation of the CH z rocking split on the basis of two crystal phases is not considered. It is known that the 721 cm-1 rock has a crystalline origin in n-paraffins and polyethylene [11]. The splitting in the asynchronous plot (Fig. 3) suggests that it has a disordered (or incompletely ordered) component associated with it. In polyethylene, the C H 2 crystalline rocking mode splits into two peaks and has an disordered component under the lower frequency peak [12]. This implies a disordered origin to the 725 cm-1 band and a crystalline origin to the 715 cm -~ band of Fig. 3, since in the case of liquid n-paraffins, it has been observed that the in-phase
A. Singhal, L.J. F i n a / Vibrational Spectroscopy 13 (1996) 75-82
CH2 rocking vibrations of sequences of four or more t r a n s bands have absorption at frequencies below 719 cm-f [13,14]. However, it is also possible that the splitting of the 721 cm -~ peak in the asynchronous spectrum (Fig. 3) is due to the presence of amide groups rather than a crystal/amorphous split of the methylene rocking vibrations. Evidence to the contrary is derived from two sources: (1) in the asynchronous correlation plot of deuterated nylon 11 (Fig. 5) in the methylene rocking-amide V region, the split is observed; and (2) the synchronous and asynchronous crossplots between the methylene rocking-amide V and the amide I-II regions (Figs. 6 and 7) shows ordered and disordered components in the methylene rocking region. The amide V peak at 675 c m - l splits into three bands at 690, 675, and 660 cm -I in the asynchronous plot (Fig. 3) where the middle 675 cm-1 peak is out-of-phase with the outer two peaks at 690 and 660 cm- ~, Several pieces of evidence lead to the fact that the splitting of the amide V peak can also be assigned to ordered and disordered origins. The presence of an asynchronous crosspeak between the 725 cm -I (disordered) and the 675 cm -I peaks implies that the 675 cm-~ has a crystalline origin. Also, the amide V band at 660 cm-i is out-of-phase with the band at 715 cm-~ (crystalline), suggesting a disordered origin to the 660 cm -I band. This is consistent with the fact that in the molten state the amide V band shifts to a lower frequency [4]. Also, the 690 cm-~ band of the amide V splitting is out-of-phase with the 710 cm -~ band, implying a disordered origin to the former. To further confirm the morphological assignments of the CH 2 rocking and amide V bands, this region is crossed with the amide I-amide II region. For the amide I-amide II region it is known that the 1636, 1525, and 1560 cm 1 are primarily crystalline bands and 1640 cm -1 band results from disordered or partially ordered amide groups [1]. The crystalline CH 2 rocking band ( ~ 715 cm -j ) and amide V band (675 cm- 1) are in-phase with the crystalline bands at 1525 cm ~, 1560 cm - l , and the low frequency amide I component (Fig. 6), and out-of-phase with a high frequency amide I component (disordered) and the 1540 cm- ~ component of the amide II band (Fig. 7). Further, the bands at 725 and 660 cm-1 show an out-of-phase relationship with the crystalline bands
81
at 1636 and 1560 cm t (Fig. 7). This analysis of crosspeaks corroborates the morphological phase assignments of the CH 2 rocking and amide V vibrations. In melt-crystallized nylon 11 where the c~-phase predominates, four peaks at 1480, 1460, 1435 and 1420 cm-~ are observed in the CH 2 bending region. When the y-phase is present, only two peaks at 1460 and 1435 c m - l are observed [2]. The four peaks in the absorbance spectrum of Fig. 1 show the presence of the c~-phase. In the synchronous plot (Fig. 8) autopeaks are observed for the peaks at 1460 and 1415 cm ~, and there is an indication of the presence of weak peaks at about 1480 and 1440 cm -~. The weak peaks also show