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A structural and dynamic study of poly(vinyl alcohol) in the gel state by solid-state 13C NMR and 1H pulse NMR
Journal of
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular Structure 447 (1998) 49-59
A structural and dynamic study of poly(vinyl alcohol) in the gel state by solid-state 13C NMR and 1Hpulse NMR Masahito Kanekiyo a, Masatoshi Kobayashi a, Isao Ando a'*, Hiromichi Kurosu h, Takahiro Ishii c, Shigetoshi Amiya c ~Department of Polymer Chemistry, Tokyo Institute of Technology, Okayama, Meguro-ku, Tokyo, Japan ~Department of Textile and Apparel Science, Nara Women's University, Kitauoya-Nishimachi, Nara, Japan CAnalytical Research Center, Kuraray Co., Ltd., Kurashiki, Okayama, Japan
Received 20 November 1997; accepted 24 December 1997
Abstract 1H pulse NMR and high-resolution solid-state n3C NMR spectra of poty(vinyl alcohol) (PVA) gel were measured to clarify the structure and dynamics of the immobile component of the gel. From IH pulse NMR experiments, it was found that the T: signal is mainly composed of three components. The long T2 component was assigned to the mobile component which comes from the noncrosslinked region, the intermediate T2 component corresponds to the intermediate immobile component which comes from the vicinity of the crosslinked region, and the short T2 component corresponds to the immobile component which comes from the crosslinked region in the PVA gel. From high-resolution solid-state ~3C NMR experiments, the mechanism of gel formation was satisfactorily elucidated. Further, dynamic viscoelastic modulus measurements were carried out. These experimental results were satisfactorily explained on the basis of the NMR results. © 1998 Elsevier Science B.V. All rights reserved Keywords: Solid state NMR spectroscopy; Poly(vinyl alcohol) gel; Dynamics; Shear viscosity
I. Introduction It is known that poly(vinyl alcohol) (PVA) in aqueous solution forms a gel by undergoing freezethaw cycles which lead to the formation of intermolecular hydrogen bonds between the different chains [1]. From high-resolution solid-state 13C NMR experiments on PVA gels [2,3], it has been demonstrated that there are two components of the gel with different molecular motions, and further that the CH carbon in the slow motion region provides three split signals like the case of solid PVA [4], which come * Corresponding author.
from the CH carbon with two hydrogen bonds, one hydrogen bond and no hydrogen bond. It might be expected that the amount of intermolecular hydrogen bonding affects the structure and dynamics of PVA gel. However, at this stage inconclusive results on the structure and dynamics of PVA gel system are obtained because the system is complicated by changes in the concentration of the polymer. As a continuation of previous work [2,3], we have tried to elucidate the structure and dynamics of the PVA gel system by means of 1H pulse NMR, highresolution solid-state Lac NMR, and dynamic the viscoelasticity method. Further, the mechanism of gel formation will be discussed.
0022-2860/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PH S0022-2860(98)00300-7
50
M. Kanekiyo et al./Journal of Molecular Structure 447 (1998) 49-59
2. Experimental section 2.1. Preparation of PVA gel Atactic PVA used in this work was supplied by Kuraray Co. Ltd. The fractions of mm, mr and rr triads evaluated from the 13C NMR spectra of PVA in deuterated dimethylsulfoxide (DMSO) solution are 0.29, 0.44 and 0.27, respectively, where m and r indicate meso and racemic diads, respectively. The degree of polymerization and the degree of saponification are 1700 and 99.9% respectively. For IH pulse NMR experiments, PVA with deuterated hydroxyl groups (d-PVA) was prepared by deuterium exchange between hydroxyl proton and deuteron in PVA and D20 by dissolving PVA in D20. By adding PVA/ D20 solution to excess acetonitrile, PVA with the hydroxyl proton substituted by deuterium was precipitated. The precipitated d-PVA was dried under vacuum. The d-PVA obtained was re-dissolved in D20 and the procedure repeated three times. The gel was prepared from aqueous PVA or d-PVA solution by repeated freeze-thaw cycles. The solution was frozen at - 20°C for 20 h, and then melted at 25°C and kept for 4 h. This cycle was repeated four times. PVA gel samples with different polymer concentrations were prepared by evaporation of the water. Also, the number of freeze-thaw cycles of PVA/D20 solution was changed and so gels were obtained.
accumulated 200-1000 times to achieve a reasonable signal-to-noise ratio. The rotor was spun at 3.84 kHz. A cylinder-type rotor with a rubber O-ring is used to prevent evaporation of water from the gel during the experiments. 13C chemical shifts were calibrated indirectly through the upfield peak (29.5 ppm) of adamantane relative to tetramethylsilane (TMS). H pulse NMR measurements were carried out with a Bruker Minispec PC20 spectrometer operating at 20 MHz. The pulse sequences used for measurement of long and short IH T2s (spin-spin relaxation time) were the CPMG (Carr-Purcell-Meiboom-Gill) [5,6] and solid-echo [7,8] methods, respectively. The decomposition of the T2 signal into components with different molecular motion was carried out by the nonlinear least squares method with an NEC PC9801 microcomputer.
