J. FOCK, A. HENGLEIN, AND W, SCHNABEL
310
The Effects of Linear Energy Transfer in the Radiation-Induced Polymerization of Several Vinyl Compounds
by J. Fock, A. Henglein, and W. Schnabel Huhn-Meitner-Institut fCr Kernforschung, Berlin- Wannsee, Germany
(Received August $3, 1966)
The radiation-induced polymerization of styrene, methyl methacrylate, and vinyl acetate containing small amounts of triethyl borate has been studied. At the same absorbed dose rate, the rate of polymerization by the high LET particles from the reaction B10(n,a)Li7 was found to be much lower than by y-rays, while the average molecular weight of the polymer formed was higher. The results are explained by assuming that only free radicals in the tracks of fast &electrons are able to initiate long-chain polymerization. The fraction f of the energy of the heavy particles that is effective in producing polymerization amounts to 0.16 for styrene, 0.09 for methyl methacrylate, and 0.04 for vinyl acetate (if f is made equal to unity in the case of y-rays) . It is shown that f decreases with increasing radiation sensitivity (G(R) value) of a monomer. The minimum kinetic energy of the &-electrons which participate in the initiation of long-chain polymerization is calculated to be 170-240 e.v., depending on the nature of the monomer.
Introduction In a recent paper,' an investigation of the polymerization of acrylonitrile containing 5 wt. % of triethyl borate under the influence of ?-rays and of thermal neutrons was reported. A very pronounced LET effect was found, since a 15 times higher absorbed dose of the He-Li particles from the nuclear reaction BO ' n -P He4 Li7 was necessary to initiate the polymerization with the same rate as with Co60 y-radiation. The average molecular weight of the polymer formed by heavy particle radiation was found to be higher than in the case of y-rays a t the same absorbed dose rate. These results were interpreted as a homogeneous polymerization of acrylonitrile, i.e., the growing of the chains far away from the tracks of the He-Li particles. The failure of a large fraction of the absorbed heavy particle energy to initiate the polymerization could be understood if it is assumed that only fast &electrons are able to produce long chains of the polymer. Radicals which are formed within the main tracks of the He-Li particles are deactivated by mutual interaction before they reach any appreciable chain length. It could be calculated that as an average only those &electrons which have a kinetic energy exceeding 215 e.v. escape the tracks and initiate polymerization. It seems interesting to
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The Journal of Physical Chemistry
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compare these results with the classical work of Dale, Gray, and Meredith2 on the deactivation of carboxypeptidase in aqueous solution by X- and a-rays. In this case, the low LET component of the a-rays was also found to be more effective than the primary particles. Similar effects of the linear energy transfer of the radiation have now been observed in the polymerization of other monomers, such as styrene, methyl methacrylate, and vinyl acetate. The principal experimental procedure consisted of irradiating (a) the pure monomer by Co6O y-radiation, (b) the monomer containing a few per cent of triethyl borate by y-rays in order to study the influence of the additive on the polymerization, (c) the monomer containing triethyl borate by thermal neutrons from a nuclear reactor, and (d) the pure monomer in the same position in the reactor in order to correct the results from (c) for the contribution of the y-background.
4.Henglein, J. Fock, and W. Schnabel, Mukromol. Chem., 6 3 , 1 (1963). (2) W. M . Dale, L. H. Gray, and W. J . Meredith, Phil. Trans. R o y . Soc. London, A242, 33 (1950).
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EFFECTS OF LINEAR ENERQY TRANSFER
311
Experimental The details of the experimental procedures have been described in the earlier report and are therefore only briefly mentioned here. The ?-irradiations were carried out in the field of a Coca source of 150 c. The neutron irradiations were carried out in the thermal column of the Hahn-Rleitner-Institut nuclear reactor where the intensity of fast neutrons was negligible. All samples were surrounded by bismuth in order to decrease the intensity of the y-background of the thermal column. As can be seen from Table I, the
The polymers formed were separated by precipitation and their yields determined by a gravimetric method. The intrinsic viscosity was used as a measure of the average molecular weight. Viscosities were determined in methanol in the case of polyvinyl acetate and in benzene in the cases of polystyrene and polymethyl met hacrylate.
