Reactive Intermediates in the Radiation Chemistry of Polyethylene

3 Reactive Intermediates in the

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 17, 2016 | http://pubs.acs.org Publication Date: June 1, 1967 | doi: 10.1021/ba-1967-0066.ch003

Radiation Chemistry of Polyethylene MALCOLM DOLE Northwestern University, Evanston, Ill. D A V I D M . BODILY University of Arizona, Tucson, Ariz.

The types and reactions postulated for reactive intermediates in the radiation chemistry of polyethylene are reviewed. Ultraviolet spectroscopy is an important tool in complementing data obtained from electron spin resonance studies. Finally, the kinetics of growth and decay of the allyl and polyenyl free radicals as inferred from ultraviolet spectra are discussed.

" O eactive intermediates in the radiation chemistry of polyethylene have been postulated to be positive ions of various types, trapped elec trons, electronically excited groups, alkyl and allyl type free radicals, diffusible species such as atomic hydrogen, or the methyl radical, and so on. To this list we should like to add negative ions, carbanions. The electron must always be considered to be present, trapped, or otherwise when positive ions exist, but it has not yet been identified in polyethylene as a chemically reactive intermediate as has the hydrated electron in water, although the postulate of trapped electrons in irradiated poly ethylene has been invoked by Partridge (33) working with Charlesby to explain their luminescence decay curves. Ionic

Intermediates

Positive Ion. Many ion-molecule reactions have been suggested to explain unsaturation and crosslinking in polyethylene. The subject has recently been reviewed by the author (10). However, there has been practically no direct evidence of ion-molecule reactions. The author (JO) 31 Irradiation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

32

IRRADIATION OF POLYMERS

proposed that production of irarw-vinylene unsaturation in polyethylene may occur by means of Reaction 1 — C H s — C H — -» H H


2

C H = C H — (trans)

(1)

+

2

because of the small effect of temperature or phase on G(t—VI) and be cause Reaction 1 is exothermic (Table I). G(t—VI) is the number of frans-vinylene groups produced per 100 e.v. of energy absorbed. How ever, the production of molecular hydrogen and the vinylene group might equally well be independent of temperature and phase if the reactive intermediate were an electronically excited species, — C H C H * — . There are other processes, of course, which give rise to hydrogen, as discussed below. 2

2

Table I. Some Postulated Ion-Molecule Reactions (10) Reaction

e.v.

(1) — C H C H — - » H., + — C H = C H — 2

2

—0.105 (cis)

+

—0.145 (trans) (2) — C H — + —CHo— - » — C H — + — C H — 2

+

0

3

(3) — C H C H = C H + — C H C H = C H -> — C H C H = C H 2

2

+ —CH CHCH 2

2

+

2

2

+0.25

a

(4) —CHoCHCH;, + — C H C H = C H 2

2

—0.61

—CH CHCH CHCH — 2

2

CH

2

3

Inasmuch as the production of frans-vinylene unsaturation is one of the important processes occurring during the γ-ray irradiation of poly ethylene, it is interesting to compare G(t—VI) in polyethylene with G(Vl) values in other substances. Data are presented in Table II where G(Vl) represents a monoene group, whether cis or trans. Table II. G Values for Production of Unsaturation at Room Temperature Substance

Phase

G(t - VI)

Linear polyethylene Low density polyethylene n-Hexane Cyclohexane Dioxane

Solid Solid Liquid Liquid Liquid

2.4 1.7 1.2

G(Vl)

Ref.

3.2 ~0

(10,15) (IS) (9) (21) (26, 35)

The results of Table II are interesting from several standpoints. In the first place the vinylene groups in polyethylene (PE) are all trans,

Irradiation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

3.

DOLE AND BODILY

Reactive Intermediates

33

as would be expected because the trans-configuration can be formed with fewer chain rearrangements in the solid than the cis. However, in cyclohexane all of the vinylene groups must be cis, yet the yield is quite high. This may partly be the result of the liquid state of the sample, although Hamashima, Reddy, and Burton (20) have shown that G ( H ) for cyclohexane, which is equal to the high value of 6.0, is independent of temperature from —60° to -f-25°C. (melting point of cyclohexane is 6.5°C). More recent determinations of G ( H ) from liquid cyclohexane have yielded the values 5.55 (16) and 5.3 (38). This constancy of G ( H ) with temperature is in contrast to polyethylene, whose G ( H ) rises from 4.01 at 100°C. to 5.4 above the melting point at 140°C. (22). If G ( H ) is constant with change of phase, G(VI) of cyclohexane is probably also constant or nearly so. The fact that the double-bond yield from cyclohexane is high and that dioxene has not been observed in the radiolysis of dioxane (26, 35), a geometrically similar molecule, is inter esting but not surprising when it is remembered that organic chemists find it most difficult to make dioxene and that the free radical shown be low is undoubtedly stabilized by resonance of the electrons of the lone 2