2.3. Dynamic viscoelasticity measurements Dynamic viscoelasticity measurements on five kinds of hard gel sample with different water content were carried out by means of a tension module DMS210 Seiko Instrument; those on soft gel samples were carried out by means of a Rheometrix mechanical spectrometer.
3. Results and discussion 3.1. The structure and dynamics of PVA gel
2.2. High-resolution solid-state pulse NMR measurements
13C NMR and IH
High-resolution solid-state 13C CP/MAS (cross polarization/magic angle spinning) and 13C PST/ MAS (pulse saturation transfer/magic angle spinning) NMR spectra were measured by means of a Jeol GSX270 NMR spectrometer operating at 67.8 MHz. In the CP method, enhancement of ~3C magnetization is effective for relatively immobile components such as solids; in the PST method, on the other hand, the nuclear Overhauser effect (NOE) enhances 13C magnetization in the mobile components of gels. The r/2 pulse widths for lH and 13C nuclei were 4.5 and 4.6/~s, respectively. The contact time was 2 ms, and the repetition time was 5 s. The spectral width was 27 kHz and 8 k data points were taken. Spectra were
The J3C PST/MAS and J3C CP/MAS NMR spectra of PVA gel samples as a function of PVA concentration are shown in Figs. 1 and 2, respectively. The spectral assignments were made as described elsewhere [2]. As seen from Fig. 1, in 13C PST/MAS spectra at low polymer concentrations the mm, mr and rr peaks for the CH carbon appear in the 6 5 - 6 9 ppm region. Because these signals correspond to those in the high-resolution solution-state spectrum, it is apparent that they arise from the mobile component. As the polymer concentration is increased, the triad peaks become weak and then disappear. At high polymer concentrations, the broad peaks assigned to peaks I, II and III from downfield as reported previously [2] appear at about 77, 71 and 65 ppm, respectively. Peak I is assigned as arising
M. Kanekiyo et al./Journal of Molecular Structure 447 (1998) 49-59
from the formation of two intramolecular or intermolecular hydrogen-bonds, peak II from the formation of one intramolecular or intermolecular hydrogen-bond, and peak III from no hydrogen bond. On the other hand, in the 13C CP/MAS NMR spectra (Fig. 2), peaks I, II and III for the CH carbon are prominent. At low polymer concentrations, the triad peaks are barely apparent. Therefore, it can be said that peaks I, II and III come from the immobile component. This shows that the total fraction of the immobile component increases as the concentration of polymer is increased. The fraction of these peaks
51
was determined by decomposing into individual peaks by computer fitting, as shown in Fig. 3. The fraction of peak III is independent of polymer concentration. As the polymer concentration is increased, the fractions of peaks I and II increase and decrease, respectively. From these results it can be said that the crosslinking region in the gel increases with increasing concentration of polymer. According to BPP theory [9], T2 decreases continuously as the correction time rc in molecular motion is increased. Thus, T2 can provide dynamic information about the gels through the BPP theory. A typical t H T2
CH II
CH2 Ill
PVA concentration /wt% 81.3
76.3
36.1
27.4
~
I00
,
~
90
¢
x
~
BO
"' ~ v ~ j /
70
60
~
5o
12.0
40
30
20
Fig. 1.67.8 MHz 13C PST/MAS NMR spectra of PVA gel as a function of PVA concentration.
M. Kanekiyo et al./Journal of Molecular Structure 447 (1998) 49-59
52
signal of PVA gel is shown in Fig. 4. IH T2 values are determined from the slope of the plot of In M against time t, where M is the amplitude of the spin-echo signal. This IH T2 curve is composed of three components: a long T2 component corresponding to the mobile component (component C); a short T2 component corresponding to the immobile component (component A); and an intermediate T2 component corresponding to the interfacial component (component B) which exists between the mobile and immobile components. By use of computer fitting the JH T2 value was obtained from the T2 signal with small experimental error.
The ]H T 2 values obtained for the three components were plotted against polymer concentration as shown in Fig. 5. With increasing polymer concentration, the T2 values slowly decrease. This means that the molecular motion of the three components is restrained as polymer concentration increases. Fig. 6 shows the fractions of the three components as a function of polymer concentration. The fractions of the mobile and immobile components, respectively, decrease and increase substantially as the polymer concentration is increased. The fraction of the intermediate component over a wide range of polymer concentrations is very small.
