NMR Spectroscopic Study of Ion-Conducting

Chapter 31

NMR Spectroscopic Study of Ion-Conducting

N

15

-LabeledPolyphosphazenes

Downloaded by MONASH UNIV on February 25, 2016 | http://pubs.acs.org Publication Date: December 10, 2002 | doi: 10.1021/bk-2003-0834.ch031

ThomasA.Luther, Mason K. Harrup, and Frederick F. Stewart Energy and Environmental Sciences, Idaho National Engineering and Environmental Laboratory, P.O. Box 1625, Idaho Falls, ID 83415-2208

To investigate the nitrogen nuclei involvement in metal ion complexation in polyphosphazenes, two N labeled polyphosphazenes, N poly-[bis(2-(2methoxyethoxy)ethoxy)phosphazene] ( N MEEP) and N poly-[((2-allylphenoxy) (4-methoxyphenoxy) (2-(2methoxyethoxy)ethoxy) )phosphazene] ( N HPP), were studied by NMR spectroscopy ( H, C{ H}, N, and P) at magnetic field strengths of 7.04 and 17.63 Tesla. The complexation of lithium ion in the polymer matrix was probed by changes in chemical shifts and variable temperature spin-lattice relaxation (T ) studies. After a 39 mol% addition of lithium trifluoromethanesulfonate (LiOTf), observable changes in chemical shifts are exhibited in the N M R spectra of N MEEP in CDCl . The N M R spectra of N HPP in CDCl only exhibit changes in the chemical shifts of resonances that correspond to 2-(2-methoxyethoxy)ethoxy pendant groups after addition of 39 mol% LiOTf. Most significantly, a sharp decrease in the N NMR minimum T value measured for N M E E P reveals that significant lithium ion complexation occurs with the nitrogen nuclei. 15

15

15

15

0.12

1.02

15

0.86

1

13

1

15

31

1

15

3

15

3

15

1

15

© 2003 American Chemical Society

In NMR Spectroscopy of Polymers in Solution and in the Solid State; Cheng, H. N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

409


410 Polyphosphazenes are a class of materials that have been studied as solid polymer electrolytes for the past 16 years. The first polyphosphazene shown to have ion conducting properties when complexed with a metal salt was poly[bis(2-(2-methoxyethoxy)ethoxy)phosphazene] (MEEP) (J). The proposed transport mechanism of the metal ions through the M E E P matrix has been modeled on the transport mechanism of metal ions through poly(ethylene oxide) (PEO) (2). This proposed model characterizes the ionic transport in terms of "jumps" between neighboring polymer strands utilizing the lone electron pairs of the oxygen atoms on the side chains and that the backbone nitrogen atoms are not significantly involved (3). This study set out to investigate metal ion eomplexation in polyphosphazenes by N M R spectroscopy. In particular, the involvement of the nitrogen atoms with the metal ions was explored through changes in chemical shift and variable temperature spinlattice relaxation (Γι) N M R experiments. Unfortunately, the natural abundance of the spin active N nuclei that can be observed by N M R spectroscopy is too low to be effectively studied (natural abundance N is only 0.345%). To overcome the natural abundance deficiency, the polyphosphazenes in this study were synthesized from N labeled precursors. 1 5

1 5

1 5

Experimental 3 1

l3

l

15

N M R Spectroscopy. N M R data (*H, P , C{ VL}, and N ) were acquired on a Bruker D M X 300WB spectrometer with a magnetic field strength of 7.04 Tesla corresponding to operating frequencies of 300.13 M H z (*H) 121.49 M H z ( P), 75.48 M H z ( C), and 30.41 M H z ( N). Additional N M R spectra were acquired on a Bruker D M X 750 spectrometer with a magnetic field strength of 17.63 Tesla corresponding to operating frequencies of 750.13 M H z (*Η), 303.66 M H z ( P), 188.64 M H z ( C), and 76.01 M H z ( N). The N M R spectra were referenced internally to T M S (*H and ^ C ^ H } ) or externally to H P 0 ( P) or to ( NH4) S0 ( N). Spin-lattice relaxation measurements were performed using standard inversion-recovery 180°-τ-90° pulse sequence experiments. Variable temperature N M R experiments were conducted using a Bruker D M X 300WB spectrometer equipped with a Bruker B V T 3000 temperature control module. Temperature calibration was accomplished by following the Van Geet methanol calibration method (4). 31