2

2

2

2

Ο

/\

H,C

I

I

HC 2

CH-

CH

\ /

2

Ο

pair on the oxygen atom with the unpaired electron of the free radical group. The dimer formed by recombination of the free radicals in dioxane is chemically equivalent to practically all of the hydrogen liberated; hence, material balance requires that little, if any, dioxene be formed. As far as the major events in dioxane are concerned, it does not seem necessary to invoke ion-molecule reactions to explain the chemical effects observed. There is some ring degradation, however, as indicated by the significant value of G ( C H ) equal to 0.50 and of G ( C O ) equal to 0.23. These compounds probably arise from the decomposition of the energyrich dioxane formed on recapturing the electron by the positive ion, as suggested by Llabador and Adloff (26). In the case of cyclohexane ion-molecule reactions may contribute to some of the radiolysis products, but we believe ( for reasons given below ) that in this case also the ion-molecule yields are small if not completely negligible. The radiological behavior of dioxane shows that when a 2

4


34


reactive intermediate is more stable, in this case the C H ( V free radical, it will form and react in preference to other possible intermediates.


4

7

Possibly the most convincing evidence for positive ion-molecule re actions in polymers is the high rate of decay of vinyl unsaturation during the radiolysis of polyethylene, as recently discussed by Dole, Fallgatter, and Katsuura (13). The ideas of these authors with respect to the carbonium ion mechanism for vinyl decay by means of a dimerization reaction were largely suggested by the mechanisms proposed by Collinson, Dainton, and Walker (5) for vinyl decay (polymerization) in the radiolysis of n-hexa-l-decene, Reactions 3 and 4 of Table I. Although the ion-molecule theory of Libby (25) for the crosslinking mechanism in polyethylene is extremely attractive, there is no solid evi dence to date that crosslinking occurs other than by free radical recom bination. Inasmuch as the G value for producing free ions in a liquid n-paraffinic hydrocarbon is only about 0.1 as found by several workers [see Williams (39) for a review] one would not expect ion-molecule reactions to be important in the radiation chemistry of polyethylene un less a chain reaction is possible, the ions and electrons become stabilized in some way, or there are reactive scavengers present that can react with the electrons or positive ions before the positive ions recapture the electrons (38). A double bond, such as a vinyl or vinylene group, rep resents a location where a positive charge could become slightly sta bilized because the ionization potential of a saturated paraffinic chain, 10.55 e.v., is about 1.3 e.v. higher than that of an olefinic vinyl group (9.24 e.v.) [gas phase data from Field and Franklin (18)]. In a system such as n-hexa-l-decene which was investigated by Collinson, Dainton, and Walker (5), the G values for vinyl group decay were so high that chain reactions must have been caused by irradiation. Under these conditions ion-molecule reactions can become important. The G value for total possible ions is estimated to be about 3 (I). Another example of a significant ion-molecule reaction which can occur during radiolysis is the proton scavenging by ammonia in the γ-ray radiolysis of liquid cyclohexane as studied by Williams (38). In this case N H is formed at a rate that can compete with the rate of geminate recombination of the ions. An analogous reaction in the case of pure polyethylene (or of pure cyclohexane) is Reaction 2 of Table I. For such a reaction to occur it is necessary that the methylene groups in polyethylene have a high proton affinity, but the proton affinity, which is high in the case of methane, is less in ethane and and still less in pro pane. Derwish et al. (8) could find no evidence for the ion C H» in the mass spectrum of propane even at the high pressure of 0.12 torr in the ionization chamber of the mass spectrometer. Thus it seems unlikely 4

+

3


+

3.

