Polymer Characterization - ACS Publications

26 Characterization of Molecular Motion in Solid Polymers by Variable Temperature Magic Angle Spinning C-NMR Spectroscopy Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 6, 2016 | http://pubs.acs.org Publication Date: June 1, 1983 | doi: 10.1021/ba-1983-0203.ch026

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W. W. FLEMING, J. R. LYERLA, and C. S. YANNONI IBM Research Laboratory, San Jose, CA 95193 The inclusion of a variable temperature magic-angle spinning capability for solid-state C-NMR spectros copy makes feasible the investigation by C-relaxation parameters of structural and motional features of poly mers above and below T and in temperature regions of secondary relaxations. We report variable temperature (77 Κ to 323 K) spectral data on polytetrafluoroethylene and polypropylene. Illustrative of the data are the T and T results for isotactic polypropylene over the temperature range 77—300 K. All carbons in the repeat unit show minima in T and T that reflect methyl group reorientational motion at the appropriate measur ing frequencies (15 MHz and 57 kHz). The T data for CH and CH carbons indicate the importance of spin— spin as well as spin—lattice pathways in their rotating frame relaxation over much of the temperature interval studied. An interesting spectral observation is the strong motional broadening of the methyl group in the tem perature region of the T minimum. These and other facets of the polypropylene data as well as similar data for other polymers are discussed with respect to their implications for insight into polymer chain dynamics in the solid state. 13

13

g

1

1ρ

1

1ρ

1ρ

2

1ρ

M A N Y O F T H E E A R L I E S T N M R experiments were per formed on solid samples, technical advances i n magnetic f i e l d homo geneity and p u l s e d F o u r i e r transform N M R have resulted i n N M R b e i n g considered a h i g h resolution analytical tool for studying l i q u i d -

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0065-2393/83/0203-0455$06.00/0 © 1983 AmericanChemical Society

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

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 6, 2016 | http://pubs.acs.org Publication Date: June 1, 1983 | doi: 10.1021/ba-1983-0203.ch026

456

POLYMER CHARACTERIZATION

state samples. U n t i l recently, s o l i d materials c o u l d be studied only b y so-called broad-line N M R and pulse relaxation N M R measurements on abundant n u c l e i , usually hydrogen and fluorine. H o w e v e r , the re cent advances i n the application of magic angle sample s p i n n i n g , d i polar d e c o u p l i n g , cross-polarization, and m u l t i p u l s e N M R have brought s o l i d samples into the realm of h i g h resolution N M R i n w h i c h i n d i v i d u a l resonances of the s o l i d can be resolved. T h e usefulness of the h i g h resolution experiments is now w e l l established. I n particular, h i g h resolution carbon N M R of solids has shown considerable pro mise, as e v i d e n c e d by the numerous publications i n w h i c h magic angle s p i n n i n g N M R has b e e n used to study polymers a n d other s o l i d organic and inorganic materials (1—4). I n p r i n c i p l e , resolution of i n d i v i d u a l carbon resonances i n b u l k polymers allows relaxation experiments to be performed w h i c h can be interpreted i n terms of main-chain and side-chain motions i n the solid. T h i s is a distinct advantage over the more c o m m o n Ή - N M R relaxa tion experiments where efficient s p i n diffusion usually results i n the averaging of the relaxation behavior over the ensemble of protons. T h u s , a direct interpretation of O - r e l a x a t i o n data i n terms of u n i q u e motions of the s o l i d p o l y m e r is often not possible. Relaxation parameters of interest for the study of polymers i n clude the C s p i n - l a t t i c e relaxation time i n H , T ; the spin—spin relaxation time, T ; the nuclear Overhauser enhancement, N O E ; the proton and carbon rotating frame relaxation times, T%; the C - H cross-polarization or cross-relaxation t i m e , T ; and the proton relaxa tion time i n the dipolar f i e l d T . N o t a l l of these parameters provide information directly; nonetheless, a l l the inferred information is i m portant i n characterizing motional frequencies and amplitudes i n mac romolecules. A l t h o u g h i n i t i a l studies on solids b y cross-polarization ( C P ) , magic angle s p i n n i n g ( M A S ) , and C N M R have b e e n carried out almost exclusively at ambient temperature (5), f u l l exploitation of this spectroscopy requires variable temperature capability. T h i s is partic ularly true of macromolecules where the accessibility of variable t e m perature magic angle s p i n n i n g ( V T - M A S ) makes feasible the investi gation of structural and motional features of polymers above and b e l o w the glass transition temperature, T , and i n temperature regions of secondary relaxations. T h e spectral data and T\ measurements b y Garroway et al. (6) on epoxy resins over the l i m i t e d temperature inter val from - 3 1 to 51 °C represent the only other V T - M A S C N M R re sults to date. A spinner assembly has b e e n described that is suitable for routine operation over a w i d e range of temperatures (7). T h i s chap ter examines the C - r e l a x a t i o n behavior of two c o m m o n polymers: polytetrafluoroethylene ( P T F E ) and isotactic polypropylene (PP). 1 3

