The Radiation Degradation of Poly(2-methyl-1-pentene sulfone)

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13 The Radiation Degradation of Poly(2-methyl-1 -pentene sulfone) Radiolysis Products T. N. BOWMER and M. J. BOWDEN Bell Laboratories, Murray Hill, NJ 07974 Poly(2-methyl-1-pentene sulfone), PMPS, is a 1:1 alternating copolymer of 2-methyl-1-pentene with sulfur dioxide. Upon irradiation extensive main chain scission occurs via C-S bond cleavage, followed by depolymerization to monomers. These properties make it attractive for use as an electron beam resist. In this paper, the radiolysis products are reported as a function of dose. At low doses the SO/olefin ratio is —2:1. However, the ratio of SO/hydrocarbon moieties in the initial copolymer has a value of unity. The distribution of product yields and species from PMPS is explained in terms of a general mechanism for degradation of poly(olefin sulfone)s involving free radical and cationic depropagation pathways as well as oligomerization of free olefin by cationic intermediates. A post-exposure isomerization of the product olefin is described and is attributed to rearrangement in a SO/olefin charge transfer complex. 2

2

2

Copolymerization of sulfur dioxide and olefins generally yields 1:1 alternating copolymers (1). Upon exposure to ionizing radiation, the weak C-S bond in the main chain is cleaved and rapid depropagation occurs from the fractured chain ends (2-6). Values for G (scission) of 7-10 have been reported for poly (olefin sulfone) s (4—6). G (scission) is the number of main chain scissions per 100 eV of energy absorbed per gram of material. These G (scission) values are among the highest reported for polymer radiolysis (7) and have resulted in a number of poly (olefin sulfone) s being used as electron beam resists in the manufacture of integrated circuits (8). Bowmer and O'Donnell studied the degradation of a variety of poly (olefin sulfone)s with different olefin structures (2,3,9-11). The predominant process was C-S bond scission producing free radical and cationic fragments followed by depropagation. A general mechanism was presented which involved (1) depropagation via both radical and cationic species, (2) oligomerization of free 0097-6156/ 84/ 0242-0153S06.00/ 0 © 1984 American Chemical Society Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

POLYMERS IN ELECTRONICS

154

olefin initiated by cationic species on the residual polymer and (3) isomerization of olefin via the cationic species (10,11) The extent of depropagation was correlated with the ceiling temperature of the copolymer (3). The ceiling temperature (T ) is the temperature above which liquid phase copolymerization does not occur ( 0 and may be taken as an indicator of the thermodynamic stability of the polymer. Thus the lower the value of T , the more extensive was the depropagation. PolyO-butene sulfone), P B S , and poly(2-methyl-l-pentene sulfone), P M P S , are used widely as electron beam resists (8,12-14). P B S degrades according to the mechanism of Bowmer and O'Donnell (10,11). The degradation of P M P S using 1.0 M e V and 10-20 keV electrons has been studied by Bowden and Thompson (12—14) and found to undergo random chain scission followed by depropagation. However, Pacansky et a l . (15,16) degraded P M P S with electron beams and from infrared studies suggested a different mechanism. They proposed that S 0 was exclusively produced at low doses with no concomitant formation of olefin. The residual polymer was considered to be essentially pure poly(2-methyl1-pentene) and this polyolefin underwent depolymerization after further irradiation. Generally a chain reaction is postulated to account for high yields of S 0 . However a chain reaction that produces only S 0 and no olefin is difficult to envisage. The rapid depolymerization of poly(2-methyl-l-pentene) to monomer is unexpected, since G (total gas) for poly(isobutylene), a related polymer, is only 2.0 (7). The volatile products from the radiolysis of P M P S are reported in this paper. The product distribution and its dose dependence are used to decide whether P M P S degrades like other poly (olefin sulfone) s or whether a novel mechanism is required to explain its degradation. c

c

2

2

2

EXPERIMENTAL P M P S was prepared at -50 °C in methylene chloride using U V initiation (Γ7). The structure of P M P S is: CH

3

{S0 -CH -Ch 2

2

I

CH

2

CH

CH3

2

The polymer was dissolved in acetone, precipitated into methanol, and then dried in a vacuum oven at 35 °C for 24 hr. The powdered polymers (20-40 mg) were placed in glass ampoules, degassed at less than 1 0 " m m H g for 40 hr at ambient temperature and then sealed under vacuum. The samples were irradiated in a C o Gamma-cell at a dose rate of 0.29 M r a d / h r at ambient temperatures (2530 ° C ) . Absorbed doses up to 10 M r a d were used. The volatile products formed by the radiolysis were analyzed by gas chromatography (18). A sealed ampoule was placed in an injection system attached to a Hewlett Packard-5734A Gas Chromatograph ( G C ) . The ampoule was broken in line by depression of a plunger and the products were injected into 4

6 0

Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

13.