DOLE AND BODILY

35



that Reaction 2 of Table I can occur to any significant extent in either polyethylene or cyclohexane. Negative Ions and Trapped Electrons. At the present time we can only speculate on the nature of electron traps in irradiated polyethylene. Partridge (33) suggests that electrons are trapped between molecular chains because luminescence in irradiated polyethylene occurs in the temperature intervals where mechanical losses occur. For large doses, free radicals, R*, are abundant enough to be significant in trapping electrons. The reaction: R" + e- -» R:" to form a carbanion is exothermic to the extent of about 1 e.v. or more, as judged by analogy with the electron affinity of the methyl free radi cal, C I V , which is 1.08 ± 0.4 e.v. (30). Because of polarization of the surrounding medium, the electron affinity may be even greater than the gas-phase value given above. Williams (39) has estimated the polariza tion energy of an electron in a hydrocarbon of static dielectric constant equal to 2 and in a cavity of radius 4.7A. to be 0.38 e.v. At present there is no definite evidence that carbanions, R:", or trapped electrons enter into specific chemical reactions during the irradi ation of polyethylene. Diffusible

Intermediates

Atomic Hydrogen. Although there is much evidence for reactions produced by "hot" hydrogen atoms and by hydrogen atoms which have become "thermalized" by collisions with the matrix atoms, hydrogen atoms as such have never been observed in irradiated polyethylene even at temperatures as low as —195°C. Inasmuch as the ESR spectrum of atomic hydrogen is a doublet with the large separation of 508 gauss, hydrogen atoms should be readily detectable if they exist. They have been seen in irradiated ice at liquid hydrogen temperatures, but they recombine at measurable rates when the ice is warmed to 20°K. or higher (19). Nevertheless, we know that atomic hydrogen must exist if only momentarily because free radicals such as R* have been shown to cause the exchange of deuterium between D and polyethylene at room temperature after the irradiation, presumably by a chain reaction of the type (12): 2

R* + D - * R D + D D* + R H -» R* + H D 2

#

Further results of Dole and Cracco (12) indicated that the deuterium atoms of the above reactions did not diffuse far through the polyethylene before abstracting a hydrogen atom from the polyethylene chain.



36


Methyl Radicals. Yoshida and Ranby ( 41 ) have recently shown by ESR measurements that the methyl free radical is produced in poly propylene by irradiation with ultraviolet light at liquid nitrogen tem perature but decays at that temperature. The reaction is slow at —196°C, which suggests that all other hydrocarbon free radicals of larger molecular weight than the methyl free radical can be frozen in and immobilized at that temperature. The mechanism of the decay reaction of the methyl free radicals at — 196° is not known; however, the γ-ray irradiation of polypropylene at —196 °C. produces only methane and no ethane (36), as demonstrated by gas analysis after warming to room temperature after irradiation. It may be that the methyl free radicals abstract hydrogen atoms on warm ing to room temperature or that "hot" methyl radicals are produced during the radiolysis with sufficient excess energy to abstract hydrogen atoms at liquid nitrogen temperature. Aliphatic

Free

Radicals

Alkyl Free Radicals. The primary alkyl free radical, R C H \ has been postulated to exist in gamma-irradiated polyethylene (23), but its pres ence has never been unambiguously demonstrated. If all the a and β protons interacted equally with the unpaired electron, the primary alkyl free radical ESR spectrum should consist of five lines, in contrast to those of the secondary and tertiary alkyl radicals whose ESR spectra should consist of six and eight lines, respectively (the latter calculated for the tertiary free radical in polypropylene). The existence of the secondary 2

free radical, —CH.»CHCHo—, has been well documented in polyethylene by several workers (23, 24, 37), while the tertiary alkyl free radical, —CHoC—CHo—, is believed to be formed initially during the low temperature γ-ray irradiation of polypropylene (28) but is converted (27) to other types of free radicals on heating above 263°K., probably to the allyl tvpe radical, —CHoCCH=C—CH>—.

Ί

ι

CH, C H Methylene Free Radicals. As suggested by my associate, G . G . A. Bôhm, the methylene free radical, — C H j C C H — , may be produced momentarily in polyethylene by eliminating molecular hydrogen during the irradiation. This process could replace or exist in addition to that represented by Equation 1. Such a biradical (or Lewis acid) would not be expected to be stable but could revert to the vinylene group 3

2


3.

