Pulse radiolysis of polystyrene

Part I, Georg Thieme Verlag, Stuttgart, Germany, 1961, p 762. (3) Initially ... three times, giving an effective path length of 4.9 cm, and then enter...
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PULSE RADIOLYSIS OF POLYSTYRENE

4527

Pulse Radiolysis of Polystyrene'

by S. K. Ho, Seymour Siegel, Aerospace Corporation, El Sepundo, California

and Harold A. Schwarz Chemistry Department, Brookhaven National Laboratory, Upton, New York

11976

(Received June 28, 1967)

Absorption spectra following the pulse radiolysis of polystyrene containing various solutes are reported. An absorption in samples containing triphenylamine is due to the positive ion of the solute. The lifetime of the ion is of the order of milliseconds and its disappearance is attributed to recombination with negative ions in the spur. Triplet states of anthracene, diphenyl, naphthalene, perylene, and p-terphenyl are identified. The lifetimes of the triplets are never greater than 20 msec at room temperature. Much longer lifetimes for naphthalene, diphenyl, and p-terphenyl triplets have been observed when they are produced by photolysis. The short lifetime in radiolysis is attributed to paramagnetic quenching in the spur.

Introduction Very little is known about transients produced in the radiolysis of polymers. Homogeneous reactions between intermediates should require several minutes and hence the existence of short-lived transients would indicate either first-order reactions or a peculiarity of radiation chemistry such as interactions in a spur. We have chosen polystyrene for study as it is easily prepared in transparent form with and without solute. Experimental Section Materials. Styrene (Eastman Kodak), after being freed from stabilizing agent by washing, successively, with 10% sodium hydroxide solution and water, was dried in an atmosphere of nitrogen, successively, with magnesium sulfate and calcium hydride, and was distilled under reduced pressure in a nitrogen atmosphere in the presence of calcium hydride immediately before use. ilnthracene, triphenylamine, 1-chlorooctane (all from Eastman Kodak) and perylene (K & K) were used as received. Diphenyl, naphthalene, and p-terphenyl were purified by recrystallization from absolute alcohol. Sample Preparation. Polystyrene was prepared by vacuum bulk polymerization of degassed styrene.2 The polymerization conditions were 48 hr a t 80", then 9 hr a t 80-150", 10 hr at 150°, and 10 hr a t 180".

Samples containing solutes were prepared similarly from solutions in styrene. Each sample was prepared by degassing and polymerizing in a rectangular glass cell, 3 X 1.6 X 0.8 cm (internal dimensions). After polymerization was complete, the cell was cracked under nitrogen, and the polymer transferred to a storage vessel, which was kept under high vacuum. These samples were used directly for irradiation with no additional polishing of the two sides serving as optical faces (3 X 0.8 cm). Transmission of light through such samples was about 80% that of polished sample^.^ Irradiation. A 16-psec 2.0-RIev electron pulse was generated by a Van de Graaff accelerator operating at 10-ma beam current. The polymer sample was placed inside a rectangular quartz cell (internal dimensions: 3 X 1 X 2 cm) with two suprasil optical windows (1 X 3 cm). The electron beam entered (1) Research performed under the auspices of the U. S. Atomic Energy Commission and Air Force Contract USAF 04(695)-669. (2) "Methoden der Organischen Chemie," E. Moller, Ed., Vol. 14, P a r t I, Georg Thieme Verlag, Stuttgart, Germany, 1961, p 762. (3) Initially, styrene solutions were polymerized in glass ampoules, the polymer was cut and machined into blocks of the appropriate dimensions, and the optical faces were obtained by polishing. Such a lengthy exposure to air resulted in a much shorter lifetime for the transients than found in samples which had been only briefly (less than 5 min) exposed to air.

