Cross-Linking of Hydrocarbon Polymers and Their Model Compounds

Chapter 2

Cross-Linking of Hydrocarbon Polymers and Their Model Compounds Linear-Energy-Transfer Effects Y. Tabata

Downloaded by CORNELL UNIV on June 16, 2017 | http://pubs.acs.org Publication Date: November 12, 1991 | doi: 10.1021/bk-1991-0475.ch002

Department of Nuclear Engineering, School of Engineering, Tokai University, 1117 Kitakaname, Hiratsuka, Kanagawa, Japan

Radiation effects on a series of n-paraffins and hydrocarbon poly mers such as polyethylene and ethylene—propylene rubber have been investigated. A clear linear energy transfer (LET) effect on those materials has been demonstrated by various experimental methods. It has been found that crosslinking is not distributed homogeneously. So-called microheterogeneous spatial distributions by γ and e-beams and macroheterogeneous distributions by high LET ion beams have been shown to be formed.

Linear energy transfer (LET) effects on hydrocarbon compounds, including a series of n-alkanes, squalane, polyethylene, and ethylene-propylene rubber, have been studied. A heterogeneous crosslinking formation in the hydrocarbon polymers by γand electron irradiation has been extensively investigated. Measurements of transient species by pulse radiolysis and product analysis by various methods have been carried out. γ-rays from Co, electrons from accelerators, fast neutrons from a nuclear reactor, and ions from various accelerators including a cyclotron and Van de Graaff were used for the irradiation.

60

Experimental Methods The pulse radiolysis were carried out by using a 35-MeV linear accelera tor including, the Twin Linac Pulse Radiolysis System at the Nuclear Engineering Research Laboratory, The University of Tokyo (1). Measure ments of gel formation were made by direct product analysis (2). Gas

0097-6156/91/0475-0031$06.00/0 © 1991 American Chemical Society

Clough and Shalaby; Radiation Effects on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

32

RADIATION EFFECTS ON POLYMERS

chromatography of evolved gases and mass spectrometry of the irradiated products were performed (3). Electron spin resonance measurements were also carried out at low temperatures (4). Results and Discussion


Results of the pulse radiolysis both in bulk n-dodecane (n-Cj^H^) and in the presence of carbon tetrachloride (100 mM) are shown in Figure 1. As was already reported (5,6), most transient species can be observed and have been assigned: n-dodecane

RH

alkyl radical

R-

olefinic cation-radical

R(-H)

singlet excited state

RH*

alkane cation radical

RHt

+

trapped electron The measurements were extended to higher number of paraffins up to C and ethylene-propylene rubber. Pulse radiolysis of thin films of ethylene-propylene rubber in the absence and presence of CC1 was carried out, and the results are sum marized at 2 ns after the pulse in Figure 2. Time profiles at 860 nm are shown in the same figure. Three transient species, singlet polymer excited state P*, polymer cation radical Ρ ·, and olefinic cation radical Ρ(-Η)"*· are formed in a shorter wave length region of 240—350 nm. These have been confirmed by electron spin resonance measurements at very low temperatures of various hydrocarbons including polymers (6) as well as the pulse radiolysis experiments (7, 8). It was readily observed by our Twin Linac Pulse Radiolysis System with time resolution of 20 ps for the absorption experiment that alkyl radicals Rformed very fast within the time resolution of 20 ps for various saturated hydrocarbons (1,8). >From those experimental results, the mechanism of alkyl radical for mation in hydrocarbons including polymers has been proposed. The scheme is presented in Figure 3. Highly excited cation radical is most important for the formation of alkyl radical. About 10% contribution for the radical formation come from the superexcited state of hydrocarbons, RH**. The crosslinking mechan ism could be described as follows: 3 0

4


TABATA

Cross-Linking of Hydrocarbon Polymers & Model Compounds

n-Ci!