up as shoulders in the in-phase spectrum of Fig. 1. As expected of crystalline bands, the CH 2 bending bands are in-phase with each other (Fig. 8). Also, the bands at 1460 and 1415 cm-~ are out-of-phase with the bands at 1475 and 1430 cm -1 (Fig. 9). This implies that a disordered component is associated with the crystalline bands of CH 2 bending region and is consistent with the fact that in the melt two broad peaks are observed at 1470 and 1430 cm -~ [15]. The crystalline assignment of CH 2 bending bands and the presence of disordered phase components in this region is further confirmed by crossing the region with the amide I-amide II and CH 2 rocking-amide V regions (Fig. 10 and Fig. 11). Evidence to support the contention that some peaks in the methylene bending region are due to vicinal amide groups is not found here since all four peaks observed in the synchronous plot (Fig. 10) show the same behavior. The crystalline bands at 1636, 1560, and 1525 cm-f of amide I-amide II region show a general in-phase character with the four crystalline CH~ bending bands (1480, 1460, 1435, and 1415 c m - l ) (Fig. 10) and the 1636 and 1560 cm i an out-ofphase character with the CHz bending 'disordered' bands (1430 and 1475 cm-~) (Fig. 11). Further, the amide I band ( ~ 1640 cm -L) and amide I! band (1540 cm -~) due to disordered or partially ordered amide groups show general out-of-phase relationships with the crystalline bands of the CH 2 bending region. The morphological phase assignments of the C H 2 rocking-amide V and the CH 2 bending regions are consistent with each other. The crystalline bands of the CH: bending region are broadly in-phase (Fig.
82
A. Singhal, L.J. Fina / Vibrational Spectroscopy 13 (1996) 75-82
12) and the disordered bands of the same region are out-of-phase (Fig. 13) with the crystalline bands of the CH 2 r o c k i n g - a m i d e V region.
5. Conclusions The CH 2 rocking peak observed at 721 cm -1 in the normal infrared spectrum of nylon 11 splits into two bands at 725, and 710 cm -~ and amide V band observed at 675 cm-~ splits into three bands at 690, 675, and 660 cm -~, when viewed as 2D plots. A cross-correlation between the C H 2 r o c k i n g - a m i d e V and amide I - a m i d e II regions of nylon l 1 shows that the bands at 715 and 675 cm -~ can be assigned to an ordered morphological origin and the bands at 725, 690 and 660 cm - l to a disordered (or partially ordered) origin. The splittings observed in the CH 2 bending region are also explained on the basis of morphological features. Four 'ordered' bands at 1480, 1460, 1440 and 1415 cm -1 and two 'disordered' bands at 1470 and 1430 cm -1 are observed in this region. These assignments are confirmed by crosscorrelation of the C H 2 bending region with the
amide I - a m i d e II and CH 2 r o c k i n g - a m i d e V regions of nylon 11.
References [1] A. Singhal and L.J. Fina, Appl. Spectrosc. 49 (1995) 1073. [2] I. Matsubara and J.H. Magill, Polymer7 (1966) 199. [3] G. Heidemann and H. Zahn, Makromol. Chem. 62 (1963) 123. [4] I. Matsubara, Y. Itoh and M. Shinomiya, Polym. Lett. 4 (1966) 47. [5] J. Jakes and S. Krimm, Spectrochim. Acta A 27 (1971) 35. [6] A. Miyake, J. Polym. Sci. XLIV (1960) 223. [7] I. Noda, Appl. Spectrosc. 44 (1990) 550. [8] J. Jakes, P. Schmidt and B. Schneider, Collect. Czechoslov. Chem. Commun. 30 (1965) 996. [9] D.J. Skrovanek, P.C. Painter and M.M. Coleman,Macromol. 19 (1986) 699. [10] H.H. Yu and L.J. Fina, Macromol. 47 (1994) 6192. [11] R.G. Snyder, J. Chem. Phys. 47 (1967) 1316. [12] H. Hagemann,R.G. Snyder, A.J. Peacock and L. Mandelkern, Macromol. 22 (1989) 3600. [13] H. Hagemann,H.L. Strauss and R.G. Snyder, Macromol. 20 (1987) 2810. [14] A. Singhal and L.J. Fina, Polymer, submitted. [15] C.G. Cannon, Spectrochim. Acta 16 (1960) 302.
