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Structure and molecular motion of poly(ethylene oxide) in the solid state as studied by variable-temperature high-resolution solid-state 13C NMR
Journal of Molecular Structure, 239 (1990) 149-159 Elsevier Science Publishers B.V., Amsterdam
14!3
STRUCTURE AND MOLECULAR MOTION OF POLY(ETHYLENE OXIDE) IN THE SOLID STATE AS STUDIED BY VARIABLE-TEMPERATURE HIGH-RESOLUTION SOLIDSTATE 13C NMR
HIROMICHI
KUROSU
and ISA0 AND0
Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo (Japan) (Received
15 January
1990)
ABSTRACT High-resolution 13C NMR spectra of poly(ethylene oxide) in the solid state were measured between - 105.5”C and room temperature by the variable temperature i3C cross polarizationmagic angle spinning (CP-MAS) and pulse saturation transfer-magic angle spinning (PST-MAS) techniques. Based on these results, the temperature changes of the isotropic i3C chemical shift and the principal values of the 13C chemical shift tensor were used to elucidate dynamic and conformational aspects of poly (ethylene oxide) in the solid state.
INTRODUCTION
Very recently, much information about the molecular structure and dynamics of semicrystalline polymers has been provided by solid state 13CNMR [l41. Although the physical properties of polymers in the solid state are strongly affected by temperature, most of these studies has been carried out only at ambient temperature. Therefore, variable temperature NMR techniques should provide much useful data on the structural and dynamic aspects of polymers in the solid state [ 5-81. The advantage of variable temperature (VT) solidstate NMR techniques is that the temperature-dependent conformation and molecular motion can be studied by observing the isotropic chemical shift and chemical shift tensor and by using the cross polarization (CP) technique [ 91. Previously, from X-ray diffraction studies [ 10,111, it was reported that poly (ethylene oxide) (PEO) exists in two crystallographic forms; one is the monoclinic form (Type I) with (TTG), as the repeated unit (T and G stand for trans and gauche, respectively) and the other is the triclinic form (Type II) with an all-tram conformation. The former modification is very stable, but the latter is remarkably unstable and exists only under tension. As reported from broad-band NMR [ 121, PEO chains undergo rotational
0022-2860/90/$03.50
0 1990 -
Elsevier Science Publishers
B.V.
150
motion along the chain axis at room temperature. Because of the polymer’s fast motion and short TlpH [ 131, it is difficult to observe the 13C CP-MAS spectrum at room temperature. However, we can record 13C CP-MAS spectra at lower temperature because the molecular motion is then frozen, in the NMR time scale, and so the CP efficiency becomes high. The aim of the present work is to obtain precise information on the structure and dynamics of PEO in the solid state as a function of temperature by observing the isotropic 13C chemical shift and 13C chemical shift tensors by VT solid-state NMR. EXPERIMENTAL
Materials The PEO sample used had an average molecular weight of ca. 5000. It was obtained from Wako Pure Chemical Industries and was prepared by melt-crystallization. The melting point and glass transition temperature were about 60’ C and - 40’ C, respectively. NMR measurements The solid state 13C NMR spectra were recorded at 67.8 MHz with a JEOL GSX-270 NMR spectrometer, equipped with CP and VT accessories. CP-MAS NMR spectra were recorded with contact time of 2.0 or 0.15 ms during CP and the repetition time was 5 s. Samples were held in cylindrical rotors machined from zirconia and spun at 4.0-4.5 kHz. The magic angle (54.7” ) was set by increasing the symmetry and maximizing the intensity of the peak of the quarternary carbon atoms of hexamethylbenzene. The spectral width and number of data points were 27 kHz and 8 k, respectively. The spectra were usually accumulated 200-400 times to achieve a reasonable signal-to-noise ratio. The 13C NMR chemical shifts were calibrated indirectly using adamantane (29.5 ppm relative to tetramethysilane (TMS ) ) as external standard. RESULTS AND DISCUSSION
13C CP-MAS spectra The efficiency of the CP process is determined by the relative values of several relaxation times, such as TlpH, and the contact time. In the CP experiment, once the 13C signal has built up, it tends to be depleted by T1,H. If TlpH is very short, then the CP process can be short-circuited, preventing build-up of a 13C magnetization. Figure 1 shows 13C VT CP-MAS NMR spectra of PEO as a function of temperature. The solid-state spectrum at room temperature (ca. 24” C) cannot be observed under normal measurement conditions (con-
15:1
;;
100
RT
90
80
70
60
50
(wm)
Fig. 1. Variable temperature
13C CP-MAS
NMR spectra of poly (ethylene
oxide).
