C NMR Observation of the Effects of High Energy Radiation and

11 C N M R Observation of the Effects of High

13

Energy Radiation and Oxidation on Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on June 22, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0169.ch011

Polyethylene and Model Paraffins F . A . B O V E Y and F . C . S C H I L L I N G Bell Laboratories, Murray Hill, N J 07974 H. N. CHENG G.A.F.

Corp., 1361 Alps Road, Wayne, N J 07470

Carbon-13 NMR enables one to identify the products of t γ-irradiation of a model hydrocarbon, n-C H ,and of poly ethylene thermal oxidation. Irradiation ofn-C H in the molten state produces cross-links (Η-shaped molecules), long branches (T-shaped molecules), and trans-vinylene groups. Irradiation in the crystalline state produces none of these. Instead, it end-links the molecules to produce linear dimers, detected by gel permeation chromatograph (GPC). Mechanisms are proposed to account for these results. Branched polyethylene thermal oxidation gives rise to long chain ketones and carboxylic acids as the principa products, with smaller amounts of long chain secondary alcohols and hydroperoxides, esters of long chain carboxyli acids with long chain secondary alcohols, and long chain γ-lactones. Branch points are about 10-fold more reactive than linear chains. The results generally agree with ac cepted mechanisms. 44

90

44

90

/^arbon-13 N M R is powerful for determining paraffinic polymer struc^ tures because of the sensitivity of carbon chemical shifts to branches and chain ends. Carbon shieldings are also strongly dependent on oxygen-containing groups. These C N M R features suggest its use for the qualitative detection and quantitative estimation of the chemical effects of polyethylene and model paraffin exposure to high energy radia tion and thermal oxidation. This chapter deals w i t h its application to these problems. 1 3

0-8412-0381-4/78/33-169-133$05.00/l © 1978 American Chemical Society Allara and Hawkins; Stabilization and Degradation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

134

STABILIZATION

A N DD E G R A D A T I O N

C

4-41

C

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on June 22, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0169.ch011

11 1

C

2

fin

1 γ w L_

I

JIT

4-41

C

CRYSTAL

c

C (C ) 3

I

40

I

I

1

I

I

35

CH

2

3

(C43)

42

45

P O L Y M E R S

H90 (60 Mrad)

IRRADIATED

Ί

O F

I

I

I

30

_i—I—ι

ι ι ι I 1 1 1 ι I 1 1 L

25 ppm vs. TMS

20

15

1 ••

10

1

Figure 1. 25 MHz C spectra of n-C H after exposure to 53 Mrad of y-radiation in the molten state (top) and in the crystalline state (bottom). Peaks CH , C , and C correspond to the first, second, and third carbons from the end of the chain; the fourth and successive carbons give the large peak at ca. 30 ppm. 13

S

Gamma

g

4Il

90

s

Irradiation

The relative susceptibilities of the crystalline and amorphous por tions of polyethylene to high energy radiation has been a moot question. Present evidence indicates that for linear polyethylenes without v i n y l end groups the G-values for the production of hydrogen, cross-links, and transvinylene groups are identical for the melt at 133 °C and the crystalline state at 130°C ( I ). (Scission is now believed to be essentially n i l ; at least, its occurrence is not demonstrated).

Allara and Hawkins; Stabilization and Degradation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

11.

B O V E Y

E T

135

Carbon-13 NMR Observations

A L .

Chemical and physical evidence indicates that cross-linking i n the solid polymer occurs i n the amorphous phase; this is suggested b y the recent studies of Patel and Keller (2,3). T o reconcile these findings, one may conclude that the crystalline phase, while resisting the conspicuous effects of radiation, can transmit the absorbed energy to the amorphous phase. C N M R has been applied to this problem b y examining the spectra of n - C 4 H after exposure to γ-irradiation. This work is an extension of earlier studies b y Salovey, Falconer, and Hellman (4,5,6,7) i n which gas chromatography and G P C were used to identify the cross-linked products from model paraffins. C a r b o n - N M R can provide more specific and con clusive information concerning the structure of such products. M o d e l paraffins have no amorphous phase i n the crystalline state and are able to accept large doses without gel formation. Figure 1 compares the spectra of C 4 4 H 9 0 after exposure to 53 M r a d of ^ C o irradiation i n the molten state at 115°C (top) and i n the crystalline state at 25°C (bottom). (The spectra were run at 25 M H z on neat melts at 9 6 ° C ; 20,000 scans were accumulated for each). In the molten state at least three new structures were formed. These can be identified by calculation and reference to model compounds (8) as corresponding to the expected one-bond cross-links ( H structures) and frans-vinylene groups ( V ) ; there are also long branches ( T ) i n numbers about equal to the crosslinked units. These structures may arise i n the following way:


1 3

4

9 0

—CH2CH2CH2CH2CH2— —CH CH2CH2CH CH — 2

2

—CH

11° radicals

2

\

—CH2CH2 C H C H 2 C H 2 — CH — 2

—CH2CH2CHCH2CH2—

—CH2CH2CHCH0CH2—

2

(V)

CH

2

CH

2

long branch (T)

H

\

H

CH

/

2

—CH CH CHCH CH2— 2

2

2

cross-link (H)


136

STABILIZATION

A N D D E G R A D A T I O N

O F

P O L Y M E R S


The crystal appears to undergo none of these reactions. G P C observa tion of the product shows, however, that about 1 % of the molecules have doubled in molecular weight. This is the result of end-linking through methyls either i n adjacent rows (as shown below) or adjacent layers i n the crystal structure: it implies a migration of energy to the chain ends. A possible mechanism follows:

CH2

/

CH2

\

/

\

/

CH2 CH2

/

CH

/

CH

CH

/ \

CH

/

/ CH

3

CH

2

\

CH2

2

CH

\

CH

2

/ CH

2

/

/

\

CH2

CH

/ \ /

/

CH2

/ CH2

2

\ /

CH2

CH

3

\

CH

2

CH2

\ CH

\

3

CH

2

CH2

CH2 CH2 . / \ / \ / CH2 CH2 CH2

\

CH3

CH2

\

/

CH2

CH3 \

\

CH

2

/ \

CH2

/

/

CH

2

2

CH3

CH2

\

CH2

\

\

CH2

CH2

\

CH3

CH2

CH2

CH

2

\ / CH2

\

/ CH2

3

radical migration CH / \ 2

•CH

CH CH / \ / \ CH2 CH2 CH2 ' 2

2

end-linking

\

CH

CH

2

/

2

\

2

CH

2

/

\

CH

CH CH CH CH CH \ / \ / \ / \ / \ / CH2 CH2 CH2 CH2 CH2 CH2

\ /

2

2

2

2

/

CH

2

(doubling)

ψ /

CH

2

2


2

11.

BOVEY E T A L .

137


The absence of cross-linking confirms conclusions of Keller and Patel. Recently Bennett, Keller, and Stejny (9) have prepared the hydro carbon l,l,2,2-tetra(tridecyl)ethane as a polyethylene cross-link model: C13H27

\


/

CH—CH

C13H27

y

C13H27

\ C13H27

In this compound's C spectrum, the resonances for the cross-link car bons, and for those a and β to them, appear i n the same positions as those shown i n Figure 1, confirming the assignments made. They also reported the spectra of eicosane ( C o ) and hexacosane ( C g ) which h a d been exposed to 500 M r a d . The principal resonances of the irradiated material corresponded to those reported here, but the very large dose caused the appearance of some additional, unidentified small peaks. 1 3