13

15

31

13

15

3

31

15

4

15

2

4

Materials. N M R solvents tetrahydrofuran-dg (THF-dg), D 0 , and chloroformd (CDC1 ) were obtained from Cambridge Isotope Laboratories. Lithium trifluoromethanesulfonate (LiOTf) was purchased from Aldrich Chemical Co. and used as received. The two isotopically N labeled polyphosphazenes 2

3

1 5


411 1 5

1 5

investigated for this work, N M E E P and N poly[((2-allylphenoxy) .i2(4methoxyphenoxy)i.o2(2-(2-methoxyethoxy)ethoxy)o.86)phosphazene] ( N HPP), were prepared from N labeled polydichlorophosphazene following published procedures for the unlabeled materials (5-7). 0

1 5

1 5

Results and Discussion


1 5

Synthesis of N Labeled Polyphosphazenes. Linear polyphosphazenes are generally synthesized by a low yielding ring opening polymerization of hexachlorocyclotriphosphazene to generate polydichlorophosphazene followed by substitution reactions utilizing organic nucleophiles to displace the chlorine nuclei (8). The N labeled polyphosphazenes in this study were synthesized using an alternate route to generate the polydichlorophosphazene with the backbone nitrogen nuclei fully N labeled. The N polydichlorophosphazene was formed directly from ammonium sulfate following published procedures using N ammonium sulfate in place of N ammonium sulfate (Scheme 1) (5). The N M E E P and N HPP were then prepared from N polydichlorophosphazene using sodium salts of 2-(2-methoxyethoxy)ethanol (MEE), methoxyphenol (MeOP), and 0-allylphenol (o-Al) following published procedures for similar unlabeled polyphosphazenes (6, 7). 1 5

1 5

1 5

1 5

1 4

1 5

1 5

1 5

Scheme 1.

α 15,

15,'N

R—CI

α—R>, CII

CI

5 15,

NH4*+PC1

5

Direct Method

Nu

Nu

NMR Characterization. The N M R samples were typically prepared with approximately 300 mg N M E E P or N HPP and 0.5 m L solvent in a 5 mm N M R tube. THF-d , CDC1 , and D 0 ( N M E E P only) were used as solvents. 1 5

1 5

1 5

8

3

2


412 As expected, some solvent effects are exhibited in the solution spectra primarily as a small change in the observed chemical shifts along with some minor resolution differences. The sharpness of the resonances in the *H N M R spectrum of N M E E P , with T H F - d as the solvent, allows for measurable proton-proton coupling of 5.0 Hz. The N N M R spectrum of N M E E P at an operating frequency of 30 M H z exhibits a major resonance at δ 63.1 (THF-d ) with a full-width at half maximum linewidth ( Δ ν ) of 75 Hz (Figure 1). A minor resonance in the P N M R spectra downfield (0 to -5 ppm region) of the major resonance is also observable in the N N M R spectra (6 71.9). Similar resonances are observed in the P and N N M R spectra of the related N HPP. These downfield resonances are consistent with polymer chain-end effects. 1 5

8

1 5

1 5

8

3 1

1/2

1 5


3 1

1 5

1 5

ι ν lilllllllllllllllllflllllllllllllltllMIIIIMMlllMllllllllllllllllllllllllllllllllllllllllllllHHiniHIIHHIIlllll

110

100

90

80

70

60

SO

40

30

20

1

10

(ppm ) 15

15

Figure 1. Ν NMR spectrum of Ν MEEP (THF-d ). (Reproduced from reference 13. Copyright 2002 American Chemical Society.) 8