PMPS

BOWMER AND BOWDEN

Degradation:

155

Radiolysis Products

the G C . A thermal conductivity detector and a flame ionization detector were used in series to measure the product yields. H e l i u m was used as the carrier gas. A Porapak Q column (80/100 mesh 6' length, 1/8" diam) was used with the oven temperature programmed from 80 to 190°C to optimize resolution of chromatographic peaks. The detectors were calibrated for quantitative measurement of yields with pure gases (Matheson) and reagent grade chemicals (Aldrich). Products with large yields (e.g., 2-methyl-l-pentene) were collected for H N M R analysis. The effluent gas from the thermal conductivity detector was bubbled through a cold C C 1 solution (-5°C) and the proton spectra of the solutions recorded on a V a r i a n T - 6 0 A N M R spectrometer. P M P S samples, 30-40 mg, were degassed and sealed i n N M R tubes under vacuum. After irradiating up to doses of 10 M r a d , some samples were opened immediately, dissolved in C C 1 D and the * H N M R spectra recorded. Spectra were recorded for up to —300 hours after irradiation. Other samples were kept at room temperature for 100-200 hours and then opened, dissolved and their spectra recorded. D u r i n g these various processes, viz., opening the tube, adding solvent and capping the tube, the vessel was maintained at -78 °C to minimize loss of the products from the tube. A modified ampoule preparation vessel was used to study the effect of gaseous products trapped in the polymer. A side a r m with glass break-seal was added through which a solvent was introduced after irradiation (1.0 M r a d ) of the polymer. Chlorobenzene (—0.3 c m ) was used as the solvent since its chromatographic retention time was significantly longer than the retention times for the radiolysis products. After dissolution, the irradiated polymer, volatile products and solvent were sealed into an ampoule and analysed as outlined above. The addition of solvent, dissolution and ampoule preparation were a l l performed i n a closed system, i.e., the irradiated sample was never exposed to air. Mixtures of sulfur dioxide ( S 0 ) and 2-methyl-l-pentene ( 2 M 1 P ) were prepared by (1) bubbling sulfur dioxide into a N M R tube containing 2 - m e t h y l - l pentene at -78°C, and (2) condensing sulfur dioxide and 2-methyl-l-pentene into a N M R tube which was sealed under vacuum. M o l e ratios for the 2 M 1 P / S 0 mixtures of 20:1 (method (1) above) and 1:1 (method (2)) were prepared and the samples irradiated i n the C o gamma-cell. l

4

3

3

2

2

6 0

RESULTS The major products upon irradiation were sulfur dioxide and olefin with small yields of hydrogen, C to C hydrocarbons and C hydrocarbons. The S 0 and the olefin were produced by depropagation after C - S bond scission and their yields as a function of dose are shown in Figure 1. A l l other products have linear dependence on radiation dose. The G-values are listed in Table 1. x

4

1 0


2

156


T A B L E 1. PRODUCT so

Initial G-values for Products INITIAL G-VALUE 3000 ± 300

2

OLEFIN CH

0.02

4

C H 2

C

1500 ± 300

0.06

4

0.005

2 6 H

C H

8

-0.1

isobutene

-0.4

dimers

-0.5

3

The initial G-value is the slope of the yield/dose plots at low doses. A t low doses, the S 0 / o l e f i n ratio was —2 and this ratio decreased expotentially towards unity as the radiation dose increased. In the unirradiated P M P S the ratio of — S 0 — to — C H — moieties was unity. 2