DOLE AND BODILY

37


— C H = C H C H — or —CH CH==CH— by the intrachain migration of 2

2

a single hydrogen atom or to a crosslink — C H C H C H — by the inter chain migration of a single hydrogen atom, followed by coupling of the two secondary free radicals so produced. The latter is equivalent to the reaction observed in the methane radiolysis at 77 °K. in which ethane is formed by the insertion of C H into C H (2). 2


2

2

4

Allyl Free Radicals. Ayscough and Evans (3) have recently studied, by ESR measurements, the types of allylic free radicals produced by gamma-irradiation of several monomeric olefins. In irradiated polyeth ylene the allyl free radical is quite stable, persisting for several months at room temperature (31). The presence of these allyl free radicals is most noticeable in the case of high density polyethylene, and this type of free radical is undoubtedly the cause of the slow oxidation of poly ethylene at room temperature, which lasts for 40 or more days after irradiation (10). Williams and Dole (40) could observe little if any oxidation of low density polyethylene when it was exposed to air after irradiation. By oxidation we mean formation of carbonyl groups as de tected by infrared absorption studies at 1725 cm" . Parenthetically, it should be noted that adding an oxygen molecule to a free radical pro duces initially another type of free radical, a peroxy free radical, but in this paper we shall not discuss free radicals of this or any other types except those of hydrocarbons. 1

Polyenyl Free Radicals. As the number, n, of conjugated double bonds in the polyenyl free radical, — C H ( — C H = C H ) — , increases, the ESR signals merge to a singlet, thus making it impossible to identify and measure quantitatively the type and concentration of the individual polyenyl free radicals present. However, in the case of polyethylene irradiated to high doses at room temperature, relatively stable polyenes of the structure —CH (—CH=CH)„—CH —with η values as high as 5, and reactive polyenyl free radicals of the structure — C H ( C H " = C H ) — C H — with η also as high as 5 can be separately recognized and studied by ultraviolet spectroscopy (17). By observing the ultraviolet absorption bands at liquid nitrogen temperature, the spectra become considerably sharper, and the broad diene band, for example, breaks up into three well-resolved peaks ( I I ) . By irradiation at liquid nitrogen temperature, however, the production of polyenes and polyenyl free radicals with η greater than 3 is considerably reduced. n

2

2

W

2

Up to now we have been able to observe 13 separate absorption bands in the ultraviolet region of the spectrum between 220 and 400 τημ. The shift in the A of polyenyl free radicals to longer wavelengths with increasing η would be expected to be linear with n, similar to the m a x


38


increase of A with η in the case of the symmetrical odd-atom polymethine cyanine dyes studied by Brooker (4). On the other hand, the Amax of the even-atom polyene chains tends to converge with increasing n. Piatt (34) has discussed this behavior and shown that the convergence occurs in systems in which one ground state resonance structure is strongly stabilized compared with other possible structures. The differ ence between linear as compared with convergent behavior has greatly aided us in assigning the ultraviolet absorption bands summarized in Table III.


m a x

Table III.

Assignment of Ultraviolet Absorption Bands in Irradiated Polyethylene ( I I )

Polyenes Name

Polyenyl Free Radicals Μμ

Diene Triene Tetraene Pentaene

229,236,245 264,275,288 310 340

Name

mμ

Allyl Dienyl Trienyl Tetraenyl Pentaenyl

258 285 323 359 396

Kinetics of the Allyl Free Radical Growth. Bodily and Dole (11) were able to demonstrate that during irradiation at liquid nitrogen tem perature few allyl free radicals were formed, but most of them were created on warming to room temperature after γ-ray irradiation at —196 °C. Evidence for this behavior was obtained by measuring the ultraviolet absorption spectra under several different conditions as illus trated in Figure 1, where Curve 1 is the ultraviolet spectrum taken at liquid nitrogen temperature after irradiation in vacuum to 421 Mrad. There is a small peak at 258 η\μ which we believe is caused by the allyl free radical. On heating to room temperature Curve 2 was obtained. After standing at room temperature for 42 hours in a vacuum, Curve 4 was taken. The intensity of the absorption band had decreased, indi cating that the allyl radical concentration had decreased on standing at room temperature. On cooling to —196°C, however, the 258-m/i, band sharpened considerably, as shown by Curve 3. Since the intensity at 258 ϊημ of Curve 3 is much greater than that of Curve 1, the allyl con centration must have grown considerably on the first warming to room temperature. It also must have decreased somewhat on standing at room temperature, but not enough to fall to the initial allyl free radical con centration shown by Curve 1. As other workers have shown (6, 27, 32), the concentration of the allyl free radical decreases on exposure to ultraviolet light at liquid


3.


DOLE AND BODILY

1

\\\

ν \

/; ·

» !i l i 1

\v

V?


39

J

i3

Λ * Λ \ · I Λ NΛ

V•\/ %

A

4

Ζ Ο Ι

ο. QC

i\

Ο CO

m

1

Reactive Intermediates in the Radiation Chemistry of Polyethylene

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