Volume 71, Number 16

December 1967

4528

through a 2 X 3 cm face, ground to 0.5 mm, to minimize beam attenuation. All samples were evacuated. The analyzing light beam from a 150-w quartz-iodine lamp traversed the polymer by a mirror arrangement three times, giving an effective path length of 4.9 cm, and then entered a grating monochromator. The photomultiplier signal (either an RCA 7200 or an RCA 7102) was amplified and displayed on a Tetronix 555 oscilloscope. The relative dose delivered by each electron pulse was monitored by determining the fraction of the charge collected on a 0.25411. aluminum plate with a 2 X 1.6 cm opening placed in front of the sample. Ferrous sulfate dosimetry a t low beam intensity (about 100 rads sec-l) and corrected for relative depth-dose effects in polystyrene and water was used to calibrate the charge measurements. The dose per pulse in polystyrene was about 1 X 10'' ev rnl.-I For low-temperature irradiation, a cell similar to the one described above was surrounded by a quartz vacuum jacket with a 0.012-mm duPont H-film window for the electron beam. The cell was constantly purged by nitrogen. and the temperature was governed by the flowrate of the gas, which passed through copper coils immersed in liquid nitrogen. Temperature was measured by an iron-constantan thermocouple placed next to the sample.

Results Pure Polystyrene. Pulse radiolysis of pure polystyrene gave a weak absorption spectrum extending from 320 to 800 mp. There was a maximum a t 405 mp with a Ge value of 5.5 X lo3,where G is the 100-ev yield and e the extinction coeficient of the absorbing species. The rest of the spectrum was structureless with Ge values of about 2 X lo3. The decays of the absorptions were complex, i.e., not first or second order. For comparison purposes, the time taken for the absorption to decay to one-half of its initial value, which we will call the initial half-life, is used in this paper. This time varied from one sample to another. Values obtained in the beginning of the investigation were about 4 msec for the band maximum a t 405 mp, but all later experiments gave a lower value of 1 msec. The broad structureless absorp 'ion had an initial half-life of several tenths of a second, and did not completely disappear even a t times as great as 1 hr. A sample containing 0.37 M 1-chlorooctane, an electron scavenger, did not exhibit the peak a t 405 mp. The slowly decaying structureless absorption was unaffected. Polystyrene Cmtainin,g Aromatic Solutes. The transient absorption found in pure polystyrene was rather The Journal of P h p k a l Chemistry

S. K. Ho, S. SIEQEL,AND H. A. SCHWARZ

weak and yielded little information about its origin. Since triplet states and ions with known absorption spectra can be formed from polynuclear aromatic hydrocarbons, polystyrene containing five such hydrocarbons were selected for investigation. In addition, triphenylamine, which has a known positive ion spectrum, was also employed. Diphenyl. Polystyrene containing diphenyl gave, on pulse radiolysis, a spectrum with a peak a t 375 mp and a shoulder near 400 mp, beyond which the absorption was broad and weak up to 800 mp. The Ge values for the entire visible region were about 1.3 X lo3. The peak a t 375 mp had an initial half-life of 20 msec. The shoulder had a faster decaying component and appeared to be identical with the peak a t 405 mp in pure polystyrene. Subtraction of the pure polystyrene spectrum from the absorption in a sample containing 0.08 M diphenyl yielded a spectrum with a maximum at 375 mp, which was similar to that of the diphenyl t r i ~ l e t . ~ The decay of the absorption did not obey any simple law. The published diphenyl negative ion absorption spectrum has peaks a t 400 mp ( e = 4.0 X lo4) and a t 625 mp ( e = 1 X 104).5 The 400-mp peak would be obscured by the diphenyl triplet and the 405 mp peak found for pure polystyrene, but the value of Ge at 625 mp was only 1.3 X lo3 and seemed to originate in the polystyrene. Therefore, the diphenyl negative ion does not seem to be produced in appreciable yields. Naphthalene. The spectrum obtained from the pulse radiolysis of polystyrene containing naphthalene exhibited maxima a t 428, 400, and 380 mp, and can be compared with that of the naphthalene trip:et obtained by photolysis as shown in Table I. In contrast to diphenyl, there did not appear to be any contribution to absorption in the 400-mp region from the transient observed in pure polystyrene. The slowly decaying components in the naphthalene-containing samples were very similar to the long-lived components obtained in pure polystyrene. Subtraction of such contributions gives the ratios of the intensities of the three maxima as 1:0.53 :0.33. These can be compared with similar values for the naphthalene triplet, 1:0.60:0.15.6 The Ge values, at the maxima, increase with concentration of naphthalene, as shown in Table I. The decay of absorption was complex, the lifetime is ap-

(4) G. Porter and M.W. Windsor, Proc. Roy. SOC.(London), A245, 238 (1958). (5) P.Balk, G.J. Hoijtink, and J. W. H. Schreurs, Rec. Trav.Chim., 76,813 (1957). (6) G.Porter and F . Wilkinson, Proc. Roy. SOC. (London), A264, 1 (1961).