Vibrational Spectroscopy 13 (1996) 75-82
Dynamic two-dimensional infrared spectroscopy. Part II: Morphological assignments in melt-crystallized nylon 11 spectra Anu Singhal, Leslie J. Fina * Department of Chemical and Biochemical Engineering, College of Engineering, Rutgers University, Busch Campus. P.O. Box 909. Piscataway, NJ 08855, USA
Received 6 December 1995; accepted6 May 1996
Abstract
Melt-crystallized and deuterated nylon 11 are studied using dynamic two-dimensional FTIR spectroscopy in the region below 1500 cm I of the infrared spectrum. The splittings observed in the methylene rocking-amide V (760-640 cm- 1) and methylene bending (1500-1400 cm - I ) regions are explained on the basis of morphological features. These morphological assignments are corroborated by correlating methylene rocking-amide V and methylene bending regions with the morphological assignments of amide I-amide II (1680-1500 cm-1) region of nylon 11 established in an earlier work. Keywords: Nylon 11; 2D-IR spectroscopy;Morphology
1. Introduction
In a preceding paper [1], two-dimensional infrared spectroscopy was used to study in detail the N - H stretching and amide I - a m i d e II regions of meltcrystallized nylon 11. 2D-IR correlation spectroscopy confirmed the splitting of the amide I region on the basis of ordered and disordered components and on different strengths of hydrogen bonding between the amide groups. The amide II region also splits into ordered and disordered peaks. With a cross-correlation of the N - H stretching and amide I-amide II regions, it was found that the prominent dynamic peak observed in the N - H stretching region is similar in morphological character to the ordered component (1636 cm -1) of the amide I band. The Corresponding author.
goal of this study is to analyze in detail the region below 1500 cm -1 in melt-crystallized nylon 1 1 and to correlate this with the established assignments of the amide I-amide II region using 2D-IR spectroscopy. Results are presented for the CH 2 rocking-amide V (760-640 cm -I ) and CH 2 bending (1500-1400 cm -1) regions. A detailed study of the methylene bending region of the infrared spectrum of different nylons was presented by Matsubara and Magill [2]. This region shows a sensitivity to the crystal phase in all diaciddiamine and w-amino acid nylons. Four peaks are observed in the a-phase nylon 11 at 1475, 1465, 1440 and 1420 c m - 1 and two peaks in the y-phase at 1460 and 1440 cm -1. In amorphous nylon 11 the sharp crystalline methylene bending bands are replaced by two overlapping bands at 1460 and 1430 c m - 1. In addition to crystal phase assignments, some
0924-2031/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0924-2031(96)00031-8
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A. Singhal, L.J. Fina / Vibrational Spectroscopy 13 (1996) 75-82
of the peaks in this region have also been assigned to methylene groups vicinal to the amide groups. Heidemann and Zahn [3] assigned the 1475 cm -~ and 1416 cm -~ bands to the bending vibrations of the N-vicinal and the CO-vicinal CH 2 groups, respectively. They concluded that the bending vibrations of the N-vicinal and the CO-vicinal CH 2 groups coincide at 1435 cm -~ upon rotation of the - C H 2C O N H - C H 2- group out of a t r a n s planar conformation. The 760-640 c m - ~ region of the infrared spectra of nylon 11 contains the CH 2 rocking fundamental peak at about 721 cm-~ and the amide V absorption at 690 cm -~. Like the methylene bending region, this region is phase sensitive [4-6]. When the a or /3 structure predominates, the amide V band is located at 690 c m - l and when the y structure predominates, the amide V and VI peaks are at 715 and 630 cm -~, respectively. The peaks at 721 cm -~ and 690 cm-~ originate from the crystalline regions. The amide V band of the amorphous region exists below 680 cm -1 because in molten nylon the bands at 730-680 c m - 1 diminish and the position of the peak shifts to a lower frequency. These assignments are explored in this work.