2

6

1 2

Methane and propane were produced by fracture of the side chains from the hydrocarbon moiety, followed by hydrogen abstraction. Isobutene, ethylene and ethane were formed by the secondary reactions of the methyl fragment ( C H ) with itself and the propyl fragment ( C H ) . A multiplet peak on the G C trace at high retention time was assigned to dimers of the parent olefin, i.e., branched C hydrocarbons. Impurities of acetone (0.2%) and methanol (0.005%) were detected in the P M P S from its preparation, and were retained in the glassy polymer even after 40 hr degassing at less than 10~ m m H g . Figure 2 shows the proton N M R spectra obtained after a dose of 1.47 M r a d . Spectrum (a) in Figure 2 was recorded immediately after irradiation. This spectrum was attributed to 2-methyl-l-pentene ( 2 M 1 P ) plus residual P M P S . The following resonances were assigned to 2-methyl-l-pentene: 3

3

7

1 2

4

(A)

4.70 ppm, vinylic protons;

(B)

2.2-1.8 ppm, allylic methylene protons;

(C)

1.7 ppm, allylic methyl protons;

(D)

1.3-2.0 ppm, multiplet due to - C H - C H 2

3

protons;

(E) 0.9 ppm, triplet due to - C H - C H protons. Resonances at the same chemical shift as peaks ( B ) - ( E ) were present in the P M P S spectrum and were assigned to the corresponding protons. For example, the 1.7 ppm peak had contributions from the allylic methyl protons of 2 M 1 P and from the protons on the methyl side branch in P M P S . The 3.68 ppm resonance 2

3


13.

BOWMER AND BOWDEN

PMPS Degradation:

Radiolysis Products

157

DOSE (Mrad)

Figure 1.

S 0 ( · ) and olefin (O) yields i n m o l e s / K g versus radiation dose i n Mrad.

Figure 2.

* H N M R spectra of P M P S irradiated to 1.47 M r a d . Spectrum recorded (a) 0.5 hrs and (b) 193 hrs after irradiation. The solvent was C C 1 D .

2

3



158

was assigned to the methylene protons adjacent to the — S 0 — moiety in P M P S . 2-Methyl-l-pentene from depropagation of P M P S and residual P M P S were the only species present in spectrum (a) i n Figure 2. If the N M R spectrum was recorded —190 hr after irradiation, spectrum (b) in Figure 2 was obtained. N e w resonances at 5.15 ppm (triplet) and 1.65 ppm were found. In addition, the 4.7 ppm peak due to the vinylic protons of 2 M 1 P decreased in magnitude. These changes are consistent with the formation of 2methyl-2-pentene ( 2 M 2 P ) which has the following resonances: 2

(F)

triplet centered at 5.15 ppm, vinylic proton;

(G)

multiplet centred at 2.1 ppm, allylic methylene protons;

(H)

doublet at 1.7 and 1.65 ppm, allylic methyl protons;

(I)

triplet centred at 1.0 ppm, due to — C H — C H 2

3

protons.

The resonances at 3.68, 4.7 and 5.15 ppm are due to P M P S , 2 M 1 P and 2 M 2 P respectively. The kinetics of the 2-methyl-2-pentene formation was quantitatively studied using these resonances. The magnitude of the residual polymer resonance (3.68 ppm) remained constant, i.e., no further depolymerization occurred. The total concentration of olefin remained constant as the 2-methyl-l-pentene was isomerized to 2-methyl-2-pentene. Figure 3 shows the kinetic data for the isomerization reaction after 1.47 M r a d . The post-exposure isomerization reaction took place both in the solid state (symbols • , • in Figure 3) and in solution (symbols O , · in Figure 3). Similar results were found for samples after a dose of —10 M r a d . However the 10 M r a d sample had significant amounts of 2 M 2 P present immediately after irradiation, see Figure 4. DISCUSSION In a previous study, Bowmer and O'Donnell (3) had found that radiation product yields were correlated to the ceiling temperature for a number of poly (olefin sulfone) s. Their results are summarized in Figure 5 which shows that the lower the T for a particular poly (olefin sulfone), the higher is the gas yield from irradiation, i.e., the lower the T the more strongly the right-hand side of the equilibrium: c

c

(R-S0 ) ^nR + η S0 2

n

2

is favored. In Figure 5 the correlation reported for G ( S 0 ) versus T is shown. In Figure 6 this plot is expanded to include the present results for P M P S . T for P M P S is -34 °C and as seen in Figure 6, irradiation well above T (25 °C) highly favors the depropagation reaction as expected. 2

c

c

c

The degradation scheme suggested by Bowmer and O'Donnell for poly (olefin sulfone) s is summarized in Figure 7. It includes (1) depropagation via free radical and cationic intermediates, (2) oligomerization of the free olefin initiated by


13.