tact time (CT) = 2 ms) since PEO has a minimum in TlpH[111, due to molecular motion along the chain axis, at about this temperature. Such a short TlpH destroys proton ordering before CP of the carbon atoms has proceeded to a significant degree. However, the spectrum can be observed at CT = 150 pus,as shown in Fig. 1, although the line is rather broad because of molecular motion. Since the CP efficiency for complete polarization transfer depends on TlpH, which changes as a function of temperature, measurements below room temperature provide a better spectrum with relatively sharp peaks. At temperatures below - 27.5’ C, 13CCP-MAS spectra of PEO can be observed at CT = 2 ms, as shown in Fig. 1. At temperatures below -275°C the CP-MAS spectrum consists of three peaks, one major and two minor, as shown in Fig. 1. The 13CCP-MAS spectrum at - 105.5’ C clearly shows two minor peaks which appear upfield and downfield of the main peak, respectively. These three peaks are designated by I, II and III from downfield to upfield. The 13Cchemical shift values of these three peaks cannot be determined easily from the spectrum, so theoretical and ex-
152
perimental spectra were fitted by a computer-based non-linear least squares analysis (Fig. 2). The results are listed in Table 1. From an X-ray study [l41, it is known that the most stable crystal form is monoclinic, so the main peak (II) can be assigned to this form. It is obvious that the 13Cchemical shift
A OBSERVED
-___
THEORETICAL DECOMPOSE0
SPECTRUM SPECTRUM SPECTRUM
Fig. 2. Deconvolution of 13CCP-MAS NMR spectrum of poly(ethylene oxide) at - 105.5”C by computer-fitting with Lorentzian functions. TABLE 1 Observed i3C NMR chemical shifts, half-height widths and relativepeak intensities of poly (ethylene oxide) in the solid state at various temperatures” Temp. (“C)
RT -27.5 -66.5 - 105.5
Peak III
II
I fib
&2c
Peak intensityd
6
&I2
Peak intensity
6
61/Z
Peak intensity
80.0 75.1 74.4 74.4
16.0 2.2 1.1 1.0
29.9 7.3 10.3 6.7
71.6 72.2 72.6 72.6
12.0 2.8 1.0 1.0
70.1 92.7 66.0 66.6
71.7 71.6
1.0 1.0
23.1 26.7
“By the CP-MAS method. bChemical shift (ppm from TMS). “Half-height width (ppm). din percent.
153
values determined by the CP-MAS method are constant (72.6 ppm) from - 66.5 - 105.5 ‘C. At room temperature the peak width is very broad, possibly because of contributions from various polymer chain conformations and configurations. (By configurations we mean the spatial positions of the polymer chains, undergoing rotational motion, along the chain axis.) The peak width at half-height becomes smaller as the temperature is decreased, indicating that the distribution of conformations has become narrower, because the molecular motion is gradually frozen. Since the increase in molecular motion leads to a decrease in the C-H dipolar interactions, the efficiency of the CP process in the mobile region decreases and hence the CP-MAS experiment emphasises the peak intensity of the CH, carbons in the immobile phase. Therefore, it can be said that the main peak of PEO comes from the CH2 carbons in the phase in which the molecular motion is frozen at temperatures below - 66.5’ C. This will be clarified by subsequent relaxation time measurements. Pulse saturation transfer-MAS spectra In the pulse saturation transfer (PST)-MAS method, 13C magnetizations are enare induced by a single, direct 13C 90” pulse and the magnetizations hanced by saturated protons, through the nuclear Overhauser effect (NOE) (Fig. 3). Since the repetition time of the pulse is set at 5 s, the build-up of magnetization from the immobile phase is inadequate for efficient observation. Therefore, the 13C signal obtained by the PST-MAS method comes from the CH, carbons in the mobile phase which have relatively short TIC values. Figure 4 shows the VT 13C PST-MAS spectra of PEO as a function of temperature. The highest field peak (62 ppm) which does not appear in the 13C CP-MAS spectra is designated by IV. The 13C chemical shift values and the peak widths determined by computer-fitting are listed in Table 2. The main peak moves gradually downfield from 71.1 to 72.6 ppm as the temperature is decreased from room temperature to - 105.5’ C. The chemical shift value (72.6 ppm) of PEO at - 105.5 oC agrees with that of PEO determined by the 13C CP-
Fig. 3. Schematic representation of a pulse sequence in the pulse saturation transfer method.
