Linear-Energy-Transfer Effects on Polymers - ACS Publications

Chapter 3

Linear-Energy-Transfer Effects on Polymers W. Schnabel, Q. Q. Zhu, and S. Klaumünzer

Downloaded by PENNSYLVANIA STATE UNIV on August 9, 2012 | http://pubs.acs.org Publication Date: November 12, 1991 | doi: 10.1021/bk-1991-0475.ch003

Hahn Meitner Institut, Berlin GmbH, D-1000, Berlin 39, Germany

Various polymers (polymethacrylonitrile, PMCN, polyvinylacetate, PVAc, poly(ethylene oxide), PEO, poly(ethylene—co-propylene), CP-EP, and polystyrene, PSt) have been irradiated at room temperature and in the absence of oxygen witn γ-rays, and various ions of different kinetic energy E. Thus, a wide range of electronic stopping power, dE/dx, from 2x10 to 2.5 MeV/μm was accessible. PMCN predominantly undergoes main-chain scission and, in analogy to the previously examined polymethylmethacrylate, PMMA, the corresponding radiation chemical yield G(S), decreases with increasing stopping power. PSt predominantly crosslinks and the corresponding yield, G(X), is small and independent of dE/dx. PVAc, PEO and CP-EP also crosslink predominantly under the action of ionizing radiation and the gel doses increase with increasing stopping power. Thisfindingindicates that, for these polymers, both radiation chemical yields, G(S) and G(X) decrease with increasing dE/dx. Moreover, for PEO and PVAc, irradiated with Ar ions, the gel dose increases by a factor of 4 upon increasing Ε from 6.5 MeV to 150 MeV although dE/dx remains unaltered at 2.2. MeV/μm. These effects are discussed within theframeworkof the track structure model of Chatterjee and Magee, and some difficulties in applying this model to polymers are pointed out. -4

40

General considerations. Radiation chemical research on polymers concentrated for several decades on the effects of radiations of low linear energy transfer (LET) such as γ-rays and electron beam radiation. At present, however, there is growing interest in effects caused by high LET radiation such as ion-beam radiation (1-17). This is due to the fact that ion– beam radiation can be applied in surface modification and techniques used in the production of wave guides and microelectronic devices. The interest in practical applications also initiated various activities concerning the fundamentals of radiation effects caused by high LET radiation. The essential 0097-6156/91/0475-0044$06.00/0 © 1991 American Chemical Society

In Radiation Effects on Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

3. SCHNABEL ET AL.

Linear-Energy-Transfer Effects on Polymers 45

questions arising in comparing chemical effects on polymers induced by different kinds ot radiation concern the dependence of chemical yields, G, on stopping power, dE/dx, initial particle energy, E, and absorbed dose rate, D:


G = f(dE/dx, E, D)

(1)

Research work concerning the dependence of G-values of crosslinking and main-chain scission, G(X) and G(S), respectively, on dE/dx and Ε was carried out with several polymers at the Hahn-Meitner—Institute in Berlin using ion—beam radiations generated by the accelerator VICKSI (van de Graaff Isochron Cyclotron Kombination fur schwere Ionen). Prior to the presentation and discussion of the experimental data a brief review concerning the dissipation of the energy of heavy ions in polymers appears to be suitable. As has been shown previously by other authors (18) not only the stopping power but also the size of the volume around the trajectory of the particle in which its energy is initially dissipated (in the following denoted as track) depend significantly on the particle energy. Therefore, the structure of the tracks varies upon changing the particle energy. This can give rise to differences in the distribution of chemically reactive intermediates (radicals, ions, excited species) and thus influence the radiation chemical yields. Dependence of the stopping power on the particle energy. Principally, heavy particles lose energy by various modes of interaction with the absorbing material. With respect to particles dealt with in this article the important modes are: (a) interaction with shell electrons referred to as "electronic" enery loss and (b) interaction via the Coulombfieldof the target nuclei, i.e. elastic collisions referred to as "nuclear" energy loss: (dE/dx)total

=

(dE/dx)ei

+

(dE/dx) l nuc

(2)

The loss fractions applying to nuclear and electronic interaction can be calculated. Typical results of such calculations obtained with the aid of the computer code TRIM (19) for argon ions and polystyrene as stopping material are presented in Fig.l. It can be seen that, at relatively low particle energies, energy loss by nuclear collisions is predominant. At high particle energies, on the other hand, the nuclear energy loss is relatively small and can

Ε (eV)

Fig.l.