2. Experimental The nylon 11 powder used in this study is provided by the Rilsan Corporation (Glen Rock, NJ). Melt-crystallized films are prepared by melting the powder between aluminum foil sheets in a hot press at 210°C for 5 min followed by slow cooling to room temperature. Previous work has shown that this sample preparation treatment does not lead to polymer chain orientation [10]. Deuterated samples are prepared by annealing the melt-crystallized nylon 11 films at 150°C with deuterium oxide (D20) in an acid digestion bomb (Parr Instruments) under 1800 psi pressure for 4 h and then cooling to room temperature. Before scanning, each sample is sanded to remove the interference fringe effects. A BioRad FTS-60A FTIR spectrometer is used for all measurements. The nylon 11 film used for scanning is approximately 2.5 cm long and 1.5 cm wide. A phase modulation frequency of 400 Hz with an amplitude of 2AlaeNe (1.26 /zm) is used. In these
experiments, a 25 mm diameter sapphire low-pass filter with an undersampling ratio of 4 (2 AHeNedata spacings) is used to scan the 1800-400 cm-1 region of the nylon 11 film. A room temperature triglycine sulphate (TGS) photoconductive detector is used to detect the infrared beam. After detection, the signal is first demodulated by the first of two lock-ins (PAR, model 5209) referenced to the phase modulation frequency. The output of this lock-in produces the reference interferogram. It is equivalent to the 'normal' infrared transmission spectrum and used to normalize the dynamic spectra produced from the second lock-in amplifier (PAR, model 5210). The dynamic spectra thus obtained show only changes in the infrared absorption which are associated with the reorientation of the transition dipoles in response to the mechanical perturbation. The spectral resolution is set at 16 cm l in order to be compatible with past work [1]. A scan rate of 0.25 Hz is used, and a total of 16,000 digital samples are collected from each of the three output channels: lock-in l, lock-in 2 (inphase) and lock-in 2 (quadrature) outputs. Twelve scans are coadded to reduce the noise in the spectra. Each sample is scanned in four separate sets and the reproducible features are reported here. In order to determine the possible influence of sample thickness changes on the dynamic and two-dimensional spectra, nylon 11 is also scanned in a triple modulationdemodulation experiment where polarization modulation was added in the optical train. Polarization modulation tends to minimize the observation of sample thickness changes. The results reported herein are based on observations repeatable in both double and triple modulation experiments.
3. Results Fig. 1 shows the 1500-500 cm -1 region of the normal absorbance spectrum, and the in-phase and quadrature dynamic spectra of a melt-crystallized nylon 11 film. From the dynamic spectra it can be observed that a very small response is observed in the quadrature spectrum relative to the in-phase spectrum, which establishes that the polymer film in this region is responding mostly in-phase to the applied perturbation.
A. Singhal, L.J. Fina / Vibrational Spectroscopy 13 (1996) 75-82
77
3.1. CH 2 rocking-amide V region In the 760-640 cm -1 range of the normal absorbance spectrum (Fig. 1), two peaks are observed at 721 and 675 cm -1. The peak 721 cm -1 has been assigned to CH 2 rocking vibrations and the band at 675 cm-1 to N - H out-of-plane deformation vibrations [4]. The latter has been termed the amide V band. Fig. 2 shows the synchronous correlation plot in this region generated from the raw dynamic infrared spectra shown in Fig. 1. More information on the theory and basis for the correlation analysis can be found elsewhere [7]. In the synchronous correlation plot, diagonal peaks are referred to as autopeaks. Their presence represents the overall extent of dynamic fluctuations of spectral intensity with respect to the intensity of the reference spectrum. Off-diagonal peaks are called crosspeaks and their presence indicates that the transition dipole moment of the peaks move in-phase with each other. The synchronous plot in Fig. 2 shows that the bands at 721 and 675 cm t are moving in-phase with each other in response to the applied stretch. No frequency shifting or splittings as a result of stress-induced effects are visible in this region, or in the other regions presented. Fig. 1 also indicates that the methylene rocking and amide V bands move in-phase with the dynamic stretch as seen by their large in-phase response. Fig. 3 shows the asynchronous
740 ~ / / ~
~___~
640 "~', ~ 640 660 680
700
, "/ 720 740
Wavenumbers
Fig. 2. 2D-FT1R synchronous correlation plot of a melt-crystallized nylon 11 film in the CH 2 rocking-amide V region.