PULSE RADIOLYSIS OF POLYSTYRENE

4529

Table I: Comparison of Triplet Absorption Obtained in Pulse Radiolysis and in Photolysis Radiolysia Amax,

Compound

Diphenyl Naphthalene

p-Terphenyl Anthracene Perylene

Conon,

M

0.081 0.005 0.052

0.52 0.003 0.010 0.024 0.034 0.0074

mr

375 428 428 400 380 428 470 470 470 435 495

Gc X

-Photolysis Half-life,” mwc

Initial half-life, maec

Amax,

20

372

2.0

415 392 372

1 . 5 X 10*

5J

450

1 . 3 X 108

7J

426 490

35 6

1.1 0.3 1.5 0.8 0.5 1.9 1.3 3.2 5.9 7.35 0.65

mfi

x

Ref

4, d

108

7 10 10 10

1 20 20 20 18 6

4, e 4, e

, After subtraction of 0.2 x lo4for polystyrene absorption. * This value, different from that shown in Figure 1, was considered more Phosphorescence lifetime in poly(methy1 methacrylate) a t room temperature. reliable. The difference could be due to dosimetry. d R. E. Kellogg and R. P. Schwenker, J. Chem. Phys., 41,2860 (1964). e R. E. Kellogg and N. C. Wyeth, &d., 45, 3156 (1966).

parently dependent on the concentration of naphthalene (Table I). The published naphthalene negative ion spectrum has an intense maximum at 323 mp and a minor one a t 360 mpa6 Unfortunately, the polystyrene absorbs too strongly at 323 mp to allow a study of the principal negative-ion absorption, and the 360-mp peak would be obscured by one of the absorptions of the triplet (380 mr>. p-Terphenyl. The spectrum obtained on pulse radiolysis of p-terphenyl in polystyrene is shown in Figure 1. There is a maximum at 470 my, and in some runs, a shoulder a t 445 mp also appeared. The initial half-life was 20 msec, and the decay was complex. There was a prominent slowly decaying component with a lifetime of the order of 300 msec. The Ge values increased with p-terphenyl concentration (Table I), but the initial half-life appeared to be independent of concentration. The p-terphenyl mono-negative ion has a spectrum6 with three maxima in the same region as the triplet. The absorption spectrum of the triplet state of p-terphenyl obtained from flash photolysis in 3-methylpentane at -60” is shown in Figure 1. The differences in appearance of the two spectra might suggest the presence of the mono-negative ion, but the addition of 0.37 M l-chlorooctane did not eliminate the differences, only giving a somewhat broader peak. Anthracene. Anthracene, having active 9, 10 posi~~~ tions, tends to copolymerize with ~ t y r e n e . We analyzed our polymer mixtures by absorption spectroscopy of samples dissolved in carbon tetrachloride and found that 28% of the anthracene remained free, independent of anthracene concentration, in agreement

*

3

I

0

G

1

I_ /,q 2-

-

I-

0

I

/A\\\ : /

\

\

I

I

I

I

-

‘I

1

I

WAVELENGTH, m p

Figure 1. Transient absorption spectrum in the pulse radiolysis of polystyrene containing 0.024 M p-terphenyl (full curve); dashed curve, p-terphenyl triplet spectrum.

with Cherkasov and Voldaikina. The concentrations used in this paper refer to free anthracene, ie., 28% of the added anthracene. Pulse radiolysis of polystyrene containing anthracene gave an absorption spectrum with a peak at 435 mp. This is very similar to the anthracene triplet spectrum obtained by flash photolysis4 (Table I). The decay of absorption waspredominantly first order. However, there was also a minor longlived component present, with a lifetime of the order of several hundred milliseconds. These two components (7) Unpublished data. (8) C. S. Marvel and B. D. Wilson, J. Org. Chem., 23, 1479 (1958). (9) A. 9. Cherkasov and K. G. Voldaikina, Vysokomolekd. Soedin., 5,79 (1963).

