Polymer Characterization - American Chemical Society

25 C- and F-NMR Chemical Shifts and the Microstructures of Fluoropolymers 13

19

A. E. TONELLI, F. C. SCHILLING, and R. E . CAIS Bell Laboratories, Murray Hill, NJ 07974 Simultaneous H- and F-decoupled C-NMR and H -decoupled F-NMR spectra are recorded for poly(vinylidene fluoride) (PVF ), poly(fluoromethylene) (PFM), poly(vinyl fluoride) (PVF), and poly(trifluorethylene) (PF E). Observed C- and F-NMR chemical shifts are compared to those calculated as a function of stereoregularity and/or defect structure, i.e., head-to-head:tail -to-tail (H—H:T—T) addition of monomers. Calculated chemical shifts are obtained through enumeration of the numbers and kinds of γeffects (three-bond gauche ar rangements) that involve each carbon or fluorine atom as dictated by the conformational characteristics of each polymer. Agreement between measured and predicted C- and F-NMR chemical shifts is observed for each fluoropolymer when the following γeffects are assumed: γ = -2 to -5 ppm; γ = 10 to 25 ppm; and γ = 20 to 35 ppm, where γ is the upfield shift at carbon or fluorine a produced by carbon or fluorine b when in a gauche arrangement with a. This agreement permits detailed assignments of C- and F-NMR resonances in the spectra of all four fluoropolymers including identifi cation of those resonances belonging to the carbon and fluorine atoms in the H—H:T—T defects present in PVF PVF, and PF E, whose abundance increases in this order.

Downloaded by TUFTS UNIV on December 11, 2015 | http://pubs.acs.org Publication Date: June 1, 1983 | doi: 10.1021/ba-1983-0203.ch025

1

19

13

1

19

2

13

3

13

19

19

C,C

or

F,F

F

F,C

a,b

13

19

2,

3

, / Y . N A L Y S I S O F T H E N U M B E R S A N D KINDS of y interactions i n v o l v i n g each carbon atom type i n a p o l y m e r chain can a l l o w for the p r e d i c t i o n of C - N M R spectra of v i n y l homopolymers a n d copolymers (J). E a c h nonhydrogen y substituent i n a three-bond gauche arrangement (see F i g u r e 1) w i t h a g i v e n carbon atom produces an u p f i e l d c h e m i c a l shift 1 3

0065-2393/83/0203-0441$06.00/0 © 1983 American Chemical Society

In Polymer Characterization; Craver, C.; Advances in Chemistry; American Chemical Society: Washington, DC, 1983.


F

r *"°pP m

c

2 T 0

5 p , ) m

y

F #

ρ•

10 το 25 ppm

>b»C0RF^ Xp» « 20 TO 35 ppm

9

Figure 1. Scheme of γ-gauche interactions between observed (F*;C*) and y substituent (F C) fluorine and carbon nuclei in fluoropolymers (17).

c

r *.

F.C

g+(^«

Ν

w δ

Η

η

η >

M

ο r

25.

Ο and F-NMR

TONELLI ET A L .