correlation plot for the same spectral region. The presence of asynchronous crosspeaks indicates that the movement of the corresponding chemical groups is out-of-phase with each other. From the figure it can be observed that the CH 2 rocking band at 721 cm-~ splits into two bands at 725 and 715 cm -~, and the amide V band at 675 cm-~ splits into three bands at about 690, 675, and 660 c m - ~. The analysis of the crosspeaks shows that the middle amide V peak (675 c m - 1) is out-of-phase with the 725 c m - J. These results are consistent with 2D-IR studies of
740
Absorbance
700
i
E 680
<
Quadrature 641
1500
1300
It00 900 Wavenumbers
700
660
680
700
720
740
500
Fig. 1. Static absorbance spectrum and in-phase and quadrature step-scan dynamic spectra of a melt-crystallized nylon 11 film.
Wavenumbers
Fig. 3. 2D-FTIR asynchronous correlation plot of a melt-crystallized nylon 11 film in the CH 2 rocking-amide V region.
78
A. Singhal, L.J. Fina / Vibrational Spectroscopy 13 (1996) 75-82
740 7
2
0
~
(~
v
v
720.~ ~
7 0 0 ~ - - ~
~ ~.
~680 ~
,
~
E
~
700680
660 660 640 640
660
680
700
720
740
t/ti \
640 1480 1520 1560 1600 1640 1680 i
Wavenumbers Fig. 4. 2D-FT1R synchronous correlation plot of a deuterated melt-crystallized nylon 11 film in the CH 2 rocking-amide V region.
nylon 11 films upon deuteration of the nitrogen in the amide group. Fig. 4 and Fig. 5 show the synchronous and asynchronous correlation plots of the deuterated melt-crystallized nylon 11 film. Deuteration shifts the amide V band from 685 to 486 cm-1 in the infrared spectrum [8]. In the synchronous plot, the response of the CH 2 rocking modes are observed (autopeak at ~721 cm-~). Note that 675 cm -1 peak is missing due to the deuteration. In the asynchronous plot (Fig. 5) a weak response in the amide V compared to the methylene bending region is also
Wavenumbers
Fig. 6. 2D-FTIR synchronous correlation crossplot between the CH 2 rocking-amide V and amide I-amide II regions of a meltcrystallized nylon 11 film.
seen, which implies that the nitrogen in the meltcrystallized nylon 11 is not completely deuterated. Fig. 6 and Fig. 7 show the synchronous and asynchronous crossplots between the CH 2 rockingamide V and amide I-amide II regions. In the previous work with the amide I and II region [1,9,10], the 1636 cm -1, 1560 cm -I and 1525 cm -1 peaks have been assigned as primarily crystalline and the 1640 cm -~ as a peak resulting from structures of
740
740-
720
720.
700
700-
680
680
660
660
t-
640 640
660
680
700
720
740
6401480 1520 1560 1600 1640 1680
Wavenumbers
Wavenumbers
Fig. 5. 2D-FTIR asynchronous correlation plot of a deuterated melt-crystallized nylon 11 film in the CH 2 rocking-amide V region.
Fig. 7. 2D-FTIR asynchronous correlation crossplot between the CH 2 rocking-amide V and amide I-amide II regions of a meltcrystallized nylon 11 film.
79
A. Singhal, L.J. Fina / Vibrational Spectroscopy 13 (1996) 75-82
1500 1480
/
1460
E
E 1440 1420
/
1400 1400 1420 1440 1460 1480 1500
1480 15'20 15'60 16'00 16'40 1680 Wavenumbers
Wavenumbers Fig. 8. 2D-FTIR synchronous correlation plot of a melt-crystallized nylon 11 film in the CH 2 bending region.
intermediate order between crystalline and true amorphous. The synchronous crossplot (Fig. 6) shows that the CH 2 rocking and amide V bands are in-phase with the 1525 and 1560 cm -l bands of the amide H peak and the 1636 cm -1 peak of the amide I vibrations. The relationship between the splittings of the CH 2 rocking and amide V bands and amide 1-amide II is further elucidated in the asynchronous plot shown in Fig. 7, where it can be seen that the bands at 725, 690 and 660 cm-~ are out-of-phase with the bands at 1560 and 1636 cm 1. Also, the bands at
Fig. 10. 2D-FTIR synchronous correlation crossplot between the CH 2 bending and amide l-amide II regions of a melt-crystallized nylon 11 film.