CH
CH 2
III
PVA concentration /wt%
_
76.3
36.1
1
I
I00
r
¢
'
'
I
90
Fig. 2. 67.8 MHz
' ' '
f
I
' ' ' '
80
13CCP/MAS
I
70
*
~
~
'
I
60
~
'
''
I ' ' '
50
~
'
I
40
27.4
' ' ' '
l '
30
'"
'
I
20
NMR spectra of PVA gel as a function of PVA concentration.
M. Kanekiyo et al./Journal of Molecular Structure 447 (1998) 49-59
53
1
0.8
0.6 m
0.4
m
m
•
=
•
-w
0.2 A
A
I
v
0
0
i
I
i
i
20
40
60
80
100
PVA concentration / wt %
Fig. 3. Plots of the fraction for the components I, II and III in PVA gel, as determined by 13C CP/MAS NMR, against PVA concentration. Fig. 7 shows plots of shear viscosity E' against frequency for PVA gels at various polymer concentrations. It is apparent that E' increases slowly as frequency is increased but, on the other hand, increases substantially as the PVA concentration is increased.
This means that the mechanical properties of the PVA gel are highly dependent on polymer concentration. The PVA gel becomes much harder with increasing polymer concentration, and the frequency dependence of the gel is quite large at lower concentrations.
C
0
I
i
i
i
0.1
0.2
0.3
0,4
0.5
time / ms
Fig. 4. I H T2 signal of PVA gel for a PVA concentration of 27.4% w/w by the solid-echo method.
54
M. Kanekiyo et al./Journal of Molecular Structure 447 (1998) 49-59 1000
100
C
u~
B :E
8
10
1
0
•
--
_
I
I
I
I
20
40
60
80
A
100
PVA concentration / wt %
Fig. 5. Plots of ~H T~, as determined by tH pulse NMR signal decay, against polymer concentration. 0, component (A) (immobile); II, component (B) (interfacial); A, component (C) (mobile).
The fraction of each of the three components determined by pulse 1H NMR is plotted against the E' value in Fig. 8. It is shown that the vale of E' is highly dependent on the fraction. The fraction of component (C), that with fast molecular motion,
decreases with increasing E' value, and the fractions of component (A), that with slow molecular motion, and of component (B), that with intermediate molecular motion, increase as the E' value increases.
0.8
A
0.6
& 0.4
0.2
B
C 0
J 20
~
~
4O
60
~A 8O
100
PVA concentration / wt %
Fig. 6. The relationship between the fraction of the three PVA chain components of the gel with different molecular motion and polymer concentration. 0, component (A) (immobile); II, component (B) (interfacial); A, component (C) (mobile).
55
M. Kanekiyo et al./Journal of Molecular Structure 447 (1998) 49-59 109 81.30 %
10 e
---'
•
_----
_
76.34 % 107
r,,
36.10 % 106
_- -
=
_- -
-"
----_.
_.
,-""
=
--
27.40 % 10 5
1 o' 0.001
i
i
i
i
0.01
0.1
1
10
100
frequency / Hz
Fig. 7. Plots of shear viscosity, E', of PVA gel against measurement frequency,f, at 25°C. O, polymer concentration 27.40% w/w; II, polymer concentration 36.10% w/w; A, polymer concentration 76.34% w/w; x, polymer concentration 81.30% w/w. samples is constant in the frequency range 0.1 to 100 rad s -~. On the other hand, the E' value increases as the number of f r e e z e - t h a w cycles is increased. This shows that the f r e e z e - t h a w process leads to the formation of hard gel and implies that the fraction of
3.2. Formation mechanism o f PVA gel
Fig. 9 shows plots of the shear viscosity E' against frequency for P V A gels with different numbers of f r e e z e - t h a w cycles. The E' value for each of the gel 10 9
108
•
0 A
10 7
I
•
h,
1°'//1
11Oo i/"L 0
0.2
0.4
0.6
0.8
1.0
fraction Fig. 8. The relationship between the fraction of the three IH T2 components and dynamic viscoelasticityE': 0, component (A) (immobile); II, component (B) (interfacia]); A, component (C) (mobile).
56
M. Kanekiyo et aL/Journal of Molecular Structure 447 (1998) 49-59 104s
n=6
AA AA
~. 10 4
10 35
0.01
~1~1~1i.I l l I Iii I IIi ~1, i I I OIDOIDOtOOIDOl OtlOID
J
0.1
t
I
a
1
10
100
°:2 n=l
1000
frequency / Rad/s
Fig. 9. The relationship between dynamic viscoelasticity E' and the measurement frequency: 0, one freeze-thaw cycle; I , two freeze-thaw cycles; A, six freeze-thaw cycles.