40

Dependence of the stopping power of polystyrene against Ar ions on the particle energy. (Calculated with the aid of the computer code TRIM (19).


46

RADIATION EFFECTS ON POLYMERS

be neglected in discussions concerning possible differences in the mode of action of ion-beam radiations. Additional results are presented in Table I and Fig.2. Table I contains data relevant to ion-beams of relatively low energy that were utilized for surface modifications of polymers in several laboratories. Obviously, the nuclear energy lossfractionis rather high in the


Table I.

Total stopping power of polystyrene andfractionsof electronic and nuclear energy loss as calculated with the aid of the computer code TRIM (19) for He, Ne and Ar ions of relatively low energy

Particle Radiation

**)

( dE/dx )totai (MeV//*m)

(%)

He (E = 100 keV)

0.142

98.6

1.4

Ne (E = 200 keV)

0.37

78

22

Ar (E = 400 keV)

0.70

71

29

fnucl

(%)

'

0

0

0

*)

(dE/dx)el/(dE/dx) tal to

^^dE/dxJnucl/idE/dxJtotal

χ (μιη) 20

Fig.2.

74T

Stopping power of methacrylonitrile against Ne ions (Eo = 275 MeV) vs. the particle penetration depth x. The total particle range is 425 /mi. The range on the abscissa applies approximately to the thickness d of irradiated polymerfilms.Left ordinate: electronic stopping power. Bight ordinate: nuclear stopping power. In Radiation Effects on Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

3. SCHNABEL ET AL.

Linear-Energy-Transfer Effects on Polymers 47

cases of 0.2 MeV Ne and 0.4 MeV Ar ions. Fig.2 presents data concerning the energy absorption of 275 MeV Ne ions in polymethacrylonitrile. Here, (dE/3x)ei and (dE/dx) ci are plotted vs. the penentration depth of the projectiles in the target. In this case, up to a total target thickness of 30 μιη, the fraction of energy loss due to nuclear collisions contributes only very little to the total energy loss. Collisions of heavy particles with nuclei might lead to chemical alterations in the absorbing material differing qualitatively and/or quantitatively from those resulting from interactions with shell electrons. This problem has not been addressed in detail in previous publications and deserves certainly future attention. In this work, however, the ion energy and the target thickness have been chosen so that contributions of (dE/dx) i to radiation chemical yields can be safely neglected.


nU

nuc

Dependence of the track radius on the particle energy. Regarding possible track structure effects the reader's attention is to be drawn again to Fig.l. For Ε > 1 MeV, dE/dx passes through a maximum, and, notably, argon ions of quite different energy are dissipating energy in the polymer with tne same dE/dx. For example, about equal stopping power values apply for the particle energies Ε = 6.5 MeV and Ε = 150 MeV. Since the ion energy determines the velocity distribution of the secondary electrons the initial energy distribution around the ions' path is quite different for these two ion energies. This difference can cause differences in G values. In other words, the influence of track structure effects on radiation chemical yields could be principally investigated by carefully measuring the dependence of G(S) and G(X) on the particle energy. The problem of the local energy distribution induced by the passage of fast heavy ions along their trajectories has been dealt with generally by Chatterjee and Magee (18). According to their model, which summarizes both experimental and theoretical work, the track structure is grossly characterized by two regions: (1) a track core region of radius r , in which more than 50 % of the energy is deposited, resulting in an extremely high local concentration of chemically active intermediates, and (2) a penumbra region of radius r , in which the remainingfractionof the energy is deposited in ionization and excitation events by energetic secondary electrons generated by the primary particle in the center of the core. As can be seenfromFig.3 the two radii depend quite differently on the particle energy E, which is here expressed in units of MeV per nucléon. The drastic increase of rp relative to r implies that chemical effects induced by secondary electrons should become more and more important as the particle energy increases. In Table II, core and penumbra radii being of interest with respect to the ions examined in this work are presented. Obviously, r becomes unrealistically small for low kinetic energies per nucléon. In this case, the energy lost by the ion is concentrated in the small volume corresponding to r which envelopes the ion trajectory. Provided the radiation chemical yields would directly reflect the energy distribution given immediately after the passage of an ion, significant differences in G—values are expected regarding irradiations by 6.5 MeV and 180 MeV Ar ions. On the other hand, G-values should differ less significantly if polymers are irradiated by 150 MeV Ar ions and 275 MeV Ne ions. In this connection, it should be pointed out that both r nd r are based on purely physical considerations. Both radii characterize the energy distribution —15 immediately, i.e. about 10 s after the passage of the ion. Subsequent processes like conversion and dissipation of electronic energy, generation and American Chemical Society Library 1155 16th St, N.W.Clough, R., et al.; In Radiation Effects on Polymers; ACS Symposium Series; American Chemical Society: Washington, DC, 1991. Washington, D.C. 20036 c

p

c

c

p

c

p


48


E (MeV/u) e

Fig.3.

