Irradiation of Polymeric Materials - American Chemical Society

Chapter 3

Heterogeneous Nature of Radiation Effect on Polymers Yoneho Tabata

Downloaded by YORK UNIV on October 29, 2012 | http://pubs.acs.org Publication Date: April 13, 1993 | doi: 10.1021/bk-1993-0527.ch003

Faculty of Engineering, Tokai University, 1117, Kitakaname, Hiratsuka, Kanagawa 359—112, Japan

The radiation chemical phenomena caused by either gamma ray or high energy electrons are essentially inhomogeneous, in spite of the fact that those radiations have low LET values in an order of 10 ev/A. As one typical example, radiation effects on saturated hydrocarbons including model compounds of polyolefins and ethylene-propylene rubber (EPR) is discussed. It is made clear that crosslinking takes place very inhomogeneously in those materials. -2

Experimental Methods A series of model compounds of linear alkanes, squarane, and ethylene-propylene rubber were used as the examples. Those samples are listed in Table 1. Transient species were detected by pulse radiolysis at the University of Tokyo. Electron pulses with lOps and 2ns from an electron linear accelerator of 35MeV were used. The dose per pulse was about 1 krad for lOps and 4-5 krad for 2ns one (1). Gas chromatography (2), mass-spectrometry (3), liquid chromatography (4), were carried out for determining and measuring the radiolysis products, and crosslinking. Results and Discussion (1) Transient Species. In our pulse radiolysis work on saturated hydrocarbons RH including model compounds of poly-olefins, and a copolymer of ethylenepropylene rubber, the following transient species have been directly observed (5,6).

0097-6156/93/0527-0027$06.75/0 © 1993 American Chemical Society

In Irradiation of Polymeric Materials; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

IRRADIATION OF POLYMERIC MATERIALS

Table 1. Samples for Experiments


Sample naae

Foriula

eye I ohexane(c-Ce Hi 2 ) •etylated cyclohexane(Me-CeHi1) CH (CH )4CH CH (CH )eCH

n-hexaneCn-CeHt*)

3

n-octane(n-CeHte) m-nonaneCn-Ce^e) n-undecane(n-C H24) n-dodecaneCn-Ci H )

2

3

2

3

3

CH (CH2)tCH 3

3

n-pentadecane(n-CtçH 2)

CH (CH ) CH CH (CH2)ieCH CH Excited radical cation R ( - H ) -> - C = C R ( - H ) -> - C = C - C - Allyl radical

Figure 5:

RH

R(-H)

#

2

2

e

3

2

2

Scavengeable

Non-Scavengeable

Rl -> -CH -CH End radical R -> - CH-CH Tertiary radical Cm -> -CH - CH-CH - Internal radical R = Rm -» alkyl radical n

2

JR(-H )


3.

Heterogeneous Nature of Radiation Effect

TABATA

#+

#+

35

+

RH > RH *, {RH }, RH* + e" RH > RH**,RH* {RH* } > R(-H) + H major process RH* + e" > RH* RH * > R(-H) +H + e~ minor process RH** > R(-H) + H R(-H) + e" > R(-H)* neutralization R(-H)* >R(-H )» + Η· dissociation R(-H)* > R(-H) relaxation +

#+

2

+

e+

#+

2

2

# +

2

Effect of CCI4 (S) and C H O H (A) on the formation of olefinic cation radical can be expressed as follows.


2

5

> R ( - H ) +e" + H > S" enhancement of absorption R ( - H ) due to e" capture { R H } + c" >R(-H) + e- + H { R H } + e" + S + A > R* + S~ + A H scavenging of formation of R ( - H ) R(-H) + A > R(-H )# + A H enhancement of decay R ( - H ) +

e +

{RH* } + e~ S + e"

2

# +

#+

e +

2

#+

+

# +

+

+

2

# +

The scheme of the radiolysis of saturated hydrocarbons is summarized in Figure 5. (2) G-Values of H , Crosslinking and C=C(10). G-values of hydrogen formation obtained from gas chromatography and those of crosslinking obtained from liquid chromatography are summarized in Table 2. The G-value of crosslinking was determined from the slope of monomer consumption. G-value of crosslinking from that of H . 2