the microcrystalline structure of PVA increases as the number of freeze-thaw cycles is increased. Therefore, PVA gel can be made more rigid repeated freezethaw cycles. Fig. 10 shows high-resolution solid-state ~3C CP/ MAS NMR spectra of PVA gel with different numbers of freeze-thaw cycles. The intensities of peaks I, II and III for the CH carbon increase as the number of freeze-thaw cycles is increased. The change of these peak intensities shows that the amount of the crosslinking region which is formed by hydrogen bonding increases with increasing numbers of freeze-thaw cycles, and that the molecular motion is sufficiently strongly restrained to enhance peaks I, II and III by an increase in the CP efficiency. The relative intensities of peaks I, II and III do not change regardless of an increase in the number of freeze-thaw cycles (Fig. 11). The relative rates for the formation of the individual CH carbons corresponding to peaks I, II and III in the gel are similar to each other regardless of the number of freeze-thaw cycles. Fig. 12 shows plots of l H T2s against the number of freeze-thaw cycles for each of the components (A), (B) and (C). Their IH T2 values decrease as the number of freeze-thaw cycles is increased. This shows that the
increase in the number of freeze-thaw cycles leads to a slight reduction in their mobilities. As the number of freeze-thaw cycles increases, the fractions of the immobile component (A) and the intermediate component (B) increase and decrease, respectively (Fig. 13). The fraction of the immobile component (C) is decreases slightly. This shows that the intermediate component (B) is substantially affected by an increase in the number of freeze-thaw cycles, and this leads to a change in the amount of immobile component (A). As shown in Figs. 9, and 13, it can be said that the increase in the fraction of the immobile component (A) and the decrease of the fractions of the intermediate and mobile components (B) and (C) lead to an increase in the shear viscosity E'. Therefore, the increase in the shear viscosity E' is closely related to the fraction of the crosslinking region.
4. Conclusion It can be concluded from ]H pulse NMR and highresolution solid-state 13C NMR experiments that there are three kinds of component in PVA gel, the mobile, immobile and intermediate components, which result
57
M. Kanekiyo et al./Journal of Molecular Structure 447 (1998) 49-59
CH
CH 2
III
r \
The number of freeze~kx
thaw cycles
n=2 IT mr
~A
n=l
I ' ' ' '
I ' ' ' '
100
I
90
B0
I
'
'
'
I ' ' ' '
70
I ' ' ' '
60
I
' ' ' '
I'
50
' ' '
I
40
'
30
'
'
'
I
20
Fig. 10.67.8 MHz 13C CP/MAS NMR spectra of PVA gel as a function of the number of freeze-thaw cycles: (A), one freeze-thaw cycle; (B), two freeze-thaw cycles; (C), six freeze-thaw cycles.
0.8
0.6
g
II 0.4 •
III
0.2 A
w
0
i
0
1
2
3
4
number of freeze-thaw
5
6
cycles
Fig. 1 I. Plots of the fraction for the peaks I, II and Ill in PVA gel against the number of freeze-thaw cycles as determined by ~3C CP/MAS NMR.
M. Kanekiyo et al./JournalofMolecular Structure 447(1998)49-59
58
10000
I000
•
.I.
•
A_
m,
C
•
--
--
"
B
w
I
•
A
100 l-"r"
|0
0
4
1
5
6
2number o~ freeze-thaw cycles
7
Fig. 12. Plots of sH T2 for PVA gel against the number of freeze-thaw cycles as determined by ~H pulse NMR signal decay. O, component (A) (immobile); II, component (B) (interfacial); A, component (C) (mobile).
0.8
0.6
d= 0.4 •
A
0.2
•
•
3
4
•
•
C
A
_
0 0
1
2
5
6
number of freeze-thaw cycles Fig. 13. Plots of the fractions of the three components of PVA gel against the number of freeze-thaw cycles as determined by t H pulse NMR signal decay. O, component (A) (immobile); II, component (B) (interfacial); A, component (C) (mobile).
M. Kanekiyo et al./Journal of Molecular Structure 447 (1998) 49-59
from the formation o f h y d r o g e n bonds, and the structures and d y n a m i c s o f these c o m p o n e n t s are closely related to m e c h a n i c a l properties such as the shear viscosity E ' . Further, the m e c h a n i s m o f P V A gel formation by f r e e z e - t h a w cycles was elucidated.
References [1] A. Takahashi, S. Hiramatsu, Polymer J. 6 (1974) 103.
59
[2] M. Kobayashi, i. Ando, T. Ishii, S. Amiya, Macromolecules 28 (1995) 6677. [3] M. Kobayashi, I. Ando, T. Ishii, S. Amiya, J. Mol. Struct. 440 (1998) 155. [4] T. Terao, S. Maeda, A. Saika, Macromolecules 16 (1983) 1535. [5] Y.H. Carr, E.M. Purcell, Phys. Rev. 94 (1954) 630. [6] G. Meiboom, D. Gill, Rev. Sci. Instr. 29 (1958) 588. [71 J.G. Powles, J.H. Strange, Proc. Phys. Soc. 82 (1963) 6. [8] P. Mansfield, Phys. Rev. 80 (1950) 580. [9] N. Bloembergen, E.M. Purcell, R.V. Pound, Physical Review 73 (1948) 679.