PMPS

BOWMER AND BOWDEN

Degradation:

Radiolysis Products

159

TIME (hrs) Figure 3.

Kinetic curve for post-exposure isomerization of product olefin after dose of 1.47 M r a d . 2-methyl-l-pentene, •, ·; 2-methyl-2-pentene, • , O . Circles, tube opened immediately after irradiation. Squares, tube opened 193 hrs after irradiation.

100r

ο I

0

Figure 4.

'

•

.

I

100

.

.

.

TIME (hrs)

1

200

.

•

— - J

300

Kinetic curve for post exposure isomerization reaction of olefin after 9.97 M r a d dose. 2-methyl-l-pentene, ·; 2-methyl-2-pentene, O.


160


ι

•

ι

•

^

.

0

50 —

Figure 6.

T

C

m

m

>

ι--

100

—

G ( S 0 ) versus ceiling temperature, P M P S data from this work ( T - - 3 4 ° C ) . Other radiolysis yields from reference 3. 2

c


PMPS

BOWMER AND BOWDEN

Degradation:

Radiolysis

Products

® ^H-CH -CH-CH -CH2CH-S0 ^ 2

Figure 7.

2

2

Reaction scheme for poly (olefin sulfone) degradation.


162


cationic sites and (3) isomerization of the olefin via the cationic intermediates (10,11). W e attribute the discrepancy between G ( S 0 ) and G (olefin) in P M P S to the fact that oligomerization of the free olefin occurs at low doses. The oligomerization appears to be reversible as seen from the reduction of the S 0 / o l e f i n ratio from two at low doses towards unity at high doses (see Figure 1). It is possible the free olefin is physically trapped in the polymer and is not chemically retained as the oligomer. Such trapped gases would be released by dissolution of the polymer. Upon dissolving P M P S in chlorobenzene prior to analysis, we found no significant change in the S 0 / o l e f i n ratio. After a dose of 1.0 M r a d the ratio was found to be 1.6 ± 0.2 with dissolution and 1.4 ± 0.1 without dissolution. Therefore observation of a S 0 / o l e f i n ratio greater than unity can not be attributed to trapped olefin. 2

2

2

2

Infrared studies of the radiation-induced degradation of P M P S by Bowden et al. (19) supports the oligomerization process and also shows that the oligomers can be removed by post-exposure baking. These effects have not been seen for other poly (olefin sulfone) s (2,3). Figure 8 and Figure 9 show the yield versus dose curves for irradiation of poly(l-butene sulfone) and polyicyclohexene sulfone) respectively (20). N o comparable shift of the S 0 / o l e f i n ratio towards unity is observed i n the radiolysis of these polymers. In contrast to other poly (olefin sulfone) s, little isomerization of the olefin occurs during cationic depropagation. The degradation of P M P S is extremely rapid and it is possible that the lifetime of the cationic intermediates may be too short for appreciable isomerization to occur. However, isomerization of the 2methyl-l-pentene product to 2-methyl-2-pentene did occur during the post exposure period. The isomerization proceeded both i n solution and i n the solid state. It seems unlikely that the reactive intermediates (e.g. cations and free radicals) produced by irradiation catalyse the isomerization reaction since they would be expected to be destroyed during dissolution. However sulfur dioxide is well known to form charge transfer complexes ( C T C ) with olefins 0 ) and we propose that the isomerization occurs via such an intermediate: 2

2M1P + S 0

2

^ [olefin - S 0 J 2

C T C

^ 2M2P + S 0

2

Figure 10 shows the isomerization kinetics for a —20/1 mixture of 2-methyll-pentene with S 0 after a dose of 0.8 M r a d . N o significant isomerization occurred upon irradiation of pure 2-methyl-l-pentene, even after 40 M r a d of absorbed dose. However, after the small dose of 0.8 M r a d 2 M 1 P (in the presence of S 0 ) underwent isomerization to 2 M 2 P as seen i n Figure 10. The kinetics are similar to that observed following P M P S radiolysis (compare Figure 3). The equi-molar mixtures prepared under vacuum also showed accelerated isomerization after irradiation, see Figure 11. D u r i n g preparation of the 1:1 mixtures upto 20% isomerization occurred prior to any irradiation. This effect was not observed for the 20/1 mixture (Figure 10) and its cause is not known at present. 2

2


13.