2

(3) Effect of Pyrene on Irradiation Effect of Ethylene-Propylene Rubber (7). In the presence of pyrene in ethylene-propylene rubber, charge and energy transfer take place from the medium to pyrene. The pulse radiolysis results are shown in Figure 6. It is obvious from the figure that charge transfer from the polymer medium to the additive of pyrene occur very effectively. It is well known that three peaksfromshorter wavelength to the longer are assigned to be triplet excited state, cation and anion radicals of pyrene, respectively. It is obvious from the osciloscope trace in the figure that charge transfer occur very fast. As the absorption coefficients are known for those species, their G-values could be easily estimated. Emission spectra have been also observed, as shown in Figure 7. Fast formation of both the monomer singlet excited state and the excimer can be seen.


36


Table 2. G-Values of Compounds Irrad.

-77°C

n-C o 42 H

2

n-C iH


2

n-C

n

C

c

2 3

H

Temp

4 4

4 8

H

" 24 50

G(H ) 2

G(C=C)

G(X)

Phase

2.14

1.01+

0.05

1.13+0.05

crystal

25

2.26

1.15+

0.05

1.11+0.05

crystal

55

3.32

1.66+

0.07

1.66+0.07

liquid

-77

2.16

1.02+

0.05

1.14+0.05

crystal

25

2.38

1.15+

0.05

1.23+0.05

crystal

55

3.22

1.72+

0.07

1.50+0.07

liquid

-77

2.25

1.00+

0.05

1.25+0.05

crystal

25

2.45

1.14+

0.05

1.31+0.05

crystal

55

3.28

1.82+

0.07

1.46+0.07

liquid

-77

2.16

1.01+

0.05

1.15+0.07

crystal

25

2.52

1.15+

0.05

1.37+0.05

crystal

55

3.22

1.70+

0.07

1.52+0.07

liquid

2.79

1.3+

0.1

1.49+0.1

glass

25

3.27

1.6+

0.1

1.67+0.1

liquid

55

3.26

1.6+

0.1

1.66_0.1

liquid

-77

H

30 62

G(X), and G(C=C) in a Series of Model

After the fast formation of both species, a slow process for formation of the additional excimer is seen from the figure. Those processes mentioned above play an important role for stabilization of the rubber against radiation. It is shown that the formation of polymer radicals and unsaturated double bonds, and the evolution of hydrogen molecules in the rubber are scavenged and inhibited significantly by pyrene. G-values of P y and Py~ as a function of pyrene concentration are shown in Figure 8, together with G values of the evolution of hydrogen molecule. It is evident from thefigurethat G value of both ions of Py , Py" approaches to 3.8 and almost all charges formed in the medium transfer to pyrene molecules at higher concentrations of pyrene more than about 3wt%. According to our experiments, it has been made clear that the evolution of hydrogen molecules is saturated at a concentration of pyrene more than 2wt%. Gvalue of the non-scavengeable hydrogen production is 1.70. This is shown in Figure 9. +

+


3.

TABATA

37


+

Py 455nm|

.12 .08 •

0.2

.04 •


0• O.D.I

c φ Û * 0.11 υ α Ο

Py~ 475nm

.20

—) 40ns \~— —*

400 Figure 6:

500 600 WAVELENGTH (nm)

700

Transient absorption spectra of pyrene in ethylene propylene rubber and osciloscope traces of pyrene cation (455nm) and anion (475nm). 1.07 weight percent of pyrene. at 480nm

ι

300

Figure 7:

ι

ι

400 500 WAVELENGTH (nm)

Transient emission spectra of pyrene in ethylene propylene rubber and osciloscope trace at the excimer peak (480nm). 1.0 weight percent of pyrene.