Ct,

C*2/

,'"^CCU(IOOmM)

neat >_--f" 80ns ""* eiôi t /


C C U

ο 200

400

600

800

1000

WAVELENGTH (nm)

Figure 1. Pulse radiolysis of n - C H after the pulse. 12

0.10

O.D. 0.07 0.06*| 0.05 J 0.04 0.03 0.02 0.01 0

26

at 2 ns and 80 ns

860nm Η 40ns h Neat CCI4^

lei*

0.08

850nm Neat ν

> ·£ 0.06 c φ Û α Ο 0.02

0.0

600

j 800 700nm Wave Length

Figure 2. Pulse radiolysis of ethylene—propylene rubber films in the absence and presence of CC1 . 4



34

RADIATION EFFECTS O N POLYMERS

R(-H ),R 2

R π, RIII Non Scav. Scav.

RH**

•

R · +ΗΫ

RH+H*

=

R · +H2

RH**

•

R ( - H ) +H2

Figure 3. Mechanism of radical formation in saturated hydrocarbons.


2.

Cross-Linking of Hydrocarbon Polymers & Model Compounds 35

TABATA

A. Deprotonation RH RH* RH t RH + +

2

+

RH + e~ H

-»RX- + R H t -»RH + H · -* II,III2

Pair-wise Formation

2

R

+

H

B. Dissociation of Excited Cation Radical


+

+

RH*

>· R l

Ri* + e"

> Rj*

+ H

RH + Η·

> ^11,III· + H

Pair-wise Formation 2

It is known that in crystalline alkanes, more than 90% of alkyl radi cals are formed as pair radicals at very low temperatures (6). About 40% of pair radicals are located at the end of chains in the crystal. It has been demonstrated by gel permeation chromatography that 50% of dimers are almost linear due to the crosslinking at the ends of chains. C. Olefinic Cation Radical RH ^ v w — » RH**, RHt* + e" RH** -> R(-H) + H Pair-wise *+ >R(-H)t + H Formation R ( - H ) t + e^ R ( - H ) * — > R(-H) R(-H)* >R(-H )+ HRH + Η · » R- + H 2

RH

2

2

2

D. Charge Transfer R(-H) + RHt R ( - H ) t + e" R(-H)* RH + Η·

> > » »

R ( - H ) t + RH R(-H)* R ( - H ) - + HR* + H 2

Pair-wise Formation

2

This process can be scavenged by cation scavengers. E. Thermal Process of RH* RH** RHt* * RH + H · RHt + e" R H

» RH* r e l a x a t i o n ¥ RH"" » R- + H> R»+ H > RH* 1

2

„ . Pair-wxse Formation


36


F. Radical Migration (intra- and inter-) R(-H) RH

+R.

+ R.

^

R(-H )-

>

R. + RH

2

+ RH


(mobile)

Processes A and Β are mainly not scavengeable ones, and process C is partially scavengeable. Processes D, E, and F are almost scavengeable. Usually radicals are formed pairwise. Therefore, there is a tendency that crosslinking occurs between the radical pairs. The majority of the crosslinking takes place through the pairs inhomogeneously, not through a homogeneous diffusion process. Of course, the process depends largely upon the condition of the molecular aggregation states, either in the crys tal, the amorphous, or the molten state. If double bonds are formed, selective charge transfer occurs from cation radical to the site, then radi cals are formed in the vicinity of the double bond pairwise. On the other hand, it is known that radicals can migrate through either intramolecular or intermolecular processes. Then the radical is trapped stably as an allyl radical. Another alkyl radical can reach the allyl radical through migra tion in the medium to give crosslinking. In addition, once a crosslink is formed, both olefinic cation radical and tert-alkyl radical are easily formed in that location. This makes further crosslinking in the same site easier. The easier formation of terti ary alkyl radical has been demonstrated by electron spin resonance (spin trapping technique; 9,10), and olefinic cation radical is shown to be formed easier in branched hydrocarbons. It is strongly suggested that through those crosslinking processes men tioned above, crosslinking in hydrocarbon polymers, particularly linear hydrocarbons, occurs rather heterogeneously. This is an intrinsic tendency for the crosslinking process of hydrocarbon polymers. Formation of dimer, trimer, tetramer and higher oligomers can be detected and analyzed by both gel permeation chromatography and mass spectroscopy. The quantity of unsaturated bonds can be also analyzed. These experimental results indicate clearly that inhomogeneous crosslinking occurs. Through the processes of C, D, and F, an enrichment of unsaturation in higher oligomers goes on. From those experimental results mentioned above and analysis of the results, heterogeneous crosslinking has been concluded; the schematic representations are shown in Figures 4 and 5, which represent crystalline and molten states, respectively. These can be used to simulate crosslinking of polyethylene or hydrocarbon poly mers, that is, the former would be able to simulate crosslinking in the crystalline phase and the latter in the amorphous phase of polyethylene. LET Effects on Hydrocarbon Polymer Almost 15 year ago, LET effect on n-eicosane was studied by means of electron spin resonance at 77K (4). The sample was irradiated with 7-rays