Volume 71, Number 13 December 1067

4530

were separable by the Guggenheim methodlo (with the approximation that the long-lived component is constant) and the half-life of the first order process was 18 f 2 msec. This value was independent of the concentration of anthracene and not affected by a tenfold increase in dose rate (from about 10 to 100 Mrads sec-l). The value of Ge for the long-lived component varied from 3 X lo3 to lo4 and could not be measured with much precision. A large part of it is attributable to species produced from the polystyrene host. The formation of the triplet in a 0.017 M anthracene sample was at least 90% complete in 5 psec. The variation of Ge values at 435 mp with anthracene concentration is shown in Figure 2. The anthracene mono-negative ion has a principal absorption at 370 mp16a region not accessible in this system because of the strong absorption of anthracene itself. There is a minor peak of the anthracene negative ion at 714 mp, with a reported extinction coefficient of 1 X lo4. The pulse radiolysis spectrum of a 0.018 M anthracene sample showed an absorption between 680 and 800 mp (Ge = 8.3 X loa), which was larger than observed in pure polystyrene (Ge = 1.7 X lo3), and the difference (Ge = 6.6 X lo3)may arise from the negative ion. P e r y h e . For the above hydrocarbons, there was difficulty in identifying absorptions due to the negative ion, either because the principal absorption of the negative ion lies outside the accessible region, or because of overlap between the triplet and negative-ion spectra. E'erylene has a triplet absorption maximum a t 490 mp4 and a negative ion absorption peak at 580 mp (e = 4.3 X lo4, half-width of 32 mp);6 overlap of the two spectra is slight. The spectrum obtained from a perylene-containing sample is shown in Figure 3. It is divisible into two regions. Above 520 mp, there appeared to be only one dominant species with an initial half-life of 300 msec. Below 520 mp, there was a component with a 6-msec half-life. Analysis of the decay curves at several wavelengths resulted in the resolution of the observed spectra into the dotted spectra shown in Figure 3. The shortlived component, which exhibited first-order decay, is the perylene triplet. The spectrum of the long-lived component, with a maximum at 550 mp and a shoulder at 580 mk, bears little resemblance to that of the chemically prepared mono-negative ion. For further characterization of this species, 0.37 M 1-chlorooctane was added; there was practically no change in the spectrum compared to that without chloride. However, the initial half-life of the long-lived species decreased to SO msec in the presence of chloride. The increase of decay rate is consistent with the increase of diffusion The Journal of Physical Chemistry

S. K. Ho, S . SIEQEL, AND

I

SI-

I

0

H.A. SCHWARZ

I

76-

7 0

54-

W

32-

/ 0.02 0.03 0.04

OO

0.0 I

CONCENTRATION O F ANTHRACENE I N P O L Y S T Y R E N E , M

Figure 2. Variation of yield of anthracene triplet with anthracene concentration. Dashed line, fit of data to mechanism A; solid line, fit of data to mechanism B.

3 t

WAVELENGTH, mp

Figure 3. Transient absorption spectrum in the pulse radiolysis of polystyrene containing 0.0074 M perylene (full curve); dashed curves, spectra obtained after resolution by kinetic analysis.

constants with solute concentration." The half-life of the triplet at 495 mp was not affected. Triphenylumine. Polystyrene containing triphenylamine gave a spectrum (Figure 4a) in the visible region with maxima at 408 and 630 mp. The initial half-life over the entire spectral range was about 1msec. Tri-p-tolylamine is known to produce a positive ion with an absorption peak at 656 mp.I2 In the presence of an aromatic hydrocarbon as an additional solute, the absorption associated with triphenylamine was affected very little. Superimposed on it was the spectrum found with the hydrocarbon in (10) E. A. Guggenheim, Phil. Mag., 2,538 (1926). (11) G. S. Park, Trans. Faraday Soc., 47, 1007 (1951). (12) S. Granick and L. Michaelis, J. Am. Chem. SOC., 6 2 , 2241 (1940).