154
100
80
90
70
60
50
(pm)
Fig. 4. Variable
temperature
i3C! PST-MAS
NMR
Observed i3C NMR chemical shifts, half-height ide) at various temperatures”
widths
spectra
of poly(ethylene
oxide)
in the solid
state. TABLE
Temp.
2 and relative peak intensities
of poly(ethylene
ox-
Peak
(“Cl II
I
RT -21.5 -66.5 - 105.5
IV
III
6b
8,,2c
Peak intensityd
6
6,,,
Peak intensity
6
6,,,
Peak intensity
6
ai/,
Peak intensity
74.5 75.4 74.3 74.2
5.0 4.0 1.0 1.0
15.9 2.4 2.2 9.7
71.1 71.9 72.4 72.6
4.8 3.7 1.8 1.2
38.2 73.0 66.7 77.9
67.0 68.5 71.0 71.2
6.0 5.0 3.0 1.0
20.5 14.8 31.1 12.3
61.9 62.0 -
4.2 5.0 -
13.4 9.9
“By the PST-MAS
method.
bChemical
shift (ppm from TMS 1. ‘Half-height
width
(ppml.
-
din per cent.
155’
[PPm) “I
r....I...‘I..“I”“I”“I”“,’ 100
90
80
70
80
70
60
50
40
30
(b)
(ppm r’,‘I
100
I,,
90
,‘,
‘,,I
/
,‘I
60
,/,I
50
,I,,
I .,
40
30
Fig. 5. 13Cpowder pattern of poly(ethylene oxide) in the solid state: (a) room temperature; (b) - 105.5”C. TABLE 3 Isotropic 13CNMR chemical shifts and chemical shift tensors of poly(ethylene oxide) (ppm from TMS)
%
Ull
&2
%I
011-0333
71.1 72.6
79 92
72 86
62 40
17 52
-
Room temperature - 105.5”C
MAS method at temperatures below - 66.5’ C. This result shows that the molecular motion is frozen, in the NMR time scale, at - 105.5 ‘C. The 13Cpowder patterns observed at room temperature and - 105.5” C are shown in Fig. 5. The averaged value of the three principal values of the shielding tensors determined by the computer simulation [ ( oll f az2 -t ~7~~)/3] is equal
156
‘H:
(j&d DELAY
TI h2
x
ACQ
I A,,
,3c-
V"-
Fig. 6. Schematic
representation
of a pulse sequence in the inversion-recovery
method.
to the isotropic chemical shift value (ai,,), determined from the 13CPST-MAS spectrum. The principal values of the shielding tensors and isotropic chemical shifts are listed in Table 3. The breadth of the powder pattern ( oll - ~7~~)at room temperature (17 ppm) is narrower than that at - 105.5”C (52 ppm), indicating that the restriction of molecular motion at low temperature increases the chemical shift anisotropy. If only rotational motion along the chain axis is assumed to exist at room temperature, it would be expected that the principal shielding tensor value which has its direction along the chain axis
A
0.6
0.8
-m I”~‘,““,““,~!, 100 90
80
,,~‘~~(I 60
70 (pm)
I, 50
40.0 8, 40
157
Delay Time 1 (xc)
(b)
1
I”“I”“I”“I”“I”’
100
90
80
70 (w-n)
60
50
Fig. 7. Y! inversion-recovery spectra of poly(ethylene NMR: (a) room temperature; (b) -105.5”C.