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular Structure 447 (1998) 49-59
A structural and dynamic study of poly(vinyl alcohol) in the gel state by solid-state 13C NMR and 1Hpulse NMR Masahito Kanekiyo a, Masatoshi Kobayashi a, Isao Ando a'*, Hiromichi Kurosu h, Takahiro Ishii c, Shigetoshi Amiya c ~Department of Polymer Chemistry, Tokyo Institute of Technology, Okayama, Meguro-ku, Tokyo, Japan ~Department of Textile and Apparel Science, Nara Women's University, Kitauoya-Nishimachi, Nara, Japan CAnalytical Research Center, Kuraray Co., Ltd., Kurashiki, Okayama, Japan
Received 20 November 1997; accepted 24 December 1997
Abstract 1H pulse NMR and high-resolution solid-state n3C NMR spectra of poty(vinyl alcohol) (PVA) gel were measured to clarify the structure and dynamics of the immobile component of the gel. From IH pulse NMR experiments, it was found that the T: signal is mainly composed of three components. The long T2 component was assigned to the mobile component which comes from the noncrosslinked region, the intermediate T2 component corresponds to the intermediate immobile component which comes from the vicinity of the crosslinked region, and the short T2 component corresponds to the immobile component which comes from the crosslinked region in the PVA gel. From high-resolution solid-state ~3C NMR experiments, the mechanism of gel formation was satisfactorily elucidated. Further, dynamic viscoelastic modulus measurements were carried out. These experimental results were satisfactorily explained on the basis of the NMR results. © 1998 Elsevier Science B.V. All rights reserved Keywords: Solid state NMR spectroscopy; Poly(vinyl alcohol) gel; Dynamics; Shear viscosity
I. Introduction It is known that poly(vinyl alcohol) (PVA) in aqueous solution forms a gel by undergoing freezethaw cycles which lead to the formation of intermolecular hydrogen bonds between the different chains [1]. From high-resolution solid-state 13C NMR experiments on PVA gels [2,3], it has been demonstrated that there are two components of the gel with different molecular motions, and further that the CH carbon in the slow motion region provides three split signals like the case of solid PVA [4], which come * Corresponding author.
from the CH carbon with two hydrogen bonds, one hydrogen bond and no hydrogen bond. It might be expected that the amount of intermolecular hydrogen bonding affects the structure and dynamics of PVA gel. However, at this stage inconclusive results on the structure and dynamics of PVA gel system are obtained because the system is complicated by changes in the concentration of the polymer. As a continuation of previous work [2,3], we have tried to elucidate the structure and dynamics of the PVA gel system by means of 1H pulse NMR, highresolution solid-state Lac NMR, and dynamic the viscoelasticity method. Further, the mechanism of gel formation will be discussed.
0022-2860/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PH S0022-2860(98)00300-7
50
M. Kanekiyo et al./Journal of Molecular Structure 447 (1998) 49-59
2. Experimental section 2.1. Preparation of PVA gel Atactic PVA used in this work was supplied by Kuraray Co. Ltd. The fractions of mm, mr and rr triads evaluated from the 13C NMR spectra of PVA in deuterated dimethylsulfoxide (DMSO) solution are 0.29, 0.44 and 0.27, respectively, where m and r indicate meso and racemic diads, respectively. The degree of polymerization and the degree of saponification are 1700 and 99.9% respectively. For IH pulse NMR experiments, PVA with deuterated hydroxyl groups (d-PVA) was prepared by deuterium exchange between hydroxyl proton and deuteron in PVA and D20 by dissolving PVA in D20. By adding PVA/ D20 solution to excess acetonitrile, PVA with the hydroxyl proton substituted by deuterium was precipitated. The precipitated d-PVA was dried under vacuum. The d-PVA obtained was re-dissolved in D20 and the procedure repeated three times. The gel was prepared from aqueous PVA or d-PVA solution by repeated freeze-thaw cycles. The solution was frozen at - 20°C for 20 h, and then melted at 25°C and kept for 4 h. This cycle was repeated four times. PVA gel samples with different polymer concentrations were prepared by evaporation of the water. Also, the number of freeze-thaw cycles of PVA/D20 solution was changed and so gels were obtained.
accumulated 200-1000 times to achieve a reasonable signal-to-noise ratio. The rotor was spun at 3.84 kHz. A cylinder-type rotor with a rubber O-ring is used to prevent evaporation of water from the gel during the experiments. 13C chemical shifts were calibrated indirectly through the upfield peak (29.5 ppm) of adamantane relative to tetramethylsilane (TMS). H pulse NMR measurements were carried out with a Bruker Minispec PC20 spectrometer operating at 20 MHz. The pulse sequences used for measurement of long and short IH T2s (spin-spin relaxation time) were the CPMG (Carr-Purcell-Meiboom-Gill) [5,6] and solid-echo [7,8] methods, respectively. The decomposition of the T2 signal into components with different molecular motion was carried out by the nonlinear least squares method with an NEC PC9801 microcomputer.