715 and 675 c m - ' are out-of-phase with the bands at 1540 and 1640 cm -~. Further analysis of these relationships is covered in Section 4. 3.2. CH 2 bending region The infrared absorption spectrum shown in Fig. 1 shows the presence of four peaks in the methylene bending region. The synchronous correlation plot in the CH 2 bending region depicted in Fig. 8 shows
1500 -
~
1500
1480 ~
~
~
~
\
E
1480-
)
E
1460 1440 1420 1400 1480 1520 1560 1600 1640 1680
1400 1420 1440 1460 1480 1500
Wavenumbers Fig. 9. 2D-FTIR asynchronous correlation plot of a melt-crystallized nylon l I film in the CH 2 bending region.
Wavenumbers Fig. 11. 2D-FTIR asynchronous correlation crossplot between the CH 2 bending and amide I-amide II regions of a melt-crystallized nylon 11 film.
A, Singhal, L.J. Final VibrationalSpectroscopy 13 (1996) 75-82
80
740
.g
1400 1420 1440 1460 1480 1500 Wavenumbers Fig. 12. 2D-FTIR synchronous correlation crossplot between the CH 2 bending and CH 2 rocking-amide V regions of a melt-crystallized nylon 11 film.
autopeaks at 1415 and 1460 cm -1 and the presence of crosspeaks between them. The asynchronous plot in the same region (Fig. 9) shows that the absorption in this region splits into four bands at 1475, 1460, 1430, and 1415 cm -1. This region is crossed with the amide I-amide II region in Fig. 10 and Fig. 11, the synchronous and asynchronous crossplots, respectively. The synchronous crossplot in Fig. 10 shows that the CH 2 bending peaks at 1460 and 1415 cm -~ are in-phase with the bands at 1636, 1560, and
"760 ~
,
?
°
/ I 1400 1420 1440
1460
1480 lg00
Wavenumbers Fig. 13. 2D-FTIR asynchronous correlation crossplot between the CH 2 bending and CH 2 rocking-amide V regions of a melt-crystallized nylon 11 film.
1525 (very low intensity) cm -1. The asynchronous plot in Fig. 11 shows that the CH 2 bending bands at 1475 and 1430 are out-of-phase with the bands at 1636 and 1560 cm -1 and the other two CH 2 bending bands at 1460 and 1415 cm -1 are out-of-phase with the bands at 1640 and 1540 cm -I. Figs. 12 and 13 show the synchronous and asynchronous crossplots between the CH 2 bending and C H 2 rocking-amide V regions. The synchronous plot (Fig. 12) shows that the CH 2 bending bands at 1460 and 1415 cm -1 are in-phase with the bands at 721 and 678 cm -l and asynchronous plot (Fig. 13) shows that the former are out-of-phase with the bands at 721,690 and 660 c m - ) . Also, the bands at 1475 and 1430 cm- ~ are out-of-phase with the bands at 715 and 675 cm -~.