oxide) in the solid state by 13CPST-MAS
TABLE 4 13CT, values (s) for poly(ethylene
oxide) in the solid state
Peak
Room temperature - 105.5”C
I
II
III
4
3 17
5 9
would become constant as the temperature changes. However, none of the three principal values is the same at room temperature as it is at - 1055°C. This indicates that the molecular motion of PEO at room temperature is not restricted to rotational motion along the chain axis but includes other motions such as librational rotation of the chain axis. TIC measurements TIC measurements of PEO were carried out at room temperature and - 105.5 oC by the inversion-recovery method. The pulse sequence used is shown
158
in Fig. 6 and is similar to that of the 13CPST-MAS method except for insertion of a x pulse and a delay time. Figure 7 shows inversion-recovery spectra as a function of the delay time z. The intensities of the main and two minor peaks were obtained by computer-fitting, and the Tic values determined for all three peaks at room temperature and for the main peak and one minor peak at - 1055°C are listed in Table 4. These Tic values are subject to some error because the PST-MAS spectra include background signals from the NMRprobe and because of the error inherent in the analysis of sharp lines. It is difficult to calculate a Tic value for the minor peak at lower field at - 105.5’ C because the intensity of the peak is small. However, we can estimate the magnitude of its molecular motion because the measurements also show that the CH, carbons contributing to the main peak and to the two minor peaks have TICvalues over 9 s at - 105.5’ C. This shows that the mobilities of the two minor peaks are almost the same as that of the main peak at room temperature, indicating that the two minor peaks do not originate from the non-crystalline phase. Tabeta and Saito [ 141 reported that the 13C chemical shifts of ethylene oxide oligomers and PEO complexed with HgCl, or CdCl, depend on the dihedral angle. According to their results, the two minor peaks must come from crystallographic forms having conformations different from the (TTG), conformation of the monoclinic form. As seen from Fig. 7, the intensity of the main peak at room temperature decreases as the delay time z is decreased and at r=0.2 s the main peak is inverted. However, the intensity of peak IV inverts at r= 0.05 s, so its TICvalue is shorter than that of the main peak. This means that peak IV comes from the CH, carbons in the non-crystalline phase. In order to eludicate the origin of such chemical shifts and chemical shift anisotropy and to reveal the effect of molecular motions on 13C chemical shift behaviour, quantum chemical calculations using the tight-binding molecular orbital method have been carried out on all seven PEO chains model, taking into account the molecular packing and the molecular motion. Results of such calculations will be published soon.
CONCLUSIONS
Measurements of the temperature-dependent 13Cchemical shifts for the main peak in the 13C NMR spectra of PEO show that, at room temperature, the polymer chains are in the monoclinic form with the (TTG), conformation. Besides the main peak, which comes from the monoclinic form, two minor peaks which may come from other crystallographic forms were observed. A fourth peak was assigned to the non-crystalline phase.