Penumbra radius r and core radius r vs. the particle energy. (After Chatterjee and Magee (18))

Table Π.

Penumbra and core radii calculated on the basis of the track structure theory (18) for polystyrene

p

Particle Radiation 0.1 MeV

4

6.5 MeV

4 0

He

4 0

180 MeV 275 MeV

2 0

Ai

ε (MeV/u) 2 +

2 +

Ar

8 +

Ne

7 +

c

(nm)

(nm)

0.025

0.8

—

0.16

10

0.2

4.5

700

1.1

13.75

2700

2.0


3. SCHNABEL ET AL.

Linear-Energy- Transfer Effects on Polymers 49

migration offreeradicals as well as their reactions are rather complex and, at present, essentially unknown for most polymers. Thus, a comprehensive interpretation of radiation chemical yields is impossible. Nevertheless, a search for track structure effects in polymers may provide valuable information being of general importance for thefieldof radiation chemistry. RESULTS Polymers undergoing predominantly crosslinking. Polystyrene, PSt. Gel doses, Dg i, and 100 eV-yields for crosslinking, G(X), calculated according to the relationship G(X) = 50 N / M D i are shown in Table ΙΠ (N : Avogadro's number, M : initial weight average molar mass). Obviously, neither a track structure nor a stopping power effect was detected in this case: G(X) is independent of dE/dx and Ε within the error limit of our measurements. Notably, this result is at variance with results of other authors who found G(X) increasing with increasing stopping power (7,8,13—17). Moreover, G(X) was reported to depend on Ε (6). Possible reasons for these discrepancies have been discussed elsewhere (1).


?

a

a

w

ge

w

Other polymers. Gel doses obtained by irradiating poly (vinyl acetate), PVAc, poly(ethylene oxide), PEO, and an ethylene-propylene copolymer, CP-EP with Co—7-rays or Ar ions of different energy are shown in Table IV. In all cases, gel doses are significantly larger for Ar ion-beam irradiation than for 7-4rracuation. It is knownfromformer work that PVAc and PEO undergo, apartfromcrosslinking, also main-chain scission to an appreciable extent if irradiated with radiation of low stopping power. According to eq.(3) 80

Dgei = 100 N /M [2G(X) - G(S)/2] a

(3)

W

gel formation is possible only if G(S}/G(X) < 4. Principally, the higher gel doses found for Ar ion-beam irradiations could be due either to a decrease in G(X) or to an increase in G(S) with increasing stopping power. For ^^-irradiation G(S)/G(X) » 1 was found earlier in the case of PEO (21). Therefore, the increase in D i from 0.2xl0 (?-rays) to 2.1xl0 eV/g (150 MeV Ar ions) could be due to an increase in G(S). However, an increase in Dgei by a factor of 10, at constant G(X), would result in G(S)/G(X) > 4 with the consequence of predominant main-chain scission. From the fact that gel formation was observed under all circumstances it is inferred that, also at high stopping power, G(S)/G(X) < 4 indicating a decrease in both G(S) and G(X). The most interesting result of these experiments concerns the finding that, at almost constant stopping power, the gel doses for PVAc and PEO depend on E: the gel doses at Ε = 150 MeV are about 4 times larger than those at Ε = 6.5 MeV. This result seems to indicate the existence of a track effect. Notably, a track effect was not found in the case of CP-EP, where the gel dose does not depend on E, within the error limit of our measurements. 21

21

ge

Polymers undergoing predominantly main-chain scission 60

/

Polymethacrylonitrile (PMCN). Both ion-beam and Co- Hrradiation lead to main-chain cleavage. Results are presented in Table V. G(S) decreases with increasing stopping power. Obviously, PMCN exhibits the same beha vior as polymethylmethacrylate PMMA (3). The dependence of G(S) on Ε has not yet been investigated in this case.


50


Table ΠΙ.

Crosslinking of polystyrene induced by radiation of different stopping power in the absence of oxygen (1)

Radiation

Ε (MeV)

dE/dx M» (MeV//im)

6

ca.l*)

2x1ο"

^Co-7-rays

4

0.05 5


2 0

l (N Te

|1.6xl0

7 +

275

e

5

180

A Ar

40 2+ 18

6.5

A Ar

3.6

0.05

1.1

0.05

2.0

0.03

0.37 \5.3xl0

40 8+ 18

G(X)

Dgei (10îieV/g)

2.3

5

4.8xl0

Î2.7xl0

5

2.3

0.05

(l.8xl0

6

0.4

0.04

2.2

' energy of 7-photon Table IV.