2. TABATA

Figure 5. Crosslinking of n-alkane in the molten state.


38

RADIATION EFFECTS ON POLYMERS +

from a ^Co or H (60 MeV) from a synchro-cyclotron at the University of Maryland, H e (23 MeV), C " (85 MeV), and N (82 MeV) from a cyclotron at the Institute of Physical and Chemical Research. After irradi ation with a dose of 4.3 χ 10 * ev by C " ion, the ESR was measured at 77K. A very broad spectrum due to strong spin-spin interaction among radicals formed in the track, which is quite different from that of isolated radicals formed by 7-irradiation, was obtained. The decay of the broad component is faster than that of 7-irradiated isolated radicals by a factor of 5.5. About 30-40% of the radicals disappear below 150K. This is indicating that one-third of the radicals which survived at 77K combine with each other within the track of C " " ion to make crosslinks. This must give rise to heterogeneous crosslinking in n-eicosane crystals. (See Figure 6.) According to our other experiments on proton (60 MeV) (13) irradia tion of n-eicosane at 77K, only 5% of isolated radicals decay up to 200K; on the other hand, about 80% of pair radicals decay up to the same tem perature. Pair radicals disappear completely at 230K, and only 20% of isolated radicals decay at the same temperature. As the experimental con ditions are different to some extent between H and C " irradiations, it is difficult to conclude accurately the difference. However, one can say that three different decay processes occur in different temperature regions, that is, first there is decay of high-density radicals in the track of Cr* up to ca 130K, second is decay of pair radicals up to 210K, and finally there is decay of isolated (single) radicals at higher temperatures up to room temperature. Fast heterogeneous crosslinking in the track of ion C " ", then crosslinking through recombination of pair radicals, and finally slow crosslinking through encounter of isolated radicals occur successively. As certain amounts of the high-density radicals and the pair radicals are already reacted at 77K, these must be taken into consideration for estima tion of the overall crosslinking yield. Experimental results on irradiation by fast neutrons from a nuclear reactor (a fast neutron source reactor, Yayoi, at Nuclear Engineering Research Laboratory at the University of Tokyo) are shown for polyethylene (PE) and ethylene-propylene rubber (ET-PP), together with experimental results using Co 7-rays. Gel fractions as a function of irra diation doses are shown in Figure 7. It is quite clear from the figure that no difference between fast neu tron and ^-irradiations has been found in a wide range of irradiation doses, as far as the gel content measurements are concerned. These experiments have been extended to rather heavier ions, like H e and Ν , by Sasuga et.al. One of their experimental results is shown in Fig ure 8. A big difference among different charged particles has been found with respect to mechanical properties. Within 1 MGy irradiation, residual elongation of the irradiated samples depends significantly on the kind of charged particles, that is, on the LET of the radiation. Above 1 ev/A, the 2+

44

4+

2

4


4

4-

1

+

44

4

1

2+


TABATA

Cross-Linking of Hydrocarbon Polymers & Model Compounds


Irradiated at 77 Κ

Temp ( Κ ) +

Figure 6. Decay of alkyl radical in n-eicosane for η, H , and irradiations.

