NOTES
246 Fig. 1 and Fig. 2 and by the equations L=~ ( 1 2) [-IO0 4-8 9 0 ( 2 ~- 1) - 400(22
+
- 1)'
700 (22 - 1)3]cal./mole, 25" (1) CpE = ~ ( 1 Z) 19.5 - 4 . 6 ( 2 ~- 1) 11.0(22 - l)a]cal./oC.mole, 25" (2) CpE = ~ ( 1 2) [10.9 - 6 . 4 ( 2 ~- 1) (3) 9 . 6 ( 2 ~- 1)2]ca1./oC. mole, 40"
+ +
for acetone-water, and by ~ (1 2) [660
L
CpE = 3.72(1 CPE= 3.22(1
+ 110(22 - 1) +
25(2z - l)e]cal./mole, 25' - z)cal./"C. mole, 25'
- z)cal./'C.
(4) (5) (6)
mole, 40"
Vol. 62
search grade, 99.87 mole % ' minimum purity) were used without any further purification. 2.2 Sample Preparation and Irradiation.-Desired amounts of samples in the irradiation cells (ea. 10 ml. Pyrex glass cells) were cooled in liquid nitrogen and evacuated. Degassing was by the distillation method; i.e., after the usual degassing the sample was distilled into a trap immersed in liquid nitrogen and then distilled back into the cell and sealed off under a high vacuum. Irradiation in a Ghormley-Hochanadel type Cow source was at the rate of about 2 X 1018 e.v. m1.-1 min.-l Dosage was established with the Fricke dosimeter, with the electron density correction applied to each sample. Irradiation time was in the range 100 to 200 minutes. I n the case of low temperature irradiation, the temperature of the sample was controlled by a stream of cooled nitrogen gas d!a constancy of temperature was maintained within f l . The liquid Sam les of neopentane exposed at room temperature were at agout 2 atm. pressure. 2.3 Gas Collection and Analysis.-Gas collection and analysis followed techniques conventional in this Laboratory. The collected gaseous samples were anaIyzed massspectrometrically .
for acetone-methanol. As can be seen from the figures, the agreement with Sandonnini' and Hirobe2 is fairly good but Sandonnini's data do not describe the complete curve. The large amounts of the heats of mixing and the complicated nature of the functions 1 to 3 are of course due to the effect 3. Results and Discussion of hydrogen bonding. Figure 1 shows that temperature has an imWe gratefully acknowledge - the advice of Dr. Otto perceptible effect on G(H2) from cyclohexane alRedlicrh. though there may be a small effect on G(CH4). (1) C. Sandonnini, Atti Accad. Lincei, 161 1, 448 (1925); 4, 63 On the other hand, there appear to be distinct ef(1926); C. Sandonnini and G. Gerosa, Cam. chim. itaE., 66,916 (1925). fects of temperature both on G(H2) and G(CH4) (2) H. Hirobe, J . Fac. Sci. I m p . Uniu. Tokyo, [I] 1, 191 (1926). from neopentane. Unlike effects in other hydrocarbon liquids previously G(CH4) becomes greater than G(H2) a t room temperature in RADIOLYSIS OF NEOPENTANE AND case. The results near room temperature for CYCLOHEXANE BY C060 -/-RADIATION : this liquid and vapor at 50 mm. pressure are compared EFFECT OF TEMPERATURE' below BY MOTOMEHAMASHIMA,'M. P. REDDY AND MILTON BURTON Department of Chemistry, University of Notre Dame, Notre Dame, Ind. Received A U Q U $8, R ~ 1067
1. A recent report by LampeS makes timely the result of a comparison of the effects of temperature on G(HJ and G(CH4) from liquid neopentane and cyclohexane, irradiated by Coco gammas. 2. Experimental Materials.-Cyclohexane (Eastman Kodak spectrograde) and 2,2-dimethylpropane (neopentane, Phillips re2.1
i
4 t
d
I
A
-I
-60
-
-_
OlJ-
-40
-20
-0
I
a
-
20
Temp., OC. Fig. 1.-Effect of temperature on 100 e.v. yields from neopentane and cyclohexane: O, G(H& neopentane' 0, G(H2), cyclohexane; m, G(CH4), neopentane; 0 , G(6H4), cyclohexane. (1) Contribution from the Radiation Project operated by the University of Notre Dame and supported in part under Atomic Energy Commission Contract AT(l1-11-38. (2) Fellow of the Atomio Energy Bureau of Japan, 1856-1867. (3) F. W. Lampe, TAIBJOURNAL, 61, 1016 (1967).
Neopent ane :
Liquid (This work)
Vapor (Lampes)
G(H2) G(CHd
1.40 3.18
4 . 3 f 1.1 1.8 f 0.6
In neopentane, the very large value of G(CH4) in the liquid as compared with that from 2,5-dimethylhexane or 2,2,4-trimethylpentane4 is somewhat startling. The suggestion that a FranckRabinowitch cage7 is ineffective because of the shape of the molecule is inadmissible on the basis of Lampe's results. The decrease in G(CH4) in going from liquid t o gas is contrary to what may be expected on simple removal of the surrounding molecules from a cage. Some type of molecular process appears required to explain the results obtained for the liquid. The molecular process need not be unimolecular. A convenient explanation of the greater effectiveness of the liquid as compared with the vapor in CH4 production is that one of the processes involved in CHI production in the liquid is bimolecular. Two possibly important bimolecular processes have been re-emphasized recently as a result of further studies on cyclohexane8 and methanoLg The first is a reaction between two excited mole(4) B. M. Tolbert and R . M. Lemmon, Radiation Research, 8 , 52 (1955). (5) E. Collinson and A. J. Swallow, Chem. Reus., 66, 473 (1956). (6) M. Burton, Chapter on "Radiation Chemistry of Organic Liquids," in "Actions Chimiques et Biologiques des Radiations," Vol. 111, edited by M. Haissinsky, Masson et Cie., Paris, 1957. (7) J. Franck and E. Rabinowitch, Trans. FaradaV Soe. 40, 120 (1934). (8) M. Burton, J. Chang, S. Lipsky and M. P. Reddy, Radiation Rcararchl, in press. (Q) C;. Meshitsuka and M. Burton, ibid., in press.