Experimental Results The rate of polymerization generally depends on the absorbed dose rate DR according to
r Table I : Conditions in the Irradiation of Vinyl Acetate Containing 5 wt. 7, of Triethyl Borate in the Thermal Column of the Berlin Nuclear Reactor
Distance from core,
Thermal neutron flux, (n./cm.z sec.)
cm.
x
127 132 141 146
10.7 8.97 3.25 2.07
10-8
ICl X DR“
(2)
where a is a constant exponent over a wide range of the absorbed dose rate. A similar relationship is observed for the dependence of the average molecular weight of the polymer on the dose rate
B
Absorbed
=
k2 x D R - ~
(3)
He-Li
Absorbed y dose rate, radhec.
dose rate, rad/aec.
0.59 0.39 0.23 0.08
58.2 48 8 17.6 11.3
absorbed dose due to the y-background amounted to only a few per cent of the absorbed He-Li dose. In order to correct the observed rate of polymerization for the contribution of the y-background, a sample containing the pure monomer without boron ester was also irradiated as described in the earlier report, The sample (1 cc.) was irradiated in an evacuated and sealed soft glass vessel. Since the boron content of the samples was rather low, the attenuation of the neutron flux amounted t o less than 15% and the distribution of the neutron flux within the sample can be regarded as uniform. The absorbed dose rate was calculated from the relationship dose rate (rad/hr.)
=
I n order to determine the exponent a, the polymerization rate is plotted as a function of the absorbed dose rate in Fig. 1, 2, and 3. The data include the results obtained in the yirradiation of the pure and boroncontaining monomers ( X 0)as well as the results of the He-Li irradiations already corrected for the conIn the case of tribution of the r-background (0). vinyl acetate, an exponent a of exactly 0.5 was obtained. This is as expected if the growing chains are deactivated by reactions among themselves. I n the cases of styrene and methyl methacrylate, the slope of the curves decreases with increasing dose rate. This effect is well known and explained by the inefficiency of the monomer to scavenge all primary free radicals )
lo4
lo5
rad/h-
=
+
where is the slow neutron flux in n./cmS2sec, u the cross section of the B1”(n,a)Li7reaction (7.55 X cme2),N the number of boron atoms in 1 cc. of the sample, E the kinetic energy (2.35 Mev.) of the product nuclei of the nuclear reaction, and p the density of the sample. Both gold foil activation and the Fricke dosimeter containing boric acid3were used to determine the thermal neutron flux. 7-Doses were determined with the normal Fricke dosimeter,
-
4 6 0 10 Dose rate [rad/sec]
2
B
-
20
40
60
Figure 1. The rate of polymerization of styrene as a function of the absorbed dose rate by 7-rays and He-Li particles.
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R. H. Schuler and N. F. Barr, J . Am. Chem. SOC.,78, 5756 (1956).
Volume 68, Number 2
Februaru, 1964
J. FOCK, A. HENGLEIN, AND W. SCHNABEL
312
’*
lo4
a
v)
I b: meihylmethacrylate;
3% B(OCzH&
t-kadiationl
x : pure methyl methacrylate, y
0:methyl methacrylate
I V
’” 1
lo5
radlh -c
2
I
I
l
t
i
l
t
I
4 6 810 Dose rate trad/secl-
30
Figure 2. The rate of polymerization of methyl methacrylate a8 a function of the absorbed dose rate by y-rays and He-Li particles.
speaking, we are not investigating LET effects in pure monomers but in monomers containing small amounts of an additive. This difference is not significant in the cases of vinyl acetate and methyl methacrylate, but it is important in the case of styrene. If a comparison is carried out a t constant absorbed dose rate, it is found that the rate of polymerization always is smaller in the case of the He-Li particles than y-rays. It is more convenient for the quantitative discussion to compare the dose rates UR, and D R H e - L i which lead to the same rate of polymerization (see figures). The ratio f of these dose rates may be called the energy utilization of the He-Li particles with respect to the initiation of polymerization by y-rays. The value off is listed in Table I1 for the He-Li particle-induced polymerization of acrylonitrile as well cis of the monomers mentioned in Fig. 1-3. Table I1 : G(R) Value (Number of Radicals Produced per 100 e.v. of Absorbed ?-Radiation), f-Factor (Energy Ctilization), and W 1(Minimum &Electron Energy) in the Polymerization of Various Vinyl Compounds by I-Ie-TJi Particles
a 0
Dose rate [radlsec]
-
Figure 3. The rate of polymerization of vinyl acetate aa a function of the absorbed dose rate by y-rays and He Li particles.