2.3. Dynamic viscoelasticity measurements Dynamic viscoelasticity measurements on five kinds of hard gel sample with different water content were carried out by means of a tension module DMS210 Seiko Instrument; those on soft gel samples were carried out by means of a Rheometrix mechanical spectrometer.
3. Results and discussion 3.1. The structure and dynamics of PVA gel
2.2. High-resolution solid-state pulse NMR measurements
13C NMR and IH
High-resolution solid-state 13C CP/MAS (cross polarization/magic angle spinning) and 13C PST/ MAS (pulse saturation transfer/magic angle spinning) NMR spectra were measured by means of a Jeol GSX270 NMR spectrometer operating at 67.8 MHz. In the CP method, enhancement of ~3C magnetization is effective for relatively immobile components such as solids; in the PST method, on the other hand, the nuclear Overhauser effect (NOE) enhances 13C magnetization in the mobile components of gels. The r/2 pulse widths for lH and 13C nuclei were 4.5 and 4.6/~s, respectively. The contact time was 2 ms, and the repetition time was 5 s. The spectral width was 27 kHz and 8 k data points were taken. Spectra were
The J3C PST/MAS and J3C CP/MAS NMR spectra of PVA gel samples as a function of PVA concentration are shown in Figs. 1 and 2, respectively. The spectral assignments were made as described elsewhere [2]. As seen from Fig. 1, in 13C PST/MAS spectra at low polymer concentrations the mm, mr and rr peaks for the CH carbon appear in the 6 5 - 6 9 ppm region. Because these signals correspond to those in the high-resolution solution-state spectrum, it is apparent that they arise from the mobile component. As the polymer concentration is increased, the triad peaks become weak and then disappear. At high polymer concentrations, the broad peaks assigned to peaks I, II and III from downfield as reported previously [2] appear at about 77, 71 and 65 ppm, respectively. Peak I is assigned as arising
M. Kanekiyo et al./Journal of Molecular Structure 447 (1998) 49-59
from the formation of two intramolecular or intermolecular hydrogen-bonds, peak II from the formation of one intramolecular or intermolecular hydrogen-bond, and peak III from no hydrogen bond. On the other hand, in the 13C CP/MAS NMR spectra (Fig. 2), peaks I, II and III for the CH carbon are prominent. At low polymer concentrations, the triad peaks are barely apparent. Therefore, it can be said that peaks I, II and III come from the immobile component. This shows that the total fraction of the immobile component increases as the concentration of polymer is increased. The fraction of these peaks
51
was determined by decomposing into individual peaks by computer fitting, as shown in Fig. 3. The fraction of peak III is independent of polymer concentration. As the polymer concentration is increased, the fractions of peaks I and II increase and decrease, respectively. From these results it can be said that the crosslinking region in the gel increases with increasing concentration of polymer. According to BPP theory [9], T2 decreases continuously as the correction time rc in molecular motion is increased. Thus, T2 can provide dynamic information about the gels through the BPP theory. A typical t H T2
CH II
CH2 Ill
PVA concentration /wt% 81.3
76.3
36.1
27.4
~
I00
,
~
90
¢
x
~
BO
"' ~ v ~ j /
70
60
~
5o
12.0
40
30
20
Fig. 1.67.8 MHz 13C PST/MAS NMR spectra of PVA gel as a function of PVA concentration.
M. Kanekiyo et al./Journal of Molecular Structure 447 (1998) 49-59
52
signal of PVA gel is shown in Fig. 4. IH T2 values are determined from the slope of the plot of In M against time t, where M is the amplitude of the spin-echo signal. This IH T2 curve is composed of three components: a long T2 component corresponding to the mobile component (component C); a short T2 component corresponding to the immobile component (component A); and an intermediate T2 component corresponding to the interfacial component (component B) which exists between the mobile and immobile components. By use of computer fitting the JH T2 value was obtained from the T2 signal with small experimental error.
The ]H T 2 values obtained for the three components were plotted against polymer concentration as shown in Fig. 5. With increasing polymer concentration, the T2 values slowly decrease. This means that the molecular motion of the three components is restrained as polymer concentration increases. Fig. 6 shows the fractions of the three components as a function of polymer concentration. The fractions of the mobile and immobile components, respectively, decrease and increase substantially as the polymer concentration is increased. The fraction of the intermediate component over a wide range of polymer concentrations is very small.
CH
CH 2
III
PVA concentration /wt%
_
76.3
36.1
1
I
I00
r
¢
'
'
I
90
Fig. 2. 67.8 MHz
' ' '
f
I
' ' ' '
80
13CCP/MAS
I
70
*
~
~
'
I
60
~
'
''
I ' ' '
50
~
'
I
40
27.4
' ' ' '
l '
30
'"
'
I
20
NMR spectra of PVA gel as a function of PVA concentration.