1 5

1 5

Characterization of N HPP as N poly[((2-allylphenoxy)o.i (4methoxyphenoxy)i.o2(2-(2-methoxyethoxy)ethoxy)o.86)phosphazene] was accomplished by integration of the *H N M R spectrum to determine the relative ratios of the pendant groups (7). The P N M R spectrum of N HPP acquired at an operating frequency of 121 M H z exhibits three resonances that are due to the phosphorus nuclei that have 2 M E E , 1 M E E and 1 aryloxy (OAr), or 2 O A r pendant groups attached. The resolution of the spectrum at this operating frequency is such that OAr corresponds to either MeOP or 0-Al. The P N M R spectrum acquired at an operating frequency of 304 M H z exhibits further 2

3 1

1 5

3 1


413 resolution of these resonances. Two separate resonances are now observed for the phosphorus nuclei with 1 M E E and 1 MeOP versus 1 M E E and 1 o-A\ pendant groups attached along with a distinct resonance for the phosphorus nuclei with 2 MeOP pendant groups. The only unresolved resonance at this frequency corresponds to the phosphorus nuclei with either 1 MeOP and 1 o-A\ or 2 o-A\ pendant groups. The N N M R spectrum of N HPP at 30 M H z is a single very broad resonance (AVi/ ~ 260 Hz). However, the N N M R spectrum acquired at 76 M H z exhibits five separate resonances. These resonances arise from the adjacent phosphorus environments that relate the number of O A r pendant groups versus the number of M E E pendant groups attached to the phosphorus nuclei. When both neighboring phosphorus nuclei have two O A r pendant groups attached, the N resonance exhibits spin coupling of 40 Hz. As the number of aryloxy pendant groups decrease, the resolution decreases and the chemical shift of the resonance moves further upfield. The resonance that is the furthest upfield has the immediate phosphorus environment similar to M E E P and has an observed chemical shift at approximately 62.3 ppm. 1 5

1 5

1 5


2

1 5

1 3

1 5

C NMR Peak Assignment for N MEEP. The use of THF-d as a solvent for N M E E P has a distinct advantage over the other solvents for structural determination by N M R spectroscopy experiments. The increased resolution and chemical shift dispersity that is evident in the *H N M R spectrum also occurs in the ^ C ^ H } N M R spectrum. This allows for the unambiguous assignment of the C N M R resonances of N M E E P via various N M R techniques including two-dimensional proton homonuclear (COSY) and proton-carbon heteronuclear (HETCOR) correlation experiments. The assignments are consistent with the results of N M R spin-lattice relaxation (7Ί) experiments where the relaxation times of carbon atoms in flexible side chains increase with distance from the polymer backbone (Figure 2) (9). The assignments agree with published data for the unlabeled M E E P (3). 8

1 5

1 3

1 5

15

N MEEP/Lithium Trifluoromethanesulfonate Complexes. Lithium trifluoromethanesulfonate (LiOTf) was used to study metal ion complexation within the polymer matrices of the N polyphosphazenes. L i O T f was added to N M E E P in incremental amounts in an N M R tube. Additional N M R spectra were acquired to observe changes in the resonances of the ^ C ^ H } , P , and N nuclei. The addition of approximately 100 mol% of L i O T f resulted in no significant change in the chemical shift of the resonance in the P N M R spectra with THF-d as the solvent. This indicated that, as expected, the THFd has the ability to out compete the polymer for solvation of the lithium ions. Since L i O T f is soluble in the polymer matrix but it is not appreciably soluble in CDC1 , further N M R experiments were conducted using CDC1 . Linear changes in the chemical shift in the resonances of the C { ' H } , P , and N N M R spectra were observed for the incremental addition of LiOTf. After a 39 1 5