2

Thermal

2

Oxidation

Although the general outlines of polyethylene thermal oxidation w i t h molecular oxygen are understood generally, traditional investigative methods have left some questions unanswered. The exact nature of the oxidation products is not completely clear (10,11,12,13). This is caused by heavy reliance on ir spectroscopy. I R spectroscopy suffers from band overlap, particularly i n the important carbonyl stretch region near 1725 cm" , and from the necessity of establishing reliable extinction coefficients. Because of the large range of C chemical shifts, peak overlap usually does not present a severe problem. Since C chemical shifts are sensitive to local structure, a more positive and detailed identification of oxygencontaining groups is possible. If spin-lattice relaxation is considered, i.e., b y selecting pulse intervals equal to 3 T i or greater, oxidation products can be estimated quantitatively from peak intensity measurements, assuming that at the observing temperature of 110°C a l l nuclear Overhauser enhancements have reached their maximal value of 3. W e have studied the oxidation of branched polyethylene at 140 °C using circular films ca. 5 m i l thick exposed to oxygen i n a conical flask held i n an oil bath and connected to a mercury manometer (14). Figure 2 shows a branched polyethylene's C spectrum before (top) and after (middle and bottom) thermal oxidation to the extent of 108ml 0 per gram of polymer, corresponding to about 4.5% toward complete oxida tion. ( Some H 0 is lost but probably little C 0 . ) The resonances asso ciated with branches are known (8) and are indicated on the spectrum. In spectra b and c the new peaks resulting from oxidation are shaded. Most of these can be unequivocally assigned b y comparison with appro1

1 3

1 3

1 3

2

2

2


138

STABILIZATION

A N D DEGRADATION

O F

P O L Y M E R S

ppm vs CS2 13

140


II I I

60

I I I

I

150

I I I I

I

I

I I I I

1 1 1 ,L 1 1 1 1 1 50

I I I I

1'

I

»

170

I

I I I I

. ι

ι.

• C-C-Ç-0H

1

1

yLM

lactone

ester

ι ι I ι ι ι ι I ι ι ι ι

180

-c-g-C-

1

190

I ι ι ι ι I ι ι ι ιI ι ι ι I I ''I I I

ι ι ι I ι ι ι ι I ι ι ι ι

111

/

.

180

I I I I

1

II

M i l l

I

30 20 ppm vs. TMS

0

ι ,

1 1 1 1 1 1.1

I I M

ι I ι 1 ι "1ι 1

40

••MINIMUM M •c-ç-c

215

160

11

1

1

I

11 10

1

111 11 1 11 111 1 0

1 1 1 1 I11 I 1 1 1 1

lactone ester

OH

1

_

ι ι I ι ι ι ι I ι ι ι ι I ι ι

ι ι ι ι ι I

170 85 ppm vs. TMS

65

Figure 2. 25 MHz C spectra of branched polyethylene: top, before oxidation. The peaks are labeled according to the designations of carbons on and near a branch shown at the upper left. The middle spectrum shows the new peaks (shaded) resulting from oxidation to the extent of 108 ml 0 per gram of polymer. The bottom spectrum shows peaks of oxidation products appearing below 65 ppm. Peaks not yet identified are indicated with question marks. 13

2


ι ι ι • ι


11.

BOVEY E T A L .

139


priate long chain model compounds. The groups thus observed are: long chain ketone, carboxylic acid, secondary alcohol, and secondary hydro peroxides; esters of long chain carboxylic acids with long chain secondary alcohols, and long chain y-lactones. Not observed at the present detection level (ca. 0.3% ) were: aldehydes, conjugated ketones, olefins, peresters, primary and tertiary hydroperoxides, and primary and tertiary alcohols and their esters. Figure 3 shows the distribution of established oxidation products as a function of time and extent of oxidation, expressed as m L 0 per gram of polymer. The secondary hydroperoxide reaches a maximum at ca. 40 m L - g and then decreases as other products accumulate. The results appear to agree with those of model reaction calculations (15) and support the generally accepted oxidation scheme: 2

+ 1

0

—CH CHCH2—

2

—CH2CHCH2—

2

00· —CH CHCH2— + —CH2— 2

00·

—CH CHCH — + — C H — 2

2

OOH (secondary hydroperoxide) —CH2CHCH2—

0· + -OH (recombine)

—CH CCH2— + 2

—CH2CHO +

CH2—

II

H 0 2

Ο (ketone) —CH C00H (acid) 2

—CH CHCH2— + —CH— 2

OH (Secondary alcohol)

—CH2—C—0—CH

II ο


140

STABILIZATION

A N D D E G R A D A T I O N

O F

P O L Y M E R S

Over 8 0 % of the oxygen consumed can be accounted for by the known products observed However, the scheme is not complete since there are products that have not been considered. One of these products is ylactone, also reported by Adams (11) i n polyolefin oxidation.

