broadband probe at either 395K or 445K, single pulse acquisition and WALTZ 'H decoupling. For "0 NMR, standard conditions included a I O ps d 2 pulse, 2000 ppm spectral window and 8K complex points. Signal averaging of 32K to 64K points was accomplished using a 100 ms recycle delay. Spectra were obtained using 100 Hz line broadening with

"0 NMR INVESTGATIONS OF OXIDATIVE DEGRADATION

s m - - qb- aq 7 6 (*, IN POLYMERS Todd M. Alam, M. Celina, R. A. Assink,

K. T. Gillen, and R. L. Clough

Aging and Reliability, Bulk Properties Department Sandia National Laboratories

Albuquerque, New Mexico 87185-1407

INTRODUCTION

An improved understanding of mechanisms for oxidative degradation in polymers may be realized by identification of the resulting chemical species. There have been numerous studies of polymer degradation utilizing NMR, including the recent 'H investigation of thermal degradation in polyisobutylene' and the "C study of oxidative thermal degradation of epoxy composites.' These studies are complicated by the small concentration of degradation products in comparison to the native polymer NMR signal. We have initiated studies utilizing both solution and solid state magic angle spinning (MAS) "0 NMR for a series of oxidatively aged polymers. Due to the low natural abundance (0.037%) and a nuclear spin of I = 5/2 possessing an appreciable electrical quadrupolar moment, the use of "0 NMR as a structural probe has been limited.',' By using synthetically enriched materials"0 NMR may prove valuable for investigating polymer and material systems. In this short note we report the solution "0 NMR for oxidatively degraded polypropylene, ethylene-propylene-diene, polyisoprene and nitrile rubber. By utilizing enriched 0, during the accelerated aging process, the "0 NMR spectra provide a unique probe since all the observed NMR resonances result directly from the oxidative degradation.

EXPERIMENTAL

Materials: The polypropylene (PI') used in this study was obtained as an

unstabilized reactor powder from Moechst AG. The nitrile rubber was based on a typical commercial formulation (including carbon black as a filler) and was manufactured by Burke Rubber Co.' The ethylcnc- propylene-diene (EPDM) rubber (carbon black filled) was also an industrial material. The polyisoprene polymer sample was obtained from Aldrich.

Oxidative Aging: Appropriate weights of the materials were placed in ampoules

and tilled to approximately 250 Torr with 85% enriched "02 (Isotec). The saniples were then aged at temperatures ranging from 1O0-14OoC, depending on their relative sensitivity to oxidation (Le. the more reactive nitrile rubber, and polyisoprene at 100°C. and the EPDM and polypropylene at 140'C). During aging the oxygen pressure decreased to approximately 30 Torr. The residual oxygen as well as CO and CO, formed during degradation were accurately determined with gas clvomatography. a technique commonly employed for precise oxygen uptake measurements.' Knowing the consumed oxygen and volatiles produced, the approximate amount of oxygen (weight %) present in the samples as oxidation products was calculated.

Solution NhlR For all "0 and 'IC spectra reponed here IO0 mg of the thermally

osidized material was swollen or dissolved in 1 ml of trichlorobenzene. Trace alcohol natural abundance spectra were obtained by dissolving -2% (vlv) of the various alcohols in trichlorobenzene. "0 and "C NMR

zero filling to 16K complex points. Linear prediction for the first 8 dala points was used to remove the baseline roll distortion resulting from acoustic ring in the probe. All "0 chemical shifts are referenced in ppin relative to external doubly distilled H,O (6 = 0 ppm), while "C chemical shifts are referenced to the upfield peak of the solvent trichlorobenzene (6 = 127.9 ppm).

RESULTS

The "0 solution NMR spectra of oxidatively degraded polypropylene for different percentages of 0, incorporation are shown in Figure I . Resonances over a wide range of chemical shifts are observed, and can be quickly assigned tu dillcrent oxygen functiunillitics in carbon containing compounds.

700 600 500 400 300 200 100 0 -100

I tppml Figure I . Solution "0 NMR spectra of polypropylene (PP) at different levels of oxidative degradation: A) 0.9 %. B) 2.0 and C) 4.0% by weight oxygen incorporation (R = C or H, X = C or 0).

In Fig. I the "0 resonances observed between +600 and +550 ppm can be assigned fo aldehydes and ketones. The broad resonances observed between +400 and +300 ppni result from carbonyl oxygens in esters and lactones. The carbonyl resonance for anhydrides is also expected to appear in this chemical shift range, but typically resonate downfield in the narrower range 6 = +370 to +400 ppm. The large resonances between +280 and +250 pprn are typically associated with alkyl hydrogen peroxides. peroxides. anhydrides and carboxylic acids. Between +250 ppin and + I 70 ppiii the singly bonded oxygen of lactones is observed. while +210 to +IO0 ppm the oxygens of ester groups resonate. Between +I IO and -20 ppm acetals and saturated or unsaturated ethers (including epoxides) would be observed, while the"0 chemical shift of alcuhols are between +SO and -50 ppm.