PULSERADIOLYSIS OF POLYSTYRENE

4531

polystyrene. Three hydrocarbons were investigated, diphenyl (0.08 M ) , p-terphenyl (0.024 M ) , and anthracene (0.018 M ) . The spectrum found for p-terphenyl and triphenylamine is illustrated in Figure 4b. There was a marked change in the lifetime of the 630mp absorption associated with triphenylamine, depending on the identity of the second solute. Diphenyl had no effect on the lifetime, i.-.,it was about 1 msec, but in mixtures containing triphenylamine and either anthracene or p-terphenyl, the initial half-life was increased to 0.5 see. I

I

I

I

I

Table 11: Variation of Rate of Decay of Triplet Absorption with Temperature 4 . 0 1 8 M AnthracenFirstorder Temp, half-life, OK msec

89 156 226 276 296

I

28.2 27.0 24.0 21.7 21.2

-0.054 M NaphthalenInitial Initial

half-life Temp,

(radiolysis),

sec

OK

77 98 172 210 24 1 268 295

hau-iife5 (photolysis), BBC

1.1 0.78 0.62 0.51 0.32 0.125 0.029

0.5

a Phosphorescence lifetimes of triplets excited by ultraviolet light were measured on several of our samples at Aerospace Corporation, El Segundo, Calif.

Discussion Homogeneous diffusion-controlled reactions between intermediates should proceed at a negligible rate in polystyrene. The rate constant for a diffusion-controlled reaction is given by13

k = 4rDRN ___ lo00

I I I 500 600 WAVELENGTH, rnp

I

I

too

I

Figure 4. Transient absorption spectra in the pulse radiolysis of polystyrene containing triphenylamine: (a) 0.046 M triphenylamine, and (b) 0.046 M triphenylamine and 0.024 M p-terphenyl.

Low-Temperature Irradiation. The effect of temperature on the naphthalene and anthracene triplets was followed at 425 and 435 mp, respectively. For both samples, the optical density remained unchanged with a decrease in temperature down to -184”. For the anthracene-containing sample, the decay curves, when analyzed by the Guggenheim method, gave linear first-order plots, with half-lives which increased only slightly at low temperatures (Table 11). For the sample containing naphthalene, the decay curves were not first order over the entire temperature range; however, the initial half-life increased markedly with a decrease in temperature (Table 11).

where D is the diffusion constant, R is the reaction radius, and N is Avogadro’s number. The diffusion constant of benzene at low concentration in polystyrene is 2 X 10-l2 cm2 sec-l-ll Assuming this value as typical of radiation-induced intermediates, and assuming R to be 5 X lo-* cm, k would be about lo3M-’ sec-’. The concentration of intermediates in our experiments would be about M , and the half-life for homogeneous reaction between intermediates, (kc)-l, would be 100 sec. Similarly, impurity effects should be negligible. The diffusion constant does depend on solute size, being 7 X cm2 sec-l for krypton14 and about 4 X cm2 sec-’ for oxygen.16 Still, even for oxygen, the rate constant would be at most 106, and M oxygen would not produce transient effects on a millisecond time scale. The inhomogeneous reaction between intermediates in a spur can occur on a much shorter time scale. In (13) 9. W. Benson, “The Foundations of Chemical Kinetics,” McGraw-Hill Book Co., Inc., New York, N. Y., 1960,p 498. (14) H. Schaefer and H. Maywald, Kolloid-Z., 204, 11 (1965). (15) This is an order-of-magnitude estimate, based on the rate of decolorization of samples made brown by radiation in Vacuo and then exposed to air. A decolorized boundary moves through the sample on a time scale of weeks and the depth of the boundary is proportional to the square root of the time. The proportionality factor, by a simple diffusion theory, should be about 3Dl’*.