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5

10

20

ml. 0 absorbed per g. polymer 2

Figure 3. Distribution of polyethylene oxidation products as a function of the extent of oxidation (lower scale: 0 absorbed; upper scale: time of reac tion). The vertical scale refers to the intensity of the backbone CH resonance at 30 ppm. The hydroperoxide decomposes gradually at 110°C; the concen trations shown above are the estimated values extrapolated back to zero time assuming first-order decay. 2

2

The data provide an estimate of the ratio of reactivity to oxidative attack of branch points compared with linear hydrocarbon chains. T h e butyl C carbon resonance intensity at 23.4 p p m (Figure 2) decreases from 9.7 to 6.6 per 1000 C H upon absorption of 53 m l - g " of oxygen. Oxidative cleavage at an η-butyl (or longer) branch point occurs as fol lows (15,16,17) (the tertiary alkoxy radical having been generated by steps parallel to those shown above): 2

2

1


11.

B O V E Y

E T

. · — C H — C — C H — · · + -Bu 2

Bu

I

-CH —C—CH 2

2

(a)

o 2

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141


A L .

—CH —C—Bu + ·C H — · 2

2

(b)

II 0

Reactions a and b occur with statistical probability giving long chain ketone and butyl ketone, which are observable i n these experiments. M o d e l compound measurements show that in the η-butyl ketone group, the butyl C carbon resonance moves from 23.4 to 22.7 ppm, coinciding with the C resonance of "amyl + long" branches, the intensity of which increases upon oxidation. B y comparing these results quantitatively with the overall production of oxidized structures, the reactivity ratio of branch points to linear chains is calculated to be 9.8 ± 1.0. This result is in agree ment with the value of 8 derived from model hydrocarbon oxidation studies (15). 2

2

Literature Cited

1. Mandelkern, L., in "The Radiation Chemistry of Macromolecules," Vol. I, M. Dole, Ed., p. 329, Academic, New York, 1972. 2. Patel, G. N., Keller, Α., J. Polym. Sci., Polym. Phys. Ed. (1975) 13, 303, 323, 333. 3. Patel, G.N.,J. Polym.Sci.,Polym. Phys. Ed. (1975) 13, 339, 351, 361. 4. Salovey, A. R., Falconer, W. E.,J.Phys. Chem. (1965) 69, 2345. 5. Salovey, A. R., Falconer, W. E.,J.Phys. Chem. (1966) 70, 3203. 6. Falconer, W. E., Salovey, R.,J.Chem. Phys. (1966) 44, 3151. 7. Salovey, R., Hellman, M. Y., Macromolecules (1968) 1, 456. 8. Bovey, F. Α., Schilling, F. C., McCrackin, F. L., Wagner, H. L., Macro molecules (1976) 9, 76. 9. Bennett, R. L., Keller, Α., Stejny, J., J. Polym.Sci.,Polym. Chem. Ed. (1976) 14, 3027. 10. Luongo, J. P.,J.Polym. Sci. (1960) 42, 139. 11. Adams, J. H.,J.Polym. Sci., Polym. Chem. Ed. (1970) 8, 1077, 1279. 12.Émanuél,M. M., Izv. Akad. Nauk. SSSR., Ser. Khim. (1974) 5, 1056. 13. Tabb, D. L., Sevick, J. J., Koenig, J. L., J. Polym. Sci., Polym. Phys. Ed. ( 1975) 13, 815, and references therein. 14. Cheng, H. N., Schilling, F. C., Bovey, F. Α., Macromolecules (1976) 9, 363. 15. Allara, D. L., Edelson, D., Rubber Chem.Technol.(1972) 45, 437. 16. Chien, J. C. W., Vandenberg, E. J., Jabloner, H., J. Polym. Sci., Polym. Chem. Ed. (1968) 6, 381. 17. Mill, T., Richardson, H., Mayo, F. R., J. Polym. Sci., Polym. Chem. Ed. (1973) 11, 2899. RECEIVED May 26, 1977.


C NMR Observation of the Effects of High Energy Radiation and

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