1 5

3 1

1 5

3 1

8

8

3

3

13

3 1

1 5


414 1 5

l3

l

mol% addition of L i O T f to N M E E P , the C{ H] N M R chemical shift changes were 0.16 ppm for C I , -0.14 ppm for C2, 0.63 ppm for C3, -0.62 ppm for C4, and -0.01 ppm for C5 (Figure 3). The observed downfield P and N N M R chemical shift changes were 1.51 and 0.70 ppm respectively (Figures 4 and 5). Amounts greater than approximately 40 mol% addition of L i O T f to either N M E E P or N HPP in CDC1 resulted in incomplete solvation of the metal salt. 3 1

l 5

1 5

1 5


3

-τ 74

1 72

1

«

1 70

1

1 68

1

1 66

«

1 64

1

1 62

·

1 60

»

r 58

(PP»)

CI

C2

C3

C4

C5

0.30

0.37

0.78

1.13

3.61

13

1

15

Figure 2. C{ H} NMR spectrum of Ν MEEP (THF-d ) with carbon peak assignments and C{ H} NMR (CDCl 75 MHz) room temperature spin-lattice relaxation times (Tj) of Ν MEEP (seconds). (Reproduced from reference 13. Copyright 2002 American Chemical Society.) 8

13

1

3>

15

1 5

The " C ^ H } N M R spectra of N HPP acquired at 75 and 189 M H z exhibit resolvable resonances for the carbon nuclei in the pendant groups. After a 39 mol% addition of LiOTf, only the C4 resonance of the M E E pendant groups shows a chemical shift change of -0.6 ppm. The other resonances of N HPP exhibit smaller chemical shift changes, in the -0.2 to 0.2 ppm range. After the 39 mol% addition of L i O T f to N HPP, the P N M R spectrum acquired at 121 M H z shows a chemical shift change of 0.39 ppm for the resonance corresponding to the phosphorus nuclei with 2 M E E pendant groups 1 5

1 5

3 1



415

C2

C3 C4 Carbon Atom Number

13

C5

15

Figure 3. Change in C NMR chemical shifts for Ν MEEP in CDCl after lithium ion complexation. (Reproduced from reference 13. Copyright 2002 American Chemical Society.) 3

-5.00

_

-5.50

-6.00

1 Έ φ .c Ο rr

Ζ α.

-6.50

-7.00

-7.50 10

31

20 30 LiOTf (mol%)

40

50

15

Figure 4. Change in the Ρ NMR chemical shift of Ν MEEP (CDCl , 121 MHz) with the addition of LiOTf. (Reproduced from reference 13. Copyright 2002 American Chemical Society.) 3


416


63.8

20 30 LiOTf (mol%) 15

15

Figure 5. Change in the N NMR chemical shift of N MEEP (CDCl , 30 MHz) with the addition of LiOTf (Reproduced from reference 13. Copyright 2002 American Chemical Society.) 3

attached (Figure 6). The resonance for the nuclei with 1 M E E and 1 O A r pendant groups exhibits a smaller chemical shift change of 0.17 ppm. There was no significant change in chemical shift for the resonances corresponding to the phosphorus nuclei with 2 OAr pendant groups attached. In the N N M R spectrum of N HPP, the 39 mol% addition of L i O T f results in resonance Ε to shift downfield ~1 ppm and the resolution of resonances B - D decreases (Figure 7). The only resonance that does not undergo any observable change is resonance A , which has four O A r pendant groups attached to the two adjacent phosphorus nuclei. 1 5

1 5

Temperature Dependence of Spin-Lattice Relaxation. The spin-lattice relaxation rate (T{ ) is comprised of various contributions to the relaxation process, including homo- and heteronuclear dipolar interactions, quadrupolar interactions, chemical shift anisotropy, spin-rotation, and others (10). When the relaxation mechanism is dominated by inter- and intramolecular dipoledipole interactions, the T{ will increase with temperature, pass through a maximum, and decrease with increasing temperature. Since the relaxation rate is the inverse of the relaxation time, the Γι will decrease, pass through a minimum (Τ ), and then increase with increasing temperature (77). The 7\min values are proportional to the internuclear distances. In the temperature range studied, the ^ C ^ H } N M R T data of N M E E P in CDC1 only exhibit distinct minimum values for carbons C I and C2, which is approximately 0.11 seconds at 226 Κ for both nuclei. At the lowest 1