0

t

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C R

1 D

1 3

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p

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F L E M I N G ET A L .

Magic Angle Spinning C-NMR 13

Spectroscopy

457

Experimental The C - N M R data at 15.1 M H z were acquired on a modified Nicolet TT-14 N M R system. The features of this spectrometer and of the spinning assembly have been reported previously (4, 7, 8). Samples were machined into the shape of Andrew-type rotors (9) and used directly for the various studies. Temperature variation was achieved by cooling or heating the helium gas used for driving the rotor. The temperature was controlled to ±2 °C with a home-built temperature sensing and heater/feedback network. Spin-lattice relaxation times T were collected using a pulse sequence developed by Torchia (10) that allows C P enhancement of the signals. The T data were deter mined at 40 and 58 k H z using T methodology described elsewhere (5). Polypropylene, PP, samples were made from compression-molded ProFax PP (Hercules). The material was quenched slowly and had a 70% crystalUnity, as determined by Ή N M R , for the 95% isotactic material. The polytetrafluoroethylene, P T F E , was machined from commercial Teflon from du Pont. Data based on an IR band intensity ratio analysis (IJ) indicated the P T F E sample to be 67% crystalline. 13

t

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Results and

Discussion

T h e proton-decoupled C P / M A S C - N M R spectra of P P as a func tion of temperature are shown i n F i g u r e 1. A t ambient temperature, a l l three carbons of the P P repeat u n i t are resolved. H o w e v e r , as the temperature is l o w e r e d , the m e t h y l resonance begins to broaden sig nificantly and is completely lost i n the baseline at about - 1 4 3 °C. T h e broadening, w h i c h is also seen to occur to a degree for the methine and methylene carbons, arises p r i m a r i l y from incomplete d e c o u p l i n g as the reorientation rate of the m e t h y l group about the C - a x i s be comes comparable to the strength of the d e c o u p l i n g f i e l d . H e t e r o n u clear dipolar c o u p l i n g characterized by correlation frequencies near the d e c o u p l i n g frequency is not d e c o u p l e d efficiently (12). T h i s phenomenon has b e e n observed (6) for m e t h y l groups i n epoxy resins, and is the same mechanism responsible for the motional broadening i n crystalline regions of P T F E (13). T h e T data for P P over a temperature range from - 1 9 5 to 24 °C are summarized i n F i g u r e 2a. A s indicated i n the figure, each of the car bons displays i n d i v i d u a l relaxation rates. T h e C H and C H carbons have a T m i n i m u m at about - 1 1 3 °C, nearly the same temperature as that reported for the proton T m i n i m u m i n isotactic P P (14). I f it is assumed that a C - H heteronuclear dipolar relaxation mechanism is operative, the m e t h y l protons probably dominate the relaxation behavior of these carbons over m u c h of the temperature range studied despite the 1/r dependence of the mechanism. T h e shorter T± for the C H as compared to the C H then arises from the shorter C - H distances. A p p a r e n t l y , the contributions to spectral d e n 1 3

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Figure 2. C-NMR T, relaxation times for the methyl (A), methylene (%), and methine (O) carbons of PP as a function of temperature at 1.4 T(a), and C-NMR T relaxation times for the same carbons of PP as a function of temperature at 1.4 Τ (b). 13

13

l p


26.