Et-Pr Rubber

1

2

3

Py Concentration (wt %)

Figure 8:

G-values of H 2 production (from Figure 9), and of pyrene cation P y and anion Py~ in ethylene-propylene rubber as a function of pyrene concentration. Rough estimation of Gvalues of alkyl radical and trans-vinylene R(-H) which can be scavenged (S) and not be scavenged (Non-S). R(-H) corresponds to C=C and X to the crosslinking. Nonscavengeable crosslinking yield was roughly estimated from G-values of squalane at low temperature. +

0 Figure 9:

1 2 Py concentration (wt%)

G-values of hydrogen evolution G(H ) from e ethylenepropylene rubber as a function of concentration of the additive, pyrene. 2


3.

TABATA


(4) Heterogeneous Crosslinking in a Series of Paraffins. Dimer, trimer, tetramer and higher oligomers are formed through crosslinking reaction successively and can be analyzed by means of liquid chromatography, massspectroscopy. The results are shown in Figure 10 and Figure 11. In the LC analysis, both infrared and ultra-violet detection methods were used. By the infrared method, a series of oligomers can be separated by the molecular weight as well as the shape of molecules. On the other hand, by the UV method, the contents of unsaturated bonds in thefractioncan be estimated. Three examples of n - C 2 o H , n - C 2 i H and n-C24H5g are cited in Figure 10. The irradiation was made in both crystalline and molten states. It is interesting to note that a single mode of dimer formation is observed in the molten state, while two different modes of dimer formation in the crystalline state. About 50% of dimer is an extended form without double bond structure, and another 50% of the dimer is a less extended form with double bond structure. For comparison, the L C spectrum for n-C4oH which is corresponding to the complete linear dimer of n-C4oH is indicated in the figure. It is emphasized that two different modes of dimerization exist in the crystalline state. Based upon the selective alkyl radical formation in the edge of crystals (which is explained later), crosslinking of about 40 to 50% occurs at the end of monomer chains. Transvinylene and allyl radical seem to be formed randomly and distributed rather homogeneously in the crystal matrices, in contrast with alkyl radicals. Crosslinking in the crystalline phase may take place by combining of radicals formed at double bond site with radicals migrating in the matrices. In the molten state, alkyl radicals are formed randomly along the molecular chain because of TG conformation in the medium. Crosslinking occurs by encountering of both alkyl radicals, alkyl and allyl radicals, and both allyl radicals through the molecular motion in the molten medium. In the crystal, it is evident from the LC experiment that certain amounts of double bonds are formed in the monomer without dimerization. This is indicating thatrecombinationof olefinic cation radical with electron occurs without further dissociation of the species. This process is much favorable in the crystalline state than in the molten state. One of our experimental results on the massspectrometry of irradiation alkanes is shown in Figure 11 (11). n - C ^ H ^ was irradiated at room temperature and the irradiated sample was subjected to the mass-analysis. Distribution of double bonds in the series of oligomersfromdimer to hexamer are shown in the figure. It is clear from the figure that the higher the number of monomer units in the oligomer, the higher the concentration of double bonds. Crosslinking proceeds, step by step, from dimer to trimer, trimer to tetramer and so on. It is indicating that as the crosslinking proceeds, double bonds are enriched and accumulated in the crosslinked oligomers. This phenomena could be explained by assuming that if once a double bond is formed, the site becomes the location where the next step of crosslinking starts. The scheme is discussed later. The other factor affecting the successive crosslinking after the first step of crosslinking will be a specific structure of the crosslinking site. 42


39

44

82

42


40


70


80

60

(a) n-C H o, 25°C (crystal) (b) n-C 4H , 55°C (liquid), 2MGy 24

2

5

50

Figure 10: Product analysis by liquid chromatography (LC) of n-C oH44 and n - C 2 4 H at 25°C in crystalline state and at 55°C in molten state. LC spectra from the refractive indes (RJ) and ultraviolet (UV) detectors. EV: elution volume (ml). LC spectrum for n-C4oHg is shown in the figure for comparison with the dimer product D: dimer, T: trimer, Te: tetramer 2

50

2


3. TABATA

41


LU

Irradiation of Polymeric Materials - American Chemical Society

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