1

ληύη

1 5

x

3


417 0.50

0.40

0.30

I "ε

0.20


0.10

0.00

-0.10 20

30

LiOTf (mol %) 31

15

Figure 6. Change in Ρ NMR chemical shifts of Ν HPP (CDCl , 121 MHz) with increasing amounts of LiOTf added. The symbols correspond to the following pendant group configurations attached to the phosphorus nuclei: • = (MEE) , · = (MEE)(OAr), • = (OAr) . (Reproduced from reference 13. Copyright 2002 American Chemical Society.) 3

2

2

R R

"I

Ί

72

71

""I 70

R R

I

I

I

I

I

I

I

"I"

"I'

69

68

67

66

65

64

63

62

61

(ppm)

15

15

Figure 7. Ν NMR spectrum of Ν HPP after 39 mol% addition of LiOTf (CDCl , 76 MHz). The pendant group assignments for the observed resonances are A: R = 4 OAr, B:R = 3 OAr and 1 MEE, C: R = 2 OAr and 2 MEE, D: R = 1 OAr and 3 MEE, E: R = 4 MEE. (Reproduced from reference 13. Copyright 2002 American Chemical Society.) 3


418 temperature recorded (215 Κ), the T\ for C3 had reached a constant value of 0.15 seconds. The Γι values for C4 and C5, 0.16 and 0.58 seconds respectively, were still decreasing with the decrease in temperature. The complexation of L i O T f to the polymer matrix results in a decrease in the flexibility of the polymer. This decrease in flexibility is evidence by an increase in the glass transition temperature (7g) values (12, 13). The decrease in flexibility is also observed as an increase in temperature where the Γ ι ^ occurs. After the 39 mol% addition of LiOTf, the r values showed the expected increase in temperature, approximately 285 Κ for all the carbon resonances, except for the C5 resonance. The T\ values for C5 were still decreasing at the lowest obtainable temperature (1.30 seconds at 262 K). The ^imin values showed increases in relaxation times of approximately 0.04 seconds for C I and C2, and approximately 0.07 seconds for C3 and C4 (Table I).


l m i n

13

!

1 5

Table L C{ H} NMR T Values of N M E E P (CDC1 , 75 MHz) before and after LiOTf complexation (Seconds). lmin

1 5

1 5

N MEEP N MEEP/Li*

3

CI

C2

C3

C4

C5

0.11 0.15

0.11 0.15

0.15° 0.22

0.16* 0.23

0.58 1.30*

fl

a

T\ value still decreasing at the lowest temperature recorded (215 K). SOURCE: Reproduced with permission from reference 13. Copyright 2002 American Chemical Society. 3 1

The P N M R Τ data indicate a Γ value of 1.55 seconds at 256 K . After the addition of the LiOTf, the r increases to 1.76 seconds at 309 Κ (Figure 8). The N N M R 7 value decreases with the addition of LiOTf, from approximately 7.5 seconds at 255 Κ to 5.0 seconds at 300 Κ (Figure 9). This relaxation rate increase, or 7 ^ decrease, is consistent with lithium ion coordinating to the nitrogen nuclei and the corresponding heteronuclear and quadrupolar interactions contributing to the relaxation process. λ

1 π ύ η

l m i n

1 5

l m i n

Conclusions 1 5

The N M R data for N HPP indicate that the solvation energies of the less polar MeOP and o-AL pendant groups are not sufficient to successfully compete with the M E E pendant groups for complexation with the lithium ion. However, coordination of the lithium ion in the M E E environments is not limited to the oxygen nuclei on the etherial pendant groups as proposed by the P E O ion transport model for M E E P (3). Examination of the P and N N M R data reveal that when four M E E pendant groups are attached to the adjacent phosphorus nuclei, a pocket is formed that allows coordination of the lithium ion with the nitrogen nuclei. 3 1