Magic Angle Spinning C-NMR


13

Spectroscopy

sity i n the megahertz region of the frequency spectrum due to backbone motions are m i n o r relative to the side-group motion. T h e T data for the C H and C H carbons also give an indication of methyl group rotational frequencies (Figure 2b). As the temperature is l o w e r e d b e l o w - 1 1 3 °C, the c o n t r i b u t i o n of the m e t h y l protons to megahertz spectral density decreased, yet increased i n the kilohertz regime. Consequently, the T decreases by roughly 10 times b e t w e e n - 1 0 3 and - 1 9 5 °C. T h e interpretation of carbon T data is complicated by the fact that s p i n - s p i n (cross-relaxation) processes, as w e l l as rotating frame s p i n - l a t t i c e processes, may contribute to the relaxation (15). O n l y the latter process provides direct information o n molecular motion. F o r the C H and C H carbons of P P , the T s do not change greatly over the temperature interval from —113 °C to ambient temperature a n d , as opposed to the T behavior, the C H carbon has a shorter T than the C H carbon. These results suggest that s p i n - s p i n processes dominate the T . H o w e v e r , b e l o w - 1 1 8 °C, the T s for both carbons shorten and t e n d toward equality. A proton T m i n i m u m ( w h i c h reflects m e t h y l group reorientation of kilohertz frequencies) has been re ported at - 1 8 0 °C (14). N o clear m i n i m u m is observed i n the C - N M R data, perhaps due to an interplay of spin—spin and spin—lattice pro cesses. Nonetheless, it is apparent that the m e t h y l protons are respon sible for the s p i n - l a t t i c e portion of the T relaxation for the C H a n d C H carbons. F i g u r e 3 shows semilog plots of typical Ί\ a n d T° relaxation data (intensity vs. time) at - 2 °C for P T F E . T h e decays are clearly nonexponential, indicative of m u l t i p l e relaxation behavior. H o w e v e r , for both plots, the long-time behavior of P T F E can be characterized b y one relaxation t i m e , associated w i t h about 6 5 - 7 0 % of the total signal intensity as j u d g e d by the y-intercept, and i n agreement w i t h the I R analysis reported i n the experimental section. O n this basis and p r e v i ous F - N M R relaxation studies (16), the long-time relaxation compo nent is ascribed to crystalline regions of the polymer. T h e faster re laxing component of the resonance l i n e is attributed to noncrystalline regions and is not described b y a single time constant, but b y a d i s tribution of relaxation times i n accord w i t h the results on glassy p o l y mers (5). T h e C - N M R T data for the crystalline component of P T F E are plotted vs. reciprocal temperature i n the region from - 4 3 to 47 °C i n F i g u r e 4. A shortening of T b y two orders of magnitude is observed i n l p

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C P - T Data PTFE -2°C; 1 ms CP -I 1

Τ =31 sec


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15 20 Time (sec)

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30

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T Data PTFE -2°C; 1 ms CP l p

60 80 Time (ms)

140

Figure 3. C-NMR T and T relaxation data (intensity vs. time) for PTFE at -2 °C and 1.4 T. The sample had a crystallinity of 67%. 13

x

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this temperature interval. T h e sharp decrease i n T b e t w e e n 0 and 20 °C c a n be attributed to the w e l l - k n o w n crystal phase transition at 19 °C (17). T h e increase i n specific volume accompanying the unit c e l l change and slight u n w i n d i n g of the helix apparently allows rotational motion characterized b y correlation times i n the range of 10~ —10~ s. T h e motional broadening of the C - N M R resonance l i n e i n the tern1