at high dose rate^.^ The average of the slopes here amounts to about 0.4 over the dose range studied. The exponent a is found to have the same value in the y and He-Li particle irradiation of each of the monomers studied. The diagrams also show how the y-polymerization is influenced by the addition of small amounts of triethyl borate. ’ The rate of polymerization in styrene is strongly increased, which is not unexpected since the radiation stability of boron ester is much lower than that of the aromatic styrene molecule. The rate of polymerization of the two other monomers is slightly decreased by the addition of boron ester. Strictly The Journal of Phvsical Chemistru
Monomer
G(R)
!
Wl
Styrene Methyl methacrylate Acrylonitrile Vinyl acetate
0 69 5 5-11 5 2.46,O 9-1 2
0 16 0 09 0.07 0 04
170 205 215 240
The table shows that thef-factor is quite different for the monomers studied. The most pronounced LET effect (small value of f) was found to occur in vinyl acetate while the smallest difference in the rates of polymerization by y and He-Li radiation was observed in styrene. The intrinsic viscosities of the polymers formed in the experiments described by Fig. 1-3 are plotted IIS, the absorbed dose rate in Fig. 4. It can be seen that the average molecular weight always decreases with increasing dose rate as is expected from eq. 3. The slope of the straight lines in Fig. 4, L e . , the exponent b in eq. 3, has the same value for y and heavy particle irradiation. In the cases of styrene and methyl methacrylate, the average molecular weight is considerably higher in the formation of the polymer by He-Li particles if the comparison is carried out a t the same over-all absorbed dose rate. In the case of vinyl acetate, only a slight increase of the intrinsic viscosity (4)
A. Chapiro, “Radiation Chemistry of Polymeric Systems,” Interscience Publishers, New York, N. Y., London, 1962, p. 163.
EFFECTS OF LINEAR ENERGY TRANSFER
20t
313
vinyl acetate
methyl methacrylati
QbL.~ 1
2
,
I
4
,
I
I , I
6 8 10
I
,
I
I
+
k,DR,“
1
LO 60 dose rate [rad /see] 20
ular weight of the polymer would be epected t o be much lower than in the case of y-irradiation. Since a was found to be independent of the type of radiation and since the average molecular weight increases with decreasing dose rate and is higher than in the case of y irradiation, it must be concluded that the chains are growing and are deactivated far away from the tracks of the He-Li particles. In order to explain the low rate of polymerization a t high linear energy transfer, the absorbed dose rate may be presented as the sum of the two terms ( D R H + L ~X f) ( D R H + L , ( ~- f ) ) . The first term is the dose rate which effectively initiates long chain polymerization. It is identified as the rate of energy dissipation of the He-Li particle for the production of fast &electrons which sufficiently escape the tracks. The fraction DR(1 - f) is the rate of energy dissipation within the main track,of the particles. It is used to produce secondary electrons of less than about 100 e x . of kinetic energy which cannot escape the tracks. If the dose rates for equal rates of polymerization by y-rays and He-Li particles are compared, one obtains the relation
Figure 4. The intrinsic viscosity of the polymers formed by yand by He-Li radiation as a function of the absorbed dose rate: ( X ) pure monomer, ?-rays; 0, monomer containing boron ester, ?-rays; E, rnononier containing boron ester, He-Li particles).
with increasing LET is observed. This may be due to the fact that the growing radicals are mainly deactivated by chain transfer in vinyl a ~ e t a t e . ~ The effect of‘ triethyl borate on the average molecular weight of the polymers formed by y-irradiation is rather difficult to understand. For instance, the molecular weight of polystyrene is a little higher in the presence of 5 wt. yo of the additive, although one would expect from the strong increase in the rate of polymerization (Fig. 1) that shorter chains are formed in the plesence of triethyl borate. It seems, therefore, that the effect of triethyl borate on the rate of initiation is complex. However, the influence of the triethyl borate on the mechanism of the polymerization is not important in the comparisons between the effects of y-rays and He-Li particles since the additive was present a t the same concentration.