4. Discussion The asynchronous plot in Fig. 3 shows that the C H 2 rocking peak, observed in the normal infrared
spectrum at 721 cm -) splits into two bands at 725 and 715 cm -1. There could be two reasons for the observed split, a crystal phase split (a-y phases) or morphological phase split (order-disorder). A splitting due to bond stretching is unlikely since this effect will show up in the synchronous correlation spectra rather than the asynchronous. The y-phase results in an amide VI band at 630 cm -1 in the absorbance spectrum of nylon 11 [4]. The absence of a 630 cm -~ peak in the absorbance spectrum of melt-crystallized nylon 11 in the films studied here (Fig. 1) suggests that the y-phase is not present. Therefore, an explanation of the CH z rocking split on the basis of two crystal phases is not considered. It is known that the 721 cm-1 rock has a crystalline origin in n-paraffins and polyethylene [11]. The splitting in the asynchronous plot (Fig. 3) suggests that it has a disordered (or incompletely ordered) component associated with it. In polyethylene, the C H 2 crystalline rocking mode splits into two peaks and has an disordered component under the lower frequency peak [12]. This implies a disordered origin to the 725 cm-1 band and a crystalline origin to the 715 cm -~ band of Fig. 3, since in the case of liquid n-paraffins, it has been observed that the in-phase
A. Singhal, L.J. F i n a / Vibrational Spectroscopy 13 (1996) 75-82
CH2 rocking vibrations of sequences of four or more t r a n s bands have absorption at frequencies below 719 cm-f [13,14]. However, it is also possible that the splitting of the 721 cm -~ peak in the asynchronous spectrum (Fig. 3) is due to the presence of amide groups rather than a crystal/amorphous split of the methylene rocking vibrations. Evidence to the contrary is derived from two sources: (1) in the asynchronous correlation plot of deuterated nylon 11 (Fig. 5) in the methylene rocking-amide V region, the split is observed; and (2) the synchronous and asynchronous crossplots between the methylene rocking-amide V and the amide I-II regions (Figs. 6 and 7) shows ordered and disordered components in the methylene rocking region. The amide V peak at 675 c m - l splits into three bands at 690, 675, and 660 cm -I in the asynchronous plot (Fig. 3) where the middle 675 cm-1 peak is out-of-phase with the outer two peaks at 690 and 660 cm- ~, Several pieces of evidence lead to the fact that the splitting of the amide V peak can also be assigned to ordered and disordered origins. The presence of an asynchronous crosspeak between the 725 cm -I (disordered) and the 675 cm -I peaks implies that the 675 cm-~ has a crystalline origin. Also, the amide V band at 660 cm-i is out-of-phase with the band at 715 cm-~ (crystalline), suggesting a disordered origin to the 660 cm -I band. This is consistent with the fact that in the molten state the amide V band shifts to a lower frequency [4]. Also, the 690 cm-~ band of the amide V splitting is out-of-phase with the 710 cm -~ band, implying a disordered origin to the former. To further confirm the morphological assignments of the CH 2 rocking and amide V bands, this region is crossed with the amide I-amide II region. For the amide I-amide II region it is known that the 1636, 1525, and 1560 cm 1 are primarily crystalline bands and 1640 cm -1 band results from disordered or partially ordered amide groups [1]. The crystalline CH 2 rocking band ( ~ 715 cm -j ) and amide V band (675 cm- 1) are in-phase with the crystalline bands at 1525 cm ~, 1560 cm - l , and the low frequency amide I component (Fig. 6), and out-of-phase with a high frequency amide I component (disordered) and the 1540 cm- ~ component of the amide II band (Fig. 7). Further, the bands at 725 and 660 cm-1 show an out-of-phase relationship with the crystalline bands
81