159 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14
J.R. Lyerla and C.S. Yannoni, IBM J. Res. Dev., 27 (1983) 302. R.A. Komoroski (Ed.), High Resolution NMR Spectroscopy of Synthetic Polymers in Bulk, VCH Publishers, Florida, 1986. H. Saito and I. Ando, in G.A. Webb (Ed.), Annual Reports on NMR Spectroscopy, Academic Press, London, 1989, p. 209. J. Schaefer and E.O. Stejskal, in G.C. Levy (Ed.), Topics in Carbon-13 NMR Spectroscopy, Wiley-Interscience, New York, 1979, p. 284. I. Ando, T. Yamanobe, S. Akiyama, T. Komoto, H. Sam, T. Fujito, K. Deguchi and M. Imanari, Solid State Commun., 62 (1987) 785. T. Yamanobe, M. Tsukahara, T. Komoto, J. Watanabe, I. Ando, I. Uematsu, K. Deguchi, T. Fujito and I. Imanari, Macromolecules, 21 (1988) 48. M. Okabe, T. Yamanobe, T. Komoto, J. Watanabe, I. Ando and I. Uematsu, J. Mol. Struct., 213 (1989) 213. H. Tanaka, M.A. Gomez, A.E. Tonelli and M. Thakur, Macromolecules, 22 (1989) 1208. W.W. Fleming, C.A. Fyfe, R.D. Kendrick, J.R. Lyerla, H. Vanni and C.S. Yannoni, ACS Symp. Ser., 142 (1980) 193. Y. Takahashi and H. Tadokoro, Macromolecules, 6 (1973) 672. Y. Takahashi, I. Sumita and H. Tadokoro, J. Polym. Sci., Polym. Phys. Ed., 11 (1973) 2113. K. Hikichi, T. Shibata, A. Tsutsumi and J. Furuichi, J. Polym. Sci., A-2,6 (1968) 653. T.M. Connor and A. Hartland, J. Polym. Sci., Sect. A-2,7 (1969) 1005. R. Tabeta and H. Saito, Bull. Chem. Sot. Jpn., 58 (1985) 3215.
14!3
STRUCTURE AND MOLECULAR MOTION OF POLY(ETHYLENE OXIDE) IN THE SOLID STATE AS STUDIED BY VARIABLE-TEMPERATURE HIGH-RESOLUTION SOLIDSTATE 13C NMR
HIROMICHI
KUROSU
and ISA0 AND0
Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo (Japan) (Received
15 January
1990)
ABSTRACT High-resolution 13C NMR spectra of poly(ethylene oxide) in the solid state were measured between - 105.5”C and room temperature by the variable temperature i3C cross polarizationmagic angle spinning (CP-MAS) and pulse saturation transfer-magic angle spinning (PST-MAS) techniques. Based on these results, the temperature changes of the isotropic i3C chemical shift and the principal values of the 13C chemical shift tensor were used to elucidate dynamic and conformational aspects of poly (ethylene oxide) in the solid state.
INTRODUCTION
Very recently, much information about the molecular structure and dynamics of semicrystalline polymers has been provided by solid state 13CNMR [l41. Although the physical properties of polymers in the solid state are strongly affected by temperature, most of these studies has been carried out only at ambient temperature. Therefore, variable temperature NMR techniques should provide much useful data on the structural and dynamic aspects of polymers in the solid state [ 5-81. The advantage of variable temperature (VT) solidstate NMR techniques is that the temperature-dependent conformation and molecular motion can be studied by observing the isotropic chemical shift and chemical shift tensor and by using the cross polarization (CP) technique [ 91. Previously, from X-ray diffraction studies [ 10,111, it was reported that poly (ethylene oxide) (PEO) exists in two crystallographic forms; one is the monoclinic form (Type I) with (TTG), as the repeated unit (T and G stand for trans and gauche, respectively) and the other is the triclinic form (Type II) with an all-tram conformation. The former modification is very stable, but the latter is remarkably unstable and exists only under tension. As reported from broad-band NMR [ 121, PEO chains undergo rotational
0022-2860/90/$03.50
0 1990 -
Elsevier Science Publishers
B.V.