Volume 71, Number 13 December 1987

4532

water, the initial half-life for reactions in the spur is expected to be 3 X 10-lo sec for species with diffusion cm2 sec-l.16 For similar spur constants of 5 X geometry, Dtl,2 would be similar, and in polystyrene, the initial half-life of a spur would be expected to be sec. Many of the data presented here can be interpreted in terms of spur reactions. Ions. The spectrum in Figure 4 suggests that the triphenylamine (TPA) positive ion is formed. The extinction coefficient of the ion is not known, but following Skelly and Hamill," it may be assumed that the value of E is the same as for tri-p-tolylamine, or 1 X lo4. Since, experimentally, Ge at 650 mp is 2 X lo4, the yield of the TPA positive ion in 0.046 M TPA solution is 2 ions per 100 ev. Schenck'" has observed the TPA+ ion as a stable product at -80" in poly(methy1 methacrylate) solutions of TPA. The absorption peak found at 405 mp in pure polystyrene is probably the polystpene negative ion, since it disappears in the presence of 1-chlorooctane. The lifetime of the negative ion is about 1 msec, which agrees with the lifetime of the TPA+ ion. As pointed out earlier, this lifetime cannot be due to homogeneous recombination of the ions, but it is of the right magnitude for spur recombination. The lifetime of the TPA+ ion is increased by a factor of 500 when produced in the presence of electron-trapping hydrocarbons. This suggests that in the polystyrene negative ion, the electron can move along the chain rapidly, compared to the diffusive movement of hydrocarbon negative ions. The fact that this effect is not noted in diphenyl solutions is in agreement with the relative electron affinities. Poplelg has theoretically estimated that the styrene negative ion is more stable than the diphenyl ion, while all the other hydrocarbon ions are more stable than the styrene ion. This model of ionic reactions is only poorly supported by observations of negative ions. It is unfortunate that the negative-ion absorption peaks tend to be obscured by other absorptions or by the polystyrene itself. The negative-ion absorption peak in anthracene at 714 mp may be present with a yield of 0.G per 100 ev, which is not in disagreement with the TPA+ yield, in view of the rough estimate of extinction coefficients. The strong absorptions in perylene solutions at 550 and 580 mp may be due to ions, but both the positive and negative ions absorb in this region.20 1-Chlorooctane had no effect on this absorption, but the dissociative electron capture of this solute is exothermic to only about 0.5 ev, and the electron affinity of perylene is probably greater than 1 ev121so that reaction with the perylene negative ion would not be expected. The l-chlorooctane should have shown some effect by competing The Journal of Physical Chemistry

S.K. Ho, S.SIEGEL, AND H. A. SCHWARZ

for the electrons, however. Christodouleas and Hami1122have concluded that the perylene positive ion was the absorbing species in methyltetrahydrofuran glasses. The long lifetime of the 550-and 58O-mfi peaks, if attributed to the perylene positive ion, indicates that the negative ion was also stabilized by the presence of the solute in some manner. Triplet states. It may be seem from Table I that when photolytically produced triplet states have short lifetimes (less than 20 msec), the lifetimes of the radiation-produced triplets are similar, but when the photolytic lifetimes are long, the radiation-produced species still have short lifetimes. In the case of naphthalene containing solutions, a 0.5-sec photolytic lifetime and a 10-msec radiolytic lifetime were observed on the same sample. McCollum and Wilson23 have observed a similar shortening of triplet lifetime in radiolysis and attributed the effect to quenching by radicals, but we observed no dose-rate effect, and, as shown earlier, homogeneous reaction with radicals would be impossible on this time scale. Quenching of triplets by radicals and other triplets in the spur would appear to be a reasonable explanation for the effect, as the initial halflife would be limited to the half-life for spur reactions. The explanation is supported by the observed temperature effects. The lifetime of the radiolytic anthracene triplet is similar to the photolytic triplet and is not appreciably affected by temperature (Table 11). The lifetime or the radiolytic naphthalene triplet is strongly dependent on temperature, suggesting that diffusion of intermediates is involved. The yield of triplets depends on the concentration of the hydrocarbon. Porter and Windsor' gave the extinction coefficient of the anthracene triplet as 7 X lo4 at the peak. From Figure 2, the maximum yield of anthracene triplets is about 1 per 100 ev. A yield of 4 triplet naphthalene molecules per 100 ev has been observed in 0.008 M naphthalene in polystyrene a t 100°K by an esr technique.24 Keller and Hadley2bgave the extinction coefficient of the naphthalene triplet as 1.4 (16) A Kuppermann in "Actions Chimiques et Biologiques des Radiations," M. Haissinsky, Ed., Vol. 5, Academic Press, London, 1961,p 122. (17) D. W. Skelly and W. H. Hamill, J . Chem. Phys., 43, 3497 (1965). (18) G. 0. Schenck, W. Meder, and M. Pape, Proc. Second Intern. Conf. Peaceful Uses At. Energy, 29, 352 (1958). (19) N.S. Hush and J. A. Pople, Trana. Faraday soc., 51,600 (1955). (20) W. I. Aalberberg, G . J. Hoijtink, E. L. Mackor. and W. P. Weijland, J . Chem. SOC.,3049 (1959). (21) R. V. Slatas and M. Srwarc, J . Phys. Chem., 69,4124 (1965). (22) N. Christodouleas and W. H. Hamill, J. Am. Chem. SOC.,86, 5413 (1964). (23) J. D.McCollum and W. A. Wilson, U. 9. Department of Commerce, 05ce of Technical Services Report AD268,637 (1981).