19

Chemical Shifts

443

of that carbon resonance relative to their trans arrangement. T h e fre quency w i t h w h i c h these γ-gauche interactions occur can be evaluated from the conformational characteristics of the p o l y m e r c h a i n as m a n i fested by calculated b o n d rotation probabilities. T h e sensitivity of v i n y l homopolymer a n d copolymer C-NMR c h e m i c a l shifts to stereomonomer and comonomer sequence results from the dependence of b o n d rotation probabilities on the same m i crostructural features, and leads, v i a the y-gauche effect, to a disper sion of C - N M R c h e m i c a l shifts reflecting the different possible m i croenvironments found along the v i n y l p o l y m e r c h a i n . A l l that is needed to calculate the effects of p o l y m e r microstructure on C - N M R c h e m i c a l shifts are the b o n d rotation probabilities obtained from the polymer's conformational characteristics and the magnitudes of the u p f i e l d y effects produced by the gauche arrangements of a g i v e n carbon atom w i t h its y substituents. T h i s approach, w h i c h fully utilizes a l l the microstructural infor mation p r o v i d e d by C - N M R spectroscopy, has b e e n a p p l i e d suc cessfully to the calculation of the C - N M R c h e m i c a l shifts i n a variety of v i n y l homopolymers a n d copolymers (I). P o l y p r o p y l e n e oligomers (2,3) and homopolymer (4) and its copolymers w i t h ethylene (5, 6) and v i n y l chloride (7), p o l y ( v i n y l chloride) oligomers (8) and homopoly mer and its copolymers w i t h ethylene (9) and propylene (7), and polystyrene (10) and its oligomers have a l l b e e n treated v i a the γ effect method of p r e d i c t i n g C - N M R c h e m i c a l shifts i n polymers. T h i s chapter extends the application of the y effect method to the calculation of the C - and, for the first time, F - N M R c h e m i c a l shifts expected for the carbon and fluorine atoms i n fluoropolymers. P o l y (vinylidene fluoride) ( P V F ) , p o l y v i n y l fluoride) ( P V F ) , poly(fluoromethylene) ( P F M ) , and poly(trifluorethylene) ( P F E ) are treated. T h i s study was prompted for three p r i n c i p a l reasons. F i r s t , the polymers studied possess fluorine atoms whose F n u c l e i exhibit magnetic resonance and whose c h e m i c a l shifts are at least as sensitive to p o l y m e r micro structure as their C n u c l e i . W e sought to determine i f the sensitivity of F - N M R c h e m i c a l shifts to microstructure has its origin, as do C - N M R c h e m i c a l shifts, i n y-gauche interactions. Sec ond, the magnitude of the u p f i e l d C - N M R c h e m i c a l shift produced at a carbon atom by a γ fluorine substituent w h e n i n a gauche arrange ment was u n k n o w n . T h i r d , w i t h the exception of P V F (II), the con formational characteristics of these fluoropolymers had never been studied experimentally. W e hope to test the recently p r e d i c t e d con formational characteristics (12) of P V F , P F E , and P F M b y comparing their observed and predicted C - and F - N M R c h e m i c a l shifts, the latter of w h i c h are d e r i v e d from b o n d rotation probabilities as ob tained (13) from their conformational models. 1 3

1 3


1 3

1 3

1 3

1 3

1 3

1 9

2

3

1 9

1 3

1 9

1 3

1 3

2

3

1 3

1 9


444

POLYMER CHARACTERIZATION

Materials T h e synthesis of the P F M e m p l o y e d i n this study has b e e n re ported previously (14). P F E was p r o v i d e d through P e n n w a l t C o r p o ration. P V F was purchased from A l d r i c h C o m p a n y , a n d the P V F e m p l o y e d is K y n a r 301 (Pennwalt Corporation). 3

2

Experimental Generally, spin—spin coupling between directly bonded C and F nu clei obliterates the detailed structure of the dispersion of C - N M R chemical shifts produced by different fluoropolymer micro structures. Only in the methylene carbon regions of the usual ^-decoupled C - N M R spectra of P V F and P V F can we begin to separate the effects of F coupling and mi crostructure on the C - N M R chemical shift dispersion. Consequently, we have performed C - N M R measurements on these fluoropolymers with si multaneous decoupling of both the H and F nuclei. The details of this triple-resonance experiment are published separately (15). The N M R spectra were recorded on Bruker WH-90 and Varian XL-200 spectrometers at C and F frequencies of 22.62, 84.6, and 188 M H z . Com plete experimental details for the C - N M R spectra (16) and F - N M R spectra (17) are published elsewhere (15). 1 3