150
motion along the chain axis at room temperature. Because of the polymer’s fast motion and short TlpH [ 131, it is difficult to observe the 13C CP-MAS spectrum at room temperature. However, we can record 13C CP-MAS spectra at lower temperature because the molecular motion is then frozen, in the NMR time scale, and so the CP efficiency becomes high. The aim of the present work is to obtain precise information on the structure and dynamics of PEO in the solid state as a function of temperature by observing the isotropic 13C chemical shift and 13C chemical shift tensors by VT solid-state NMR. EXPERIMENTAL
Materials The PEO sample used had an average molecular weight of ca. 5000. It was obtained from Wako Pure Chemical Industries and was prepared by melt-crystallization. The melting point and glass transition temperature were about 60’ C and - 40’ C, respectively. NMR measurements The solid state 13C NMR spectra were recorded at 67.8 MHz with a JEOL GSX-270 NMR spectrometer, equipped with CP and VT accessories. CP-MAS NMR spectra were recorded with contact time of 2.0 or 0.15 ms during CP and the repetition time was 5 s. Samples were held in cylindrical rotors machined from zirconia and spun at 4.0-4.5 kHz. The magic angle (54.7” ) was set by increasing the symmetry and maximizing the intensity of the peak of the quarternary carbon atoms of hexamethylbenzene. The spectral width and number of data points were 27 kHz and 8 k, respectively. The spectra were usually accumulated 200-400 times to achieve a reasonable signal-to-noise ratio. The 13C NMR chemical shifts were calibrated indirectly using adamantane (29.5 ppm relative to tetramethysilane (TMS ) ) as external standard. RESULTS AND DISCUSSION
13C CP-MAS spectra The efficiency of the CP process is determined by the relative values of several relaxation times, such as TlpH, and the contact time. In the CP experiment, once the 13C signal has built up, it tends to be depleted by T1,H. If TlpH is very short, then the CP process can be short-circuited, preventing build-up of a 13C magnetization. Figure 1 shows 13C VT CP-MAS NMR spectra of PEO as a function of temperature. The solid-state spectrum at room temperature (ca. 24” C) cannot be observed under normal measurement conditions (con-
15:1
;;
100
RT
90
80
70
60
50
(wm)
Fig. 1. Variable temperature
13C CP-MAS
NMR spectra of poly (ethylene
oxide).
tact time (CT) = 2 ms) since PEO has a minimum in TlpH[111, due to molecular motion along the chain axis, at about this temperature. Such a short TlpH destroys proton ordering before CP of the carbon atoms has proceeded to a significant degree. However, the spectrum can be observed at CT = 150 pus,as shown in Fig. 1, although the line is rather broad because of molecular motion. Since the CP efficiency for complete polarization transfer depends on TlpH, which changes as a function of temperature, measurements below room temperature provide a better spectrum with relatively sharp peaks. At temperatures below - 27.5’ C, 13CCP-MAS spectra of PEO can be observed at CT = 2 ms, as shown in Fig. 1. At temperatures below -275°C the CP-MAS spectrum consists of three peaks, one major and two minor, as shown in Fig. 1. The 13CCP-MAS spectrum at - 105.5’ C clearly shows two minor peaks which appear upfield and downfield of the main peak, respectively. These three peaks are designated by I, II and III from downfield to upfield. The 13Cchemical shift values of these three peaks cannot be determined easily from the spectrum, so theoretical and ex-
152
perimental spectra were fitted by a computer-based non-linear least squares analysis (Fig. 2). The results are listed in Table 1. From an X-ray study [l41, it is known that the most stable crystal form is monoclinic, so the main peak (II) can be assigned to this form. It is obvious that the 13Cchemical shift
A OBSERVED
-___
THEORETICAL DECOMPOSE0
SPECTRUM SPECTRUM SPECTRUM
Fig. 2. Deconvolution of 13CCP-MAS NMR spectrum of poly(ethylene oxide) at - 105.5”C by computer-fitting with Lorentzian functions. TABLE 1 Observed i3C NMR chemical shifts, half-height widths and relativepeak intensities of poly (ethylene oxide) in the solid state at various temperatures” Temp. (“C)
RT -27.5 -66.5 - 105.5
Peak III
II
I fib
&2c
Peak intensityd
6
&I2
Peak intensity
6
61/Z
Peak intensity
80.0 75.1 74.4 74.4
16.0 2.2 1.1 1.0
29.9 7.3 10.3 6.7
71.6 72.2 72.6 72.6
12.0 2.8 1.0 1.0
70.1 92.7 66.0 66.6
71.7 71.6
1.0 1.0
23.1 26.7
“By the CP-MAS method. bChemical shift (ppm from TMS). “Half-height width (ppm). din percent.