PULSERADIOLYSIS OF POLYSTYRENE

X lo4,so that we find G(trip1et naphthalene) to be 0.2 in 0.005 M naphthalene (Table I). The discrepancy is not due to temperature, as we find, for a 0.052 M solution GE is 1.43 X lo4 a t room temperature and is 1.5 X lo4 a t 100°K. Since E increases somewhat with decreasing temperature, the G value cannot be greater at low temperature. The fact that solute triplet-state formation is complete in 5 psec indicates that the triplets are not formed by the recombination of the stabilized ions (which proceeds in tenths of a second). The lack of a temperature effect on triplet yields also shows that no material diffusion is involved. The excitation must move from the solvent to the solute in one of two ways: (A) when the solute is present within a distance r of the localized excitation in the solvent, bimolecular transfer can occur directly in a single step with unit probability by an electron exchange, or (B) the excitation can move rapidly along the polymer chain (or between different chains) and finally become trapped on a solute molecule by a random walk process. Ilechanism A would give the concentration dependence26

Ge = (Ge),,(l

- e--OC)

The constant a is related to r by a = 47rr3N/3000. The best fit to the the anthracene data is given by the dashed line in Figure 2, from which a is determined to be 74 l./ mole. The corresponding value of r is 30 A. This value is large compared with the value of 13 A obtained by

4533

the same equation for triplet-triplet transfer in organic glasses a t 77°K,27indicating that mechanism A is not involved. Mechanism B gives the concentration dependence Ge=

(GdO ~

kl

1+k2C where kl is the first-order rate constant for disappearance of the excited solvent molecules and kz is the second-order rate constant for transfer of the excitation energy to anthracene. The satisfactory fit of this equation to the anthracene data is shown as the solid line of Figure 2. The best value of kl/lcz is 0.017 M . There is no direct evidence to indicate whether singlet or triplet states are involved in the transfer. A lower limit to kl may be obtained by noting that anthracene triplet formation was complete in less than 5 X sec in a 0.017 M anthracene sample. This time must be longer than (ICl k2C)-l or kl kzC > 2 X los sec-l. Since kl/k2 = .017 M , IC1 > 1 X 106 sec-' or the half-life of the polystyrene excited state is less than 7 X sec. Such a short lifetime indicates that a singlet state is probably involved in the transfer.

+

+

(24) M. V. Alfimov, N. Ya. Buben, A. I. Pristupa, and V. N. Shamshev, O p t . Spectry. ( U S S R ) ,20,232 (1966). (25) R. A. Keller and S. G. Hadley, J. Chem. Phys., 42, 2382 (1965). (26) F. H. Brown, M. Furst, and H. Kallmann, Trans. Faraday SOC., 27,43 (1959). (27) A. Terenin and V. Ermolaev, ibid., 52, 1042 (1956).

Volume 71, Number IS DecembeT 1067