1 9

1 3

1 3

1 9

2


1 3

1 3

1

1 3

1 9

1 9

1 3

Results and

1 9

Discussion

Comparisons of calculated C - N M R c h e m i c a l shifts w i t h the C - N M R spectra observed for P V F , P F M , P V F , and P F E are pre sented i n Figures 2 - 5 . T h e agreement is generally good, w i t h the calculated C - N M R c h e m i c a l shifts reproducing faithfully the effects of stereosequence a n d monomer addition defects (18, 19) [head-tohead: tail-to-tail ( H - H : T - T ) ] observed i n the C - N M R spectra. T h e γ effects i n the range - 2 5 p p m u p f i e l d , reflecting the s h i e l d i n g pro d u c e d at the observed carbon b y γ-carbon and/or fluorine substituents i n a gauche arrangement (see F i g u r e 1), permit this agreement. Prior to our triple-resonance (15) C - N M R study of P V F , the Η—H:T—Τ C H defect resonance 4 observed (16) at —0.8 p p m u p f i e l d from Η—Τ C H carbons (see F i g u r e 2) was obscured by long-range F - C - s p i n - s p i n c o u p l i n g i n the usual ^ - d e c o u p l e d C-NMR spectrum of P V F (20). I n a d d i t i o n , the C F region of the C - N M R spectrum is a c o n t i n u u m of resonances without F - d e c o u p l i n g re sulting i n the complete obliteration of H - H : T - T defect resonances A , B , and C , so clearly apparent i n F i g u r e 2. O n the other hand, even the triple-resonance C - N M R spectrum of P F M reveals a near c o n t i n u u m of c h e m i c a l shifts resulting from the long-range sensitivity to stereosequence (see F i g u r e 3). C-NMR c h e m i c a l shifts calculated for the pentads i n P F M are sensitive to nonad stereosequences l e a d i n g to a C - N M R c h e m i c a l shift disper1 3

1 3

2

3

1 3

1 3

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2

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1 9

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1 3

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19

1 3

1 3

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25.

TONELLI ET AL.

C- and F-NMR

13

Chemical Shifts

19

445


H-T

ι I

ι

ι ι

ι ι

135

I

ι

ι

ι

ι

130

» '

» » '

125

I

ι ι

ι

ι

120

I

ι ι

ι

ι—I—L

115

110

1 A B 2 3 C 4 -CFj-CHg-CFj-CHg-CFg-CFg-CHg-CHj-CFg-CHg-CFg-CHj-

H-TI

t

1 ι

t

ι

90

Figure 2. C-NMR 13

ι

I

ι

t

45

ι

ι

I

40

ι

ι

ι

ι

1 ι

35 8 ppm

ι

ι

ι

»

30

ι

ι

ι

ι

I

ι

I 1

25

spectrum (22.62 MHz) and calculated shifts for PVF (16).

chemical

2

sion of 0 . 2 - 0 . 5 p p m calculated for each pentad. A comparison of ob served and calculated C - N M R c h e m i c a l shifts shows that the sample of P F M can be characterized (14) as predominantly atactic (discussed later). Without recording the C - N M R spectrum of P V F i n the triple resonance mode (see F i g u r e 4) the sensitivity of the H—Τ C H F a n d C H resonances to stereosequence w o u l d not be revealed. Sensitivity 1 3

1 3

2



446


ι

ι

ι

I

ι

ι

95 Figure 3. C-NMR 13

ι

ι

I

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90

I

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85 8 ppm

ι

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I

80

ι

ι

ι

ι

I

ι

ι

ι

75

spectrum (22.62 MHz) and calculated shifts for PFM (16).

chemical

to triad stereosequences i n the C H F region and tetrad sensitivity i n the C H region was observed. W i t h increasing f i e l d the three reso nances observed i n the H - T C H F region are assigned to the rr, mr, m m triads by comparison to our calculated shifts. T h u s , our P V F sam ple is predominantly atactic w i t h P = 0.43 (17). T h e C - N M R spectrum of P F E i n F i g u r e 5 does not permit a separation of the effects of stereosequence and defect structure on the observed C - N M R c h e m i c a l shifts. A comparison of observed and calculated C - N M R c h e m i c a l shifts enables us to conclude that H - H : T - T addition i n our P F E sample is more prevalent than i n our P V F and P V F samples. T h e agreement achieved between the observed and calculated C - N M R c h e m i c a l shifts i m p l i e s that the b o n d rotation probabilities used to evaluate the frequencies of the various y effects occurring for the carbon atoms i n each of the possible micro structural environments 2

m

1 3

3

1 3

1 3

3

2

1 3


TONELLI ET AL.