153
values determined by the CP-MAS method are constant (72.6 ppm) from - 66.5 - 105.5 ‘C. At room temperature the peak width is very broad, possibly because of contributions from various polymer chain conformations and configurations. (By configurations we mean the spatial positions of the polymer chains, undergoing rotational motion, along the chain axis.) The peak width at half-height becomes smaller as the temperature is decreased, indicating that the distribution of conformations has become narrower, because the molecular motion is gradually frozen. Since the increase in molecular motion leads to a decrease in the C-H dipolar interactions, the efficiency of the CP process in the mobile region decreases and hence the CP-MAS experiment emphasises the peak intensity of the CH, carbons in the immobile phase. Therefore, it can be said that the main peak of PEO comes from the CH2 carbons in the phase in which the molecular motion is frozen at temperatures below - 66.5’ C. This will be clarified by subsequent relaxation time measurements. Pulse saturation transfer-MAS spectra In the pulse saturation transfer (PST)-MAS method, 13C magnetizations are enare induced by a single, direct 13C 90” pulse and the magnetizations hanced by saturated protons, through the nuclear Overhauser effect (NOE) (Fig. 3). Since the repetition time of the pulse is set at 5 s, the build-up of magnetization from the immobile phase is inadequate for efficient observation. Therefore, the 13C signal obtained by the PST-MAS method comes from the CH, carbons in the mobile phase which have relatively short TIC values. Figure 4 shows the VT 13C PST-MAS spectra of PEO as a function of temperature. The highest field peak (62 ppm) which does not appear in the 13C CP-MAS spectra is designated by IV. The 13C chemical shift values and the peak widths determined by computer-fitting are listed in Table 2. The main peak moves gradually downfield from 71.1 to 72.6 ppm as the temperature is decreased from room temperature to - 105.5’ C. The chemical shift value (72.6 ppm) of PEO at - 105.5 oC agrees with that of PEO determined by the 13C CP-
Fig. 3. Schematic representation of a pulse sequence in the pulse saturation transfer method.
154
100
80
90
70
60
50
(pm)
Fig. 4. Variable
temperature
i3C! PST-MAS
NMR
Observed i3C NMR chemical shifts, half-height ide) at various temperatures”
widths
spectra
of poly(ethylene
oxide)
in the solid
state. TABLE
Temp.
2 and relative peak intensities
of poly(ethylene
ox-
Peak
(“Cl II
I
RT -21.5 -66.5 - 105.5
IV
III
6b
8,,2c
Peak intensityd
6
6,,,
Peak intensity
6
6,,,
Peak intensity
6
ai/,
Peak intensity
74.5 75.4 74.3 74.2
5.0 4.0 1.0 1.0
15.9 2.4 2.2 9.7
71.1 71.9 72.4 72.6
4.8 3.7 1.8 1.2
38.2 73.0 66.7 77.9
67.0 68.5 71.0 71.2
6.0 5.0 3.0 1.0
20.5 14.8 31.1 12.3
61.9 62.0 -
4.2 5.0 -
13.4 9.9
“By the PST-MAS
method.
bChemical
shift (ppm from TMS 1. ‘Half-height
width
(ppml.
-
din per cent.
155’
[PPm) “I
r....I...‘I..“I”“I”“I”“,’ 100
90
80
70
80
70
60
50
40
30
(b)
(ppm r’,‘I
100
I,,
90
,‘,
‘,,I
/
,‘I
60
,/,I
50
,I,,
I .,
40
30
Fig. 5. 13Cpowder pattern of poly(ethylene oxide) in the solid state: (a) room temperature; (b) - 105.5”C. TABLE 3 Isotropic 13CNMR chemical shifts and chemical shift tensors of poly(ethylene oxide) (ppm from TMS)
%
Ull
&2
%I
011-0333
71.1 72.6
79 92
72 86
62 40
17 52
-
Room temperature - 105.5”C
MAS method at temperatures below - 66.5’ C. This result shows that the molecular motion is frozen, in the NMR time scale, at - 105.5 ‘C. The 13Cpowder patterns observed at room temperature and - 105.5” C are shown in Fig. 5. The averaged value of the three principal values of the shielding tensors determined by the computer simulation [ ( oll f az2 -t ~7~~)/3] is equal
156
‘H:
(j&d DELAY
TI h2
x
ACQ
I A,,
,3c-
V"-
Fig. 6. Schematic
representation
of a pulse sequence in the inversion-recovery
method.