100h

Dose (Mrad)

Figure 7. Gel fraction as a function of dose in η and fastneutron irradiation for polyethylene and ethylene-propylene rubber. (Reproduced with permission fromRadiat.Phys. Chem. Copyright 1991 Pergamon.)


40


Stoping Power

Ions

Ο 80 MeV Ν

1.Οο-


φ 80 MeV C

C ο CO CD C O Φ Τδ 3 "Ο

φ θ

Ο.8

•

4 +

3136 MeV'cmVg

4 +

2215

8 MeV Η

LET

224

30 MeV He"

53

+

-

o-o--

2 MeV e"

0.6

sf

0.4

Ίλ φ

ce

0.2 0.1

0

0

1

10

LET(ev/À)

u_

1.5

1

0.5

Dose (MGy)

(Sasuga)

Figure 8. Residual elongations of polyethylene as a function of irradiation dose for 5 different LET. (Reproduced with permission from Radiat. Phys. Chem. 1991, 37, 135—140. Copyright 1991 Pergamon.) effect becomes significant for polyethylene. This phenomena could be explained by macroheterogeneous crosslinking in the track due to highdensity radical formation by high-LET charged particles, compared with low-LET radiation (and high-energy electrons) which produce rather microheterogeneous crosslinking. Concerning the residual elongation, there is no difference between 7-rays or electron beam and proton H . There exists, however, a big difference among H , H e , C " ", and N , as shown in the figure under the experimental conditions. For the irradiated polyethylene sheets, heterogeneous crosslinking under a fixed number of crosslinks below 1 MGy irradiation makes easier the stretching and is more favorable for stretching than homogeneous crosslinking. Figure 9 shows the scheme qualitatively. For gel formation, no differences among different radiation sources have been observed. This is suggesting that the G-value of crosslinking is almost the same for all kinds of radiations examined. In a dose range up to 2 MGy, a significant difference in tensile strength of the irradiated samples between e~ and H irradiations has been found. This is suggesting that a suitable condition for higher tensile strength does exist for the proton irradiation in a dose range of 1-2 MGy, compared with that of electron irradiation. Therefore, it may be possible to distinguish the difference of radiation effect between e~ and H by this method, in spite of the fact that no differences can be found by measurements of the gel fraction and the stretching. These results are summarized in Table I. +

+

2+

4 +

+

+


4

1

2. TABATA



j

Residual Elongation

I

I

Low LET

High LET

Figure 9. Schematic representation of heterogeneous crosslinking for low and high LET radiations

Table I. LET Effect on Polyethylene ^\Radiation 7(Co) Test^^" Method Residual Elraction

a

Gel Fraction

b )

S0

e

n

H

S

S

S

S

Ε

E

E

E

S

S

L

L

\

L

r c e

+

He

J+

c

4+

N

4 +

)

M

L

L

c)

Swelling

Tensile Strength

d

*

a) 0.1~1MGy,

b) 0.02~2MGy,

L

\

H

c) ~2MGy.

d ) 1~2MGy

S : small, M : medium, L : large, E : equivalent, L'.low, H : high


42


Conclusion Radiation effects on a series of η-paraffins have been studied as model compounds of hydrocarbon polymers. It has been pointed out that heterogeneous crosslinking in both crystalline and molten (amorphous) states occur in different ways, even if radiation sources are 7-rays from ^Co or high-energy electrons with a low LET. An LET effect has been clearly observed among different radiations of e"~, H , He*", C " ", and N . The effect was detected and confirmed by various methods. Radia tion effects on polymers could be well simulated by that of their model compounds. In saturated linear hydrocarbons, no chain scission occurs above a certain number of hydrocarbons, probably ca 20 carbons. The crosslinking yield is not much affected by LET under our experimental conditions, because there is no LET dependence on the gel formation. The main LET effect comes from difference in spatial distribution of active species (including alkyl radicals and unsaturated bonds) and crosslinks. +