M. Kanekiyo et al./Journal of Molecular Structure 447 (1998) 49-59
53
1
0.8
0.6 m
0.4
m
m
•
=
•
-w
0.2 A
A
I
v
0
0
i
I
i
i
20
40
60
80
100
PVA concentration / wt %
Fig. 3. Plots of the fraction for the components I, II and III in PVA gel, as determined by 13C CP/MAS NMR, against PVA concentration. Fig. 7 shows plots of shear viscosity E' against frequency for PVA gels at various polymer concentrations. It is apparent that E' increases slowly as frequency is increased but, on the other hand, increases substantially as the PVA concentration is increased.
This means that the mechanical properties of the PVA gel are highly dependent on polymer concentration. The PVA gel becomes much harder with increasing polymer concentration, and the frequency dependence of the gel is quite large at lower concentrations.
C
0
I
i
i
i
0.1
0.2
0.3
0,4
0.5
time / ms
Fig. 4. I H T2 signal of PVA gel for a PVA concentration of 27.4% w/w by the solid-echo method.
54
M. Kanekiyo et al./Journal of Molecular Structure 447 (1998) 49-59 1000
100
C
u~
B :E
8
10
1
0
•
--
_
I
I
I
I
20
40
60
80
A
100
PVA concentration / wt %
Fig. 5. Plots of ~H T~, as determined by tH pulse NMR signal decay, against polymer concentration. 0, component (A) (immobile); II, component (B) (interfacial); A, component (C) (mobile).
The fraction of each of the three components determined by pulse 1H NMR is plotted against the E' value in Fig. 8. It is shown that the vale of E' is highly dependent on the fraction. The fraction of component (C), that with fast molecular motion,
decreases with increasing E' value, and the fractions of component (A), that with slow molecular motion, and of component (B), that with intermediate molecular motion, increase as the E' value increases.
0.8
A
0.6
& 0.4
0.2
B
C 0
J 20
~
~
4O
60
~A 8O
100
PVA concentration / wt %
Fig. 6. The relationship between the fraction of the three PVA chain components of the gel with different molecular motion and polymer concentration. 0, component (A) (immobile); II, component (B) (interfacial); A, component (C) (mobile).
55
M. Kanekiyo et al./Journal of Molecular Structure 447 (1998) 49-59 109 81.30 %
10 e
---'
•
_----
_
76.34 % 107
r,,
36.10 % 106
_- -
=
_- -
-"
----_.
_.
,-""
=
--
27.40 % 10 5
1 o' 0.001
i
i
i
i
0.01
0.1
1
10
100
frequency / Hz
Fig. 7. Plots of shear viscosity, E', of PVA gel against measurement frequency,f, at 25°C. O, polymer concentration 27.40% w/w; II, polymer concentration 36.10% w/w; A, polymer concentration 76.34% w/w; x, polymer concentration 81.30% w/w. samples is constant in the frequency range 0.1 to 100 rad s -~. On the other hand, the E' value increases as the number of f r e e z e - t h a w cycles is increased. This shows that the f r e e z e - t h a w process leads to the formation of hard gel and implies that the fraction of
3.2. Formation mechanism o f PVA gel
Fig. 9 shows plots of the shear viscosity E' against frequency for P V A gels with different numbers of f r e e z e - t h a w cycles. The E' value for each of the gel 10 9
108
•
0 A
10 7
I
•
h,
1°'//1
11Oo i/"L 0
0.2
0.4
0.6
0.8
1.0
fraction Fig. 8. The relationship between the fraction of the three IH T2 components and dynamic viscoelasticityE': 0, component (A) (immobile); II, component (B) (interfacia]); A, component (C) (mobile).
56
M. Kanekiyo et aL/Journal of Molecular Structure 447 (1998) 49-59 104s
n=6
AA AA
~. 10 4
10 35
0.01
~1~1~1i.I l l I Iii I IIi ~1, i I I OIDOIDOtOOIDOl OtlOID
J
0.1
t
I
a
1
10
100
°:2 n=l
1000
frequency / Rad/s
Fig. 9. The relationship between dynamic viscoelasticity E' and the measurement frequency: 0, one freeze-thaw cycle; I , two freeze-thaw cycles; A, six freeze-thaw cycles.