The insprcfion of Fig. IA-IC show that in PP the oxidalive dcgradatiun products contain a variety of oxygen lunctionalities. 'lhesc functionalities are also observed ar flrc lowest oxygen incorporation (0.9%). succestinE that thc decmdofion aathwav remains the same for all -- - extents of o-ddtioii The lack of apprcciable signal intensity between

R

degradation product and that lactones and anhydrides are the functionalities responsible for the "0 NMR resonances between 6 = +400 and +300 pprn. Lactones have been proposed as secondary oxidation products in PP previously by FTIR.6 Model compound studies and polarization transfer experiments are presently being pursued in order to identify the chemical identity of the major I7O NMR resonances in PP. A dominant degradation product observed for the 4.0% 0, aged PP occurs at 6 - -13 ppm. It has been established from polarization transfer INEPT experimcnts (not presented here) that thc -13 ppm resonance results from a hydrogen bearing oxygen (0-H), limiting the identity of this degradation species to an alcohol. Previous investigations have shown that primary, secondary and tertiary saturated alcohols have characteristic chemical shift ranges of approximately: 6 = -3 to +IO (except methanol which is 6 = -37 ppm), 30 to 40 and 55 to 70 ppm, respectively.' These trends were confirmed by obtaining I7O NMR spectra of trace amounts of various alcohols in tricholorbenzene. From these studies it was concluded that the unassigned alcohol (6 - - I 3 ppm) is not a simple saturated primary alcohol. Investigations into the effect of unsaturation, and other functional groups on the "0 chemical shift of alcohols are presently being pursued. The other dominant degradation species are observed between 6 = 275 and 285 pprn. Experiments to unequivocally distinguish between a carboxylic acid, alkyl hydrogen- peroxide. peroxide or a anhydride are in progress.

The real benefit of utilizing "0 NMR for polymer degradation studies becomes apparent when one contrast these results (even qualitative in nature) with information obtainable using 'IC NMR. In Fig. 2 the "C spectra for unaged and oxidatively degraded PP (4% 0,) are shown. The changes observed in the "C NMR spectra are few and extremely small, with no changes in the carbonyl area of the spectrum observed under the acquisition conditions used (7114 pulses and 12 s interpulse delay). At high levels of oxidation "C NMR spectroscopy has been used to study the degradation products in pol!propylene.' In PP mechanical failure occurs even at lower extents of oxidation, emphasizing the importance of identifying degradation species and mechanisms at low concentrations.

- - , ~ . ~ - , _ _ , .__ . , __ , 5 5 5 0 4 5 4 0 3 5 3 0 2 5 2 0 1 5

ij I3c ( p p m I

Figure 2. Solution "C NMR spectra at 395K of A) uuaged and B) oxidatively aged (4% 0,) polypropylene (expanded 200x ). The resulting degradation products are marked (*).

The "0 NMR spectra for isoprene, nitrile and EPDM rubber samples are shown in Fig. 3 and allow differences and similarities in the oxidative degradation mechanism to be addressed. For example, the alcohol present in polypropylene (- -13 ppm) is also produced during degradation of these three polymers. In the nitrile rubber (Fig. 3B) there is very little ketone or aldehyde (6 = 600 - 500 ppm) production, as compared to polyisoprene (Fig 3A), EPDM (Fig. 3C) or polypropylene (Fig. 1C). The increased "0 NMR signal between +I50 and +lo0 pprn in polyisoprene (Fig IA) supports the role of ester formation in the oxidative degradation of this material.

.

600 500 400 300 200 100 0 - 1 0 0

8 ( p p m )

Figure 3. Solution "0 NMR spectra at 445K for different degrees of polymer oxidative degradation for. A) Polyisoprene 59 days at 100" C with 1.8% 0, incorporation, B) Nitrile rubber. 59 days at 100" C with 1.5% 0, incorporation, C) EPDM (black), 59 days at 140" C with 1.0% 0, incorporation.

ACKNOWLEDGEMENTS

Work supported by the United States Department of Energy under Contract DE-AC04-94AL85000. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy.

REFERENCES

1. T. Sawaguchi. M. Seno, Polymer, 1996.37.3697. 2. L. Yang, F. Heatley, T. G. Blease, R. I. Thompson, Eur. Polyrn. J.. 1996,32,535. 3. W. G. Klemperer.. Angege,~. Cltenz. btl., 1978, 17, 246. 4. D. W. Boykin 1 7 0 NMR Speclroscopy in Organic Chemisrry, 1991. CRC Press, Bow Raton, FI. 5. J. Wise, K. T. Gillen. R. L. Clough. Polym. Deg Stab. 1995, 49.403. 6. G. A. George, M. Celina, A. M. Vassallo, P. A. Cole-Clarke, P o l p . Deg. Stab., 1995, 48. 199. 7. D. Vaillant, J. Lacoste, G. Dauphin. Polym. Deg. SId. 1991. 45, 355.

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liabili- ty or responsibility for the accuracy, completeness, or usefulness of any information, appa- ratus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessan'ly constitute or imply its endorsement, mmmendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessar- ily state or reflect those of the United States Government or any agency thereof.

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