Discussion If the growing chains are deactivated by chains or radicals originating from the same track of a high LE?’ particle, the exponents a and b should be equal to 1.0 and 0, respectively. Furthermore, the average molec-
=
IC~DRH,-I,,~~~
(4)
from which it follows that
as already mentioned in the Experimental part. This reaction mechanism, in a rather schematic way, postulates that only those free radicals will initiate long chains of polymerization which are formed in the &ray tracks. However, it may occasionally occur that a free radical produced in the main track escapes the interaction with another radical during the expansion of the track by diffusion and contributes to the formation of a chain that grows into the bulk of the liquid. The probability of this contribution will increase with decreasing linear free radical density along the track. This quantity, however, is determined by the radiation sensitivity of the monomer. The G(R) value, ie., the number of free radicals produced per 100 e.v. of absorbed energy, may be taken as a measure of the radiation sensitivity. It must therefore be expected that there exists a relationship between the factor f and the G(R) value of a vinyl compound, L e . , f should become smaller with increasing free radical yield. The relationship between f and G(R) can be recognized from the data in Table 11. G(R) has been measured by various authors using different techniques (5)
L. Kachler, “Polymerisationskinetik,” Springer-Verlag, Berlin, 1951, p. 118.
Volume 68,Number 8
F&ruary, 1904
R. R. HAMMER AND N. W. GREGORY
3 14
such as the polymerization method or the consumption of dissolved DPPH.* In some cases, the G(R) values reported to date differ considerably. However, the predicted dependence off on G(R) is readily recognized from Table 11. The fraction of particle energy which is used to produce &electrons of kinetic energies between the upper value W,,, = (4m/M) E and a lower value W1can easily be calculated by using the classical approximation of the energy transfer in a heavy particleelectron collision' 1 In W,,, 2 In W,,,
9s = -
- In W1 - In I
(6)
Here M and E are the mass and kinetic energy of the
heavy particle, m the electron mass, and I the average ionization potential of the stopping atoms (about 50 e.v. in organic materials). Since the initial kinetic energy of the He-Li particles amounts to about 1 Mev., a value for E of 0.5 Mev. may be taken as representative. By substituting qs in eq. 6 by the f-factor, W , can be calculated. This energy then may be interpreted as the minimum kinetic energy of a &electron to participate in the initiation of long-chain polymerization. Table I1 shows the value of W1 calculated from the observed f-factor for several monomers. These numbers, of course, have only a formal meaning in terms of schematicized description of the tracks of a heavy particle. At least, they indicate the order of magnitude of the energy of secondary electrons necessary to initiate polymerization.
An Effusion Study of the Decomposition of Copper(I1) Bromide
by R. R. Hammer and N. W. Gregory Department of Chemistry, University of Washington, Seattle 6 , Washington
(Receiaed August $6,1069)
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A torsion effusion study of the reaction 2CuBr2(s) = 2CuBr(s) Brz(g) leads to values of AH" = 23.4 kcal. and AS" = 43 e.u. a t 298°K. Results are compared with earlier investigations a t higher temperatures. The condensation coefficient for the process appears to be of the order of 0.1 a t 50' and to decrease as the temperature increases. Apparent values for the activation enthalpies and entropies for the vaporizafion and condensation processes are calculated.
The decomposition of copper(I1) bromide has been 2CuBr2(s) = 2CuBr(s)
+ Brz(g)
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studied by a number of investigators. Equilibrium pressures of bromine reach 1 atm. at ca. 280'. At this and lower temperatures, neither of the solid compounds has a vapor pressure which contributes significantly to the total equilibrium pressure in the system. No evidence to suggest that the two solids are significantly soluble in each other has been found. Equilibrium data have been reported by Jackson,' Shchukarev and The Journal of Phy8icaE Chemistry
Oranskaya, and Barret and Guenebaut-Thevenot, all of whom used a diaphragm gage technique in the temperature interval between 130 and 316'. The results are not in good agreement. Studies of the rate of decomposition have been made by Barret and coworkers.. C. G. Jackson, J . Chem. SOC.,99, 1066 (1911). (2) S. M.Shchukarev and M. A. Oranskaya, Zh. Obshch. Khim., 24, 1926 (1954). (3) P. Barret and N . Guenebaut-Thevenot, Bull. BOC. chim. France, 409 (1957). (1)