NOTES
Feb., 1958
247
cules suggested also by Dewhurst, Samuel and 2M*
+Products
t-
(1)
Mageelo as a possible reaction in radiolysis of water. Such a reaction occurs with any reasonable probability only in the liquid phase in the dense region of excitation represented by a spur. The second reactionll can occur with significant probability M
+ M + + e--+
Products
(2)
only in the liquid phase, where the ion and its associated electron can be considered to be in a state of collision with a neighboring molecule during the neutralization process. Reaction 2 is akin to Lind's. cluster theory but is considerably more restrictive. The residual CH4 yield in gaseous neopentane must be bv a reaction other than 1 or 2. The increase in G(H2) in going to the gas may be taken as evidence that reactions producing CH4 and H2 compete a t an early stage in the liquid; e.g., reaction 2 may compete very effectively with M+
+ e- -+
M*
+ R H + CHI + R + RH +Hz + R
I
0
I
I
5
I
I
I
I
I
10 15 ELECTRON V O L T S / q I
20
I
I
I J
25
change of intrinsic viscosity Fig. 1.-Polyacrylonitrile: (dimethylformamide solutions) with energy absorbed, in nitrogen and air (1000 kvp. electrons); A represents the dose above which gel formation is noted; triangles indicate the intrinsic viscosity of the soluble polymer isolated from the nitrogen-irradiated system (the sol fraction is given in parentheses).
(3)
The latter reaction goes without competition from 2 in the gas and is presumably the source of the increased Hz yield there. Another effect of state may perhaps be revealed by the G(CHr) curve for neopentane shown in Fig. 1. The rapid rise in G(CH4) begins near the melting point of neopentane, -20°, but it is too early to insist on the reality of such an effect, particularly when the cyclohexane data show no important change near 6.5",the m.p. of that compound. I n the case of neopentane Fig. 1 shows a real effect of temperature on both CH4 and Hz. The simplest explanation is that free radical reactions CH, H
Nitrogen
(4) (5)
compete with radical or atom combination processes. The greater effect of temperature on CH4 yield is consistent with the fact that activation energies for reactions such as 4 and 5 usually bear the relationship E4 > E6.
t0
I
' . ,
Nitrogen
I
I
CI
I
IO
I
I 20
15
ELECTRON V O L T S / q
25
x
Fig. 2.-Poly-~+methacrylonitrile: change of intrinsic viscosity with energy absorbed; solvent acetone.
nitrile (PMAN) were examined. The role of oxygen in these irradiations was briefly studied.
Chemistry Depaitment, Scientific Laboratory, Ford Motor Company, Dearborn, Michigan Received August 19,i967
Experimental The procedures for the irradiations (1000 kvp. electrons), dosimetry and apparatus for the infrared and mass spectrometric analyses have been described;2 Debye-Scherrer diffraction patterns were obtained from powdered samples with a G. E. XRD-4 X-ray unit. PAN (American Cyanamid) was reprecipitated from dimethylformamide. PMAN, prepared by the benzoyl peroxide catalyzed (0.1%) p q l y merization of monomer to 20% conversion, was reprecipitated from acetone. About 0.5-g. samples of powdered polymer of particle size 10-15 were employed for the irradiations. PAN films were cast from 1% dimethylformamide solutions; after drying in vacuo for several days, some solvent was still detectable with the infrared spectrometer. It was assumed that one gram of material absorbed 58 X 1OL8 e.v. when given 1 megaroentgen.
It has been noted' that polymers which predominantly cross-link when subjected to ionizing radiation (absence of oxygen) have higher heats of polymerization and afford less monomer when pyrolyzed than those that degrade when irradiated ( H Pand behavior upon pyrolysis reflect the steric configuration of the polymer chain). I n light of this correlation, the effects of 1 Mev. electrons on polyacrylonitrile (PAN) and poly-a-methacrylo-
Results and Discussion Polyacrylonitrile Powder.-In nitrogen the net initial effect is one of cross-linking (Fig. 1); gelation is observed a t doses above 10 X 106 rep. That chain cleavage occurs simultaneously, at least above this dose, is indicated by the lower intrinsic viscosity of the soluble fraction. With oxygen present, radicals initially produced form oxidized species less stable than the original
(10) H. A. Dewhurst, A . H. Samuel and J. L. Magee, ibid., 1, 62 (1954). (11) M. Burton and B. Lipsky, THISJOURNAL, 61, 1461 (1957).
EFFECT OF RADIATION ON POLYACRYLONITRILE AND POLY-a-METHACRYLONITRILE BY W. J. BURLANT A N D C. R. TAYLOR
(1) L. Wall, J . Polymer Bci., 17, 141 (1955).
(2)
W. Burlant and A. Adiooff, ibid., in press (1957).