to the isotropic chemical shift value (ai,,), determined from the 13CPST-MAS spectrum. The principal values of the shielding tensors and isotropic chemical shifts are listed in Table 3. The breadth of the powder pattern ( oll - ~7~~)at room temperature (17 ppm) is narrower than that at - 105.5”C (52 ppm), indicating that the restriction of molecular motion at low temperature increases the chemical shift anisotropy. If only rotational motion along the chain axis is assumed to exist at room temperature, it would be expected that the principal shielding tensor value which has its direction along the chain axis
A
0.6
0.8
-m I”~‘,““,““,~!, 100 90
80
,,~‘~~(I 60
70 (pm)
I, 50
40.0 8, 40
157
Delay Time 1 (xc)
(b)
1
I”“I”“I”“I”“I”’
100
90
80
70 (w-n)
60
50
Fig. 7. Y! inversion-recovery spectra of poly(ethylene NMR: (a) room temperature; (b) -105.5”C.
oxide) in the solid state by 13CPST-MAS
TABLE 4 13CT, values (s) for poly(ethylene
oxide) in the solid state
Peak
Room temperature - 105.5”C
I
II
III
4
3 17
5 9
would become constant as the temperature changes. However, none of the three principal values is the same at room temperature as it is at - 1055°C. This indicates that the molecular motion of PEO at room temperature is not restricted to rotational motion along the chain axis but includes other motions such as librational rotation of the chain axis. TIC measurements TIC measurements of PEO were carried out at room temperature and - 105.5 oC by the inversion-recovery method. The pulse sequence used is shown
158
in Fig. 6 and is similar to that of the 13CPST-MAS method except for insertion of a x pulse and a delay time. Figure 7 shows inversion-recovery spectra as a function of the delay time z. The intensities of the main and two minor peaks were obtained by computer-fitting, and the Tic values determined for all three peaks at room temperature and for the main peak and one minor peak at - 1055°C are listed in Table 4. These Tic values are subject to some error because the PST-MAS spectra include background signals from the NMRprobe and because of the error inherent in the analysis of sharp lines. It is difficult to calculate a Tic value for the minor peak at lower field at - 105.5’ C because the intensity of the peak is small. However, we can estimate the magnitude of its molecular motion because the measurements also show that the CH, carbons contributing to the main peak and to the two minor peaks have TICvalues over 9 s at - 105.5’ C. This shows that the mobilities of the two minor peaks are almost the same as that of the main peak at room temperature, indicating that the two minor peaks do not originate from the non-crystalline phase. Tabeta and Saito [ 141 reported that the 13C chemical shifts of ethylene oxide oligomers and PEO complexed with HgCl, or CdCl, depend on the dihedral angle. According to their results, the two minor peaks must come from crystallographic forms having conformations different from the (TTG), conformation of the monoclinic form. As seen from Fig. 7, the intensity of the main peak at room temperature decreases as the delay time z is decreased and at r=0.2 s the main peak is inverted. However, the intensity of peak IV inverts at r= 0.05 s, so its TICvalue is shorter than that of the main peak. This means that peak IV comes from the CH, carbons in the non-crystalline phase. In order to eludicate the origin of such chemical shifts and chemical shift anisotropy and to reveal the effect of molecular motions on 13C chemical shift behaviour, quantum chemical calculations using the tight-binding molecular orbital method have been carried out on all seven PEO chains model, taking into account the molecular packing and the molecular motion. Results of such calculations will be published soon.
CONCLUSIONS
Measurements of the temperature-dependent 13Cchemical shifts for the main peak in the 13C NMR spectra of PEO show that, at room temperature, the polymer chains are in the monoclinic form with the (TTG), conformation. Besides the main peak, which comes from the monoclinic form, two minor peaks which may come from other crystallographic forms were observed. A fourth peak was assigned to the non-crystalline phase.
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