5

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Magic Angle Spinning C-NMR


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Spectroscopy

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T(C°) 50

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Figure 4. C-NMR 13

Ί \ relaxation times for PTFE as a function of tem perature at 1.4 T .

perature range from 17 to 27 °C has b e e n reported p r e v i o u s l y (13) a n d also reflects the phase transition. T h e C - N M R T relaxation times for P T F E decreased from about 185 ms at - 1 1 7 °C to 53 ms at 47 °C. I n contrast, the F - N M R T relaxa tion times for crystalline P T F E change 2 0 - 4 0 fold over the same temperature range (16). T h e smaller change i n the C T is probably due i n part to the contribution of the spin—spin interactions to the rotating frame relaxation process. 1 3

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Conclusion Significant i n f o r m a t i o n about d y n a m i c processes i n the s o l i d state using variable temperature dipolar d e c o u p l e d magic angle s p i n n i n g C - N M R can be obtained. T h e results are i n substantial agree ment w i t h the results of p u l s e d F - a n d Ή - N M R relaxation mea surements. A l t h o u g h the p o l y p r o p y l e n e data suggest that relaxation i n methyl-containing polymers can be dominated by the m e t h y l protons, the use of deuterated m e t h y l groups can overcome this contribution i f necessary. Possibly the strong m e t h y l contribution to relaxation may be exploited to study intermolecular phenomena such as p o l y m e r compatibility a n d m i s c i b i l i t y . T h u s , the C - N M R technique offers the opportunity to study details of dynamic processes that were p r e v i ously intractable u s i n g most other techniques. 1 3


1 9

1 3

Literature Cited 1. Fyfe, C. Α.; Rudin, Α.; Tchir, W. Macromolecules 1980, 13, 1320. 2. Dixon, W. T.; Schaefer, J.; Sefcik, M. D.; Stejskal, E . O.; McKay, R. A. J. Magn. Reson. 1981, 45, 173. 3. Brown, C. E . ; Jones, M. B.; Kovacic, P. J. Polym. Sci., Polym. Lett. Ed. 1980, 18, 653. 4. Lyerla, J. R. In "Contemporary Topics in Polymer Science"; Shen, M., Ed.; Plenum: New York, 1979; Vol. 3, p. 143. 5. Schaefer, J.; Stejskal, E. O.; Buchdahl, R. Macromolecules 1977, 10, 384. 6. Garroway, A. N.; Moniz, W. B.; Resing, H. A. Faraday Discuss. Chem. Soc. 1979, 13, 63. 7. Fyfe, C. Α.; Mossburger, H.; Yannoni, C. S. J. Magn. Reson. 1979, 36, 61. 8. Fyfe, C. Α.; Lyerla, J. R.; Volksen, W.; Yannoni, C. S. Macromolecules 1979, 12, 757. 9. Andrew, E . R. Int. Rev. Sci.: Phys. Chem., Ser. Two 1976, 4, 1973. 10. Torchia, D. A. J. Magn. Reson. 1978, 30, 613. 11. Rabolt, J. F., IBM, San Jose, private communication. 12. Rothwell, W. P.; Waugh, J. S. J. Chem. Phys. 1981, 74, 2721. 13. Fleming, W. W.; Fyfe, C. Α.; Lyerla, J. R.; Vanni, H.; Yannoni, C. S. Mac romolecules 1980, 13, 460. 14. McBrierty, V. J.; Douglas, D. C.; Flacone, D. R. J. Chem. Soc., Faraday Trans. 2 1972, 68, 1051. 15. VanderHart, D. L.; Garroway, A. N. J. Chem. Phys. 1979, 71, 2773. 16. McCall, D. W.; Douglas, D. C.; Falcone, D. R. J. Phys. Chem. 1967, 71, 998, and references therein. 17. For example, see Clark, E . S.; Muus, L. T. Z. Kristallogr. 1960, 117, 119. RECEIVED for review October 14, 1981. ACCEPTED March 8, 1982.


Polymer Characterization - ACS Publications

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