447

C- dnd F-NMR Chemicdl Shifts 19

13


-CHF-

H-T

Fl M J

»

100

ι

ι

t

I

ι

95 δ

A

t

I

t

1

1

90

I

1

!_

ppm

A I B C 2 3 0 - C H F - C H - CHF- C H - C H F - CHF- C H - C H - C H F - C H - C H F 2

2

2

2

H-T

t

ι

» 45

R ι

»

»

*

» 40

t

ι

8 Figure 4. C-NMR 13

2

ι

ι

1 35

1

1

1

1

1 1 30

L

ppm

spectrum (22.62 MHz) dnd cdlculdted chemical shifts for PVF (16).

.•terfiâiii -homîcaï ^dety If rai ^55 166SL.N. W. Washington, 0. S. 20031 In Polymer Characterization; Craver, C.; Advances in Chemistry; American Chemical Society: Washington, DC, 1983.

448



M-T

2

ffll 1

1

1

1

1

1

1

'

95

100

Figure 5. C-NMR 13

I

'

t

I

i

t

t

ι

I

ι

90 85 8 ppm

ι

ι

ι

I

ι

ι

ι

80

spectrum (22.62 MHz) and calculated chemical shifts for PF E (16). 3


25.

TONELLI ET A L .

C- and F-NMR

13

19

Chemical Shifts

449

i n these polymers are realistic. T h i s provides, for the first time, direct microstructural support for the conformational models (11,12) d e r i v e d previously for these chains (17, 21). Traditional measures (13) of the conformational characteristics of p o l y m e r chains, such as end-to-end distance and dipole moment, were predicted (11, 12) to be relatively insensitive to micro structure (both stereosequence and defect content) for the fluoropolymers considered (11,12). T h u s , the successful testing of their conformational models as p r o v i d e d by the correct p r e d i c t i o n of their C - N M R c h e m i c a l shifts takes on added importance. W i t h the confidence gained i n the conformational models of these fluoropolymers through the successful p r e d i c t i o n of C - N M R c h e m i cal shifts, F - N M R c h e m i c a l shifts were also calculated, and the results are presented i n Figures 6—9. T h e agreement between observed and calculated F - N M R c h e m i c a l shifts is good and leads to y-gauche F - s h i e l d i n g interactions of 10 to 25 p p m for fluorine and 20 to 35 p p m for carbon y substituents. T h e y effects on fluorine n u c l e i are nearly an order of magnitude larger than those experienced by carbon n u c l e i (see F i g u r e 1). T h e larger magnitude of the y effects operating on F n u c l e i , as compared to C n u c l e i , results i n a greater sensitivity of the F - N M R spectra of fluoropolymers to their microstructure. I n F i g u r e 6 we see F resonances ( 1 8 8 - M H z spectrum) due to defect structures other than H - H : T - T addition i n P V F (resonances 5, 6, and 7). Instead of a continuum of c h e m i c a l shifts as observed i n the C - N M R spectrum of P F M (Figure 3) the F - N M R spectrum of P F M (Figure 7) manifests detailed information regarding the stereosequences present i n our sample. A d o p t i o n of B e r n o u l l i a n p o l y m e r i z a t i o n statistics a n d comparison of observed and calculated c h e m i c a l shifts permits a faithful simulation (14) of the observed spectrum and indicates clearly the predominantly atactic nature (P = 0.42) of our P F M . C o m p a r i s o n of Figures 5 and 9 reveals the greater sensitivity of F - N M R , relative to C - N M R spectroscopy, to the microstructure of P F E . T h i s greater sensitivity enables us to estimate the content of monomer units added i n a H - H : T - T manner. T h e detailed assignment of resonances i n the C - and F - N M R spectra of P V F , P V F , and P F E achieved b y comparison to p r e d i c t e d C - and F - N M R c h e m i c a l shifts permits a quantitative estimate of the H - H : T - T defect content i n each fluoropolymer. Integration of defect ( H - H : T - T ) and normal ( H - T ) resonances to obtain peak areas leads to estimates of the defect content i n each fluoropolymer. W e observed 3, 12, and 20 m o l % H - H : T - T monomer addition i n the samples of P V F , P V F , and P F E , respectively. 1 3