2

1

4

1


4 +

Literature Cited 1. Kobayashi, H.; Tabata, Y.Radiat.Phys. Chem. 1989, 34, 447-451. 2. Seguchi, T.; Hayakawa, N.; Tamura, N.; Hayashi, N.; Katsumura, Y.; Tabata, Y. Radiat. Phys. Chem. 1989, 33, 119-128 3. Seguchi, T.; Hayakawa, N.; Tamura, N.; Hayashi, N.; Katsumura, Y.; Tabata, Y. Radiat. Phys. Chem. 1988, 32, 753-760. Seguchi, T.; Arakawa, K.; Tamura, N.; Katsumura, Y.; Hayashi, N.; Tabata, Y. Radiat. Phys. Chem. 1990, 36, 259-266. 4. Hamanoue, K.; Kamantauskas, V.; Tabata, Y.; Silverman, J. J. Chem. Phys. 1974, 61, 3439-3443. Kimura, K.; Matsui, M.; Karasawa, T.; Imamura, M. Ogawa, M.; Tabata, Y.; Oshima, K. J. Chem. Phys. 1975, 63, 1797-1802. 5. Tagawa, S.; Hayashi, N.; Yoshida, Y.; Washio, M.; Tabata, Y. Radiat. Phys. Chem. 1989, 34, 503-511. 6. Iwasaki, M.; Toriyama, K.; Fukaya, M.; Muto, H,; Nunome, K. J. Phys. Chem. 1985, 89, 5278. Miyazaki, T.Radiat.Phys. Chem. 1991, 37, 11-14. Toriyama, K.; Nunome, K.; Iwasaki, M. J. Chem. Phys. 1986, 90, 6836. Trifunac, A. D.; Werst, D. W.; Percy L. T. Radiat. Phys. Chem. 1989, 34, 547. 7. Brede, O.; Bös, J.; Naumann, W.; Mehnert, R. Radiochem. Radioanal. Lett. 1978, 35, 85. Jonah, C. D.Radiat.Phys. Chem. 1983, 21, 53. Klassen, Ν. V.; Teather, G. G. J. Phys. Chem. 1985, 89, 2048. Mehnert, R.; Brede, O.; Cserep, G. Radiat. Phys. Chem. 1985, 26, 353. Clough and Shalaby; Radiation Effects on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

2.

TABATA


8. Tabata, Y.; Kobayashi, H.; Washio, M.; Yoshida, Y.; Hayashi, N.; Tagawa, S. J. Radioanal. Nucl. Chem. 1986, 101, 163. 9. Tabata, M.; Sohma, J.; Yamaoka, H.; Matsuyama Chem. Phys. 1985, 119, 256.

Lett.

10. Katsumura, Y., The University of Tokyo, unpublished data.


11. Katsumura, Y.; Tabata, Y.; Seguchi, T.; Hayakawa, N.; Yoshida, K.; Tamura, N.Radiat.Phys. Chem. 1985, 26, 211. Seguchi, T.; Hayakawa, N.; Yoshida, K.; Tamura, N.: Katsumura, Y.; Tabata, Y. Radiat. Phys. Chem. 1985, 26, 221.

12. Seguchi, T.; Sasuga, T.; Kawakami, W.; Hagiwara, M.; Kohno, I.; Kamitubo, H. Proc. 11th Int. Conf. Cyclotronon and Their Applicatio 1987, 667. Sasuga, T.; Kawanishi, S.; Kohno, I. Proc. Int. Conf. Radiation Damage to Organic Materials in Nuclear Reactors and Radiation Enviro ments, Takasaki, Japan, July 17-20, 1989. Sasuga, T ; Kawanishi, S.; Seguchi, T.; Kohno, I. Polymer 1989, 30, 2054-2059. RECEIVED May 21, 1991


Cross-Linking of Hydrocarbon Polymers and Their Model Compounds

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