at 1636 and 1560 cm t (Fig. 7). This analysis of crosspeaks corroborates the morphological phase assignments of the CH 2 rocking and amide V vibrations. In melt-crystallized nylon 11 where the c~-phase predominates, four peaks at 1480, 1460, 1435 and 1420 cm-~ are observed in the CH 2 bending region. When the y-phase is present, only two peaks at 1460 and 1435 c m - l are observed [2]. The four peaks in the absorbance spectrum of Fig. 1 show the presence of the c~-phase. In the synchronous plot (Fig. 8) autopeaks are observed for the peaks at 1460 and 1415 cm ~, and there is an indication of the presence of weak peaks at about 1480 and 1440 cm -~. The weak peaks also show up as shoulders in the in-phase spectrum of Fig. 1. As expected of crystalline bands, the CH 2 bending bands are in-phase with each other (Fig. 8). Also, the bands at 1460 and 1415 cm-~ are out-of-phase with the bands at 1475 and 1430 cm -1 (Fig. 9). This implies that a disordered component is associated with the crystalline bands of CH 2 bending region and is consistent with the fact that in the melt two broad peaks are observed at 1470 and 1430 cm -~ [15]. The crystalline assignment of CH 2 bending bands and the presence of disordered phase components in this region is further confirmed by crossing the region with the amide I-amide II and CH 2 rocking-amide V regions (Fig. 10 and Fig. 11). Evidence to support the contention that some peaks in the methylene bending region are due to vicinal amide groups is not found here since all four peaks observed in the synchronous plot (Fig. 10) show the same behavior. The crystalline bands at 1636, 1560, and 1525 cm-f of amide I-amide II region show a general in-phase character with the four crystalline CH~ bending bands (1480, 1460, 1435, and 1415 c m - l ) (Fig. 10) and the 1636 and 1560 cm i an out-ofphase character with the CHz bending 'disordered' bands (1430 and 1475 cm-~) (Fig. 11). Further, the amide I band ( ~ 1640 cm -L) and amide I! band (1540 cm -~) due to disordered or partially ordered amide groups show general out-of-phase relationships with the crystalline bands of the CH 2 bending region. The morphological phase assignments of the C H 2 rocking-amide V and the CH 2 bending regions are consistent with each other. The crystalline bands of the CH: bending region are broadly in-phase (Fig.
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A. Singhal, L.J. Fina / Vibrational Spectroscopy 13 (1996) 75-82
12) and the disordered bands of the same region are out-of-phase (Fig. 13) with the crystalline bands of the CH 2 r o c k i n g - a m i d e V region.
5. Conclusions The CH 2 rocking peak observed at 721 cm -1 in the normal infrared spectrum of nylon 11 splits into two bands at 725, and 710 cm -~ and amide V band observed at 675 cm-~ splits into three bands at 690, 675, and 660 cm -~, when viewed as 2D plots. A cross-correlation between the C H 2 r o c k i n g - a m i d e V and amide I - a m i d e II regions of nylon l 1 shows that the bands at 715 and 675 cm -~ can be assigned to an ordered morphological origin and the bands at 725, 690 and 660 cm - l to a disordered (or partially ordered) origin. The splittings observed in the CH 2 bending region are also explained on the basis of morphological features. Four 'ordered' bands at 1480, 1460, 1440 and 1415 cm -1 and two 'disordered' bands at 1470 and 1430 cm -1 are observed in this region. These assignments are confirmed by crosscorrelation of the C H 2 bending region with the
amide I - a m i d e II and CH 2 r o c k i n g - a m i d e V regions of nylon 11.
References [1] A. Singhal and L.J. Fina, Appl. Spectrosc. 49 (1995) 1073. [2] I. Matsubara and J.H. Magill, Polymer7 (1966) 199. [3] G. Heidemann and H. Zahn, Makromol. Chem. 62 (1963) 123. [4] I. Matsubara, Y. Itoh and M. Shinomiya, Polym. Lett. 4 (1966) 47. [5] J. Jakes and S. Krimm, Spectrochim. Acta A 27 (1971) 35. [6] A. Miyake, J. Polym. Sci. XLIV (1960) 223. [7] I. Noda, Appl. Spectrosc. 44 (1990) 550. [8] J. Jakes, P. Schmidt and B. Schneider, Collect. Czechoslov. Chem. Commun. 30 (1965) 996. [9] D.J. Skrovanek, P.C. Painter and M.M. Coleman,Macromol. 19 (1986) 699. [10] H.H. Yu and L.J. Fina, Macromol. 47 (1994) 6192. [11] R.G. Snyder, J. Chem. Phys. 47 (1967) 1316. [12] H. Hagemann,R.G. Snyder, A.J. Peacock and L. Mandelkern, Macromol. 22 (1989) 3600. [13] H. Hagemann,H.L. Strauss and R.G. Snyder, Macromol. 20 (1987) 2810. [14] A. Singhal and L.J. Fina, Polymer, submitted. [15] C.G. Cannon, Spectrochim. Acta 16 (1960) 302.