1 3


1 9

1 9

19

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m

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3



450


1

2

3

4

CH2 "CF2"CH2~CF^~CFg"CHg**CH2~CFg~CH2" 5 CH2 ~ CF2~CH2~CH2"CF2 "CH2" CF2 " CF2-CH2 " 6 -

CH2 " - C F g ^ C r ^ - C F ^ CF2~CH2" CFg ~CH2~ CF^ " 7 CH2 ~CFg"CH2™CH2"CF2 ~CF2 "CH2~CH2~CF2 "

(C) H-T

6

J

I

L

100

90

7,

110 Φ

120

ppm

Figure 6. F-NMR spectra (84.6 MHz) (a), (188 MHz) (b), and calculated chemical shifts for PVF (see Ref. 22) (17). 19

2



25.

C- and F-NMR

TONELLI ET A L .

1

ι

ι

ι

0

13

ι

Chemical Shifts

19

I

5

ι

l

l

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I

10

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I

1

1

451

ι

I

I

15

I

I

20

8 Figure 7. F-NMR 19

spectrum (84.6 MHz) and calculated chemical shifts for PFM (17).


L


452


rr |A(mm) |D(m)

J

I

I

L

D(r)

J

A(rr)

I

L

180 -CH

185

H-T A B C D - C H F - C H -CHF - C H -CHF - C H F - C H - C H -CHF - C H ·

2

2

B(mm)

J

2

B(rm)

L

2

C(m)

J

I

2

2

B(mr)C(.r)

J

L

190

I

B(r_r)

L

195

200

Φ ppm Figure 8. F-NMR 19

spectrum (188 MHz) and calculated chemical shifts for PVF (17).


TONELLI ET AL.

C- and F-NMR

13

19

Chemical Shifts


25.


453

454


Acknowledgment T h e . authors are i n d e b t e d to D r . G o r d o n R. L e a d e r of the P e n n w a l t Corporation for p r o v i d i n g samples of P F E .


3

Literature Cited 1. Tonelli, A. E.; Schilling, F. C. Acc. Chem. Res. 1981, 14, 223. 2. Tonelli, A . E. Macromolecules 1978, 11, 565. 3. Tonelli, A . E. Macromolecules 1979, 12, 83. 4. Schilling, F. C.; Tonelli, A . E. Macromolecules 1980, 13, 270. 5. Tonelli, A . E. Macromolecules 1978, 11, 634. 6. Tonelli, A . E. Macromolecules 1979, 12, 255. 7. Tonelli, A . E.; Schilling, F. C., in preparation. 8. Tonelli, A . E.; Schilling, F. C.; Starnes, W. H., Jr.; Shepherd, L.; Plitz, I. M. Macromolecules 1980, 13. 9. Tonelli, A . E.; Schilling, F. C. Macromolecules 1979, 12, 252. 10. Tonelli, A . E. Macromolecules 1979, 12, 252. 11. Tonelli, A . E. Macromolecules 1976, 9, 547. 12. Tonelli, A . E. Macromolecules 1980, 13, 734. 13. Flory, P. J. In "Statistical Mechanics of Chain Molecules"; Wiley: New York, 1969; Chaps. II and IV. 14. Cais, R. E. Macromolecules 1980, 13, 806. 15. Schilling, F. C.J. Magn. Reson. 1982, 47, 61. 16. Tonelli, A. E.; Schilling, F. C.; Cais, R. E. Macromolecules 1981, 14, 560. 17. Tonelli, A. E.; Schilling, F. C.; Cais, R. E. Macromolecules 1982, 15, 849. 18. Wilson, C. W., III J. Polym. Sci., Part A 1963, 1, 1305. 19. Wilson, C. W.; Santee, E. R., Jr. J. Polym. Sci., Part C 1965, 8, 97. 20. Bovey, F. Α.; Schilling, F. C.; Kwei, T. K.; Frisch, H. L. Macromolecules 1977, 10, 559. 21. Tonelli, A . E., unpublished data. 22. Ferguson, R. C.; Brame, E. G., Jr. J. Phys. Chem. 1979, 83, 1397. RECEIVED for review January 15, 1982. ACCEPTED February 28, 1982.


Polymer Characterization - American Chemical Society

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