PMPS

BOWMER AND BOWDEN

Degradation:

Radiolysis Products

163

DOSE (Mrad) Figure 8.

Y i e l d (moles/kg) versus radiation dose ( M r a d ) for polyG-butene sulfone). S 0 , · and 1-butene, •; for irradiations at 0° and 50°C. 2

Ot Ο

Figure 9.

Yield

ι 2

(moles/kg)

ι ι ι U 6 8 DOSE (Mrad) versus

poly(cyclohexene sulfone).

Ι10

radiation

dose

(Mrad)

for

S 0 , · and cyclohexene, • irradiated 2

at 0 and 50 °C.



164

Figure 11.

Kinetic curve for isomerization of equi-molar mixture of 2-methyl1-pentene and sulfur dioxide. 2-methyl-l-pentene, ·; 2-methyl-2pentene, O .


13. BOWMER AND BOWDEN

PMPS Degradation: Radiolysis Products 16

CONCLUSION PMPS has been found to depropagate rapidly upon exposure to ionizing irradiation producing S0 and 2-methyl-l-pentene. Oligomerization of the free olefin was observed probably occurring via cationic intermediates. This degradation scheme is consistent with that proposed by Bowmer and O'Donnell (10,11). No evidence was found for the mechanism suggested by Pacansky et al. (15,16). A post-exposure isomerization reaction was observed in which 2-methyl-l pentene isomerized to 2-methyl-2-pentene. We propose that this reaction proceeds via S0-olefin charge transfer complexes. 2

2

ACKNOWLEDGMENTS The author wishes to thank A. Novembre for preparation of the polymer and L. Shepherd for discussions of the degradation mechanism.

REFERENCES 1. Ivin, K. J. and Rose, J. B. "Advances in Macromolecular Chemistry" (Editor, W. M. Pasika) Vol. 1., p. 335 (1968). 2. Bowmer, T. N. and O'Donnell; J. H.; Jour. Polym. Sci., Polym. Chem. Ed., 19, 45 (1981). 3. Bowmer, T. N. and O'Donnell, J. H.; Jour. Macromol. Sci.-Chem., A17, 243 (1982). 4. Stillwagon, L. ACS Symposium Series No. 184, "Polymer Materials for Electronic Applications" (Editors, E. D. Feit and C. W. Wilkins, Jr.) p. 19 (1982). 5. Brown, J. R. and O'Donnell, J. H.; Macromolecules, 3, 265 (1970). 6. Brown, J. R. and O'Donnell, J. H.; Macromolecules, 5, 109 (1972). 7. Dole, M. "The Radiation Chemistry of Macromolecules", Academic Press (1973). 8. Bowden, M. J. CRC Critical Reviews in Solid-State Sciences,8,223 (1979). 9. Bowmer, T. N., O'Donnell, J. H. and Wells, P. R. Makromol. Chem., Rapid Communication,1,1 (1980). 10. Bowmer, T. N., O'Donnell and Wells, P. R.; Polymer Bulletin 2, 103 (1980). 11. Bowmer, T. N. and O'Donnell, J. H., Polymer, 22, 71, (1981). 12. Bowden, M. J. Polym. Sci.,12,499 (1974). 13. Bowden, M. J. and Thompson, L. F., Polym. Eng. and Sci., 14 (7), 525 (1974).


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14. Bowden, M. J. and Thompson, L. F., Polym. Eng. and Sci., 17(4), 269 (1977). 15. Gutierrez, Α., Pacansky, J. and Kroeker, R. Org. Coatings and Appl. Polym. Sci. Proceedings, 46, 520 (1982). 16. Pacansky, J. paper presented at Electrochemical Society Meeting 10th Intl. Conf. on Electron and Ion Beam Sci. and Technol., Montreal, May 9-14, (1982). 17. Bowden, M. J. and Novembre, T., Org. Coatings and Appl. Polym. Sci. Proceedings, 46 681 (1982). 18. Bowmer, T. N. and O'Donnell, J. H., Polymer, 18, 1032 (1977). 19. Bowden, M. J., Allara, D. L., Vroom, W. I., Frackoviak, J., Kelley, L. C. and Falcone D. R.; preceding paper. 20. Bowmer, T. N., Ph.D. Thesis, University of Queensland, Australia (1980). RECEIVED September

14, 1983


The Radiation Degradation of Poly(2-methyl-1-pentene sulfone)

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