the microcrystalline structure of PVA increases as the number of freeze-thaw cycles is increased. Therefore, PVA gel can be made more rigid repeated freezethaw cycles. Fig. 10 shows high-resolution solid-state ~3C CP/ MAS NMR spectra of PVA gel with different numbers of freeze-thaw cycles. The intensities of peaks I, II and III for the CH carbon increase as the number of freeze-thaw cycles is increased. The change of these peak intensities shows that the amount of the crosslinking region which is formed by hydrogen bonding increases with increasing numbers of freeze-thaw cycles, and that the molecular motion is sufficiently strongly restrained to enhance peaks I, II and III by an increase in the CP efficiency. The relative intensities of peaks I, II and III do not change regardless of an increase in the number of freeze-thaw cycles (Fig. 11). The relative rates for the formation of the individual CH carbons corresponding to peaks I, II and III in the gel are similar to each other regardless of the number of freeze-thaw cycles. Fig. 12 shows plots of l H T2s against the number of freeze-thaw cycles for each of the components (A), (B) and (C). Their IH T2 values decrease as the number of freeze-thaw cycles is increased. This shows that the
increase in the number of freeze-thaw cycles leads to a slight reduction in their mobilities. As the number of freeze-thaw cycles increases, the fractions of the immobile component (A) and the intermediate component (B) increase and decrease, respectively (Fig. 13). The fraction of the immobile component (C) is decreases slightly. This shows that the intermediate component (B) is substantially affected by an increase in the number of freeze-thaw cycles, and this leads to a change in the amount of immobile component (A). As shown in Figs. 9, and 13, it can be said that the increase in the fraction of the immobile component (A) and the decrease of the fractions of the intermediate and mobile components (B) and (C) lead to an increase in the shear viscosity E'. Therefore, the increase in the shear viscosity E' is closely related to the fraction of the crosslinking region.
4. Conclusion It can be concluded from ]H pulse NMR and highresolution solid-state 13C NMR experiments that there are three kinds of component in PVA gel, the mobile, immobile and intermediate components, which result
57
M. Kanekiyo et al./Journal of Molecular Structure 447 (1998) 49-59
CH
CH 2
III
r \
The number of freeze~kx
thaw cycles
n=2 IT mr
~A
n=l
I ' ' ' '
I ' ' ' '
100
I
90
B0
I
'
'
'
I ' ' ' '
70
I ' ' ' '
60
I
' ' ' '
I'
50
' ' '
I
40
'
30
'
'
'
I
20
Fig. 10.67.8 MHz 13C CP/MAS NMR spectra of PVA gel as a function of the number of freeze-thaw cycles: (A), one freeze-thaw cycle; (B), two freeze-thaw cycles; (C), six freeze-thaw cycles.
0.8
0.6
g
II 0.4 •
III
0.2 A
w
0
i
0
1
2
3
4
number of freeze-thaw
5
6
cycles
Fig. 1 I. Plots of the fraction for the peaks I, II and Ill in PVA gel against the number of freeze-thaw cycles as determined by ~3C CP/MAS NMR.
M. Kanekiyo et al./JournalofMolecular Structure 447(1998)49-59
58
10000
I000
•
.I.
•
A_
m,
C
•
--
--
"
B
w
I
•
A
100 l-"r"
|0
0
4
1
5
6
2number o~ freeze-thaw cycles
7
Fig. 12. Plots of sH T2 for PVA gel against the number of freeze-thaw cycles as determined by ~H pulse NMR signal decay. O, component (A) (immobile); II, component (B) (interfacial); A, component (C) (mobile).
0.8
0.6
d= 0.4 •
A
0.2
•
•
3
4
•
•
C
A
_
0 0
1
2
5
6
number of freeze-thaw cycles Fig. 13. Plots of the fractions of the three components of PVA gel against the number of freeze-thaw cycles as determined by t H pulse NMR signal decay. O, component (A) (immobile); II, component (B) (interfacial); A, component (C) (mobile).
M. Kanekiyo et al./Journal of Molecular Structure 447 (1998) 49-59
from the formation o f h y d r o g e n bonds, and the structures and d y n a m i c s o f these c o m p o n e n t s are closely related to m e c h a n i c a l properties such as the shear viscosity E ' . Further, the m e c h a n i s m o f P V A gel formation by f r e e z e - t h a w cycles was elucidated.
References [1] A. Takahashi, S. Hiramatsu, Polymer J. 6 (1974) 103.
59
[2] M. Kobayashi, i. Ando, T. Ishii, S. Amiya, Macromolecules 28 (1995) 6677. [3] M. Kobayashi, I. Ando, T. Ishii, S. Amiya, J. Mol. Struct. 440 (1998) 155. [4] T. Terao, S. Maeda, A. Saika, Macromolecules 16 (1983) 1535. [5] Y.H. Carr, E.M. Purcell, Phys. Rev. 94 (1954) 630. [6] G. Meiboom, D. Gill, Rev. Sci. Instr. 29 (1958) 588. [71 J.G. Powles, J.H. Strange, Proc. Phys. Soc. 82 (1963) 6. [8] P. Mansfield, Phys. Rev. 80 (1950) 580. [9] N. Bloembergen, E.M. Purcell, R.V. Pound, Physical Review 73 (1948) 679.