Gel doses for polyvinylacetate, polyfethylene oxide) and poly(ethylene-co-propylene) in tne absence of O2 D i(10"eV/g) ge

Radiation

E (MeV)

dE/dx PVAc (MeV//im) a)

^°Co-7-ray8

ca.l

2X10

A Ar

6.5

A Ar

150

40 2+ 18 40 8+ 18 a J

PEO b)

CP-EP c)

0.9

0.2

0.07

2.3

2.4

0.5

0.62

2.2

8.5

2.1

0.52

-4

M = 2x10»; ^ M = 8.7x108; % b

w

w

Table V.

eocc—T-raye Ne 10 40 8+ 18 7 +

iNe

A

Ar

= 3.58x10*

100 eV-yields of main-chain scission of polymethacrylonitrile in the absence of O 2

Radiation

2 0

w

E (MeV) *\ ca.l ι 275 180

dE/dx (MeV//an)

G(S)

2xl0~* 0.43

3.3 0.5

(20)

2.5

0.4

(2)

References

(2)

) energy of photon In Radiation Effects on Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

3. SCHNABEL ET AL.

Linear-Energy- Transfer Effects on Polymers51


DISCUSSION As already pointed out above general conclusions regarding LET and track structure effects in polymers cannot yet be arrived at as far as chemical mechanisms are concerned. From a theoretical point of view there is a considerable lack of data input concerning the formation, the spatial distribution, and the chemical reactions of free radicals in the tracks of fast ions. Theoretically, the gap rangingfromthe very short period of energy deposition (t » 10~^s) to the period of the formation of thefinalproducts (t > 10~~^s) has to be bridged. Experimentically, the latter are farfrombeing completely characterized. In particular, the available results usually do not include radiation chemical yields of products of low molar mass. It appears that the important question whether densely ionizing radiation produces bond scissions in neighboring repeat units of the polymer chain cannot be answered until a material balance comprising all radiolysis products is available. However, what can be concluded on the basis of the results obtained until now applies to the efficiency of heavy ions with respect to crosslinking and main-chain scission. It seems that there is a general trend regarding G(X) and GfS) values. Both yields were found to decrease with increasing LET for all polymers investigated apart from polystyrene. On the basis of the assumption that densely ionizing particles are as effective as low LET radiations in producing primary products (ions, radicals, electronically excited species) the inefficiency of heavy ions to reduce the molar mass of polymers such as PMMA and PMCN or to increase the gel doses of polymers such PEO, PVAc and CP-EP might be related to the very high local concentration of reactive species. This leads to early deactivation reactions, especially to the combination of radicals (cage reactions). Simultaneous bond scissions in neighboring repeat units snould contribute to the origin of inefficiency of heavy ions. This appears to be quite feasible as far as the high fraction (> 50 %) of energy absorbed in the track core is concerned. Notably, a theoretical approach, on the basis of the Chatterjee-Magee track model, to track effects concerning the Fricke dosimeter system resulted in a rather satisfying agreement with the experimental data (18). In particular, this agreement refers to the general trend of decreasing G—values with increasing stopping power. Although the mechanisms concerning the chemical reactions occurring in the Fricke dosimeter system and in polymers are quite different, there might be common physical features that could explain the similar trend in the decrease of G—values. For example, in the case of the fluid Fricke dosimeter system, which consists of an aqueous solution, the magnitude of the radiation chemical yields is strongly dependent on the diffusion rate offreeradicals. In the case ofrigidpolymers,freeradical diffusion is certainly not an important factor. However, in this case energy dissipationfromthe core to rather remote regions in the form of transfer of excitation energy might lead to similar results as the diffusion offreeradicals in fluid systems. This applies of course, to the case that energy dissipaion ultimately results in the formation of reactivefreeradicals or radical ions. It is also feasible that radical migration mechanisms involving, for example, hydrogen abstraction become operative inrigidpolymers. Finally it should be pointed out that the independence of GiX) on dE/dx observed in our laboratory in the case of polystyrene is still a matter of controversy with researchers of other laboratories. For the sake of clarity it appears necessary to carry out again additional crucial experiments in this case.


52


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Linear-Energy-Transfer Effects on Polymers - ACS Publications

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