1 5


419


2.2 h

1.6

220

240

260

280

300

320

340

Temperature (Κ) 31

15

Figure 8. Ρ NMR spin-lattice relaxation times of Ν MEEP (CDCl , 121 MHz) before and after lithium ion complexation. The symbols correspond to: • = Ν MEEP, • = Ν MEEP + LiOTf. (Reproduced from reference 13. Copyright 2002 American Chemical Society.) 3

15

15

12

Τ (s)

340 Temperature (Κ) 15

15

Figure 9. N NMR spin-lattice relaxation times of Ν MEEP (CDCl , 30 MHz) before and after lithium ion complexation. The symbols correspond to: • = Ν MEEP, • = Ν MEEP + LiOTf. (Reproduced from reference 13. Copyright 2002 American Chemical Society.) 3

15

15


420 1 5

3 1

1 3

Changes in the chemical shifts in the N and P N M R spectra of N M E E P following lithium ion addition are consistent with lithium ion complexation with the etherial oxygen nuclei or the nitrogen nuclei of the polyphosphazene backbone. The sharp decrease in the N N M R Γι min value measured for N M E E P reveals that significant lithium ion complexation preferentially occurs with the nitrogen nuclei rather than the oxygen nuclei. 1 5

1 5


Acknowledgements The authors thank Dr. Tom Pratum for his assistance with the Bruker D M X 750 N M R Spectrometer at the University of Washington. The work described in this paper was supported by the United States Department of Energy through contract DE-AC07-99ID13727. References (1) Blonsky, P. M.; Shriver, D . F.; Austin, P.; Allcock, H . R. J. Am. Chem. Soc. 1984, 106, 6854-6855. (2) Ratner, Μ. Α.; Shriver, D. F. Chem. Rev. 1988, 88, 109-124. (3) Allcock, H . R.; Napierala, M. E.; Olmeijer, D. L.; Best, S. Α.; Merz, Κ. M. Macromolecules 1999, 32, 732-741. (4) Van Geet, A . L. Analytical Chemistry 1970, 42, 679-680. (5) Allen, C. W.; Hneihen, A . S. Phosphorus Sulfur Silicon Relat. Elem. 1999, 146, 213-216. (6) Harrup, M. K.; Stewart, F. F. J. Appl. Polym.Sci.2000, 78, 1092-1099. (7) Stewart, F. F.; Harrup, M. K.; Luther, T. Α.; Orme, C . J.; Lash, R. P. J. Appl. Polym. Sci. 2001, 80, 422-431. (8) Mark, J. E.; Allcock, H . R.; West, R. Inorganic Polymers; Prentice Hall: Englewood Cliffs, New Jersey, 1992. (9) Krajewski-Bertrand, Μ.-Α.; Lauprêtre, F.; Monnerie, L . Dynamics of Solutions and Fluid Mixtures by NMR; John Wiley & Sons: Chichester, 1995. (10) Günther, H . NMR Spectroscopy: Basic Principles, Concepts, and Applications in Chemistry; Second ed.; John Wiley & Sons: Chichester, 1995. (11) Desrosiers, P. J.; Cai, L . H . ; L i n , Z. R.; Richards, R.; Halpern, J. J. Am. Chem. Soc. 1991, 113, 4173-4184. (12) Allcock, H . R.; Napierala, M. E.; Cameron, C . G . ; O'Connor, S. J. M. Macromolecules 1996, 29, 1951-1956. (13) Stewart, F. F.; Singler, R. E . ; Harrup, M. K.; Peterson, E . S.; Lash, R. P. J. Appl. Polym. Sci. 2000, 76, 55-66. (14) Luther, T. Α.; Stewart, F. F.; Budzien, J. L.; LaViolette, R. Α.; Bauer, W. F.; Harrup , M. K.; Allen, C. W.; Eiayan, A . "On the Mechanism of Ion Trans port Through Polyphosphazene Solid Polymer Electrolytes 1: N M R , IR, and Raman Spectroscopic Studies and Computational Analysis of N-Labeled Polyphosphazenes," J. Amer. Chem. Soc., Submitted for Publication, 2002. 15


NMR Spectroscopic Study of Ion-Conducting

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